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GSM Events

IMSI Attach  IMSI Detach  Location Update  Mobile-Originated Call  Mobile-Terminated Call 

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Mobile Terminated Call

The term Mobile Terminated Call refers to when the MS is the receiver of a

call. In this example, the call is originating from outside the PLMN.

Route Establishment1. The calling party

dials the MSISDN for the mobile subscriber.

The PSTN identifies

the network (PLMN)that the dialed

MSISDN belongs to

and will locate aGMSC for that

network. The PSTNsends an Initial 

 Address message tothe GMSC.

2. The GMSCforwards the MSISDN

to the HLR and

requests routinginformation for it. The

HLR looks up the

MSISDN anddetermines the IMSI

and the SS7 address

for the MSC/VLR that

is servicing the MS.

3. The HLR then

contacts the servicing

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MSC/VLR and asks it

to assign a Mobile

Station Routing Number (MSRN) to

the call.

4. The MSC/VLR 

allocates the MSRN

and forwards it to theHLR.*Note: It is important to

remember that the

MSC/VLR assigns a

MSRN to thecall not to the

MS itself. 

5. The HLR forwards

the MSRN as well asrouting information

for the servicing

MSC/VLR to theGMSC.

6. The GMSC sends

an Initial Addressingmessage to the

servicing MSC/VLR 

and uses the MSRN to

route the call to theMSC/VLR. Once the

servicing MSC/VLR receives the call, the

MSRN can be

released and may bemade available for 

reassignment.

*Note: This is

implementationdependent.

Paging the Mobile Station

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7.The MSC/VLR then

orders all of its BSCs

and BTSs to page theMS. Since the

MSC/VLR does not

know exactly whichBSC and BTS the MS

is monitoring, the

 page will be sent out

across the entireLocation Area.

Initial Setup8. The MS receives

the Page

 Request (PAG_REQ)

on the PCH. The MS

recognizes that the

 page is intended for it, based on a TMSI or 

an IMSI.

9. The MS sends

a Channel 

 Request (CHAN_REQ) message on the

RACH.

10. The BSS responds

on the AGCH bysending an Immediate

 Assignment (IMM

ASS) message which

assigns an SDCCH to

the MS. At this point,the network does not

know that the MS is

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the one that it is

 paging, it only knows

that this MS wantsaccess to the network.

11. The MSimmediately switches

to the assigned

SDCCH and sendsa Paging 

 Response (PAG_RES)

message on theSDCCH. This lets the

network know that the

MS is responding to

its page.

Authentication12. Before thenetwork will provide

any services to the

MS, the network willrequire the MS to

authenticate itself.

The BSS sendsan Authentication

 Request (AUTH_REQ) message to the MS.

The RAND serves asthe "challenge" for 

authentication.

13. The MS calulates

the proper SRES

 based on the RANDthat was given and

sends the SRES to the

BSS inan Authentication

 Response (AUTH_RE

SP) message.

14. The BSS verifies

the SRES. If the

SRES is correct then

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the MS is

authenticated and

allowed access to thenetwork.

15. Once theMSC/VLR has

authenticated the MS,

it will order the BSSand MS to switch to

cipher mode using the

CIPH_MOD_CMDmessage. Once the

MS in in encryption

mode, the VLR will

normally assign a new

TMSI to the MS.

*See the Authentication

and Encryption

Tutorial for more

information on this process.

Establishing a Channel16. Once the MS is

authenticated and in

encryption mode, TheMSC sends a Setup

Message to the BSS,the BSS forwards the

SETUP message to

the MS on theassigned SDCCH.the

assigned SDCCH. The

SETUP message may

include the Calling 

 Line Identification

 Presentation (CLIP),which is essentiallycaller ID.

17. The MS responds by sending a Call 

Confirmed (CALL_C

ON) message; which

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indicates that the MS

is able to establish the

requested connection.The BSS relays the

message up to the

MSC.,br>

Call Setup18. The BSS then

sends an Assignment Command (ASS_CMD) message to the MS

on the assigned

SDCCH. The

ASS_CMD messageassigns a Traffic

Channel (TCH) to the

MS.

19. The MS

immediately switchesto the TCH and

responds with

an Assignment 

Complete (ASS_COM

) message on theFACCH. The MS

 begins ringing once ithas established the

TCH.*Remember that allsignaling that occurs on

the traffic channel actually

occurs on a FACCH,

which is a time slot that is

stolen from the TCH and

used for signaling. 

20. The MS sends anALERT message to

the MSC on the

FACCH. The BSSforwards the ALERT

message through the

PSTN to the calling

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 party and the caller 

hears the line ringing.

Establishing the Call

21. Once the user 

answers the call (by pressing the send

 button), the MS will

send a Connect CON

message to the MSC.The Connect message

is forwarded back to

the caller's switch toactivate the call.

22. The MSC sendsa Connect 

 Acknowledge CON_A

CK message to the

MS and the call isestablished.

Disconnecting the Call23. A disconnect

happens the same wayas for any other call.

In this example, the

calling party initiatesthe disconnect.*Only the Air Interface is

shown here. 

24. When the calling

 party hangs up, thecalling party's switch

initiates

a Release (REL)message. The message

is forwarded to the

serving MSC, which

is then forwarded to

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the BSS.

25. The BSS will senda Disconnect (DISC)

message to the MS on

the FACCH.

26. The MS confirms

release of the call bysending

a Release(REL)

message on theFACCH, which is

forwarded to the

MSC.

27. The MSC sendse ReleaseComplete (REL_COM) message through the

BSS to the MS. As far 

as call control (CC) isconcerned, the

connection has been

terminated.

28. The MS still has a

TCH assigned to it, sothe BSS sendsaChannel 

 Release (CHAN_REL

) message to the MS.This releases the radio

resource on the Air 

Interface.

29. The MS responds

 be sending a

final Disconnect message and returns to

idle.

IMSI Attach  IMSI Detach  Location Update  Mobile-Originated Call  Mobile-Terminated Call 

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