2.6 The Network Switching Subsystem (NSS)

2.6.1 NSS OVERVIEW

Network Switching System (NSS)

• Key elements of the NSS:
• Mobile Switching Centre (MSC) with:
• Visitor Location Register (VLR)
• Home Location Register (HLR) with:
• Authentication Centre (AuC)
• Equipment Identity Register (EIR)
• Gateway MSC (GMSC)
• These elements are interconnected by means of an SS7 network
EIR
PSTN/ISDN
SS7
Network
MSC
VLR
HLR
AuC
GMSC

2.6 The Network Switching Subsystem (NSS)

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The NSS combines the call routing switches (MSCs and GMSC) with database registers
required to keep track of subscribers’ movements and use of the system. Call routing between
MSCs is taken via existing PSTN or ISDN networks. Signalling between the registers uses
Signalling System No. 7 protocol.

2.6.2 THE MOBILE SWITCHING CENTRE (MSC)

The Mobile-services Switching Centre is an exchange which performs all the switching and
signalling functions for mobile stations located in a geographical area designated as the MSC
area.
The main difference between a MSC and an exchange in a fixed network is that the MSC has
to take into account the impact of the allocation of radio resources and the mobile nature of
the subscribers and has to perform in addition, at least the following procedures:
· procedures required for location registration (details in GSM 03.12);
· procedures required for handover (details in GSM 03.09).
Mobile Switching Centre (MSC)
Functions of the MSC:
• Switching calls, controlling calls and logging calls
• Interface with PSTN, ISDN, PSPDN
• Mobility management over the radio network and
other networks
• Radio Resource management - handovers between
BSCs
• Billing Information
MSC
VLR

2.6.3 THE VISITOR LOCATION REGISTER (VLR)

A Visitor Location Register is a database serving temporary subscribers within an MSC area.
Each MSC in the network has an associated VLR but a VLR may serve many MSCs. A mobile
station roaming in an MSC area is controlled by the VLR associated with that MSC.
When a Mobile Station (MS) enters a new location area it starts a registration procedure. The
MSC in charge of that area notices this registration and transfers the identity of the location
area where the MS is situated to the VLR. If this MS is no yet registered, the VLR and the
HLR exchange information to allow the proper handling of calls involving the MS.
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Visitor Location Register ((VLR)

• Each MSC has a VLR
• VLR stores data temporarily for mobiles served by the MSC
• Information stored includes:
• IMSI
• MSISDN
• MSRN
• TMSI
• LAI
• Supplementary service parameters
MSC
VLR
The VLR contains also the information needed to handle the calls set-up or received by the
MSs registered in its data base. The following elements are included:
· the International Mobile Subscriber Identity (IMSI);
· the Mobile Station International ISDN number (MSISDN);
· the Mobile Station Roaming Number (MSRN)
· the Temporary Mobile Station Identity (TMSI), if applicable;
· the Local Mobile Station Identity (LMSI), if used;
· the location area where the mobile station has been registered. This data item will be
used to call the station.

2.6.4 THE HOME LOCATION REGISTER (HLR)

The HLR is a database in charge of the management of mobile subscribers. A PLMN may
contain one or several physical HLRs depending on the number of mobile subscribers, the
capacity of the equipment and the organization of the network. However, even if the HLR
comprises geographically separated hardware, it logically forms a single virtual database.
Two kinds of information are stored there:
· the subscription information;
· location information enabling the charging and routing of calls towards the MSC
where the MS is located (e.g. the MS Roaming Number, the VLR address, the MSC
address, the Local MS Identity).
Two types of number are attached to each mobile subscription and are stored in the HLR:
· the International Mobile Station Identity (IMSI);
· one or more Mobile Station International ISDN number(s) (MSISDN).
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Home Location Registerr (HLR)

• Stores details of all subscribers in the network , such as:
• Subscription information
• Location information: mobile station roaming number, VLR, MSC
• International Mobile Subscriber Identity (IMSI)
• MS ISDN number
• Tele-service and bearer service subscription information
• Service restrictions
• Supplementary services
• Together with the AuC, the HLR checks the validity and
service profile of subscribers
HLR
AuC
The IMSI or the MSISDN may be used as a key to access the information in the database for a
mobile subscription.

HLR Implementation

• One HLR in a network
• May be split regionally
• Stores details of several thousand subscribers
• Stand alone computer - no switching capabilities
• May be located anywhere on the SS7 network
• Combined with AuC
HLR
AuC
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The data base can also contain other information such as:
· teleservices and bearer services subscription information;
· service restrictions (e.g. roaming limitation);
· supplementary services; the HLR contains the parameters attached to these services.
Supplementary services parameters need not all be stored in the HLR. However, it is
considered safer to store all subscription parameters in the HLR even when some are stored in
a subscriber card.
The organization of the subscriber data is outlined in the ETSI GSM 03.08 Recommendation.
Notice that the VLR stores the current Location Area of the subscriber, while the HLR stores
the MSC/VLR they are currently under. This information is used to page the subscriber
when they have an incoming call.

2.6.5 THE AUTHENTICATION CENTRE (AuC)

The Authentication Centre (AuC) is associated with an HLR, and stores an identity key for
each mobile subscriber registered with the associated HLR.
This key is used to generate:
· data which are used to authenticate the IMSI;
· a key used to cipher communication over the radio path between the mobile station
and the network.
The procedures used for authentication and ciphering are described more fully in the security
section of this course and in the ETSI GSM 03.20 recommendation.

2.6.6 THE GATEWAY MOBILE SWITCHING CENTRE (GMSC)

Gateway Mobile Switching Centre (GMSC)

• A Gateway Mobile Switching Centre (GMSC) is a device
which routes traffic entering a mobile network to the correct
destination
• The GMSC accesses the network’s HLR to find the location
of the required mobile subscriber
• A particular MSC can be assigned to act as a GMSC
• The operator may decide to assign more than one GMSC
GMSC
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The GMSC routes calls out of the network and is the point of access for calls entering the
network from outside.
If a network, delivering a call to the PLMN cannot interrogate the HLR directly, the call is
routed to an MSC. This MSC will interrogate the appropriate HLR and then route the call to
the MSC to which the mobile station is affiliated. The MSC which performs the routing
function to the actual location of the MS is called the Gateway MSC (GMSC).
The choice of which MSCs can act as Gateway MSCs is for the operator to decide (i.e. all
MSCs or some designated MSCs).

2.6.7 THE EQUIPMENT IDENTITY REGISTER

Equipment Identity Register (EIR)

• EIR is a database that stores a unique International
Mobile Equipment Identity (IMEI) number for each
item of mobile equipment
• The EIR controls access to the network by returning the status of a
mobile in response to an IMEI query
• Possible status levels are:
• White-listed The terminal is allowed to connect to the network.
• Grey-listed The terminal is under observation by the network
for possible problems.
• Black-listed The terminal has either been reported stolen, or is not a
type approved for a GSM network.
The terminal is not allowed to connect to the network.
EIR
The EIR contains one or several databases which store(s) the IMEIs used in the GSM system.
The mobile equipment may be classified as "white listed", "grey listed" and "black listed" and
therefore may be stored in three separate lists. An IMEI may also be unknown to the EIR.
The EIR contains, as a minimum, a "white list" (Equipment classified as "white listed").
There is an optional implementation that may be used by the operator to control access to the
network by certain types of equipment or to monitor lost or stolen handsets.