half rate in bsc

Half Rate in BSC

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# Nokia Siemens Networks 1 (80)

BSC3153Nokia GSM/EDGE BSS, Rel. BSS13, BSC andTCSM, Rel. S13, Product Documentation, v.1

The information in this document is subject to change without notice and describes only theproduct defined in the introduction of this documentation. This documentation is intended for theuse of Nokia Siemens Networks customers only for the purposes of the agreement under whichthe document is submitted, and no part of it may be used, reproduced, modified or transmitted inany form or means without the prior written permission of Nokia Siemens Networks. Thedocumentation has been prepared to be used by professional and properly trained personnel,and the customer assumes full responsibility when using it. Nokia Siemens Networks welcomescustomer comments as part of the process of continuous development and improvement of thedocumentation.

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Half Rate in BSC

Contents

Contents 3

List of tables 4

List of figures 5

Summary of changes 7

1 Overview of Half Rate 9

2 Technical description of Half Rate 112.1 Capacity and channel resources 152.2 Requirements for Half Rate 16

3 Channel rate and speech codec control in handovers 19

4 Channel rate selection based on cell load 27

5 Half Rate and radio resource allocation 335.1 The A interface in radio resource allocation 455.2 The Abis interface in radio resource allocation 515.3 The Radio interface in radio resource allocation 52

6 TCH allocation schemes in Half Rate 55

7 User interface of Half Rate 79

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Contents

List of tables

Table 1. Conversion of RTSL types on the Abis O & M interface 12

Table 2. Software requirements 16

Table 3. Circuit modes supported by TCSM2 47

Table 4. Circuit modes supported by TCSM3i 48

Table 5. A interface circuit pools supported by the BSC 48

Table 6. Uplink DTX control 53


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List of figures

Figure 1. Transmission example 17

Figure 2. Cell load based TCH allocation 28

Figure 3. Assignment, mobile terminated call 56

Figure 4. Assignment, mobile terminated call (continued) 57

Figure 5. Assignment, change of the A interface circuit pool 60

Figure 6. Internal handover, intra-cell 63

Figure 7. Internal handover, intra-cell (continued) 64

Figure 8. Internal handover, inter-cell (asynchronous) 66

Figure 9. Internal handover, inter-cell (asynchronous) (continued) 67

Figure 10. External handover, source BSS 70

Figure 11. External handover, target BSS 72

Figure 12. External handover, change of the A interface circuit pool 74

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List of figures


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Summary of changes

Changes between document issues are cumulative. Therefore, the latestdocument issue contains all changes made to previous issues.

Changes made between issues 11-0 and 10-0

Section Fault recovery in chapter Technical description of Half Rate hasbeen updated because now also non-control channels can bereconfigured in fault recovery situations.

Information on Nokia Flexi EDGE BTS has been added to table Softwarerequirements in chapter Technical description of Half Rate.

Information on Nokia PrimeSite BTSs and Nokia 2nd generation BTSs hasbeen removed.


Chapter Overview of Half Rate

The list of related topics has been modified and updated.

Chapter Technical description of Half Rate

Subsection Capacity and channel resources

Information referring to TRCU and SM2M has been deleted andinformation referring to TCSM3i has been added.

Subsection Requirements for Half Rate

Figure Transmission example has been updated.

Information referring to TCSM2 and the explanation of the function ofGroup Switches has been deleted.

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Summary of changes

Chapter Channel rate and speech codec control in handovers

A reference to internal handover to external has been added.

Chapter Half Rate and radio resource allocation

More information on the channel allocation for mobile-terminating calls(MTCs) has been added to TCH type requirements and to Restrictionscaused by FACCH call set-up.

Minor editorial changes throughout the document.


Subsection RXLev Min Access has been added to Chapter TCH allocationschemes in Half Rate.

Half Rate data qualities have been modified due to HR data support.

Nokia licence control information has been added.


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1 Overview of Half Rate

With Half Rate (HR) coding it is possible to maximise the spectrumefficiency. HR coding almost doubles the amount of radio resources,compared with the use of full rate traffic channels (TCHs). Whenever HR ismentioned in Half Rate in BSC, it applies to speech or signalling casesonly. HR coding for data is not supported.

Full rate (FR) speech and data is coded and transferred using 16 kbit/schannels in the BSS. With the HR coding, 8 kbit/s transmission can beused on the Abis interface. This is also possible on the Ater interface whenthe BSS (BTS, BSC, transcoder) and mobile stations all support HR.

Each radio timeslot of the BTS TRX can be configured to be a FR, HR, ordual rate (DR) TCH resource on the Abis interface. In the case of dual rate,the BSC dynamically allocates an idle radio timeslot either for HR or FRcoding on a call basis.

It is possible to introduce HR coding to existing full rate GSM networksgradually. The A interface supports different types of transcoders capableof FR coding, HR coding, or both. The BSS is able to co-operatesimultaneously with both old phase 1 FR-only mobile stations and withphase 2 mobile stations that support both FR and HR. HR can, therefore,also be installed in the network without replacing the old transcoders withnew ones that support the HR coding.

Half Rate is an application software in the BSC.

Related topics in Half Rate in BSC

. Technical description of Half Rate

. Channel rate and speech codec control in handovers

. Channel rate selection based on cell load

. Half Rate and radio resource allocation

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Overview of Half Rate

. TCH allocation schemes in Half Rate

. User interface of Half Rate

Other related topics

. Functional descriptions. Radio network performance

. Radio Channel Allocation. Operability

. Radio Network Configuration Management

. Radio Network Supervision in BSC

. Feature descriptions. Radio network performance

. Dynamic Frequency and Channel Allocation

. Enhanced Speech Codecs: AMR and EFR

. FACCH Call Set-up

. Intelligent Underlay-Overlay. Data

. GPRS/EDGE in BSC. Value added services

. Radio Resource Pre-emption and Queuing

. Product descriptions. Product Description of Nokia BSC2i and BSCi High Capacity

Base Station Controller. Product Description of Nokia BSC3i High Capacity Base

Station Controller

. Test and activate. Radio network performance

. Activating and Testing BSS6115: Half Rate

. Operate and maintain. BSC Statistics Administration

. Reference. Alarms

. Base Station Alarms (7000–7999)

. Parameters. BSS Radio Network Parameter Dictionary


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2 Technical description of Half RateHalf Rate and radio resource allocation

Introduction of Half Rate (HR) to a network has significant effects on theTCH allocation algorithm of the BSC. With the most optimised allocation oftraffic channels, the BSC must be able to ensure the penetration of mobilestations which do or do not support HR. The BSC must also be able to co-operate with the other network elements having different kinds ofcapabilities in supporting HR.

For more information on radio resource allocation and channel rate, see

. Overview of Half Rate



. Half Rate and radio resource allocation

. Activating and Testing BSS6115: Half Rate under Test and activate/Radio network performance in the PDF view

. Channel rate for AMR calls in Enhanced Speech Codecs: AMR andEFR under Feature descriptions/Radio network performance in thePDF view

. Radio Channel Allocation under Functional descriptions/Radionetwork performance in the PDF view

. Radio Resource Pre-emption and Queuing under Featuredescriptions/Value added services in the PDF view

. Dynamic Frequency and Channel Allocation under Featuredescriptions/Radio network performance in the PDF view

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Technical description of Half Rate

Effects on the Abis O & M interface

A logical radio timeslot (RTSL) type dual rate (DR) TCH is introduced onthe Abis O & M interface. The BSC converts RTSL TCH types of the BSCdatabase for the Abis O & M interface configuration messages accordingto table Conversion of RTSL types on the Abis O & M interface.

Table 1. Conversion of RTSL types on the Abis O & M interface

BSC database Abis O&M BTS

TCHF TCHF TCHF

TCHH TCHD TCHD

TCHD TCHD TCHD

If a TRX has one or more DR TCH RTSLs, all the RTSLs of the TRX arehandled as DR TCH RTSLs in the BTS.

In the TRX reset and restart procedures, the BTS checks that the logicalchannel configuration given by the BSC in the TRX configuration messagematches the actual hardware/software configuration of the BTS. If thelogical configuration of a TRX contains HR TCH or DR TCH RTSLs andthe TRX hardware or BTS software does not support HR, the BTSgenerates a fatal TRX-wide alarm.

When a TRX test using RTSL(s) configured as a HR TCH or a DR TCH isrun, the BTS configures the RTSL dynamically as a full rate (FR) TCH. TheFR path is tested and after the test the BTS returns the original channelconfiguration of the RTSL into use.

Effects on the Q3 interface

Two values (HR TCH and DR TCH) for the channel type parameter of theradio channel object are introduced on the Q3 interface.

A TRX-specific parameter is needed to indicate whether the TRX supportsHR or not.

Radio network supervision

The radio network supervision reports on the network capacity problemsand certain types of repeated or continuous errors and disturbances inradio channel transactions on the BTS level. The success of HR and FRTCH transactions is monitored by both channel and BTS-specificsupervisions. The channel-specific supervisions are:


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. mean duration of calls on a specific channel

The measurement reveals possible hanging calls and sudden calldrops immediately after call establishment.

. channel failure rate

The measurement reveals abnormally ended transactions on aspecific channel.

. interference estimation of a TCH RTSL

The measurement is based on the interference-level informationwhich is received per TCH subchannel from the BTS.

The traffic channel congestion level is measured per BTS. HR and FRtraffic portions in congestion can be observed separately.

For more information, see Radio Network Supervision in BSC underFunctional descriptions/Operability in the PDF view.

Supervision of available traffic channels

The system supervises the ratio of available traffic channels to theconfigured traffic channels on the BTS, in case the ratio falls below thegiven threshold. The availability of HR TCHs and FR TCHs per sectormust be supervised separately, and an alarm is triggered if the availabilityof either HR TCHs or FR TCHs falls below the user-defined limit.

If the number of HR TCHs is below the given threshold, the alarm 7717WORKING HALF RATE TCH RATIO BELOW THRESHOLD is triggered. Ifthe number of FR TCHs is below the given threshold, the 7711 WORKINGFULL RATE TCH RATIO BELOW THRESHOLD alarm is triggered. Thesystem cancels these alarms when the given threshold is exceeded.

You can define the supervision alarm limits with the Base StationController Parameter Handling in BSC MML commands (command groupEE). The alarm limit for full rate TCH availability (ALFRT)parameter defines the lower limit value for the available FR TCHs. Thealarm limit for half rate TCH availability (ALHRT) parameterdetermines the corresponding threshold value for HR TCHs.

For more information on alarms, see Base Station Alarms (7000–7999)under Reference/Alarms in the PDF view.

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Fault recovery

During a recovery situation when HR is used, each RTSL holding controlchannels is included in the number of TCHs as two TCH resources. In thisway, it is possible to control the number of real TCHs in a recoverysituation.

The radio network recovery system handles the HR TCH resources (HRTCH and DR TCH) in the same way as the FR TCH resources. Therefore,if a TRX becomes faulty, the system must take into account the TRX'scapability to support HR TCHs. The system reads this from the TRX halfrate support (HRS) parameter of the TRX object.

Depending on the BTS configuration in a recovery situation:

. all timeslots of the TRX are reconfigured or

. the user defined traffic timeslots of the TRX are restored.

The RNW recovery reconfigures all timeslots of the TRX in TRXreconfiguration, if Soft Channel Capacity and AMR are used and allunlocked TRXs of the BTS have been defined to support HR. The RNWrecovery can restore the operator-defined original traffic channelconfiguration of the TRX in TRX reconfiguration, if Soft Channel Capacityis not in use, or AMR HR is not in use in the BTS or not all unlocked TRXsof the BTS have been defined to support HR.

The 7727 TRAFFIC CHANNEL NUMBER DECREASED alarm isgenerated by the radio network recovery system to indicate that becauseof recovery actions some HR TCHs are missing. Alternatively, the TCHsare changed to NOTUSED channels, because TCH resources have runout. The channel types changed by the system start from CH7 downwards.The BSC cancels the alarm automatically when the original TCH count isreturned or the user modifies the channel configuration of the TRX.

