Tuesday, December 23, 2008

Concept of Parity Bits

Fortunately,few communication systems experience frequent experience.More important,the interference on a long distance communication circuit may be so small that modems can handle all problem automatically.Despite the low probability of error,scientists and engineers who design networks understand that transmission errors do occur,and provide hardware and software mechanisms to detect and correct such problems.But now a days most RS-232 circuits use the different mechanism to ensure that each character arrives intact.This mechanism is known as Parity Check mechanism,this mechanism requires the sender to compute an additional bit,called a parity bit,and to attach it to each character before sending.After all bits of a character arrives,the receiver removes the parity bit,performs the same computation as the sender,and verifies that result agrees with the value of the parity bits.The parity computation chosen is such that if one of the bits in the character is damaged in transit,the receiver's computation will not agree with the parity bit and the receiver will report that an error occurred.

There are two forms of parity,even or odd.Both the sender and receiver must agree on which form is being used.In either form,the computation of the parity bit for a given character is straightforward.To achieve even parity,the sender sets the parity bit to 0 or 1 whichever will make the total number of 1 bits(including parity bit)an even number.Thus,when using even parity ,the parity bit for 0100101 is 1 because the character contains an odd number of 1 bits,and the parity bit for 0101101 is 0 because the character already contains an even number of 1 bits.Similarly to achieve odd parity,the sender chooses a parity bit such that it makes the total number of one in the character is odd.If all bits from the character arrive intact,the receiver's computation of parity will agree with the sender's.If interference changes one of the bits during transmission,the receiver's computation will not agree with the sender's and the receiver will report a parity error.

Sunday, December 21, 2008

Transmission errors in Computer Networks

Lightning,Power Surges,and other electro-magnetic interference can introduce unwanted electrical currents in the electronic components or wires used for communication.Interference that is severe especially lightning can cause permanent damage to network equipment.More often,however interference merely changes the signal used for transmission without damaging the equipment.A small change in the electrical signal can cause the receiver to misinterpret one or more bits of the data.In fact,interference can completely destroy a signal,meaning that although the sender transmits,the receiver does not detect that any data has arrived.Surprisingly,interference on a completely idle transmission circuit can create the opposite effect that means the sender does not transmit any thing,a receiver might interpret the incoming interference as a valid sequence of bits or characters.All these types of errors are known as transmission errors.In the last we can summarize the whole thing in this way that much of the complexity in the computer networks arises because digital transmission system are susceptible to interference that can cause random data to appear or transmitted data to be lost or changed.

Saturday, December 20, 2008

Concept of Packets

Most computer networks do not transfer data as an arbitrary string of continuous bits.Instead the network system divides the data into small blocks called packets,which it sends individually.Computer networks are often called packet networks or packet switching networks because they use packet technology.The concept of using data packets for data transmission is supported by two facts,first a sender and receiver need to coordinate transmission to ensure that data arrives correctly.Dividing the whole data into small blocks helps a sender and receiver to determine which blocks arrive correctly and which blocks do not.

The second fact that supports packet technique is as all we know that communication circuits and modem hardware are expensive,multiple computers often shares underlying connection and hardware.To ensure that all computers receive fair,prompts access to a shared communication facility,a network system cannot allow one computer to access deny to other computers.Using small packets helps ensure fairness.Let understand this more clearly.The networks that do not packet technology allows an application program(computer) to hold a shared communication resource arbitrarily long that means an application was permitted to finish before another application could begin using the resource.But now the networks that uses packet technology have a solution of this problem.Here the networks permits one computer to send a packet of data and then blocks that computer from sending another packet.Meanwhile the network permits another computer to send a packet and so on.A single computer can hold a shared resource only long enough to send single packet,and must wait until other computers have a turn before sending a second packet.