Understand Data Transmission

      There are so many people using Internet in the world, and a large amount of information is produced every second. Thus, transmitting the data containing the information is a significant task for the network. These few weeks, I have learned about the how to transmit the data correctly in the classes of the course Computer Network.  

      Automatic repeat-request (ARQ) is one of the protocols that correct the errors in Data Link Layer in OSI model. It plays an important role in data transmission. There are many kinds of ARQ, such as Stop-and-Wait ARQ, Go-Back-N ARQ and Selective-Repeat ARQ.

Fig.1 Stop-and-Wait ARQ

      For Stop-and-Wait ARQ, the sender will not send the next frame until it receives the Acknowledge (ACK) of the frame just sent. 







Fig.2 Go-Back-N ARQ



 

      For Go-Back-N ARQ, the sender will keep on sending the frame unless it finds that the ACK of the Nth frame before the current frame has not been received, then it will go back to that frame to retransmit from it. 



Fig.3 Selective-Repeat ARQ





      For Selective-Repeat ARQ, the sender performs similarly to Go-Back-N ARQ except that when it receives an ACK that is not wanted currently, it will only retransmit the frame related to the wanted ACK and then go on transmitting. 

 

     Besides ARQ, Flow Control acts importantly, as the sender, the network and the receiver do not perform of the same speed, if there is no Flow Control the data easily get lost, which can result in serious problems.



Fig.4 On-Off Flow Control



      For On-Off Flow Control, the receiver will send an Off to tell the sender to stop sending data when it finds that its buffer is going to be filled up. After the data in the receiver's buffer have been transmitted to the upper layer, and the buffer is available again, the receiver will send an On to tell the sender to send the following data.



Fig.5 Sliding Window Flow Control

 

      For Sliding Window Flow Control, •receiver sends an ACK only after the corresponding frame has been removed from the receiver buffer, and thus the sender will not send too many data once.

 

      There is much other knowledge about how the data could be transmitted correctly, such as Bit Buffing, Timing Recovery, Packet Multiplexing, etc. It's really good to know about these, as they have raised my more interest to learn Computer Network!