gsm signaling and protocols architecture

GSM Signaling and Protocols

Architecture

GSM signaling and protocol architecture Mobile initialization Location update Mobile identification Mobile authentication Mobile registration MS- PSTN call PSTN-MS call MS-MS call Intra-MSC handover Inter-MSC handover

Contents

GSM signaling and protocol architecture

Interfaces in GSM

MS BTS BSC MSC

MSC or HLR or

Um interface A-bis

A- interfaceSS7 GSM MAP

GSM signaling and protocol architecture

GSM signaling protocol layer 1

GSM signaling protocols

GSM signaling & SS7

SS7 Network Component

SS7 protocol stack

GSM Protocol layers To perform network control operations unique to

personal communications, GSM adds a Mobile Application Part (MAP) to SS7.

MAP makes use of the Transactions Capabilities Part (TCAP) transport protocol.

MAP functions: ◦ Updating of residence information in VLR◦ Storage of routing information in HLR◦ Updating and supplementing of user profiles in HLR◦ Handoff of connections between MSCs

GSM Mobile Application Part

Contents

Frequency Synchronization Time Synchronization System Synchronization

Mobile initialization

Frequency Synchronization

Time Synchronization

System Synchronization

Mobile initialization MS In idle

Contents

GSM NETWORK LAYOUTGSM Network (PLMN)

MSC region

Location areaBSC

Location area

BTS BTS

GSM NETWORK LAYOUT

EIRHLR

AUCVLR

MSC GMSC

PSTNISDN

PLMNINTERNATIONAL

What is a location area (LA)?

A powered-on mobile is informed of an incoming call by a paging message sent over the PCH channel of a cell

One extreme is to page every cell in the network for each call, which results in a waste of radio bandwidth

Other extreme is to have a Mobile notify the system via location updating messages of its current location at the individual cell level. This requires paging messages to be sent to exactly one cell, but this is wasteful due to the large number of location updating messages.

Hence, in GSM, we group cells into Location Areas (Neighborhoods).

Addresses and Identifiers International Mobile Station Equipment Identity (IMEI)

- It is similar to a serial number. It is allocated by equipment manufacturer, registered by network, and stored in EIR International Mobile Subscriber Identity (IMSI)

-CC: Country Code-MNC: Mobile Network Code-MSIN: Mobile Subscriber Identification Number (ID in

home net.)When subscribing for service with a network,

subscriber receives (IMSI) and stores it in the SIM (Subscriber Identity Module) card. The IMSI has the unique subscriber id that identifies the HLR of the MS. It is never made public

CC MNC MSIN

Addresses and Identifiers Mobile Subscriber ISDN (MSISDN)

-NDC: National Destination Code, SN: Subscriber Number, CC: Country Code-The “real telephone number”, assigned to the SIM-The SIM can have several MSISDN numbers for selection of different services like voice, data, fax Mobile Station Roaming Number (MSRN)

-It is temporary location dependent ISDN number-It is assigned by local VLR to each MS in its area.

CC NDC SN

Addresses and Identifiers

Location Area ID(LAI)- CC: Country Code, MNC: Mobile Network Code, LAC: Location Area Code-LAI is broadcast regularly by Base Station on BCCH-Each cell is identified uniquely as belonging to an LA by its LAI Temporary Mobile Subscriber Identity (TMSI)

-It is an alias of the IMSI and is used in its place for privacy.-It is used to avoid sending IMSI on the radio path.It is an temporary identity that is allocated to an MS by the VLR at inter-VLR registration, and can be changed by the VLR--TMSI is stored in MS SIM card and in VLR. MSCs and location registers (HLR,VLR) are

addressed with ISDN numbers. In addition, they may have a Signaling Point Code (SPC) within a PLMN to address them uniquely in SS#7.

CC MNC LAC

TMSI vs IMSI vs MSRN vs MSISDN TMSI is used during location update and registration for find,

paging and call routing. Instead of using IMSI, the MS sends the TMSI to the BSS, which forwards it to the MSC.

