A timing diagram of a Call Flow is shown below. And a small description is also included after the image. Consider a situation where in a mobile telecommunication network Mobile Subscriber 1 (MS1) is originating a call for Mobile Subscriber 2 (MS2). (Call from MS1 to MS2)

1.  CM Service Request

Sent to MSC as request to access the network. MSC may ask MS to authenticate itself or MSC may allow the MS to access and use the resources of the network. Temporary Mobile Subscriber Identity (TMSI) is also attached to this message.

2.  Ciphering

The process which creates a logical tunnel between the MSC and MS.

3.  Set up message

Contains the number of called party and calling party.

4.  MSC checking VLR profile

To identify allowed services for MS1. If outgoing calls are allowed MSC will send an 
assignment request to the corresponding BSC to allocate a TCH for the call (CIC- Circuit 
Identification Code).

5.  Send Routing Information (SRI)

SRI message consists of IMSI of MS1 and MSISDN of MS2. HLR will look up for MS2 s 
profile and extracts current VLR/MSC.

6.  Provide Roaming Number (PRN)

PRN consists MS2 s IMSI and then it will be forwarded to the MS2 s serving MSC (MSC2).

7.  MSC2 will allocate an MSRN number from its MSRN pool to the IMSI of MS2.

8.  PRN response

Contains the MSRN of MS2 allocated by MSC2.

9.  SRI response

Contains the MSRN number.

10. Initial Address Message

After MSC1 receives the MSRN it will check the routing table to resolve the routing path for 
that number. After resolving the routing path MSC1 will be instructed to communicate with 
MSC2. IAM contains MSRN, Called party number, CIC.

11.  After receiving the IAM message MSC2 will execute following,

·         Check the MSRN table and resolve IMSI for MS2

·         Check MS2 VLR profile

·         Resolve MS2 s LAC and check the LAC table

·         Resolve the BSCs inside that LAC

12.  As requested from MSC2, paging is done in all the BSCs inside that LAC using the 
TMSI of MS2

13. Paging response

When MS2 receives paging request it will send a paging response to the MSC2.

14. Setup

MSC will identify the response and then it will originate setup message to MS2.

15. Alert setup

This message is sent to the MSC2 and MS2 will start ringing.

16.  MSC2 will send an assignment request to the BSC of MS2. Therefore a CIC is

allocated to proceed with the call.

17. Address Complete Message

This is sent to MSC1 to notify that the voice path is established.

18. Alerting

MS1 starts hearing ringing tone of MS2.

19.  Connect

When MS2 answers the call, connect message will be sent.

20. Answer Message

MSC2 will connect the call to MS2 and at the same time it will send the answer message to 
MSC1.

21.  Disconnect

MS1 is disconnecting the call (MS2 may also disconnect before MS1).

22. Release

With the disconnect a release message will be sent to MSC2. Allocated CIC will be released.

23. Release complete

An acknowledgement for the release message.