16-02-2013, 04:02 PM
Global system for mobile communication (GSM)
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INTRODUCTION-:
The main task of telecommunication network is call Processing. Call Processing means how a call is connected between ms-1 to ms-2 in GSM network is known as call Processing. This includes identifying incoming calls, establishing a communication path for the duration of the connection, and disconnecting the call after the conversation has ended.
The first part to mobile call processing is initialization. It's what happens when you first turn on your phone. You get a connection to a nearby cell site, then the cellular network checks your account. If you have a valid telephone number and your account is good then your call proceeds. First of all a connection is to be established with nearby BTS. It is not possible to make a call unless your mobile has a link to a cell site.
The main elements of cellular network are the base stations, a switching network, land lines, and the mobile users. In a simple cellular system, there is only one base station per cell. It consists of a transmitter/receiver, which communicates directly with the users in its cell. Each base station is connected via land lines to a central switching network. The switching network makes important decisions such as channel allocation and cell hand off.
WHAT IS GSM-:
Global system for mobile communication (GSM) is a globally accepted standard for digital cellular communication. GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz It is estimated that many countries outside of Europe will join the GSM partnership.
Throughout the evolution of cellular telecommunications, various systems have been developed without the benefit of standardized specifications. This presented many problems directly related to compatibility, especially with the development of digital radio technology. The GSM standard is intended to address these problems.
From 1982 to 1985 discussions were held to decide between building an analog or digital system. After multiple field tests, a digital system was adopted for GSM. The next task was to decide between a narrow or broadband solution. In May 1987, the narrowband time division multiple access (TDMA) solution was chosen.
GSM NETWORK-:
GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to limit the designers as little as possible but still to make it possible for the operators to buy equipment from different suppliers. The GSM network is divided into three major systems: the switching system (SS),
the base station system (BSS), and the operation and support system (OSS).The figure is shown below.
CELLS:-
Cells mean Area covered by the BTS which transmits and receives signals on Radio Channels.
Cell provides the complete coverage for the designed area. Each cell is allocated a number of Radio Channels. The MS and BTS communicate over these radio channels.
Cell is identified by a number called CGI –Cell Global Identification.
Types of Cells:-
Different density of population gives the need for different types of cells.
• Macro Cells
• Micro Cells
• Selective Cells
• Umbrella Cells
GSM Voice & Channel Coding
In order to send our voice across a radio network, we have to turn our voice into a digital signal. GSM uses a method called RPE-LPC (Regular Pulse Excited - Linear Predictive Coder with a Long Term Predictor Loop) to turn our analog voice into a compressed digital equivalent. Once we have a digital signal we have to add some sort of redundancy so that we can recover from errors when we trams our digital voice over the radio channel. GSM uses a convolution codes to encode digital speech representations.
Here we will consider two forms of coding techniques used within the GSM system.
Firstly the process used to convert human speech into a digital equivalent and secondly the coding processes for compressing and protecting the data for transmission over the air interface.
CHANNEL CODING:
• Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors occurred during the transmission.
• The channel coding is performed using two codes: a block code and a convolution code.
• The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits.
• A convolution code adds redundancy bits in order to protect the information. A convolution encoder contains memory. This property differentiates a convolution code from a block code.
• A convolution code can be defined by three variables : n, k and K.
The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block and K to the memory of the encoder
CALL FLOW:
In an example scenario to make a call, the user enters the phone number and presses the send button on the phone. Base stations reserve a few control channels through which the user initially communicates with the base station. All base stations within the cellular network constantly monitor these control channels and when a “send” command is requested, the base station with the stronger received signal takes control. It then routes the user request to the switching network.
CONCLUSION:-
This paper presents a call processing model for wireless network. A call from a mobile user is set up through a base station and a number of switches. A switch has the input queue for the source and the output queue. The packets are held in the input queue if the output queue in the switch is full. The input queue is a buffer that holds the packets if the required Bandwidth exceeds the available bandwidth. The switch design allows for collision free packet transfer between the input and the output queue. A variable buffer is employed based upon the protocol carried, with a priority technique used to service time sensitive protocols. A multiplexing mechanism is used to allow other protocols to entire the queue during time-outs.