Global System for Mobile communications (GSM) is a European standar- disation incentive with digital voice encoding on the air interface, devel- oped as part of a standards effort to ensure compatibility for roaming subscribers. In the UK, Vodafone and BT Cellnet (now branded mmO2)
offer GSM services. As do One2One/T-Mobile and Orange, except strictly speaking they offer digital cellular system 1800 (DCS-1800). This is essen- tially GSM, but with an air interface operating at 1800 MHz. With a large number of roaming agreements with other network operators throughout the world GSM is arguablytheglobal standard. It is this compatibility and roaming potential that makes GSM far superior to the first-generation analogue systems such as TACS.
GSM, IS-54 (D-AMPS), IS-95, IS-136 (IS stands for interim standard and are the standards applicable to US networks) and DCS-1800 networks all share a similar approach and topology to solve the issue of terminal mobility, and rely on the network architectures developed for analogue systems.
All mobile networks, like their fixed networked cousins, rely on the services of signalling system number 7 (SS#7). In the case of GSM, the addition of the Mobile Application Part (MAP) to the top layer of the SS#7 signalling stack enables the communication and control of mobility. In North America (and essentially all other countries that don’t use GSM), Telecoms Industry Association (TIA) interim standard IS-41 finalised as the (American National Standards Institute) ANSI-41 standards for inter- system messaging is the equivalent in function to MAP. MAP and ANSI- 41 sit at the same level in the protocol stack as the Intelligent Network Application Part (INAP) protocol (see Chapter 3).
The digital mobile network standards mainly differ around the speci- fication of the radio interface; the US standards listed are in fact mostly specifications of the air interface. GSM has both digital voice (with time division multiple access) and digital (packetised) signalling. D-AMPS has digital voice encoding, with analogue signalling (AMPS signalling in fact). IS-95 uses Code Division Multiple Access (CDMA) as a mechanism for sharing the bandwidth to the base station. CDMA allows all the cellu- lar phones to transmit at the same time, voice channels from different customers are separated in the base station by the use of a shared code value that allows the original voice to be reconstructed. It’s like lots of people of different nationalities all speaking simultaneously to a partner in their national language.1
MOBILE NETWORKS 46
1
Lots of battles have since raged over the use of CDMA for the future 3G networks, which were all financially driven around patents owned by Qualcom for the CDMA technology. These battles where eventually sorted out to everyone’s satisfaction and hopefully for the better good of 3G networks.
The key components of a cellular network are:
† Mobile stations (handset plus smart card subscriber ID module).
† Base stations, the actual cellular masts (Base Station Transceivers (BTS)) and Base Station Controllers (BSCs).
† Mobile Switching Centres (MSCs), essentially the equivalent of a Public Switched Telephone Network (PSTN) service switching point (SSP). There is a variant of the MSC called a Gateway MSC (GMSC) which deals with the interconnect between the PSTN and the Public Land Mobile Network (PLMN).
† Mobility and management databases, Home Location Register (HLR) and Visitor Location Register (VLR). The HLR can also incorporate an Authentication Centre (AUC) or this is sometimes a separate data- base, to validate a handset on the network. An Equipment Identity Register (EIR) is also present in GSM. For the storage of the mobile station equipment identity.
† Where additional intelligent network services are used, an IN Service Control Point (SCP) is also present.
They are the main elements, Figure 4.3 shows how they fit together. In order for calls to reach the mobile terminals, the network must know of their existence. When a mobile handset is turned on it initiates a registration and location update process. This process not only informs the network of the device’s presence, but also uses the information in the EIR and AUC to ensure the device and user are valid, this process involves the VLR, HLR EIR and AUC. Once the device and subscriber details have been validated, the device is given a temporary roaming address (MSRN – Mobile Station Roaming Number). The reason for this temporary address is so that the network can constantly update the location of the device, whilst maintaining a fixed address to reach it, the fixed address being the mobile number called the MSISDN (Mobile Station ISDN Number).
Registration only takes place once, when the device is powered up, however, as the device moves around its home network it must constantly change its location information. All this information exchange is kept secret by the use of encryption performed both in the network and in the smart card Subscriber Identification Module (SIM).
Now if someone wishes to reach a mobile device, the address they use is the MSISDN. The MSISDN has no location-specific context; the only thing known is that a particular number range is allocated to a particular PLMN. If the call is originating from the PSTN the call will ingress through a GMSC. The GMSC will have to interrogate the HLR to discover the location of the handset which will invariably be roaming somewhere on the network. The HLR returns the MSRN, this number points the call to the MSC that is handling the group of BSCs that the mobile handset is on. The ‘local’ MSC then determines which BSC the handset is currently
located with. The device is then paged by transmitting the paging message from all the MTSs that are in the mobile handset’s location.
Clearly as a mobile device continues to move around a PLMN, proce- dures must be in place to handle the movement of the device from area to area and across different BSCs and even different parent MSCs.
This is a very simple description of the procedures necessary to call a mobile device in a GSM network. Other networks operate in a similar way. The main points to note here are that the PSTN only knows that a particular MSISDN number belongs to a specific PLMN if the owner of that number has ported their number to another PLMN, then the original PLMN must forward the call on to the new network. The alternative is to have a national database of all ported numbers that the PSTNs and PLMNs can access before routing calls. The point worthy of note is that a similar situation exists as the one previously described for customers that are roaming on a partner network (generally internationally). In order for the caller to be reached, the call is routed to their home PLMN. If you would like a more complete description of GSM, then I suggest you look up [EBERS]. All the transactions discussed above rely on the services of the Mobile Application Part (MAP) protocol (either GSM MAP of ANSI-41).
4.3
BEYOND GSM, THE PATH TO UMTS
GSM has undergone enhancements to its specifications with the aim to move it closer to UMTS. This work is being done by the 3GPP. This work is part of ETSI GSM and UMTS strategy and takes GSM networks through release 2 and 2.5. Release 2.5 introduces Customised Applications for Mobile Networks Enhanced Logic (CAMEL), General Packet Radio Service (GPRS) data services. Enhanced Data Service for GSM Evolution (EDGE) picks up where GPRS stops and forms the longer term data service for UMTS networks.