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Elite Member
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FYI:以下引自 Multicast in 802.11 WLANs: An Experimental Study....
引用:
Multicasting video instead of streaming individually each video results in a much more efficient use of the shared wireless medium. Whereas all these new applications are very likely to appear soon with upcoming WiMAX or DVB-H enabled devices, the IEEE 802.11 standard does not comply with multicast data requirements. In particular, the current MAC layer sends multicast packets in open-loop as broadcast packets, i.e., without any possible acknowledgements.
This open-loop transmission mechanism causes three main problems. First, without feedback mechanism, it is not possible to adapt the contention window according to the network state, as it is done with regular point-to-point connections. Consequently, multicast achieve a higher priority than concurrent unicast and the network may become severely congested and could collapse. Second, it is not possible to adapt the physical (PHY) transmission rate to the channel characteristics, so the packets are broadcast over the wireless medium at one of the rates included in the
basic rates set. Third, there is no way to retransmit lost packets at the MAC layer, so the transmission is more lossy than for unicast , which degrades performance of the multicast application.
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看來802.11先天上還真是不適合直接拿來送multicast就是了,不過有些AP有強調適應multimedia的部份,不知是不是有針對這些部份避開802.11的先天限制  ... |