Archive for February 3rd, 2010
February 3rd, 2010 | 
Author: Dear all! To ensure the data transmitted in a network has no errors, an error detection and correction technique has to be employed. The errors caused by the noise during transmission can be detected by an error detection system and rectified and original signal is reconstructed by an error correction system.
There are several methods to construct an error detection system. Repetition code technique repeats the bits to be transmitted again and again to ensure an error free communication. Parity bits technique introduces an additional bit with the original data to ensure the number of bits. But this can able to detect only the odd number of bits that are incorrect.
Checksum technique produces an arithmetic sum of the message code words of finite word length. Cyclic Redundancy Check (CRC) technique uses a long division operation in which the remainder gives the result while the quotient is discarded. For error correction, the way to go is retransmission. But the type of request varies in different techniques.
In automatic repeat request an acknowledgement message is from the receiver is sent to transmitter after the receiver has correctly received the data. Otherwise the transmitter will resend. Forward error correction technique employs the addition of redundant codes with the original data and can be recovered at the receiver. Here no retransmission is required and hence this is well suited for simplex communication.
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Dear all! The most popular packet-switched LAN technology invented at the Xerox PARC is known as the Ethernet. After it was standardized by the Xerox, Intel and digital equipment corporations, it has become the most familiar LAN technology and hence it exists in different forms.
Now it has replaced all the other technologies and used in many of the corporate and personal networks. A coaxial cable had been used to connect all the computers in the original wiring system. But the twisted pair Ethernet is the advanced technology in which Ethernet can be accessed by unshielded copper wires as in telephones.
With the twisted pair wiring, installation becomes easy and hence it requires very low capital cost. Usually every computer and the Ethernet hub are connected by a set of eight wires. But out of eight wires, only four are effective. These four wires are divided into two pairs.
One pair is used to carry data from the computer to the hub and the other to carry data from the hub to the computer. All the signals carried by the Ethernet cable are simulated by the hub. For reliable operation, the distance between a hub and a computer should not exceed 100 meters.
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