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# V Communication Error Detection And Correction

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### V Communication Error Detection And Correction

1. 1. COMMUNICATION ERROR DETECTION AND CORRECTION DATA COMMUNICATION ERRORS F Data communication errors are undesirable changes in the bit pattern of data that occur after the data goes from the internal PC data bus enroute to an external device or computer. F They can appear at any point along a communication link. F Data communication errors can produce catastrophic results if not quickly detected and corrected. REDUNDANCY ERROR-DETECTION TECHNIQUES F To detect and correct communications errors, a sender must provide a receiver with information that allows the receiver to verify proper receipt of information or data. F This additional information is called error checking data or redundancy data. F The sender must use a mathematical technique to calculate this error checking data based on the bit stream of information it will send. Communication Error Detection and Correction 1
2. 2. ERROR-DETECTION TECHNIQUES F Parity Checking This technique is primarily for detecting errors when the number of information bits is small and the probability of an error being present is small. Two Kinds of Parity Checking: 1. Even Parity. The number of 1’s in a word should be even. Example: A = 1000001 B = 1000010 In ASCII C = 1000011 Applying the parity bit: A = 0 1000001 B = 0 1000010 C = 1 1000011 Communication Error Detection and Correction 2
3. 3. 2. Odd Parity. The number of 1’s in a word should be odd. Example: A = 1000001 B = 1000010 In ASCII C = 1000011 Applying the parity bit: A = 1 1000001 B = 1 1000010 C = 0 1000011 Take note the parity checking can only detect any odd number of digit corruptions Communication Error Detection and Correction 3
4. 4. F Block Sum Checking Parity checking is done on a per block basis. Example: Transmit DATACOM D = 1000100 C = 1000011 A = 1000001 O = 1001111 T = 1010100 M = 1001101 (odd) 1 1 0 0 0 1 0 0 D 1 1 0 0 0 0 0 1 A 0 1 0 1 0 1 0 0 T 1 1 0 0 0 0 0 1 A Traverse (Row) Parity Bits 0 1 0 0 0 0 1 1 C 0 1 0 0 1 1 1 1 O 1 1 0 0 1 1 0 1 M 0 1 0 1 0 0 0 1 (even) Longitudinal (Column) Parity Bits Communication Error Detection and Correction 4
5. 5. F Polynomial Codes In using polynomial codes, a single set of check digits referred to as the frame check sequence or cyclic redundancy check is generated or computed for each frame. Polynomial codes are based upon treating bit strings as representations of polynomials with coefficients of 0 and 1 only. A k-bit string or frame is regarded as the coefficient list for a polynomial with k terms, ranging from Xk-1 to X0. The MSB is the coefficient of Xk-1 while the LSB is the coefficient of X0. Example: 110001 - 6 bits (represents a 6-term polynomial) M(X) = 1ŸX5 + 1ŸX4 + 0ŸX3 + 0ŸX2 + 0ŸX1 + 1ŸX0 = X5 + X4 + X0 = X5 + X4 + 1 Degree of 110001 = 5 (highest exponent) Communication Error Detection and Correction 5
6. 6. In using the polynomial code, the sender and receiver agree upon a generator polynomial G(X) in advanced. Algorithm for computing the FCS: 1. Let r be the degree of G(X). Append r zero bits to the low-order end of the message frame M(X), so it now contains m + r bits where m is the number of bits in M(X). The resulting frame will be called M ’(X). 2. Divide the bit string corresponding to G(X) into the bit string corresponding to M‘(X) using modulo 2 arithmetic. Modulo 2 Arithmetic (XOR) Examples: 10011011 00110011 + 11001010 - 11001101 01010001 11111110 3. Subtract the remainder (which is always r or fewer bits) from M’(X) using modulo 2 subtraction. The result is the checksummed frame to be transmitted T(X). Communication Error Detection and Correction 6
7. 7. Example: Message Frame M(X) = 1101011011 Generator G(X) = 10011 r = 4 (degree) M’(X) = 11010110110000 1100001010 10011 11010110110000 10011 10011 10011 00001 00000 00010 00000 00101 00000 01011 00000 10110 10011 01010 00000 10100 10011 01110 00000 Remainder 1110 Communication Error Detection and Correction 7
8. 8. To compute for T(X): 11010110110000 - 1110 11010110111110 = Frame to be Transmitted, T(X) When the receiver gets the transmitted frame T(X), it tries dividing it by G(X). If there is a remainder, there has been a transmission error. Some Points Regarding Polynomial Codes: 1. T(X) is exactly divisible (modulo 2) by G(X). In any division problem, if the remainder is subtracted from the dividend, what is left over is divisible by the divisor. Example: M’ = 11 G = 5 Remainder of M’ / G = 1 T = 11 -1 = 10 Exactly divisible by G Communication Error Detection and Correction 8
