Summary computer networks

COMPUTER NETWORK - UNIT IV 1 Error :-  An error occurs when the output information does not match the input information.  The interfaces can change the shape of a signal and such changes can alter the meaning of data.  An error is a situation when data received at the destination machine is not the same as data that is actually sent by the source machine.  Types of Errors  Basically, there are two types of errors that can occur during the transmission of data. The types of errors are as follows. o Single Bit Error: The term single bit error means that only one bit of a data unit is changed from 1 to 0 or from 0 to 1. o Burst Error: The term burst error means the two or more two bits of the data unit is changed from 1 to 0 or vice versa. There may be three types of errors:  Single bit error In a frame, there is only one bit, anywhere though, which is corrupt.  Multiple bits error Frame is received with more than one bits in corrupted state.  Burst error  Frame contains more than1 consecutive bits corrupted. Error control mechanism may involve two possible ways:  Error detection  Error correction COMPUTER NETWORK - UNIT IV 2 Error Detection  When a message is sent, it may be jumbled by noise or the data may be damaged.  To avoid this, we employ error-detecting codes, which are bits of extra data appended to a digital message to assist us detect whether an error occurred during transmission.  In networking, error detection refers to the techniques used to detect noise or other impairments introduced into data while it is transmitted from source to destination.  Error detection ensures reliable delivery of data across vulnerable networks.  The oldest method of error correction involves using parity. It works by adding an additional bit to each character word transmitted.  The state of the bit is determined by a number of factors such as the type of parity and the number of logic-one bits in the data character. Error Detecting Codes (Implemented either at Data link layer or Transport Layer of OSI Model)  Whenever a message is transmitted, it may get scrambled by noise or data may get corrupted.  To avoid this, we use error-detecting codes which are additional data added to a given digital message to help us detect if any error has occurred during transmission of the message.  Basic approach used for error detection is the use of redundancy bits, where additional bits are added to facilitate detection of errors. Some popular techniques for error detection are: 1. Simple Parity check 2. Two-dimensional Parity check 3. Checksum 4. Cyclic redundancy check 1. Simple Parity check : -  It is also known as a parity check.  It has a cost-effective mechanism for error detection.  In this technique, the redundant bit is known as a parity bit. It is appended for every data unit. The total number of 1s in the unit should become even, which is known as a parity bit.  This scheme makes the total number of 1’s even, that is why it is called even parity checking. 2. Two-dimensional Parity check Parity check bits are calculated for each row, which is equivalent to a simple parity check bit.  Parity check bits are also calculated for all columns, then both are sent along with the data.  At the receiving end these are compared with the parity bits calculated on the received data. COMPUTER NETWORK - UNIT IV 3 3. Checksum  The data is split into k segments of m bits each in the checksum error detection technique.  To get the total, the segments are summed at the sender’s end using 1’s complement arithmetic. To obtain the checksum, a complement of the sum is taken.  The checksum segment is sent with the data segments.  To obtain the total, all received segments are summed using 1’s complement arithmetic at the receiver’s end. The sum is then calculated.  If the result is 0, the data is accepted; otherwise, it is rejected. Example 02 COMPUTER NETWORK - UNIT IV 4 Cyclic redundancy check (CRC)  CRC is an alternative method for determining whether or not a received frame includes valid data. The binary division of the data bits being delivered is used in this approach. Polynomials are used to generate the divisor.  The sender divides the bits that are being transferred and calculates the remainder.  The sender inserts the remainder at the end of the original bits before sending the actual bits. A codeword is made up of the actual data bits plus the remainder.  The transmitter sends data bits in the form of codewords.  The receiver, on the other hand, divides the codewords using the same CRC divisor.  If the remainder consists entirely of zeros, the data bits are validated; otherwise, it is assumed that some data corruption happened during transmission.  . Checksum  A checksum in computer networks is another error detection technique. The concept of redundancy is used in this technique to identify errors. The major two components of the checksum are the checksum generator and checksum checker.  A checksum generator is used on the sender’s side and it divides the whole data into k segments and each segment has n bits. All the segments are added using 1’s complement, and the sum is calculated.  Then the sum is complemented to generate the checksum. The data segments are then transmitted along with the checksum field from the sender to the receiving machine.  