GSM Communication Flow

HUAWEI TECHNOLOGIES CO., LTD.

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GSM Communication Flow

GBSS Training Team

HUAWEI TECHNOLOGIES CO., LTD. Page 2All rights reserved

After the completion of this course, the trainees

should understand the following contents: GSM security management Call flow in GSM Knowledge about Location update SMS flow in GSM Handover flow in GSM


•GSM Security ManagementGSM Security Management

•GSM Basic Call SequenceGSM Basic Call Sequence

•Location Update SequenceLocation Update Sequence

•SMS SequenceSMS Sequence

•Handover SequenceHandover Sequence


PLMN’s need a higher level of protection than traditional telecommunication networks. Therefore, to protect GSM systems, the following security functions have been defined:Subscriber authentication: By performing authentication, the network ensures that no unauthorized users can access the network, including those that are attempting to impersonate others.Radio information ciphering: The information sent between the network and an MS is ciphered. An MS can only decipher information intended for itself.Mobile equipment identification: Because the subscriber and equipment are separate in GSM, it is necessary to have a separate authentication process for the MS equipment. This ensures, e.g. that a mobile terminal, which has been stolen, is not able to access the network.Subscriber identity confidentiality: During communication with an MS over a radio link, it is desirable that the real identity (IMSI) of the MS is not always transmitted. Instead a temporary identity (TMSI) can be used. This helps to avoid subscription fraud.

The AUC and EIR are involved in the first three of the above features, while the last is handled by MSC/VLRs.

GSM Security Management


Authentication and Ciphering Sequence

TMSI Reallocation

Equipment Identification

GSM Security Management


Authentication

Authentication may be executed during setup, location updating

and supplementary services. Authentication is done by AUC.

The primary function of an AUC is to provide information,

which is then used by an MSC/VLR to perform subscriber

authentication and to, establish ciphering procedures on the radio

link between the network and MS’s. The information provided is

called a triplet and consists of:

A non predictable RANDom number (RAND)

A signed RESponse (SRES)

A ciphering key (Kc)


At subscription time, each subscriber is assigned a subscriber authentication Key (Ki). Ki is stored in the AUC along with the subscriber’s IMSI. Both are used in the process of providing a triplet. The same Ki and IMSI are also stored in the SIM. In an AUC the following steps are carried out to produce one triplet:1. A non-predictable random number, RAND, is generated.2. RAND and Ki are used to calculate SRES and Kc, using two different algorithms, A3 and A8 respectively.3. RAND, SRES and Kc are delivered together to the HLR as a triplet.

RAND -Random number SRES-Signed ResponseKc-Ciphering keyKi-Subscriber authentication keyIMSI-International Mobile Subscriber Identity

Provision of Triplets


1. The MSC/VLR transmits the RAND to the MS.

2. The MS computes the signature SRES using RAND and the subscriber authentication key (Kii) through the A3 algorithm.

3. The MS computes the Kc by using Ki and RAND through A8 algorithm. Kc will thereafter be used for ciphering and deciphering in MS.

4. The signature SRES is sent back to MSC/VLR, which performs authentication, by checking whether, the SRES from the MS and the SRES from the AUC match. If so, the subscriber is permitted to use the network. If not, the subscriber is barred from network access.

Authentication Procedure


Authentication can by operator’s choice be performed during:

• Each registration

• Each call setup attempt

• Location updating

• Before supplementary service activation and deactivation

MSC/VLR

1. RAND

3. SRES

2. MS calculates SRES using RAND + Ki (SIM-card) through A3 and Kc using RAND+Ki through A8.

4. Compare SRES received from MS with SRES in triplet. If they are equal access is granted.


Confidentiality means that user information and signaling exchanged between BTS’s and MS’s is not disclosed to unauthorized individuals, entities or processes.A ciphering sequence is produced using Kc and the TDMA frame number as inputs in the encryption algorithm A5. The purpose of this is to ensure privacy concerning user information(speech and data) as well as user related signaling elements.In order to test the ciphering procedure some sample of information must be used. For this purpose the actual ciphering mode command (M) is used.

