Several research projects have been proposed in the last few years, such as: the WATMnet project, developed by NEC for personal multimedia networks in a micro-cellular environment with fixed and mobile terminals [28]; the Magic Wand (Wireless ATM Network Demonstrator) project, proposed by European Union ACTS (Advanced Communications Technologies and Services), for indoor environment with mobile terminals [2]; the BAHAMA project proposed by Bell Laboratories, for ad hoc local area networks with mobile terminals [3]; the SWAN (Seamless Wireless ATM Networks) developed by Bell Laboratories for indoor networks with picocellular environment and mobile terminals [4]; the ATMmobil project, proposed by a pool of universities and enterprises in Germany, considering four different scenarios: local area networks, mobile broadband cellular networks, residential networks, and fixed access networks [5].Abstract -Wired ATM (Asynchronous Transfer Mode) networks are designed taking into account an optical link in the physical layer. The main problem to implement a wireless ATM network is the fact that its radio link bit error rate is high and variable, demanding the use of an additional error control mechanism. The usual techniques for error control are ARQ (Automatic Repeat Request) and FEC (Forward Error Correction). ARQ works better than FEC if the channel bit error rate is low, and is worse if it is high, thus indicating that hybrid ARQ-FEC techniques can improve the radio link performance. A new hybrid ARQ-FEC scheme with multiple FEC states is proposed in this paper. The performance of the proposed scheme is compared with non-adaptive ARQ and FEC schemes, which have similar complexities, and with an ideal adaptive FEC scheme. It is shown that the proposed scheme works better than the nonadaptive schemes, and that the hybrid and the ideal adaptive FEC schemes have similar performances. Moreover, the hybrid scheme does not require an estimate of the bit error rate in the channel, leading to a less complex implementation.In general, the goal of wireless ATM networks is to provide the same characteristics of the wired ATM networks, in a transparent way, through a wireless link, for fixed or mobile terminals. Thus, wireless ATM networks must integrate several kinds of traffic with guaranteed quality of service. The main problems to make the wireless ATM networks feasible are related to the following research areas [3][6][7]: mobility management, multiple access protocol and error control in the wireless link.

Resumo As redes ATM (Asvnchronous Transfer Mode) foram concebidas consider;ndo a utiliza«ii'a de meios de transmissiio 6pticos, que apresentam taxa de erro de bit pequena. Urn dos principais problemas de se utilizar 0 ATM em redes sem fio esta relacionado a taxa de erro, que e variavel e pode assumir valores elevados. Para contornar este problema, mecanismos adicionais de controle de erro devem ser inclufdos no enlace ATM sem fio. as esquemas tradicionais de controle de erro siio a corre«iio por retransmissao (ARQ-Automatic Repeat Request) e a corre«iio automatica no receptor do enlace sem fio (FEC Forward Error Correction). a uso de FEC ou ARQ no enlace sem fio da rede ATM resulta na diminui«iio da taxa de erro de bit residual (taxa de erro de bit ap6s a aplica«iio do mecanismo de controle de erro), no aumento do atraso para transmissiio dos pacotes de informa«iio e na diminui«iio da vazao, devido ao overhead dos mecanismos de controle de erro. Neste artigo, compara-se, analiticamente, 0 FEC e 0 ARQ no ambiente das redes ATM sem fio considerando sistemas adaptativos e nao adaptativos e aplica«oes em tempo real e niio tempo real. as criterios de compara«ao utilizados siio: 0 tempo para se transmitir uma PDU (Protocol Data Unit), para aplica«oes de comunica«iio de dados niio tempo real, e, para aplica«oes em tempo real, a vazao e os tempos medio e maximo para se transferir uma celula.