Date: Wed, 19 Jun 2002 08:13:12 -0700 From: "Kevin Oberman" <oberman@es.net> To: freebsd-mobile@freebsd.org Subject: Re: wireless lans with multiple accesspoints Message-ID: <20020619151312.063275D06@ptavv.es.net> In-Reply-To: Your message of "Wed, 19 Jun 2002 10:56:17 EDT." <20020619145617.GB23903@pir.net>
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> Date: Wed, 19 Jun 2002 10:56:17 -0400 > From: Peter Radcliffe <pir@pir.net> > Sender: owner-freebsd-mobile@FreeBSD.ORG > > Kevin Oberman <oberman@es.net> probably said: > > The use of that term goes way back and it really is no longer > > appropriate in the era of full-duplex where collision simply don't > > exist. But it matters here. > > It is still appropriate in the era of full-duplex, and collisions do > exist even with full-duplex switches. > > Switches are still the same broadcast domain, you can still get > collisions for any packet that is not unicast or is broadcast to all > ports by the switch for another reason. If the per port buffering > is small or there is just too much traffic ... it's much, much > less likely to happen, but can still happen. > > Some switches produce collisions to signal the kit on that port to > back off if the egress port is highly congested, for example. Simply stated, no. Per 802.3, collisions are a local event. There is no way for a switch to "send a collision" and the section of 802.3 on full-duplex is specific that collision detection MUST be disabled. A switch does not divide a broadcast domain, but does divide a collision domain. There is a push-back signaling system in 802.3, but it is not a collision. Collisions do back-off and retransmit at the MAC level. Switches are "store and forward" devices, even if they do cut-through forwarding, and they handle deferrals due to busy ports in firmware and not the MAC. There simply are no collisions here. R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-mobile" in the body of the message
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