# Flow Control Methods in Computer Networks

Till now you must have gotten a fair idea of what is computer networks and why we study it. The concept of classes must be clear to you. You should be able to Subnet and supernet a given network using both classful addressing and Classless Interdomain Routing . Now, we will study flow control methods in computer networks. Have you ever thought why do we need it?

Flow control methods are techniques that determine the data flow between sender and receiver. It determines how much data should be send by the sender before receiving acknowledgement. It makes sender to wait for some sort of an acknowledgement before continuing to send some more data.

There are different flow control methods – Stop and Wait ARQ, Go Back N and Selective Repeat methods.

We will study each method separately. But, before that we need to get familiar with some terms to understand the topic more clearly.
So, we will start with the understanding of various delays in computer networks in this tutorial.

## Delays in Computer Networks

There are different types of delays in computer networks. For example,
Transmission delay, propagation delay, queuing delay, processing delay etc.
Now, we will look into each delay one by one.

### 1. Transmission Delay (TT):

Time required to place the data on the transmitting medium is called transmission delay.

We can calculate transmission delay using below formula –
TT = ${\frac {length of the data to be sent}{bandwidth}}$ Transmission delays

– Example 1.
Calculate transmission delay if data size is 2kbits and bandwidth is 10Mbps.
Solutions –
TT = ${\frac {length of the data to be sent}{bandwidth}}$
So,
TT = ${\frac {2*10^3 }{ 10 * 10^6 } = 200}$ microseconds

### 2. Propagataion Delay (PT):

Time required for data to travel from one host to another is called propagation delay.

We can calculate propagation delay using below formula –
PT = ${ \frac {distance of the transmission media}{velocity of data} }$ Propagation delays

– Example 1.
If length of transmitting media is 2 km and velocity is 2 *108 m/s. Then, calculate propagation delays.

Solutions –
PT = ${ \frac {distance of the transmission media}{velocity of data} }$
So,
PT= ${ \frac {2*10^3 }{2 * 10^8 } } = 10$ microseconds

### 3. Queuing Delay

The time spent by the packet in the queue waiting before being processed is called queuing delay. This is to be assumed as 0.

### 4. Processing Delay

Time taken to process a packet is processing delay. This is also negligible and thus assumed to be 0.

## Important points to remember

1. Total time taken to send data from one host to another =
Transmission time of data
+ Acknowledgement time of transmission
+ Propagation time of data
+ Acknowledgement time of propagation

Also,
– Propagation time of data = Propagation time of acknowledgement
– Acknowledgement time of transmission is negligible.

Thus, total time of transfer = TT+ (2 * PT)

– % Link Utilization = ${ \frac {TT * 100}{TT + 2 * PT} }$
Where TT = Transmission delay,
PT = Propagation delay

Example 1.
If % Link Utilisation = 50%. Find the relation between TT and 2PT.

Solution –
– % Link Utilization = ${ \frac {TT * 100}{TT + 2 * PT} }$

So,
– 50 = ${ \frac {TT * 100}{TT + 2 * PT} }$
On solving, we get TT=2PT

3. Throughput
Throughput is a measure of how many units of information a system can process in a given amount of time.

We can calculate throughput using below formula –

Throughput = $\frac {data size}{total time}$

4. Bandwidth
Maximum capacity of the transmitting media is called bandwidth.

Note:
– Transmission time is characteristic of computer.
– Propagation time is characteristic of transmitting media.

Till Now, we have covered the fundamentals terms and formula for flow control. Now, we will solve questions using above formula –

## Practice Questions:

###### Q.1. Find the throughput and link utilization.
Given below data,
- Length = 200m
- Velocity = 2 * 10^8 m/s
- Data Size = 200 bits
- Bandwidth = 10Mbps

###### Q.2. Calculate one bit delay.
Given below data,
- Bandwidth = 10Mbps

###### Q.3. Calculate 1-bit delay in meters of cable?
Given below data,
- Length = 200m
- Velocity = 2 * 108m/s
- Data Size = 200 bits
- Bandwidth = 10Mbps

###### Q.4. Calculate no of bits in RTT?
Given below data,
- Round trip time (= 2TP) = 50 microsecond
- Bandwidth = 100Mbps


Till now we have covered the delays, some terms and formulas used in flow control methods. In this next tutorial, we will learn about one of flow control methods – Stop and Wait ARQ flow control technique .
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