Data Link Layer Protocols - Networking - Part-I

📌Highlights: In this post we cover data link layer protocols such as Unrestricted Simplex Protocol  OR Unrestricted Utopia protocol, Stop and wait protocol, Stop and wait protocol for noisy channel and also cover problems arise with these protocols, We also discuss about there solution. We explain with the help of diagrams so that it is easily understandable.

Protocol #1 : Unrestricted Simplex Protocol  OR Unrestricted Utopia protocol.

Features:

1. Data traffic is simplex.

2. Channel is Noise free.

3. Both sander and receiver have infinite amount of buffer.

4. Both sander and receiver have infinite processing speed.

Protocol #2: Stop and wait protocol

Features:

1. Data traffic is simplex.

2. Channel is noise free.

3. Both sender and receiver have one buffer each.

4. Receiver has finite and limited processing speed.

In this type of protocol there is a limited number of buffer. So sender can send data or frame one by one  that mean  S can not send data until R not receive it. Actually this data is stored in buffer at receiver end there is only one buffer.  So when the buffer is empty than S will send the next frame, But the problem is that how S will judge that the buffer is empty. So for this R will send acknowledgement to S and A can understand that buffer is empty and send the next frame.

Note: In this protocol flow control is needed.

Protocol #3: Stop and wait protocol for noisy channel.

Features:

1. Data traffic is simplex

2. Channel is noisy.

3. Sander and receiver have one buffer each.

4. Receiver have finite and limited processing speed.

In this type of protocol one timer is used. When first bit of frame is send than the timer is ON Suppose in CASE-I sender S send. S0 frame to R and timer will ON and the sender will not send the next frame to R until S will not get the Acknowledgement(ACK) from R. If ACK is received by S with in time limit than S will send the next frame S1.

But if time is out and S will not get the ACK with in time limit then it will re transmitted the same frame as in CASE-II.

Note S will send ACK when the buffer at receiving end R will send the frame to corresponding network layer.

If any error will occur in frame that is send by sender than receiver will discard that frame and no ACK will send.

PROBLEM WILL ARISE IN THIS PROTOCOL ARE:

1. DELAYED ACKNOWLEDGEMENT: 

When sender send one frame, If the frame is currently send and stored in the receiving buffer than timer will ON. But if processor at receiving end is busy and delay in processing and time is out ( that mean R will not send ACK with in time limit) than duplicate transmission of same frame will occur.

here S0 will again transmit by sender S due delay in acknowledgement (ACK).

2. ACKNOWLEDGEMENT IS LOST OR DAMAGED:

In this case if error will occur in ACK than duplicate frame will occur.

here if the ACK is lost then S0 will again transmit the same frame such as S0.

SOLUTION OF ABOVE PROBLEM:

For solving the above problems we must follow some method such as :

The rule is that one extra bit is added in frame which is sequence bit ( which is 0 or 1) sender send alternative sequence number of 0 and 1 such as 0,1,0,1,0,1,0,1 ..... and receiver will also wait for such sequence of bit is 0,1,0,1,0,1,0,1 ....

If time out then again duplicate frame will send by sender then discard that frame and ACK will send.

To identify the frame we use the sequence bit.

FAILURE OCCUR:

When S send frame S0 with 0 sequence bit and timer is ON.

If the ACK is delayed and time is out than S will again send S0 frame with 0 sequence bit. But R is waiting for 1 sequence bit then discard this S0 frame.

Description and Solution of above problem:

We understand by some conditions such as suppose first S will send S0 frame with 0 sequence bit if timeout occur and S will not get acknowledgement of S0 then S will again send S0 frame with 0 sequence bit but here buffer is waiting for sequence bit -1 and getting bit is 0 so due to duplicate bit buffer will discard it. After some time S will get acknowledgement to S0 then S will send S1 frame with sequence bit -1 but unfortunately now some error will occur in frame so R will discard that frame Simultaneously next frames are also discard due to disturbance in sequence bit. 

So for overcoming this type of problem we also add sequence bit with acknowledgement. 

📌Now this post contain three protocols of Data link layer. I hope is it easy to understand to all my friends. We will meet in next post with next protocols of Data link layer.