Global system for mobile communication (GSM) is a globally accepted standard for digital cellular communication which is based on Time Division Multiple Access (TDMA) technique.
Newer GSM phones support digital transmission through GPRS, a protocol which allows packets of data to be sent and received at moderate rates. It is this GPRS extension which allows new features such as MMS and WAP/web browsing capability.
GSM uses a variation of Time Division Multiple Access (TDMA) and is the most widely used of the three digital wireless telephone technologies (TDMA, GSM, and CDMA). It digitizes and compresses data and then sends it down a channel with streams of user data, each in its own time slot and operates at either at900 MHz or 1,800 MHz frequency band.
In their effort to design a pan-European mobile technology,Confederation of European Posts and Telecommunications, (CEPT) formed the Groupe Speciale Mobile (GSM) in 1982. Though it was initailly designed for and confined only to Europe, it is now used worldwide.
Basic Principle of TDMA
Time division multiple access (TDMA) is a technology for shared medium (usually radio) networks. It allows several users to share the same frequency within the medium, by dividing it into different timeslots and transmits voice and non – voice data in rapid succession, each using their own time slot.
This allows multiple users to share the same transmission medium (e.g. radio frequency) whilst using only part of its required bandwidth.
The digital transmission technology that allows a number of users to access a single radio-frequency (RF) channel without interference by allocating unique time slots to each user within each channel. The TDMA digital transmission scheme multiplexes three signals over a single channel. The current TDMA standard for cellular technology divides a single channel into six time slots, with each signal using two slots and each caller being assigned a specific time lot for transmission.
The receiving equipment must be able to decode the received signal by decoding the received signal and reconstitute it using the same time slot selection algorithm which was used when it was encoded and transmitted. TDMA is used by the GSM digital cellular system.
The GSM network is divided into three major systems: The switching system (SS), the base station system (BSS), and the operation and support.
The following are some of the basic components of GSM network
The Switching System
The switching system (SS) is responsible for performing call processing and subscriber-related functions. It includes the following functional units.
Home Location Register (HLR) -The HLR is a database used for storage and management of subscriptions. It is considered the most important database, as it stores permanent data about subscribers, including 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 telephonic switching functions of the system. It controls calls to and from other telephone and data systems and also performs such functions as network interfacing, 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 provide service to subscribers. The VLR is integrated with the MSC. When a mobile station enters 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 confidentiality of each call. The AUC protects network operators from different types of fraud found in the 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.
Base Station
The Base Station System (BSS)
All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).
BSC-The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.
BTS- The BTS handles the radio interface to the mobile station. It is the radio equipment (transceivers and antennas) that is needed to service each cell in the network. A group of BTSs are controlled by a BSC.
The Operation and Support System
The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS) from which the network operator monitors and controls the system. It’s purpose is to offer the customer cost-effective support for centralized, regional and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.
GSM uses a process called Circuit Switching which is a method of communicating a communication path that is established between two devices through one or more switching nodes. The digital data is sent as a continuous stream of bits, allowing the data to be heard from the receiving end whilst the data is still being sent. This makes circuit switching an ideal method of sending real time audio. This is different from GPRS which sends packets of information which need to be reassembled before they can be heard and is therefore not suitable for real time audio broadcast applications.
One of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smartcard containing the user’s subscription information and phonebook. This allows the user to retain his information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM.
Advantages of GSM over the conventional Telephone sets
1.The main advantage of GSM mobiles are International Roaming ability, good sound quality, small cheaper handsets and the ability to handle high volumes of users. Unlike analogue phones, GSM mobile phones within the digital network are more safe and secure for conversations.
2. GSM mobile phones offer roaming facility, which means that one can use the same mobile number around the world through the GSM network or by using the SIM card in another GSM cell phone.