02 mn1789eu12mn 0001 overload handling

7/13/2019 02 Mn1789eu12mn 0001 Overload Handling

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MN1789EU12MN_0001 2007 Nokia Siemens Networks

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Contents

1 Introduction 3

2 BSC overload 5

2.1 BSC overload conditions 6

2.2 BSC overload reactions 7

2.3 Timer control of overload reduction 8

2.4 Traffic reduction algorithm 10

3 MSC overload 15

4 BTS overload 19

4.1 BTS overload conditions 20

4.2

Traffic reduction algorithm 22

4.3 Simultaneous BTS and BSC overload 24

4.4 Parameters for overload handling 25

5 Traffic reduction algorithm 27

5.1 Preemption, queuing 28

Overload handling


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2007 Nokia Siemens Networks2


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1 Introduction

The aim of the overload control mechanism is to preserve the traffic handlingcapabilities of the mobile base station system (BSS) under adverse conditions.

The overload control mechanism is implemented in the BSC and consists of trafficdetection mechanism and defensive actions.

Traffic detection mechanismconsists of overload messages coming either fromthe various parts of the network (BTS and/or MSC) of from the BSC itself (overloaddetection).

Defensive actionsaimed at reducing both mobile originating (MOC) and/or mobileterminating (MTC) calls (overload handler); the strategy is here presented and follows

the GSM recommendation 08.08.

GSM rec 08.08gives the following definition of the general overload handlingstrategy: The overload causing traffic is reduced in several timer guarded steps untilthe overload is prevented. Otherwise, if for some time no overload is indicated, thetraffic is increased again in timing steps until full load has been resumed.

The time-out values of the timers, number of steps, amount and type of trafficreduced in each step, overload recognition and threshold parameters are allconsidered implementation dependent and have not been specified.

Essentially there are two ways to reduce overload:

reducing the mobile terminating traffic by discarding paging messages,

reducing the mobile originating traffic by discarding CHANNEL REQUIRED orCHANNEL REQUEST messages (dependent on which NE detects the overload).

To reduce overload in moderate way, incoming traffic is not completely rejected, butreduced by several steps of escalation controlled by timers.


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2 BSC overload


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2.1 BSC overload conditions

The following overload conditions are recognized:

Tx buffers for CCS7 links are congested

Buffering is a CCS7 level 2 function handled by PPXL. Congestion may occurbecause of high traffic volume or frequent retransmission (in case of badtransmission quality).

Percentage of busy level 3 radio registers to handle incoming call requestsexceeds a hard-coded threshold of 90 %

For dedicated transactions (e.g. MOC, MTC, SMS, incoming handover, location

update), a (release-dependent) number of level 3 call processing transactionregisters is available.

Real time TDPC load exceeds the threshold OVLSTTHR

The database parameter OVLSTTHR determines the start of TDPC real timeprocessor overload, OVLENTHR gives the end of this overload condition.

Note: This overload condition is only detected if the database flag BSCOVLH is set to"TRUE".

Lack of TDPC memory detected

If the memory occupation of TDPC exceeds 70 %, the overload condition starts. Forvalues below this hard-coded threshold, the overload condition has ended.Oscillations are avoided through the usage of timer T18.

Real time processor load of PPXX exceeds 70 %

The threshold for PPXX processor overload is hard-coded to 70 %.

Lack of PPXX memory resources

If the usage of the PPXX memory goes above the hard-coded value of 70 %, thisoverload condition is detected.

Overflow in BSC paging queue

PAGING messages from MSC are queued in the BSC buffer (the number of bufferedpagings depends on number of the LACs that the BTSs are grouped in and on CCCHconfiguration).

After successful delivery of PAGING COMMAND via Abis, the PAGING messagesare removed from the paging queue.


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2.2 BSC overload reactions

For all overload conditions (with the exception of "Overflow in BSC Paging Queue"),the overload actions taken by the BSC are

alarm message "BSC Overload Detected" (incl. specific overload cause value) issent to RC/LMT/event log file

BSSMAP OVERLOAD message is sent to MSC (with cause and without cellidentifier what means general overload situation)

mobile terminating traffic (PAGING messages from MSC) as well as

mobile originating traffic (CHANNEL REQUIRED messages from BTS) are

discarded.

For the overload cause " BSC Paging Queue Overflow", neither mobile originatingnor mobile terminating traffic is reduced (directly, e.g. BSC will discard just thosemessages that have arrived in the BSC and can not be put in the paging queue).

