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

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.

enter image description here

Please log in to add an answer.

Continue reading...

The best way to discover useful content is by searching it.