# Full Duplex and Collision Domain

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 not, 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 a switch). In this scenario can two computers ping each other simultaneously?

Does it matter on the type of cable: UTP or Coaxial?

• Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer. Commented Jan 5, 2021 at 17:31

You seem to be confused about logical topology (bus, ring, star). Ethernet has a couple of different topologies, but token ring has only the logical ring topology.

Full-duplex ethernet doesn't use CSMA/CD because it has separate send and receive paths between two devices. When you share send and receive paths, then you must use CSMA/CD in order to detect collisions (two devices sending at the same time). With full-duplex, there is no possibility of collisions; the send path on one device is the receive path on the other end, and vice versa.

A hub means you must use half-duplex because the possibility of collisions exists, and you must detect the collisions, so you use CSMA/CD. The devices connected to the hub take turns sending (and resending when there are collisions). Each will have the opportunity to get frames on the wire, so, yes, they can ping each other at the same time.

For ethernet: Coaxial cable only has a single path, so it must be half-duplex. UTP cable as point-to-point between two devices can use full-duplex because each device has separate send and receive paths, but on a hub, you have multiple devices trying to send on on the same path, and receive on the same path, so you must use half-duplex with CSMA/CD. Fiber will have separate send and receive paths.

For token ring: there can be no collisions because only one device possesses the token at a time, and no device can send without possession of the token.

For Wi-Fi: all devices share the medium, so there are going to be collisions. Wi-Fi doesn't use CSMA/CD. CSMA/CA was developed for Wi-Fi to try to avoid collisions.

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.

Unfortunately there is a lot of reference material out there that is horribly outdated and/or doesn't make a clear distinction between "original Ethernet" and "modern Ethernet".

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?

It will work

if it works then why is there no CSMA/CD involved?

There is no CSMA/CD in full duplex mode. Collisions simply cannot happen.

Does it matter on the type of cable: UTP or Coaxial?

Yes, coaxial Ethernet cannot support full duplex mode. Only the twisted pair and fiber physical layers can.

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

No, hubs cannot support full duplex mode.

• Minor point: strictly speaking if there's only 2 computers on the hub, there cannot be collision (it is functionally equivalent to a cross over cable between the two hosts). With 3 computers, collisions will happen.
– JFL
Commented Mar 8, 2017 at 22:51
• A hub will force the links to half-duplex mode. In half-duplex mode two stations transmitting at the same time is considered a collision (even if nothing actually collided "on the wire") Commented Mar 9, 2017 at 23:52
• Ho, I thought even on a hub you can have full-duplex. Thanks for correcting me.
– JFL
Commented Mar 10, 2017 at 7:30