Flow Control in Data Link Layer

Flow control is design issue at Data Link Layer. It is a technique that generally observes the proper flow of data from sender to receiver. It is very essential because it is possible for sender to transmit data or information at very fast rate and hence receiver can receive this information and process it. This can happen only if receiver has very high load of traffic as compared to sender, or if receiver has power of processing less as compared to sender. Flow control is basically a technique that gives permission to two of stations that are working and processing at different speeds to just communicate with one another. Flow control in Data Link Layer simply restricts and coordinates number of frames or amount of data sender can send just before it waits for an acknowledgement from receiver. Flow control is actually set of procedures that explains sender about how much data or frames it can transfer or transmit before data overwhelms receiver. The receiving device also contains only limited amount of speed and memory to store data. This is why receiving device should be able to tell or inform the sender about stopping the transmission or transferring of data on temporary basis before it reaches limit. It also needs buffer, large block of memory for just storing data or frames until they are processed.

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flow control can also be understand as a speed matching mechanism for two stations.

Approaches to Flow Control : Flow Control is classified into two categories:  

• Feedback ' based Flow Control : In this control technique, sender simply transmits data or information or frame to receiver, then receiver transmits data back to sender and also allows sender to transmit more amount of data or tell sender about how receiver is processing or doing. This simply means that sender transmits data or frames after it has received acknowledgements from user.
• Rate ' based Flow Control : In this control technique, usually when sender sends or transfer data at faster speed to receiver and receiver is not being able to receive data at the speed, then mechanism known as built-in mechanism in protocol will just limit or restricts overall rate at which data or information is being transferred or transmitted by sender without any feedback or acknowledgement from receiver.

Techniques of Flow Control in Data Link Layer : There are basically two types of techniques being developed to control the flow of data

1. Stop-and-Wait Flow Control : This method is the easiest and simplest form of flow control. In this method, basically message or data is broken down into various multiple frames, and then receiver indicates its readiness to receive frame of data. When acknowledgement is received, then only sender will send or transfer the next frame. This process is continued until sender transmits EOT (End of Transmission) frame. In this method, only one of frames can be in transmission at a time. It leads to inefficiency i.e. less productivity if propagation delay is very much longer than the transmission delay and Ultimately  In this method sender sent single frame and receiver  take one frame at a time and sent acknowledgement(which is next frame number only) for new frame. 

 Advantages '

• This method is very easiest and simple and each of the frames is checked and acknowledged well.
• This method is also very accurate.

Disadvantages '

• This method is fairly slow.
• In this, only one packet or frame can be sent at a time.
• It is very inefficient and makes the transmission process very slow.

2. Sliding Window Flow Control : This method is required where reliable in-order delivery of packets or frames is very much needed like in data link layer. It is point to point protocol that assumes that none of the other entity tries to communicate until current data or frame transfer gets completed. In this method, sender transmits or sends various frames or packets before receiving any acknowledgement. In this method, both the sender and receiver agree upon total number of data frames after which acknowledgement is needed to be transmitted. Data Link Layer requires and uses this method that simply allows sender to have more than one unacknowledged packet 'in-flight' at a time. This increases and improves network throughput. and Ultimately  In this method sender sent multiple frame but  receiver  take one by one and  after completing one frame acknowledge(which is next frame number only) for new frame. 

Advantages '

• It performs much better than stop-and-wait flow control.
• This method increases efficiency.
• Multiples frames can be sent one after another.

Disadvantages '

• The main issue is complexity at the sender and receiver due to the transferring of multiple frames.
• The receiver might receive data frames or packets out the sequence.

Flow control, in technical terms, is a methodology that allows two stations operating at different rates to interact with one another. For example, when a high-speed station provides data, flow control gets involved to ensure that the high speed does not affect the slow receiver, resulting in efficient data transmission. 

When a sender delivers data to a receiver, the receiver gets the data in a network. The operation may appear straightforward to us, but the systems may encounter a significant hurdle if the pace at which the incoming data arrives is incompatible with the receiver's speed. This anomaly will result in substantial data loss.

In this scenario, flow control saves the day by controlling the speed and ensuring that the data is supplied at a rate that the receiver can understand and receive without difficulty.

Flow control in the data link layer limits the number of frames a sender can send before waiting for a response from the receiver.

What Does Flow Control do?

As previously stated, flow control is a mechanism that ensures that the receiver does not experience data loss due to speed disparities between them and the sender.

It is a mechanism that allows two stations that are operating and processing at different speeds to communicate with one another simply. 

Flow control at the Data Link Layer essentially limits and coordinates the number of frames or quantity of data that a transmitter can send before waiting for an acknowledgement from the receiver. It is a series of procedures instructing the transmitter on how much data or frames it can move or broadcast before data overloads the receiver.

