Computer Networks and it's features

Computer Network means an interconnected collection of autonomous computers. Two computers are said to be interconnected if they're able to exchange information and they use transmission media like copper, fibre optics , microwave etc.

Features of Computer Networks:

1) Resource sharing

2) High reliability

3) Saving Money

4) Scalability

5) Communication Medium

Uses of Computer networks:

1) For people, video conferencing, person to person communication through computer network.

2) Accessing for remote information

3) For interactive entertainment like video on demand and playing game.

4) It is used for social issues.

Data communication:

Data communication based transfer data from one point to another and in other words data communication means of transport data from source to destination.

The data is transmitted from source to destination through a transmission medium in a form of electromagnetic waves. The transmission medium either guided media or unguided media. 

• Guided media- wires and cables (connection oriented)
• Unguided media-like air (connection less)

The following component of data communication are:

Source and destination: who sent same data and who receives same data from source.

Data communication: device which converts the data into electromagnetic signal that can be transmitted and we can need a device which converts the electromagnetic signals to original data form.

Transmission media: transmission media communication channels that transport signal from one point to another.

Communication channels:

Channels means path, a line, a link that interconnects a source to it's destination.

Three types of communication channel:

(1) Simplex

(2) Half Duplex

(3) Full Duplex

(1) Simplex: in simplex transmission signal, data or signal is always in one direction.

Example: keyboard attached to a computer can only send data to the computer.

For two way transmission of data two such lines will be attached one transmitting in each direction.

Half duplex: half duplex allows transmission in either direction but not simultaneously.

Example the transmission of data to and from a hard disk is done in half duplex.

                                 Half Duplex

Full duplex: This allows transmission in both directions simultaneously like telephone lines which provides this facility. This mode is also called two way simultaneously.

Data transmission signal:

Signalling is the process of communicating information.

Type of data transmission:

1) Analog data communication

2) Digital data communication

1) Analog data communication: Analog data communication is the transmission of analog signal ,have ability to capture the nature of real world.

Disadvantages of analog signal:

1) Analog signal has high error prone .

2) Analog signals accuracy is less.

3) Analog signal is high noise signal.

2) Digital data communication: Digital data transmission is the transmission of data using on and off electrical state or 0 & 1 signals.

Different modes of data transmission:

We have two different modes of data transmission:

(1) Asynchronous Transmission

(2) Synchronous Transmission

Asynchronous transmission: Asynchronous transmission refers to the case when the sending end start transmission of bytes at any instant of time. Only one byte is sent at a time and there is no time relation between consecutive bytes, i.e., after sending a byte , the next byte can be sent after arbitrary delay. In the idle state, when no byte is being transmitted the polarity of the electrical signal corresponds to 'I'.

Due to arbitrary delay between consecutive ides the time occurrences of the clock pulses at the receiving end need to be synchronized repeatedly for each byte. This is achieved by providing two extra bits, a start bit at the beginning and a stop bit at the end of the byte.

Start bit: The start bit is always zero and is prefixed to each byte. At the onset of transmission of a bite it ensures that the electrical signal changes from idle state 1 to 0 and remains at 0 for one bit duration. The leading edge of the start bit is used as time reference for generating the clock pulses at the required sampling instants. Thus each onset of a byte results in the resynchronisation of the receiver clock.

Stop bit: To ensure that the transition from '"1" to '"0" is always presented the beginning of the byte it is necessary that polarity of the electrical signal should correspond to "1" before occurrence of the start bit. That is why the idle state is kept at "1". But there maybe two bytes one immediately following the other and if the last bit of the first byte is "0" the transition form "1" to "0" will not occur. Therefore a stop it is also suffixed to each byte. It is always "1" and its duration is usually 1, 1.5 or 2 bits.

Synchronous transmission: A synchronous action unlike an asynchronous action, is carried out under the control of a timing source. In synchronous transmission, bits are always synchronised to reference clock irrespective of the bytes they belong to. There are no start or stop bits. Bytes are transmitted as a block in continuous stream of bits. Even the internal block idle time is filled with the idle characters.

continuous transmission of bits enables the receiver to extract the clock from the incoming electrical signal. As a clock is inherently synchronised to the bits the job of the receiver becomes simpler.

However there is still one problem the bytes lose their identity and their boundaries need to be identified. Therefore a unique sequence of which number of bits, called flag comma is prefixed to each block. The flag identifies the start of a block. the receiver first detects the flag and then identifies the boundaries of different bytes using a counter.