Again: Collisions on switches

[Photogregor]

Hi,

I read a lot about that, but surprisingly the question still is not answered: How can a collision occur on a switched network, using twisted pair cables with different wires for sending and receiving? Most answers simply state, that with half duplex CSMA/CD is used, but that's no answer to the question. Whether on full duplex, half duplex or (erroneously) mixed settings pakets are sent and received on different wires, the switch stores pakets in different buffers and forwards them on separate receive or transmit lanes. So even if both hosts send at the same time these pakets never see each other.

So why ports on a switch still are called collision domain? Why do they, at least with half duplex, need CSMA/CD?

Regards,
Stefano

[Richard Burts]

Stefano

Part of the definition of half duplex is that collisions are possible. If a switch port is configured for half duplex then the interface operates the function to check for collisions. Perhaps the question might better be why would a switch port with a transmit wire and a receive wire be configured for half duplex?

HTH

Rick

[Photogregor]

Hi Richard, machines not capable of full duplex are dying out, sure. But I never understood it in the theory, because on RJ-45, twisted pair and switch technique I don't see any shared media. So how could CSMA/CD ever detect colliding frames? Is it that what you mean? They do CSMA/CD because it is part of the definition, even if collisions never can occur? Regards, Stefano

[ahmedshoaib]

Hi;

As half duplex definition mentioned data can be transmitted in both directions on a signal carrier, but not at the same time. means to transmit or received data we have only 1 path. UTP/Fiber cable are just media which carry the data from 1 device to another device. Traffic forward on the media is responsibility of transmit/received circuit. 

If my device/switch port is half-duplex means I have only 1 circuit which is responsible for transmit/received data and required CSMA/CD protocol to avoid collision on half duplex or Hub. 

If my device/switch port is full-duplex means I have 2 separate circuit for transmit & received data.

Thanks & Best regards;

[Richard Burts · Accepted solution]

I am not sure that I understand the point from Stefano. Perhaps it comes down to semantics in how we define collision. If you interpret an Ethernet collision as a situation where incoming signal physically impacts outgoing signal the on twisted pair it would not be possible to have an Ethernet collision. But if you interpret collision as receiving data at the same time that you are sending data then it is quite possible to have an Ethernet collision on twisted pair.

HTH

Rick 

[Photogregor]

That's exactly the point. Thanks for clearing that. I always was searching for the place where the signals could collide as they did on the former coax cable.