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 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.