1. M and Kc are sent from the MSC/VLR to the BTS.2. M is forwarded to the MS.3. M is encrypted using Kc (calculated earlier with SRES in the authentication procedure) and the TDMA frame number which are fed through the encryption algorithm, A5.4. The encrypted message is sent to the BTS.5. Encrypted M is decrypted in the BTS using Kc, the TDMA frame number and the decryption algorithm, A5.6. If the decryption of M was successful, the ciphering mode completed message is sent to the MSC. All information over the air interface is ciphered from this point on.

Ciphering Procedure


MSC/VLR

Decryption process using A5

Encryption process using A5

1. M+Kc 2. M

4. Encrypted

M’c

M’

Kc

TDMA Frame no.

3. Encrypt M

TDMAFrame no. Kc

5. Decryption of M’ successful

If yes

6. Ciphering mode complete

MS

Ciphering Procedure

A5 Encryption and decryption algorithmM Ciphering Mode CommandM’ Ciphering Mode CompleteM’c Ciphering Mode Complete, cipheredKc Ciphering keyMSC Mobile services Switching CenterVLR Visitor Location Register


HLRMS BSS MSC VLR

1 Pre-send Triples to VLR



HLRMS BSS MSC VLR

RAND

RAND<SDCCH>


2 AuthenticateAuthentication Request



HLRMS BSS MSC VLR

RAND

RAND<SDCCH>

<SDCCH>

(SRES)



3 Authenticate Response



HLRMS BSS MSC VLR

RAND

RAND<SDCCH>

<SDCCH>

(SRES)



3 Authenticate Response

4 Start Ciphering



HLRMS BSS MSC VLR

5 Cipher Mode Command

Cipher Mode Complete

<SDCCH>

<SDCCH>



RAND

RAND<SDCCH>

3 Authenticate Response <SDCCH>

(SRES)

4 Start Ciphering



MSVLR

Location Update Req

Location Update Acc

TMSI Reallocation Comp

(TMSI)

TMSI ReallocationThe Temporary Mobile Subscriber Identity (TMSI) is a temporary IMSI number made known to an MS at registration. It is used to protect the subscriber’s identity on the air interface. The TMSI has local significance only (that is, within the MSC/VLR area) and is changed at time intervals or when certain events occur such as location updating. Every operator can chose TMSI structure, but should not consist of more than 8 digits.


The equipment identification procedure uses the identity of the equipment itself (IMEI) to ensure that the MS terminal equipment is valid.1. The MSC/VLR requests the IMEI from the MS.2. MS sends IMEI to MSC.3. MSC/VLR sends IMEI to EIR.4. On reception of IMEI, the EIR examines three lists:

• A white list containing all number series of all equipment identities that have been allocated in the different participating GSM countries.• A black list containing all equipment identities that has been barred.• A gray list (on operator level) containing faulty or non -approved mobile equipment.

5. The result is sent to MSC/VLR, which then decides whether or not to allow network access for the terminal equipment.

MSC/VLR

1. IMEI Request

2. IMEI

3. Check IMEI

4. Access/Barring info

EIR

EIR FunctionEquipment Identification Procedure


MS BSS MSC VLR HLR PSTN1 Equipment ID

Request<SDCCH>

EIR




Request<SDCCH>

2 ID Response<SDCCH>

IMEI)

EIR




Request<SDCCH>

2 ID Response<SDCCH>

IMEI)

3 Check IMEI

Check IMEIResponse

EIR









Mobile to Land Sequence

Land to Mobile Sequence

GSM Basic Call Sequence



The process for calling MS and called MS are two

independent flow. The calling party begins with

channel request and ends with TCH assignment

competition. In general, the calling party includes

following several stages: access process,

authentication and ciphering process, TCH assignment

process. So, we take the sequence from mobile to land

as example, in this sequence, we mainly devote to the

calling party.


MS BSS MSC VLR HLR1 CHANNEL REQUEST <RACH>

<AGCH>DCCH ASSIGN

SIGNALING LINK ESTABLISHED

PSTN




<AGCH>DCCH ASSIGN

<SDCCH>


2 REQ. FOR SERVICECR

CC

PSTN




<AGCH>DCCH ASSIGN

<SDCCH>



CC

3 AUTHENTICATION

SET Cipher MODE

PSTN




<AGCH>DCCH ASSIGN

<SDCCH>



CC

3 AUTHENTICATION

SET Cipher MODE

4 SET-UP <SDCCH>

Call Info

PSTN

SFOC




<AGCH>DCCH ASSIGN

<SDCCH>



CC

3 AUTHENTICATION

SET Cipher MODE

4 SET-UP <SDCCH>

Call Info5 EQUIP. ID REQ.

