Flow & Error Control

Sender will send data packets to Receiver and Receiver will reply by sending Acknowledgement(Ack). Packets are lost during transmission due to errors . To overcome this errors there are Flow & Error control mechanism .

Flow and Error mechanism :-

1. Stop and Wait Protocol
2. Sliding Window Protocol :-

• 1. Go Back N Protocol
• 2. Selective Repeat Protocol

Stop and Wait :-

Sender will send one data packet at a time and Receiver will reply with sending acknowledgment after receiving & consuming of data packet . No other data packet can be send until the Receiver sends back a reply .

There are some problems in using Stop & Wait Protocol :-

• Lost Data :- Both sender and receiver will waits for infinite amount time .
• Lost Acknowledgement :- Sender waits for Acknowledgement for infinite amount of time .

For overcome this problem there are Stop & Wait ARQ(Automatic Repeat Request):-

1. It performs both error control and flow control .
2. It is a combination of four concepts :- Stop & Wait , Timeout [Loss Data], Sequence No. for data [Loss Ack], Sequence No. for Ack[delayed Ack] .

• Timeout:- Sender waits for few minutes after sending data , if the Acknowledgement will not come from Receiver side . Sender Thought that the data will lost . It will send same data packet and Receiver will send Acknowledgement .
• Sequence No. (data):- Sender will send data packet and Receiver will send Acknowledgement will lost then the sender will wait for a timeout and again send same data , because of sequence no. for data is attached with the data packet . Sender will check the sequence no. of data packet which sender will send .
• Delayed Acknowledgement:- This is resolved same as Sequence No. (data) by giving sequence no. to acknowledgement .

Stop & Wait Protocol

Advantages :-

• Noise Channels .
• Flow & Error Control mechanism .
• Timer Implementation .

Disadvantages :-

• Efficiency is very less .
• Only one frame is send at a time .

2. Go Back N :-

1. Several data packets are send by sender before receiving acknowledgement by receiver .
2. Receiver sends Negative acknowledgement along with data packet sequence no.
3. After Negative acknowledgement receiver discarded all the data packets .
4. If the data packets is lost then just checks the sequence of each data packet .
5. If sender does not receive acknowledgement within a time , it re-sends all the data packets .

Go Back N Protocol

3. Selective Repeat :-

1. It re-transmits only that data packet which is damaged and lost .
2. The receiver is capable of sorting the data packets in proper sequence .
3. The sender must be capable of searching the data packets for which Negative acknowledgement has been send by receiver .
4. It requires less window size as compared to Go Back N protocol .

Selective Repeat Protocol