9. 9. 2. Assume that a transmission error occurs so that instead of the polynomial for the transmitted message T(X) arriving, T(X) + E(X) arrives. Those errors that happen to correspond to polynomials containing G(X) as a factor will slip by unnoticed, but all other errors will be caught. Example: T(X) = 11010110111110 E(X) = 10011 T(X) + E(X) = 11010110101101 If the receiver tries to divide this by G(X), there will be no remainder (error was not detected). Communication Error Detection and Correction 9
10. 10. An important characteristic of the polynomial code method is that all burst of errors with fewer terms or bits than the generator polynomial are detected. Standards in CRC (For Generator Polynomial) 1. CRC - 12 X12 + X11 + X3 + X2 + X + 1 2. CRC - 16 X16 + X15 + X2 + 1 3. CRC - CCITT X16 + X12 + X5 + 1 4. CRC - 32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10 + X8 + X7 + X5 + X4 + X2 + X + 1 Communication Error Detection and Correction 10
11. 11. ERROR-CORRECTION TECHNIQUES F Error-correction techniques allow the receiver to determine which bit or bits is/are erroneous thereby allowing it to make the necessary corrections. F One popular error-correction technique uses the Hamming codes. F In using Hamming codes, the bits of the message to be transmitted are numbered consecutively, starting with bit 1 at the left end. The bits of the message that will be transmitted are therefore numbered as: b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 . . . Those bits that are powers of 2 (b1, b2, b4, b8, etc.) are the check bits (the exact values of which will be computed) while the rest of the bits (b3, b4, b5, b6, b7, b9, etc.) are the data bits. Each check bit forces the parity of some collection of bits, including itself, to be even (or odd). A bit may be included in several parity computations. Communication Error Detection and Correction 11
12. 12. To determine which check bits the data bit in position k contributes to, rewrite k as a sum of powers of two. Example: 11 = 1 + 2 + 8 This means that b11 is checked by b1, b2, and b8. F Case Study: Transmit the byte 1101101 The actual bits that will be transmitted are: b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 1 1 0 1 1 0 1 The values of the check bits b1, b2, b4, and b8 will have to be computed. b3 is checked by bits b1 and b2. b5 is checked by bits b1 and b4. b6 is checked by bits b2 and b4. b7 is checked by bits b1, b2 and b4. b9 is checked by bits b1 and b8. b10 is checked by bits b2 and b8. b11 is checked by bits b1, b2 and b8. Communication Error Detection and Correction 12
13. 13. Summarizing which data bits are checked by a certain check bit: b1 checks data bits b3, b5, b7, b9, b11 b2 checks data bits b3, b6, b7, b10, b11 b4 checks data bits b5, b6, b7 b8 checks data bits b9, b10, b11 By using the values of the data bits and by assuming that even parity is used, the values of the check bits are computed as: b1 = 1 b2 = 1 b4 = 0 b8 = 0 The actual bits that will be transmitted are therefore: b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 1 1 1 0 1 0 1 0 1 0 1 Communication Error Detection and Correction 13
14. 14. F Correcting Errors Assume that the transmitted code is: 1 1 1 0 1 0 1 0 1 0 1 However, due to impulse noise, b9 was corrupted. The received message is therefore: 1 1 1 0 1 0 1 0 0 0 1 The receiver checks for parity errors as follows: 1st group: b1, b3, b5, b7, b9, b11 1 1 1 1 0 1 2nd group: b2, b3, b6, b7, b10, b11 1 1 0 1 0 1 3rd group: b4, b5, b6, b7 0 1 0 1 4th group: b8, b9, b10, b11 0 0 0 1 There is a parity error in groups 1 and 4. By method of elimination, the erroneous bit cannot be b4, b5, b6, b7, b8, b10, and b11 since all of these bits are present in groups 2 and 3 and yet there these groups do not indicate any error. Therefore, the erroneous bit is b9. Communication Error Detection and Correction 14
