Data Link Control Protocols

• Node to code communications Dynamic Graphics Group/Dynamic Graphics Group/Getty Images

  Data link control includes framing, flow and error control, and software-implemented protocols functions. To implement data link control, you need protocols -- rules implemented in software -- to provide a smooth and reliable transmission of frames between nodes involved in the data exchange at the data link layer. There are five protocols designs, two for noiseless channels and three for noisy channels. Designs for noiseless channels include the Simplest and Stop-and-Wait protocols and for noisy channels include the Stop-and-Wait ARQ, Go-Back-N ARQ and the Selective Repeat ARQ protocols.

Simplest

• A Simplest protocol is a unidirectional protocol with no flow or error control, where data frames are traveling in only one direction, from the sender to the receiver. In the Simplest protocol, the sender sends its frames one after another with no regard to the receiver. The data link layer of the receiver then removes the header from the frame and hands the data packet to its network layer without hesitation, where it is accepted.

Stop-and-Wait

• The Stop-and-Wait protocol is a unidirectional protocol, which has no error control. In the Stop-and-Wait protocol, the sender sends one frame, stops until it receives confirmation from the receiver and then sends the next frame. The feedback to the sender prevents the receiver from becoming overwhelmed with frames and gives the sender an indication that a slowdown is necessary.

Stop-and-Wait ARQ

• Both corrupted and lost frames are re-sent when using the Stop-and-Wait ARQ -- or Automatic Repeat Request -- protocol. This protocol includes a simple error control mechanism to detect and correct corrupted frames. When a frame is transmitted, it may be correct, a duplicate or a frame out of order; when it arrives at the receiver's site, it is checked and if it is corrupted, it is silently discarded. When a frame is out of order, it usually means that it is lost or duplicated. Lost frames are more difficult to handle than corrupted frames.

Go-Back-N ARQ

• A Go-Back-N ARQ -- or Automatic Repeat Request -- protocol offers an efficiency improvement. To achieve this goal, several frames are sent before receiving acknowledgments. A copy of the frame is kept until the acknowledgments are received. If the frame is not acknowledged, the sender will go back and resend.

Selective Repeat ARQ

• Although the processing of the Selective Repeat ARQ -- or Automatic Repeat Request -- protocol at the receiver is more complex, it is more efficient for noisy links. In this protocol, only the damaged frames are resent and not N frames, which avoids unnecessary transmissions that use up the bandwidth.