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Call Routing Process in GSM:

      Assume you are a BSNL user calling Airtel user, when you  make a call, your mobile requests BTS to allocate a channel (Random Access Channel)   , then your call request goes to Mobile Switching Center via Base Switching Center. In MSC, Home Location Register is located which checks whether you have sufficient balance and eligible for making outgoing call, Equipment Identity Register checks IMEI of your mobile, Authentication Center decrypts the signal from your mobile (key is stored in both Sim card and Authentication Center) , after successful, your call request is sent to  Airtel BSC which encrypts the call using the key in Authentication Center and sends it to the person's Mobile System via Paging Channel(PCS), then the person's mobile starts ringing, when the person accepts the call, both are connected.

 

Call routing exampleCall routing example

          

             Call from BSNL MSC will use the Interconnection Point of Airtel and sends the call to Airtel BSC. Airtel BSC will check with the MSC whether the person you called has incoming facility and call gets routed.