Short note on Call procedures in GSM.

Mumbai University > Electronics and telecommunication engineering> Sem 7 > mobile communication

Marks: 10M

Year: May 2016

1 Answer

Solution: Mobile call origination in GSM:

  • First, the subscriber unit must be synchronized to a nearby base station as it monitors the BCH.
  • By receiving the FCCH, SCH, and BCCH messages, the subscriber would be locked on to the system and the appropriate BCH.
  • To originate a call, the user first dials the intended digit combination and presses the "send" button on the GSM phone.
  • The mobile transmits a burst of RACH data, using the same ARFCN as the base station to which it is locked.
  • The base station then responds With an AGCH message on the CCCH which assigns the mobile unit to a new channel for SDCCH connection.
  • The subscriber unit, which is monitoring TS 0 of the BCH, would receive its ARFCN and TS assignment from the AGCH and would immediately tune to the new ARFCN and TS.
  • This new ARFCN and TS assignment is physically the SDCCH {not the TCH).
  • Once tuned to the SDCCH, the subscriber unit first waits for the SACCH frame to be transmitted (the wait would last, at most, 26 frames or 120ms), which informs the mobile of any required timing advance and transmitter power command.
  • The base station is able to determine the proper timing advance and signal level from the mobile's earlier RACH transmission and sends the proper value over the SACCH for the mobile to process.
  • Upon receiving and processing the timing advance information in the SACCH, the subscriber is now able to transmit normal burst messages as required for speech traffic.
  • The SDCCH sends messages between the mobile unit and the base station, taking care of authentication and user validation, while the PSTN connects the dialled party to the MSC, and the MSC switches the speech path to the serving base station.
  • After a few seconds, the mobile unit is commanded by the base station via the SDCCH to retune to a new ARFCN and new TS for the TCH assignment.
  • Once retuned to the TCH, speech data is transferred on both the forward and reverse links, the call is successfully underway, and the SDCCH is vacated.
  • When calls are originated from the PSTN, the process is quite similar. The base station broadcasts a PCH message during TS0 within an appropriate frame on the BCH.
  • The mobile station, locked on to that same ARFCN, detects its page and replies With an RACH message acknowledging receipt of the page.
  • The base station then uses the AGCH on the CCCH to assign the mobile unit to a new physical channel for connection to the SDCCH and SACCH while the network and the serving base station are connected.
  • Once the subscriber establishes timing advance and authentication on the SDCCH, the base station issues a new physical channel assignment over the SDCCH and the TCH assignment are made.
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