## Delays in Computer Networks-

Consider-

• Two hosts A and B are connected over a transmission link / transmission media.
• A data packet is sent by the Host A to Host B.

Following different types of delay occur during transmission-

1. Transmission delay
2. Propagation delay
3. Queuing delay
4. Processing delay

## 1. Transmission Delay-

 Time taken to put the data packet on the transmission link is called as transmission delay.

Mathematically,

• Transmission delay ∝ Length / Size of data packet
• Transmission delay ∝ 1 / Bandwidth

Thus,

## 2. Propagation Delay-

 Time taken for one bit to travel from sender to receiver end of the link is called as propagation delay.

Mathematically,

• Propagation delay ∝ Distance between sender and receiver
• Propagation delay ∝ 1 / transmission speed

Thus,

## 3. Queuing Delay-

 Time spent by the data packet waiting in the queue before it is taken for execution is called as queuing delay.

• It depends on the congestion in the network.

## 4. Processing Delay-

 Time taken by the processor to process the data packet is called as processing delay.

• It depends on the speed of the processor.
• Processing of the data packet helps in detecting bit level errors that occurs during transmission.

## Note-01:

 Total delay in sending one data packet or End to End time= Transmission delay + Propagation delay + Queuing delay + Processing delay

## Note-02:

 In optical fibre, transmission speed of data packet = 2.1 x 108 m/sec

• In optical fibre, signals travel with 70% speed of light.

70% speed of light

= 0.7 x 3 x 108 m/sec

= 2.1 x 108 m/sec

• So, consider transmission speed = 2.1 x 108 m/sec for calculations when using optical fibre.

## Note-03:

 Both queuing delay and processing delay are dependent on the state of the system.

This is because-

• If destination host is busy doing some heavy processing, then these delays will increase.
• If destination host is free, then data packets will be processed immediately and these delays will decrease.

## Note-04:

• For any particular transmission link, bandwidth and transmission speed are always constant.
• This is because they are properties of the transmission medium.

## Note-05:

 Bandwidth is always expressed in powers of 10 and data is always expressed in powers of 2.

(Remember while solving numerical problems)

## Examples-

• 1 kilo bytes = 210 bytes
• 1 kilo bits = 210 bits
• 1 Mega bytes = 220 bytes
• 1 kilo bytes per second = 103 bytes per second
• 1 kilo bits per second = 103 bits per second
• 1 Mega bytes per second = 106 bytes per second

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