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Processor Interfaces (GSM Originating Call)

Cell Mobile Network Fixed Network

Mobile Station Base Stations NSS PSTN

EventStudio System De signer 4. 0

13-Sep-08 21:38 (Page 1)

LEG: GSM M obile Originated Call

This sequence diagram w as generated with EventStudio System Designer 4.0 (http:// ww w.EventHelix.com /EventStudio ). Copyrigh t 2008EventHelix.com In c. All Rights Reserved. The EventStudio so urce files for this docum ent can be downloaded fromhttp://www .eventhelix.com/call-flow/gsm-call-setup.zip.

This scenario describes the call setup for a GSM originating call. A mobile user calling a land line subscriber is c overed here.

Copyright 2000-20 08 EventHelix.com Inc. All Rights Reserved.

Begin RR Connection Establishment

Call related information needs to be transported from the mobile phone to the Mobile Switching Center (MSC). This requires the establishmeof a Radio Resource (RR) con nection to M SC. The first phase of the call setup just sets up this RR conn ection.

RR CHANNEL REQUEST

RACH

RR connection establishment is triggered by sending Channel Request m essage. This message requests t heBase Station System (BSS) for allocation for radioresources for the RR connection setup. The mobile nowaits fo r an assignm ent on th e Access Grant Channel(AGCH). At this p oint t he mobile is list ening to the AGfor a reply.

Note: The RR CHANNEL REQUEST is sent on a Random Access Channel (RACH). This is a slotted aloha channel that can be used at randomwithout any coordination between the mobiles. Any mobile can transmit on this channel whenever it wishes. If two mobiles transmit on the

channel at the same time, their messages will be lost in a collision. The mobiles will detect the collision via a timeout and retransmit them essage after a random back off.

RR IM MEDIATE ASSIGNMENT

AGCH, Radio_Resou rce = ( TCH, Frequency,Timeslot), Time Correction, Frequency Correction

The BSS transmits the radio resource assignment to tMo bile via the AGCH channel. The m essage also contathe time and frequency corrections. The time correctioallow the m obile to time it's transmissions so that thereach the BSS only in th e specified slot. The frequenccorrections correct for the Doppler shift caused by themobile's motion.

RR SABM + MM CM SERVICE REQUEST

TCH, SAPI = 0

This is the first message that is sent after tuning to thechannel. The Mobile initiates a LAPm connection withBSC by sending a Set Asynchronou s Balanced Mo de(SABM) m essage. The service request m essage m eanfor th e MSC is also sent in this m essage.

RR UATCH, SAPI = 0

The BSS replies with Unnum bered Acknowledge (UA)complete the LAPm setup handshake

SCCP CONNECTION REQUEST + MM CM SERVICE REQUEST

SS7

The BSS receives the CM Service Request message frthe m obile and f orm s a "BSSMAP COM PLETE LAYER INFORMATION". The BSS then piggy backs the messaon th e SCCP connection request m essage.

LEG: Skip Authenticati on Procedure

Enable Ciphering

BSSMAP CIPHER M ODE COMM AND Since the subscriber has been successfullyauthenticated, the MSC initiates ciphering of the databeing sent on t he channel. The channel is ciphered soso protect the call from eavesdropping.

RR CIPHERING MODE COMMANDmo de = CLEAR

The BSS sends the CIPHERING MODE COMMAND to mo bile. The mo bile will be able to receive this m essagas the transmission from the BSS is still in clear.

RR CIPHERING MODE COMPLETE

mode = CIPHERED

Ciphering has already b een enabled, so this m essage transmitted with ciphering. The BSS will receive thismessage as it is already expecting ciphered data in thereceive direction.

BSSM AP CIPHER M ODE COM PLETE BSS replies back to the M SC, indicating that cipheringhas been successfully enabled.

RR Connection Establishm ent Completed

At this point a connection has been setup between the Mobile and the MSC. From this point onward, the BSS is just acting as a conduit fortransport ing the signaling m essages between the Mobil e and the M SC.

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Processor Interfaces (GSM Originating Call)

Cell Mobile Network Fixed Network

Mobile Station Base Stations NSS PSTN

EventStudio System De signer 4. 0

13-Sep-08 21:38 (Page 2)

Call Setu p

CC SETUP

Dialed Digits

The Mobile sends the setup m essage to establish a vocall. The message contains the dialed digit s and otherinformation needed for call establishment.

CC CALL PROCEEDING The m obile is informed that the call setup is in progre

Mode Modify

BSSMAP ASSIGNM ENT REQUEST

Voice circuit

MSC informs the BSS about the allocated voice circuitThe call is also switched from signaling to voice.

RR CHANNEL M ODE MODIFY The BSS notif ies the Mobile about the changeover tovoice mode.

RR CHANNEL MODE M ODIFY ACKNOWLEDGE Mobile acknowledges.

BSSMAP ASSIGNM ENT COMPLETE The BSS responds back to the MSC.

ISUP INITIAL ADDRESS MESSAGE

SS7, Dialed Digits

The MSC routes the call and sends the call tow ards thcalled subs criber

ISUP ADDRESS COMPLETE MESSAGE

SS7

The PSTN indicates to the M SC that it has received alldigits and the called subscriber is being rung.

CC ALERTING The M SC inform s the m obile that the called subscribebeing alerted via a ring

ISUP ANSWER

SS7

The called subscrib er answers the call.

CC CONNECT The M SC inform s the m obile that the call has beenanswered.

CC CONNECT ACKNOWLEDGE Acknowledge the recipt of CC CONNECT.

Conversation

Speech The call has entered the conversation phase. The speepath has been setup between the mobile sub scriber anthe land-line subscriber.

Call Release

LEG: Mobile initiates call release

CC DISCONNECT The m obile sends the disconnect message to the MSC

ISUP RELEASE

SS7

The MSC initiates release on the PSTN side.

CC RELEASE The M SC inform s the M obile that it has initiated callrelease

ISUP RELEASED

SS7

The MSC inform s the PSTN that t he call release has bcompleted.

ISUP RELEASE COMPLETE

SS7

The PSTN infor ms that call release has been com pleteat its end.

CC RELEASE COM PLETE Mobile indicates that the call has been released.

RR Connection Release

BSSMAP CLEAR COM MAND Call release has been com pleted, now the RR connectis released by the M SC.

RR CHANNEL RELEASE The BSS initiates RR release with the m obile.

BSSMAP CLEAR COM PLETE The BSS informs the the M SC that the RR connectionhas been released.

RR DISC The m obile sends a disconnect m essage to release theLAPm connection.

RR UA The BSS replies w ith an Unnum bered Acknowledge

message.