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Explain how mobile terminated call works detailing the role of HLR and VLR.

This question appears in Mumbai University > Mobile communication and computing subject

Marks: 10 M

Year: May 2015

1 Answer
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When a station calls a mobile station, we can term it as a Mobile Terminated Call (MTC). For e.g. when a call is made from a MTNL landline to an Airtel mobile phone we call it as an MTC.

The mobile terminated call set-up is described below.

  • Initially the user dials the mobile number. It reaches the PSTN/ISDN/Switching office where it is identified as a GSM call (by the destination code mentioned) and forwards it to the gateway MSC i.e. GMSC.

  • The GMSC identifies the HLR for the subscriber (which is coded in the phone number) and signals the call set-up to the HLR.

  • The HLR then checks if the number is a valid number and whether that user has subscribed to this particular service. If so then an MSRN (Mobile Subscriber Roaming Number) is requested from the subscriber’s current VLR.

  • After receiving the MSRN, the HLR determines the MSC responsible for the mobile station and sends this information to the GMSC.

  • The GMSC then forwards the call setup request to the concerned MSC.

Till now we were finding the phone. Now since we have partially-located it we try calling it so as not to waste more time.

  • MSC requests the current status of Mobile Station (i.e the subscriber’s cell-phone) from VLR.
  • Now the MSC initiates paging in all cells it is responsible for (i.e location area). It searches in all cells as finding the exact cell would be time consuming.
  • If any response is found by any BTS, the VLR performs a security check (encryption etc.). The VLR then asks the MSC to connect to the MS.

Role of HLR in Mobile terminated call Set-up:

  • The HLR is contacted by the gateway MSC (GMSC) for the MS location area.
  • The HLR current VLR number of a particular MS.
  • The HLR basically acts just as a parent guide towards a MS.

Role of VLR in Mobile terminated call Setup:

  • The MSC checks with the VLR for the list of available service for a particular subscriber.
  • The VLR provides a temporary number called as MSRN (Mobile Subscriber Roaming Number)
  • The VLR also contains other parameters with respect to a MS like Location area Code (LAC) and TMSI.
  • The VLR basically acts as the central point of contact for the MSC.
  • It is also responsible for the authentication of a MS once it has been located by the MSC.

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