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  (GSM) Mobile to Mobile Call Flow, Within Same MSC/BSC Hi Friends, Here I tried to explain very basic call flow from mobile to mobile. To make it simple following assumptions have been taken. A) Call is in same MSC and BSC. B) No Authentication, C)No Ciphering , D) Messages on Ater and Air interface are not consider, etc. 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 voice channels which were seized during the assignment complete message. 17.   Connect Acknowledge -  To acknowledge the connection. Read more at
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