PSTN

SFOC




<AGCH>DCCH ASSIGN

<SDCCH>



CC

3 AUTHENTICATION

SET Cipher MODE

4 SET-UP <SDCCH>

Call Info5 EQUIP. ID REQ.

6 COMPLETE CALL

CALL PROCEEDING <SDCCH>

PSTN

SFOC



7 ASSIG. COMMAND <SDCCH>

ASSIG. COMPLETEcircuit<FACCH>

MS BSS MSC VLR HLR PSTN



7 ASSIG. COMMAND<SDCCH>

ASSIG. COMPLETE

Address Complete(ACM)

Alerting

circuit<FACCH>

Initial and Final Address 8 Message (IFAM)

MS hears ring tone from land phone

<FACCH>





ASSIG. COMPLETE


Alerting

<FACCH>

9 Answer (ANS)Connect

circuit<FACCH>



<FACCH>

Ring tone stops





ASSIG. COMPLETE


Alerting

<FACCH>

9 Answer (ANS)Connect

10 Connect Acknowledge<FACCH>

circuit<FACCH>

<TCH>



<FACCH>

Ring tone stops

HELLO!


BILLING STARTS




For the called party, the flow for the called party

begins when MSC sends paging command to the

called party, ends when two party start talk. In

general, this call flow includes several stages:

access process, authentication and ciphering

process, TCH assignment process, talk process,

release process.


MS BSS MSC VLR HLR GMSC

(MSISDN)

Initial and Final 1 Address Message

PSTN




(MSISDN)

(MSISDN) (IMSI)


PSTN

2 Send Routing Info




(MSISDN)

(MSISDN) (IMSI)

(MSRN)


PSTN

(MSRN) (MSRN)

2 Send Routing Info

3 Routing Info Ack

Initial and Final Address Message




(MSISDN)

(MSISDN) (IMSI)

(MSRN)

(MSRN)


PSTN

(MSRN) (MSRN)

2 Send Routing Info

3 Routing Info Ack


4 Send Info For I/C Call Setup




(MSISDN)

(MSISDN) (IMSI)

(MSRN)

(MSRN)

(LAI & TMSI)(TMSI)(TMSI)


PSTN

(MSRN) (MSRN)

2 Send Routing Info

3 Routing Info Ack


4 Send Info For I/C Call Setup

5 PagePaging Request

<PCH>



6 Channel Request

DCCH Assign

Signaling Link Established

Page Response

*Authentication

<RACH>

<AGCH>

<SDCCH>

<SDCCH>

(TMSI)(TMSI & Status)

(Status)

CR

MS BSS MSC VLR HLR GMSC PSTN



6 Channel Request

DCCH Assign


Page Response

*Authentication

7 Complete Call

Setup

<RACH>

<AGCH>

<SDCCH>

<SDCCH>


(Status)

<TMSI>

<SDCCH>

CR




6 Channel Request

DCCH Assign


Page Response

*Authentication

7 Complete Call

Setup

8 Call Confirmation

<RACH>

<AGCH>

<SDCCH>

<SDCCH>


(Status)

<TMSI>

<SDCCH>

<SDCCH>

CR


Ring Tone at the land phone



(channel) (circuit)

<FACCH>

9 Assignment Command

Assignment Complete

Alert<TCH>

Address Complete





<FACCH>

(channel) (circuit)

<FACCH>

9 Assignment Command

Assignment Complete

Alert<TCH>

<FACCH>10 Connect

Connect ACK ANS

<TCH>

Hello...

Address Complete


Billing starts


Ringing stops at land phoneSubscriber

picks up









Brief Introduction to Location Update

Several Typical Location Update Sequences

Location Update Sequence


The types of Location Update :

Normal Location Update

IMSI Attach/Detach

Periodic Location Update

Brief Introduction to Location Update

Mobile powers on and is idle.

Reads the LAI broadcast on the BCCH.

Compares with the last stored LAI and if it is different does

a location update.

Normal Location UpdateNormal Location Update


IMSI AttachIMSI Attach Saves the network from paging a MS which is not active in the system.

When MS is turned off or SIM is removed the MS sends a detach signal to the Network. It is marked as detached.

