## Discuss and explain the following- (i) Throughput (ii) Delay (iii) Bandwidth delay product.

Ans. (i) Throughput- The throughput- is the measurement of how fast

data can pass through a point. In other words, if we consider any point in the

transmission medium as a wall through which bits pass. throughput is the

number of bits that can pass this wall in one second. Fig. 1.14 shows the

concept of throughput.

(ii) Delay- The latency or delay defines how long it takes for an entire message to

completely arrive at the destination from the time the first

bit is sent out from the source. We can say that latency is made of four

components – propagation time, transmission time, queuing time and

processing delay.

Delay= Propagation time + Transmission time + Queuing time + Processing delay.

(a) Propagation Time – Propagation time measures the time

requred for a signal (or a bit) to travel from one point of the transmission

medium to another. The propagation time is calculated by dividing the distance

by the propagation speed. Fig. 1.15 shows the concept of propagation time.

(b) Transmission Time- In data communications we donot send just 1 bit, we send a message. The first bit may take equal to the propagation

time to reach its destination; the last bit also may take the same amount of

time. However, there is a time between the first bit leaving the sender and the

last bit arriving at the receiver. The first bit leaves earlier and arrives earlier·

the last bit leaves later and arrives later. The time required for transmission of

a message depends on the size of the message and the bandwidth of the

channel.

Transmission time = Message size/Bandwidth

(c) Queuing Time-The third component in delay is the queuing

time, the time needed for each intermediate or end device to hold the message

before it can be processed. The queuing time is not a fixed factor, it changes

with the load imposed on the network. When there is heavy traffic on the

network, the queuing time increases. An intermediate device, such as a router,

queues the arrived message and processes them one by one. If there are many

message, each message will have to wait.

(iii)Bandwidth Delay Product- The bandwidth-delay product defines the number of bits that can fill the link.

Bandwidth and delay are two

performance metrics of a link, what is very important in data communications

is the product of the two, the bandwidth-delay product. Here two hypothetical

cases are taken to describe bandwidth-delay product.

Case-1 – Let us assume that we have a link with a bandwidth of 1 bps.

We also assume that the delay of the link is 5 s. We want to see what the

bandwidth- delay product means in this case. Looking at figure, we can say

that this product I x 5 is the maximum number of bits that can fill the link.

there can be no more than 5 bits at any time on the link.

Case-II -Now assume we have a bandwidth of 4 bps. Fig. 1.17 shows

that there can be maximum 4 x 5 = 20 bits on the line. The reason is that, at

each second there are 4 bits on the line; the duration of each bit is 0.25 s.

The above two cases show that the product of bandwidth and delay is

the number of bits that can fill the link. This measurement is important if we

need to send data in bursts and wait for the acknowledgement of each burst

before’ sending the next one. To use the maximum capability of the link, we