FLOW AND ERROR CONTROL

Flow Control 

Flow control coordinates the amount of data that can be sent before receiving an acknowledgment and is one of the most important duties of the data link layer. In most protocols, flow control is a set of procedures that tells the sender how much data it can transmit before it must wait for an acknowledgment from the receiver

Error Control.

Error control is both error detection and error correction. It allows the receiver to inform the sender of any frames lost or damaged in transmission and coordinates the re-transmission of those frames by the sender.In the data link layer, the term error control refers primarily to methods of error detection and re-transmission 


Flow Control




 Stop-and-wait 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 the discarding of frames or denial of service. To prevent the receiver from becoming over-whelmed with frames,we somehow need to tell the sender to slow down. There must be feedback from the receiver to the sender



It is still very simple. The sender sends one frame and waits for feedback from the receiver. When the ACK arrives, the sender sends the next frame It is Stop-and-Wait Protocol because the sender sends one frame, stops until it receives confirmation from the receiver (okay to go ahead), and then sends the next frame. We still have unidirectional communication for data frames, but auxiliary ACK frames (simple tokens of acknowledgment) travel from the other direction. 

Stop-and-Wait Automatic Repeat Request

To detect and correct corrupted frames, we need to add redundancy bits to our data frame. When the frame arrives at the receiver site, it is checked and if it is corrupted, it is silently discarded. The detection of errors in this protocol is manifested by the silence of the receiver. Lost frames are more difficult to handle than corrupted ones. In our previous protocols, there was no way to identify a frame. The received frame could be the correct one, or a duplicate, or a frame out of order. The solution is to number the frames. When the receiver receives a data frame that is out of order, this means that frames were either lost or duplicated

The lost frames need to be resent in this protocol. If the receiver does not respond when there is an error, how can the sender know which frame to resend? To remedy this problem, the sender keeps a copy of the sent frame. At the same time, it starts a timer. If the timer expires and there is no ACK for the sent frame, the frame is resent, the copy is held, and the timer is restarted. Since the protocol uses the stop-and-wait mechanism, there is only one specific frame that needs an ACK Error correction in Stop-and-Wait ARQ is done by keeping a copy of the sent frame and re-transmitting of the frame when the timer expires

In Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmetic. In Stop-and-Wait ARQ, the acknowledgement number always announces in modulo-2 arithmetic the sequence number of the next frame expected.









Comments

Post a Comment

Popular posts from this blog

Image
  OSI ·          OSI stands for Open Systems Interconnection ·          Created by International Standards Organization (ISO) ·          Was created as a framework and reference model to explain how different networking technologies work together and interact ·           Each layer has specific functions it is responsible for ·          All layers work together in the correct order to move data around a network  Physical Layer  The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to Occur.  The physical layer is al...
Read more

Data communications system components

Image
  A data communications system has five components 1      Message . The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. 2      Sender . The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on. 3      Receiver . The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on. 4          Transmission medium . The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves. 5        Protocol . A protocol is a set of rules that govern data communications. It represents an agreement betw...
Read more

Introduction

  NETWORK- A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network.   Computer network mean a collection of autonomous computers interconnected by a single technology. Two computers are said to be interconnected if they are able to exchange information.   The connection need not be via a copper wire; fiber optics, microwaves, infrared, and communication satellites can also be used. USES OF COMPUTER NETWORKS Business Applications   1.       To distribute information throughout the company ( resource sharing ). sharing physical resources such as printers, and tape backup systems, is sharing information 2.       Client-server model . It is widely used and forms the basis of much network usage.   3.       C...
Read more