Basic Call Flow For Same Operator

A Subscriber calls B Subscriber Both are of same Operator(Posted In or Normal Subscriber)

Both In Same PLMN (Local)

1) Subscriber A dials Subscriber B

2) MSC1 receives B number and SRI is sent to MNP server instead of HLR as in case of traditional call flow.

3) MNP checks its database and founds that this is own subscriber and relays that SRI to HLR.

4) HLR on receiving SRI check the VLR address of subscriber and send PRN to MSC2.

5) In response to PRN MSC2 returns with MSRN.

6) HLR forward that MSRN number to MSC1 in SRI Response Message

7) MSRN is now dialled out from MSC1 to MSC2 to establish the voice path between two MSCs

 (GSM) Mobile to Mobile Call Flow, Within Same MSC/BSC

Call Flow Mesage Details:-

1. CM Service Request – Message contains Global Cell Identity, Kind of Service required by subscriber e.g Voice, SMS, Fax, USSD. Mobile Identity e.g TMSI/IMSI.
2. CM Service Accept – Message is in reponse of CM Service Request.
3. Set Up - Message contains dailed number from Mobile A (i.e Mob B number) and speech Codec.
4. Send Routing Info-Request (SRI) - After analysis of dialed numbers received from A in set up message, MSC recognises its own MSISDN series and send SRI to HLR to get the Mobile station roaming number(MSRN). This messge contains B MSISDN, Interrogation type e.g Basic Call etc.
5. Assignment Request - Message is to seize a channel for voice call for Mob A between MSC and BSC. Message contains Circuit Identification Code(CIC).
6. Assignment Complete - Acceptance from BSC end that same CIC has been seized.
7. Provide Roaming Number Request - HLR checks in its database to which VLR, Mob B is registered & ask for roaming number from that VLR. Message contains IMSI, MSISDN, VLR ID of Mob B.
8. Provide Roaming Number Response - Message is response of roaming no. request and contains roaming number of Mob B. This message is for HLR. In this VLR checks its database & allocate one free MSRN from its MSRN pool for B number.
9. Send Routing Info-Response - In this HLR forward same roaming number which it receives in above message to MSC.
10. Paging Request - In this VLR checks the MSRN recieved from HLR & search which MSISDN is against that MSRN, now MSC checks the VLR database of that MSISDN to get the LAC & TMSI/IMSI of that MSISDN (Mob B) & initiates the paging request message to that particular LAC on the basis of TMSI/IMSI in that BSC. BSC now page to that LAC on the basis of IMSI/TMSI of Mob B. Message contains LAC, IMSI/TMSI, DPC of BSC.
11. Paging Response - Mob B response to that paging with GCI (Global Cell Identity) to BSC that inturn forwarded to MSC.
12. Assignment Request - Message is to seize a channel for voice call for Mob B between MSC and BSC. Message contains Circuit Identification Code(CIC).
13. Assignment Complete - Acceptance from BSC end that same CIC has been seized.
14. Set Up - It is used to display Mob A number on Mob B handset.
15. Alert - This message indicates Mob A that Mob B is connected & its ringing. The Ring Back Tone is send from MSC to Mob A.
16. Connect - Mob B answers the call & connect message from Mob B is send to MSC, inturn MSC sends connect message to Mob A indiacting that phone has been answered & connects those 2 voicechannels which were seized during the assignment complete message.
17. Connect Acknowledge - To acknowledge the connection.

GSM Mobile Terminated Call

1. The PSTN subscriber dials the MS’s telephone number (MSISDN), the MSISDN is analyzed in the PSTN, which identifies that this is a call to a mobile network subscriber. A connection is established to the MS’s home GMSC. The PSTN sends an Initial Address message (IAM) to the GMSC.
2. The GMSC analyzes the MSISDN to find out which HLR, the MS is registered in, and queries the HLR for information about how to route the call to the serving MSC/VLR. The HLR looks up the MSISDN and determines the IMSI and the SS7 address for the MSC/VLR that is servicing the MS. The HLR also checks if the service, “call forwarding to C-number” is activated, if so, the call is rerouted by the GMSC to that number.
3. The HLR then contacts the servicing MSC/VLR and asks it toassign a MSRN to the call. [MSRN - Mobile Station Routing Number].
4. The MSC/VLR returns an MSRN via HLR to the GMSC.
5. The GMSC sends an Initial Addressing message (IAM) to the servicing MSC/VLR and uses the MSRN to route the call to the MSC/VLR. Once the servicing MSC/VLR receives the call, the MSRN can be released and may be made available for reassignment.
6. The MSC/VLR then orders all of its BSCs and BTSs to page the MS. Since the MSC/VLR does not know exactly which BSC and BTS the MS is monitoring, the page will be sent out across the entire Location Area(LA). 
7. When the MS detects the paging message to the BTS’s in the desired LA. The BTS’s transmit the message over the air interface using PCH. To page the MS, the network uses an IMSI or TMSI valid only in the current MSC/VLR service area.
8. When the MS detects the paging message, it sends a request on RACH for a SDCCH.
9. The BSC provides a SDCCH, using AGCH.
10. SDCCH is used for the call set-up procedures. Over SDCCH all signaling preceding a call takes place. This includes: Marking the MS as “active” in the VLR. Authentication procedure (Start ciphering, Equipment identification).
11. The MSC/VLR instructs the BSC/TRC to allocate an idle TCH. The BTS and MS are told to tune to the TCH. The mobile phone rings. If the subscriber answers, the connection is established.

MNP Prepaid Call Flow

 

Assumptions: A and B both are Vodafone Delhi Subscriber in different MSC/MSS coverage area.

Let us discuss call flow step by step.

1) Subscriber A (prepaid) call B.

2) Since A is prepaid first query(IDP) has to go IN/SCP with "calling party number" Subscriber A MSISDN and "called party Subscriber" Subscriber B MSISDN. Here is change from normal prepaid call flow, in normal case IDP would have gone straight to serving SCP but in case of MNP IDP will sent to MNP server.

3) MNP server will check its database for B MSISDN and add LRN/RN according to operator to which B subscriber is registered, in above case it is Vodafone Delhi.
After addition of LRN/RN IDP is forwarded to SCP.

4) IDP received by SCP contains LRN/RN + B MSISDN in "called party number" field and "calling party field" contains A MSISDN. Charging is done on the basis of LRN/RN. Here LRN is of Vodafone Delhi so local call rates apply to this call. In normal scenario charging would have be done on the basis on B party MSISDN.
In response to IDP SCP revert with Connect/Continue message to MSC which contains "called party number" as LRN+B MSISDN.

5) MSC check called party number and removes LRN (as its own LRN) and forward SRI to MNP server.

Hereafter normal MNP call flow is followed which is already discussed in detail in earlier post.

6) MNP server checks B MSISDN and forward SRI to HLR.

7) HLR queries with MSC B and provide MSRN to MSC A

8) IAM is send out to MSC B with called number at B party MSRN.