Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE) PDF Download

FlowControl

It refers to a set of procedures used to restrict the amount of data flow between sending and receiving stations. It tells the sender how much data it can transmit before it must wait for an acknowledgement from the receiver.
There are two methods are used. They are,
1.     stop and wait
2.     sliding window

Stop And Wait:
In this method the sender waits for acknowledgment after every frame it sends. Only after an acknowledgment has been received, then the sender sends the next frame.
Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

The advantage is simplicity. The disadvantage is inefficiency.

Sliding Window:
In this method, the sender can transmit several frames before needing an acknowledgment. The receiver acknowledges only some of the frames, using a single ACK to confirm the receipt of multiple data frames.

The sliding window refers to imaginary boxes at both the sender and receiver. This window provides the upper limit on the number of frames that can be transmitted before requiring an acknowledgement. To identify each frame the sliding window scheme introduces the sequence number. The frames are numbered as 0 to n-1. And the size of the window is n-1. Here the size of the window is 7 and the frames are numbered as 0,1,2,3,4,5,6,7.

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)S

Ender Window:
At the beginning the sender‟s window contains n-1 frames. As frames are sent out the left boundary of the window moves inward, shrinking the size of the window. Once an ACK receives the window expands at the right side boundary to allow in a number of new frames equal to number of frames acknowledged by that ACK.

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

Error Control
Error control is implemented in such a way that every time an error is detected, a negative acknowledgement is returned and the specified frame is retransmitted. This process is called automatic repeat request (ARQ).

The error control is implemented with the flow control mechanism. So there are two types in error control. They are,
1.     stop and wait ARQ
2.     sliding window ARQ

Stop And Wait ARQ:
It is a form of stop and wait flow control, extended to include retransmission of data in case of lost or damaged frames.

Damaged Frame:
When a frame is discovered by the receiver to contain an error, it returns a NAK frame and the sender retransmits the last frame.

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

Lost Data Frame:
The sender is equipped with a timer that starts every time a data frame is transmitted. If the frame lost in transmission the receiver can never acknowledge it. The sending device waits for an ACK or NAK frame until its timer goes off, then it tries again. It retransmits the last data frame

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

 

Lost Acknowledgement
The data frame was received by the receiver but the acknowledgement was lost in transmission. The sender waits until the timer goes off, then it retransmits the data frame. The receiver gets a duplicated copy of the data frame. So it knows the acknowledgement was lost so it discards the second copy.

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

 

Sliding Window ARQ
It is used to send multiple frames per time. The number of frame is according to the window size. The sliding window is an imaginary box which is reside on both sender and receiver side.
It has two types. They are,
1.     go-back-n ARQ
2.     selective reject ARQ

Go-Back-N ARQ:
In this method, if one frame is lost or damaged, all frames sent since the last frame acknowledged or retransmitted.

Damaged Frame:

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

SELECTIVE REPEAT ARQ
Selective repeat ARQ re transmits only the damaged or lost frames instead of sending multiple frames. The selective transmission increases the efficiency of transmission and is more suitable for noisy link. The receiver should have sorting mechanism.

DAMAGED FRAME:
Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE)

The document Flow Control Link Layer | Computer Networks - Computer Science Engineering (CSE) is a part of the Computer Science Engineering (CSE) Course Computer Networks.
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FAQs on Flow Control Link Layer - Computer Networks - Computer Science Engineering (CSE)

1. What is flow control in the link layer?
Ans. Flow control in the link layer refers to the techniques used to manage the rate of data transmission between two devices in a computer network. It ensures that the sender does not overwhelm the receiver with data and prevents data loss or congestion.
2. How does flow control work in the link layer?
Ans. Flow control in the link layer typically involves the use of acknowledgments and windowing. The sender sends a certain number of data packets and waits for acknowledgments from the receiver. The receiver maintains a buffer to store incoming data and sends acknowledgments to inform the sender about the successful receipt of data. This two-way communication helps regulate the flow of data.
3. What are the advantages of flow control in the link layer?
Ans. Flow control in the link layer provides several advantages. It helps prevent data loss by ensuring that the receiver can handle the incoming data at its own pace. It also prevents congestion in the network by regulating the rate of data transmission. Flow control improves the overall efficiency of the network by optimizing the data flow between devices.
4. What are the different flow control techniques used in the link layer?
Ans. There are several flow control techniques used in the link layer, including stop-and-wait, sliding window, and selective repeat. Stop-and-wait flow control involves sending a single data packet and waiting for an acknowledgment before sending the next packet. Sliding window flow control allows multiple packets to be sent without waiting for individual acknowledgments. Selective repeat flow control retransmits only the lost or corrupted packets, reducing unnecessary retransmissions.
5. How does flow control relate to error detection and correction in the link layer?
Ans. Flow control and error detection/correction are closely related in the link layer. Flow control techniques often rely on error detection mechanisms, such as checksums or cyclic redundancy checks (CRC), to ensure the integrity of the received data. If errors are detected, the receiver can request retransmission of the corrupted packets, thereby maintaining the flow control and preventing data loss.
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