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ERROR DETECTION AND CORRECTION, FLOW AND ERROR CONTROL AND TRASMISSION MEDIA

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WHAT IS ERROR?Whenever bits flow from one point to another, they are subject to unpredictable changes because of interference. This interference can change the shape of the signal.

There are two types of errors

Single bit error Burst errorTYPES OF ERROR

Single-Bit Error:-The term single-bit error means that only 1 bit of a given data unit (such as a byte, character, or packet) is changed from 1 to 0 or from 0 to 1.In single bit error, only 1 bit the data unit has changed.

Burst Error:-The term burst error means that 2 or more bits in the data unit have changed from 1to 0 or from 0 to 1.A burst error means that 2 or more bits in the data unit have changed.

ERROR DETECTION AND CORRECTIONData can be corrupted during transmission. Some applications require that errors be detected and corrected.

Redundancy

The central concept in detecting or correcting errors is redundancy. To be able to detect or correct errors, we need to send some extra bits with our data. These redundant bits are added by the sender and removed by the receiver. To detect or correct errors, we need to send extra (redundant) bits with data.

Error detection1) Error detection means to decide whether the received data is correct or not without having a copy of the original message.

2) Error detection uses the concept of redundancy, which means adding extra bits for detecting errors at the destination.

Make sense of message. Make sense of message.

Error Detection:-How can errors be detected by using block coding? If the following two conditions are met, the receiver can detect a change in the original codeword.

The receiver has (or can find) a list of valid codewords. 2. The original codeword has changed to an invalid one.

Error CorrectionIt can be handled in two ways:receiver can have the sender retransmit the entire data unit.The receiver can use an error-correcting code, which automatically corrects certain errors.

Error correction:-Error correction is much more difficult than error detection. In error detection, the receiver needs to know only that the received codeword is invalid; in error correction the receiver needs to find (or guess) the original codeword sent. We can say that we need more redundant bits for error correction than for error detection.

Single-bit error correctionTo correct an error, the receiver reverses the value of the altered bit. To do so, it must know which bit is in error.Number of redundancy bits neededLet data bits = mRedundancy bits = rTotal message sent = m+rThe value of r must satisfy the following relation:2r m+r+1

Error Correction

Hamming CodesOne of the most effective codes for error-recoveryUsed in situations where random errors are likely to occurError detection and correction increases in proportion to the number of parity bits (error-checking bits) added to the end of the information bits

code word = information bits + parity bitsHamming distance: the number of bit positions in which two code words differ.1000100110110001 * * *Minimum Hamming distance or D(min) : determines its error detecting and correcting capability.

Hamming Code

Example of Hamming Code

FLOW AND ERROR CONTROL

Data communication requires at least two devices working together, one to send and the other to receive. The most important responsibilities of the data link layer are flow control 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. Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment.

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 retransmission of those frames by the sender. In the data link layer, the term error control refers primarily to methods of error detection and retransmission. Error control in the data link layer is often implemented simply: Any time an error is detected in an exchange, specified frames are retransmitted. This process is called automatic repeat request (ARQ).Error control in the data link layer is based on automatic repeat request, which is the retransmission of data.

TRANSMISSION MEDIAThe transmission media that are used to convey information can be classified as guided or unguided. Guided media provide a physical path along which the signals are propagated; these include twisted pair, coaxial cable and optical fiber. Unguided media employ an antenna for transmitting through air, vacuum, or water.There are two types of transmission media:-

Guided (wired)Unguided (wireless)

GUIDED MEDIAGuided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber - optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium.

TWISTED-PAIR CABLE:- A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together.

Physical Description:- A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pair acts as a single communication link. Over longer distances, cables may contain hundreds of pairs. The twisting tends to decrease the crosstalk interference between adjacent pairs in a cable. Neighbouring pairs in a bundle typically have somewhat different twist lengths to reduce the crosstalk interference. On long-distance links, the twist length typically varies from 5 to 15 cm. The wires in a pair have thicknesses of from 0.4 to 0.9 mm.

UNSHIELDED AND SHIELDED TWISTED PAIR

Twisted pair comes in two varieties: unshielded and shielded. Unshielded twisted pair (UTP) is ordinary telephone wire. This is the least expensive of all the transmission media commonly used for local area networks and is easy to work with and easy to install. Unshielded twisted pair is subject to external electromagnetic interference ,including interference from nearby twisted pair and from noise generated in the environment. A way to improve the characteristics of this medium is to shield the twisted pair with a metallic braid or sheathing that reduces interference. This shielded twisted pair (STP) provides better performance at higher data rates. However, it is more expensive and more difficult to work with than unshielded twisted pair.

COAXIAL CABLE:-Coaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it to operate over a wider range of frequencies. It consists of a hollow outer cylindrical conductor that surrounds a single inner wire conductor. Coaxial cable can be used over longer distances and support more stations on a shared line than twisted pair.

APPLICATIONS

Coaxial cable is perhaps the most versatile transmission medium and is enjoying widespread use in a wide variety of applications. The most important of these are:-

Television distribution Long-distance telephone transmission

OPTICAL FIBER:-An optical fiber is a thin (2 to 125 micro m), flexible medium capable of guiding an optical ray. Various glasses and plastics can be used to make optical fibers. The lowest losses have been obtained using fibers of ultrapure fused silica.

Five basic categories of application have become important for optical fiber:

Long-haul trunks (average 1500 km in length) Metropolitan trunks(average length of 12 km) Rural exchange trunks (lengths ranging from 40 to 160 km) Local area networks

UNGUIDED OR WIRELESS TRANSMISSIONFor unguided media, transmission and reception are achieved by means of an antenna.

RADIO TRANSMISSION:-Radio waves frequency are easy to generate, can travel long distances, and can penetrate buildings easily, so they are widely used for communication, both indoors and outdoors. Radio waves also are omnidirectional, meaning that they travel in all directions from the source, so the transmitter and receiver do not have to be carefully aligned physically. It is used for multicast communication such as radio and television.

MICROWAVE TRANSMISSION:-Above 100 MHz, the waves travel in nearly straight lines and can therefore be narrowly focused. Concentrating all the energy into a small beam by means of a parabolic antenna (like the familiar satellite TV dish) gives a much higher signal-to-noise ratio, but the transmitting and receiving antennas must be accurately aligned with each other. These are used for unicast communication such as cellular telephones ,satellite networks and wireless LANs.

INFRARED TRANSMISSION:-Infrared waves lies in between visible light spectrum and microwaves. It has wavelength of 700 nm to 1 mm and frequency ranges from 300 GHz to 430 THz. Infrared waves are used for very short range communication purposes such as television and its remote. Infrared travels in a straight line so they are directional by nature. Because of high frequency range, Infrared do not cross wall like obstacles.

Thank YouRegards :Pankaj Kohli