Date: Sat, 27 Sep 1997 09:16:17 -0400 (EDT) From: "David E. Cross" <dec@phoenix.its.rpi.edu> To: sthaug@nethelp.no Cc: tlambert@primenet.com, hackers@FreeBSD.ORG, stable@FreeBSD.ORG Subject: Re: 'fxp' driver/hardware lossage (was Re: Alexander B. Povol's mail) Message-ID: <Pine.BSF.3.96.970927091018.16019C-100000@phoenix.its.rpi.edu> In-Reply-To: <17057.875363985@verdi.nethelp.no>
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On Sat, 27 Sep 1997 sthaug@nethelp.no wrote: > > > That's only true of coaxial media. Over twisted pair, if you see > > > data start arriving on the receive pair while your transmitting then > > > you have to assume that the data is being sent by another station > > > and you've got a collision situation. > > > > Isn't this the hub's responsibility to distinguish and prevent? > > Nope. A hub doesn't do anything with collisions - it just propagates > them bit by bit. The NICs sense the collision. > > Steinar Haug, Nethelp consulting, sthaug@nethelp.no Technically this is not even a colission situation. Many new NICs and Hubs (both must support it to work) support full-duplex 10BaseT, allowing 20MBits/sec. I am not sure what happens when it gets into the hub and needs to be propogated to other ports though *shrug*. Anyway... multiple packets being transmitted at the same time is detected by the NIC, and the NIC then sends a jamming signal which stops all transmissions on the network for a random time (each card that was transmitting durring this time sets a ranom timer for how long to wait, and they all try to re-transmit, using exponential backoff if there are additional collisions). This jamming signal is the actual 'collision', and is what shows up as a colision light on hubs/external transcievers. Excellent and page-turning reading that 802.* ;) -- David Cross
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