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Circuit Switching

Circuit switching is normally done at physical layer where a complete physical connection is first set up between source and destination computers and then only data transmission takes place. The data follows a dedicated path reserved for this communication. Data lost in the transmission cannot be recovered here. The connection breaks only when the data transfer is over and after this only that path can be assigned to another session.

Dedicated, non-shared communication path

Packet Switching

It is usually done at network layer .In Packet Switching, data is divided into small data packets which are independent of one another. All the data packets may take different routes to reach the destination where they are assembled again to form a complete message. The data packets may reach at the destination out of order and some may be lost also but certain techniques are incorporated to resolve these issues.

Data Encoding

Data Encoding is a technique of converting digital data into digital signals that can be transmitted over a medium.

Important Definitions:

a) Digital Signal- It is a sequence of discrete, discontinuous pulses.

b) Bit duration : the time taken by the transmitter to emit a bit.

Following techniques are used for data encoding:-

Non Return to Zero (NRZ)- It uses 0 volts for a data bit of ‘0’ and +V for a data bit of ‘1’. It faces synchronization issues since the signal never returns to zero. The average DC voltage is 1/2 V so there is high power output. Two bits are transmitted for every cycle.

Return to Zero(RZ)- Here a ‘0’ bit is represented by 0 Volts and a ‘1’ by +V volts for half the cycle and 0 volts for the second half cycle. Average DC voltage is ¼ V and a voltage change always occurs even for a series of 1’s.

There are two variations to this:

A)NRZ-L( NRZ-Level)- here, the voltage level is determined by the value of the bit.

B) NRZ-I- Here, the change in voltage determines the value of the bit. If there is a change the bit is 1, else it is 0.

This can be demonstrated in the following examples for the binary patterns 10110 and




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