PREPARED BYSAIF UL ISLAM SAJID

CS CORE- CALL FLOWS

MS Authentication

Whenever an MS attaches to the network or its inter-MSS location update is required, authentication is performed

a) MS attaches to the network or LU is performed

b) MSS requests the HLR for authentication triplets (RAND, Kc & SRES) which are held/calculated with AuC

c) HLR forwards these triplets to MSS

d) MSS sends RAND to MS.

e) MS sends SRES which it generates by using A3 algorithm on RAND & Ki. The two SRES are then compared in MSS and if they are equal, the user is authenticated.

MSS also sends TMSI and LAI to MS and updates its location in VLR.

f) Ciphering is enabled between the MS & BSS using 3-way handshake. Kc is generated using A8 algorithm on RAND & Ki

Voice Call Setup(step by step)

1. Radio Resource (RR) and Mobility Management (MM) parameters are negotiated between the MS & BSS

2. Cellular Management (CM) request is received by the BSS, which then forms ‘BSSMAP complete layer3 information’ msg. The BSS then piggybacks it on the SCCP connection request msg and sends to MSS

3. MSS checks whether MS is authentic. Since authentication has already been performed, the MSS does not re-authenticate.

4. MSS initiates ciphering and sends a ‘BSSMAP ciphering mode command’ to the BSS, which then ciphers (using A5 algorithm) the data being sent on TCH to prevent eavesdropping

5. BSS sends ‘BSSMAP ciphering mode complete’ msg to the MSS. After this point, the BSS becomes transparent for communication between MSS & MS (thus BSS will not be shown in following slides unless necessary)

0. A-party dials the number

Voice Call Setup(step by step-contd.)

6. MS sends ‘Call Control (CC) call setup’ msg which contains the digits of dest #

7. MSS replies with ‘CC call proceeding’ msg which tells the MS that it is establishing link with the B-party

8. MSS reserves a circuit on CGR and informs the BSS, which then informs the MS to switch to voice mode.

9. MSS sends Initial Detection Point (IDP) msg to IN

10. IN responds with Request Return BCSM Event (RRBE) and Apply Charging (AC) msg

11. MSS sends Send Routing Info (SRI) msg to HLR

12. HLR sends Provide Roaming Number (PRN) to MSS of party-B

13. MSS of party-B responds with Mobile Subscriber Roaming Number (MSRN).

14. HLR responds with SRI-Ack

MSS-A

MSS-B

15. MSS of party-A sends Initial Address Msg (IAM) to MSS-B

16. MSS-B responds with Address Complete Msg (ACM) and establishes communication with party-B17. MSS-B then sends Answer Msg (ANM) when party-B answers the call and communication is established between the two parties.

Voice Call Setup(step by step-contd.)

MSS-A MSS-B

Voice Call Termination

18. Assuming that Subs-A drops the call, ‘CC disconnect’ msg is sent to MSS

19. ‘ISUP release’ msg is sent to MSS-B

21. MSS-A releases the circuits and sends ‘ISUP released’ msg to MSS-B

20. Voice circuits with BSS are released

22. MSS-B responds with ‘ISUP release complete’ msg

23. MSS responds with ‘CC release’ msg

24. MS sends ‘CC release complete’ msg to MSS to indicate that the call has been released.

25. MSS sends ‘BSSMAP clear command’ to BSS

26. RR release is negotiated and the reserved radio resources are freed

27. BSS sends ‘BSSMAP clear complete’ msg to MSS to indicate that all occupied resources are now free

MSS-A MSS-B

SMS Call Flow

i) MS sends the SMS to MSS

ii) MSS consults with IN if the user has sufficient balance and if so, it responds to MSS

iii) MSS sends ‘MO-SMS’ msg to SMC (aka SMSC)

iv) SMC sends ‘MO-SMS ack’ msg to MSS

v) SMC performs SRI for SMS with HLR

vi) SMC sends ‘MT forward short msg (which contains the original SMS)’ to MSS-B

vii) MSS-B sends ‘MT forward short msg ack’ to SMC

MSS-A

MSS-B

Location Update

LU can be done in 3 cases:•IMSI attach (already covered in slide 2)•Periodic LU•Inter LAC LU

•Intra-MSS LU•Inter-MSS LU

Periodic LU:•It is done to keep a check on the presence of MS in the network. •There is an IMSI detach timer in the VLR, which if times out, will detach the subscriber, delete its entry and inform the HLR. •Thus, to keep the user attached a timer, T3212, is implemented in the BSS. •This timer is network dependent and for TP it is 6 hours.•BSS keeps the MS informed about this timer through its BCCH channel and MS keeps track of time expiry through its own counter.•When the time expires, the MS performs an LU.

