GSM uses 25MHz frequency bands, that is, 890MHz to 915MHz band is used for mobile subscriber unit to base station transmissions reverse-link transmissions, and the 935MHz to 960MHz frequency band is used for base station to mobile subscriber unit transmission (forward-link transmissions). GSM uses frequency-division duplexing (FDD) and a combination of TDMA and FDMA technique to provide simultaneous access to multiple mobile subscriber units.
The GSM network architecture consists of three major subsystems:
Mobile station (MS)
Base station subsystem (BSS)
Network and switching subsystem (NSS)
The wireless link interface is between the MS and the base transceiver station (BTS), which is a part of BSS. Many BTSs are controlled by base station controller (BSC). BSC is connected to the mobile switching center (MSC), which is a part of NSS.
Mobile station (MS)
A mobile station communicates across the air interface with a base station transceiver in the same cell in which the mobile subscriber unit is located. The MS communicates the information with the user and modifies it to transmission protocol of the air-interface to communicate with the BSS. The users voice information interfaced with the MS through microphone and speaker for the speech, keypad and display for short messaging, and the cable connection for other data terminals. The MS has two elements. The mobile equipment (ME) refers to the physical device, which comprises of the transceiver, digital signal processors, and the antenna.
The second element of the MS in the GSM is the subscriber identity module (SIM) that is as smart card issued at the subscription time identifying the specification of a user such as unique number and the type of service. The SIM card is unique to the GSM system. It is about postage-stamp size with 32 k bytes of memory that can be plugged into any GSM mobile phone. From the users point of view, one of the most remarkable feature of GSM is the SIM card, which portable device in the form of smart card or plug-in module memory device that store information such as subscribers identification number, privacy keys, the cellular network and region where the subscribers is authorized to service and other user specific information.
The GSM subscriber units are totally generic until a SIM is inserted. Therefore, a subscriber needs only to carry a SIM card to use a wide variety of mobile equipment simply by inserting SIM in the device to be used. In fact, except for certain emergency communication, the subscriber units will not work without a SIM inserted. Thus, the mobile equipment does not roam, it is the SIM which roam. The calls in the GSM are directed to the SIM inserted in any mobile phone. Short messages are also stored in the SIM card.
The SIM card allows a mobile subscriber to use any GSM mobile phone anywhere in the world where GSM services available. Alternatively, people visiting different GSM-enabled countries those are not keen on making calls at their home number can always carry their own mobile phone and purchase a SIM card in any other country. This way they avoid roaming charges and the expense of having different contact number. Several users can also share mobile phone with different SIM card.
Because SIM card carry the private information for a user's, a security mechanism is implemented in the GSM that asks for four digit PIN number to make the information on the SIM card available to the user. The SIM also offers some protection against fraudulent use. A GSM mobile phone is useless without a SIM.
For example, if the mobile subscriber removes the SIM card when leaving mobile phone in a vehicle, the mobile cannot be use unless another person has a valid SIM. Unfortunately, the SIM cards can be stolen too. The SIM can be set up to require the user to enter a Personal Identification Number (PIN) whenever the mobile phone is switched on to provide some security in case the card is lost or stolen.
Once a mobile phone user has a valid SIM, buying a new GSM mobile phone is easy. No setup or programming is required. Similarly, a user can have a permanently vehicle-installed mobile phone and a handled mobile phone with same number, provided that only one is used at a time. The phone number of mobile subscriber is usually of 10-15 digits. The first three digits are country code; the next two are the digits for the specific MSC, and rest are the telephone number. The IMSI of the same user is totally different from the ISDN number. The first three digits of the IMSI identify the country, and next two digits, the service provider.
Besides the SIM card, the next most remarkable feature of GSM is the on-the-air privacy which is provided by the system. Unlike analog FM cellular phone systems which can be readily monitored, it is virtually impossible to eavesdrop on a GSM radio transmission. The privacy is made possible by encrypting the digital bit stream sent by a GSM transmitter, according to a specific secret cryptographic key which changes with time for each user that is known only to the service provider.
Base station Subsystem (BSS)
A base station subsystem consists of a base station controller and one or more base transceiver stations. Each base transceiver station (BTS) defines a single cell. A cell can have a radius of between 100m and 35km, depending on the environment. A Base Station Controller (BSC) may be collocated with a BTS. It may control multiple BTS units and hence multiple cells. The BSC reserves radio frequencies, manages the hand-off of a mobile unit from one cell to another within the BSS, and controls paging. The BSS manages radio interface between the mobile stations and all other subsystems of GSM such as MSC.
The BSS translates between the wireless-interface and fixed wired infrastructure protocols. The need for the wireless and wired media are different because the wireless medium is unreliable, bandwidth limited, and needs to support mobility. As a result, protocols used in the protocols used in the wireless medium and wired medium are different. The BSS provides for the translation among these protocols.
There are two main architectural elements in the BSS- the transceiver subsystem (BTS), and the base station controller (BSC). The BSS consists of many BSCs which connect to single MSC, and each BSC typically controls up to several hundred BTSs. Some of the BTSs may be co-located at the BSC, and others may be remotely distributed and physically connected to the BSC by microwave links or dedicated leased lines. The BTSs is the counterpart of the MS for physical communication over the air-interface. The BTS components include a transmitter, a receiver, and a signaling equipment to operate over the air-interface, and it is physically located at the center of the cells where the BSS antenna is installed. One BSS may have from one up to several hundred BTSs under its control. The hand-offs of calls between two BTSs under the control of the same BSC are handled by the BSC, and not the MSC. This greatly reduces the switching burden of the MSC.
The interface that connects BTS to a BSC is called the A-bis interface. The A-bis interface carries traffic and maintenance data. The main function of the BSC is to look over a certain number of BTSs to ensure proper operation. The BSC is a small switch inside the BSC in charge of frequency administration, maintains appropriate power levels of the signal and hand-off among the BTSs inside a BSS. The hardware of the BSC in a single BTS site is located at the antenna and in the multi-BTS systems, in a switching center where other hardware elements of NSS located. The interface between BSC and an MSC is called the A interface, which is standardized within GSM.
The users speech signal is converted into 13kbps-digitised voice with a speech coder and communicated over the air-interface to provide bandwidth efficient air-interface. The backbone wired network uses 64kbps PCM digitized voice in the PSTN hierarchy. Conversion from analog speech signal to 13kbps digitized voice signal take place at the mobile station, and the change from 13kbps to 64kbps coding take place at the BSS. The call is established through the exchange of number of packets.
Network & switching subsystem (NSS)
The NSS is responsible for the network operation. It provides the link between the cellular network & the Public Switched Telecommunication Networks (PSTN or ISDN or Data Networks). The NSS controls hand-offs between cells in different BSSs, authenticates users & validates their accounts, & includes functions for enabling worldwide roaming of mobile subscribers. The NSS could be interpreted as a wireless specific switch that communicates with other switches in the PSTN & at the same time supports functionalities that are needed for a cellular mobile environment. The NSS interconnects to the PSTN through ISDN protocols. The NSS provides communication with other wired & wireless networks, as well as support for registration & maintenance of the connection with the MSs via BSCs in the radio subsystem.
The network & the switching subsystem together include switching function of GSM as well as the databases needed for subscriber data & mobility management. In particular, the switching subsystem consists of
-Mobile Switch Center (MSC)
-Home Location Register (HLR)
-Visitor Location Register (VLR)
-Authentication Center (AuC)
- Equipment Identity Register (EIR)
-Interworking Function (IWF)
The NSS is the most elaborate element of the GSM network, & it has hardware, Mobile Switching Center (MSC), & four software database elements: Home Location Register (HLR), Visitor Location Register (VLR), Equipment Identity Register (EIR),& Authentication Center (AuC). An MSC is the hardware part of the wireless switch that can communicate with PSTN switches using the signaling system-7 (SS-7) protocol, as well as other MSCs in the coverage area of a service provider. If the MSC has an interface to the PSTN then it is called a Gateway MSC (GMSC). The MSC also provides the network the specific information on the status of the mobile terminals. The MSC basically the switching function of the system by controlling call to & from other telephone & data systems. It also does functions such as network interfacing & common channel signaling.