In all cases, the recovery system tries to find such a new TRX that the HRTCHs would not be missed.

The HR TCH disappearance can be avoided by configuring the BTSs insuch a way that there is no combination of TRXs that support HR andTRXs that do not support HR in the same cell.

For more information on alarms and parameters, see Base Station Alarms(7000–7999) under Reference/Alarms and BSS Radio Network ParameterDictionary under Reference/Parameters in the PDF view.


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Errors detected by call control in the BSC

If the BSC detects channel type or state mismatches concerning HR TCHsor DR TCHs, the radio network recovery system runs recovery actions forthe entire RTSL. The same applies to recovery requests if the BTS hasrejected the HR channel allocation request too many times during asupervision period.

BSS radio network traffic administration

The aim of the traffic administration is to provide relevant information onthe traffic capabilities of the network. Especially in this case you must havethe means to ascertain the functioning of HR. Furthermore, an aid is givenfor dimensioning the network, for example, for ascertaining the blocking ofFR BTSs to upgrade them to HR BTSs.

Performance management

Performance management provides information on the success of HRcalls and their share of the traffic. It is possible to compare the amount andsuccess of HR and FR traffic to find out the capacity of the network as wellas the effects that the implementation of HR has on the penetration ofcalls.

For more information, see BSC Statistics Administration under Operateand maintain in the PDF view.

Allocation of an RTSL for BSS radio network testing

Radio Network tests are run for FR paths only, which means that if theRTSL is configured as a HR TCH or a DR TCH, the entire RTSL (both HRTCHs) has to be allocated for the test.

2.1 Capacity and channel resources

Together with HR-related routing and switching and the second generationtranscoder submultiplexer (TCSM2) it is possible to configure 210 HRchannels (seven HR transcoder PCMs connected to one transcoder unit).TCSM3i and TCSM3i for combined BSC3i/TCSM3i installation allow theconfiguration of 120 HR channels (four HR transcoder PCMs connected toone transcoder unit).

In a FR configuration, the maximum number of FR channels is 116 (fourFR transcoder PCMs connected to one transcoder unit).

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It is also possible to create mixed configurations with the accuracy of oneA interface PCM. An example is presented in figure 1 Transmissionexample. One FR, one dual rate (DR) and three HR A interface PCMs arecarried over one Ater PCM link, 150 traffic channels in total.

For more information on capacity and channel resources, see the followingdocuments under Product descriptions in the PDF view:

. Product Description of Nokia BSC2i and BSCi High Capacity BaseStation Controller

. Product Description of Nokia BSC3i High Capacity Base StationController

For more information on signalling capacity, see Radio NetworkConfiguration Management under Functional descriptions/Operability inthe PDF view.

2.2 Requirements for Half Rate

Software requirements

Table 2. Software requirements

Network element Software requirements

BTS Nokia Talk-family BTS SW release D2.1 or later

Any release of Nokia MetroSite BTS, Nokia InSiteBTS, or Nokia UltraSite EDGE BTS or Nokia FlexiEDGE BTS

Nokia Flexi EDGE BTS EP1.0

MSC M7 or later

Circuit pool support is required by the MSC.

NetAct OSS4.1 or later

BSC S5 or later

Hardware requirements

HR can be employed only when the BSC is equipped with the bit groupswitch (GSWB). In the GSWB, which can handle 8 kbit/s sub-timeslots, allconnections between outgoing and incoming bits can be made with one-bitaccuracy.


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Transmission management requirements

Figure Transmission example presents an example how the transcoderPCMs are configured on the A interface.

Figure 1. Transmission example

The transcoder PCMs of the first TCSM unit are all in HR mode. Thesecond TCSM unit includes all three types of transcoder PCMs: HR, FR,and DR. The third unit is an HR transcoder.

From the MSC point of view, all PCMs are normal PCMs which consist of64 kbit/s timeslots.

PCM 1BTS BSC TCSM HR MSC

ET PCM 2GSWB HR GSW

PCM 3HR

PCM 4BTS ET HR

PCM 5HR

PCM 6HR

PCM 7HR

BTSPCM 1

TCSM HRPCM 2

FRPCM 3

ET DRPCM 4

HRPCM 5

HR

PCM 1TCSM HR

PCM 2HR

ET PCM 3HR

PCM 4HR

ET_PCM transcoder PCM

AterInterface

AInterface

AbisInterface

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3 Channel rate and speech codec controlin handovers

The traffic channel rate and the speech codec that are used in a call canbe changed when a new channel is allocated for the call during ahandover. The change can be made if only a preferred channel ratewithout any rate change prohibitions is given in the handover request. Theused speech codec can also be changed within a TCH rate, if a handoveris performed between BTSs with different speech encoding capabilitiesand several speech codec alternatives have been given for the rate in therequest. In speech call cases this may require that the A interface circuittype is changed to another one if the circuit used on the old channel doesnot support the new channel rate.

There can be a need to deny channel rate and speech codec changesduring the handovers for speech calls to avoid sudden tone changesduring a call. Therefore, if the channel rate selection is not restricted by thesystem, you can set extra constraints for the channel rate and speechcodec changes. The changes can be restricted both on the BSS-level (forinter-cell and intra-cell handovers) and on the BTS-level (for intra-cellhandovers).

When the speech codec is changed during an internal handover, there is ashort break in the downlink speech path because, in this case, aunidirectional connection cannot be established. However, the BSC canchange these handovers to external handovers to enable a unidirectionalconnection and in this way avoid downlink muting. For more information,see TCH rate in internal handover in Radio Channel Allocation underFunctional descriptions/Radio network performance in the PDF view.

Parameters

In handovers for speech calls, the parameters TCH in handover (HRI)and TCH rate intra—cell handover (TRIH)control both channel rateand speech codec selection. In the case of signalling, a half rate (HR) TCHis allocated primarily unless a full rate (FR) TCH is particularly requested.

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Channel rate and speech codec control in handovers

. TCH in handover (HRI)

This BSC-specific parameter controls the TCH rate determinationand speech codec selection in TCH allocation during handovers.The parameter is applied to each handover for which the type of thesource TCH and the source speech version is known, that is, at leastto all BSC-controlled handovers. It can also be applied to the MSC-controlled, external handovers when the channel type or the speechversion that the call is using in the cell of the source BSS is known inthe target BSS. By default, the call-maintaining type of TCH isallocated and the actual speech codec used by the call is kept up inthe handover.

The parameter can have the following values:. 1

In the handover, the best candidate cell in which the same typeof TCH and the same speech version can be used as in thesource cell is selected for a target cell.

If such a cell does not exist, the other speech version is usedor finally the other type of TCH is allocated from the bestpossible cell.

This means that in the handover candidate cell list, the cell inwhich it is possible to use the same type of TCH, HR or FR thatis used by the call in the source cell of the handover has thehighest priority.

In case one or more cells of that type exist, the best cell inwhich the same speech version is used for the call as in thesource cell of the handover is selected. If the same speechversion is not used in any of these cells, the TCH is allocatedfrom the best of these cells. In the initial radio resourcerequest, the MSC determines the order of preference,according to which the speech version representing the type ofthe TCH to be allocated is selected.

If all the handover candidate cells are such that it is notpossible even to allocate the TCH of the type which is used forthe call in the source cell, the other type of TCH is allocatedfrom the best possible candidate cell. In the initial radioresource request, the MSC determines the order of preferenceaccording to which the speech version representing the type ofthe TCH to be allocated is selected.

The actual TCH resource situation, that is, the traffic load ofthe cells, also affects the type of the target TCH when the callis maintained in a FR TCH in the source cell. When the FRTCH is searched for among the candidate cells, those cells inwhich the traffic load requires allocation of the HR TCH, are


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ignored. The FR TCH is allocated from the best cell in whichthe traffic load situation makes it possible. If all the candidatecells are overloaded, an HR TCH is allocated from the best ofthe candidate cells.

When the type of the source TCH is HR and, therefore, an HRTCH is attempted to be allocated primarily, the actual trafficload of the candidate cells does not affect the procedure. TheHR TCH is allocated from the best of the candidate cellsregardless of the amount of possible available FR TCHresources in the cell.

External handover: The current Channel and Speech Version(used) elements are used, when present, as the primarydecisive factor when allocating a TCH for a speechconnection. For a data call, FR TCH is the only alternative. Ifthe Current Channel and Speech Version (used) elements arenot present or they are not in line with the received ChannelType element, the decisive factor is the requirements in theChannel Type element.

Internal handover: The same applies with the chosen channeland speech codec.

. 2

During the speech connection the call serving type of the TCHis allocated primarily and the original speech version is used.In the case of data connection, the TCH is allocated from thebest cell in the candidate list.

For the speech calls, the procedure is similar to that of value 1.For data calls, see the description of value 5.

External handover: Current Channel and Speech Version(used) elements are used, when present, as the primarydecisive factor when allocating a TCH for a speechconnection, if the received Channel Type element supportsCurrent Channel and Speech Version (used). If the CurrentChannel and Speech Version (used) elements are not presentor they are not in line with the received Channel Type element,the decisive factor is the requirements in the Channel Typeelement.

Internal handover: The same applies with the chosen channeland speech codec.

. 3

Channel rate and speech version changes are denied totally.

This means that an attempt is made to find a cell in thehandover candidate list in which it is possible to use the sametype of TCH (HR or FR), which is used by the call in the sourcecell of the handover.

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The call-serving type of channel is the only alternative in theTCH allocation. It is not allowed to change the actual speechversion used by the call either.

The traffic load of the candidate cells is not of any importancein this case. The only requirement is that there is a cell inwhich the TCH can be allocated so that the speech version isnot changed. The best of those handover candidates isselected regardless of its traffic load.

External handover: Current Channel and Speech Version(used) elements are used, when present, as the only decisivefactor when allocating a TCH for a speech connection. For adata call, FR TCH is the only alternative. If the CurrentChannel and Speech Version (used) elements are not presentor they are not in line with the received Channel Type element,the decisive factor is the requirements in the Channel Typeelement.

Internal handover: The same applies with chosen channel andspeech codec.

. 4

The TCH is allocated from the best handover candidate cell inwhich the call can use the type of TCH and speech versionthat the MSC has set to be the preferred ones in the initialradio resource request. The speech version can be changed inorder of preference if the allocation of the TCH requires it.

This means that a cell which uses the type of TCH (HR or FR)that the MSC has originally set to be the preferred one, isprimarily looked for from the handover candidate cell list. TheTCH type and the speech version which the call is using in thesource cell of the handover is ignored.

If such cells exist, the best of them is selected in such a waythat the speech version which best meets the preference orderof the codecs determined by the MSC is used for the call.

If all the handover candidate cells are such that even the TCHof the type set preferred by the MSC cannot be allocated fromany of them, the other type of TCH is allocated from the bestpossible cell. The speech version corresponding to the type ofthe TCH is selected in the preference order which the MSChas determined in the initial radio resource request.

The actual TCH resource situation, that is, the traffic load ofthe cells, affects the type of the target TCH when the MSC hasinitially preferred the FR TCH. The FR TCH is searched fromthe candidate cells. Those cells in which the traffic loadrequires allocation of an HR TCH are ignored and the FR TCHis allocated from the best cell in which the traffic load situationmakes it possible. If all the candidate cells are overloaded, anHR TCH is allocated from the best of the candidate cells.


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When the MSC has preferred the HR TCH and, therefore, theHR TCH is attempted to be allocated primarily, the actual trafficload of the candidate cells does not affect the procedure. TheHR TCH is allocated from the best of the candidate cellsregardless of the amount of possible available FR TCHresources in the cell.

