3310 Computer Network(3)

3310 Computer Network(3)

    The main topic the course recently talking about is the peer-to-peer protocols, which contains Service Models and ARQ.A Service Model in a given layer specifies the manner in which information is transferred. There are two broad categories of service models: connection oriented and connectionless. In connection-oriented services a connection setup procedure precedes the transfer of information while connectionless do not involve a connection setup procedure.

Figure-1 Connection-oriented transfer service

Figure-2 Connectionless transfer service

    ARQ protocols includes Stop-and-Wait ARQ, Go-Back-N ARQ and Selective Repeat ARQ. The finite state machine (FSM) diagram of the above three ARQ protocols are as following:

Figure-3 Sender and Receiver Algorithms of Stop-and-Wait ARQ

              
               Figure-4 Sender and Receiver Algorithms of Go-Back-N ARQ

Figure-5 Sender and Receiver Algorithms of Selective-Repeat ARQ

    Also, we begin to learn some kinds of procedures and methods to measure the delay time, throughput and efficiency of the network transmission. For all these methods, we have certain assumptions respectively.

    In this section, I come to realize the details about how some of the network protocols work and how people gradually develop them. As we consider peer-to-peer protocols and the services the provide, they are trying to set up the interaction of two or more processes or entities through the exchange of PDUs. In order to accomplish this goal and also considering the cost and so on, different ideas show up and gradually improve the efficiency of network communication. This is also the way for us to research for developing faster, more reliable and efficiency achievements in the future. It is also the fundamental job for an engineer.

TOPIC : Comparison Between ARQ Protocols

Stop-and-Wait ARQ : works well on channels that have low propagation delay. However, the protocol becomes inefficient when the propagation delay is much greater than the time to transmit a frame.

Go-Back-N ARQ : Go-Back-N ARQ is efficient than Stop-and-Wait when the channels are long and as well the error rate is low.

Selective Repeat ARQ: It is the most efficient of the ARQ protocols as it uses the minimum number of retransmissions. However, if the window size is too small, then the transmitter may run out of sequence numbers from time to time and the efficiency will then reduced.

The relationship between transmission efficiency of ARQ Protocols among these ARQs are shown below :

                    
                     Figure-6 Transmission efficiency of ARQ Protocols

                    (where p is the frame error rate )

What’s else, the relationship between the frame size and the efficiency is like:

           Figure-7  Transmission efficiency of ARQ Protocols(frame size)

        -- Where p equals to 1.E-04.