2. NETWORK SUBSYSTEM
O & M
The Network Subsystem (NSS) is a part of the GSM network taking care of the
following major functions:
; Call Control
; Network Inter Working
; Subscriber Data and Service Handling
; Collection of Statistical Raw Material
; Mobility Management
; Security Management
; Interface Signalling A & PSTN
; BSS Control
2.1 NSS ELEMENTS
The NSS task list requires some specialised equipment. The following picture illustrates the contents of the Network Subsystem NSS.
Subscriber Data and Service HandlingHLRStatisticsMobility Management
MSCVLRMSC: Call ControlMSC: BSS (Radio Network) ControlMSC: Network Inter WorkingMSC: ChargingMSC: StatisticsO & MMSC: Interface Signalling A & PSTNVLR: Subscriber Data and Service HandlingVLR: Mobility ManagementVLR: Security Management
MSC (Mobile Services switching Centre) is the main element of the NSS from the
call control point of view. MSC is responsible for call control, BSS control functions, inter working functions, charging, statistics and interface signalling towards A and PSTN interfaces.
HLR (Home Location Register) is the place where all the subscriber information is stored permanently. The HLR also provides a known, fixed location for the variable routing information. The main functions of the HLR are subscriber data and service handling, statistics and mobility management.
VLR (Visitor Location Register) provides a local store for all the variables and
functions needed to handle calls to and from mobile subscribers in the area related to the VLR. Subscriber related information remains in the VLR as long as the mobile subscriber visits the area. The main functions of the VLR are subscriber data and service handling, mobility management and security management. A typical solution is that the VLR is integrated together with the MSC. In this case, the MSC area is the same as the VLR area and this makes mobile subscriber addressing easier.
AC (Authentication Centre) and EIR (Equipment Identity Register) are NSS
network elements taking care of security-related issues. The AC maintains subscriber identity-related security information together with the VLR. The EIR maintains mobile equipment identity (hardware) related security information together with the VLR.
2.2 NSS FUNCTIONS
2.2.1 Call Control
The MSC must be capable of making certain decisions concerning the call. Roughly,
these decisions are:
; Where does this call come from? Is it coming from a Mobile
Station (Mobile Originated Call, MOC)? Is it coming from
the PSTN (Mobile Terminated Call, MTC)?
; Where is this call to be connected? To a Mobile Station or to
a PSTN? Or should it be connected to another kind of
; What kind of call is it: a normal call, data call or emergency
In addition, the MSC must perform the connections needed. In principle, the MSC
has four possibilities to handle a call. These are: normal speech path through
connection, connection of tones, announcement connection and call rejection.
2.2.2 Network Inter Working
Network Inter Working Functions (IWF) is a functional entity associated with the
MSC. The IWF is used whenever there is a need to adapt a GSM network to another,
external network like PSTN, PSPDN (Public Switched Packet Data Network) or
ISDN (Integrated Services Digital Network). The IWF entity consists of echo
cancelling equipment, modems and rate adapters.
In practice, the IWF is needed when, for instance, a mobile subscriber calls to a
PSTN. The GSM network always generates a certain speech processing delay which
can be realised as an echo. In order to prevent this echo, echo cancellers are used.
ECHOEC = Echo Cancelling
2.2.3 Subscriber Data and Service Handling
Subscriber data is permanently stored in the HLR. When a mobile subscriber moves in the network, the relevant part of her information is also additionally stored in the VLR concerned.
In GSM Technical Specifications, all the services to be offered to a mobile subscriber are defined. The most common service is called Teleservice T11 meaning normal speech. In addition, there are several dozen services which may be used if needed and if the operator wishes them to be implemented in the network. The most common services are introduced in the chapter 'Subscriber Services'.
Normally, when a subscriber uses a GSM network, this use is not free. The network operator collects data about the calls made and, based on this collected data, the subscriber is then billed. Every call or other transaction generates typically several charging records in the NSS. An average size of a charging record is about 200 bytes of information. The charging record is also called a Toll Ticket. One toll ticket
contains for instance the following call-related items: Calling and Called subscriber numbers, Start and stop time of the transaction, Transaction type (see for instance call type above), Charging pulses (if used), Circuits allocated for the transaction, etc.