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Advance Repeat reQuest (ARQ)
Automatic Repeat reQuest (ARQ) is a protocol that uses acknowledgements (ACKs) and timeouts to ensure reliable data transmission over unreliable lines. There are three main types: Stop-and-Wait ARQ where the transmitter waits for each ACK before sending again; Go-Back-N ARQ where the transmitter can send multiple frames before waiting but must retransmit all unacknowledged frames if an error occurs; and Selective Repeat ARQ where only the frame in error is retransmitted. Go-Back-N and Selective Repeat can achieve higher throughput than Stop-and-Wait when there are no errors by keeping the transmission line busy.
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Advance Repeat reQuest (ARQ)
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- 3. • Automatic RepeatreQuest (ARQ) • A Protocol to control error on unreliable data transmission lines • Uses timeouts & ACKs (or NAKs) for each data packet • In case timeout expires before sender receives an ACK from the receiver, sender resends the packet • Receiver will not send an ACK if – Data packet is lost in transmission – An error in the packet is detected • Three Types of ARQ ACKs are messages sent by a receiver to acknowledge that it has correctly received a data packet. Negative ACKs or NAKs are their counterpart. When Data is to be transmitted it is broken down into like data structures also called data packets. They are reassembled at destination.
- 4. CRC stands for cyclicredundancy code and is an error-detecting code attached here with both the packet and the ACK The header is required as a sequence number. When an ACK is transmitted, it tells the sender to which data packet is next in sequencePacket Sequence Information frames Control frames Error-free Packet Sequence CRC Information packet Header CRCHeader Transmitter Receiver 0 11
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- 6. • In thismethod the transmitter and receiver work on delivering one frame at a time through an alteration of actions. Here B receives error free F4, but its ACK is lost in transmission. After timeout, A resends F4, B recognizes it has received a double, deletes one and resends the ACK for F4 F0 process was successful. F1 is sent. There is an error so B sends no ACK. After Timeout, A resends F1. This time ACK is received so A sends F2. TimeA B F0 ACK1 F1 F1 F2 ACK2 ACK3 TimeA B F3 ACK4 F4 F4 F5 ACK5 ACK6ACK5 Timeout Timeout error
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- 8. • Here, thetransmitter continues sending frames unlike ‘stopping and waiting’ for the ACK to arrive and then proceed. Thus the transmission line is kept busy and more data is transferred if line remains reliable • Called a sliding window protocol as with each arriving error free frame the receive window slides forward. When the number of outstanding frames crosses N (Here N=3) the sender is forced to ‘go back N’ frames. Thus here it will retransmit frames from F2 onwards. A sends F0. Without waiting for an ACK, it sends F1 and then F2. F2 encounters an error. B sends no ACK and then continues to ignore later frames TimeA B F0 ACK1 F1 F2 F3 F4 F2 F3 F4 F5 ACK2 ACK3 ACK4 Three Frames Outstanding; Go Back 3 Out-of-sequence frames error
- 9. Compare and Contrast Stop-and-WaitARQ • Error causes loss of transmission time equal to timeout period. • Utilization and thus transfer speed is low even with no error as sender waits for ACK before sending new F. Go-Back-N ARQ • Error causes loss in transmission time equal to N frames. • When no error occurs, utilization is nearly 100%. Transfer speeds are thus high on relatively reliable connections.
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- 11. • Unlike Go-Back-N,here a NAK is sent by the receiver if sees a skipped frame due to error. Subsequent frames are not ignored and the sender selectively repeats transmission of only the frame that encountered error. B continues to receive subsequent frames but it sends an ACK for 2 until F2 arrives. It then sends an ACK for the next frame. Here it is for F6. A sends F0. Without waiting for an ACK, it sends F1 and then F2. F2 encounters an error. B sends a NAK for F2 when it receives F3. F3 is saved at B TimeA B F0 ACK1 F1 F2 F2 F6 F7 ACK2 ACK6 error F3 F4 F5 ACK2 NAK2 ACK2
- 12. Compare and Contrast Go-Back-NARQ • Error causes loss in transmission time equal to N frames. • When no error occurs, utilization is nearly 100%. Transfer speeds are thus high on relatively reliable connections. Selective Repeat ARQ • Error causes loss in transmission time equal to the time taken to begin transmitting a frame. • When no error occurs, utilization is nearly 100%. Transfer speeds are thus high.
- 13. Automatic Repeat reQuest Advantages •Provides a high throughput in good signal conditions. • Asking the sender to to send packets again instead of correcting them results in high data reliability. Disadvantages • Requires a reverse channel for transmission of ACKs/NAKs • Poor signal conditions result in low transfer speeds as errors will cause delay variations due to retransmitted data.