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Reliability Of Information Transmission

Author: Mr D Wilson

Source of document: Open University

Date: 12/5/96

LANs are setup to comply with standards.  The Open Systems Interconnection Reference Model known as the OSI model covers hardware and software through seven layers.  Each layer provides documented services.  When a communication is performed the sequence of events is known as a protocol.

Data integrity is preserved by:
 

Each file is broken into short messages known as frames.
Individual addressing of nodes (device on the network).
Only one node can transmit at a time.
A specification known as the IEEE 802 is a group of standards that achieve the above strategies in different ways.  The Data Link layer is split into two sub layers.  The Logical Link Layer and the Media Access Control (MAC) layer.  This uses one of three protocols; Carrier Sense Multiple Access with Collision Detection (CSMA/CD), Token Bus or Token Ring.

Token passing uses a specially formatted MAC layer frame that acts as a token.  Only the node that has the token can transmit for a specific amount of time.  A frame containing data is passed from node to node.  The status of the frame and number of times sent are used for reception reliability checks.  The originating node eventually receives the original frame to confirm receipt.  The  server monitors the token.

CSMA/CD which is used primarily on Ethernet LANs.  Ethernet is a broadcast network developed by Xerox.  In this network frames are transmitted by one node and received simultaneously by all other nodes on the network.  Each node listens to the network waiting for a break in network traffic (carrier sense).  When the break is detected, a timer is started.  The node waits for a randomly generated period and then broadcasts its frame to all listeners (multiple access).  All listening nodes receive the frame and check the destination address.  The addressee node decodes the frame.  The transmitting node monitors the network to check for a collision indicated by a characteristic pattern of electrical activity.  If detected, the transmit process starts again.

Frames can be corrupted by electrical interference.  Both Token passing and CSMA/CD use Cyclic Redundancy Checking to detect errors in frames.  The transmitting node uses an algorithm to process the data that results in a number.  This is stored in the frame and sent to the receiving node.  The receiving node performs the same algorithm and compares the number generated against the number in the Frame Check Sequence filed.  If the numbers are the same then there is a high probability, 99.9%+ that there was no error.  The % of probability depends on the algorithm used.

Error correction is conducted by the Logical Link Control layer in the IEEE 802 standard.  This is done by the transmitting node and receiving node agreeing on how many frames make up a message and the sequence in which the frames should be received.  Any frames that are corrupt are retransmitted if a receipt acknowledgement is not received by the transmitting node.

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