Access Network

Access network is a connection between MS and NSS, comprise of BTSs & BSCs. It is responsible for radio management.BSC looks towards MSC through single A-interface as being the entity responsible for communicating with Mobile Stations in a certain area. The radio equipment of a BSS

may support one or more cells.A BSS may consist of one or more base stations, where an A-bis-interface is implemented.

BSS ConfigurationSS Configuration

• Collocated BTS
• BTS
• Daisy Chain BTS
• Star Configuration
• Loop Configuration

Collocated BTS:
BTS is situated along with BSC or the MSC and no additional E1 link is required.
Remote BTS :
BTS is situated in a stand alone position and additional E1 links are required to connect to BSC.
Daisy Chain

Star Configuration

Loop Configuration

Comparison of Different Configurationsomparison of Different Configurations Daisy Chain:

Easy to implement, effective utilization of transmission links but if one of the link fails, all the BTSs connected in the chain will went off.
Star Configuration:
Easy to implement but poor utilization of links. Each BTS require one E1 to connect to BSC. But if link goes down only individual BTS will be affected.
Loop Configuration:
Slightly difficult to implement but effective utilization of E1 links. Even if one link goes off BTS will continue to communicate with the network from the other side.

Home location register (HLR)

The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribersincluding a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.

Mobile services switching center (MSC)

The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.

Visitor location register (VLR)

The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.

Authentication center (AUC)

A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.

Equipment identity register (EIR)

The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber. These services are provided in conjuction with several functional entities, which together form the Network Subsystem. The MSC provides the connection to the fixed networks (such as the PSTN or ISDN). Signalling between functional entities in the Network Subsystem uses Signalling System Number 7 (SS7), used for trunk signalling in ISDN and widely used in current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile. The location of the mobile is typically in the form of the signalling address of the VLR associated with the mobile station. The actual routing procedure will be described later. There is logically one HLR per GSM network, although it may be implemented as a distributed database.

The Visitor Location Register (VLR) contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR. Although each functional entity can be implemented as an independent unit, all manufacturers of switching equipment to date implement the VLR together with the MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, thus simplifying the signalling required. Note that the MSC contains no information about particular mobile stations --- this information is stored in the location registers.

The other two registers are used for authentication and security purposes. The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is not type approved. The Authentication Center (AuC) is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and encryption over the radio channel.

Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the user can have access to subscribed services irrespective of a specific terminal. By inserting the SIM card into another GSM terminal, the user is able to receive calls at that terminal, make calls from that terminal, and receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to identify the subscriber to the system, a secret key for authentication, and other information. The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card may be protected against unauthorized use by a password or personal identity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). These communicate across the standardized Abis interface, allowing (as in the rest of the system) operation between components made by different suppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio-link protocols with the Mobile Station. In a large urban area, there will potentially be a large number of BTSs deployed, thus the requirements for a BTS are ruggedness, reliability, portability, and minimum cost.

The Base Station Controller manages the radio resources for one or more BTSs. It handles radio-channel setup, frequency hopping, and handovers, as described below. The BSC is the connection between the mobile station and the Mobile service Switching Center (MSC).

Mobile service Switching Center (MSC)

The combination of a cellphone and the SIM card creates a special digital "signature" - that includes your subscriber number - which is sent from your cellphone to the nearest BS asking that you as a subscriber of a particular network be allowed to use the network. The request is passed on along the network of BSs to the multifaceted heart of a cellular network - the Mobile Switching Center (MSC).

The MSC also routes all your incoming and outgoing calls to and from the fixed-line networks or other cellular networks.

The GSM radio interface

The radio interface [13] is the interface between the mobile stations and the fixed infrastructure. It is one of the most important interfaces of the GSM system.The Radio Interface (Um) is split into several channels: traffic channels and signaling channels. The traffic channels carry user data. The signaling channels carry management and control information.

Frequency allocation:

Two frequency bands, of 25 Mhz each one, have been allocated for the GSM system:

The band 890-915 Mhz has been allocated for the uplink direction (transmitting from the mobile station to the base station)FDMA and TDMA

The band 935-960 Mhz has been allocated for the downlink direction (transmitting from the base station to the mobile station).

Access Scheme :

The multiple access scheme defines how different simultaneous communications, among different mobile stations located in some different cells, share the GSM radio spectrum. A mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with frequency hopping, has been adopted as the multiple access scheme for GSM.

Using FDMA, a frequency is assigned to a user. So, the larger the number of users in a FDMA system, the larger the number of available frequencies must be.

