Automatic Repeat Request (ARQ)
- A Layer-2 sliding-window based flow control mechanism.
- Per connection basis.
- Only effective to non-real-time applications.
- Uses a 11-bit sequence number field.
- Uses CRC-32 checksum of MAC PDU to check data errors.
- Maintain the same fragmentation structure for Retransmission.
- Used to detect errors in an OFDM symbol.
- Usually used with FEC.
Introduction
Automatic Repeat Request(ARQ) is one of the most widely used technique for error control in data communication, the other being Forward error correction? (FEC). ARQ is not meant for correcting errors, but using this technique, the receiver requests for repeated transmission of errored codewords until they are received error-free.
ARQ Methodology
The MAC SDUs received are fragmented or packed into required block size. The blocks are then transmitted at the receiver end the block received are checked for errors. If the received block is error free, ACK is sent to the transmitter using feedback channel. If the received block is in error, a retransmission request is sent to the transmitter using the feedback channel. The transmitter then retransmits the same block.
ARQ Techniques
ARQ techniques can be broadly classified into two categories
· Stop and Wait ARQ
· Continuous ARQ
Stop-and-wait ARQ
With stop-and-wait ARQ, the transmitter sends a single codeword and waits for a positive acknowledgment (ACK) or negative acknowledgment (NAK) before sending any more codewords. The advantage of stop-and-wait ARQ is that it only requires a half duplex channel. The main disadvantage is that it wastes time waiting for ACKs, resulting in low throughput. The throughput of such system is given by
T=nn–tr1–P(n)
where n is the block length P(n) is the error probability for the block r is the signaling rate t is the time between the beginning and end of subsequent blocks
The above equation ignores the loss in throughput due to parity bits. From the above equation we can see that the throughput is degraded when P(n) is high and when t.r is considerable fraction of block length. Stop and wait ARQ are very unsuitable for long delay channel as the effective information rate is drastically low because of the idle time spent for acknowledgment for each transmitted blocks.
Continuous ARQ
Continuous ARQ requires a full duplex channel because codewords are sent continuously until a NAK is received. A NAK can be handled using one of the two ways.
Go Back-N ARQ
Using Go Back-N ARQ the transmitter on receipt of NAK retransmits the errored codeword along with all the codewords that followed until the NAK was received. The parameter N is determined from the round trip channel delay. For satellite channels, N is very large because of the very large round trip time. The transmitter stores N codewords at a time and large values of N result in expensive memory requirements.
The throughput of GBN ARQ protocol
T=kn(1–(Tr–1)N)
Selective Repeat ARQ
With selective-repeat ARQ, only the errored codeword is retransmitted, thus increasing the throughput over go back-N ARQ. The throughput efficiency for the ideal selective ARQ is defined as the average number of information per transmitted channel symbol
T=kNTr
where k/n is the loss in the throughput due to added parity bits for error detection where Tr is the average number of transmission for given data packet.
Both types of continuous ARQ offer greater throughput efficiency than stop-and-wait ARQ at the cost of greater memory requirements.
Drawbacks and need for the better technique
The severe drawback of the ARQ mechanism is that the througput efficiency falls rapidly with high channel error rate.
The drawback with FEC scheme is it is hard to achieve high system reliability because the received data is delivered to the sink even if the FEC is unable to decode the packet correctly. Moreover the long powerful code are hard to implement and very expensive. The drawbacks of both ARQ and FEC system could be overcome if the two error control schemes are combined. The combination of two basic error control scheme is referred to as Hybrid ARQ.
Performance
In terms of error performance, ARQ outperforms forward error correction because codewords are always delivered error-free (provided the error detection code doesn't fail). This advantage does not come free the price we pay we pay for this is decreased throughput. The trade of lies between throughput and reliabilty. The chief advantage of ARQ is that error detection requires much simpler decoding equipment than error correction. ARQ is also adaptive since it only re-transmits information when errors occur. ARQ schemes require a feedback path.
The key figure of merit in the evaluation of ARQ scheme is Throughput. Throughput refers to the ratio of average number of bits accepted by the receiver per unit time to the data rate or bit rate of the channel. The uncertainty in computing this parameter is because of the unreliable channel which has varying error probability.
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