MSRN is the routing number that identifies the current location of the called MS.

MSRN is temporary network identity assigned during the call establishment to a mobile subscriber.

MSRN is the address to the serving MSC/VLR. MSRN is used during call termination (Incoming call to the

MS). MSISDN is dialed during call termination. MSISDN points to the subscribers records in the HLR that

contains information to locate the MSC where the subscriber is currently located.

MSISDN is the telephone number. There is an association between IMSI and MSISDN in the

Location management Set of procedures to:

◦ track a mobile user◦find the mobile user to deliver it calls

Current location of MS maintained by 2-level hierarchical strategy with HLRs and VLRs.

Location registrationMS BSS/MSC VLR HLR AUC

IMSI Ki

A3 & A8

Generate TMSI

Loc.Upd.Req(IMSI,LAI)

Upd Loc.Area(IMSI,LAI)

Auth.Para.Req

(IMSI)

Auth.Info.Req

(RAND)Authenticat

e (IMSI,Kc,RAND,SRES)

Aut. Info.(IMSI)

(RAND)Authentic. Req

(IMSI,Kc,RAND,SRES)

Auth.Info

Auth.Resp.(SRES) (SRES)

Auth.Resp

Update Location(IMSI,MSRN)

SRESRANDKi

Kc SRES

Contd...

(…contd) Location registration.

MS BSS/MSC VLR HLR AUC

Generate TMSI

(Kc)Start Ciph.

Ciph.Mod.Com.

Message M

Ins.Subsc.Data(IMSI)

Forw. New TMSI(TMSI) Subs.Dat.Ins.Ack

Loc.Upd.Accept(IMSI)Loc.Upd.Accept

Ciph.Mod.Kc(M)

A5Kc(M)Kc

TMSI Realloc.Ack

TMSI Realloc.Cmd.

TMSI.Ack

Loc.Upd.Accept

Location registration MS has to register with the PLMN to get communication services Registration is required for a change of PLMN MS has to report to current PLMN with its IMSI and receive new

TMSI by executing Location Registration process. The TMSI is stored in SIM, so that even after power on or off,

there is only normal Location Update. If the MS recognizes by reading the LAI broadcast on BCCH that it

is in new LA, it performs Location Update to update the HLR records.

Location update procedure could also be performed periodically, independent of the MS movement.

The difference in Location Registration and Location Update is that in location update the MS has already been assigned a TMSI.

Location updateMS BSS/MSC VLR HLR AUC

IMSI, TMSIKi, Kc, LAI

Start ciphering.

Authentication

Loc.Upd.Req(TMSI,LAI)

Update Loc.Area

(TMSI,LAI)

Generate (option)TMSI

Start ciphering(Kc) IMSI

Insert Subscriber. data

Subs. Data Insert Ack(contd..)

(..contd) Location update.

MS BSS/MSC VLR HLR AUC

(IMSI)Auth.Info.Req

(IMSI,Kc, RAND,SRES)Auth.Info

Start ciphering.

Forward new TMSI

Auth. Para. Req

Loc. Upd. Acept

TMSI AckTMSI ReallocationComplete

TMSI Realloc. Cmd.

(TMSI)

Auth. Info.(IMSI,Kc, RAND,SRES)

(IMSI)

Loc. Upd. Acept

Location update for inter LA, inter VLRMS BSS/MSC HLR

Loc.Upd.Req(TMSI,LAI)

Update Loc.Area(TMSI,LAI)

Start ciphering(Kc) IMSI

Subs. Data Insert Ack

Authentication

Generate TMSI

IMSI, TMSIKi, Kc, LAI

VLR new

Send para. From VLR new(TMSI, LAI)

IMSI response(IMSI,RAND,SRES,Kc

Cancel Location(IMSI)

Cancel location ack(IMSI)

