Collsion Domain and Full Duplex

[Pratyush Sinha]

Hello All,

I have a very basic query on Collision Domain and Full Duplex Mode. There are two types of logical topology ; Ethernet and Token Ring .

Whenever we read about Ethernet the first thing that we are taught is CSMA/CD. It says that in Ethernet two computers can not send data simultaneously. I am trying to figure out why it is not possible.

Scenario 1. Two computers connected directly to each other via ethernet cable . No switch or hub in between. Both the adapters are in full duplex mode. In this scenario if both the computers started pinging each other will it not work. If no then why not and if it works then why is there no CSMA/CD involved.

Scenario 2. Two computers connected to each other via a plain hub( not switch).  In this scenario can two computers ping each other simultaneously.

Does it matter on the type of cable ; CAT or Coaxial .

Please let me know if I am missing anything.

[Joseph W. Doherty]

Does it matter on the type of cable ; CAT or Coaxial .

Yes it does.  If you're not already familiar, you might read up on Ethernet's 10Base-5 and 10Base-2, then CSMA/CD's restrictions on sending and receiving at the same time will make more sense.

In your two scenarios, especially scenario #1, I assume you're using 10Base-T.  In scenario #1, if both port are configured for full duplex, then they should be able to ping each other concurrently, as you really do have a full duplex path between them.

In scenario #2, normally you would have all hosts configured in half duplex mode.  This because a hub will take one port's received input and replicate it to all the other ports output.  I.e. two ports transmitting at the same time can collide on another port.

However, if you only had just two hosts attached to a hub, you should be able to run both in full duplex mode, as you've effectively recreated your scenario #1 situation.

The difference between half duplex mode and full duplex mode, using twisted pair cable, the former expects there might be collisions, the later does not.

[pwwiddicombe]

With a true hub, you can't really have stations running full duplex - even if you only have 2.  You can't have both transmitting at the same time.  Now, if they were both full duplex and connected with a crossover cable, you could.

The reason you can with a SWITCH is the switch will buffer incoming frames, and then pass them off to either the "learned" destination port, or to all ports if the destination MAC address isn't known.

[Joseph W. Doherty]

Now, if they were both full duplex and connected with a crossover cable, you could.

Agreed, because with twisted pair there's a transmit and receive pin, and a cross over connects one host's transmit pin with the other host's receive pin.

With a true hub, you can't really have stations running full duplex - even if you only have 2.  You can't have both transmitting at the same time.

That depends on whether the hub, itself, detects a collision and generates a jam signal or it relies on the connected hosts to detect it, generate their own jam signal, and just propagates it.  If the latter, then for two hosts set to full duplex mode, operation should be like a direct connect.

Doing some quick research, I found mention that hubs can, themselves, detect collisions and generate a jam signal, but I also found mention that hubs do not detect collisions.  So it's unclear whether hubs, themselves, detecting and generating a jam signal was a required standard or it was only an enhanced feature of some "better" hubs.  I suspect the latter after reading this: http://www.google.com/patents/US5978383.