Hybrid Error Control Mechanism for Wireless ATM

WATM (Wireless Asynchronous Transmission Mode) experience burst errors, which needs flexible error control scheme. There are two types of basic error schemes ARQ(Automatic Error Request) send NACK (Negative Acknowledgment) for retransmission of packets when it is lost and cannot be recovered, second type id FEC (Forward Error Correction) transmits redundant bits with original data to  recover the corrupted packets.
Most of the time error control scheme use same type of redundancy for every service of ATM networks whether it is real time or non real time, increases overhead and consumes bandwidth. The flexible approach should be adopted to support several classes of services in ATM networks. HARQ (Hybrid automatic repeat request) is the combination of ARQ and FEC is the suitable error control scheme can be applied in WATM networks to minimum the amount of redundancy.
The architecture of HARQ is intended to minimize the redundancy and delay by distinguishing class of service. First, the data is fed into buffer and forwarded to FEC encoder that break the block into k packets and apply a redundancy of n packets. A packet scheduler transmits packets to wireless fading channel, which retransmits packets if the packet receiver sends NACK to packet scheduler if the number of packets is less than expected.  If entire packets received by packet receiver module it forwards the packets to FEC error check, in case of error FEC recover it by using parity bits.
Three different schemes are simulated namely HARQ, HARQ with different redundancy and adaptive HARQ. The results shows taken as a whole the adaptive HARQ give better results applied in diverse situations, the scheme uses the c amount of redundancy in low bit rate error and increases the performance results in better of Quality of Service. In other conditions HARQ and HARQ with different redundancy also shows improved results

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