CSMA\CD Question

(snip)

Repeaters need to detect receive mode collisions (even when they aren't transmitting), but ordinary end stations don't.

-- glen

The IEEE-standard algorithm specifies that stations only adjust their retransmission timers based on collisions to which they are a party. Thus, while both A and B may be able to detect the fact that there was a collision (not involving themselves), they will not use this information to adjust their retransmission timers.

In the original (coaxial Ethernet) design, it could not always be guaranteed that a listening station would always detect collisions between 3rd-parties; thus, the algorithm was specifically designed not to depend on that ability.

Transceivers do not detect collisions by comparing the transmitted data to received data. The loopback of TX to RX came "naturally" in the design of the original coaxial Ethernet. To maintain backwards compatibility with existing controller silicon, we included this capability in later 10BASE-T systems.

We actually considered the behavioral impact of using so-called "receive-based collision detection" in the original design, and concluded that any performance improvements were minimal, and not worth the added design complexity.

-- 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

"Jam signal" is a misnomer. There is absolutely no difference between the signal transmitted for "jam" and the signal transmitted for ordinary data. "Jam" simply means "continue sending for some additional time," to ensure that all parties to the collision properly detect it.

Stations detect collision in a medium-specific manner (the method of detection is different in coaxial vs. twisted-pair vs. fiber systems, etc.), through circuitry in the transceiver. Every transmitting station, upon detecting a collision, "jams", that is, continues to send data for another 32 bit times, before ceasing transmission and rescheduling the frame.

Of course, all of this is now somewhat moot, as the majority of devices operate in full-duplex mode using switched connections. Full-duplex mode eliminates the entire CSMA/CD algorithm: no collision detect, no jam, no backoff, no retransmission. Any discussion of the details of CSMA/CD is of purely historic interest.

-- 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

In what way? Do repeaters have to detect the collision itself or just the jam signal?

The way it's being presented to me says that any station that the first station to detect the collision sends out the jam signal. If this is the case, wouldn't then pretty much everyone detect it 'first' because of propagation delays?

(snip)

There is a National Semiconductor application note describing both transmit mode and receive mode collision detect for coaxial cable ethernet.

It was my understanding that repeaters required receive mode collision detect. In the application note they determine the parameters for running ethernet on 75 ohm and 93 ohm coax, and they do both modes separately.

Using external resistors on the 8392 they can properly detect collisions on such cables.

-- glen

They only detect a collision if their workstation sends and collides.