Dedicated to Dosen Ardi
Paula Dewanti - Made Heriyasa
Prima Rangga
In telecommunication a data linkis the means of connecting one location to another for the purpose of transmitting and receiving digital information. It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver [two data terminal equipments (DTEs)] and the interconnecting data telecommunication circuit. These are governed by a link protocol enabling digital data to be transferred from a data source to a data sink.
There are at least three types of basic data-link configurations that can be conceived of and used:
- Simplex communications, most commonly meaning all communications in one direction only.
- Half-duplex communications, meaning communications in both directions, but not both ways simultaneously.
- Duplex communications, communications in both directions simultaneously.
FLOW CONTROL
In computer networking, flow controlis the process of managing the rate of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with data from transmitting node.
Flow control should be distinguished from congestion control, which is used for controlling the flow of data when congestion has actually occurred. Flow control mechanisms can be classified by whether or not the receiving node sends feedback to the sending node.
Flow control is important because it is possible for a sending computer to transmit information at a faster rate than the destination computer can receive and process them. This can happen if the receiving computers have a heavy traffic load in comparison to the sending computer, or if the receiving computer has less processing power than the sending computer.
PARITY CHECK
A parity bit is a bit that is added to ensure that the number of bits with the value one in a set of bits is even or odd. Parity bits are used as the simplest form of error detecting code.
There are two variants of parity bits: even parity bit and odd parity bit. When using even parity, the parity bit is set to 1 if the number of ones in a given set of bits (not including the parity bit) is odd, making the entire set of bits (including the parity bit) even. When using odd parity, the parity bit is set to 1 if the number of ones in a given set of bits (not including the parity bit) is even, making the entire set of bits (including the parity bit) odd. In other words, an even parity bit will be set to "1" if the number of 1's + 1 is even, and an odd parity bit will be set to "1" if the number of 1's +1 is odd.
Even parity is a special case of a cyclic redundancy check (CRC), where the 1-bit CRC is generated by the polynomial x+1.
If the parity bit is present but not used, it may be referred to as mark parityspace parity (the bit is always 0). (when the parity bit is always 1) or
| 7 bits of data (number of 1s) | 8 bits including parity | |
|---|---|---|
| even | odd | |
| 0000000 (0) | 00000000 | 10000000 |
| 1010001 (3) | 11010001 | 01010001 |
| 1101001 (4) | 01101001 | 11101001 |
| 1111111 (7) | 11111111 | 01111111 |
Parity
In mathematics, parity refers to the evenness or oddness of an integer, which for a binary number is determined only by the least significant bit. In telecommunications and computing, parity refers to the evenness or oddness of the number of bits with value one within a given set of bits, and is thus determined by the value of all the bits. It can be calculated via an XORsum of the bits, yielding 0 for even parity and 1 for odd parity. This property of being dependent upon all the bits and changing value if any one bit changes allows for its use in error detection schemes. In quantum mechanics, parity corresponds to the behavior of the wave function under reflection.ERROR DETECTION AND CORRECTION
If an odd number of bits (including the parity bit) are transmittedincorrectly, the parity bit will be incorrect and thus indicates that an error occurred in transmission. The parity bit is only suitable for detecting errors; it cannot correct any errors, as there is no way to determine which particular bit is corrupted. The data must be discarded entirely, and re-transmitted from scratch. On a noisy transmission medium, successful transmission can therefore take a long time, or even never occur. However, parity has the advantage that it uses only a single bit and requires only a number of XOR gates to generate. See Hamming code for an example of an error-correcting code.
Parity bit checking is used occasionally for transmitting ASCII characters, which have 7 bits, leaving the 8th bit as a parity bit.
