System Architecture (Global System for Mobile Communication (GSM)) Part 2

Network switching system

The role of the network switching system (NSS) is to set up call connections in a mobile environment. The NSS achieves this by using the following switching and database nodes:

■ Mobile switching center (MSC and gateway MSC)

■ Home location register (HLR)

■ Visitor location register (VLR)

■ Equipment identity register (EIR)

■ Authentication center (AuC)

In addition, short message service center (SMSC) is required to support short message service.

In almost all the implementations, the HLR and the AuC functionality is implemented in one physical node usually referred to as the HLR/AuC. In the following sections, the HLR functionality as described includes AuC features.

Gateway mobile switching center. The gateway mobile switching system (MSC) is the central component of an NSS. It is like a switching node/ central office of the PSTN with additional capabilities to support mobility.

Figure 3-9 Cell global identity.

The GMSC supports all the GSM-specific interfaces defined in the next section. In addition, it also provides an interface to external networks, i.e., PSTN, ISDN, and other PLMNs. The services provided by GMSC include call setup, call routing, registration, authentication, location updating, handovers, and billing. The GMSC offers these services in conjunction with other NSS entities such as HLR, VLR, MSC, AuC, and EIR. The main functions of a GMSC are:

■ Registration, location updating

■ Authentication and other security functions

■ Paging

■ Handover management

■ Switching and signaling

■ Billing

■ BSS management

The number of GMSCs in a GSM network varies, depending upon the size of the network and its interfacing requirements with PSTN. Usually, one GMSC suffices. Each GMSC is associated with one or more HLR.

Mobile switching center. The MSC has the same functionalities as GMSC, with a few exceptions. The MSC has no direct interface with PSTN/other PLMNs. Also, it has no associated HLR. The number of MSCs in a network depends on the size of the network. The MSC/GMSC and the BSSs are connected via a standard and well-defined interface. Therefore, a BSS and an NSS from two different vendors can coexist.

Home location register and authentication center. The home location register (HLR) stores the identity and subscriber data of all the users registered in a GSM network. The information stored in the HLR includes permanent data such as the IMSI, MSISDN, authentication keys, permitted supplementary services, and some temporary data. Examples of temporary data stored in the HLR are the current address of the serving MSC/VLR and the roaming number to which the calls must be forwarded. The temporary data is required to support mobility. Table 3-1 lists some of the important data stored in the HLR/AuC.

Most of the vendors have implemented the authentication center (AuC) in the same node as the HLR. The AuC calculates and provides the authentication triplets, i.e., Kc, RAND, and signed response.

TABLE 3-1 Important Data Stored in the HLR/AuC

IMSI	International mobile subscriber identity (IMSI), also stored in the SIM
Ki	Authentication key, also stored in the SIM
Service restrictions	Operator-determined barring (ODB)
SS	List of permitted supplementary services
MSISDN	Mobile subscriber ISDN number; one subscription may be associated with multiple MSISDNs
Security data
A3/A8	Authentication algorithm, also stored in the SIM
RAND	Randomly generated number; it is used by the SIM as an input to calculate SRES
SRES	Signed RESponse
Kc	Ciphering key
Subscriber location
VLR address	The address of the VLR in which the mobile is currently located
MSC address	The address of a serving MSC
LMSI	Local mobile subscriber identity

These are used for authentication and encryption over the radio channel. The HLR connects to an AuC, in cases where it is implemented as a separate node.

It is desirable to keep access time to retrieve the data from the HLR to a minimum. The access time directly impacts the call completion time. Hence, the number of HLRs in a GSM network is determined by several factors such as the access time and the amount of data stored for the number of subscribers. The HLR can be implemented as a distributed database for security, reliability, and performance reasons but logically one HLR exists per PLMN.

Visitor location register (VLR). The visitor location register (VLR), like an HLR, is a database. It contains selected administrative data for all the mobiles currently located in a serving MSC associated with the VLR. As can be seen in Table 3-2, the permanent data stored in the VLR is the same as the data stored in the HLR. However, there are a few additional parameters mainly of temporary data type—for example, TMSI and MSRN.

The main function of a VLR is to support GMSC/MSC during authentication and call establishment. To enable this functionality, the HLR updates the VLR with the relevant subscriber information on a need basis. A MSC is always associated with only one VLR. However, a VLR can serve several MSCs.

TABLE 3-2 Important Data Stored in the VLR

Subscription data
IMSI	International mobile subscriber identity (IMSI), also stored in SIM
TMSI	Temporary mobile subscriber identity (TMSI)
SS	List of permitted supplementary services
MSISDN	Mobile subscriber ISDN number
Security data
RAND	Randomly generated number; it is used by SIM as an input to
	calculate SRES
SRES	Signed RESponse
CKSN	Ciphering key sequence number
Kc	Ciphering key
Subscriber location
HLR address	The address of the VLR in which the mobile is currently located
MSC address	The address of serving MSC
LAI	Location area identity
MSRN	Mobile subscriber roaming number
LMSI	Local mobile subscriber identity

Equipment identity register (EIR). The equipment identity register is a database that contains the list of IMEIs for all mobile equipment. As shown in Figure 3-10, it maintains three lists:

White list. Contains IMEIs of all valid mobile equipment.

Black list. Contains IMEIs for stolen mobile equipment. It also contains the list of IMEIs for mobile equipment that need to be barred because of technical reasons.

White list. Contains the list of all IMEIs that need to be traced.

Figure 3-10 Equipment identity register.

The implementation of an EIR is not critical from the services point of view. It is an option and left to the network operators. From a technical point of view, the EIR is interrogated at the time of location update or any time during call setup.