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Now we will discuss how the data link layer can combine framing, flow control, and error control to achieve the delivery of data from one node to another. The protocols are normally implemented in software by using one of the common programming languages. To make our discussions language-free, we have written in pseudocode a version of each protocol that concentrates mostly on the procedure instead of delving into the details of language rules.

So for the noiseless Chanels, the protocols we use are the following:

  • Stop – N – Wait Protocol
  •  Sliding window Protocol


If data frames arrive at the receiver site faster than they can be processed, the frames must be stored until their use. Normally, the receiver does not have enough storage space, especially if it is receiving data from many sources. This may result in either discarding of frames or denial of service. To prevent the receiver from becoming overwhelmed with the frames, we somehow need to tell the sender to slow down. There must be feedback from the receiver to sender. For this, the protocol we will discuss is the Stop – N – wait protocol because the sender sends one frame, stops until it receives the confirmation from the receiver (okay to go ahead), and then sends the next frame. We still have a unidirectional communication for data frames, but auxiliary ACK frames (simple tokens of acknowledgement) travel from the other direction.

Design and Implementation

Assumption: Error free communication channel The sender in this protocol simply retrieves a packet from the network layer, copies it into a frame, and then transmits it. After transmission, the sender busy waits until an acknowledgement is received from the receiver, then the loop starts over again.



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