Telecom Champ: prepaid in call flow & Roaming MT Call Flow

Roaming Call Flow
Today we are discussing about MT Call Flow, When Subscriber B is in Roaming.
Authentication, Ciphering, Paging, are taken for simple understanding.

Postpaid Subscriber MT Call Flow :-

- When Customer (MS A) dials Mobile (MS B) Number (which home Circle is 2 but currently in roaming at Circle 1).
- After "Connect" msg is sent from MS A to MSC A, which analyse B-Number & NPDB query found it's of different circle, LRN+B no is routed towards GMSC A.
- From GMSCA we route the LRN to B party circle.
- After receiving IAM from GMSC A, GMSC B initiates the HLR SRI request to HLR B.
- In Normal Scenario, i.e. Postpaid MT call flow, GMSC B will initiate the SRI query to HLR B & HLR B will ask for PRN from MSC B (in which MS B currently present), & MSC B will respond to that query by providing MSRN number to GMSC B via HLR B, but here,
- Since MS B is in Roaming at Circle 1, so HLR B will ask for PRN from MSC C, via GMSC B & GMSC C.
- MSC C provides the MSRN to GMSC A via GMSC C & GMSC B
- After receiving MSRN by GMSC A, MSC A routes the call to MSC C.
- Now, ISUP msgs (IAM, ACM, ANM, REL, RLC) is sent from MSC A to MSC B, to complete the call.

Prepaid Subscriber MO Call Flow :-

When a user dials a no., it originates a call from it’s mobile phone. The calling party is known as A-Party and called party is known as B-party

First of all call goes to the BTS which is the nearest tower of that operator. BTS forwards it to the MSC/MSS through BSC( Base Station Controller). MSC is the Mobile Switching Centre, which routes the call to its destination.

To reach to its destination, call has to travel the entire circuit defined by the operator. Which mainly consists of Signal Switch Part (SSP), Signal Transfer Part (STP) and Signal Control Part (SCP) followed by the Base Station Subsystem (BSS).

1.IDP: MSC/MSS sends first signal to STP to initiate a prepaid call. This signal is called Initial Detection Point. , it contains –

• A-Party no

• B-Party no.

• Service key=90

• A Party Location.

• Time stamp.

2. IDPm(Manipulated IDP): STP receives this signal. It checks the B-Party no. with its database and adds respective LRN to the no. STP converts the B-Party no in (91-LRN-MSISDN) format and forwards it to IN.

3. AC: IN receives this signal IDP, analyses the A-Party no, checks it’s balance and the entire tariff related facilities available to that no. Depending on these observation IN decides whether A party can make this call or not, If IN finds everything is Ok then it responds with a signal AC (Applying Charges) to STP, which contains maximum granted time provided by IN.

4. AC: STP just receives this AC signal and forwards it to the MSS as it is.

5.CIQ: Call Information Request.

Now IN requests to the MSS for Call Information through STP which consist of-

• Call Attempt Elapsed Time. (CAET)

• Call Connect Elapsed Time. (CCET)

• Call Stop Time.

• Release cause.

6.CIQ: STP simply receives the CIQ request from IN and pass it to the MSS.

7. RRB: Request Report BCSM Event.

IN again requests to MSS for the detailed release cause through STP, which is Request Report Basic Call State Module Event and the request is called RRB

8. RRB: STP simply forwards this request RRB to the MSS.

9. CONNECT: Finally after getting everything Ok and when all the requests from IN side have been done then IN sends a CONNECT signal(91+LRN+MSISDN) to the MSS through STP, which allows MSS to connect the call further.

10. CONNECT: STP forwards the same CONNECT signal to the MSS.

101.PRN: Provide Roaming No.(Send MSRN)

Now MSS is eligible to establish the call after getting CONNECT signal from IN. Then it checks the B-Party’s NDC and sends a signal to the corresponding B-HLR. MSS request to the HLR to Provide MSRN(PRN) of B-Party.

102.PRN : HLR checks the VLR of the called no. and sends the PRN signal.

103. TIMSI: By checking the LAC CI of the called party, exact cell in which the B-Party no is latched has been found. So VLR can send a signal to the exact BTS of B-Party. Signal is the TMSI of the called no.

104. PAGING: After getting the TMSI of the B-Party BTS broadcasts its TIMSI in the respective cell in which the B-Party no is latched. This signal is broadcasted to all the nos latched in that cell.

105. Paging Response: When the B-Party no is free and receives the broadcasted TIMSI it suddenly responds to the signal and sends a Paging Response back to the BTS.

106. Ping: BTS simply forward that PING signal to the corresponding VLR.

107. MSRNB: When VLR received that PING, it assured that the called no is available on the other end and ready to receive the call. Now VLR sends an MSRN Block signal to the HLR depending upon the available free MSRN blocks (MSRNB)it have.

108. MSRNB: HLR simply forwards this signal back to the MSS. Now MSS got the confirmation from HLR that B-Party no. has been found and is ready to receive the call. After getting this confirmation MSS is now able to make direct ISUP-connection with B-VLR.

CALL CONNECTS: After getting MSRNB, A-VLR initiates ISUP traffic towards B-VLR (including messages IAM,ACM,ANM,REL,RELC) and actual voice traffic starts.

ACTIVITY TEST: After IN provides “connect” message to A-VLR, no standard messages are communicated b/w IN & VLR other than “Activity test” message. It is a sort of ping message which is one directional sent by IN to VLR.

During and after the call MSS notes:

• The duration for which B-Party’s phone rings. Time between cell start ringing and user picks up the phone. This is called Call Attempt Elapsed Time. (CAET)

• The duration as long as they talks is called as Call Connect Elapsed Time (CCET), or call duration.

• The exact time when call terminates, which is called Call Stop Time.

• The exact cause due to which call got terminated.

11. ACR: - Apply Charging Report. When call gets terminated MSS sends the Apply Charging report to STP to forward it to IN for appropriate charging.

12. ACR: - STP forwards this report to the IN. IN receives this report and makes charging on basis of it.

13. CIR: Call Information Report. By using the details which was noted down containing CAET(Call attempt elapsed time), CCET(Call connect elapsed time), Call Stop Time and Release Cause; MSS makes a report i.e. Call Information Report and sends to IN through the STP.

14. CIR: STP forwards the same CIR message to IN.

15. ERB: -Event Report BCSM. This is another report sends by MSS to IN through STP which contains the actual release cause in detail. The release cause may be one of following -