Collision Domains 1

[NFI]

Hello,

sorry if this is a bit of a basic question;

I understand the concept of collision domains, in they they form electrical sub-networks to avoid the problem of multiple machines connecting to the network media at the same time.

What happens, though, in this situation;

1. Machine A sends data to machine B via a switch
2. Switch sets up a collision domain between A and B
3. Machine C tried to talk to machine B
4. ?

If a collision domain is already in place between two hosts and a third host wants to talk to either of them, what happens? Does it have to wait for the switch to "deconstruct" the collision domain, or does it become part of the same collision domain?

Any thoughts will be much appreciated,

Thanks,

Paul

 

[jimbopalmer]

OK, once you have full duplex using switches, there ARE no collisions. (some vendors still report buffer overflows as collisions) If a packet from C to A starts before a packet from B to A finishes, it is buffered in the switch, at that port, until the B packet is all sent, then it follows it.

If you are using still half duplex with switches, there is more buffering, as it also has to delay packets if A is sending, but no collisions between B and C. If A and the switch port send 'at the same time' there is a collision, but B and C's packets get buffered. If effect the collision domain is from the half duplex device to the switch, not from the half duplex device all the way to the other device. (and yes, a switch WILL speed up a large network that has mostly half duplex devices, as the number of collisions is much lower than a hub)

Port buffering also allows high speed ports to talk to low speed ports, packets just back up in the buffer.

Putting priorities in the port buffer allows a switch to support 802.1p

I tried to remain child-like, all I acheived was childish.

 

[NFI]

Oh, I *see*...

I knew switches buffered packets, but it had never struck me that this was why...

Thanks for explaining that

Paul