For example, the parity bit can be computed as follows, assuming we are sending a simple 4-bit value 1001 with the parity bit following on the right, and with ^ denoting an XOR gate:
Transmission sent using even parity:
A wants to transmit: 1001
A computes parity bit value: 1^0^0^1 = 0
A adds parity bit and sends: 10010
B receives: 10010
B computes parity: 1^0^0^1^0 = 0
B reports correct transmission after observing expected even result.Transmission sent using odd parity:
A wants to transmit: 1001
A computes parity bit value: ~(1^0^0^1) = 1
A adds parity bit and sends: 10011
B receives: 10011
B computes overall parity: 1^0^0^1^1 = 1
B reports correct transmission after observing expected odd result.This mechanism enables the detection of single bit errors, because if one bit gets flipped due to line noise, there will be an incorrect number of ones in the received data. In the two examples above, B's calculated parity value matches the parity bit in its received value, indicating there are no single bit errors. Consider the following example with a transmission error in the second bit:
Transmission sent using even parity:
A wants to transmit: 1001
A computes parity bit value: 1^0^0^1 = 0
A adds parity bit and sends: 10010
*** TRANSMISSION ERROR ***
B receives: 11010
B computes overall parity: 1^1^0^1^0 = 1
B reports incorrect transmission after observing unexpected odd result.B's calculated parity value (1) does not match the parity bit (0) in its received value, indicating the bit error. Here's the same example but now the parity bit itself gets corrupted:
A wants to transmit: 1001
A computes even parity value: 1^0^0^1 = 0
A sends: 10010
*** TRANSMISSION ERROR ***
B receives: 10011
B computes overall parity: 1^0^0^1^1 = 1
B reports incorrect transmission after observing unexpected odd result.Once again, B's computes an odd overall parity, indicating the bit error.There is a limitation to parity schemes. A parity bit is only guaranteed to detect an odd number of bit errors. If an even number of bits have errors, the parity bit records the correct number of ones, even though the data is corrupt. (See also error detection and correction.) Consider the same example as before with an even number of corrupted bits:
A wants to transmit: 1001
A computes even parity value: 1^0^0^1 = 0
A sends: 10010
*** TRANSMISSION ERROR ***
B receives: 11011
B computes overall parity: 1^1^0^1^1 = 0
B reports correct transmission though actually incorrect.B observes even parity, as expected, thereby failing to catch the two bit errors. In information theory and coding theory with applications in computer sciencetelecommunication, error detection and correction or error control are techniques that enable reliable delivery of digital data over unreliable communication channels. Many communication channels are subject to channel noise, and thus errors may be introduced during transmission from the source to a receiver. Error detection techniques allow detecting such errors, while error correction enables reconstruction of the original data. and
The general definitions of the terms are as follows:
- Error detection is the detection of errors caused by noise or other impairments during transmission from the transmitter to the receiver.
- Error correction is the detection of errors and reconstruction of the original, error-free data.
- Automatic repeat request (ARQ) (sometimes also referred to as backward error correction): This is an error control technique whereby an error detection scheme is combined with requests for retransmission of erroneous data. Every block of data received is checked using the error detection code used, and if the check fails, retransmission of the data is requested – this may be done repeatedly, until the data can be verified.
- Forward error correction (FEC): The sender encodes the data using an error-correcting code (ECC) prior to transmission. The additional information (redundancy) added by the code is used by the receiver to recover the original data. In general, the reconstructed data is what is deemed the "most likely" original data.
Error detection schemes
Several schemes exist to achieve error detection. The general idea is to add some redundancy(i.e., some extra data) to a message, which enables detection of any errors in the delivered message. Most such error-detection schemes are systematic: The transmitter sends the original data bits, and attaches a fixed number of check bits, which are derived from the data bits by some deterministic algorithm. The receiver applies the same algorithm to the received data bits and compares its output to the received check bits; if the values do not match, an error has occurred at some point during the transmission. In a system that uses a "non-systematic" code, such as some raptor codes, the original message is transformed into an encoded message that has at least as many bits as the original message.In general, any hash functionmay be used to compute the redundancy. However, some functions are of particularly widespread use because of either their simplicity or their suitability for detecting certain kinds of errors (e.g., the cyclic redundancy check's performance in detecting burst errors).
Error-correcting codescan provide a suitable alternative to hash functions when a strict guarantee on the minimum number of errors to be detected is desired. Repetition codes, described below, are special cases of error-correcting codes: although rather inefficient, they find applications for both error correction and detection due to their simplicity.
REDUNDANCY CHECK
A cyclic redundancy check (CRC) or polynomial code checksum is a non-secure hash function designed to detect accidental changes to raw computer data, and is commonly used in digital networks and storage devices such as hard disk drives. A CRC-enabled device calculates a short, fixed-length binary sequence, known as the CRC code or just CRC, for each block of data and sends or stores them both together. When a block is read or received the device repeats the calculation; if the new CRC does not match the one calculated earlier, then the block contains a data error and the device may take corrective action such as rereading or requesting the block be sent again, otherwise the data is assumed to be error free.
CRCs are so called because the check (data verification) code is a redundancycyclic codes. The term CRC may refer to the check code or to the functionthat calculates it, which accepts data streams of any length as input but always outputs a fixed-length code. CRCs are popular because they are simple to implement in binary hardware, are easy to analyse mathematically, and are particularly good at detecting common errors caused by noise in transmission channels. The CRC was invented by W. Wesley Peterson, and published in his 1961 paper. The IEEE-recommended 32-bit CRC, used in Ethernet and elsewhere, appeared at a telecommunications conference in 1975. (it adds zero information) and the algorithm is based on
continue..
continue..
No comments:
Post a Comment