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2. NETWORK SUBSYSTEM
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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
; Charging
; Collection of Statistical Raw Material
; Mobility Management
; Security Management
; Interface Signalling A & PSTN
; BSS Control
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2.1 NSS ELEMENTS
The NSS task list requires some specialised equipment. The following picture illustrates the contents of the Network Subsystem NSS.
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EIRSecurity Management
ACSecurity Management
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.
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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
destination?
; What kind of call is it: a normal call, data call or emergency
call?
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.
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EC
ECHOEC = Echo Cancelling
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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'.
2.2.4 Charging
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.
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Charging Information
Transfer
BC
BC = Billing Center
The MSC and the HLR collect these toll tickets together and send them to the Billing
Centre (BC). The Billing Centre is a separate charging post processing platform which, in practice, produces a bill for the mobile subscriber.
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2.2.5 Collection of Statistical Raw Material
Both the MSC and all the registers VLR, HLR, AC and EIR are collecting statistical raw material for network control purposes. Naturally, different kinds of statistics are collected in different places. The nature of all statistical information collected in these network elements is short-term, i.e. a certain measurement covers only a reasonably short time period.
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EIR
AC
HLR
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Statistical Raw
Material fromAlfaskop m346MSC, VLR, HLR,
AC and EIRAlfaskop m346
Alfaskop m346
When the operator wants overall control of the statistics, this raw material is transferred to the NMS and it is post processed by using the processing tools available in the NMS. Due to the interface structure coming from the GSM Technical Specifications (see introduction part) it is essential to collect statistical information from both sides of a defined interface, for instance, A interface. By doing this and combining the results, the operator may have reliable measures for, for instance, network quality and usability.
2.2.6 Mobility Management (MM)
When a mobile subscriber moves in the network she often causes situations difficult to control. For instance, a network wants to establish a mobile terminated call. Now the question is: how to find out where the subscriber is? In this context, Mobility Management is an entity including all the actions and functions needed to control the situation where a mobile subscriber moves in her own network (HPLMN = Home
Public Land Mobile Network) or even in another network (VPLMN = Visited
Public Land Mobile Network). The following terms and procedures are regarded as part of Mobility Management:
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; Location Registration. This term means all the functions the
network needs to maintain a subscriber's location data.
; Location Update (Procedures). Location Update is a
procedure the MS uses to inform the network about its
location. Location Update may be either normal or periodical.
; Handover (Procedures). Handover is a function where the
radio channel used in Air interface is changed to another
without cutting the call.
; Paging. Paging is a procedure the network uses to find out a
subscriber's location before actual call establishment.
Because this is quite a large topic as such, it is handled separately in the chapter
'Mobility Management'.
2.2.7 Security Management
The main topics under this entity are:
; Mobile Subscriber Identification (Authentication Procedure)
; Mobile Equipment Identification (IMEI Checking Procedure)
; Air Interface Ciphering (Ciphering Procedure)
; Secure Subscriber Identity Handling (Temporary Identity
Allocation Procedure)
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AC
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The authentication procedure is handled by the Mobile Station, the AC and the VLR. The IMEI checking procedure is taken care by the Mobile Station and the VLR. The ciphering procedure is initiated by the VLR but actually implemented in the Mobile Station and the BSS. Temporary identity allocation procedures are handled by the VLR. More detailed description of these procedures is included in the chapter 'Subscriber Services'.
2.2.8 Interface Signalling A & PSTN
For network traffic the NSS must maintain two interfaces: A and PSTN. All procedures to be used over the A interface are strictly defined in GSM Technical Specifications. PSTN interface varies from country to country as far as signalling details are concerned.
The MSC is the network element maintaining the above-mentioned interfaces. The basic signalling used through these interfaces is CCS7 (Common Channel Signalling
#7). CCS7 is a message based signalling scheme designed according to the OSI
(Open System Interconnection) model. This model defines several layers forming a stack. In this layered stack, the upper layer in principle controls at least the next layer below. The lowest layer in this model is the physical signalling link. In practice, this
is the physical connection through the interface concerned. All signalling link and signalling network related controls and functions are handled with the levels being above the signalling link level. Specific signalling applications are then on top of this
layered model. A signalling application is a user of this model, and this kind of application always has some specific signalling functions to be performed.
APSTN
MAP
TCAPBSSAPBSSAPTUPISUPNUPTUPISUPNUP
SCCPSCCPSCCP
MTPMTPMTP
L 1L 1L 1L 1L 1
The picture above illustrates the protocol stacks needed in the MSC in order to handle A interface and the interface towards PSTN. The protocol stack of the MSC is located in the middle of the picture.
L 1 (Layer 1) is the physical link through the interface. This can be for instance fibre optics cable, radio link, etc.
The MTP (Message Transfer Part) is an entity defined by the CCS7 signalling
specification. The MTP is responsible for signalling link and signalling network
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control. In practice, the MTP as such can handle signalling networks which are located in one country.
The SCCP (Signalling Connection Control Part) is an additional and optional part of
the CCS7 scenario. In the GSM system the SCCP is obligatory due to some signalling requirements the MTP cannot offer. For example, international roaming
needs telephone number type of addresses for signalling routing; the SCCP is capable of this kind of signalling routing.
The BSSAP (Base Station Subsystem Application Part) is a signalling application
maintaining the A interface. The BSSAP can relay signalling information between an MS and the NSS. In addition, the BSSAP controls speech channels going through the A interface.
The MAP (Mobile Application Part) and the TCAP (Transaction Capabilities
Application Part) handle mobility management and call establishment related signalling through the A interface.
The TUP (Telephone User Part), the ISUP (ISDN User Part) and the NUP (National
User Part) are three examples of how to handle a country-specific PSTN interface.
2.2.9 BSS Control
The interface between the BSS and the NSS is the A interface. Through this interface, the NSS controls the Base Station Subsystem at a general level. This control is carried out by the MSC. In order to complete the mobile terminated call, the MSC must know the structure of the radio network located below itself. In addition, every radio network part visible in the MSC has an administrative state, meaning the MSC
is capable of controlling the condition of the radio network.
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