Location updateInter-LAC, Intra-MSS

ii) Radio Resources assignment is negotiated between MS & BSS

iii) MM ‘location update request’ is sent by MS to BSS. This msg contains LAI & TMSI.

iv) BSS sends ‘BSSMAP complete layer 3 msg’ to the MSS which contains ‘location update request’

v) MSS notes that the LAC is covered by itself and updates the VLR with new LAC

i) MS continuously monitors the BCCH channel and upon changing cells during movement when it notices that broadcasted LAC has changed, it will initiate LU

vi) ‘BSSMAP clear command’ is sent to the BSS

vii) ‘BSSMAP clear complete’ is sent to the MSS

viii) Ciphering is enabled and MM & RR are released by the BSS with a 3-way handshake

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Location updateInter-MSS

ii) Radio Resources assignment is negotiated between MS & new BSS

iii) MM ‘location update request’ is sent by MS to new BSS. This msg contains LAI & TMSI.

iv) BSS sends ‘BSSMAP complete layer 3 msg’ to the MSS which contains ‘location update request’

v) MSS notes that old LAC was covered by another MSS, so it contacts that MSS using TMSI of the subs and HLR for its Database

i) MS continuously monitors the BCCH channel and upon changing cells during movement when it notices that broadcasted LAC has changed, it will initiate LU

viii) MM & RR are released by the BSS with a 3-way handshake

vi) New MSS contacts the old one using ‘MAP/G send parameters’ msg which contains TMSI and requests the IMSI of the subs

vii) Old MSS sends ‘MAP/G send parameters result’ msg to the new MSS

viii) MSS sends ‘MAP/D update location’ msg to the HLR

ix) HLR sends ‘MAP/D insert subs data’ which contains authentication triplets

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Location updateInter-MSS (contd.)

x) MSS replies with ‘MAP/D insert subs data result’ msg

xiii) HLR sends ‘MAP/D update location result’ after updating its record

xi) HLR sends ‘MAP/D cancel location’ msg to the old MSS

xii) MSS sends ‘MAP/D cancel location result’ msg to HLR and releases TMSI & deletes subs entry from its VLR

xiv) MS is authenticated by MSS.xv) Ciphering mode is enabled on radio channels for secure communication.xvi) MSS sends ‘MM location updating accept’ msg to MS which contains new TMSI.

xvii) MS sends ‘MM TMSI reallocation complete’ msg to MSSxviii) RR are cleared between MS & BSS upon request of MSS.

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Inter-MSS HO During Call

1) MS sends periodic RR measurement report of its current and neighboring cells to the BSS

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2) BSS analyses the measurements received and when the signal quality of its serving cell falls below threshold, HO is requested. A list of target cells is provided to the MSS.

3) ‘BSSMAP HO required’ msg is sent to MSS. It contains target cells and origin cell

4) Anchor MSS sends ‘MAP prepare HO’ msg to the new MSS. It also contains target and origin cells

5) ‘BSSMAP HO request’ msg is sent to the new BSS. It contains target cells and origin cell

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6) BSS allocates a TCH.

7) ‘BSSMAP HO request ack’ msg is sent to the MSS. It contains ‘RR HO command msg’ which contains all the parameters which the MS will need to switch to the best cell.

8) MSS allocates a HO number through which inter-MSS call will be initiated.

9) New MSS sends ‘MAP prepare HO response’ msg. It contains ‘RR HO command’ and ‘HO number’ msg

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Inter-MSS HO During Call(contd)

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10) IAM msg sent to the new MSS using HO number

11) ACM msg sent to the anchor MSS and call is setup.

12) ‘BSSMAP HO command’ msg is sent to BSS. It ‘RR HO command’.

13) ‘RR HO command’ is sent to MS which contains HO reference and TCH

14) ‘RR HO accept’ is sent to the BSS through access burst and it contains HO reference

15) BSS sends ‘BSSMAP HO detected’ to the MSS and then MSS switches to the voice path.

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Inter-MSS HO During Call(contd)

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16) ‘MAP access signaling request’ is sent to the old MSS to inform that HO has been detected

17) BSS sends ‘RR physical info’ to the MS. This msg contains time & freq correction

18) Signaling connection is completed between the MS & BSS. Then, MS sends ‘RR HO complete’ msg to the BSS.

19) ‘BSSMAP HO complete’ msg is sent to MSS. It ‘RR HO command’.

20) ‘MAP send end signal’ is sent to the old MSS. It contains ‘HO complete’

21) ‘ISUP Ans’ msg is sent to the old MSS to establish voice call.

22) ‘BSSMAP clear command’ msg is sent to the old BSS to release the RR connection

23) ‘BSSMAP clear complete’ msg is sent to the MSS to indicate that RR connection have been released.

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24) SUBS HAVE HUNG UP:‘ISUP Release’ msg is sent by the anchor MSS.

Inter-MSS HO During Call(Call Cleared)

25) ‘ISUP RLC’ msg is sent by the new MSS.

26) ‘MAP send end signal response’ is sent to indicate that HO call has ended.

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Thank you