Because the GSM represent an independent network, it must dispose of entities which provide connection to other users. Therefore, the main component of the witching subsystem is the Mobile Switching Center, MSC. The main role of the MSC is to manage the communication between the GSM users & other telecommunication network users. The basic switching function is performed by the MSC, whose main function is to coordinate setting up calls to & from GSM users. The MSC has interface with the BSS on one side (through which MSC VLR is in contact with GSM users) & the external networks on the other side (ISDN/PSTN). An MSC is generally connected to several BSSs, which provide radio coverage to the MSC area. The MSC is also connected to other GSM public Land Mobile Network (PLMN) entities such as other MSCs & HLR through a fixed network.
The MSC is the telephone witching office for mobile-originated or terminated traffic. The MSC controls the call set-up & routing procedures in a manner similar to the function of a land network end office. The MSC provides call set-up. Routing, & handover between BSCs in its own area & to/from other MSCs; an interface to the fixed PSTN; & other function such as billing. It also performs such as toll ticketing, network interfacing, common channel signaling & others.
The HLR is database software that handles the management of the mobile subscriber account. It stores the subscriber address, service type, current location, forwarding address, authentication/ ciphering keys, & billing information. In addition to the ISDN telephone number for the terminal, the SIM card is identified with an International Mobile Subscriber Identity (IMSI) number that is totally different from the ISDN telephone number. The IMSI is used totally for internal networking applications. Each HLR is identified by the HLR number which is sent to all the required VLRs. The HLR is the reference database that permanently stores data related to subscribers, including a subscriber's service profile, location information, & activity status. When an individual users buys a subscription from one of the GSM service providers, he is registered in the HLR of that service provider.
Various identification numbers and addresses as well as authentication parameters, services subscribed, and special routing information are stored in the HLR. Current subscriber status, including a subscriber's temporary roaming number and associated VLR if the mobile is roaming, are maintained. Location registration is performed by HLR. The HLR provides data needed to route calls to all MS-SIMs home based in its MSC area, even when they are roaming out of area or in other GSM networks. The HLR provides the current location data needed to support searching for and paging the MS-SIM for incoming calls, wherever the MS-SIM may be. The HLR is responsible for storage and provision of SIM authentication and encryption parameters needed by the MSC where the MS-SIM is operating. It obtains these parameters from the AuC. The HLR maintains records of which supplementary services each user has subscribed to and provides permission control in granting access to these services.
Based on described functions, different types of data are stored in the HLR. Some data are permanent, that is, they are modified only for administrative reasons, while others are temporary and modified automatically by other network entities depending on the movements and actions performed by the subscriber, some data are mandatory, other data are optional. Both the HLR and the VLR can be implemented in the same equipment in an MSC (collocated). A PLMN may contain one or several HLRs.
The VLR is a temporary database software similar to the HLR identifying the mobile subscribers visiting inside the coverage area of an MSC. The VLR assigns a temporary mobile subscribers identity (TMSI) that is used to avoid using IMSI on the air. The location of mobile subscriber is determined by the VLR into which the mobile subscriber is entered. The visitor location register maintains information about mobile subscribers that are physically in the region covered by the switching centre. It records whether or not the subscriber is active and other parameters associated with the subscriber. For a call coming to the mobile subscriber, the systems uses the mobile phone number associated to identify the home switching centre of the mobile subscriber. The home switching centre can find in its HLR the switching centre in which the mobile subscriber is presently located. For a call coming from mobile subscriber, the VLR is used to initiate the call. Even if the mobile subscriber is in area covered by its home switching centre, it is also represented in the switching centre's VLR.
A VLR is linked to one or more MSCs. The function of the VLR is to memorie temporarily information about the mobiles which are currently located in the geographical area controlled by the linked MSC. The VLR is a database that contains temporary information about subscriber that is needed by the MSC in order to service visiting subscribers. The VLR supports a mobile paging-and-tracing subsystem in the local area where the mobile is presently roaming. The VLR is always integrated with the MSC. A VLR may be in charge of one or several MSC Las (Location Areas).