However, the following alarm messages are issued:

"BSC Paging Queue Overload" is sent to RC/LMT/event log file and

BSSMAP OVERLOAD message is sent to MSC.Therefore, mobile terminating traffic is reduced indirectly by the MSC.

2.2.1 Mechanism for reduction of MOC and MTC traffic

In the BSC overload handling is based on "overload level" mechanism.

The "overload level" determines to which extent the originating or terminating traffic isreduced. It is realized as a counter variable which is automatically increased if after adefined observation period the overload conditions still persist. When the overloadprocess receives the first trigger (BSC overload detected) the BSC overload level isset to 1 and starts overload handling (control and reduction) via timers.


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2.3 Timer control of overload reduction

When the BSC detects an overload situation, it performs the following actions:

start timers T2 and T18,

set overload level to "1",

send "OVERLOAD" message to MSC,

send "overload detected" message to RC/LMT/event log file.

Timer T2 is fixed to 2 sec.

If after the expiry of T2, the overload condition is still present, the variable"overload level" is increased by one step.

If the overload condition has disappeared (at expiry of T2), the variable isdecreased by one step.

After the overload level has reached the level "0" again and the timer T18 expires,the overload condition has ended. If T18 expires before the overload level "0" hasbeen reached it is restarted.

T2: Overload level de-/escalation timer

T18: Overload O&M alarm observation timer


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To MSC: OVERLOAD

Overload level = 1

Overload level = 2

If overload still present:

T 2 (2 sec) T 18

To O&M: overload detected

Overload

Detected

To MSC: OVERLOAD

If overload still present:To MSC: OVERLOAD

Overload level = 3

If overload NO more present:

Overload level = 2

If T 18 expires foroverload level > 0

T18 is restarted


Overload level = 1


Overload level = 0

Overload

End

To O&M: overload ceased

BSC

Fig. 1 Overload level management

The overload de-/escalation timer T2is fixed to 2 sec. If an overload condition isstill present after expiry of T2, the variable "overload level" is increased. If anoverload condition has disappeared after expiry of T2, the variable "overload level" isdecreased.

The overload observation timer T18is set in the BSC timer package:

DB Name Range

(default)

Unit Meaning

BSC T17 0...254

(HLFSEC-20).

HLFSEC, SEC5 Timer started when anMSC Overloadmessage is received

BSCT18

GSM 08.08

0...254

(HLFSEC-60).

HLFSEC, SEC5 Overload observation

timer


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2.4 Traffic reduction algorithm

Mobile originating traffic is reduced by discarding CHANNNEL REQUIRED messages(on Abis) in a specific cycle.

Mobile terminating traffic is reduced by discarding PAGING messages (from Ainterface) in a same specific cycle.

The extent to which these messages are discarded depends on the current overloadlevel as shown in the figure 2.

The BSC administers only one overload level variable which determines the extent ofdiscarding both CHANNNEL REQUEST and PAGING messages.

CHANNEL REQUIRED messages with the two following establishment causes are

notdiscarded: "EMERGENCY CALL" and

"ANSWER TO PAGING".


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Fig. 2 Paging and Channel Required messages discarding cycle


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2.4.1 BSC paging queue overflow

PAGING messages are discarded due to lack of space in the BSC paging queuesavailable for one LAC.

When the percentage of discarded paging messages has exceeded the thresholddefined by PAGQOVLIND in the last second

an alarm BSC Paging Queue Overload is sent to RC/LMT/event log file and

BSSMAP OVERLOAD message is sent to MSC.

In addition, the two following timers are started:

the BSC paging queue overload observation timer T1 (fixed at 1 sec). This timeralso defines the frequency of the BSSMAP OVERLOAD message sending

the Overload O&M alarm observation timer T18.

If during the runtime of T1, PAGING messages are discarded (again), T18 isrestarted (after expiry of T1) and a BSSMAP OVERLOAD message is sent to MSCagain.

If no PAGING messages are discarded during runtime of T1, only T1 is restarted andobservation continues.

If no PAGING messages are discarded during the runtime of T18, when it expires thetimer T1 is stopped and the alarm is ceased.

