} In near future most computer will be portable and users will access network without wire i.e., wireless
} An acceptable level of intelligibility of speech is obtained by transmitting frequencies in the range 300-3400 Hz.
} Voice is sampled at 8000 samples per second with 8 bits per sample . This results in a digital signal of 64kbps which is sent as data frames over a circuit switched network
In 1946, the first car-based telephone was set up in St. Louis, Missouri, USA. The system used a single radio transmitter on top of a tall building. A single channel was used, therefore requiring a button to be pushed to talk, and released to listen. This half duplex system is still used by modern day CB radio systems utilized by police and taxi operators. In the 1960s, the system was improved to a two-channel system, called the improved mobile telephone system (IMTS). Since frequencies were limited, the system could not support many users.
Cellular radio systems, implemented for the first time in the advanced mobile phone system (AMPS), support more users by allowing reuse of frequencies. AMPS is an analogue system, and is part of first generation cellular radio systems. In contrast, second generation systems are digital. In the USA, two standards are used for second generation systems: IS-95 (CDMA) and IS-136 (D-AMPS). Europe consolidated to one system called the global system for mobile communications (GSM). Japan uses a system called personal digital cellular (PDC).
Cellular radio is the fastest growing segment of the communications industry. Cellular companies reported a subscription base of more than 200 million people in 1997. This figure grows by an average of 150,000 new subscribers every day.
Because of Europe's early commitment to one system, it is leading the field in both its subscriber base and data transmission capabilities. GSM is used in over 100 countries by over 215 operators inside and outside of Europe. The Japanese PDC system is the second largest digital cellular system, followed by the IS-54/136 and IS-95 systems used in North America.
Current cellular radio systems are in their second generation (2G). The third generation of cellular systems (3G systems) will allow different systems to interoperate in order to attain global roaming across different cellular radio networks. The International Telecommunication Union (ITU) has been doing research on 3G systems since the mid 1980s. Their version of a 3G system is called international mobile telecommunications - 2000 (IMT-2000).
In the following explanation, a cellular telephone or any other device that can connect to a cellular radio network will be referred to as a mobile station. This is in keeping with the literature on the subject.
A cellular network consists of both land and radio based sections. Such a network is commonly referred to as a PLMN - public land mobile network. The network is composed of the following entities:
Digital circuit switched system -four elements : MS, BTS,BSC and switching subsystems
This is the structure of GSM network. It gives us a simplified overview of the GSM system.
Now , A GSM system consists of 3 subsystems-
1. RSS : Radio subsystem
2. NSS : Network & switching subsystem
3. OSS : Operation subsystem.
But , a GSM customer only notices a very small fraction of the whole network :
A. The mobile stations(MS)
B. Some antenna masts of the base transceiver stations(BTS)
The radio subsystem(RSS) comprises all radio specific entities, i.e., the mobile stations(MS) and the base station subsystem(BSS).the above fog. Shows the connection between the RSS and the NSS via the A interface and the connection to the OSS via the O interface.
● Base station subsystem(BSS): A GSM network comprises many BSSs, each controlled by a base station controller(BSC).The BSS performs all functions nessary to maintain radio connections to an MS,coding/decoding of voice.The BSS contains several BTSs.
● Base transceiver station(BTS): A BTS comprises all radio equipment, i.e., antennas,signal processing,amplifiers necessary for radio transmission.It is connected to MS via the Um interface(ISDN U interface for mobile use),and to the BSC via the Abis interface.The Um interface contains all the mechanisms necessary for wireless transmission. The Abis interface consists of 16 or 64 kbits/s connections.
● Base station controller(BSC): The BSC basically manages the BTSs.It reserves radio frequencies, handles the handover from one BTS to another within the BSS, and performs paging of the MS.
● Mobile station(MS): The MS comprises all user equipment and softwere needed to communication with a GSM network.An MS consists of user independent hard-and softwere and of the subscriber identity module(SIM), which stores all user-specific data that is relevant to GSM. MS can be identified via the international mobile equipment identity(IMEI). SIm card contains many identifiers and tables, such as card-type, serial number, list of subscribed services, a personal identity number(PIN), a PIN unblocking key(PUK).
The “heart” of the GSM system is formed by the Network and switching subsystem(NSS).The NSS consists of the following switches and databases:
● Mobile services switching center(MSC): MSCs are high-performance digital ISDN switches.They set up connections to other MSCs and to the BSCs via the A interface, and form the fixed backbone network of a GSM system.An MSC manages several BSCs in a geographical region.It handles all signaling needed for connection setup, connection release and handover of connections to other MSCs.
● Home location register(HLR): The HLR is the most important database in a GSM system .It stores all user-relevent information.
● Visitor location register(VLR): The VLR associated to each MSC is a dynamic database which stores all important information needed for the MS users currently in the LA that is associated to the MSC.
The third part of a GSM system, the Operation subsystem(OSS), contains the necessary functions for network operation and maintenance.This consists of following:
● operation and maintenance center(OMC): The OMC monitors and controls all other networks entities via the O interface.
● Authentication centre(AUC): As the radio interface and mobile stations are particularly vulnerable, a separate AUC has been defined to protect user identity and data transmission.
● Equipment identity register(EIR): The EIR is a database for all IMEIs, i.e., it stores all device identifications registered for this network.
• It includes cells ,clusters,frequency reuse and handovers
• Cells are base stations transmitting over small geographical areas that are represented as hexagons
• A cluster is a group of cells where no channels are reused
• The concept of frequency reuse is based on assigning to each cell a group of radio channels so that their frequencies do not interfere
HANDOFF MECHANISM IN CELLULAR NETWORK :
Cellular systems require handover procedures, as single cells do not cover the whole service area. It can only upto 35Km around each antenna on the countryside and some hundred meters in cities.
There are 2 basic reasons for handover-
Ø The mobile station moves out of the range of a BTS or a certain antenna. Then the received signal level decreases continuously until it falls below the minimum requirements for communication.
Ø Load balancing:-if the traffic in one cell is too high then shift some MS to other cells with a lower load.
CELLULAR SWITCHING EQUIPMENT :
} Switching Equipment is the brain of the cellular system
} It consists of two parts : the switch and the processor
} The processor controls all the functions that are specific for cellular systems viz. frequency assignment, decisions regarding handoff and monitoring of traffic
• The control is usually centralized
• Programs are stored in the memory that provides the logic for controlling the telephone calls
• The central processor performs the following functions : controls every connection at the line and trunk side , sends the order to all units(switching n/w , trunks , service ckts.).
} A decentralised system-all switches are treated equally
} A remote control switching system a main switch is used to control a remote secondary switch
NEXT GENERATION VOICE INFRASTRUCTURE :
} The transport and switching of voice will be performed in the packet domain-a VoIP .The infrastructure efficiencies that can be achieved with VoIP.
} Computing platforms physically separate from the switching systems-known as the softswitch-will provide the intelligence powering voice services
REFERENCES :
http://en.wikipedia.org/wiki/GSM"
http://www.gsmworld.com/newsroom/market-data/market_data_summary.htm.
