Flow Control | Stop and Wait Protocol

Flow Control in Computer Networks-

 

In computer networks, flow control is defined as-

 

A set of procedures which are used for restricting the amount of data that a sender can send to the receiver.

 

Flow Control Protocols-

 

There are various flow control protocols which are classified as-

 

 

In this article, we will discuss about stop and wait protocol.

 

Stop and Wait Protocol-

 

Stop and Wait Protocol is the simplest flow control protocol.

 

It works under the following assumptions-

• Communication channel is perfect.
• No error occurs during transmission.

 

Working-

 

The working of a stop and wait protocol may be explained as-

• Sender sends a data packet to the receiver.
• Sender stops and waits for the acknowledgement for the sent packet from the receiver.
• Receiver receives and processes the data packet.
• Receiver sends an acknowledgement to the sender.
• After receiving the acknowledgement, sender sends the next data packet to the receiver.

 

These steps are illustrated below-

 

 

Analysis-

 

Now, let us analyze in depth how the transmission is actually carried out-

 

• Sender puts the data packet on the transmission link.
• Data packet propagates towards the receiver’s end.
• Data packet reaches the receiver and waits in its buffer.
• Receiver processes the data packet.
• Receiver puts the acknowledgement on the transmission link.
• Acknowledgement propagates towards the sender’s end.
• Acknowledgement reaches the sender and waits in its buffer.
• Sender processes the acknowledgement.

 

These steps are illustrated below-

 

 

Also Read- Delays in Computer Networks

 

Total Time-

 

Total time taken in sending one data packet = (Transmission delay + Propagation delay + Queuing delay + Processing delay)packet + (Transmission delay + Propagation delay + Queuing delay + Processing delay)ACK

 

Assume-

• Queuing delay and processing delay to be zero at both sender and receiver side.
• Transmission time for the acknowledgement to be zero since it’s size is very small.

 

Under the above assumptions.

 

Total time taken in sending one data packet = (Transmission delay + Propagation delay)packet + (Propagation delay)ACK

 

We know,

• Propagation delay depends on the distance and speed.
• So, it would be same for both data packet and acknowledgement.

 

So, we have-

 

Total time taken in sending one data packet = (Transmission delay)packet + 2 x Propagation delay

 

Efficiency-

 

Efficiency of any flow control control protocol is given by-

 

Efficiency (η) = Useful Time / Total Time

 

where-

• Useful time = Transmission delay of data packet = (Transmission delay)_packet
• Useless time = Time for which sender is forced to wait and do nothing = 2 x Propagation delay
• Total time = Useful time + Useless time

 

Thus,

 

 

Factors Affecting Efficiency-

 

We know,

Efficiency (η)

= (Transmission delay)_packet / { (Transmission delay)_packet + 2 x Propagation delay }

 

Dividing numerator and denominator by  (Transmission delay)_packet, we get-

 

 

From here, we can observe-

• Efficiency (η) ∝ 1 / Distance between sender and receiver
• Efficiency (η) ∝ 1 / Bandwidth
• Efficiency (η) ∝ Transmission speed
• Efficiency (η) ∝ Length of data packet

 

Throughput-

 

• Number of bits that can be sent through the channel per second is called as its throughput.

 

 

Round Trip Time-

 

Round Trip Time = 2 x Propagation delay

 

Advantages-

 

The advantages of stop and wait protocol are-

• It is very simple to implement.
• The incoming packet from receiver is always an acknowledgement.

 

Limitations-

 

The limitations of stop and wait protocol are-

 

Point-01:

 

It is extremely inefficient because-

• It makes the transmission process extremely slow.
• It does not use the bandwidth entirely as each single packet and acknowledgement uses the entire time to traverse the link.

 

Point-02:

 

If the data packet sent by the sender gets lost, then-

• Sender will keep waiting for the acknowledgement for infinite time.
• Receiver will keep waiting for the data packet for infinite time.

 

Point-03:

 

If acknowledgement sent by the receiver gets lost, then-

• Sender will keep waiting for the acknowledgement for infinite time.
• Receiver will keep waiting for another data packet for infinite time.

 

Important Notes-

 

Note-01:

 

Efficiency may also be referred by the following names-

• Line Utilization
• Link Utilization
• Sender Utilization
• Utilization of Sender

 

Note-02:

 

Throughput may also be referred by the following names-

• Bandwidth Utilization
• Effective Bandwidth
• Maximum data rate possible
• Maximum achievable throughput

 

Note-03:

 

Stop and Wait protocol performs better for LANs than WANs.

This is because-

• Efficiency of the protocol is inversely proportional to the distance between sender and receiver.
• So, the protocol performs better where the distance between sender and receiver is less.
• The distance is less in LANs as compared to WANs.

 

To gain better understanding about Stop and Wait Protocol,

Watch this Video Lecture

 

Next Article- Stop and Wait ARQ

 

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