External and internal handovers: The requirements in theChannel Type element are the primary decisive factor whenallocating a TCH for a speech connection. Current Channeland Speech Version (used) elements are not used. For a dataconnection, FR TCH is the only alternative.

. 5

The TCH must be allocated from the best BTS of the handovercandidate list, regardless of the speech codec version actuallyused.

This means that the TCH is allocated from the best handovercandidate cell that has the available TCH resources.

An attempt is made in the cell to allocate the same type of TCH(HR or FR) which is used for the call in the source cell of thehandover. If this is possible, it is also checked whether thesame speech version can be used for the call as in the sourcecell of the handover. If the speech version cannot be used, thespeech version which represents the type of the TCH to beallocated is selected in the preference order which the MSChas determined in the initial radio resource request.

When the TCH of the type which is used for the call in thesource cell of the handover cannot be allocated, the other typeof TCH is allocated. The speech version corresponding to thetype of the TCH is selected in the preference order which theMSC has determined in the initial radio resource request.

The actual TCH resource situation, that is, the traffic load ofthe best cell, also affects the type of the target TCH when thecall is maintained in FR TCH in the source cell. The FR TCH isallocated when the cell is not overloaded. If the cell isoverloaded, an HR TCH is allocated.

When the type of the source TCH is HR and, therefore, the HRTCH is attempted to be allocated primarily from the best cell,the actual traffic load of the cell does not have an effect at all.The HR TCH is allocated if one is available.

External handover: Current Channel and Speech Version(used) elements are used, when present, as the primarydecisive factor when allocating a TCH for a speechconnection. For a data call, FR TCH is the only alternative. IfCurrent Channel and Speech Version (used) elements are not

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present or they are not in line with the received Channel Typeelement or they prevent the TCH allocation from the bestcandidate, the decisive factor is the requirements in theChannel Type element.

Internal handover: The same applies with chosen channel andspeech codec.

The parameter can be handled with the EEM and EEO commands.

. TCH rate intra—cell handover (TRIH)

Though the BSS-level channel rate control is valid for all the TCHhandovers, it can be readjusted with this BTS-specific parameter forthe internal intra-cell handovers in each BTS. The possibleparameterisation alternatives correspond to values 1-4 of theparameter TCH in handover (HRI).

An additional value 0 corresponds to the case where no extraconstraints for the intra-cell handovers are given and the TCH inhandover (HRI) parameter is also applied for the intra-cellhandovers in the particular BTS.

If the BSS-level parameter has the value of 5, a TCH with the samecharacteristics as the call-serving TCH is preferred primarily fromanother TRX than the one maintaining the call in the BTS. Ifpossible, the call-serving type of TCH and the used speech versionare preferred.

The parameter can be handled with the EQM and EQO commands.

Note that if the MSC has denied any channel rate changes duringhandovers in the initial resource request, this cannot be overwritten witheither of the two parameters.

These parameters control the channel rate and speech codec in internaland external handovers if the TCH is allocated for speech. For a dataconnection, FR TCH is the only alternative. If signalling is indicated in theresource request, the preferred channel rate type of channel is allocatedprimarily for internal handovers. Note that the type of the allocated TCHfulfils all the conditions determined primarily by the Channel Typeinformation element (IE) of the initial A interface message and the type ofthe A interface circuit. The BSC parameters give the extra constraints forthe channel selection. Especially if the channel rate changes are deniedafter the first TCH assignment, it is not possible to evade the prohibition bygiving a suitable value to the parameters. However, in emergency calls thechannel rate is always determined merely on the basis of the ChannelType and A interface circuit type.


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The BSC is able to send in the A interface HANDOVER REQUIREDmessage and receive two optional information elements, Current Channeland Speech Version in the HANDOVER REQUEST message. Wheneverthis information is available in the target BSC of the handover, that is,when the MSC is able to transfer it, the TCH type and speech versioncontrol, described above, is also applied for the external handovers.

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4 Channel rate selection based on cell load

Channel rate selection based on cell load offers the possibility todetermine the distribution between full rate (FR) TCH and half rate (HR)TCHs according to the cell load in TCH allocation for requests that canaccept a TCH of any channel rate.

TCH allocation control according to cell load is possible when

. the MSC does not determine the channel rate uniquely but givesonly the preferred rate, that is, HR or FR

. the BTS traffic channel configuration, the resource situation in thecell, and the parameters the operator has possibly set do not restrictthe TCH allocation

To apply cell load in TCH rate selection, the A interface circuit mustsupport both TCH rates. If there is no support for both TCH rates, the Ainterface circuit must be changed to one from another circuit pool before aTCH of the selected rate can be allocated.

If traffic channel allocation based on cell load is applied, FR TCHs areallocated until the number of free FR resources is reduced below a specificlower limit. After this the HR resources are allocated. When the number ofFR TCH resources increases above a specific upper limit, FR TCHs areallocated again. This is illustrated with Figure Cell load based TCHallocation.

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Channel rate selection based on cell load

Figure 2. Cell load based TCH allocation

HR controls the TCH allocation for dual rate (DR) requests, that is, whenboth channel rates are acceptable. For requests that determine a singleTCH rate a channel of the respective type is allocated regardless of thetraffic channel load in the BTS.

HR is applicable for speech calls and signalling with some exceptions. Fordata calls, FR TCH is the only alternative.

. In a speech call, if HR is set to be the preferred rate, thecorresponding free permanent TCH resource is allocated primarily,regardless of whether the cell-load-based TCH allocation has beenactivated or not.

. With signalling, a HR TCH is always allocated if it is set to be theprimary requirement although the cell load does not currently requireit.

Free FR TCHs

Time

Upper limit forfree FR TCHs

Lower limit forfree FR TCHs

Allocation ofFR TCHs

Allocation ofHR TCHs

Allocation ofFR TCHs


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If HR is activated and the lower limit is set to be greater than zero, at leastthe last free DR RTSL is split into two HR TCHs in TCH allocation. Thismakes it possible to determine a positive margin for the HR TCH allocationin cells equipped with only one TRX without making the lower limitunnecessarily high.

If the value of the lower limit equals zero, HR TCH resource can beallocated for a speech call only if the MSC strictly requires it, regardless ofwhether the target cell has permanent HR resources or not.

Parameters

HR is controlled by two parameters that determine the values of the upperand the lower limit for FR TCH load in a cell. The limit parameters are givenas relative amounts of free FR TCH resources in proportion to working FRTCH resources. The parameters can be defined on both the BSC level andthe SEG level.

. lower limit for FR TCH resources. BSC level: lower limit for FR TCH resources (HRL). SEG level: lower limit for FR TCH resources (FRL)

The parameter gives the lower limit percentage value of the relativeamount of free FR TCH resources. The limit includes permanent andDR types together. If a FR TCH was chosen the previous time therate determination dependent on cell load was applied, the relativeamount of free FR TCH resources must be above this limit for a FRTCH to be allocated. If the relative amount of free FR TCH resourcesis below the lower limit, a HR TCH has to be allocated.

. upper limit for FR TCH resources. BSC level: upper limit for FR TCH resources (HRU). SEG level: upper limit for FR TCH resources (FRU)

The parameter gives the upper limit percentage value of the relativeamount of free FR TCH resources. If a HR TCH was chosen theprevious time the rate determination dependent on cell load wasapplied, the relative amount of free FR TCH resources must bebelow this limit for a HR TCH to be allocated. If the relative amount offree FR TCH resources is above the upper limit, a FR TCH has to beallocated.

HR is enabled in specific cells when either of the two parameter pairs iseffective. However, when the SEG-level parameters are effective, theBSC-level parameters are not significant in TCH allocation.

If the upper limit is set smaller than the lower limit the effect of theparameters is deactivated.

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The SEG-level parameters, if active, are followed in a particular cell. If noindividual channel rate control rules have been specified in the cell, thecommon BSC-level parameters are applied. The default values of theparameters have been set so that the cell load is not initially used.

HR is disabled in specific cells when it is disabled both on the BSS leveland cell level.

These parameters are handled with the EEM, EEO, EQM, and EQOcommands.

Channel rate control in handovers

In addition to the call set-up and external handovers, the two TCHallocation thresholds can also be applied in internal handovers, but only ifthe handover request does not contain any limitations concerning thechannel rate changes, or if they are not defined on the BSS level.

The parameters that control the channel rate, TCH in handover (HRI)and TCH rate intra-cell handover (TRIH), cause the followingrestrictions for applying the cell load threshold parameters:

. If HR is the preferred channel rate of the resource request, thecorresponding available permanent HR TCH resource is allocatedprimarily, regardless of whether the cell-load-based TCH allocationhas been activated or not.

. An HR TCH is always allocated if the cell load requires it, regardlessof possible existing available permanent FR resources or thepreferred FR TCH requirement of the TCH resource request.

. When the TCH is allocated for internal handover and the channelrate changes are denied totally (either by the MSC or by the BSC),neither of them are allowed because of the cell-load-dependentresource conditions.

. When the TCH is allocated for internal handovers and the call-maintaining type of TCH is determined to be the preferredalternative, a HR channel is allocated for a HR call from the bestpossible cell, regardless of cell load.

. When the TCH is allocated for internal handover: if the rate preferredby the MSC is defined as the primary rate to be used, and if the MSChas a preferred HR TCH, a HR channel is allocated from the bestpossible cell, regardless of the traffic load in the cell.

The other values of the parameters controlling channel rate make itpossible to take into account the cell load thresholds in TCH allocation forinternal handovers because of the flexible limitations they determine.


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GPRS/EDGE

If GPRS/EDGE is in use in a BTS in which a TCH is being allocated, theTCH resources of the BTS are divided between two territories for thecircuit-switched (CS) traffic and the packet-switched (PS) GPRS/EDGEtraffic. In this case, the actual TCH load thresholds used in TCH rateselection are calculated by applying the threshold parameters to theresource amounts of the CS territory. With the CS TCH allocationcalculation (CTC) parameter, you can define how the RTSLs used forPS are seen when calculating resources for CS allocation.

For more information, see GPRS/EDGE in BSC under Featuredescriptions/Data in the PDF view.

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5 Half Rate and radio resource allocationTCH type requirements

Initially the mobile station (MS) informs the network of its transcodingcapabilities. It sends this information in two phases of the call set-up.

When the MS requires a channel for starting the connection establishmentto the network, it sends a specific establishment cause in the messagewhich indicates the reason for the call attempt to the network. Phase 2MSs also inform of the required type of TCH, that is, whether the half rate(HR) TCH is sufficient or whether the full rate (FR) TCH is the onlyalternative. This is possible only in mobile-originating calls (MOC) whenthe new establishment causes are supported by the network (NECI is setON in the BTS). In mobile-terminating calls (MTC) the type of the channelneeded is indicated to the MS in the paging request. The BSC uses theestablishment cause information when it decides on the first dedicatedcontrol channel (DCCH) to be allocated for the call. A FR TCH is alwaysallocated for an MTC because the HR capabilities of the MS cannot besolved for the call in this phase of the fast associated control channel(FACCH) call set-up. The BSC describes the allocated DCCH to the MSCin the message containing the initial layer 3 information.

When the connection to the network has been established, the MS sendsinformation of its HR capabilities on the main DCCH. The information istransferred in the Bearer Capability (BC) information element (IE) of theset-up or call confirmation messages to the MSC, depending on whetherthe call is a MOC or a MTC. Two alternatives are possible:

. the MS supports only FR transcoding (phase 1 MS)

. the MS is capable of both FR and HR transcoding

In this case, the MS also states which TCH rate is preferred, eitherFR or HR.

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The BC IE sometimes also contains information on the speech codecversions the MS supports. The MS gives all speech codec versions in theorder of preference, which further determines the requirement of thechannel rate preference.

