# Full-duplex collisions in wires

I don't know that much in detail how full-duplex works and I wonder if collisions can occur in them. Take a UTP cable for an example - are some cables within it reserved for data going one way and the other wires for data going the other way? Can two frames going in the same direction crash with each other?

• Consult the definition of full duplex. No collisions are possible; TX and RX happen at the same time. Mar 3, 2014 at 14:18
• @RickyBeam Okay, TX is transmit and RX is receive, right? But what does the 'X' stand for? Mar 3, 2014 at 14:49

Full-duplex basically means communication can happen both ways, negating the possibility of a collision. The below diagram should make it a little more clear. One pair is designated a transmit pair, and the other a receive pair. In most environments now, it isn't even necessary to match up transmit/receive pairs, as MDIX handles that for you.

This doesn't eliminate the possibility of collisions on one side as a result of a configuration mistake, such as half-duplex on one side and full-duplex on the other. That's fairly common.

Can two frames going in the same direction crash with each other?

No, frames wait in a buffer until they can be transmitted across a link. One by one.

• Okay cool :) So what does the 'X' stand for in RX and TX? @fizzle Mar 3, 2014 at 17:54
• @AxelKennedal-TechTutor I honestly don't know, the most I can discern from it's use is that it stands for "Receive Pair" and "Transmit Pair". Perhaps because there is a tip/ring configuration on each pair that twists (crosses) along the length of the wire? Mar 3, 2014 at 17:57
• Tx and Rx are common abbreviations for transmit and receive. X has no special meaning other than an abbreviation. FYI, pilots sometimes abbreviate weather as wx Mar 3, 2014 at 18:02
• Also to clarify, the Rx and Tx pairs are used in both full-duplex and half-duplex. The reason for collisions is not that the same pairs are used, but because of how the devices that connect half-duplex connections (hubs) repeat the signal. Mar 3, 2014 at 18:14

I don't know that much in detail how full-duplex works and I wonder if collisions can occur in them.

No.

Take a UTP cable for an example - are some cables within it reserved for data going one way and the other wires for data going the other way?

That depends on which version of Ethernet you are talking about.

10BASE-T and 100BASE-TX have a seperate pair of wires for each direction.

1000BASE-T uses all four pairs in both directions at the same time. They use echo cancellation techniques to seperate transmitted and received data.

Can two frames going in the same direction crash with each other?

No, Dumb multiport repeaters (hubs) are not allowed in full duplex networks. Bridges (switches) queue frames for each port in an output buffer and send them out one at a time.

Note that while frames cannot collide on the wire in a full duplex ethernet network they can be dropped if the buffers overflow either because of a speed change or because multiple sources are sending to the same destination.