Data communication is the movement of data and information from one point to another point means of electrical or electromagnetic devices, fibre-optic cables or microwave signals.
Telecommunications system has five components. These are
1. Sender: – input devices.
2. Channel & Media:-channel characteristics.
• Transmission Speed.
• Transmission Mode.
• Transmission direction
• Bounded.
• Unbounded.
3. Hardware: –
• Host Computer,
• Front end Processor,
• Modem,
• Multiplexer,
• Switch.
• Bridge
• Gateway
• Bus.
4. Software:-
• Access Control.
• Transmission control.
• Network control & Management.
• Error detection & Correction.
• Network Security.
5. Receiver:-Output Devices.
5.2.1. Telecommunications Channels
A data communications channels is a path through a medium that data can take from sending stations to receiving stations.
Characteristic of channels are:

Transmission Speed: –
It is the capacity of a telecommunication channel depending on the bandwidth. The greater bandwidth the greater amount of information transmitted over the channel. There are three types of bandwidth:-
 Voice-band (300 bps to9600 bps),
 Medium band (9600bps to 256000bps).
 Broadband (256000bps to a very large number.)
• Transmission Mode:-There are two types of transmission mode
 Synchronous:-In this transmission a group of characters transmit at a time.
 Asynchronous: – In this transmission one character transmit at a time.
• Transmission direction:- It is the direction in which data is transmitted . There are three types of directions:
 Simplex: – In this direction data communication devices that can either send or receive data, but can not do both.
 Half duplex: – In this case two devices are send or received data, but not at a time.
 Full duplex: – In this case two devices are send or received data at a time.

Telecommunications Media
There are two types of media over with data is transmitted.
• Bounded media-The signals are confined to the medium. Ex-Twisted pair cable, fibre-optic cables, coaxial cable.
• Unbounded media-The signal are not restricted to the medium. Ex-Microwaves.

 Host Computer:-The host performs the data processing for the network. The incoming messages are handled in the same manner as data received from any other type of input unit .After the processing, messages can be transmitted back to the front –end processor for routing.
 Front-end processor:-The front-end processor is a minicomputer that acts as a buffer between the client device and the host computer. It increases the operating efficiency of the network taking care of routine tasks such as coordinating peripherals and ensuring error free transmission.
 Modem:-It stands for modulator-demodulator. It is used for Digital signals are converted into analog signals and vice-versa.
 Multiplexer:-It is a device that permits the simultaneous sending and receiving of multiple messages over a signal channel. A multiplexer collects signals from several terminals and transmits them over a single channel.
 Switch:-It is a device which is determines the data transmission path. Switches can control the transmission path across national or local networks.
 Bridge and gateway:-
• A bridge is a device that separates two or more network segments with in one logical network (e.g. a single IP-subnet). A bridge is usually placed between two separate groups

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