DB Name Object Range

(default)

Meaning

PAGQOVLIND BSC 0...100

(0, disabled)

Paging queue overloadindication


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To MSC: OVERLOAD

If PAGING msg. discarded:

T 1 (1 sec) T 18

To O&M:BSC Paging Queue Overload

PAGING msg.

from MSCdiscarded

To MSC: OVERLOAD

If PAGING msg. discarded:

If NO PAGING msg. discarded:

If PAGING msg. are

discarded, T1 andT18 are restarted

Overload End

To O&M: BSC Paging Queue

Overload ceased

BSC



If NO PAGING msg.

are discarded, only

T1 is restarted

To MSC: OVERLOAD

Fig. 3 Overload handling algorithm in case of BSC Paging Queue Overflow


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3 MSC overload


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BSC overload reaction due to MSC overload is controlled with the database flagMSCOVLH.

MSC overload is signaled with the BSSMAP OVERLOAD message from MSC.

BSC reactions to MSC overload message differ in comparison with the BSC overloaddetected, i.e. :

BSC sends an alarm message "MSC overload detected" to RC/LMT/event log file,

BSC will not reduce mobile terminating traffic as the reduction of it is performed bythe MSC already

BSC starts to reduce mobile originating traffic (i.e. systematically discardsCHANNEL REQUIRED messages).

When the BSC receives the first BSSMAP message OVERLOAD from the MSC, thetimers T17 and T18 are started, the overload level is set to 1and the first step oforiginating traffic reduction is performed as the reduction of terminating traffic isperformed by MSC. The alarm "MSC Overload detected" is output.

As long as T17 is running, another MSC overload messages are ignored. If at leastone overload message was received from the MSC in the time period between T17expiry and T18 expiry, the overload level is increased by 1 when T18 expires andboth timers are restarted. If no further OVERLOAD message is received from theMSC, the expiry of T18 triggers the decrease of the load level by one step.

When the overload level has reached the value 0 and timer T18 expired for the lasttime, MSC overload alarm is cleared.


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To BSC: MSC OVERLOAD

Overload level = 1

The first step of traffic

reduction

T 17 T 18

To O&M: MSC overload detected

MSC Overload

Detected


Massage is ignored

If overload still present:To BSC: MSC OVERLOAD

If T 18 expires for

overload level > 0, T17

and T 18 are restartedIf overload NO more present:

Overload level = 1


Overload level = 0

Overload

End

To O&M: MSC overload ceased

BSC


Overload level = 2

The second step of traffic

reduction

Fig. 4 MSC Overload handling


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4 BTS overload


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4.1 BTS overload conditions

The BTS overload conditions are recognized by the BSC when some of the followingmessages are received from the BTS:

PCH OVERLOAD message (cause "CCCH overload") from BTS via Abis/RSL

In the BTS two slots (i.e. PAGING REQUEST messages) for each paging group areavailable. The OVERLOAD message is sent if PAGING COMMANDs (from Abis)cannot be placed in a paging queue and therefore the BTS discards them.

DELETE INDICATION message from BTS via Abis/RSL

For each configured CCCH the BTS has an AGCH queue with 16 slots where each

IMMEDIATE ASSIGNMENT COMMAND or IMMEDIATE ASSIGNMENT REJECTmessage requires one place. How fast this is emptied depends on the volume ofpaging messages and the amount of CCCH blocks reserved for AGCH. Already atthe MEDIUM load of the AGCH queue an Overload Indicator on16 bits is issued fromthe BTS towards the BSC to report load history of entries in the queue.

For the AGCH messages discarding in the BTS two scenarios have to bedistinguished in the BSC:

the BSC receives DELETE INDICATION messages without IE 'timeout'

the BSC receives DELETE INDICATION messages with IE 'timeout'.

Within the first scenario the receipt of the DELETE INDICATION message without IE'timeout' is the indication for an AGCH overload situationin the BTS. The DELETEINDICATION message is sent by the BTS to the BSC if an IMMEDIATEASSIGNMENT/REJECT message can not be placed in the respective AGCH queueand, if BTS overload handling is enabled (BTSOVLH=TRUE), leads to output thealarm message 'AGCH QUEUE CONGESTION DETECTED' towards the O&Moutput media.

Within the second scenario the receipt of the DELETE INDICATION message with IE'timeout' is the indication of an IMMEDIATE ASSIGNMENT/REJECT message expirywhile waiting to be transmitted. AGCH messages are not transmitted if they havebeen delayed for too long due to signaling processing and AGCH queuing. In otherwords, IMMEDIATE ASSIGNMENT/REJECT message can be rejected by the BTSimmediately after reception if due to long Abis link propagation or congestion systemcalculates that when message can be scheduled for transmission it will expire. Also,the message will be rejected immediately before its transmission via the Um due tochanges of the actual condition of the queues at the moment of the delivery.