15. 15. ERROR CONTROL IN FILE TRANSMISSION F A simple but time-consuming technique of transferring files with the assurance of file integrity is to transfer the entire file at least twice. If the files are not identical, there is no way of knowing which file contains errors. In this case, the entire file must be retransmitted again. F To make the process of file transfer error detection and correction a manageable task, the file is divided into smaller units called packets as it moves from one computer to another. F These packets include error-detection information (such as CRC) that enables the receiving computer to determine the presence of communication-induced errors. If the receiver receives an error-free packet, it sends an acknowledgement (ACK) to the sender. F The sender waits for an ACK after every packet it sends. Only when an acknowledgement has been received is the next packet sent. If a packet contains errors, then the receiver will send a negative acknowledgement (NAK) to the sender. The sender will then retransmit that particular packet. F This protocol is known as the Stop-and-Wait ARQ (Automatic Repeat Request) protocol since the transmitter has to stop and wait for the receiver to formulate and send a response. Communication Error Detection and Correction 15
16. 16. THE STOP-AND-WAIT ARQ PROTOCOL F Error control in the Stop-and-Wait ARQ protocol is implemented simply: anytime an error is detected in an exchange, a negative acknowledgement (NAK) is returned and the specified packets are retransmitted. F An error is implied as a damaged packet, a lost packet, or a lost acknowledgement. F For retransmission to work, the following features are necessary: 1. The sending device keeps a copy of the last packet transmitted until it receives an acknowledgement for that packet. Keeping a copy allows the sender to retransmit lost or damaged packets until they are received correctly. 2. For identification purposes, both data packets and ACK packets are numbered alternately 0 and 1. A data 0 packet is acknowledged by an ACK 1 packet indicating that the receiver has gotten data 0 and is now expecting data 1. 3. If an error is discovered in a data packet, a NAK packet is returned. NAK frames, which are not numbered, tell the sender to retransmit the last packet sent. Communication Error Detection and Correction 16
17. 17. 4. The sending device is equipped with a timer. If an expected acknowledgement is not received within the allotted time period (time out), the sender assumes that the last data frame was lost in transit and sends it again. F Damaged Packets When a packet is discovered by the receiver to contain an error, it returns a NAK packet and the sender retransmits the last packet. Sender Receiver Data 0 ACK 1 Data 1 ACK 0 Data 0 error in NAK packet 0 Data 0 ACK 1 Communication Error Detection and Correction 17
18. 18. F Lost Packet The sender is equipped with a timer that starts every time a data packet is transmitted. If the packet never makes it to the receiver, the receiver can never acknowledge it, positively or negatively. The sending device waits for an ACK or a NAK frame until its timer goes off, at which point it tries again. It retransmits the last frame, restarts its timer, and waits for an acknowledgement. Sender Receiver Data 0 lost time out period Data 0 ACK 1 Communication Error Detection and Correction 18
19. 19. F Lost Acknowledgement In this case, the data packet has made it to the receiver and has been found to be either acceptable or not acceptable. But the ACK or NAK packet returned by the receiver is lost in transit. The sending device waits until its timer goes off, then retransmits the data packet. The receiver checks the number of the new data packet. If the lost packet was a NAK, the receiver accepts the new copy and returns an appropriate ACK (assuming the copy arrives undamaged). If the lost frame was an ACK, the receiver recognizes the new copy as a duplicate, acknowledges its receipt, then discards it and waits for the next packet. Sender Receiver Data 0 time out lost ACK 1 period Data 0 ACK 1 Communication Error Detection and Correction 19
20. 20. F This stop-and-wait file transfer protocol is a half- duplex protocol because it allows the transmission to flow in only one direction at a time. F The advantage of stop-and-wait is simplicity: each packet is checked and acknowledged before the next frame is sent. The disadvantage is inefficiency: stop-and-wait is slow. If the distance between devices is long, the time spent waiting for ACKs between each packet can add significantly to the total transmission time. F A different approach is to allow a transmitter to send many packets before needing an acknowledgement. The receiver acknowledges only some of the packets, using a single ACK to confirm the receipt of multiple data packets. This is known as the sliding window protocol. Communication Error Detection and Correction 20