The BSC receives from the MSC the TCH channel type and raterequirement in the Channel Type IE of the ASSIGNMENT REQUESTmessage. In the external handover case the Channel Type IE is alsoreceived in the HANDOVER REQUEST message. The MSC requests astrictly FR or HR traffic channel. The former choice is the only possibilitywhen the MS supports FR transcoding only. If the MS supports HR, theMSC lets the BSC make the final decision on the TCH type. The MSC thengives only the preference for the channel rate, either FR or HR.

In a speech call, the Channel Type IE also contains the Permitted SpeechVersion Indication. One or more speech versions can be represented. Inthe latter case codecs are given in the order of preference, which may alsodetermine the requirement of the channel rate preference. The decision ofthe channel rate in TCH allocation made by the BSC depends on whichspeech codec versions of a specific channel rate the BTS supports.

The HANDOVER REQUEST message optionally includes information onthe source channel (Current Channel IE). The mode of the channel(speech or data call information) and the channel rate and type (stand-alone dedicated control channel (SDCCH), FR TCH or HR TCH) arerepresented in the IE.

In a speech call, the HANDOVER REQUEST message also optionallycontains the speech version information the MS is using on the sourceTCH. The codec information also implies the source channel rate and type.

Both the BC IE and the Channel Type IE can be used when the BSCmakes a decision on the rate and on the type of the target channel inallocation of the TCH for the call to be handed over.

As an extra requirement, the MSC also determines whether the channelrate changes are allowed or not after the first channel allocation. If theMSC has not uniquely determined the TCH type, the TCH rate changestake place during the BSS-internal handovers. The type of the call-maintaining TCH brings a new factor for TCH allocation at the time ofinternal handovers: whether the BSC must always allocate a call-maintaining type of TCH for speech quality reasons or allow the channelrate to change, for example, because of to the TCH resource situation ofthe adjacent cells. The decision procedure is simpler if the MSC denieschannel rate changes.


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In the subsequent assignment case as well as in initial assignment of theFACCH call set-up, the situation is perhaps more complicated because thecall is already maintained in a certain type of TCH, HR or FR. No problemsoccur if the MS supports only FR. However, if the MS supports bothchannel rates, problems may occur in those exceptional cases when theMSC strictly requires either an HR TCH or an FR TCH.

The BTS does not allow reconfiguration of an active channel from HR toFR or vice versa by using the Mode Modify procedure. If the channel raterequirement of the ASSIGNMENT REQUEST is totally contradictory withthe type of the call-maintaining TCH, the call modification procedure isunsuccessful. Especially alternating speech connection cases where thechannel rate changes are required cause the BSC to be unable to handlethe connection portion. However, this does not deviate from the GSMrecommendations which state that FR is used from the start of the call ifone of the connection portions requires FR.

The BSC changes the initial preference of the TCH rate stated in theChannel Type IE because of resource reasons of a specific cell, that is, theactual TCH configuration, the traffic load of a cell or constraints determinedfor the TCH channel rate at the time of internal handovers.

Circuit configuration on the A interface

The transcoders are located at the MSC site. Therefore, when the MSCallocates the A interface circuit, it simultaneously determines thetranscoder to be used. If there are transcoders of different transcodingcapabilities, the A interface circuit allocation becomes dependent on thechannel rate of the required traffic channel resource.

If Reversed Hunting is active, the A interface circuit is allocated by theBSC instead of the MSC.

A circuit pool concept has been introduced in GSM recommendations forhandling transcoders of different types (for more information, see 3GPPTS 48.008: MSC-BSS Interface, Layer 3 Specification). Each circuit poolconsists of a group of A interface circuits which all have the sametranscoding capabilities.

There are 50 predefined speech-codec-specific circuit pools. The DX 200BSC supports the following 13:

. FR pool of circuits supporting FR speech version 1 and FR data (12,6, 3.6 kbit/s)

. HR pool of circuits supporting HR speech version 1 (supported onlyby TCSM2)

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. dual rate (DR) pool of circuits supporting FR speech version 1 andHR version 1 and FR data (12, 6, 3.6 kbit/s)

. FR pool of circuits supporting FR speech versions 1 and 2 and FRdata (12, 6, 3.6 kbit/s)

. DR pool of circuits supporting FR speech versions 1 and 2 and HRversion 1 and FR data (12, 6, 3.6 kbit/s)

. DR pool of circuits supporting FR speech versions 1 and 2 and HRversion 1 and FR data (14.5, 12, 6, 3.6 kbit/s)

. DR pool of circuits supporting FR speech versions 1 and 2 and HRversion 1 and FR data (12, 6, 3.6 kbit/s) and multislot data (max 2 x12 or 6 kbit/s)

. DR pool of circuits supporting FR speech versions 1 and 2 and HRversion 1 and FR data (12, 6, 3.6 kbit/s) and multislot data (max 4 x12 or 6 kbit/s)

. DR pool of circuits supporting FR speech versions 1 and 2 and HRversion 1 and FR data (14.5, 12, 6, 3.6 kbit/s) and multislot data(max 2 x 14.5 or 12 or 6 kbit/s)

. DR pool of circuits supporting FR speech versions 1 and 2 and HRversion 1 and FR data (14.5, 12, 6, 3.6 kbit/s) and multislot data(max 4 x 14.5 or 12 or 6 kbit/s)

. DR pool of circuits supporting FR speech version 3 and HR version 3

. DR pool of circuits supporting FR speech versions 1, 2 and 3 andHR versions 1 and 3 and FR data (14.5, 12, 6, 3.6 kbit/s) (supportedby TCSM3i and TCSM3i for combined BSC3i/TCSM3i installation)

. DR pool of circuits supporting FR speech versions 1, 2 and 3 andHR versions 1 and 3 and FR data (14.5, 12, 6, 3.6 kbit/s) andmultislot data (max 4 x 14.6 or 12 or 6 kbit/s) (supported by TCSM3iand TCSM3i for combined BSC3i/TCSM3i installation)

Speech FR version 2 corresponds to the enhanced full rate (EFR) speechcodec. Speech FR version 3 and HR version 3 correspond to adaptivemultirate codec (AMR) FR and AMR HR codecs. EFR-supporting poolsare available only when the EFR option is used in the BSC. AMR-supporting pools are available only when the AMR option is used in theBSC.

For more information on AMR and EFR, see Enhanced Speech Codecs:AMR and EFR under Feature descriptions/Radio network performance inthe PDF view.


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The A interface circuit must enable the BSC to allocate a radio resource ofthe requested type. If the A interface circuit pool type and the TCHresource request are totally contradictory, the BSC interrupts the on-goingtransaction, assignment or external handover attempt, by sending theproper failure message to the MSC.

Rather than the pool of the A interface circuit allocated by the MSC (or bythe BSC if Reversed Hunting is active), the actual A interface circuit poolconfiguration determines primarily how the BSC selects the channel rate ofthe TCH to be allocated. There must be a circuit pool that supports thetype of TCH that is going to be allocated according to the requirements,including those related to the speech codec versions, received from theMSC.

There is a specific fallback procedure for cases in which the circuit that theMSC has allocated belongs to another type of pool than what is suitable forthe BSC. The fallback procedure makes it possible for the MSC toreallocate the circuit from the correct pool. If Reversed Hunting is activethe circuit reallocation is done internally by the BSC and, therefore, theMSC is not involved. For more information, see Radio Channel Allocationunder Functional descriptions/ Radio network performance in the PDFview.

This example is valid only if Reversed Hunting is not active: the MSCdefines that a HR channel is preferred, but allocates the A interface circuitfrom the FR pool. If an HR TCH is available in the BTS and a HR or a DRcircuit pool exists on the A interface, the BSC sends the MSC theappropriate transaction failure message. The message contains thespecific cause and information on the pool from where the MSC hasallocated the circuit. The message also contains a recommendation of thepool that is the most suitable for the BSC and from which the A interfacecircuit should be allocated. The MSC then sends a new ASSIGNMENTREQUEST or HANDOVER REQUEST message with new A interfacecircuit information. This method enables the BSC to allocate the TCH radioresources more efficiently because the initial allocation of the A interfacecircuit does not necessarily set also the TCH resource type.

Channel type and A interface circuit pools

The BSC makes primary checks of the ASSIGNMENT REQUESTand theHANDOVER REQUEST messages. The remaining checks are madeduring the TCH allocation. The following assumptions and principles areobserved:

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. The speech version indications of the Channel Type IE must bereasonable. This means that it must be possible to attach a specificcodec version to a specific channel rate, either FR or HR. Eachcodec version that does not meet this requirement is removed fromthe speech version indication list of the resource request.

. The A interface circuit pools must support the speech versionindication of the resource request. Any codec version that does nothave transcoding support is removed from the resource request.

. FR channel rate must be allowed in channel rate requirements if adata call is concerned.

. The A interface circuit must support FR coding for data if a data callis concerned.

The total mismatches between the requested channel type (that is, thepermitted speech version indication) and the A interface circuit pools areobserved immediately when the ASSIGNMENT REQUEST orHANDOVER REQUEST has been received. This observation is possibleonly if the circuit identification code (CIC) is included in the request, that is,the circuit has been allocated in the traditional way by the MSC (ReversedHunting is not active). The treatment of the partial contradictions dependsnot only on the A interface circuit pool configuration, but also on the TCHresource situation of the target cell.

In general, when the A interface circuit does not support the preferred TCHchannel rate requirement and the cell has the preferred type of TCHsavailable, the BSC immediately tries to change the A interface circuit poolto another pool if a suitable one exists. When the A interface circuitsupports only the preferred channel rate and the cell has configured thesame type of TCHs, the BSC first tries to allocate a TCH. If the attemptfails, the BSC tries to change the A interface circuit to a pool which bettersupports the TCH rate requirement.

The primary checking that the BSC makes for the ASSIGNMENTREQUEST depend on the call phase in which the message has beenreceived from the A interface. In the initial ASSIGNMENT REQUEST, aswell as when the HANDOVER REQUEST is received from the A interface,when speech or data is indicated in the Channel Type IE, the procedureprogresses in the following way:


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1. In a speech call, the BSC must first confirm that the speechencoding algorithm versions are valid and correspond to the channelrate requirements. If they are not, that is, there are no A interfacecircuit pools supporting any of the speech versions expected in theresource request, the transaction procedure (assignment or externalhandover) is interrupted (that is, an ASSIGNMENT or HANDOVERFAILURE message is going to be sent to the MSC) with the Ainterface cause Requested Speech Version Unavailable.

When both of the two possible channel rates given are alternativechoices but only one of them has reasonable codec versionrepresented in the CHANNEL TYPE, the resource request messageis accepted but the channel rate requirement of the Channel Type IEis revised.

2. For a data call, FR channel rate has to be allowed, otherwise thechannel cannot be allocated. If the channel rate and the radiointerface data rate are fully contradictory, the transaction isinterrupted with the A interface error cause Invalid MessageContents. This can happen, for example, if a transparent service of9.6 kbit/s data rate is required but the channel rate of the TCHrequested to be allocated is HR. If the channel rate and the radiointerface data rate are in line but the only allowed channel rate is HR,the transaction is interrupted with A interface error cause RequestedTranscoding/Rate Adaption Unavailable.

3. This statement is valid only if Reversed Hunting is not active. Whenthe pool of the A interface circuit implied by the CIC represents anexisting pool, but it is totally contradictory with the permitted speechversion indication of the resource request, the transaction isinterrupted with the A interface cause value Circuit Pool Mismatch.This is the case when, for example, an HR TCH is requested but thepool type is FR.

4. When a resource is requested for a data call but there is no suitableA interface circuit pool which supports the data rate requirements,the transaction is interrupted with cause Requested Transcoding/Rate Adaption Unavailable.

5. This statement is valid only if Reversed Hunting is not active. Whena resource is requested for a data call and the data rate requirementdoes not match with the A interface pool implied by the CIC, butthere is a suitable A interface circuit pool which supports the datarate requirements, the transaction is interrupted with the A interfacecause Switch Circuit Pool.