When a mobile subscriber roams from one LA to another, their current location is automatically updated in their VLR. When a mobile station roams into a new MSC area, if the old & new Las are under the control of two different VLRs, the VLR connected to that MSC will request data about the mobile station from the HLR. The entry on the old VLR is deleted & an entry is created in the new VLR by copying the basic data from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call set-up without having to interrogate the HLR each time. The subscriber's current VLR address, stored at the HLR, is also updated. This provides the information necessary to complete calls to roaming mobiles. These two databases, HLR & VLR, are used to keep track of the current location of an MS in GSM. Maintenance of two databases at home & at the visiting location allows a mechanism to support dialing & call routing in a roaming situation where the MS is visiting the coverage area of a different MSC.
GSM transmission is encrypted. The AuC database hold different algorithms that are used for authentication & encryption of the mobile subscribers that verify the mobile user's identity & ensure the confidentially of each call. The AuC protects network cellular operators from different types of frauds & spoofing found in today's cellular world. AuC holds the authentication & encrypting keys for all the subscriber in both the home & visitor location registers. A stream cipher, A5, is used to encrypt the transmission from subscriber to base transceiver. However, the conversation is in the clear landline network. Another cipher, A3, is used for authentication. Different classes of SIM cards have their own algorithms, & the AuC collects all of these algorithms to allow the NSS to operate with different mobile terminals from different geographic areas.
The EIR is another database that keeps the information about the identity of mobile equipment such as the International mobile equipment identity (IMEI) the reveals the details about the manufacturer, country of production, and device type. This information is used to prevent calls from being misused, to prevent unauthorized or defective MSs, to report stolen mobile phones or check if the mobile phone is operating according to the specification of its type.
Each mobile equipment is identified by IMEI which is memorized by the manufacturer and cannot be removed. By the registration mechanism the MS always sends the IMEI to the network, so that the EIR can memories and assign them to three different lists.
White List This list contains the IMEI of the phones who are allowed to enter in the network.
Black List This list on the contrary contains the IMEI of the phones who are not allowed to enter in the network, for example because they are stolen. Those phones are not able to enter in all the GSM networks which dispose of an EIR.
Grey List This list contains the IMEI of the phones momentarily not allowed to enter in the network, for example because the software version is too old or because they are in repair.
By the registration mechanism, the MSC checks if the MS is contained in the black or grey list; if so, the mobile cannot enter the network. One EIR per GSM network is enough. In the future there will be an interconnection between all the EIRs to avoid situation where a mobile stolen in one country can be used in a GSM network from a different country. Both AuC and EIR can be implemented as individual stand-alone nodes or a a combined AuC/EIR node. The implementation of the EIR is left optional to the service provider.
IWF-Interworking Function It is a subsystem in the PLMN that allows for non-speech communication between the GSM and the other network. The tasks of an particularly to adapt transmission parameters and protocol conversion. The physical manifestation of an IWF may be through a modem which is activated by the MSC dependent on the bearer service and the destination network.
The OSS supports operation and maintenance of the system and allows engineers to monitor, diagnose, and troubleshoot every aspect of the GSM network. The OSS supports one or several Operation maintenance centres (OMC) that are used to monitor and maintain and maintain the performance of each MS, BS, BSC, and MSC within a GSM system. The OSS has three main functions, which are to maintain all telecommunications hardware and network operation with a particular service area, manage all mobile equipment in the system, and manage all charging and billing procedures. Within each GSM system, an OMC is dedicated to each of these tasks and has provisions for adjusting all base-station parameters and billing procedures, as well as for providing systems operators with the ability to determine the performance and integrity of each unit of mobile subscriber equipment in the system.
GSM Signaling Protocol Architecture
Figure shows the signaling protocol architecture for communication between the main hardware elements of the GSM network architecture and the associated interfaces.
The GSM standard specifies the interfaces among all the elements of the architecture. The air-interface 'Um" which specifies communication between the MS and BTS, is the wireless related interface. Messages between the BTS flow through the A-bis interface. The support on this interface is for voice traffic at 64kbps and data/signaling traffic at 16kbps. Both of traffic are carried over LAPD (which is a data link protocol used in ISDN).
