Network Switching System (NSS)

NSS Identifier IMEI International Mobile Equipment Identifier. The IMEI is an internationally-unique serial number allocated to the MS hardware at the time of manufacture. It is registered by the network operator and (optionally) stored in the AuC for validation purposes. IMEI = TAC + FAC + SNR +sp TAC = Type Approval Code by central GSM body FAC = Final Assembly Code, identifies the manufacturer SNR = Serial Number, unique six digit number sp = spare for future use

IMSI International Mobile Subscriber Identifier When a subscriber registers with a network operator, a unique subscriber IMSI identifier is issued and stored in the SIM of the MS as well as in the HLR. An MS can only function fully if it is operated with a valid SIM inserted into an MS with a valid IMEI. IMSI consist of three parts: IMSI = MCC + MNC + MSIN MCC = Mobile Country Code MNC = Mobile Network Code MSIN = Mobile Station Identification Number

TMSI Temporary Mobile Subscriber Identity A TMSI is used to protect the true identity (IMSI) of a subscriber. It is issued by and stored within a VLR (not in the HLR) when an IMSI attach takes place or a Location Area (LA) update takes place. At the MS it is stored in the MS s SIM. The issued TMSI only has validity within a specific LA. Since TMSI has local significance, the structure may be chosen by the administration. It should not be more than four octets.

MSISDN Mobile Station ISDN Number The MSISDN represents the true or dialled number associated with the subscriber. It is assigned to the subscriber by the network operator at registration and is stored in the SIM. According to the CCITT recommendations, it is composed in the following way: CC = Country Code MSISDN = CC + NDC + SN NDC = National Destination Code SN = Subscriber Number

MSRN Mobile Station Roaming Number The MSRN is a temporary, location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility. It is stored in the VLR and associated HLR but not in the MS. The MSRN is used by the VLR associated MSC for call routing within the MSC/VLR service area.

LAI Location Area Identity Each Location Area within the PLMN has an associated internationally unique identifier (LAI). The LAI is broadcast regularly by BTSs on the Broadcast Control channel (BCCH), thus uniquely identifying each cell with an associated LA. LAI = MCC + MNC + LAC MCC = Mobile Country Code, same as in IMSI MNC = Mobile Network Code, same as in IMSI LAC = Location Area Code, identifies a location area within a GSM PLMN network. Maximum length of LAC is 16 bits.

Mobile Switching Center (MSC) The Mobile services Switching Center (MSC) performs the telephony switching functions of the system. It also controls calls to and from other telephony and data systems, such as the Public Switched Telephone Network (PSTN) and Public Land Mobile Network (PLMN). Difference between a MSC and an exchange in a fixed network is - 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:

required for location registration procedures required for handover An MSC can be connected to only one VLR. Therefore, all mobile stations that move around under base stations connected to the MSC are always managed by the same VLR. An MSC would communicate typically with one EIR. While it is possible for an MSC to communicate to multiple EIRs, this is highly unlikely since the EIR provides a centralized and geographic independent function.

The MSC consults an HLR to determine how a call should be routed to a given mobile station: For incoming calls to a mobile station, the MSC would typically consult one HLR. For mobile-to-mobile calls in larger networks, a MSC could consult HLRs of other systems to help minimize the trunk paths to the other mobile station. A given MSC can be interconnected to other MSCs to support inter- MSC handovers

The following are typical MSC functions in a cellular system: Provide switched connections with PSTN Provide switched connections between mobile subscribers Provide coordination over signaling with mobiles Coordinate the location and handover process Provide custom services to mobile users Collect billing data

Protocols MSC/BSC MSC/HLR MSC/VLR MSC/EIR MSC/GMSC VLR/VLR VLR/HLR MSC/MSC OMC/MSC OMC/HLR OMC/VLR OMC/BSS MSC/Fixed Network MSC/Voice messaging BSSMAP TCAP+MAP X.225 R2, ISUP other Signaling SCCP SCCP X.224 MTP MTP X.25 MTP SS7 SS7

Switching In MSC Signaling network is separated from the speech network and consists of signaling Links (SL) signaling Point (SP) signaling Transfer Part (STP).

Telephony system contains: Group Switch to switch the calls, ST to perform signaling in accordance with SS7 Trunk interfaces for interfacing the PCM. Group switch provides a semi permanent connection between time slot (PCM) and ST.

Signaling Point (SP) SP provides the functions of signaling and transmit and receive messages to and from different nodes. Each SP in the network will have an identification code termed as signaling Point Code (SPC).