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6. This statement is valid only if Reversed Hunting is not active. Whenmore than one alternative is given in the permitted speech versionindication of the Channel Type IE of the resource request, but thepool of the A interface circuit implied by the CIC does not support allof them, the BSC checks whether there are other pools of availablecircuits which match the codec versions that the A interface circuitpool does not support.

7. When the speech version requirements and the circuit pool (CIP)type match fully, the transaction is accepted. The message containsthe Channel Type IE, pool information of the A interface circuit, andthe statement that the pool switching is not allowed.

If the ASSIGNMENT REQUEST is received at the time of the FACCH callset-up, the checking procedure is modified in the following way:

1. The channel rate and type requirement of the Channel Type IE mustbe compatible with the channel rate of the TCH allocated by theBSC. If it is not, the assignment procedure is interrupted with thecause Invalid Message Contents.

2. Checks of the speech encoding algorithm versions are made withregard to the allocated TCH. This is also valid for the data raterequirements.

3. This statement is valid only if Reversed Hunting is not active. Thepool of the A interface circuit implied by the CIC is compared with theallocated TCH. If the pool is compatible with the permitted speechversion indication of the Channel Type IE, but it does not support theTCH that the BSC has allocated for the call or if the speech codecrequirement requires the A interface circuit pool switching, the failuremessage is sent to the MSC with the cause Switch Circuit Pool.

If the ASSIGNMENT REQUEST is received during the In Call Modificationprocedure, the following actions take place:

1. The channel rate and type requirement of the Channel Type IE mustmatch the channel rate of the allocated TCH. If it does not, theassignment procedure is interrupted with the cause Invalid MessageContents.

2. Checks of the speech encoding algorithm version are made withregard to the allocated TCH. This is also valid for the data raterequirements.

3. Because the A interface circuit is already allocated, no othercompatibility checks need to be made.


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The BSC sends the TCH resource request to the Radio ResourceManagement. The message contains the information on the Channel TypeIE, pool information of the A interface circuit, and if the A interface pooldoes not fully support the TCH type requirement, information on whetherimmediate pool switching is possible or not.

1. If the A interface circuit supports only AMR and the MS supportsboth AMR and other codecs, but the target cell has no availableresources and it does not support AMR, a possible AMR call requestis rejected with the cause No Radio Resource Available. However, ifthe target cell has other than AMR resources supported by theconfigured MS, but no free resources, and queuing or pre-emption isallowed, the AMR request is rejected with the cause Switch CircuitPool.

2. If the A interface circuit only supports the channel rate stated aspreferred in the resource request (here either the MSC or the BSCitself sets the preference), the BSC attempts to allocate a TCH.

If there are preferred types of TCHs available in the BTS, but the Ainterface circuit does not support the speech version set preferredalthough the BTS does, the following takes place, depending on thepool switching advice set by the BSC:. If the A interface circuit does not support the preferred codec

or codecs and it is possible to switch to the appropriate poolimmediately, the TCH allocation attempt fails with the causeSwitch Circuit Pool. This is the case when the circuit isallocated by the MSC. If Reversed Hunting is active, poolswitching is handled by the BSC and the MSC is not involved.Both the queuing of TCH resource and the directed retry aredenied.

. If the A interface circuit does not support the preferred codecor codecs and it is indicated that the immediate switching tothe appropriate pool is not possible, the TCH is allocated andthe most preferred codec that the A interface circuit pool andthe cell are supporting is used.

If there are no preferred types of TCHs available in the cell, thefollowing takes place, depending on the TCH configuration of theBTS and the pool switching advice set by the BSC:

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. If the available TCH resources in the cell are other than thepreferred channel rate type, the TCH allocation attempt failswith the cause Switch Circuit Pool. This is the case when thecircuit is allocated by the MSC. If Reversed Hunting is active,pool switching is handled by the BSC and the MSC is notinvolved. If immediate pool switching is possible, both TCHresource queuing and directed retry are prohibited, otherwisethey can be attempted.

. If the A interface circuit supports AMR and the MS supportsboth AMR and other codecs, but the target cell supports onlyeither of AMR codec sets but same kind of resources are notavailable, a possible AMR call request is rejected with thecause No Radio Resource Available. However, if the target cellhas other than AMR resources supported by the configuredMS, but no free resources, and queuing and pre-emption isallowed, the AMR request is rejected with the cause SwitchCircuit Pool.

3. If the A interface circuit does not support the channel rate stated aspreferred in the resource request, the decision of the TCH allocationattempt again depends on the BTS configuration:. If the BTS has the preferred type of TCHs available and

immediate pool switching is possible, the TCH allocationattempt is interrupted immediately with the cause SwitchCircuit Pool. This is the case when the circuit is allocated bythe MSC. If Reversed Hunting is active, pool switching ishandled by the BSC and the MSC is not involved.

Neither TCH resource queuing nor directed retry are allowedin this case.

. If the BTS has no preferred type of TCHs available, allocationof a TCH is attempted.

If there are non-preferred types of TCHs available in the BTSbut the A interface circuit does not support the speech versionset as preferred in the channel rate although the BTS does, thefollowing takes place, depending on the pool switching adviceset by the BSC:


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a. When the A interface circuit does not support thepreferred codec or codecs of the channel rate and it ispossible to switch to the appropriate pool immediately,the TCH allocation attempt fails with the cause SwitchCircuit Pool. This is the case when the circuit is allocatedby the MSC. If Reversed Hunting is active, poolswitching is handled by the BSC and the MSC is notinvolved. Both the queuing of TCH resource and thedirected retry are denied.

b. When the A interface circuit does not support thepreferred codec or codecs of the TCH rate and it isindicated that immediate switching to the appropriatepool is not possible, a TCH is allocated and the mostpreferred codec that the A interface circuit pool and theBTS are supporting is used.

If there are no non-preferred TCHs available in the BTS, thefollowing takes place, depending on the TCH configuration ofthe BTS:

a. If the BTS has preferred channel rate type of TCHsavailable, the TCH allocation attempt fails with the causeSwitch Circuit Pool. This is the case when the circuit isallocated by the MSC. If Reversed Hunting is active,pool switching is handled by the BSC and, therefore, theMSC is not involved.

Both the queuing of TCH resource and directed retry aredenied.

b. If the BTS has no preferred channel rate type of TCHsavailable, the TCH allocation fails with the cause NoRadio Resource Available.

The queuing of TCH resource and directed retry are bothpossible in this case.

The BSC controls that only one attempt is made to switch the A interfacecircuit pool during the same assignment or external handover. It issufficient that the circuit pool supports the TCH that the BSC is going toallocate. No pool switching attempt is made for merely changing the Ainterface circuit from one pool to another, for example, because of a circuitreservation situation in a specific pool.

When the initial cause value of the HANDOVER REQUEST is SwitchCircuit Pool, no attempt is made to change the A interface circuit pool. Thiskind of handover is interpreted to be an attempt to switch the circuit pool.

In a total pool and Channel Type mismatch cases, the A interface failuremessage is always sent with the cause Circuit Pool Mismatch. This is doneregardless of the reason for the last failure message.

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Restrictions to the Mode Modify procedure

The Mode Modify procedure cannot be used for changing the TCH ratefrom FR to HR and vice versa, because the BTS allows the reconfigurationof an RTSL only using the Channel Activation command which cannot beused for an active TCH.

Neither can the Subsequent Assignment procedure be used to change theTCH type. Subsequent Assignment attempts fail when the uniquelydetermined TCH type requirement (that is, either FR or HR TCH) iscontradictory with the channel rate of a call-maintaining TCH.

In both cases, the BSC sends an ASSIGNMENT FAILURE message witha proper cause value to the A interface.

Switching, for example, from HR transcoded speech to data transmissionin a FR TCH by using Channel Mode Modify is not supported. This ispossible only during handovers.

Restrictions caused by FACCH call set-up

The HR TCH allocation is applied in FACCH call set-up of MOCs onlywhen the network supports the new establishment causes and the channelis requested for establishing an HR speech call. The NECI for newestablishment causes is set ON or OFF for an individual cell. When it is setON, the phase 2 MSs include more accurate information about theestablishment cause and TCH rate needed for the requested service. Thedecision of the initial TCH type in FACCH call set-up is made according tothis information.

The FR TCH is always allocated for emergency calls and mobile-terminating calls (MTCs) because the HR capabilities of the MS cannot besolved for the calls in this phase of the call set-up.

When the NECI is set OFF, the FACCH call set-up of MOCs only takesplace on a FR TCH.

There is a possibility of contradictions between the TCH type allocated bythe BSC for an MS supporting HR and the TCH type request of the MSCwhen the latter is uniquely determined to be either HR or FR. This canhappen if the MSC does not accept the optional Chosen Channel IE of theinitial message containing layer 3 information sent by the BSS (forcomplete Layer 3 information, see 3GPP TS 24.008: Mobile RadioInterface, Layer 3 Specification). In these cases the BSC sends anASSIGNMENT FAILURE message with a proper cause value to the MSC.


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For more information, see FACCH Call Set-up under Feature descriptions/Radio network performance in the PDF view.

Restrictions to the Additional Assignment procedure

Additional Assignment is not supported in changing the channelconfiguration from HR TCH + ACCH to HR TCH + HR TCH + ACCH.

5.1 The A interface in radio resource allocation

Resource Indication

Both full rate (FR) and half rate (HR) TCH resources available in a specificcell are represented in the Resource Available IE. Presentation is in termsof the number of channels available in five interference bands.

The optional Total Accessible Resources IE gives the number of the FRand HR TCH resources which are accessible or in use.

Complete Layer 3 Information

The message includes the optional Chosen Channel IE which the BSScan use for giving the MSC a description of the channel (channel rate andtype) on which the first layer 3 message was received. The IE is significantespecially when the MS is assigned from the common control channel(CCCH) directly to the TCH by the Immediate Assignment procedure.

Assignment and External Handover

The ASSIGNMENT REQUEST and HANDOVER REQUEST messagescontain the mandatory Channel Type IE which represents the channel rateand type requirement. In speech call cases, the indication of permittedspeech versions is included in the request messages. Space for threecodec versions has been reserved for both FR and HR codecs. Currentlythe following speech versions are available:

Full rate:

. GSM 1

. GSM 2 (EFR)

. GSM 3 (AMR)

Half rate:

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. GSM 1

. GSM 3 (AMR)

Codec versions are presented in the order of preference in the indicationlist.

The data adaption information is included when a resource is requested fordata.

The HANDOVER REQUIRED and HANDOVER REQUEST messagesinclude the following optional information elements (IEs):

. Current Channel

Current Channel indicates both the type and the mode of thechannel allocated to the MS. When the channel is a TCH, thechannel rate is specified. The mode indicates whether it is a speechor a data call. In the case of a data call, the interface rate is alsogiven.

. Speech Version

Speech Version indicates the speech version used by the call.

In speech and data call cases, both resource request messages includethe circuit identity code (CIC) of the A interface circuit which the BSC usesin finding out the transcoding properties. The CIC is not included ifReversed Hunting is active.

The ASSIGNMENT COMPLETE and HANDOVER REQUESTACKNOWLEDGE messages contain the optional Chosen Channel IE. Itdescribes the radio resource which the BSS has allocated and in TCHcases also whether it is FR or HR. These messages also include theSpeech Version IE when the BSC chooses the speech version.

Both of the resource request acknowledge messages and the failuremessages ASSIGNMENT FAILURE and HANDOVER FAILURE containthe optional Circuit Pool IE when several circuit pools are defined on the Ainterface for indicating the type of the pool from which the circuit hasinitially been allocated.

3GPP specifications (3GPP TS 48.008: MSC-BSS Interface, Layer 3Specification) define 50 circuit pools supporting different sets of speedsand data codings.


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The transcoder and submultiplexer TCSM2 supports six circuit modes(3GPP TS 48.008: MSC-BSS Interface, Layer 3 Specification). For details,see table Circuit modes supported by TCSM2.