The ISDN between a BSC and a MSC is called the 'A' interface, which is standardized within GSM. The 'A' interface uses an SS7 protocol called the signaling connection control part (SCCP) which supports communication between the MSC and the BSS, as well as network messages between the individual mobile subscribers and the MSC. The 'A' interface allows a service provider to use base station and switching equipment made by different manufacturers.
A number of control messages are exchanged between the key entities of GSM network architecture that deal with radio resources, mobility management, and connection management. The protocol stack is divided into three layers.
Layer 1 Physical Layer
Layer 2 Data Link Layer (DLL)
Layer 3 Networking of Messaging Layer
Layer I: Physical Layer
The physical layer defined in the GSM specification is for the Um air-interface. The radio link carries higher level data inside the TDMA format between the mobile station and the base transceiver station. This layer specifies how the information from different voice and data services are formatted into packets and sent through the radio channel. It specifies the radio modem details, the packaging of a variety of services into the bits of a packet, traffic structure and control packets. This Layer specifies modulation and coding techniques, power control methodology, and the time synchronization approaches which enable establishment and maintenance of the channels. The physical layer of the A and A-bis interfaces follow the ISDN standard with 64 kbps digital data per voice user.
Layer II: Data Link Layer
The control and signaling data transfer may be through the same physical channels or through separate physical channels. Sugnaling and control data are conveyed through Layer II and Layer III messages. At the link layer, a data link control protocol known as LAPDm is used where m refers to the modifed to the modified version of LAPD adapted to the mobile environment. In essence, LAPD is designed to convert a potentially unreliable physical link into a reliable data link. It does this by using a cyclic redundancy check to perform error detection and automatic repeat request (ARQ) to retransmit damaged frames. The LAPD protocol is used for the A-bis and A interface connecting the BTS to BSC and BSC to MSC, respectively.
The overall purpose of DLL is to check the flow of packets for Layer III and allow multiple services access points (SAP) with one physical layer. The remaining links use the normal LAPD protocol. The DLL checkc the address and sequence number for layer III and manages acknowledgement for transmission of the packets. In addition, the DLL allows two SAPs for signaling and short messages (SMS). The SMS traffic channel in the GSM is not communicated through voice channels. In GSM, the SMS is transmitted through a fake signaling packet that carries user information over signaling channels. The DLL in GSM provides this mechanism for multiplexing the SMS data into signaling stream.
Signaling packets deliver to the physical layer are each 184 bits, same as that of the lenght of the DLL packets in the LAPD protocol used in the ISDN networks. The lenght of LAPDm packets, is the same as the LAPD, but the format is slightly adjusted to fit the mobile environment.
Since GSM has the time synchronisation and strong coding at the physical layer, the synchronisation bits and CRC codes in LAPD are eliminated in the LAPDm. The address feild is optional, and it identifies the SAP, protocol revision type, and nature of the message. The control feild is optional, and it holds the type of the frame (command or response) and the transmitted and received sequence number.
The linght indiactor identifies the lenght of the information field. Fill-in bits are all 1s bits extend the lenght to desired 184 bits. In peer-to-peer layer II communications, such as DLL acknowledgement, there is no layer III payload and fill-in bits cover this field. The information field carries the Layer III payload data.
The peer-to-peer layer II messages are unnumbered acknowledgement, receiver ready, receiver not ready, disconnect, and reject. These messages do not have layer III information bits and are referred to as Layer II messages. The information bits in layer II packets specify Layer III operations implemented on the logical signaling channels. These information bits are different for different operations.
Layer III: Networking or Signaling Layer
The networking or signaling layer implements the protocols needed to support the mechanism required to establish, maintain, and terminate a mobile communication session. It is also responsible for control function for supplementary and SMS services. The traffic channels are carried by normal bursts in different formats associated with different speech or data services. The signaling information uses other burts and more complicated DLL packaging. A signaling procedure sucha as the registration process is composed of a sequence of communication events or messages between hardware elements of the systems that are implemented on the logical channels encapsulated in the DLL frames.
Layer III defines the details of implementation of messages on the logical channels encapsulated in DLL frames. Among all messages communicated between two elements of the network only a few, such as DLL acknowledgement, do not carry Layer III information. Information bits of the Layer II packets specify the operation of a layer III message.