Whenever the BSC receives a DELETE INDICATION message with 'timeout', itindicates this event by the alarm message ' MESSAGES DISCARDED DUE TOTIMEOUT' towards the O&M output media (RC, LMT).


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Abis LAPD signaling overload UL

PPXL modules have layer 2 buffers for LAPD messages. All transmitted LAPD

messages are buffered until they are positively acknowledged by the remote end (inthis case from the BSC). In case of excessive signaling volume or frequent re-transmission buffer congestion may occur.

When the BTSE has detected an overload situation on the LAPD link based on theLAPD load threshold SLAPDOVLTH, it sends the O&M message LAPD OVERLOADtowards the BSC (only valid for BTSEs of generation BTSplus). If the parameterDLAPDOVL is set to TRUE, this message is the trigger for the BSC to start trafficreduction measures.

If the Abis overload situation has been detected for a particular BTSM, the BSCregards all BTS belonging to this BTSM as overloaded.

BTS detects RACH overload

In case the percentage of busy RACH exceeds the threshold TCCCHLDI, the BTSsends CCCH LOAD INDICATION message to BSC (periodicity PCCCHLDI).

No special overload actions are taken neither by BTS nor BSC.


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4.2 Traffic reduction algorithm

BTS overload handling works similar to BSC overload handling - reducing mobileterminating traffic by discarding PAGING messages and mobile originating traffic bydiscarding CHANNEL REQUIRED messages.

The difference is that OVERLOAD message contains optional IE "Cell Identifier" if theoverload to be reported only concerns specific BTS. Therefore BSC starts themechanism for the traffic reduction towards the affected cell only.

The differences between BSC and BTS overload handling are:

BTS overload handling is restricted to the BTS affected

two different thresholds are administered for discarding of CHANNEL REQUIRED

messages ("OVLD_LEVEL_UPLINK") and PAGING messages("OVLD_LEVEL_DOWNLINK").

PCH overload (after OVERLOAD message from BTS) incrementsOVLD_LEVEL_DOWNLINK only.

AGCH overload, i.e. the receipt of a DELETE INDICATION message (BTS->BSC),does not lead to any overload defense action by the BSC.

if PPXL Abis LAPD signaling overload is detected, both OVLD_LEVEL_UPLINKand OVLD_LEVEL_DOWNLINK are set to the max value ("10").

4.2.1 PCH overload

After the message OVERLOAD "CCCH Overload" has been received by the BSC, ifthe BTS overload handling is enabled (BTSOVLH=TRUE), the BSC outputs:

the alarm message 'BTS OVERLOAD HANDLING STARTED DUE TO PCHQUEUE CONGESTION' with overload cause 'CCCH overload' towards the O&Moutput media like RC, LMT, event log file, etc.,

the BSSMAP message OVERLOAD with cell identity and overload cause 'CCCH

overload' to the MSC,

the mobile terminating traffic is reduced by discarding PAGING messages for theaffected BTS based on the current value of OVLD_LEVEL_DOWNLINK (startingvalue for the variable after the first Abis overload message is "0").


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4.2.2 AGCH overload

After a message DELETE INDICATION has been received by the BSC, if the BTSoverload handling is enabled (BTSOVLH=TRUE), the BSC outputs:

the alarm message 'AGCH QUEUE CONGESTION DETECTED' or 'AGCHMESSAGES DISCARDED DUE TO TIMEOUT' with overload cause 'AGCHoverload' towards the RC, LMT, event log file, etc.,

the BSSMAP OVERLOAD message with cell identity and overload cause 'CCCHoverload' to the MSC.

No any overload defense action is taken by the BSC. Instead, the discarding ofIMMEDIATE ASSIGNMENT or/and IMMEDIATE ASSIGNMENT EXTENDED

messages is entirely left to the BTS.

4.2.3 Abis LAPD signaling overload

After the Abis LAPD signaling overload condition is detected by the BSC, if thedownlink LAPD overload is enabled (DLAPDOVL=TRUE), the BSC outputs:

the alarm message 'BTS OVERLOAD HANDLING STARTED DUE TO UPLINKLAPD LINK CONGESTION' with overload cause 'AGCH overload' towards theRC/LMT/ event log file,

the BSSMAP message OVERLOAD with cell identity and overload cause 'CCCHoverload' to the MSC.

