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ERROR CONTROL STRATEGIES

Automatic Repeat Request (ARQ).

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ERROR CONTROL STRATEGIES

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  1. Automatic Repeat Request (ARQ) In most communication systems, the information can be sent in both directions, and the transmitter also acts at a receiver (transceiver), and vice-versa. Examples: data networks, satellite communications, etc. Error control strategies for a two-way system can include error detection and retransmission, called automatic repeat request (ARQ). In an ARQ system, when errors are detected at the receiver, a request is sent for the transmitter to repeat the message, and repeat requests continue to be sent until the message is correctly received. ERROR CONTROL STRATEGIES

  2. Stop-and-Wait (SW) ARQ • Continuous ARQ The transmitter sends a block of information to the receiver and waits for a positive (ACK) or negative (NAK) acknowledgment from the receiver. If an ACK is received (no error detected), the transmitter sends the next block. If a NAK is received (errors detected) , the transmitter resends the previous block. When the errors are persistent, the same block may be retransmitted several times before it is correctly received and acknowledged. The transmitter sends blocks of information to the receiver continuously and receives acknowledgments continuously. When a NAK is received, the transmitter begins a retransmission. It may back-up to the block and resend that block plus the N-1 blocks that follow it. This is called Go-Back-N (GBN) ARQ. Alternatively, the transmitter may simply resend only those blocks that are negatively acknowledged. This is known as Selective Repeat (SR) ARQ. ERROR CONTROL STRATEGIES

  3. Remarks GBN Versus SR ARQContinuous Versus SW ARQ • SR ARQ is more efficient than GBN ARQ, but requires more logic and buffering. • Continuous ARQ is more efficient than SW ARQ, but it is more expensive to implement. • In a satellite communication, where the transmission rate is high and the round-trip delay is long, continuous ARQ is used. SW ARQ is used in systems where the time taken to transmit a block is long compared to the time taken to receive an acknowledgment. SW ARQ is used on half-duplex channels (only one way transmission at a time), whereas continuous ARQ is designed for use on full-duplex channels (simultaneous two-way transmission). ERROR CONTROL STRATEGIES

  4. Some Performance Measures (ARQ) • Throughput Efficiency: Is the average number of information bits successfully accepted by the receiver per unit of time, over the total number of information digits that could have been transmitted per unit of time. • Delay of a Scheme: The interval from the beginning of a transmission of a block to the receipt of a positive acknowledgment for that block. ERROR CONTROL STRATEGIES

  5. Lin and Costello, Error Control Coding ERROR CONTROL STRATEGIES Remarks (GBN Versus SR ARQ) Fig.1.

  6. Remarks (ARQ Versus FEC) • The major advantage of ARQ versus FEC is that error detection requires much simpler decoding equipment than error correcting. • Also, ARQ is adaptive in the sense that information is retransmitted only when errors occurs. • In contrast, when the channel error is high, retransmissions must be sent too frequently, and the SYSTEM THROUGHPUT is lowered by ARQ. In this situation, a HYBRID combination of FEC for the most frequent error patterns along with error detection and retransmission for the less likely error patterns is more efficient than ARQ alone (HYBRID ARQ). ERROR CONTROL STRATEGIES

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