21. 21. THE SLIDING WINDOW PROTOCOL F The sliding window refers to imaginary boxes at both sender and receiver. This window can hold packets at either end and provides the upper limit on the number of packets that can be transmitted before requiring an acknowledgement. Packets may be acknowledged at any point without waiting for the window to fill up and may be transmitted as long as the window is not yet full. F To keep track of which packets have been transmitted and which received, sliding window introduces an identification scheme based on the size of the window. The packets are numbered modulo-n, which means they are numbered 0 to n - 1. For example , if n = 8, the packets are numbered 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, …The size of the window is n – 1 (in this case, 7). In other words, the window cannot cover the whole module (8 packets): it covers one packet less. F When the receiver sends an ACK, it includes the number of the next packet it expects to receive. In other words, to acknowledge the receipt of a string of packets ending in packet 4, the receiver sends an ACK with the number 5. When the sender sees an ACK with the number 5, it knows that all packets up through number 4 have been received. Communication Error Detection and Correction 21
22. 22. F The window can hold n – 1 packets at either end: therefore, a maximum of n – 1 packets may be sent before an acknowledgement is required. Window 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 F The Sender Window At the beginning of the transmission, the sender’s window contains n – 1 packets. As the packets are sent out, the left boundary of the window moves inward, shrinking the size of the window. Given a window of size w, if three packets have been transmitted since the last acknowledgement, then the number of packets left in the window is w – 3. Once an ACK arrives, the window expands to allow in a number of new packets equal to the number of packets acknowledged by that ACK. Sender Window 0 1 2 3 4 5 6 7 0 1 2 3 4 Direction Direction This wall moves to the This wall moves to the right when a packet is right when an ACK is sent received Communication Error Detection and Correction 22
24. 24. Receiver Window 0 1 2 3 4 5 6 7 0 1 2 3 4 Direction Direction This wall moves to the This wall moves to the right when a packet is right when an ACK is received sent In the given figure, the window contains spaces for seven packets, meaning that seven packets may be received before an ACK must be sent. With the arrival of the first packet, the receiving window shrinks, moving the boundary from space 0 to 1. The window has shrunk by one, so the receiver may now accept six packets before it is required to send an ACK. If frames 0 through 3 have arrived but have not been acknowledged, the window still contains three packet spaces. As each ACK is sent out, the receiving window expands to include as many new placeholders as newly acknowledged packets. The window expands to include a number of new packet spaces equal to the number of the most recently acknowledged packet minus the number of the previously acknowledged packet. Communication Error Detection and Correction 24
25. 25. In a seven-packet window, if the prior ACK was for packet 2 and the current ACK is for frame 5, the window expands to by three (5 – 2). If the prior ACK was for packet 3 and the current ACK is for frame 1, the window expands to by six (1 + 8 – 3). F Example of a simple transmission that uses sliding window protocol: Sender Receiver 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 Data 0 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 Data 1 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 ACK 2 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 Data 2 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 ACK 3 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 Data 3 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 Data 4 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 Data 5 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 ACK 6 0 1 2 3 4 5 6 7 0 1 2 3 4 0 1 2 3 4 5 6 7 0 1 2 3 4 Communication Error Detection and Correction 25