An algorithm for the reduction of the mobile terminating and originating trafficwhich discards PAGING messages and CHANNEL REUIRED message for theaffected BTS in correspondence with the current value ofOVLD_LEVEL_DOWNLINK starts.

Note: On the first detection of the LAPD overload condition (Abis message LAPDOVERLOAD message received), the variable OVLD_LEVEL_DOWNLINK starts withthe value '1'.

4.2.4 RACH overload

After BTS sends CCCH LOAD INDICATION, no overload defense actions areperformed at all.


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4.3 Simultaneous BTS and BSC overload

In case of BTS and BSC overload occurring simultaneously, the discarding ofmessages for the BTS affected is based on the maximum value of the variablesrelating to BSC overload and relevant BTS overload (OVLD_LEVEL_UPLINK orOVLD_LEVEL_DOWNLINK).

CHANNEL REQUIRED messages with cause "answer to paging" are treateddifferently:

If BSC is discarding PAGING messages coming via A interface (due to BSC orBTS overload) then CHANNEL REQUIRED messages with cause "answer topaging" are not discarded.

If no PAGING messages coming via A interface are discarded (no BSC or BTS

overload condition), the channel filter applies to all CHANNEL REQUIREDmessages except for the messages relating to emergency calls.


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4.4 Parameters for overload handling

SpecificationName


(Default)

Meaning

T3122

GSM 04.08

T3122 BSC 0...255 sec

(5)

Timer to bar a new accessfor a certain time afterunsuccessful access (e.g.no SDCCH available).

EN_BSC_OVL_HAND

BSCOVLH BSC TRUE,

FALSE

(TRUE)

Flag to enable/disableBSC overload handling.

EN_MSC_OVL_HAND

MSCOVLH BSC TRUE,

FALSE

(TRUE.)

Flag to enable/disableMSC overload handling.

EN_BTS_OVL_HAND

BTSOVLH BSC TRUE,

FALSE

(TRUE)

Flag to enable/disableBTS overload handling.

OVL_START_THRESH

OVLSTTHR BSC 7000... 10000(9500)

Threshold for TDPCprocessor (in BSC)overload detection: 10000= 100 %

OVL_END_THRESH

OVLENTHR BSC 7000... 10000(8500)

Threshold for TDPCprocessor (in BSC) belowwhich the sending ofoverload messages isstopped: 10000 = 100 %

CCCH_LOAD_IND_PERIOD

PCCCHLDI BTS 0...255

Unit: 1sec

(255)

Period for sending theCCCH load indicationmessage from BTS toBSC.

CCCH_LOAD_THRESH

TCCCHLDI BTS 0...100

Unit: 1%

(100)

CCCH load threshold, ifthis threshold is exceededthe BTS informs the BSCusing the CCCH loadindication message.


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DB Name Object Range (Default) Meaning

FLAPDOVL BTSM 10100Unit: 1%

(80-60-10)

First LAPD overload threshold,represents the first load levelthresholds of the BTSE RadioSignaling Links (LPDLRs). Itconsists of three fields:

- firstLevelUpperThreshol

- firstLevelLowerThreshold

- firstLevelReductionStep.

SLAPDOVL BTSM 10100

Unit: 1%

(90-80)

Second LAPD overload threshold

defines the second load levelthresholds of the BTSE RadioSignaling Links (LPDLRs). Itconsists of two fields:

- secondLevelUpperThreshold

- secondLevelLowerThreshold.

DLAPDOVL BTSM TRUE, FALSE

(TRUE)

Downlink LAPD Overloadenables or disables theprocedure that detects thedownlink LAPD overload. If theBTSE has detected an overloadsituation on the LAPD link basedon the LAPD load thresholdsSLAPDOVLTH it sends the O&Mmessage LAPD OVERLOADtowards the BSC.

LAPDOVLT BTSM 160

(10)

Unit: 1s

LAPD overload timer defines thetime to pass between twoconsecutive LAPD OVERLOADindication messages.

LAPDOVLT0 BTSM 0,510,0

(1)

Unit:1s

LAPD overload timer 0 definesthe time to pass between twoconsecutive LAPD OVERLOADchecks.


