Explain the process of GSM to PSTN call and PSTN to GSM call.

Subject: Mobile Computing

Difficulty: Medium

Marks: 8 Marks

1 Answer

Mobile call organization in GSM

  1. The subscriber unit must be synchronised to the nearby base station as it monitors the BCH.
  2. By receiving FCCH, BCCH messages, the subscriber would be locked on to the system and the appropriate BCH.
  3. User dials the intended digit combination and presses ―Send‖ on GSM phone. 4.The mobile transmits a burst of RACH data.
  4. The base station then responds with an AGCH message on CCCH which assigns the mobile unit a channel for SDCCH connection.
  5. Once tuned to SDCCH, the subscriber will wait for SACCH frame to be transmitted which informs the mobile of any required timing advance and transmitter power command. The base station is able to determine the timing advance and the signal level from mobiles earlier RACH transmission.
  6. Upon receiving and processing the timing advance info in the SACCH, the subscriber is now able to transmit normal burst messages as required for speech traffic.
  7. The SDCCH sends message between the mobile unit and the base station, taking care of authentication & user validation.
  8. PSTN connects the dialled party to the MSC and the MSC switches the speech path to the serving base station.

    Data is transferred on both the forward and reverse links. The calls is successfully underway and SDCCH is vacated.


  • When calls are organised from PSTN, the process is similar. The base station broadcast a PCH message during TSo within an appropriate frame on the BCH.
  • The mobile station locked onto that same ARFCN, detects its page and replies with a RACH message acknowledging receipt of the page.
  • The BS then uses the AGCH on the CCCH to assign the mobile unit to a new physical channel for connection to the SDCCH & SACCH while the network and the serving base station are connected.
  • Once the subscriber establishes timing advance & authentication on the SDCCH, the base station issues a new physical channel assignment over the SDCCH, and the TCH assignment is made.
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