Table 3. Circuit modes supported by TCSM2

Mode Speech/data

A FR speech version 1

FR speech version 2

FR data (14.5, 12, 6 kbit/s)

B HR speech version 1

C HR speech version 1

FR speech version 1

FR speech version 2

FR data (14.5, 12, 6 kbit/s)

D HR speech version 1

FR speech version 1

FR speech version 2

FR data (14.5, 12, 6 kbit/s)

HSCSD max 2 * FR data (14.5, 12, 6 kbit/s)

E HR speech version 1

FR speech version 1

FR speech version 2

FR data (14.5, 12, 6 kbit/s)

HSCSD max 4 * FR data (14.5, 12, 6 kbit/s)

F HR speech version 3

FR speech version 3

The transcoder and submultiplexer TCSM3i and TCSM3i for combinedBSC3i/TCSM3i installation support three circuit modes (3GPP TS 48.008:MSC-BSS Interface, Layer 3 Specification). For details, see table Circuitmodes supported by TCSM3i.

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Table 4. Circuit modes supported by TCSM3i

Mode Speech/data

G FR speech version 1

FR speech version 2 (EFR)

FR speech version 3 (AMR)

HR speech version 1

HR speech version 3 (AMR)

FR data (14.5, 12, 6, 3.5 kbit/s)

H FR speech version 1


FR data (14.5, 12, 6, 3.6 kbit/s)

HR speech version 1

HSCSD max 2 x FR data (14.5, 12, 6 kbit/s)

I FR speech version 1



HR speech version 1



The BSC supports the A interface circuit pools listed in table A interfacecircuit pools supported by the BSC (for more information, see 3GPP TS48.008: MSC-BSS Interface, Layer 3 Specification).

Table 5. A interface circuit pools supported by the BSC

A interface circuitpool

Speech/data

1 FR speech version 1

FR data (12, 6, 3.6 kbit/s)

2 HR speech version 1

(only supported by TCSM2)



HR speech version 1


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Table 5. A interface circuit pools supported by the BSC (cont.)


Speech/data







HR speech version 1




HR speech version 1

HSCSD max 2 x FR data (12, 6 kbit/s)




HR speech version 1

HSCSD max 4 x FR data (12, 6 kbit/s)



FR data (14.5, 12, 6, 3.6 kbit/s)

HR speech version 1



FR data (14.5, 12, 6, 3.6 kbit/s)

HR speech version 1




FR data (14.5, 12, 6, 3.6 kbit/s)

HR speech version 1


23 FR speech version 3 (AMR)


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Table 5. A interface circuit pools supported by the BSC (cont.)


Speech/data




HR speech version 1


FR data (14.5, 12, 6, 3.5 kbit/s)

(only supported by TCSM3i and TCSM3i for combinedBSC3i/TCSM3i installation)




HR speech version 1



(only supported by TCSM3i and TCSM3i for combinedBSC3i/TCSM3i installation)

The following applies to cases in which Reversed Hunting is not active.

When the resource request is rejected by the BSC because the poolimplied by the CIC does not support any of the speech versions or datacoding indicated in the Channel Type, the cause Circuit Pool Mismatch isreturned to the MSC in the ASSIGNMENT FAILURE and HANDOVERFAILURE messages.

While the type of circuit pool is compatible with the speech version or datacoding requirements stated in the resource request and the BSC stillwishes to switch the terrestrial circuit to another pool, an ASSIGNMENTFAILURE or HANDOVER FAILURE message with the cause SwitchCircuit Pool is sent to the MSC.

In both of these two cases the Circuit Pool List IE is included in the failuremessages. The Circuit Pool List presents the pools in the order ofpreference, indicated by the BSC, according to which the A interface circuitshould be allocated.


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The creation method of the Circuit Pool List depends on the actual causevalue, the circuit pool configuration of the A interface and TCHconfiguration of the BTS. Not all the pool list variants are presented here indetail. There are three simple principles which are followed while formingthe circuit pool list:

. Only those pools which actually exist on the A interface arerepresented in the pool list.

. Circuit pools are switched as seldom as possible. The DR or multiplecodec pools are favoured when the order of preference of the poolsis determined.

. The circuit pool list is formed as simply as possible. Here alsopossible future pools, which will increase the complexity of formingthe circuit pool list, must be taken into account.

The BSC sends the HANDOVER REQUIRED message with the causeSwitch Circuit Pool to the MSC when it wants the A interface circuit pool tobe changed during an internal handover. The message is sent if thesuccessful allocation of a proper TCH in intra-BSS handover requires it.The message also contains the list of preferred circuit pools.

The MSC gives a HANDOVER REQUEST with the cause Switch CircuitPool (or alternatively HANDOVER REQUIRED REJECT) as a response tothe message.

5.2 The Abis interface in radio resource allocation

Channel Activation and Mode Modify

Channel rate and type of the Channel Mode IE specifies the channelcharacteristics and the possible data adaption.

Channel Activation Negative Acknowledgement

The following cause values are significant in HR:

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1. Radio channel already activated/allocated. If an attempt is made to activate for an full rate (FR) call a dual

rate (DR) RTSL, which has already been activated for an HRor FR call.

. If an attempt is made to activate for an HR call a DR RTSL,which has already been activated for an FR call.

. If an attempt is made to activate for an HR call the subchannelof a DR RTSL, which has already been activated for an HRcall.

2. Service or Option Not Implemented, Unspecified

Nokia Talk-family BTS with SW releases D1.1 and earlier when anattempt is made to activate an RTSL for an FR call but the channelmode in the message is HR.

3. General Information Element Error. Nokia Talk-family BTS with SW releases D1.1 and earlier

when an attempt is made to activate an RTSL for an HR call.. Nokia Talk-family BTS with SW releases D2.0 and later when

an attempt is made to activate an HR TCH but the TRXconfiguration does not support HR.

4. Mandatory Information Element Error

Nokia Talk-Family BTS with SW releases D2.0 and later when thechannel type is accepted but the channel mode is contradictory inthe activation command; replaces the cause Service or Option NotImplemented, Unspecified.

RF Resource Indication

The channel interference band information of an RTSL is coded persubchannel. Particularly, both the FR and the HR subchannels of the DRRTSLs are reported.

5.3 The Radio interface in radio resource allocation

System Information Type 6

In 3GPP specifications, the uplink DTX indicator included in the CellOptions IE of the SYSTEM INFORMATION TYPE 6 message has differentinterpretations when the message is sent on a FR TCH-SACCH or an HRTCH-SACCH (for more information, see 3GPP TS 44.018: Mobile RadioInterface, Layer 3 Specification).


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At the same time the representation of the DTX indicator has an extra bit tomake it possible to define more different combinations (maximum of eight)of the DTX control orders.

The TRX-specific sending of the SYSTEM INFORMATION TYPE 6 to theBTS takes place in a similar way as before (as well as the sending of theBTS-specific SYSTEM INFORMATION 3 message which contains theDTX indicator sent on the BCCH). The SYSTEM INFORMATION TYPE 6,sent per TRX, fully determines the DTX control on the FR TCH-SACCH. Italso determines the control on the HR TCH-SACCH, and that is why to getthe right interpretation of the DTX mode, the HR-call-specific SYSTEMINFORMATION TYPE 6 must always be sent to the BTS when the BTS-specific DTX indicator has a different meaning on the HR TCH-SACCHthan on the FR TCH-SACCH. Sending of the call-specific SYSTEMINFORMATION TYPE 6 takes place in the channel activation phase byloading the message data to the SACCH Information IE of the CHANNELACTIVATION message.

Table Uplink DTX control illustrates the value of the DTX indicatordepending on the uplink DTX mode and the SACCH type. The cases inwhich the HR-call-specific SYSTEM INFORMATION TYPE 6 is sent to theBTS, are also presented:

Table 6. Uplink DTX control

DTX modeUplink DTXmode of MS

DTXindicator onFR-SACCHbit 8 6 5

DTXindicator onHR-SACCHbit 8 6 5

Sys info 6 inactivation ofHR TCH

0 may use 0 0 0 1 0 0 Yes

1 uses 0 0 1 1 0 1 Yes

2 does not use 0 1 0 0 1 0 No

The columns of the table are:

. DTX mode

This is the BTS object class parameter, which defines the use of theuplink DTX. The parameter can have the three different valuespresented in the table.

. DTX indicator on FR-SACCH

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This presents the way the DTX indicator is coded on the FR TCH-SACCH. Although bit 8 (of the Cell Options IE) has been included inthe presentation, the coding is the same as earlier because it hasbeen a spare bit which has always had the value 0. Because thespare bit 8 is not actually used here, the phase 1 MSs can alsomanage with the DTX indicator.

. DTX indicator on HR-SACCH

This presents the way the DTX indicator is coded on the HR TCH-SACCH.

. Sys info 6 in activation of HR TCH

This presents the cases in which SYSTEM INFORMATION TYPE 6is loaded to the CHANNEL ACTIVATION message in activation ofHR TCH.

The discontinuous uplink transmission can be switched off for the HR callsin the BSS area with the aid of an UTPFIL parameter of the BSC.

Changes in the Assignment and Handover Commands and ChannelMode Modify because of HR

. channel type of the Channel description IE and the mode incommand messages to the MS

. RR cause Channel Mode Unacceptable in failure messages from theMS


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6 TCH allocation schemes in Half Rate

The following sections cover aspects of TCH allocation at a general level.The main aspect is how specific types of TCH are allocated depending onthe TCH rate requirement and the A interface circuit pool configuration.Some special cases on speech codec level are also handled in EnhancedSpeech Codecs: AMR and EFR under Feature descriptions/Radio networkperformance in the PDF view. The following signalling cases are describedhere:

. assignment

. external handover

. internal handover

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TCH allocation schemes in Half Rate

Assignment, basic procedure

Figure 3. Assignment, mobile terminated call

MS BTS BSC MSC

Paging part

Paging_Request Paging3 2 1

Radio access part on CCCH

Channel_Request4 5

6

7

8

9

DCCH establishment part

Sabm10 11

UA12

13

SDCCH signalling part:Ciphering and encryption...

Paging_Command

Channel_Required

Channel_Activation

Channel_Activation_Ack

Immediate_Assign_Command

Immediate_Assignment

Establish_Indication

Complete_Layer_3_Information


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Figure 4. Assignment, mobile terminated call (continued)

1. The message contains the optional Channel Neededinformation element (IE) as an indication to the MSwhich channel is needed for the transaction related tothe paging. If the TCH is indicated, the alternatives forchannel rates are also given.

2. When the Channel Needed IE is not received fromthe MSC, the BSC always includes the IE with theindicator of 'any channel' in the paging message.

MS BTS BSC MSC

SDCCH signalling part: Call set-up

Set-up14

Call_Confirmed15

Assignment_Request16

17

18

19

20

21

FACCH signalling part

Sabm22 23

UA24

25 26

Assignment complete

Physical_Context_Request

Physical_Context_Confirm

Channel_Activation


Assignment_Command


Assignment_Complete Assignment_Complete

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4. The message contains the Establishment Cause.

5. When fast associated control channel (FACCH) callset-up is applied, the BSC allocates directly a TCH,half rate (HR) or full rate (FR), if the channel rate isspecified in the Establishment Cause. However,because the basic call set-up procedure is describedhere, we assume that the stand-alone dedicatedcontrol channel SDCCH, is allocated as the channelfirst dedicated.

For more information, see FACCH Call Set-up underFeature descriptions/Radio network performance inthe PDF view.

10. Paging Response; the MS starts to establish theassociated control channel (DCCH).

13. The BSC establishes the signalling connectioncontrol part (SCCP) connection for the dedicatedsignalling to the MSC. The message contains thePaging Response and the optional Chosen ChannelIE for indicating the type of channel from which thelayer 3 message was received. The channel type canbe SDCCH, FR TCH, or HR TCH.