When the MS is powered again it reads the current LAI and if it is same does a location update type IMSI attach.

Attach/detach flag is broadcast on the BCCH sys info.

Periodic Location UpdatePeriodic Location Update Many times the MS enters non-coverage zone.

The MS will keep on paging the MS thus wasting precious resources.

To avoid this the MS has to inform the MSC about its current LAI in a set period of time.

This time ranges from 0 to 255 deci-hours.

Periodic location timer value is broadcast on BCCH sys info messages.

Location Update


Intra-VLR Location

Update Sequence

Inter-VLR Location

Update Sequence

Several Typical Location Update Sequences


MS BSS MSC VLR<RACH> 1 Channel Request<AGCH>

DCCH Assign

HLR

Only sent to HLR if this is the first time the MS has Location Updated in this VLR

Intra-VLR Location Update Sequence



DCCH Assign

2 Location Update Request<SDCCH>

HLR





DCCH Assign


LAI & TMSI 3 Authentication & Ciphering

HLR





DCCH Assign



4 Forward New TMSI

Location Update Accept

<TMSI>

<SDCCH>

<TMSI>

HLR





DCCH Assign



4 Forward New TMSI


<TMSI>

<SDCCH>

<TMSI>

5 TMSI Reallocate Complete

TMSI ACK

<SDCCH>

HLR





DCCH Assign



4 Forward New TMSI


<TMSI>

<SDCCH>

<TMSI>

5 TMSI Reallocate Complete

TMSI ACK

<SDCCH>

6 Clear Command

Clear Complete

<SDCCH>

<SDCCH>

HLR




Location Update via IMSI

Location Update via TMSI

Inter-VLR Location Update Sequence


<RACH>

1 Channel Request <AGCH>DCCH Assign

MS BSS MSC VLRn HLR VLRo

Inter-VLR Location Update Via IMSI


<RACH>


2 Location Update Request

<SDCCH>


LAI & IMSI



<RACH>



<SDCCH>

LAI & IMSI

Authentication & Ciphering

MS BSS MSC VLRn HLR

3 Authentication Para. Req

VLRo

.



<RACH>



<SDCCH>

LAI & IMSI


MS BSS MSC VLRn HLR



Insert Subscriber Data

Insert Subscriber Data Ack

Location Update accept

VLRo



<RACH>



<SDCCH>

LAI & IMSI


MS BSS MSC VLRn HLR






VLRo

5 Cancellocation

Cancellocation Ack.



6 Forward New TMSI


TMSI Reallocate Complete

TMSI ACK




6 Forward New TMSI



TMSI ACK

7 Clear Command

Clear Complete




<RACH>



Inter-VLR Location Update Via TMSI


<RACH>



<SDCCH>


LAI & TMSI



<RACH>



<SDCCH>

LAI & TMSI



3 Provide Identification

provide Identification Ack

TMSI&LAIO

TMSI,IMSI,KC,R,S)



<RACH>



<SDCCH>

LAI & TMSI



3 Provide Identification

provide Identification Ack

TMSI&LAIO

TMSI,IMSI,KC,R,S)








5 Cancellocation

Cancellocation Ack.



6 Forward New TMSI



TMSI ACK


5 Cancellocation

Cancellocation Ack.



6 Forward New TMSI



TMSI ACK

7 Clear Command

Clear Complete


5 Cancellocation

Cancellocation Ack.









MS BSS MSC VLRInterworkingMSC

1 CHANNEL REQUEST <RACH>

<AGCH>DCCH ASSIGN

SIGNALING LINK

ESTABLISHED

SC

MO SMS Transfer




<AGCH>DCCH ASSIGN

<SDCCH>

SIGNALING LINK

ESTABLISHED

2 REQ. FOR SERVICE CR

CC

SC

MO SMS Transfer




<AGCH>DCCH ASSIGN

<SDCCH>

SIGNALING LINK

ESTABLISHED

2 REQ. FOR SERVICE CR

CC

3 AUTHENTICATION

SET Cipher MODE

SC

MO SMS Transfer




<AGCH>DCCH ASSIGN

<SDCCH>

SIGNALING LINK

ESTABLISHED


CC

3 AUTHENTICATION

SET Cipher MODE

SC

4 RP_MO_DATA

SIF_MO_SMS

SIF_MO_SMS-Ack

MO SMS Transfer


5 MO_Forward_SM

Short_Message

MO_Forward_SM_Ack

Short_Message_Ack

(SC_No.)


