Please explain why an RJ45 splitter won't work

TomÃ¡s Ã hÃilidhe · 2008-03-28T18:03:42+00:00

I understand that you can't use an RJ45 splitter as an ethernet"double adapter".But could someone please explain to me why? I'm not thick, I'llunderstand. . .I was under the impression that a very basic hub was a "dumb device"that just connected certain wires together; I thought you'd be able tomake one using a wire stripplers, an ethernet cable, and a few RJ45's.Or does a hub need some sort of processing?

Mar 28, 2008 23 Replies

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glen herrmannsfeldt 18 years ago

(snip)

Note, though, that is against the rules.

If both sides are ready to send, at some point in the inter packet gap they will make the decision to send. If a signal is detected past that point, the station must send anyway to generate the collision.

This is needed to make collision resolution fair to all. Otherwise, it would be more or less fair depending on the exact point the carrier sense was done.

Well, okay, but the jam signal is the data that it is required to send.

Well, what do you mean by physical collision? A collision on TP ethernet is detected by receiving a signal during transmission. Do you not call it a physical collision as the two are on different wire pairs?

-- glen

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News Reader 18 years ago

Each switch port is its own collision domain.

Collisions on one switch port are not seen by any devices on other switch ports.

Duplex settings on one switch port (or the devices connected to it), have no bearing on any device connected to other switch ports.

If a single host is connected to a switch port, and the two are configured in half duplex mode, they will use collision detection.

If the two are configured for full duplex, collision detection is disabled.

Best Regards, News Reader

Robert Redelmeier wrote:

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Rich Seifert 18 years ago

No; in half-duplex mode (which is the condition assumed in the post), frame reception during frame transmission is *the definition* of a collision. In full-duplex mode, that same condition (receive while transmitting) is *not* defined as a collision. That's the difference.

There will indeed be collisions in a two-station LAN implemented through a crossover cable. Collisions are guaranteed any time both stations have one or more frames in their transmit queue at the same time.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

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Rich Seifert 18 years ago

No "ghost". A collision is defined as two stations attempting to transmit on the same half-duplex link at the same time. This can occur (and does, often) with a two-station *half-duplex* link implemented through a crossover cable. That crossover cable is a half-duplex link, even though it *could have* been configured as a full-duplex link. If the stations are configured in half-duplex, then collisions occur. If configured in full-duplex, then collisions do not occur. Simple.

-- Rich Seifert Networks and Communications Consulting 21885 Bear Creek Way (408) 395-5700 Los Gatos, CA 95033 (408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

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