26. 26. The given figure shows a sample transmission that uses sliding window flow control with a window of seven frames. In this example, all frames arrive undamaged. As will be shown later, if errors are found in the received frames, or if one or more frames are lost in transit, the process will become more complex. At the beginning of the transmission, both sender and receiver windows are fully expanded to include seven frames (seven transmittable frames in the sender window, seven placeholder frames in the receiver window). The frames within the windows are numbered 0 through 7 and are part of a larger data buffer, 13 of which are shown. F In the sliding window method of flow control, the size of the window is one less than the modulo range so that there is no ambiguity in the acknowledgement of the received frames. Assume that the frame sequence numbers are modulo-8 and the window size is also 8. Now imagine that frame 0 is sent and ACK 1 is received. The sender expands its window and sends frame 1, 2, 3, 4, 5, 6, 7, and 0. If it now receives an ACK 1 again, it is not sure if this is a duplicate of the previous ACK 1 (duplicated by the network) or a new ACK 1 confirming the most recently sent eight frames. But if the window size is 7 (instead of 8), this scenario could not happen. Communication Error Detection and Correction 26
27. 27. SLIDING WINDOW ARQ F Among the several popular mechanisms for continuous transmission error control, two protocols are the most popular: Go-Back-n ARQ and Selective-Reject ARQ, both based on the sliding window flow control. F To extend sliding window to cover retransmission of lost or damaged frames, three features are added to the basic flow control mechanism: 1. The sending device keeps copies of all transmitted frames until they have been acknowledged. 2. In addition to ACK frames, the receiver has the option of returning a NAK frame if the data have been received damaged. Because sliding window is a continuous transmission mechanism, both ACK and NAK frames must be numbered for identification. Unlike ACK frames, NAK frames carry the number of the damaged frame itself. Every damaged frame must be negatively acknowledged individually. If data frames 4 and 5 are received damaged, both NAK 4 and NAK 5 must be returned. However, a NAK 4 tells the sender that all frames received before frame 4 have arrived intact. Communication Error Detection and Correction 27
28. 28. 3. Like stop-and-wait ARQ, the sending device in sliding window ARQ is equipped with a timer to enable it to handle lost acknowledgements. The sliding window ARQ, n – 1 frames (the size of the window) may be sent before the acknowledgement must be received. If n – 1 frames are awaiting acknowledgement, the sender starts a timer and waits before sending any more. If the allotted time has run out with no acknowledgement, the sender assumes that the frames were not received and retransmits one or all of the frames depending on the protocol. F Go-Back-n ARQ In the sliding window go-back-n ARQ method, if one frame is lost or damaged, all frames sent since the last frame acknowledged are retransmitted. Damaged Frame Suppose that frames 0, 1, 2, and 3 have been transmitted but the first acknowledgement received is a NAK 3. Remember that a NAK means two things: (1) a positive acknowledgement of all frames received prior to the damaged frame and (2) a negative acknowledgement of the frame indicated. If the first acknowledgement is a NAK 3, it means that data frames 0, 1, and 2 were all received in good shape. Only frame 3 must be resent. Communication Error Detection and Correction 28
29. 29. As soon as the receiver discovers an error, it stops accepting subsequent frames until the damaged frame has been replaced correctly. Example of Go-Back-n (damaged data frame): Sender Receiver Data 0 Data 1 Data 2 Data 3 ACK 3 Error, Discarded Data 4 Discarded NAK 3 Data 5 Discarded Resent Data 3 Resent Data 4 Resent Data 5 . . . Communication Error Detection and Correction 29
30. 30. Lost Data Frame Sliding window protocols require that data frames be transmitted sequentially. If one or more frames are become lost in transit, the next frame to arrive at the receiver will be out of sequence. The receiver checks the identifying number on each frame, discovers that one or more have been skipped, and returns the NAK for the first missing frame. A NAK frame does not indicate whether the frame has been lost or damaged, just that it needs to be resent. The sending device then retransmits the frame indicated by the NAK, as well as any frames that it had transmitted after the lost one. Lost Acknowledgement The sender is not expecting to receive an ACK frame for every data frame it sends. It cannot use the absence of sequential ACK numbers to identify lost ACK or NAK frames. Instead it uses a timer. The sending device can send as many frames as the window allows before waiting for an acknowledgement. Once the limit has been reached or the sender has no more frames to send, it must wait. If an acknowledgement has not been received within a certain time limit, the sender retransmits every frame transmitted since the last ACK. Communication Error Detection and Correction 30
