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Multimedia transmission is widely available over wired networks. With the advent of low-cost WLAN devices, the wireless delivery of multimedia content is highly desirable. However, for media requiring low end-to-end latency, the use of WLAN technology introduces many significant challenges. These challenges are further enhanced if multicast/broadcast transmission is employed to serve a wide range of wireless terminals. This paper provides an understanding of the practical issues associated with WLAN multimedia transmission. A cross-layer measurement programme is performed to identify design issues for low-cost off-the-shelf WLAN multimedia systems. Problems identified include i) broadcast/multicast transmission using the slowest link-speed, ii) common link adaptation mechanisms for all clients, iii) lack of a call admission policy, and iv) irreducible PER even in good channel conditions. (UDP). While UDP offers a time bounded service, it does not guarantee the delivery of the video packets. To use this protocol successfully, the video codec must support strong error resilience and concealment. Furthermore, some video applications require transmission to a large number of remote terminals. Due to a lack of radio bandwidth, in these situations a dedicated point-to-point UDP-IP transmission cannot be accommodated with each remote client. Instead, Broadcast (or Multicast) transmission must be used at the WLAN. For these types of link there is absolutely no retransmission (ARQ) of errored or missing data packets (either at the WLAN MAC or the transport layer). As discussed above, wireless UDP based unicast and multicast/broadcast transmissions pose the problem of packet erasures. The lossy nature of the transmission medium can lead to unacceptable video quality at the client. As a result, it is important to consider the channel behaviour I. INTRODUCTION

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This paper presents simulation results for the transmission of unicast MAC frames over 802.11a/g. Fading channel models at various Doppler frequencies are developed to generate timecorrelated SNR waveforms. These are then used together with a bit accurate MAC/PHY simulator to estimate the frame loss rate, the transmission delay, and the jitter for a steady flow of transmit frames. Time correlated channels are required to correctly simulate the bursty nature of packet loss in a wireless channel. The Doppler spread is shown to have a strong effect on the performance of the ARQ mechanism in the MAC layer. Delay is computed as the sum of the transmission delay and the accumulated queuing delay in the MAC buffer. Delay and frame loss are compared for time correlated and time uncorrelated fading channels. Compared to the slow fading case, in a fast fading channel fewer retransmissions are required and the end-to-end delay is significantly reduced. When channel conditions are poor the simulated delay and frame loss rate are seriously underestimated when time uncorrelated fading is assumed. To analyze the performance of video codecs, we show that a time correlated channel model must be combined with a dedicated 802.11a/g MAC/PHY simulation.