Thanks! -----Original Message----- From: Juliusz Chroboczek [mailto:[email protected]] Sent: Thursday, August 06, 2015 8:43 PM To: Eric Gray Cc: Homenet Subject: Multicast in 802.11 [was: Despair] Importance: High
> I wasn't aware of the treatment of multicast packets as less than best > effort in wireless transmission. That is not exactly intuitive, given > that radio is inherently broadcast. Yes, that's counter-intuitive, but actually quite natural. 802.11 uses two different MAC sublayers: for unicast, it uses ARQ (typically 8-persistent) and varies the PHY rate (depending on e.g. pre-ARQ packet loss), while for multicast, it uses no ARQ and a fixed PHY rate. The effect is that unicast uses a higher data rate and virtually zero packet loss, while multicast uses a low data rate and suffers from significant packet loss (20% - 40% is not unusual for full-size frames, and I've seen 80% on a link that was quite usable, post-ARQ, for reading mail over IMAP). This is good enough for IPv6 (both ND and RA use little throughput and deal gracefully with reasonable packet loss), but makes multicast useless for some other applications. -- Juliusz _______________________________________________ homenet mailing list [email protected] https://www.ietf.org/mailman/listinfo/homenet
