Mobile Communications Principles
Each mobile uses a separate, temporary radio channel to talk to the cell site. The cell site talks to many mobiles at once, using one channel per mobile. Channels use a pair of frequencies for communication. One for transmitting from the cell site, the forward link , and one frequency for the cell site to receive calls from the users, the reverse link.
Communication between mobile units can be either half-duplex or full-duplex. In case of half-duplex , transmit and receive communications between the mobile units are not at the same time, i.e. talking and listening can not be done at the same time. In case of full-duplex communication, transmit and receive communication is at the same time, i.e. one can talk and listen at the same time.
When communications between mobile units are within a cell , and if the same is half-duplex , then it shall require only one pair of frequency. If the same is full-duplex , then requirement of frequency pair shall be two .
When a mobile unit is communicating with a mobile unit outside the cell, then the requirement of frequency pair shall be one per cell for both half-duplex and full-duplex communication. Hence the system resources are utilized more if the mobile units communicate with each other in full-duplex mode.
Early Mobile Telephone System Architecture
Traditional mobile service was structured similar to television broadcasting. One very powerful transmitter located at the highest spot in an area would broadcast in a radius of up to fifty kilometers. The "cellular concepts" structure the mobile telephone network in a different way. Instead of using one powerful transmitter, many low-power transmitters were placed throughout a coverage area. For example, by dividing a metropolitan region into one hundred different areas (cells) with low-power transmitters using twelve conversations (channels) each, the system capacity theoretically could be increased from twelve conversations - or voice channels using one powerful transmitter- to twelve hundred conversations (channels) using one hundred low-power transmitters.
Different Type Of Communication Systems
The different types of communication systems available today can be broadly classified into the following categories.
Ø Landline System
Ø Cellular System
Ø Satellite System
The evolution of the above Systems had been broadly as a point to point system .
Ø Two-Way Radio System
The evolution of the Two-way Systems has been both as a point to point and a point to multi point system
The System (T1540) Overview
The system consists of a number of radio sites, which are linked in groups to "Regional Nodes" (in the Western Railway System there are two radio sites, one at MX, and the other at BVI). Regional Nodes may be linked together to form a wide area network. Radio sites (base stations) are connected by fixed audio and data links (V.24 interface at 1200 bauds) in "Star Configuration around the Regional Node".
In case of wide area network the interconnection between the Regional Nodes shall be with X.25/TCP IP (9600-baud link) for data, and by an audio network consisting of fixed link bearers for audio signals.
PSTN & PABX interconnection is provided at the Regional Node now.
The expansion capability of the T1540 system is as under
Ø Each Regional Node can support upto 16 base stations connected in a star configuration.
Ø Each base station can support upto 24 channels including the control channel (both the base stations of W.R are configured for only 5 channels each).
Ø Overall System can be configured for a maximum of 32 Regional Nodes.
The overall access for management of the system is provided by the Network Provider Interface (NPI). The NPI provides an interface for external systems running applications (a Network Provider Package (NPP)) using the following facility
Ø Fault Management and Alarms
Ø Call Records
Ø NPI management
Frequency of operation of the Western Railway System is between 338 to 350 MHz
Mobile Communication today is a fast growing field. No one can deny its role in Modern Railway Operations. However there is a need of proper choice of technology looking into Railways' Operational needs. It is beyond doubt that incorporation of Mobile Communication into Railways will open new operational
avenues, there by reducing operational costs and increasing customer satisfaction by providing better services. This shall not only help in increasing productivity, but also help in increasing safety of operations. This is an age of communication. Indian Railways, which is a lifeline of the nation, is also geared up to take the requirements of the new millennium, which is knocking the door of this century