Data-link Control & Protocols

• Data-link layer is responsible for implementation of point-to-point flow and error control mechanism.

Flow Control

• When a data frame (Layer-2 data) is sent from one host to another over a single medium, it is required that the sender and receiver should work at the same speed. 
• That is, sender sends at a speed on which the receiver can process and accept the data. What if the speed (hardware/software) of the sender or receiver differs? 
• If sender is sending too fast the receiver may be overloaded, (swamped) and data may be lost.

Two types of mechanisms can be deployed to control the flow

Stop and Wait

• This flow control mechanism forces the sender after transmitting a data frame to stop and wait until the acknowledgement of the data-frame sent is received.

Sliding Window

• In this flow control mechanism, both sender and receiver agree on the number of data-frames after which the acknowledgement should be sent. 
• As we learnt, stop and wait flow control mechanism wastes resources, this protocol tries to make use of underlying resources as much as possible.

Error Control

• When data-frame is transmitted, there is a probability that data-frame may be lost in the transit or it is received corrupted. 
• In both cases, the receiver does not receive the correct data-frame and sender does not know anything about any loss.
• In such case, both sender and receiver are equipped with some protocols which helps them to detect transit errors such as loss of data-frame. 
• Hence, either the sender retransmits the data-frame or the receiver may request to resend the previous data-frame.

Requirements for error control mechanism

Error detection  

• The sender and receiver, either both or any, must ascertain that there is some error in the transit.

Positive ACK  

• When the receiver receives a correct frame, it should acknowledge it.

Negative ACK  

• When the receiver receives a damaged frame or a duplicate frame, it sends a NACK back to the sender and the sender must retransmit the correct frame.

Retransmission

• The sender maintains a clock and sets a timeout period. If an acknowledgement of a data-frame previously transmitted does not arrive before the timeout the sender retransmits the frame, thinking that the frame or it’s acknowledgement is lost in transit.
• There are three types of techniques available which Data-link layer may deploy to control the errors by Automatic Repeat Requests (ARQ).

Stop-and-wait ARQ

The following transition may occur in Stop-and-Wait ARQ:

• The sender maintains a timeout counter.
• When a frame is sent, the sender starts the timeout counter.
• If acknowledgement of frame comes in time, the sender transmits the next frame in queue.
• If acknowledgement does not come in time, the sender assumes that either the frame or its acknowledgement is lost in transit. Sender retransmits the frame and starts the timeout counter.
• If a negative acknowledgement is received, the sender retransmits the frame.

Go-Back-N ARQ

• Stop and wait ARQ mechanism does not utilize the resources at their best.When the acknowledgement is received, the sender sits idle and does nothing. 
• In Go-Back-N ARQ method, both sender and receiver maintain a window.

• The sending-window size enables the sender to send multiple frames without receiving the acknowledgement of the previous ones. 
• The receiving-window enables the receiver to receive multiple frames and acknowledge them. The receiver keeps track of incoming frame’s sequence number.
• When the sender sends all the frames in window, it checks up to what sequence number it has received positive acknowledgement. 
• If all frames are positively acknowledged, the sender sends next set of frames. If sender finds that it has received NACK or has not receive any ACK for a particular frame, it retransmits all the frames after which it does not receive any positive ACK.

Selective Repeat ARQ

• In Go-back-N ARQ, it is assumed that the receiver does not have any buffer space for its window size and has to process each frame as it comes. 
• This enforces the sender to retransmit all the frames which are not acknowledged.

• In Selective-Repeat ARQ, the receiver while keeping track of sequence numbers, buffers the frames in memory and sends NACK for only frame which is missing or damaged.

• The sender in this case, sends only packet for which NACK is received.