SC

MO SMS Transfer


5 MO_Forward_SM

Short_Message

MO_Forward_SM_Ack

Short_Message_Ack

6 RP_ACK

(SC_No.)


SC

"Send Successfully" is displayed on the mobile

MO SMS Transfer


For Forwarding a Short Message

For Forwarding Several Short Message

MT SMS Transfer


MSServicing

MSC VLR HLR GatewayMSC SC

1 Short Message

SRI_For_SM

SRI_For_SM_Ack

MT_Forward_SM

MT SMS Transfer (A Message)


MSServicing


1 Short Message

SRI_For_SM

SRI_For_SM_Ack

MT_Forward_SM

2 SIF_MT_SMS

Page

Page



MSServicing


1 Short Message

SRI_For_SM

SRI_For_SM_Ack

MT_Forward_SM

2 SIF_MT_SMS

Page

Page

3 Paging Response

Authentication and Ciphering



MSServicing


1 Short Message

SRI_For_SM

SRI_For_SM_Ack

MT_Forward_SM

2 SIF_MT_SMS

Page

Page Request

3 Paging Response


4 Short_Message

Short_Message_Ack

MT_Forward_SM_Ack

Short_Message_Ack



MSServicing


1 Short Message

SRI_For_SM

SRI_For_SM_Ack

MT_Forward_SM

MT_Forward_SM(The More message To

Send Flag is True)

MT SMS Transfer (Several Messages)


MSServicing


1 Short Message

SRI_For_SM

SRI_For_SM_Ack

MT_Forward_SM


Send Flag is True)

2 SIF_MT_SMS PagePaging Request



MSServicing


1 Short Message

SRI_For_SM

SRI_For_SM_Ack

MT_Forward_SM


Send Flag is True)

2 SIF_MT_SMS PagePaging Request

3 Paging Response




MSServicing


4 Short_Message

Short_Message_Ack

MT_Forward_SM_Ack

Short_Message_Ack



MSServicing


4 Short_Message

Short_Message_Ack

MT_Forward_SM_Ack

Short_Message_Ack

5 Short_Message

MT_Forward_SM


Send Flag is False)

Short_Message_Ack

MT_Forward_SM_Ack

Short_Message_Ack

Short_Message









Inter - BSS handover sequence

Inter - MSC handover sequence

Handover Sequence


MS oBSS nBSS MSC

1 Periodic Measurement 1 Periodic Measurement reportsreports

<SACCH>

Inter - BSS Handover Sequence


MS oBSS nBSS MSC

1 Periodic Measurement reports

<SACCH>

2 Handover required2 Handover required



MS oBSS nBSS MSC


<SACCH>

2 Handover required

3 Handover Request3 Handover Request TMSI cct. code



MS oBSS nBSS MSC


<SACCH>

2 Handover required

3 Handover Request

4 Handover REQ ACK4 Handover REQ ACK

TMSI cct. code

HO Ref. No.



MS oBSS nBSS MSC


<SACCH>

2 Handover required

3 Handover Request

5 Handover Command 5 Handover Command

4 Handover REQ ACK

TMSI cct. code

HO Ref. No.

HO Ref. No.<FACCH>



MS oBSS nBSS MSC


<SACCH>

2 Handover required

3 Handover Request

5 Handover Command

4 Handover REQ ACK

TMSI cct. code

HO Ref. No.

HO Ref. No.<FACCH>

6 Information Interchange 6 Information Interchange <FACCH>



MS oBSS nBSS MSC


<SACCH>

2 Handover required

3 Handover Request

5 Handover Command

4 Handover REQ ACK

TMSI cct. code

HO Ref. No.

HO Ref. No.<FACCH>

6 Information Interchange <FACCH>

7 Handover Complete 7 Handover Complete



MS oBSS nBSS MSC


<SACCH>

2 Handover required

3 Handover Request

5 Handover Command

4 Handover REQ ACK

TMSI cct. code

HO Ref. No.

HO Ref. No.<FACCH>


7 Handover Complete

8 Clear Command 8 Clear Command



MS oBSS nBSS MSC


<SACCH>

2 Handover required

3 Handover Request

5 Handover Command

4 Handover REQ ACK

TMSI cct. code

HO Ref. No.