The transaction identifier (TI) field is used to identify a procedure or protocol that consists of a sequence of messages. This field allows multiple procedures to operate in parallel. The protocol discriminator (PD) identifies the category of the operation (management, supllementary services, call control, and test procedure). The message type (MT) identifies the type of message for a given PD. Information elements (IE) is an optional field for the time that an instruction carries some information that is specified by an IE indentifier (IEI). The number of Layer III messages is much larger than the number of Layer II messages.
To further simplify the description of the Layer III messages, GSM standard divides the messages into three sublayers that provides specific functions;
- Radio resource management (RRM)
- Mobility management (MM)
- Communication management (CM)
The RRM sublayer of Layer III manages the frequency of operation and the quality of the radio link. Radio resource management establishes and releases connections between MSs and an MSC and maintains them despite subscriber movements. The RMM functions mainly performed by the MS and the BSC. The main reponsibilties of RRM are to assign the radio channel and hop to new channels in implementation of the slow frequency-hopping option, to manage hand-off procedure, and to adapt to timing advance for synchronisation.
The major functions of mobility management (MM) sublayer are location update, registration procedures, authentication procedure, TMSI handling, and attachment and detachment procedures for the IMSI. This sublayer handles mobility issues that are not directly related to the radio, and include management of security functions. Mobility management functions are handled by the MS/SIM, the MSC/VLR, and the HLR/AuC.
The communication management (CM) sublayeris used to establish, maintain, and release the circuit-switched connection between the calling and called subscribers of GSM network. Specific procedures for the CM sublayer include mobile-originated and mobile-terminated call establishement, change of transmission mode during the call, control of dialing using dual-tones, and call reestablishment. In addition to call management, it includes supplementry services management and SMS management.
The Mobile application part (MAP) handles most of the signaling between different entities in the fixed part of the network, such as between the HLR and VLR. It runs on top two intermediate protocols- signal connection control part (SCCP) amd message transfer part (MTP). SCCP and MTP protocols are part of signaling system number 7, which is a set of protocols designed to provide control signaling within digital circuit-switching networks.
SS7 Signaling
Common channel signaling systems no.7 (SS7 or CC7) is a global standard that defines the procedures and protocol by which network elements in PSTN exchange information over a digital signaling network to effect wireless (cellular) and wireline call set-up, routing and control. The SS7 signaling protocols are mainly used for basic call set-up, all management, wireless services such as PCS, wireless roaming, mobile subscriber authentication, local number portability, toll-free and toll wireline services, enhanced call features such as call forwarding, calling party name/number display,and three way calling, efficient and secure worldwide telecommunications. The SS7 protocol provides both error correction and retransmission capabilties to allow continued services in the event of signaling point or link failures.
SS7 messages are exchanged between nework elements over 64 kbps bi-directional channels called signaling links. Signaling occurs out-of-band on dedicated channels rather than in-band on voice channels. Compared to in-band signaling, out-of-band signaling provides faster call set-up times, more efficient use of voice circuits, support for intelligent network (IN) services which require signaling to network elements without voice trunks (For example, database systems), and improved control over fraudulent network usage.
There are three kinds of signaling points in the SS7 network:
- Service Switching Point (SSP)
- Signal Transfer Point (STP)
- Service Control Point (SCP)
SSps are switches that originate, terminate, or tandem calls. An SSP sends signaling messages to other SSPs to set-up, manage, and release voice circuits required to complete call. An SSP may also send a query message to centralised database, an SCP, to determine how to route a call. An SCP sends a response to the originating SSP containing the routing number(s) associated with the dialed number. An alternate routing number maybe used by the SSP if the primary number is busy or call is unanswered within a specifiesd time. Actual call features vary from network to network and from sevice to service.
Network traffic between signaling points may be routed vai a packet switch called STP. The STP routes each incoming message to an outgoing signaling link based on routing information contained in the SS7 message. Because it act as a network hub, STP provides improved utilisation of the SS7 network by eliminating the need for direct links between signaling points. The STP may perform global title translation, a procedure by which the destination signaling point is determined from digits present in the signaling message. The STP can also act as a firewall to screen SS7message exchanged with other networks. Because the SS7 networks is critical to call processing, SCPs and STPs are usually depolyed in pair configuration to separate physical location to ensure network-wide service in the event of an isolated failure.