Forward new TMSI

(TMSI)Location update accept

VLR old

Contents

The Identification procedure is used to identify the MS when the VLR fails to recognize the TMSI sent by MS

Such failure may be caused when the MS changes its location while powered off

The Identification procedure is initiated by VLR

The VLR send MAP/D provide IMSI message to the MSC

Mobile identification

Location update for inter LA, inter VLRMS BSS/MSC HLR

(TMSI,LAI) (TMSI,LAI)

Start ciphering

(Kc) IMSI

Subs. Data Insert Ack

Authentication Generate TMSI

TMSIKi, Kc, LAI

VLR new

Send para. From VLR new(TMSI, LAI)

IMSI response(IMSI,RAND,SRES,Kc

Cancel Location(IMSI)

Cancel location ack(IMSI)

Forward new TMSI(TMSI)

Location update accept

VLR old

(MAP/B provide IMSI)

(IMSI,LAI)

Contents

Mobile Authentication Authentication procedure occurs at:Location Update Services Request

Mobile Authentication is done every location update and every service request

The VLR send MAP/D send parameter message to the HLR

The Authentication is done using the AuC with the HLR

Mobile Authentication

Contents

GSM Attach/Detach or IMSI Attach/Detach is done only if you switch ON and switch OFF your cell phone

Lets say you switched ON your Cell phone so at this stage what happens is you first synchronize with frequency (FCCH Channel) then you go for time synchronize(SCH Channel) and then you start getting information about your network (BCCH Channel).

It is like once you enter into the IDLE mode means you have done frequency and time synchronization and now you need to inform to the network that hey….now I am in idle mode please attach so that I could make and receive calls.

Mobile registration

IMSI ATTACHED

IMSI Attach

MS registeredMS ON

When mobile is turn on, it send its TMSI to the MSC which in turn send it to the VLR

if suppose lets say you were there under this MSC/VLR before… in this case what happens is that the state changes from the detach to attach.

IMSI ATTACHED

When MSC finds that the MS is new MS that is there is no any kind of entry for this particular MS in VLR then the MSC will ask HLR…the HLR will provide the details about the MS i.e nothing but the IMSI saying that this IMSI is valid…the MSC now is going to create one entry in VLR for this new MS with the status of “IMSI ATTACHED”…inside the HLR also the same will be updated.

IMSI ATTACHED

IMSI DETACHED

IMSI Detach

MS OFF

IMSI DETACHED

Contents

Call OriginationVLRu1

MSCPSTN

VLR PSTN MSC

2. MAP_SEND_INFO_FOR_OUTGOING_CALL3.

MAP_SEND_INFO_FOR_OUTGOING_CALL_ack4. IAM

1.call origination request

Call TerminationPSTN

GMSCHLR VLR

Target MSC

Originating Switch

GMSC HLR VLRTarge

t MSC1. ISUP IAM

2. MAP_SEND_ROUTING_INFO3. MAP_PROVIDE_ROAMING_NUMBER

4. MAP_PROVIDE_ROAMING_NUMBER_ack

5. MAP_SEND_ROUTING_INFO_ack

6. ISUP IAM

MSC Directory Number Allocation

LocalExchange

MSISDN

Directory Number Spectrum in MSC

TrunksTrunks

Used to reference home subscribers

Used to reference visiting subscribers

Ways to obtain MSRN1. Obtaining at location update– MSRN for the MS is

assigned at the time of each location update which is stored in the HLR. This way the HLR is in a position to supply immediately the routing info (MSRN) needed to switch a call through to the local MSC.