Signaling Transfer Point (STP) Signaling Transfer Part is signaling point that only transfers messages from one signaling point (SP) to another. STP SP (SPC) STP SP (SPC)

Signaling Link (SL) Signaling Link is the 64kbps link interconnecting two signaling Points and provides the functions of message error control and message sequencing. Each signaling Link has an SLC (signaling Link Code), which identifies the signaling Link with in the signaling Link Set.

Service Switching Point (SSP) The MSC contains: The Service Switching Point One or more radio control point SSP handles the usual switching function and can be connected via 2Mbps PCM link with: Other exchanges of fixed PSTN or mobile PLMN, Points on the SS7 signaling network, X.25 network Continued..

The OA&M network, The Intelligent network, PSTN via user data channels and signaling channels using ISUP and R2 protocols, Other elements of the GSM

Switching Function of SSP: Main control, Switching matrix, PCM multiplex connection, Service circuits Operation and maintenance Establishing and releasing section of the links from and to mobiles, Finding circuits to the BSS; special circuit groups are created. SSP selects an incoming and outgoing circuit.

Call Routing If a number received is a national or international number, the address information is passed to the SSP. If the number received is an HPLMN (Home PLMN), the RCP asks the HLR for a roaming number (MSRN). This MSRN is passed to the SSP for routing. If the number received is an emergency service number, the originating geographic area is attached to it and the combined information passed to the SSP. Continued..

In the SSP the number received from RCP follow the standard translation process: Preliminary analysis: Selection of a translator (national, international), Translation: Determination of a routing depend on the first digits dialled, Routing: Determination of an outing circuit group.

Circuit Groups Call routes from the MSC through circuit groups. Different circuit groups are created inside it: Group for the PSTN (according to the exchange) Group for the BSCs Group for the Supplementary services Group for the IWF

CG1 CG2 BSC1 BSC2 MSC CGn CGa BSCn PSTN1 CGx CG PSTNx Supplementary Services CG IWF

Interfaces Section 3 NSS Topology

A-Interface (MSC BSC) The interface between the MSC and its BSS is specified in the 08-series of GSM Technical Specifications. The BSS-MSC interface is used to carry information concerning: BSS management; call handling; mobility management.

B-Interface (MSC VLR) The VLR is the location and management data base for the mobile subscribers roaming in the area controlled by the associated MSC(s). Whenever the MSC needs data related to a given mobile station currently located in its area, it interrogates the VLR. When a mobile station initiates a location updating procedure with an MSC, the MSC informs its VLR which stores the relevant information. This procedure occurs whenever an MS roams to another location area. Also, when a subscriber activates a specific supplementary service or modifies some data attached to a service, the MSC informs (via the VLR) the HLR which stores these modifications and updates the VLR if required.

C-Interface (HLR - MSC) The Gateway MSC must interrogate the HLR of the required subscriber to obtain routing information for a call or a short message directed to that subscriber.

D-Interface (HLR - VLR) This interface is used to exchange the data related to the location of the mobile station and to the management of the subscriber. The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area. To support this, the location registers have to exchange data. The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station. The HLR sends to the VLR all the data needed to support the service to the mobile subscriber. The HLR then instructs the previous VLR to cancel the location registration of this subscriber. Exchanges of data may occur when the mobile subscriber requires a particular service, when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means

E-Interface (MSC - MSC) When a mobile station moves from one MSC area to another during a call, a handover procedure has to be performed in order to continue the communication. For that purpose the MSCs have to exchange data to initiate and then to realize the operation. After the handover operation has been completed, the MSCs will exchange information to transfer A-interface signaling as necessary. When a short message is to be transferred between a Mobile Station and Short Message Service Centre (SC), in either direction, this interface is used to transfer the message between the MSC serving the Mobile Station and the MSC which acts as the interface to the SC.

F-Interface (MSC - EIR) This interface is used between MSC and EIR to exchange data, in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station.

G-Interface (VLR - VLR) When a mobile subscriber moves from a VLR area to another Location Registration procedure will happen. This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR.

H-Interface (HLR - AUC) When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data, the HLR requests the data from the AuC. The protocol used to transfer the data over this interface is not standardized.

Switch Modules Switch has three major types of equipment modules: Switching module (SM) Communication module (CM) Administrative module (AM)

Switching Module (SM): All external lines, trunks, and special services circuits are terminated at the switching module. The analog and digital signals are converted to the digital format used inside the switch. The SM performs almost 95% of the call processing and maintenance functions including: Line and trunk scanning Tone generation Announcements Call progress supervision Routine maintenance and self-maintenance.