HO Ref. No.<FACCH>


7 Handover Complete

8 Clear Command

9 Periodic Meas. reports9 Periodic Meas. reports <SACCH>



Basic Inter-MSC handover

Subsequent Inter - MSC handover

Inter - MSC Handover Sequence


MSCA MSCB VLRB

1 PrepareHandover1 PrepareHandover

2 Allocate HandoverNo.

4 PrepareHandover_Ack

5 Send HO Report_Ack

6 Initial

Address Message

3 Send Handover Report

Address Completed

Answer

Inter-MSC handover


MSCA MSCB VLRB

1 PrepareHandover

2 Allocate HandoverNo. 2 Allocate HandoverNo.



6 Initial

Address Message


Address Completed

Answer

Inter-MSC handover


MSCA MSCB VLRB

1 PrepareHandover




6 Initial

Address Message

3 Send Handover Report3 Send Handover Report

Address Completed

Answer

Inter-MSC handover


MSCA MSCB VLRB

1 PrepareHandover


4 PrepareHandover_Ack4 PrepareHandover_Ack


6 Initial

Address Message


Address Completed

Answer

Inter-MSC handover


MSCA MSCB VLRB

1 PrepareHandover



5 Send HO Report_Ack5 Send HO Report_Ack

6 Initial Address Message


Address Completed

Answer

Inter-MSC handover


MSCA MSCB VLRB

1 PrepareHandover




6 Initial 6 Initial Address MessageAddress Message


Address CompletedAddress Completed

AnswerAnswer

Inter-MSC handover


MSC/VLRA MSC/VLRB

7 Process Access Signal7 Process Access Signal

Send End SignalSend End Signal

Forward Access SignalForward Access Signal

Process Access SignalProcess Access Signal

8 Clear Forward

Release Guard

Send End Signal_Ack

Inter-MSC handover


MSC/VLRA MSC/VLRB

7 Process Access Signal

Send End Signal

Forward Access Signal

Process Access Signal

8 Release8 Release

Release CompleteRelease Complete

Send End Signal_AckSend End Signal_Ack

Inter-MSC handover


MSC/VLRA MSC/VLRB HLRA

9 SendAuth. Info9 SendAuth. Info

Auth InfoAuth Info

Update LocationUpdate Location

InsertSubs.DataInsertSubs.Data

InsertSubs.Data_AckInsertSubs.Data_Ack

UpdateLocation_AckUpdateLocation_Ack

Cancellocation

Cancellocation_Ack

Inter-MSC handover



1 PrepareSubsequentHOV1 PrepareSubsequentHOV

PrepareHandoverPrepareHandover

PrepareHandoverPrepareHandover

PrepareSubsequentHOVPrepareSubsequentHOV

2 Initial and FinalAddress Message

Address Complete

Answer

3 RLS

RLC

Subsequent Inter-MSC handover



1 PrepareSubsequentHOV

PrepareHandover

PrepareHandover

PrepareSubsequentHOV

2 Initial and Final2 Initial and FinalAddress MessageAddress Message

Address CompleteAddress Complete

AnswerAnswer

3 RLS

RLC




1 PrepareSubsequentHOV

PrepareHandover

PrepareHandover

PrepareSubsequentHOV

2 Initial and FinalAddress Message

Address Complete

Answer

3 RLS3 RLS

RLCRLC



MSC/VLRA MSC/VLRB MSC/VLRC

ForwardAccessSignalForwardAccessSignal

4 ProcessAccessSignal4 ProcessAccessSignal

7 Location Update

SendEndSignal_Ack

6 SendEndSignal

5 RLS

RLC




5 RLS5 RLS

RLCRLC

7 Location Update

SendEndSignal_Ack

6 SendEndSignal

4 ForwardAccessSignal

ProcessAccessSignal




7 Location Update

SendEndSignal_AckSendEndSignal_Ack

6 SendEndSignal6 SendEndSignal


ProcessAccessSignal

5 RLS

RLC





ProcessAccessSignal

5 RLS

RLC

7 Location Update7 Location Update

SendEndSignal_Ack

6 SendEndSignal



SummarySummary

1. GSM Security Management

2. GSM Basic Call Sequence

3. Location Update Sequence

4. SMS Sequence

5. Handover Sequence

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GSM Communication Flow