2. Obtaining on a per call basis– This case requires that the HLR has at least an identification for the currently responsible VLR. When routing info is requested from the HLR, it first has to obtain the MSRN from the VLR. This MSRN is assigned on a per call basis, i.e. each call involves a new MSRN assignment

Land to Mobile Call RoutingMobile Located in Non-Home MSC Area

HomeMSC

VisitedMSC

TMSI & LACMSRN

TMSIMSRN

MSISDNMSISDN

MSISDN MSRN

Signalling

Voice Path

Land to Mobile Call Routing

Mobile in Home MSC Area

HomeMSC

TMSI & LACMSRN

MSISDN MSRN

MSISDN BSS 1

TMSITMSI

MSISDN

Land to Mobile Call Routing

Routing Via a Gateway MSC

TMSI & LACMSRN

MSISDN

MSISDN BSS 1

BSS 2VisitedMSC

MSISDN

TMSITMSI

HomeMSC

GatewayMSC

Signalling

Voice Path

Phases of a Land-to-Mobile Call

Routing Analysis

Paging

Authentication*

Ciphering*

Equipment Validation*

Call Set-up

Handover(s)*

Release

*Phase might not occur

Note:: Detail for authentication, ciphering and equipment validation is not shown. It is the same as in location registration update and mobile-to-land call scenarios.

Land-to-Mobile Call Routing Analysis

VLRHLRMSCPSTN

Incoming Call MSISDN

Get Route MSISDN

Routing Information MSRN

Incoming Call MSRN

Perform PageTMSI, LAI

Land-to-Mobile Call Paging

MSCBSSMSAUm

Perform Page TMSI

Page TMSI (on PCH)

Channel Request (on RACH)

Dedicated Signalling ChannelAssignment (on AGCH)

Page Response TMSI,LAI (on SDCH)

Page Response TMSI, LAI

Land-to-Mobile Call Set-up

MSCBSSMS

Call Set-up

Call Set-up Confirm

Assign Trunk & RadioChannel TRUNK

Assign RadioChannel TCH

Radio Assignment Complete

Trunk & RadioAssignment Complete

Land-to-Mobile Call Set-upContinued

PSTNMSCMS

Mobile Alerting

Network Alerting

Connect (off-hook)

Connect

23 Connect Acknowledge

Land-to-Mobile CallNetwork Initiated Call Release

PSTNMSCBSSMSAUm

Disconnect

Release

Release Complete

Clear Command

Channel Release

Clear Complete

Network Release

Mobile-to-LandCall Scenario

Required Facilities for a Mobile-to-Land Call

1. Radio channel between Mobile Station and BTS selected by the BSC 2. BSC – BTS voice trunk selected by the BSC3. MSC – BSC voice trunk selected by the MSC4. MSC – PSTN voice trunk selected by the MSC5. Line from PSTN end switching office to Fixed Station (permanent link)

BSC - Base Station ControllerBTS - Base Transceiver Station MSC - Mobile Switching CentrePSTN - Public Switched Telephone Network

RadioChannel

Mobile StationBSC

MSC PSTN

BSCVoiceTrunk

VoiceTrunk to PSTN Line

MSCVoiceTrunk

Fixed PublicLand NetworkPublic LandMobile Network

Phases of a Mobile-to-Land Call

Request for Service

Authentication*

Ciphering*

Equipment Validation*

Call Set-up

Handover(s)*

Release

* Phase might not occur

Note: Detail for authentication and ciphering is not shown. It is the same as in the location registration update scenario.

Mobile-to-Land Call Request for Service

NewVLRB

MSCBSSMSAUm

Channel Request

Dedicated SignallingChannel Assignment

Service Request TMSI, LAI

Mobile-to-Land Call Equipment Validation

EIRMSCMS

IMEI Request

IMEI Response

Check IMEI

IMEI Check Results

(IMEI)

Mobile-to-Land Call Set-up

MSCBSSMSAUm

Call Setup Request

Access Subscriber Data

Subscriber Data

Call Proceeding

Assign Trunk & RadioTrunk No.

Assign Radio Channel TCH

Radio Assignment Complete(on TCH)

Trunk & Radio AssignmentComplete TCH

Mobile-to-Land Call Set-upContinued

PSTNMSCMS

Network Set-up(Dialled DN, Trunk No.)