The limited available radio spectrum and the fact that a user will not free its assigned frequency until he does not need it longer, explain why the number of users in a FDMA system can be "quickly" limited.

On the other hand, TDMA allows several users to share the same channel. Each of the users, sharing the common channel, are assigned their own burst within a group of bursts called a frame. Usually TDMA is used with a FDMA structure.

In GSM, a 25 Mhz frequency band is divided, using a FDMA scheme, into 125 carrier frequencies spaced one from each other by a 200 khz frequency band.

Normally a 25 Mhz frequency band can provide 125 carrier frequencies. Each carrier frequency is then divided in time using a TDMA scheme. This scheme splits the radio channel, with a width of 200 khz, into 8 bursts. A burst is the unit of time in a TDMA system. A TDMA frame is formed with 8 bursts. Each of the eight bursts, that form a TDMA frame, is then assigned to a single user.

Structure:

A channel corresponds to the recurrence of one burst every frame. It is defined by its frequency and the position of its corresponding burst within a TDMA frame. In GSM, there are two types of channels

• The traffic channels used to transport speech and data information.
• The control channels used for management messages and some channel maintenance tasks.

The GSM System Architecture

The GSM system architecture[5], [13], [1] includes four subsystems (cf. Figure 1): Mobile Station (MS), Base Station Subsystem (BSS), Network and Switching Subsystem (NSS), and Operation Sub-System (OSS).

The MS subsystem involves a radio part, an interface, and a Subscriber Identity Module (SIM). The radio part carries out all the functions related to the radio interface (Um), e.g. receiving and transmitting radio signals, signal processing, frequency hopping, and channel management. The interface to terminal equipment acts as a gateway between the terminal and the radio part. The SIM contains all the subscriber-related information on the MS side of Um to identify a subscriber and take care of the security. The SIM is implemented as a smart card.

The BSS forms cell structure of GSM network. It includes two types of network elements: the Base Transceiver Station (BTS), and the Base Station Controller (BSC).

The BTS is a transmission component. It carries out radio signal transmission and reception, signal processing, speech encoding and decoding, and transmission rate adaptation. The BSC is a managing component. It is responsible for all the management of the Um, e.g. channel allocation and deallocation, handover, and timing of radio signals. One BTS implements one cell in GSM. A BSC can manage several BTSs.

The NSS comprises the main switching functions of GSM and all the databases needed for subscriber information, mobility management, and interworking. It contains the following databases: Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AUC), and Equipment Identity Register (EIR).

The MSC performs the basic switching function by setting up calls to/from MSs.

The GSM system also communicates with other networks such as the Public Switched Telephone Network (PSTN), Integrated Services digital Network(ISDN), Circuit-Switched Public Data Network (CSPDN), and packet-switched public data network (PSPDN).

Services Provided by GSM

From the beginning, the planners of GSM wanted ISDN compatibility in services offered and control signalling used. The radio link imposed some limitations, however, since the standard ISDN bit rate of 64 kbps could not be practically achieved.

Using the ITU­T definitions, telecommunication services can be divided into bearer services, teleservices, and supplementary services. The digital nature of GSM allows data, both synchronous and asynchronous, to be transported as a bearer service to or from an ISDN terminal. Data can use either the transparent service, which has a fixed delay but no guarantee of data integrity, or a non­transparent service, which guarantees data integrity through an Automatic Repeat Request (ARQ) mechanism, but with a variable delay. The data rates supported by GSM are 300 bps, 600 bps, 1200 bps, 2400 bps, and 9600 bps . The most basic teleservice supported by GSM is telephony. There is an emergency service, where the nearest emergency­service provider is notified by dialling three digits (similar to 911).

Group 3 fax, an analog method described in ITU­T recommendation T.30, is also supported by use of an appropriate fax adaptor. A unique feature of GSM compared to older analog systems is the Short Message Service (SMS). SMS is a bidirectional service for sending short alphanumeric (up to 160 bytes) messages in a store­and­forward fashion. For point­to­point SMS, a message can be sent to another subscriber to the service, and an acknowledgement of receipt is provided to the sender. SMS can also be used in a cell­broadcast mode, for sending messages such as traffic updates or news updates. Messages can be stored in the SIM card for later retrieval . Supplementary services are provided on top of teleservices or bearer services, and include features such as caller identification, call forwarding, call waiting, multi­party conversations, and barring of outgoing (international) calls, among others.