14. Initiates call establishment. No channel typerequirement is set to the MS.

15. The Bearer Capability IE of the message includesthe MS service request. The Radio ChannelRequirement field indicates whether the MS supportsHalf Rate or not. The Bearer Capability also containsthe Permitted Speech Version Indication whichrepresents the speech codecs the MS is supporting inthe order of preference.

16. Request to allocate a TCH radio resource. TheChannel Type IE specifies the channel rate and type.In a speech call, the indication of permitted speechversions is included in the IE. In a data call, the dataadaption is represented.

When a resource is requested for speech or data, themessage contains the circuit identification code (CIC)of the A interface circuit allocated by the MSC. Withthis CIC information, the BSC determines thecorresponding circuit pool. The CIC is not included ifReversed Hunting is active; the A interface circuit isthen allocated by the BSC.

19. The Channel Number and Channel Mode of themessage together specify the channel type and modeof transcoding (speech version) or rate adaption.


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System Information Type 6 is included in the SACCHInformation IE when a special uplink DTX controlcommand must be sent in a HR call.

21. Request for TCH establishment. The Channel Typeof the Channel Description IE indicates the channelrate and type. The Channel Mode IE gives the modeof transcoding (speech version) or rate adaption.

25. Successful assignment is performed as follows:When the ASSIGNMENT COMMAND messageinstructs the MS to use a Channel Description orMode that it does not support, it returns anASSIGNMENT FAILURE message with the causeChannel Mode Unacceptable and remains on thecurrent channel and uses the old mode. AnASSIGNMENT FAILURE message with the causeRadio Interface Failure, Reversion to Old Channel isthen sent to the MSC.

26. The message contains the optional Chosen ChannelIE if the BSC has been allowed to select the channel.It gives the channel rate and the type of the radiochannel assigned to the MS.

The optional Speech Version (chosen) IE indicatesthe selected speech version. The BSC includes thisIE in the message if it has selected the speechversion.

The optional Circuit Pool IE, which indicates the Ainterface circuit pool implied by the CIC of the chosencircuit, is included when more than one circuit poolexists on the A interface.

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Assignment, change of the circuit pool

Figure 5. Assignment, change of the A interface circuit pool

MS BTS BSC MSC

SDCCH signalling part: Assignment Request

1

2

3

4

5

6

7

8

FACCH signalling part

Sabm9 10

UA11

12 13

Assignment complete

Assignment_Request

Assignment_Failure

Assignment_Request



Channel_Activation


Assignment_Command


Assignment_Complete Assignment_Complete


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1. Request to allocate a TCH radio resource. TheChannel Type IE specifies the channel rate and type.In a speech call, the indication of permitted speechversions is included in the IE. In a data call, the dataadaption is represented.

When a resource is requested for speech or data, themessage contains the CIC of the A interface circuitallocated by the MSC. With this CIC information, theBSC evaluates the corresponding circuit pool. IfReversed Hunting is activated, CIC is not included inthe message; the A interface circuit is allocated bythe BSC.

2. This step is valid only if the MSC has allocated the Ainterface circuit. If Reversed Hunting is active, thepool switching procedure is carried out internally bythe BSC and the MSC is not involved.

When the circuit pool implied by the CIC of theAssignment Request does not support any speechcodec specified in the Permitted Speech VersionIndication of the Channel Type IE and there is a poolon the A interface which supports the radio resourcerequested, an ASSIGNMENT FAILURE with thecause Circuit Pool Mismatch is sent to the MSC.

When the circuit pool is compatible with the TCHresource requirement of the Channel Type IE but theBSC still wishes to change the circuit pool, the BSCsends the ASSIGNMENT FAILURE message to theMSC with the cause Switch Circuit Pool. The reasonfor changing the pool in most cases is the actualtraffic channel resource situation of the cell and theattempt to allocate the TCH radio resources in themost efficient way. The use of the correct speechversion may also require changing of the A interfacecircuit pool.

In both failure cases the Circuit Pool List IE ofpreferred pools and the circuit pool implied by the CICof the ASSIGNMENT REQUEST (Circuit Pool IE) areincluded in the failure message.

3. This step is valid only if the MSC has allocated the Ainterface circuit. If Reversed hunting is active, thepool switching procedure is carried out internally bythe BSC and the MSC is not involved.

A NEW ASSIGNMENT REQUEST or alternatively aCLEAR COMMAND is expected from the MSC.

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If the circuit pool implied by the CIC of the newAssignment Request is totally contradictory with thechannel rate or codec requirement specified in theChannel Type IE, an ASSIGNMENT FAILUREmessage with the cause Circuit Pool Mismatch issent to the MSC.

If the circuit pool is compatible with the TCH resourcerequirement of the Channel Type IE, the BSCallocates the most suitable TCH which the circuit poolsupports. This is also the case when the pool is thesame as at the time of the first ASSIGNMENTREQUEST.

An ASSIGNMENT FAILURE message with the causeNo Radio Resource Available is sent to the MSC ifthe allocation of the TCH is not successful. The circuitpool implied by the CIC (Circuit Pool IE) is included inthe failure message as always when more than onecircuit pool is available on the A interface.

13. The message contains the optional Chosen ChannelIE if the BSC has been allowed to select the channel.It gives the channel rate and the type of the radiochannel assigned to the MS.

The optional Speech Version (chosen) IE indicatesthe selected speech version. The BSC includes thisIE in the message if it has selected the speechversion.

The optional Circuit Pool IE, which indicates the Ainterface circuit pool implied by the CIC of the chosencircuit, is included when more than one circuit poolexists on the A interface.


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Internal handover, intra-cell

Figure 6. Internal handover, intra-cell

MS BTS BSC MSC

Handover measurements and decision

1

2

Physical context information

3

4

TCH allocation and activation

5

6

Signalling on FACCH of source TCH

7

Signalling on FACCH of target TCH

Sabm Establish_Indication

Measurement_Report

Measurement_Result



Channel_Activation


Assignment_Command

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Figure 7. Internal handover, intra-cell (continued)

5. The Channel Type IE initially received from the MSCand the type of the source TCH primarily determinesthe channel rate of the TCH that the BSC tries toallocate.

The channel rate and speech version changes arecontrolled with the BSS- and BTS-specificparameters of the BSC. The MSC can also set itsown restrictions.

When the successful allocation of a TCH requireschannel rate change and it is allowed, but the Ainterface circuit pool does not support the other typeof TCH, the BSC can try to change the pool if there isa suitable one on the A interface.

During the handover, the speech version can bechanged even though the channel rate remainsunchanged. This also requires the changing of the Ainterface circuit into a pool which supports the newcodec version.

The pool switching procedure is initiated by sendingthe HANDOVER REQUIRED message to the Ainterface with the cause Switch Circuit Pool. Themessage also contains the list of preferred pools(Circuit Pool List IE) and the identifier of the cellcandidate. The list of preferred pools is not includedin the message if Reversed Hunting is active. The

MS BTS BSC MSC

8 9

10

11 12

Source TCH release in BTS

13

14

UA

Assignment_Complete

RF_Channel_Release

RF_Channel_Release_Ack

Handover_Performed


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optional IEs Current Channel and Speech Version(used), which characterise the source channel typeand mode and used speech version, can be includedin the message.

The Channel Number and Channel Mode of theCHANNEL ACTIVATION message together specifythe channel type and mode of transcoding or rateadaption.


7. The Channel Type of the Channel Description IEindicates the channel rate and type of the targetchannel. The Channel Mode IE gives the mode oftranscoding (speech version) or rate adaption.

11. A successful handover is performed as follows:

When the ASSIGNMENT COMMAND messageinstructs the mobile station to use a ChannelDescription or Mode that it does not support, it returnsan ASSIGNMENT FAILURE message with the causeChannel Mode Unacceptable and remains on thecurrent channel using the old mode.

12. The message contains the optional Chosen ChannelIE if the TCH rate has been changed during thehandover. It gives the channel rate and the type of theradio channel allocated to the MS.

The optional Speech Version (used) IE is alsoincluded when the speech codec has been changedduring the handover.

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Internal handover, inter-cell

Figure 8. Internal handover, inter-cell (asynchronous)

MS BTS1 BTS2 BSC MSC

Handover measurements and decision

1

2

TCH allocation and activation in target BTS2

3

4

Signalling on FACCH of source BTS1 TCH

5

Signalling on FACCH of target BTS2 TCH

6 7

8

9 10

11

12 13

Measurement_Report

Measurement_Result

Channel_Activation


Handover_Command

Handover_Access Handover_Detection

Physical_Information

Sabm Establish_Indication

UA

Handover_Complete Handover_Performed


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Figure 9. Internal handover, inter-cell (asynchronous) (continued)

3. The Channel Type IE initially received from the MSCand the type of the source TCH primarily determinesthe channel rate of the TCH that the BSC tries toallocate. The channel rate and speech versionchanges are controlled with the BSS-specificparameter of the BSC. The MSC can also set its ownrestrictions.

When the successful allocation of a TCH requireschannel rate change and it is allowed but the Ainterface circuit pool does not support the other typeof TCH, the BSC can try to change the pool if there isa suitable one on the A interface.

During the handover, the speech version can bechanged even though the channel rate remainsunchanged. This also requires changing the Ainterface circuit to a pool which supports the newcodec version.

The pool switching procedure is initiated by sendingthe HANDOVER REQUIRED message to the Ainterface with the cause Switch Circuit Pool. Themessage also contains the list of preferred pools(Circuit Pool List IE) and the identifier of the cellcandidate. The list of preferred pools is not includedin the message if Reversed Hunting is active. Theoptional IEs Current Channel and Speech Version(used), which characterise the source channel typeand mode and used speech version, can be includedin the message.

MS BTS1 BTS2 BSC MSC

TCH release in source BTS1

RF_Channel_Release14

RF_Channel_Release_Ack15

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The Channel Number and Channel Mode of theCHANNEL ACTIVATION message together specifythe channel type and mode of transcoding or rateadaption.


5. The Channel Type of the Channel Description IEindicates the channel rate and type. The ChannelMode IE gives the mode of transcoding or rateadaption.

12. A successful handover is performed as follows:When the HANDOVER COMMAND messageinstructs the MS to use a Channel Description orMode that it does not support, it returns aHANDOVER FAILURE message with the causeChannel Mode Unacceptable and remains on thecurrent channel and uses the old mode.

13. The message contains the optional Chosen ChannelIE if the TCH rate has been changed during thehandover. It includes the channel rate and the type ofthe radio channel allocated to the MS.

The optional Speech Version (used) IE is alsoincluded when the speech codec has been changedduring the handover.

Changing of the A interface circuit pool

Sometimes during an internal handover the TCH rate change only enablessuccessful traffic channel allocation. This naturally requires that ratechanges are allowed in general. If the A interface circuit does not supportthe new channel type of the target cell, the A interface circuit pool must bechanged.

The BSC initiates the procedure by sending the HANDOVER REQUIREDmessage with the cause Switch Circuit Pool to the A interface. Themessage also contains the preferred list of pool candidates except whenReversed Hunting is active. The cell identifier list includes only the targetcell in which the suitable TCH resource can possibly be allocated after thepool change. The procedure can be initiated both during inter-cell andintra-cell handovers.


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The TCH rate changes are not the only reason for codec change at thetime of the internal handover. The BTS may not support the transcodingused in the source cell, therefore, the speech version should be changedto a more suitable one. This may sometimes also require switching the Ainterface circuit pool.

The circuit pool change procedure is not started for an internal handoverwhen, during the same handover, an unsuccessful external handoverattempt has already been made, followed by an internal handover attempt.

Handovers to the TCHs of the super-reuse frequency area are exceptionalbecause they can be merely BSC-controlled. During these handovers thecircuit pool cannot be changed with the aid of the handover typereselection. For more information, see Intelligent Underlay-Overlay underFeature descriptions/Radio network performance in the PDF view.