31. 31. Example of Go-Back-n (lost data frame): Sender Receiver Data 0 Data 1 Data 2 Lost Data 3 Discarded NAK 2 Data 4 Discarded Resent Data 2 Resent Data 3 Resent Data 4 . . . Communication Error Detection and Correction 31
32. 32. Example of Go-Back-n (lost ACK): Sender Receiver Data 0 Data 1 Time Out Data 2 Lost ACK 3 Data 0 Data 1 Data 2 . . . Communication Error Detection and Correction 32
33. 33. F Selective-Reject ARQ In selective-reject ARQ, only the specific damaged or lost frame is retransmitted. If a frame is corrupted in transit, a NAK is returned and the frame is resent out of sequence. To make such selectivity possible, a selective-reject ARQ system differs from a go-back-n ARQ system in the following ways: 1. The receiving device must contain sorting logic to enable it to reorder frames received out of sequence. It must also be able to store frames received after a NAK has been sent until the damaged frame has been replaced. 2. The sending device must contain a searching mechanism that allows it to find and select only the requested frame for retransmission. 3. A buffer in the receiver must keep all previously received frames on hold until all retransmissions have been sorted and any duplicate frames have been identified and discarded. 4. To aid selectivity, ACK numbers, like NAK numbers, must refer to the frame received (or lost) instead of the next frame expected. 5. This complexity requires a smaller window size than is needed by the go-back-n method if it is to work efficiently. It is recommended that the window size be less than or equal to (n + 1) / 2, where n – 1 is the go-back-n window size. Communication Error Detection and Correction 33
34. 34. Damaged Frame Example of Selective-Reject (damaged data frame): Sender Receiver Data 0 Data 1 Data 2 Error in Frame 2 Data 3 Data 4 NAK 2 Data 5 Resent Data 2 . . . Communication Error Detection and Correction 34
35. 35. In the given example, frames 0 and 1 are received but are not acknowledged. Data 2 arrives and is found to contain an error, so NAK 2 is returned. Like NAK frames in go-back-n, a NAK here both acknowledges the intact receipt of any previously unacknowledged data frames and indicates an error in the current frame. NAK 2 tells the sender that data 0 and data 1 have been accepted, but that data 2 must be resent. The receiver in the selective-reject system continues to accept new frames while waiting for an error to be corrected. However, because an ACK implies the successful receipt not only of the specific frame but all of the previous frames, frames received after the error frame cannot be acknowledged until the damaged frames have been retransmitted. In the example, the receiver accepts data 3, 4, and 5 while waiting for a new copy of data 2. When the new data 2 arrives, an ACK 5 can be returned, acknowledging the new data 2 and the original frames 3, 4, and 5. Quite a bit of logic is required by the receiver to sort out-of-sequence retransmissions and to keep track of which frames are still missing and which have yet to be acknowledged. Communication Error Detection and Correction 35
36. 36. Lost Frames Although frames can be accepted out of sequence, they cannot be acknowledged out of sequence. If a frame is lost, the next frame will arrive out of sequence. When the receiver tries to reorder the existing frame to include it, it will discover the discrepancy and return a NAK. Of course, the receiver will recognize the omission only if other frames follow. If the lost frame was the last of the transmission, the receiver does nothing and the sender treats the silence like a lost acknowledgement. Lost Acknowledgement Lost ACK and NAK framers are treated by selective- reject ARQ just as they are by go-back-n ARQ. When the sending device reaches either the capacity of its window or the end of its transmission, it sets a timer. If no acknowledgement arrives in the time allotted, the sender retransmits all of the frames that remain unacknowledged. In most cases, the receiver will recognize any duplications and discard them. F Comparison between Go-Back-n and Selective-Reject Because of the complexity of the sorting and storage required by the receiver, and the extra logic needed by the sender to select specific frames for retransmission, selective-reject ARQ is expensive and not often used. Communication Error Detection and Correction 36