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5 Traffic reduction algorithm


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5.1 Preemption, queuing

This feature allows the assignment of a TCH when, due to the unavailability ofresources, no unused channel can be found within the serving cell. Three mainprocedures using a subscriber Enhanced Multi-level Precedence and Pre-emption(eMLPP) priorities and a call priority are possible:

Preemption, which is a means of providing TCH resources for high priority TCHrequests

Directed Retry, which causes the handover from a SDCCH in one cell to a TCH inanother cell (described in chapter ''Design of radio cells'')

Queuing, which allows the queuing of TCH requests on a per cell and call prioritybasis.

An extended queuing mechanismWireless Priority Service (WPS) based onpriority class of the subscribers that have no pre-emption capability but the VeryImportant Persons (VIP) service, in particular during the busy hours, is introduced inBR8.0.

WPS feature is specified to further differentiate the call establishment probability ofVIP calls based on the subscribed priority without relying exclusively on pre-emption

as eMLPP.

In case of unavailability of resources, two cases can be foreseen:

Assignment Request

Handover Request.

In case of Assignment Request, Preemption is tried first: BSC looks for a call in thecell with a priority lower than the new TCH request and, then moves the lower prioritycall to another cell performing a Forced Handover (or a Forced Release if HO fails)

in order to free the TCH. The freed TCH is then assigned to the new request. If nocall with a lower priority is using a TCH or a call with the same priority or higher isalready waiting (queued) Directed Retry is performed. If still no free TCH is available,the queuing procedure is carried out, storing the TCH request in the cell queue basedon their priority.

In case of Handover Request, Preemption is tried first. If Preemption is notsuccessful, the Directed Retry procedure will be skipped and Queuing is carried out.


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Preemption feature gives to the operator the possibility to differentiatesubscriptions per user class (e.g. different contracts for high priority class such aspolice and other emergency body, business people and children)

Queuing and Direct Retry features improve the network access and, in general,network QoS

In GSM different priorities (eMLPP, call event and MS category) and access classes(ordinary subscribers get access classes 0-9, 10 is emergency call, while classes 11-15 are special high priority subscribers like police, PLMN operator ...) are merged intothe MSC Priority Information Element being (optionally) conveyed in the ASS REQ orHO REQ message from MSC to BSC. The priority information element contains thefollowing entries:

Priority Information Element

Bit 8 Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1

spare PCI PRIORITY LEVEL QA PVI

PCI (preemption capability indicator)

It indicates if the preemption shall be applied or not.

PRIORITY LEVEL

It contains the priority of the call; GSM supports the prioritization of calls using14 priority levels (1 is the highest and 14 is the lowest priority)

QA (queuing allowed indicator)

It is used to perform the queuing procedure.

PVI (preemption vulnerability indicator)

It indicates if this connection might be preempted by another allocation request.

The Preemptive Capability Indicator (PCI) applies to the allocation of resources for anevent (e.g. ASS REQ, HO REQ) and indicates whether the event is able to trigger thepreemption of radio resources.

The Preemptive Vulnerability Indicator (PVI) applies for the entire duration of aconnection and indicates whether the connection may become a target ofpreemption.

The Queuing Allowed Indicator (QA) is used to decide on a per call basis if queuingshall be applied or not.

The Priority Level (PL) is subdivided in 14 different levels, Priority Level 1 being thehighest value.


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5.1.1 Preemption

The entire procedure is shown in the following figure. This flow-chart basicallyindicates which algorithms and which checks are necessary in which network entity(BTS, BSC or MSC) and the message flow that has to be conveyed between thenetworks entities. It can be seen immediately that, in the best case, an assignment ofa TCH in a neighbor cell takes place.


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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Flowchart of Preemption

ASS_REQ / HO_REQ (no TCH available)

Set PCI=0, PVI=0,

QA=1, PL=14

N

PCI = 1 ?

N

Y

Other queued calls with

PCI=1 and higher or

equal priority ?

Y

Other active call with

PVI=1 and lower PL

N

Successful ?Forced Release of the

lower PL call

BSC assigns the free TCH

Y

N

Y

Y

N

Directed Retry

Procedure

Does the message

contain the priority I.E.?

Send Forced Handover

for the lower PL call

Fig. 5 Flowchart of preemption


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5.1.1.1 BSC Database Parameters for Preemption

EN_PREEMPTION


(default)

Unit Description

EPRE BTS ENABLEDDISABLED

(DISABLED)

Parameter to enable/disable preemption


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5.1.2 Wireless Priority Service

Wireless Priority Service (WPS) provides priority access to authorized subscribersletting them gain priority access to the next available radio channel when requestingWPS calls without restricting the publics ability to access the same network. TheBSC receives an Assignment Request or HO request message from the MSC thatcontains the priority information element assigned to the subscriber.