Network Alerting

Alerting

Connect (answer)

Connect

Connect Acknowledgement

Note: Network Set-up, Network Alerting and Connect are generic terms. For SS7, the network set-up message would be Initial Assignment Message (IAM).

Mobile-to-Land CallMobile Initiated Release

PSTNMSCBSSMSAUm

Disconnect

Network Release

Release

Release Complete

Clear Command

Channel Release

Clear Complete

Mobile-to-MobileCall Scenario

Phases of a Mobile-to-Mobile Call

Request for Service

Authentication* Ciphering* Equipment Validation*

Call Setup

Routing Analysis

Paging

Authentication* Ciphering* Equipment Validation*

Call Setup Handover(s)* Release

*Phase might not occur

Originating Mobile

Terminating Mobile

Mobile-to-Mobile Call Set-up & Release

BSSA’MSCVLRBSSMS-A

Channel Request

MS-BUm’Um

Service RequestChannel Request

Authentication

Ciphering

Equipment Validation

Paging and Response

Authentication

Ciphering

Equipment Validation

Call Set-up

Handover

Release

Call Set-up

Handover

Release

Routing AnalysisWith HLR

Find Operation Inter-LA

◦Both LA’s belong to same MSC, call/packets will be routed directly

Inter-MSC Inter-VLR

Find operation in GSM

ISDN switch recognizes from the MSISDN that the call subscriber is a mobile subscriber. Therefore, forward the call to the GMSC of the home PLMN (Public Land Mobile Network)

GMSC requests the current routing address (MSRN) from the HLR using MAP

By way of MSRN the call is forwarded to the local MSC

Local MSC determines the TMSI of the MS (by querying VLR) and initiates the paging procedure in the relevant LA

After MS responds to the page the connection can be switched through.

GSM Handover

FromFrequency 6 Time Slot 3

ToFrequency 9 Time Slot 7MSC

SubscriberSet

• Lanline switched at MSC• Frequency and time slot changed at MS

4 types of handover

MSC MSC

BSC BSCBSC

BTS BTS BTSBTS

MS MS MS MS

1 2 3 4

Handover decisionreceive level

BTSoldreceive level

BTSold

HO_MARGIN

BTSold BTSnew

Handover procedure

HO access

BTSold BSCnewmeasurementresult

BSCold

Link establishment

MSCMSmeasurementreport

HO decisionHO required

BTSnew

HO requestresource allocation

ch. activationch. activation ackHO request ackHO commandHO commandHO command

HO completeHO completeclear commandclear commandclear complete clear complete

Speech coding Channel coding Interleaving Frame structure Modulation Logical channel Organization of logical channel Power Control

Contents

Speech coding

In order to send our voice across a radio network, we have to turn our voice into a digital signal.

GSM uses a method called RPE-LPC (Regular Pulse Excited - Linear Predictive Coder with a Long Term Predictor Loop) to turn our analog voice into a compressed digital equivalent.

Speech coding

RPE-LPC In modern land-line telephone systems, digital coding is used.

The electrical variations induced into the microphone are sampled and each sample is then converted into a digital code.

The voice waveform is then sampled at a rate of 8 kHz.

Since we sample 8000 times per second Each sample is then converted into an 8 bit binary number representing 256 distinct values.

So we have a bit rate of 8kHz X 8 bits = 64kbps.

This bit rate is unrealistic to transmit across a radio network since interference will likely ruin the transmitted waveform.

GSM speech encoding works to compress the speech waveform into a sample that results in a lower bit rate using RPE-LPC.

RPE-LPC A LPC encoder fits a given speech signal against a set of vocal characteristics.

The best-fit parameters are transmitted and used by the decoder to generate synthetic speech that is similar to the original.

Information from previous samples is used to predict the current sample.

The coefficients of the linear combination of the previous samples, plus an encoded form of the residual, the difference between the predicted and actual sample, represent the signal.

Speech is divided into 20 millisecond samples, each of which is encoded as 260 bits, giving a total bit rate of 13 kbps.

This way GSM can transmit 4 times (floor[64kbps/13kbps]) as many phone calls as a regular land-line telephone.

RPE-LPC

Contents

Once we have a digital signal we have to add some sort of redundancy so that we can recover from errors when we transmit our digital voice over the radio channel.

Channel coding adds redundancy bits to the original information to detect and correct errors occurred during transmission.

GSM uses convolution coding and interleaving to achieve this protection.

The exact algorithms used differ for speech and for different data rates

Channel coding

GSM Channel Coding

Once we have a digital signal we have to addsome sort of redundancy so that we canrecover from errors when we transmit ourdigital voice over the radio channel.

Channel codingadds redundancy bitstothe original information to detect and correct,errors occurred during transmission.

GSM usesconvolutioncoding andinterleavingto achieve this protection.

The exact algorithms used differ for speechand for different data rates

In digital transmission, the quality of the transmitted signal is often expressed in terms of how many of the received bits are incorrect .

This is called Bit Error Rate (BER). BER defines the percentage of the total number of received bits which are incorrectly detected.

Channel coding

This percentage should be as low as possible. It is not possible to reduce the percentage to zero because the transmission path is constantly changing.

Channel coding

Channel coding is used to detect and correct errors in a received bit stream.

It adds bits to a message. These bits enable a channel decoder to determine whether the message has faulty bits, and to potentially correct the faulty bits.

Channel coding for GSM

Recall that the RPE-LTP Encoder produces a block of 260 bits every 20 ms.

It was found (though testing) that some of the 260 bits were more important when compared to others.

Below is the composition of these 260 bits.◦Class Ia: 50 bits (most sensitive to bit errors)◦Class Ib:132 bits (moderately sensitive to bit

errors)◦Class II: 78 bits (least sensitive to error)

Channel coding for GSM speech

As a result of some bits being more important than others, GSM adds redundancy bits to each of the three Classes differently.

The Class IA bits are encoded in a cyclic encoder.

The Class Ib bits (together with the encoded Class IA bits) are encoded using convolutional encoding.

Finally, the Class II bits are merely added to the result of the convolutional encoder.

Class Ia bits have a 3 bit Cyclic Redundancy Code added for error detection.

These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a ½ rate convolutional encoder.

Each input bit is encoded as two output bits.

The convolutional encoder thus outputs 378 bits, which are added to the 78 remaining Class II bits, which are unprotected.

Thus every 20 m sec speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps

Class Ia

Class Ib

Class II

Contents

To further protect against the burst errors common to the radio interface, each sample is interleaved.

This method rearranges a group of bits in a particular way.

After encoding resultant sample block consists of 456 bits.

These blocks are then divided into eight blocks each containing 57 bits.

The first four blocks will be placed in the even bit positions of the first four bursts.

The last four blocks will be placed in the odd bit positions of the next four bursts.

Interleaving

Because of interleaving lost bits are part of several different packets and each packet loses only a few bits out of a large number of bits.

So Interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors.

Since the errors become less concentrated , it is then easier to correct them.

Interleaving

Contents

GSM frame structure

GSM frame offset

GSM super and hyper frame

Contents

Modulation is the process of encoding information from a message source in a manner suitable for transmition.

The ultimate goal of a modulation technique is to transport the message signal through a radio channel with the best possible quality while occupying the least amount of radio spectrum.

Digital Modulation Technique

Modulation may be done by varying the amplitude ,phase, or frequency of a high frequency carrier in accordance with the amplitude of the message signal.

Quadrature Phase Shift Keying is effectively two independent BPSK systems (I and Q), and therefore exhibits the same performance but twice the bandwidth efficiency.

Quadrature Phase Shift Keying can be filtered using raised cosine filters to achieve excellent out of band suppression.

Large envelope variations occur during phase transitions, thus requiring linear amplification.

Quadrature Phase Shift Keying

Offset quadrature phase-shift keying OQPSK is a variant of Phase Shift Keying modulation using 4 different values of the phase to transmit. It is sometimes called Staggered quadrature phase shift keying SQPSK .

OQPSK limits the phase-jumps that occur at symbol boundaries to no more than 90° and reduces the effects on the amplitude of the signal due to any low-pass filtering.

Bit transitions occur every Tb sec Smaller envelope variations

Offset Quadrature Phase-Shift Keying (OQPSK)

QPSK vs. OQPSK

MSK uses changes in phase to represent 0's and 1's, but unlike most other keying, the pulse sent to represent a 0 or a 1, not only depends on what information is being sent, but what was previously sent. The pulse used in MSK is the following:

Minimum shift Keying

Right from the equation we can see that θ(t) depends not only from the symbol being sent (from the change in the sign), but it can be seen that is also depends on θ(0) which means that the pulse also depends on what was previously sent.

To see how this works let's work through an example. Assume the data being sent is 111010000, then the phase of the signal would fluctuate as seen in the figure below.

Even though the derivation of MSK was produced by analyzing the changes in phase, MSK is actually a form of frequency-shift-keying (FSK)

MSK produces an FSK with the minimum difference between the frequencies of the two FSK signals such that the signals do not interfere with each other.

Contents

Logical Channel

A traffic channel (TCH) is used to carry speech and data traffic.

Traffic channels are defined using a 26-frame multi frame, or group of 26 TDMA frames.

The length of a 26-frame multi frame is 120ms. Out of the 26 frames, 24 are used for traffic, 1 is used for the slow associated control channel (SACCH) and 1 is currently unused.

Traffic Channels (TCH)

They can be defined as full-rate TCHs (TCH/F, 22.8 kbps) and half-rate TCHs (TCH/H, 11.4 kbps).

Half-rate TCHs double the capacity of a system effectively by making it possible to transmit two calls in a single channel.

If a TCH/F is used for data communications, the usable data rate drops to 9.6 kbps (in TCH/H: max. 4.8 kbps) due to the enhanced security algorithms.

Eighth-rate TCHs are also specified, and are used for signaling. In the GSM Recommendations, they are called stand-alone dedicated control channels (SDCCH)

Full Rate & Half Rate TCH

The signaling channels on the air interface are used for call establishment, paging, call maintenance, synchronization, etc.

There are three type of signaling channels◦Broadcast Channels◦Common Control Channels◦Dedicated Control Channel

Signaling channels

Carry only downlink information and are responsible mainly for synchronization and frequency correction.

This is the only channel type enabling point-to-multipoint communications in which short messages are simultaneously transmitted to several mobiles

Broadcast Channels (BCH)

Each cell has a designated BCH carrier All BCH timeslots transmit continuously on full power

TS 0 contains logical control channels TS1-7 optionally carries traffic BCCH block occur once each 51-frame multiframe

Each block comprises 4 frames carrying 1 message

BCH Characteristics

Broadcast Control Channel (BCCH):◦ General information, cell specific (local area code (LAC),

network operator, access parameters, list of neighboring cells, etc).

◦The MS receives signals via the BCCH from many BTSs within the same network and/or different networks.

Frequency Correction Channel (FCCH): ◦Downlink only; correction of MS frequencies;

transmission of frequency standard to MS; ◦ it is also used for synchronization of an acquisition by

providing the boundaries between timeslots and the position of the first timeslot of a TDMA frame.

Broadcast Channels (BCH) Types

Synchronization Channel (SCH): ◦Downlink only; frame synchronization (TDMA frame

number) and identification of base station. ◦The valid reception of one SCH burst will provide the

MS with all the information needed to synchronize with a BTS

Broadcast Channels (BCH) Types

A group of uplink and downlink channels between the MS and the BTS.

These channels are used to convey information from the network to MSs and provide access to the network.