37. 37. ASYNCHRONOUS FILE-TRANSFER PROTOCOLS F XMODEM Protocol Packet Format: SOH PKT # PKT # COMPL DATA CHECKSUM SOH - Start of Header or Start of Packet (00000001) PKT # - Sequential Packet Number (8 bits) PKT # COMPL - Complement of PKT # Examples: 00000001 11111110 00000010 11111101 00000011 11111100 DATA - 128 bytes of data CHECKSUM - error-detection byte (sum of the ASCII value of all 128 bytes, modulo 255) Communication Error Detection and Correction 37
38. 38. Protocol Operation: 1. Receiver sends a NAK character (00010101) to signal the transmitter to start transmitting the file. 2. Transmitter transmits a packet. 3. Receiver receives the packet and checks for errors. If packet is error-free, receiver transmits an ACK character (00000110) to inform the transmitter to start transmitting the next packet. Otherwise, receiver transmits a NAK character to inform the transmitter to retransmit the previous packet. 4. It transmitter receives an ACK, it transmits the next packet. If transmitter receives a NAK, it retransmits the previous packet. 5. Steps 3 and 4 are repeated until there are no more packets to transmit. 6. Transmitter transmits an EOT character (00000100) to inform the receiver that there are no more packets to be retransmitted. 7. Receiver sends a final ACK character. Communication Error Detection and Correction 38
39. 39. Some Points Regarding the XMODEM Protocol: 1. XMODEM can transmit a CAN character (00011000) or ^X to abort a file transfer. 2. XMODEM considers more than 10 attempts to resend the data packet a fatal error and aborts the file transfer. 3. The error-detection scheme used by XMODEM is not so reliable. 4. An ACK (00000110) can be accidentally changed (due to communication errors) to a NAK (00010101) causing the transmitter to unnecessarily retransmit the previous data packet. 5. If an ACK is changed to any other character, the transmitter will not know when to send the next sequential packet. 6. An ACK or NAK can become a CAN character thus aborting the file transfer. The file transfer has to be restarted from the beginning. 7. XMODEM is ideal for low-speed transfer through high-quality transmission lines. Communication Error Detection and Correction 39
40. 40. Communication Error Detection and Correction 40
41. 41. F YMODEM Protocol The YMODEM protocol is similar to XMODEM except: 1. YMODEM uses 1024-byte data blocks instead of 128 (less overhead). If the line is not good, YMODEM automatically decreases the data block length to 128 bytes to reduce the number of bytes that have to be retransmitted each time the protocol detects an error. Under worst of conditions, YMODEM’s performance equals that of XMODEM. 2. YMODEM must receive two sequential CAN characters before it abnormally terminates a file transfer. 3. YMODEM uses the CRC error-detection technique. 4. YMODEM transmits file-related information to the receiving computer. It transmits the filename, time, date, and size of the file in the first block. Communication Error Detection and Correction 41
42. 42. F ZMODEM Protocol 1. ZMODEM can resume a file transfer at any point that a communication link fails. 2. Before a file transfer starts, ZMODEM checks to see if the file exists on the local disk. 3. ZMODEM can perform data compression to reduce the time required to move the file. 4. ZMODEM uses CRC-32 which provides better error-detection than most protocols. 5. ZMODEM uses the sliding window technique. Communication Error Detection and Correction 42
43. 43. F Kermit Protocol Packet Format: MARK PKT LENGTH PKT # PKT TYPE DATA CHECK MARK - Start of packet PKT LENGTH - Number of bytes that follow this field PKT # - Packet Sequence Number PKT TYPE - Packet Type: Data, ACK, NAK, Send- Initiate, Break Signal, File Header, End of File, Error DATA - 7- or 8-bit data CHECK - 6-bit checksum, 12-bit checksum, or CRC- CCITT Communication Error Detection and Correction 43
44. 44. Features of Kermit: 1. Kermit uses full packets with error checking for both data and ACK/NAK response data. 2. Kermit performs feature negotiation each time it transmits a file. This takes place through the exchange of send-initiate packets. During the send-initiate process, two Kermits compare features and select the ones both ends of the link can support. This allows any kermit implementation to communicate with any other kermit implementation. 3. Kermit uses control character conversion. 4. Kermit can use 7-bit or 8-bit characters. 5. Kermit can choose which error-detection technique to use. 6. Kermit has file group transfer capability. It can move groups of files without interruption. 7. Other special capabilities: file attribute transfer, data compression, sliding windows. Communication Error Detection and Correction 44