At the receiving end, another circuit is used, which is called a checksum chec ker. The primary function of the checksum checker is to check the checksum and verify the errors in the receiving data.  It divides the data into k sections and each section has n bits. All the segments are added to each other and the complement of the sum is calculated. The working of checksum is shown in the above figure. COMPUTER NETWORK - UNIT IV 5  If the result is zero, then there is no error and the receiver will receive the data.  If the result is non zero, then it shows the data has some error and data gets rejected at the receiving end.  There are mainly five types of checksum in computer networks for error detection 1. Integer addition checksum 2. One’s complement checksum 3. Fletcher checksum 4. ATN checksum 5. Adler checksum Error Correction:  Error Correction codes are used to detect and repair mistakes that occur during data transmission from the transmitter to the receiver.  There are two approaches to error correction: 1. Backward Error Correction:  When a backward mistake is detected, the receiver requests that the sender retransmit the complete data unit. 2. Forward Error Correction:  In this scenario, the error-correcting code is used by the receiver, which automatically corrects the mistakes.  A single extra bit can identify but not repair the mistake. Error Correction Techniques: 1. Hamming Code:  Parity bits: A bit that is added to the original binary data to make sure the total number of 1s is even or odd (in case of even or odd parity respectively).  Even parity: To check for even parity, if the total number of 1s is even, the parity bit value is 0. If the total number of occurrences of 1s is odd, the parity bit value is 1.  Odd Parity: To test for odd parity, if the total number of 1s is even, the parity bit value is 1. If the total number of 1s is odd, the parity bit value is 0. COMPUTER NETWORK - UNIT IV 6 Hamming Code  Hamming code is a liner code that is useful for error detection up to two immediate bit errors.  It is capable of single-bit errors.  In Hamming code, the source encodes the message by adding redundant bits in the message.  These redundant bits are mostly inserted and generated at certain positions in the message to accomplish error detection and correction process.  Hamming code is a set of error-correction codes that can be used to detect and correct the errors that can occur when the data is moved or stored from the sender to the receiver.  It is a technique developed by R.W. Hamming for error correction.  Redundant bits – Redundant bits are extra binary bits that are generated and added to the information-carrying bits of data transfer to ensure that no bits were lost during the data transfer. History of Hamming code  Hamming code is a technique build by R.W.Hamming to detect errors.  Hamming code should be applied to data units of any length and uses the relationship between data and redundancy bits.  He worked on the problem of the error-correction method and developed an increasingly powerful array of algorithms called Hamming code.  In 1950, he published the Hamming Code, which widely used today in applications like ECC memory. Application of Hamming code  Satellites  Computer Memory  Modems  PlasmaCAM  Open connectors  Shielding wire  Embedded Processor Advantages of Hamming code  Hamming code method is effective on networks where the data streams are given for the single-bit errors.  Hamming code not only provides the detection of a bit error but also helps you to indent bit containing error so that it can be corrected.  The ease of use of hamming codes makes it best them suitable for use in computer memory and single-error correction. Disadvantages of Hamming code  Single-bit error detection and correction code. However, if multiple bits are founded error, then the outcome may result in another bit which should be correct to be changed.  This can cause the data to be further errored.  Hamming code algorithm can solve only single bits issues. COMPUTER NETWORK - UNIT IV 7 Example: If the data to be transmitted is Number of data bits = 7 Thus, number of redundancy bits = 4 Total bits = 7+4 = 11 Redundant bits are always placed at positions that correspond to the power of 2, so the redundant bits will be placed at positions: 1,2,4 and 8. Redundant bits will be placed here: Thus now, all the 11 bits will look like this: Here, R1, R2, R4 and R8 are the redundant bits. Determining the parity bits: R1:  We look at bits 1,3,5,7,9,11 to calculate R1. In this case, because the number of 1s in these bits together is even, we make the R1 bit equal to 0 to maintain even parity. R2:  We look at bits 2,3,6,7,10,11 to calculate R2. In this case, because the number of 1s in these bits together is odd, we make the R2 bit equal to 1 to maintain even parity. COMPUTER NETWORK - UNIT IV 8 R4:  We look at bits 4,5,6,7 to calculate R4. In this case, because the number of 1s in these bits together is odd, we make the R4 bit equal to 1 to maintain even parity. R8:  We look at bits 8,9,10,11 to calculate R8. In this case, because the number of 1s in these bits together is even, we make the R8 bit equal to 0 to maintain even parity.  Thus, the final block of data which is transferred looks like this: