Unit 2_ Flow & Error Control in computer networks
- 1. Flow control: It is a set of procedures that tells the sender how much data it can transmit before it must wait for an acknowledgement from the receiver. Receiver has a limited speed at which it can process incoming data and a limited amount of memory in which to store incoming data. Receiver must inform the sender before the limits are reached and request that the transmitter to send fewer frames or stop temporarily. Since the rate of processing is often slower than the rate of transmission, receiver has a block of memory (buffer) for storing incoming data until they are processed. Praveen kumar 8
- 2. There are two protocols used in flow control 1. Stop and Wait Protocol 2. Sliding Window Protocol Praveen kumar 9
- 3. 1. Stop and Wait Protocol: Source transmits frame. Destination receives frame and replies with acknowledgement (ACK). Source waits for ACK before sending next frame. Sender keeps a copy of the last frame until it receives an acknowledgement. For identification, both data frames and acknowledgements (ACK) frames are numbered alternatively 0 and 1. Sender has a control variable that holds the number of the recently sent frame. (0 or 1) Receiver has a control variable that holds the number of the next frame expected (0 or 1). Sender starts a timer when it sends a frame. If an ACK is not received within a allocated time period, the sender assumes that the frame was lost or damaged and retransmit it. Praveen kumar 10
- 4. 1. Stop and Wait Protocol: Receiver send only positive ACK if the frame is intact. ACK number always defines the number of the next expected frame. Praveen kumar 11
- 5. In stop-and-wait, at any point in time, there is only one frame that is sent and waiting to be acknowledged. This is not a good use of transmission medium. To improve efficiency, multiple frames should be in transition while waiting for ACK. Praveen kumar 12
- 6. 2. Sliding Window Protocol: This method allows the sender to transmit a specific number of frames without receiving positive acknowledgements for individual frames. ACK includes number of next frame expected. In this protocol each outbound frame contains a sequence number. If n bits are allocated the sequence number range from 0 to 2n-1. Ex: If a 3 bit number is used the sequence numbers would range from 0 to 7. The sender and receiver maintain a window. Praveen kumar 13
- 8. Example of Sliding Window Protocol: Praveen kumar 15
- 9. Sender can send up to W frames before worrying about ACKs. Sender keeps a copy of these frames until the ACKs arrive. Praveen kumar 16
- 10. Error control is implemented in such a way that every time an error is detected, a negative acknowledgement is returned and the specific frame is retransmitted. This process is called automatic repeat request. Error control in the data link layer is based on automatic repeat request, which is the retransmission of damaged or lost data. Receiver sends positive ACK if a frame arrived safe and in order. There are two protocols used in error control 1. Stop and Wait ARQ Protocol 2. Go Back N Protocol 3. Selective Repeat ARQ/Selective Reject ARQ Praveen kumar 17
- 11. 1. Stop and Wait ARQ Protocol: The sender keeps a copy of the last frame transmitted until it receives an acknowledgement for that frame. Keeping a copy allows the sender to retransmit lost or damaged frames until they are received correctly. If an error is discovered in transit, a NAK frame is returned to tell the sender to retransmit the last frame sent. The sender should wait until it receives an ack for the last frame transmitted before it transmits next frame. If an expected ack is not received within the sender’s timer expires , the sender assumes that the last data frame was lost and retransmit it While transmitting a frame there can be three situations i. Damaged frame ii. Lost data frame iii. Lost acknowledgement Praveen kumar 18
- 12. i. Damaged frame: When the frame contains error, receiver returns NAK frame and sender retransmits the last frame Praveen kumar 19
- 13. ii. Lost data frame: If the frame is not received, the receiver cannot acknowledge it. The sender waits for an ACK/NAK until its timer expires. It retransmits the last data frame, restart its timer and waits for an ACK. Praveen kumar 20
- 14. iii. Lost acknowledgement: The sender waits for ack until its timer expires. If ack is not received before the sender’s timer get expires the sender retransmits the last frame. Praveen kumar 21
- 15. 2. Go Back N Protocol: We can send up to W frames before worrying about ACKs. We keep a copy of these frames until the ACKs arrive. If the frames are damaged/out of order, receiver is silent and discard all subsequent frames until it receives the one it is expecting. The silence of the receiver causes the timer of the unacknowledged frame to expire. Then the sender resends all frames, beginning with the one with the expired timer. Praveen kumar 22
- 16. For example, suppose the sender has sent frame 6, but the timer for frame 3 expires (i.e. frame 3 has not been acknowledged), then the sender goes back and sends frames 3, 4, 5, 6 again. Thus it is called Go- Back-N-ARQ The receiver does not have to acknowledge each frame received, it can send one cumulative ACK for several frames. While transmitting a frame there can be three situations i. Damaged frame ii. Lost data frame iii. Lost acknowledgement Praveen kumar 23
- 17. • i. Damaged frame: Frame 3 is damaged. • When the receiver receives frame 4 and 5, it discards frame 4 and 5 as it is expecting frame 3. • After the timer for frame 3 expires at the sender side, the sender sends frame 3,4 and 5. Praveen kumar 24
- 18. • ii. Lost data frame: Frame 2 is lost. • When the receiver receives frame 3 and 4, it discards frame 3 and 4 as it is expecting frame 2. • After the timer for frame 2 expires at the sender side, the sender retransmits frame 2, 3 and 4. Praveen kumar 25
- 19. • iii. Lost acknowledgement: ACK 3 is lost. • When the receiver receives frame 0, 1 and 2, it sends ACK 3 for indicating that up to frame 2 all frames are successfully received and the next expected frame is 3. • But ACK 3 is lost. After the timer for frame 2 expires at the sender side, the sender retransmits frame 0, 2 and 2. Praveen kumar 26
- 20. 3. Selective Repeat ARQ/Selective Reject ARQ: • In Selective Repeat ARQ, only the damaged frame is resent. More bandwidth efficient. • It defines a negative ACK (NAK) to report the sequence number of a damaged frame before the timer expires. Praveen kumar 27
- 21. • Frames 0,1,3,4 and 5 are accepted when received. • Receiver sends a NAK 2 to show that frame 2 is damaged and then sender retransmits only frame 2. Praveen kumar 28
- 22. Networks must be able to transfer data from one device to another with acceptable accuracy. Data can be corrupted during transmission. Some applications require that errors be detected and corrected. Some applications can tolerate a small level of error. For example, random errors in audio or video transmissions may be tolerable, but when we transfer text, we expect a very high level of accuracy. Praveen kumar 29
- 23. Types of Errors: 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. Praveen kumar 30
- 24. Burst Error: The term burst error means that 2 or more bits in the data unit have changed from 1 to 0 or from 0 to 1. Praveen kumar 31
- 25. Redundancy: The central concept in detecting or correcting errors is redundancy. To detect or correct errors, we need to send extra (redundant) bits with data. These redundant bits are added by the sender and removed by the receiver. Their presence allows the receiver to detect or correct corrupted bits. Types of redundancy checks: 1. Vertical redundancy check 2. Longitudinal redundancy check 3. Cyclic redundancy check Praveen kumar 32
- 26. 1. Vertical redundancy check(VRC): ◦ Append a single bit at the end of data block such that the number of ones is even. ◦ Even Parity (odd parity is similar) 0110011 01100110 0110001 01100011 ◦ VRC is also known as Parity Check ◦ Performance: Detects all odd-number errors in a data block Praveen kumar 33
- 27. 2. Longitudinal redundancy check(LRC): Organize data into a table and create a parity for each column. Assume that if the above block hit by burst error, when the receiver checks the LRC, some of the bits do not follow even parity rule and whole block is discarded. 11000101 11010100 10111001 11101101 10101010 Block will be discarded due mismatch of LRC Praveen kumar 34 11100111 11011101 00111001 10101001 11100111 11011101 00111001 10101001 10101010 11100111 11011101 00111001 10101001 10101010 Original Data LRC
- 28. 3.Cyclic redundancy check(CRC): CRC is more powerful than VRC and LRC in detecting error. At the sender side the data unit to be transmitted is divided by predetermined divisor in order to obtain the remainder. This remainder is called CRC. The CRC has one bit less than the divisor. The sender appends this CRC to the end of data unit such that the resulting data unit becomes exactly divisible by predetermined divisor. That is remainder becomes zero Praveen kumar 35
- 29. At the destination, the incoming data unit, that is data + CRC is divided by the same number(predetermined divisor). If the remainder after division is zero then there is no error in the data unit and receiver accept it. If the remainder after division is not zero, it indicates that the data unit has been damaged in transit and therefore it is rejected. Praveen kumar 36
- 30. Sender Side: Praveen kumar 37
- 31. Receiver Side: Praveen kumar 38