CCCHs include the following channels;◦Paging Channel (PCH)◦Access Grant Channel (AGCH)◦Random Access Channel (RACH)

Common Control Channels (CCCH)

Paging Channel (PCH): ◦Downlink only; ◦ the MS is informed by the BTS for incoming calls

via the PCH Access Grant Channel (AGCH):

◦Downlink only, ◦BTS allocates a TCH or SDCCH to the MS, thus

allowing the MS access to the network. Random Access Channel (RACH):

◦Uplink only, ◦allows the MS to request an SDCCH in response to

a page or due to a call; ◦ the MS chooses a random time to send on this

channel. ◦This creates a possibility of collisions with

transmissions from other MSs

Common Control Channels (CCCH) Types

Responsible for roaming, handovers, encryption, etc.

The DCCHs include the following channels;◦Stand-alone Dedicated Control Channel (SDCCH);

Communications channel between MS and the BTS; signaling during call setup before a traffic channel (TCH) is

allocated◦Slow Associated Control Channel (SACCH);

Transmits continuous measurement reports in parallel to operation of a TCH or SDCCH

◦Fast Associated Control Channel (FACCH); Similar to the SDCCH, but used in parallel to operation of

the TCH; if the data rate of the SACCH is insufficient, “borrowing

mode” is used: Additional bandwidth is borrowed from the TCH;

this happens for messages associated with call establishment authentication of the subscriber, handover decisions, etc.

Dedicated Control Channels (DCCH)

Contents

Organization of logical channel

Contents

GSM is designed so that the MS uses only the minimum transmit power level to communicate with BTS

the power of the GSM mobiles is closely controlled so that◦ the battery of the mobile is conserved◦ the levels of interference are reduced ◦performance of the base station is not

compromised by high power local mobiles

Power Control

The base station controls the power output of the mobile keeping the GSM power level sufficient to maintain a good signal to noise ratio.

A table of GSM power levels is defined the base station controls the power of the mobile by sending a GSM "power level" number

In virtually all cases the increment between the different power level numbers is 2dB

The power level numbers vary according to the GSM band in use

GSM power levels

POWER LEVEL NUMBER POWER OUTPUT LEVEL DBM

2 393 374 355 336 317 298 279 25

10 2311 2112 1913 1714 1515 1316 1117 918 719 5

GSM power level table for GSM 900

GSM power level table for GSM 1800

POWER LEVEL NUMBER POWER OUTPUT LEVEL DBM

29 3630 3431 320 301 282 263 244 225 206 187 168 149 1210 1011 812 613 414 215 0

Not all mobiles have the same maximum power output level

In order that the base station knows the maximum power level number that it can send to the mobile, it is necessary for the base station to know the maximum power it can transmit

This is achieved by allocating a GSM power class number to a mobile

This GSM power class number indicates to the base station the maximum power it can transmit and hence the maximum power level number the base station can instruct it to use

GSM Power class

Power Control

The transmit power level of MS is dynamically controlled in steps of 2 dB to a minimum of 20 m watt (13 dBm)

MS power control is remotely controlled by BTS

The BTS monitors the received power from the MS and ordered the MS to adjust the transmit power for minimum power level for reliable transmission.

Power Control

GSM frequency hopping

In cellular urban environment, multi-path propagation exists in most cases.

Due to Rayleigh fading, short-term variations in received level are frequently observed.

This mainly affects stationary or quasi-stationary mobiles.

Frequency Hopping is able to take the advantage due to frequency selective nature of fading to decrease the number of errors and at the same time they are temporally spread.

Interference Averaging means spreading raw bit errors (BER caused by the interference) in order to have random distribution of errors instead of having burst of errors

GSM frequency hopping

timing advance value corresponds to the length of time a signal takes to reach the base station from a mobile phone

The TA value is normally between 0 and 63, with each step representing an advance of one bit period (approximately 3.69 microseconds)

the TA value changes for each 550-metre change in the range between a mobile and the base station

This limit of 63 × 550 meters is the maximum 35 kilometers that a device can be from a base station and is the upper bound on cell placement distance

Timing Advance

Thank you

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