A circuit pool change cannot be initiated during a traffic reason handovernor during a forced handover caused by pre-emption. The BSS-internalhandovers related to directed retry also have restrictions because theytake place during the assignment procedure.

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External handover, basic procedure

Figure 10. External handover, source BSS

3. The source BSC starts an external handover bysending the HANDOVER REQUIRED message tothe MSC. The message contains a candidate cell listand a handover cause.

The optional IEs Current Channel and SpeechVersion (used), which characterise the sourcechannel type and mode and the used speech version,can be included in the message.

MS BTS BSC MSC

Measurement_Report1

Measurement_Result2

3

5 4

Clear_Command6

7RF_Channel_Release_Ack

8Clear_Complete

9

Signalling on FACCH-TCH in target BSS,disconnection, TCH release in source BSS

TCH allocation and activation in target BSSHandover Command from MSC, FACCH signalling

Handover measurementsHandover decision and inquiry

Handover_Command

Handover_Required

RF_Channel_Release

Handover_Command


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A handover controlled internally by BSS can beturned into an MSC-controlled one when the TCHallocation requires an A interface pool change. Theinitial handover can be an inter-cell or an intra-cellone. In this case, the handover cause is: SwitchCircuit Pool, and the HANDOVER REQUIREDmessage contains the Circuit Pool List IE of preferredpools, except when Reversed Hunting is active. Onlyone cell is included in the cell identifier list.

The source BSS in this case is also the target BSS. AHANDOVER REQUEST with the cause SwitchCircuit Pool or a HANDOVER REQUIRED REJECTis expected from the MSC.

4. The MSC requires the MS to be handed over. Themessage contains the Layer 3 HANDOVERCOMMAND to be sent to the MS.

5. The radio interface HANDOVER COMMANDmessage is sent to the MS. The Channel Type of theChannel Description IE indicates the channel rateand type. The Channel Mode IE gives the mode oftranscoding or rate adaption.

When the HANDOVER COMMAND messageinstructs the MS to use a Channel Description orMode that it does not support, it returns aHANDOVER FAILURE message with the causeChannel Mode Unacceptable and remains on thecurrent channel and uses the old mode. AHANDOVER FAILURE message with the causeRadio Interface Failure, Reversion to Old Channel isthen sent to the MSC.

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Figure 11. External handover, target BSS

MS BTS BSC MSC

Decision of external handover in source BSSThe MSC sends the Handover Request

Handover_Request1

TCH allocation and activation in target BTS

2Channel_Activation_Ack

3

4

Signalling on FACCH-TCH

Handover_Access Handover_Detection5 6

Handover_Detect7

8Sabm Establish_Indication

9 10

UA11

Handover_Complete Handover_Complete12 13

Channel_Activation

Handover_Request_Ack



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1. A request to allocate a TCH radio resource for ahandover. The Channel Type IE specifies the channelrate and type. In a speech call, the indication ofpermitted speech versions is included in the IE. In adata call, the data adaption is represented.

The message that the MSC sends includes theoptional IEs Current Channel and Speech Version(used), which characterise the source channel typeand mode and the used speech version. With thisinformation the channel rate and speech versionchanges are controlled with the BSS-specificparameter of the BSC.

When a resource is requested for speech or data, themessage contains the CIC of the A interface circuitallocated by the MSC. With this CIC information theBSC can evaluate the corresponding circuit pool. TheCIC is not included in the message if ReversedHunting is active.

The message contains both target and source cellidentifiers and the handover cause.

2. The Channel Number and Channel Mode of themessage together specify the channel type and modeof transcoding or rate adaption.

A System Information Type 6 is included in theSACCH Information IE when a special uplink DTXcontrol command must be sent in a HR call.

4. The HANDOVER REQUEST ACKNOWLEDGEmessage contains the Layer 3 Handover Commanddirected to the source BSS in order to be sent to theMS. The Channel Type of the Channel Description IEindicates the channel rate and type. The ChannelMode IE gives the mode of transcoding or rateadaption.

The optional Circuit Pool IE which indicates the Ainterface circuit pool implied by the CIC of the chosencircuit is included when more than one circuit poolexists on the A interface.

External handover, change of the circuit pool

The following procedure takes place in the target BSS.

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Figure 12. External handover, change of the A interface circuit pool

MS BTS BSC MSC

Decision of external handover in source BSSMSC sends the Handover Request to target BSS

Handover_Request1

Handover_Failure2

Handover_Request3

TCH allocation and activation in target BTS

4

5

6

Signalling on FACCH-TCH

Handover_Access Handover_Detection7 8

Handover_Detect9

10Sabm Establish_Indication

11 12

UA13

Handover_Complete14 15

Channel_Activation


Handover_Request_Ack

Handover_Complete



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1. A request to allocate a TCH radio resource for ahandover. The Channel Type IE specifies the channelrate and type. In a speech call, the indication ofpermitted speech versions is included in the IE. In adata call, the data adaption is represented.

The optional IEs Current Channel and SpeechVersion (used), which characterise the sourcechannel type and mode and the used speech version,can be included in the message sent by the MSC.With this information the channel rate and speechversion changes are controlled with the BSS-specificparameter of the BSC.

When a resource is requested for speech or data, themessage contains the CIC of the A interface circuitwhich the MSC has allocated. With the CICinformation the BSC can evaluate the correspondingcircuit pool. The CIC is not included in the message ifReversed Hunting is active.

The message contains both target and source cellidentifiers and the handover cause.


When the circuit pool implied by the CIC of theHANDOVER REQUEST does not support anyspeech codec specified in the Permitted SpeechVersion Indication of the Channel Type IE and there isa pool on the A interface which supports the radioresource requested, a HANDOVER FAILURE withthe cause Circuit Pool Mismatch is sent to the MSC.

When the circuit pool is compatible with the TCHresource requirement of the Channel Type IE but theBSC still wishes to change circuit pools, the BSCsends the HANDOVER FAILURE message to theMSC with the cause Switch Circuit Pool. The reasonfor changing pools in most cases is the actual trafficchannel resource situation of the cell and the attemptto allocate the TCH radio resources in the mostefficient way. The use of the correct speech versionmay also require the A interface circuit pool to bechanged.

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Note that the failure message can be delayed in thepool switching case because of queuing of the TCHresource request, that is, the MSC receives it after theQueuing Indication.

In both failure cases the Circuit Pool List IE ofpreferred pools and the circuit pool implied by the CICof the HANDOVER REQUEST (Circuit Pool IE) areincluded in the failure message.

When the cause of the initial HANDOVER REQUESTis the Switch Circuit Pool, the BSC does not attemptthe pool change because the handover has alreadybeen initiated because of the change of the Ainterface circuit pool. The BSC tries to allocate a TCHsupported by the A interface circuit pool. In anunsuccessful case, a HANDOVER FAILUREmessage with the cause No Radio ResourceAvailable is sent to the MSC. However, in total poolmismatch cases, the failure message is always sent.


A NEW HANDOVER REQUEST or alternatively aCLEAR COMMAND is expected from the MSC.

When the circuit pool implied by the CIC of the newHANDOVER REQUEST is totally contradictory withthe channel rate and codec version requirementsspecified in the Channel Type IE, a HANDOVERFAILURE message with the cause Circuit PoolMismatch is sent to the MSC.

When the circuit pool is compatible with the TCHresource requirement of the Channel Type IE, theBSC allocates the most suitable TCH which thecircuit pool supports, also in the case in which thepool is the same as it was at the time of the firstHANDOVER REQUEST.

A HANDOVER FAILURE message with the cause NoRadio Resource Available is sent to the MSC if theallocation of a TCH is not successful. The circuit poolimplied by the CIC (Circuit Pool IE) is included in thefailure message as always when more than onecircuit pool is available on the A interface.


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Directed retry

Directed retry is initiated by the assignment procedure when there is atemporary traffic channel congestion in the serving cell. The MS is going tobe assigned to a traffic channel of an adjacent cell which is selected on thebasis of the radio measurement reports received from the MS.

The TCH allocation in an internal directed retry handover becomes morecomplicated than in an allocation during a normal assignment because inthe former case there can be more than one cell candidate. Thecomplexity increases when only the preferred channel rate is representedin the radio resource request. The A interface circuit pool switching can bestarted only when target cell for directed retry handover does not supportAMR, but the allocated circuit supports only AMR.

A directed retry handover is initiated in the SDCCH signalling phase.Therefore, possible channel rate change prohibitions that either the BSCor the MSC have set have no effect here. The following principles areobserved in TCH allocation for internal directed retry handover:

1. When there is a BTS in the candidate list from which the required(preferred) type of TCH can be allocated, the TCH is allocated fromthe best possible cell.

2. When the preferred type of TCH is not available in a specific cell ofthe candidate list but it has the secondary type of TCHs availableand the circuit pool supports the secondary type of TCH, thesecondary type of TCH is allocated. This takes place regardless ofthe possibility of there being a worse cell in the candidate list whichhas the preferred type of TCHs available.

RXLev Min Access

If RXLev Min Access is activated in the BSC, the carrier-to-noise ratio (C/N) based method for a speech and a data call is used to determinewhether TCH can be allocated according to the downlink RX levelmeasured by the MS.

For more information, see RXLev Min Access in Radio Channel Allocationunder Functional Descriptions/Radio network performance in the PDFview.

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7 User interface of Half RateMML commands

. Transceiver Handling (command group ER)

. Base Station Controller Parameter Handling in BSC (commandgroup EE)

. Base Transceiver Station Handling in BSC (command group EQ)

Parameters

Half Rate (HR) related parameters of the BSDATA (BSS Radio NetworkConfiguration Database) are located in the BSC, BTS, TRX and radiotimeslot (RTSL) objects. HR-related parameters are visible only if HR isactivated and the BSC is equipped with a bit group switch (GSWB).

. TRX half rate support (HRS) (ERC, ERM, and ERO commands)

. RTSL type 0–7 (CH0–CH7) (ERC, ERM, ERO, and EEI commands)

. lower limit for FR TCH resources (FRL/HRL) (EEM, EEO,EQM, and EQO commands)

. upper limit for FR TCH resources (FRU/HRU) (EEM, EEO,EQM, and EQO commands)

. TCH in handover (HRI) (EEM and EEO commands)

. TCH rate intra-cell handover (TRIH) (EQM and EQOcommands)

. initial AMR channel rate (IAC) (EEM and EEO commands)

. alarm limit for full rate TCH availability (ALFRT) (EEQand EEO commands)

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User interface of Half Rate

. alarm limit for half rate TCH availability (ALHRT) (EEQand EEO commands)

. CS TCH allocation calculation (CTC) (EEM and EEOcommands)

For more information on parameters, see



. Channel rate for AMR calls in Enhanced Speech Codecs: AMR andEFR under Feature descriptions/Radio network performance in thePDF view

. BSS Radio Network Parameter Dictionary under Reference/Parameters in the PDF view.

Alarms

. 7717 WORKING HALF RATE TCH RATIO BELOW THRESHOLD

. 7711 WORKING FULL RATE TCH RATIO BELOW THRESHOLD

. 7727 TRAFFIC CHANNEL NUMBER DECREASED

. 7741 MEAN HOLDING TIME ABOVE DEFINED THRESHOLD

The SUB_CH_ID field indicates if the HR channel generates thealarm.

. 7743 MEAN HOLDING TIME BELOW DEFINED THRESHOLD


. 7745 CHANNEL FAILURE RATE ABOVE DEFINED THRESHOLD


. 7746 CH CONGESTION IN CELL ABOVE DEFINED THRESHOLD

The REFUSED_HR_CALLS_PERCENTAGE field indicates thenumber of HR channel requests rejected because of congestioncompared with all channel requests which are rejected because ofcongestion.


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half rate in bsc

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