29-11-2012, 04:22 PM
Introduction to GSM Networks
1Introduction to GSM.pdf (Size: 131.1 KB / Downloads: 79)
INTRODUCTION
Figure 1.1 is a schematic overview of the main components in a GSM network. The various
interface labels are the formal names given to these interfaces. More details about these interfaces
are found in GSM TS 03.02 [26].
The GSM network consists mainly of the following functional parts:
• MSC – the mobile service switching centre (MSC) is the core switching entity in the network.
The MSC is connected to the radio access network (RAN); the RAN is formed by the BSCs and
BTSs within the Public Land Mobile Network (PLMN). Users of the GSM network are registered
with an MSC; all calls to and from the user are controlled by the MSC. A GSM network has
one or more MSCs, geographically distributed.
• VLR – the visitor location register (VLR) contains subscriber data for subscribers registered in
an MSC. Every MSC contains a VLR. Although MSC and VLR are individually addressable,
they are always contained in one integrated node.
• GMSC – the gateway MSC (GMSC) is the switching entity that controls mobile terminating
calls. When a call is established towards a GSM subscriber, a GMSC contacts the HLR of that
subscriber, to obtain the address of the MSC where that subscriber is currently registered. That
MSC address is used to route the call to that subscriber.
• HLR – the home location register (HLR) is the database that contains a subscription record for
each subscriber of the network. A GSM subscriber is normally associated with one particular
HLR. The HLR is responsible for the sending of subscription data to the VLR (during registration)
or GMSC (during mobile terminating call handling).
• CN – the core network (CN) consists of, amongst other things, MSC(s), GMSC(s) and HLR(s).
These entities are the main components for call handling and subscriber management. Other
main entities in the CN are the equipment identification register (EIR) and authentication centre
(AUC). CAMEL has no interaction with the EIR and AUC; hence EIR and AUC are not further
discussed.
• BSS – the base station system (BSS) is composed of one or more base station controllers (BSC)
and one or more base transceiver stations (BTS). The BTS contains one or more transceivers
(TRX). The TRX is responsible for radio signal transmission and reception. BTS and BSC are
connected through the Abis interface. The BSS is connected to the MSC through the A interface.
• MS – the mobile station (MS) is the GSM handset. The structure of the MS will be described in
more detail in a next section.
Signalling in GSM
The various entities in the GSM network are connected to one another through signalling networks.
Signalling is used for example, for subscriber mobility, subscriber registration, call establishment,
etc. The connections to the various entities are known as ‘reference points’. Examples include:
• A interface – the connection between MSC and BSC;
• Abis interface – the connection between BSC and BTS;
• D interface – the connection between MSC and HLR;
• Um interface – the radio connection between MS and BTS.
Various signalling protocols are used over the reference points. Some of these protocols for GSM
are the following:
• mobile application part (MAP) – MAP is used for call control, subscriber registration, short
message service, etc.; MAP is used over many of the GSM network interfaces;
• base station system application part (BSSAP) – BSSAP is used over the A interface;
• direct transfer application part (DTAP) – DTAP is used between MS and MSC; DTAP is carried
over the Abis and the A interface. DTAP is specified in GSM TS 04.08 [49];
• ISDN user part (ISUP) – ISUP is the protocol for establishing and releasing circuit switched
calls. ISUP is also used in landline Integrated Services Digital Network (ISDN). A circuit is the
data channel that is established between two users in the network. Within ISDN, the data channel
is generally a 64 kbit/s channel. The circuit is used for the transfer of the encoded speech or
other data. ISUP is specified in ITU-T Q.763 [137].
GSM Mobility
Roaming with GSM is made possible through the separation of switching capability and subscription
data. A GSM subscriber has her subscription data, including CAMEL data, permanently registered
in the HLR in her HPLMN. The GSM operator is responsible for provisioning this data in the HLR.
The MSC and GMSC in a PLMN, on the other hand, are not specific for one subscriber group.
The switching capability of the MSC in a PLMN may be used by that PLMN’s own subscribers,
but also by inbound roaming subscribers; see Figure 1.2.
In Figure 1.2, the GSM user who is a subscriber of PLMN-A roams to PLMN-B. The HLR in
PLMN-A transfers the user’s subscription data to the MSC in PLMN-B. The subscriber’s subscription
data remains in the MSC/VLR as long as she is served by a BSS that is connected to that
MSC. Even when the user switches her MS off and then on again, the subscription data remains
in the MSC. After an extended period of the MS being switched off, the subscription data will
be purged from the MSC. When the subscriber switches her MS on again, the subscriber has to
re-register with the MSC, which entails the MSC asking the HLR in the HPLMN to re-send the
subscription data for that subscriber.