WPS applies 5 different levels of priority to the subscribers; these priority levels aredefined in the HLR and are taken into account in the event of congestion.

The meaning of the WPS priority level based on the mobile access class and eMLPPpriority, which is mapped onto the GSM08.08 priority and the queuing allowed (QA)on the A-interface is shown in the table.

Implementation

dependent, in the range

of 7-14

40 - 9Public Users

63125 (lowest)

5213 and 124

4113 and 123

3014 and 13 and 122

2B14 and 13 and 121 (highest)

GSM08.08 Priority

Level

eMLPP Priority

Level

Access ClassWPS User Priority

Assignment

Fig. 6 WPS user priorities


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5.1.3 Queuing mechanism

The new queuing mechanism introduces two separate queuing areas:

WPS queue

Public queue

Such approach is known as PURQ-AC algorithm. The algorithm manages systemsbehavior in the event of call arrival as well as call departure.

Call arrival

When new call arrives the algorithm is checking whether radio resource is available.If so, the call is served immediately otherwise the call is queued in either WPS queue

or public queue depending on users subscription.

Call departure

In general the queued calls are served before new arriving ones.

The decision about order of preference (i.e. WPS queue will be served before publicqueue or inversely) is made first. It is determined dynamically on the basis ofmaximum allowed WPS share specified by means of appropriate attributes(parameter WPSPREF).


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5.1.3.1 BSC Database Parameters for Queuing

The flag to enable queuing (EQ) has to be set as TRUE, otherwise all queuing

parameters for WPS are meaningless.Existing parameters for queuing are modified and additional for WPS are added. It ispossible to set the

length of the queue (QL),

maximum time for Queuing the ASSIGNMENT REQUEST (T11),

maximum time for queuing the HO request (TQHO)

for the WPS and the public queues separately.


(Default)

Unit Meaning

EQ BTS ENABLED/

DISABLED

(DISABLED)

Parameter used to enable/ disable the queuingfeature.

BSCT11PUB BSC 0254

(HLFSEC-16)

HLFSEC=0.5 s

SEC5=5s

Maximum queuing time for

ASS_REQ in publicqueue.

BSCT11WPS BSC 0254

(HLFSEC-16)

HLFSEC=0.5 s

SEC5=5s

Maximum queuing time forASS_REQ in WPS queue.

BSCTQHO

GSM 08.08

BSC 0254

(HLFSEC-20)

HLFSEC=0.5 s

SEC5=5s

Maximum allowed queuingtime for incominghandover.

BSCTQHOPUB BSC 0254

(HLFSEC-16)

MS=100ms

HLFSEC=0.5 s

SEC5=5s

Maximum queuing time forHO_REQ in public queue.

BSCTQHOWPS BSC 0254

(HLFSEC-16)

HLFSEC=0.5 s

SEC5=5s

Maximum queuing time forHO_REQ in WPS queue.


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(Default )

Unit Description

QLPUB BTS 1100

(50)

Maximum number of TCHrequests that can be queuedin the public area.

QLWPS BTS 1..100

(0 means WPSdisabled )

Maximum number of TCHrequests that can be queuedin the WPS area.

LWWPSPRI BTS 1...10

(NULL)

Lowest WPS priority levelthat is considered in the

queue.WPSPREF BTS Counter Cycle

2...10

(4)

Number WPS

1 3

(1)

WPS Preference, number ofcalls per complete cycle andconsecutively served WPScalls.

Assignment request Queuing Timers

BCST11PUB/ BCST11WPS

Start: ASS_REQ is queued, BSC sends QUEUE_IND to MSC

Stop: TCH becomes free for assignment or the request is replaced by another request with higher priority.

Action expiry: remove ASS_REQ from queue, send CLEAR_REQ to MSC

HO request Queuing Timers

Start: HO_REQ is queued, BSC sends QUEUE_IND to MSC

Stop: TCH becomes free for assignment or the request is replaced by another request with higher priority.

Action expiry: remove HO_REQ from queue, send HO_FAIL to MSC

With the WPSPREF the portion of WPS calls that is served out of the queue isdefined.


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