2.7 GSM Interfaces

2.7.1 THE A (MSC-BSS) INTERFACE

The interface between the MSC and its BSS is specified in the 08-series of GSM Technical
Specifications. The BSS-MSC interface is used to carry information concerning:
· BSS management;
· call handling;
· mobility management.

2.7.2 THE Abis (BSC-BTS) INTERFACE

When the BSS consists of a Base Station Controller (BSC) and one or more Base Transceiver
Stations (BTS), this interface is used between the BSC and BTS to support the services offered
to the GSM users and subscribers. The interface also allows control of the radio equipment
and radio frequency allocation in the BTS. This interface is specified in the 08.5x-series of
GSM Technical Specifications.

2.7.3 THE B (MSC-VLR) INTERFACE

The VLR is the location and management data base for the mobile subscribers roaming in the
area controlled by the associated MSC(s). Whenever the MSC needs data related to a given
mobile station currently located in its area, it interrogates the VLR. When a mobile station
initiates a location updating procedure with an MSC, the MSC informs its VLR which stores
the relevant information.
2.7 GSM Interfaces
2. Architecture Overview
24 GSM Technology for Engineers
© AIRCOM International 2002
This procedure occurs whenever an MS roams to another location area. Also, when a
subscriber activates a specific supplementary service or modifies some data attached to a
service, the MSC informs (via the VLR) the HLR which stores these modifications and
updates the VLR if required.

2.7.4 THE C (MSC-HLR) INTERFACE

The Gateway MSC must interrogate the HLR of the required subscriber to obtain routing
information for a call or a short message directed to that subscriber.

2.7.5 THE D (HLR-VLR) INTERFACE

This interface is used to exchange the data related to the location of the mobile station and to
the management of the subscriber. The main service provided to the mobile subscriber is the
capability to set up or to receive calls within the whole service area. To support this, the
location registers have to exchange data.
The VLR informs the HLR of the location of a mobile station managed by the latter and
provides it (either at location updating or at call set-up) with the roaming number of that
station. The HLR sends to the VLR all the data needed to support the service to the mobile
subscriber. The HLR then instructs the previous VLR to cancel the location registration of this
subscriber.
Exchanges of data may occur when the mobile subscriber requires a particular service, when
he wants to change some data attached to his subscription or when some parameters of the
subscription are modified by administrative means.

2.7.6 THE E (MSC-MSC) INTERFACE

When a mobile station moves from one MSC area to another during a call, a handover
procedure has to be performed in order to continue the communication. For that purpose the
MSCs have to exchange data to initiate and then to realize the operation.
After the handover operation has been completed, the MSCs will exchange information to
transfer A-interface signalling as necessary. When a short message is to be transferred
between a Mobile Station and Short Message Service Centre (SC), in either direction, this
interface is used to transfer the message between the MSC serving the Mobile Station and the
MSC which acts as the interface to the SC.

2.7.7 THE F (MSC-EIR) INTERFACE

This interface is used between MSC and EIR to exchange data, in order that the EIR can verify
the status of the IMEI retrieved from the Mobile Station.

2.7.8 THE G (VLR-VLR) INTERFACE

When a mobile subscriber moves from a VLR area to another Location Registration procedure
will happen. This procedure may include the retrieval of the IMSI and authentication
parameters from the old VLR.

2.7.9 THE H (HLR-AuC) INTERFACE

When an HLR receives a request for authentication and ciphering data for a Mobile
Subscriber and it does not hold the requested data, the HLR requests the data from the AuC.
The protocol used to transfer the data over this interface is not standardized.

2.7.10 THE Um (MS-BTS) INTERFACE

The interface between the MS and the BSS is specified in the 04- and 05-series
of GSM Technical Specifications.