However, managing the speed is not only the responsibility of flow control. The receiver's equipment must have enough storage space to hold the incoming data when receiving data.

However, the sender is unlikely to be aware of the receiver's storage space. Their responsibility is to ensure that the requested data is sent. As a result, when data of greater size arrives at the receiver, the receiver suffers a significant loss. 

It ensures that this scenario does not occur and that data loss is avoided.

The receiving device likewise has limited speed and memory for storing data. This is why the receiving device should be able to alert or inform the sender about temporarily pausing data transmission or transfer before it approaches a limit. It also requires a buffer, which is a massive block of memory used to store data or frames until they are processed simply. 

Approaches of Flow Control 

There are generally two types of approaches when it comes to flow control. They are as follows:

Feedback'Based Flow Control 

In feedback-based flow control, the sender station delivers data or frame information to the receiving station. This informs the receiving station of the size and speed with which the data will be transmitted.

The receiver transmits data back to the sender after receiving it. The data transmitted by the receiver to the sender includes information about how the receiver processes data and whether it is ready to accept a specific amount of data.

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Rate-Based Flow Control

The rate-based flow control uses an inbuilt mechanism to alert the sender.

When the sender delivers data to the receiver, and the receiver is unable to receive it at that high rate, the built-in mechanism kicks in and limits or restricts the overall rate at which the data is arriving. Unlike the feedback-based technique, this one does not necessitate any specific acknowledgement from the sender.

What is Network Flow Control?

The flow control in the network layer is the same as the flow control in the data link layer, except that the flow control in the network layer is used to monitor data transit among multiple networks.

The objective of this process is to control the rate of data transfer between the two nodes. If the speed cannot be managed, the data transmission process becomes extremely slow, and the recipient experiences speed constraints when receiving the data. 

We employ flow control strategies for data transfer to prevent overflow at the receiver's end.

Techniques of Flow Control

To control the flow of data, two techniques are worth knowing to understand flow control more naturally: 

Stop-and-Wait Flow Control 

The stop-and-wait flow control transmits a request and then waits for an acknowledgement from the receiver.

Basically, the message or information is broken down into multiple frames in this approach, and the receiver indicates their status to receive each frame of data. The sender will only send the data to the receiver after getting the acknowledgement.

This cycle is repeated until the sender sends an EOT (End of Transmission) frame. Only one frame can be transmitted at a time using this method. If the propagation delay is substantially longer than the transmission delay, it results in inefficiency or lower production.

Transmission Time(Td)

Transmission delay (Td) is the length of time it takes for the sender to deliver all of the bits in a frame onto the cable. This is obtained by multiplying the data size by the bandwidth of the data transmission channel.

Td = D / B

Where,

D is the data size 

B is the bandwidth of the channel

Propagation Delay(Tp) 

Propagation delay is the time it takes for the last bit in a frame to go from one side to the other. It's computed by multiplying the wave propagation speed by the sender's and receiver's distances.

Tp = d / s ;

Where,

d is the distance between sender and receiver

s is the wave propagation speed.

The propagation delay for sending the data frame and the acknowledgement frame is the same as the distance, and the speed for both frames will remain constant. Hence, the total time needed to transmit a frame is:

Total time= Td (Transmission Delay) + Tp (Propagation Delay for data frame) + Tp (Propagation Delay for acknowledgment frame)

Efficiency = Useful Time/ Total Time

η=Td / (Td+2Tp)

Pros:

• This procedure is effortless, and each of the frames is thoroughly checked and acknowledged.
• It can also filter out noise in busy channels.
• This procedure is also quite precise.

Cons:

• This procedure is somewhat slow.
• Only one packet or frame can be sent at a time in this case.
• It is inefficient and slows down the transmission process.

Sliding Window Flow Control 

Sliding Window Flow Control is a point-to-point protocol that ensures no other entity attempts to intervene until the present data or frame transmission is complete. Before receiving any acknowledgement, the sender transmits or sends various frames or packets using this method.

In this approach, the sender and receiver first agree on the total amount of data frames to be transferred, and an acknowledgement is delivered to allow the sending process to begin. 

The Data Link Layer requires and uses this approach, which allows the sender to have more than one unacknowledged packet 'in-flight' simultaneously. This enhances and increases network throughput.

Pros:

• It outperforms stop-and-wait flow control significantly.
• This strategy boosts efficiency.
• Multiple frames can be sent in succession.

Cons:

• The fundamental difficulty is the complexity at the sender and receiver due to the transfer of many frames.
• Out of the sequence, the receiver may get data frames or packets.

Conclusion

Developers must understand the flow control mechanism to understand how data is transferred across networks. We hope you have a better knowledge of how it works and its significance in data transfer and transmission.

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