Addressing and Routing
Within the GSM network, two types of routing can be described:
- SS7 addressing and message signaling routing
- Call control/number routing
The SS7 MTP layer 3 provides the routing function. This layer is used to route within a local network using the signaling point code addressing. To interconnect all the local networks on the national SS7 networks. the SCCP global title translation (GTT) functionality is used. Global tilte translation is one of the strong routing capabilities of SS7 SCCP layer. This SCCP functionality allows a centralised network to hold and maintain all the addresses and routing tables, centralising the routing function. For MSC to send a message to a particular HLR, the MSC does not need to know each mobile's HLR point code. Only the adjacent STP point code and the dialed digits (MSISDN) needs to be provided to the STP in order route the message to the HLR. The STP pair after perform the translation of the dialed digits to physical point code (HLR or MSC).
The STP pair after checking the SCCP header information will determine if the message requires GTT translation. It will then exact the IMSI of the subscriber from the calling number address field in the SCCP header and from a database table determine the HLR point code where the validation/authentication should be sent. This will eliminate book-keeping on every MSC and centralise the routing/translation on the SS7 STP network.
A landline calling party dials the GSM mobile directary number (MS ISDN number). The PSTN after performing the digits translation routes the call to the home PLMN GMSC. The GMSC contains either the routing table relate the MSISDN number with the corresponding. HLR, or if the GMSC is connected to the SS7 network with the GTT functionality, the SS7 network will identify the HLR. Once the GMSC interrogates the HLR with MSISDN number, the HLR detrmines the IMSI from the MSISDN number. The HLR stores the subscribers information based on IMSI, not MSISDN. The HLR locates the visiting MSC/VLR point code and if the MSRN is available, it will return the information to GMSC. If the HLR does not have the MSRN for the subscriber it will request one from the visiting MSC/VLR. The latter can be done via GTT if an SS7 backbone with GTT (IMSI to point code) functionality is available/supported. The GMSC once it receives the MSRN and the MSC/VLR point code, will route the call to the VMSC/VLR. The MSC/VLR will then page the mobile subscriber.
The call originating information including the dialed digits will be sent to the MSC/VLR. The MSC/VLR with the subscriber profile information perform digits translation (if support) and routes the call either to the PSTN or to other MSCs. If the MSC cannot perform the digits translation it would route the call to GMSC for translation and routing.
Location update
A list of relevant functions of a mobile station includes provision of location updates. The location-updating procedures, & ubsequent call routing, use the MSC & two location registers: HLR & VLR. When a mobile ststion is switched on in a new location area, or it moves to a new location area or different operator's PLMN, it must register with the network to indicate its current location. In the normal case, a location update message is sent to the new MSC/VLR, which records the location areainformation, & then sends the location information to the subscriber's HLR. The information sent to the HLR is normally the SS7 address of the new VLR, although it may be a routing number.
If the subscriber is entitled to service, the HLR sendds a subnet of the subscriber information, needed fof call control, to the new MSC/VLR, & sends a message to the old MSC/VLR to cancle the old registration. For reliabilty reasons, the GSM also has a periodic location updating procedure. If an HLR or MSC/VLR fails, to have each mobile register simultaneously to make the dtabase up-to-date would cause overloading.Therefoer, the database is updated as location updating event occur. The enabling of periodic updating, & the time period between periodic updates, is controlled by the operator, & is a trade-off between signaling traffic & speed of recovery. If a mobile does not register after the updating time period, it is de-registered.
A procedure related to location updating is the IMSI attach & setach. A detach permits the network to known that bthe mobile station is unreachable, & avoids having to needlessly allocate channels & send paging messages. An attact is similar to a location update, & informs the system that the mobile is reachable again. The activattion of IMSI attach/detach is up to the opertor on an individual cell basis. Location update is typical example for the connection-oriented transactions in GSM. The local opertaaion code Update Location is required directly after the location for the new VLR to update the location information in the HLR. Because this is a confirmed service, it required all four variant: request, indication, response & confirmation.
