Need help with Collision domain

sid_gudos

How many collision domains are there when two different computers are connected to two different hubs and then these two hubs are connected to a bridge? how to calculate them?

MickQ

Think about it.
What is a collision domain? Where are the boundaries? What's a broadcast domain? What's the difference between the two, and why?

NetworkVeteran

Sid, this is one of those topics you must master to have any hope of landing most networkings jobs. I'd suggest answering Mick's questions and then explaining your best guess based on those answers. We're here to help if you should stumble along the way.

sizeon

The answer is 2 collision domains. Bridges and switches beak-up collision domains. In other words, every interface on a bridge or switch is it's own collision domain.

Ltat42a

Here's a visual....
1 Broadcast domain, 2 Collision domains

drkat

So much for the let him figure it out approach.

MickQ

Well since the answer for the collision domains has been given, how about we let sid answer how many broadcast domains there are?

TehToG

That's been answered too.

People need to use this forum more to help people and less to validate their own knowledge

MickQ

TehToG wrote: »

That's been answered too.

So it has. I missed that bit - mea culpa.

TehToG wrote: »

People need to use this forum more to help people and less to validate their own knowledge

I'll point out that two CCNPs have encouraged the OP to think this one through.
We are here and willing to help others. We aren't willing to spoon feed.
We want to be able to rely on the junior network admin, who's just starting with us, knowing their stuff rather than being a paper admin or even a dumper.
As NetVet said, it's a vital part of understanding networking.

TehToG

I wonder if we could get a rule change to that effect?

johnifanx98

MickQ wrote: »

Think about it.
What is a collision domain? Where are the boundaries? What's a broadcast domain? What's the difference between the two, and why?

Should duplex mode be factored here?

MickQ

Not exactly.

Remember that a collision domain exists between (a minimum of) layer 2 devices. Say a PC (up to layer 7) and a switch (layer 2). MAC addresses are used in the communications here.

A broadcast domain exists between layer 3 devices. E.g. you hooking your PC (L7) up to a router (L3) and then going out to surf techexams.net. IP addressses are the communications here.

It's simplifying it a fair bit, but it'll help you understand the differences (I hope).


Now, about duplex.
Half duplex is like a walkie talkie. Both sides can speak to each other, but only one at a time or they cut each other off.
Full duplex is like a phone. Both sides can speak to each other at the same time. Just like me telling my mother that I get the point in what she's telling me... many many times over, and trying to cut her off.

Duplex can be full when there's only two devices on the collision domain (e.g. PC to switch). It should be half duplex (only one side talking at a time) when there are more than two devices sharing the collision domain (e.g. 2/3/4/+ PCs connected to a hub which is connected to a router).

We're operating in full mode for most of our wired networks. Each PC/Switch/Router is connecting to one port of a switch (layer 2 device). This way you can get a flow of data going to your host, and also one going away from it. I'm sure you can look up the technical reasons why.

The interesting thing is that we are still using half duplex for a (relatively) new technology - wifi.
The medium (radio spectrum) is shared amongst everyone, so your devices can only transmit when no-one else is talking. It's like the old PCs connected to a hub days, but with air instead of wires.

johnifanx98

MickQ wrote: »

Think about it.
What is a collision domain? Where are the boundaries? What's a broadcast domain? What's the difference between the two, and why?

[Domain] it looks like an abstract concept. No, it is not. Mathematically a domain represents the set of elements inside it.
[broadcast domain] as above, a broadcast domain represents the set of all work stations that a broadcast can reach.
[collision domain] similarly, a collision domain represents the set of all work stations that could collide with each other.

So, it looks like duplex mode does impact the collision domain but not broadcast domain.

TehToG

You've got it backwards. Can you have an environment where you can have full duplex AND still have collisions? Explain your answer.

johnifanx98

TehToG wrote: »

You've got it backwards. Can you have an environment where you can have full duplex AND still have collisions? Explain your answer.

Just thought it twice. I realize that it's inappropriate to state "duplex mode does impact the collision domain". In the case of switch/bridge, each port is a segment, or, collision domain. A full-duplex connection simply eliminates the collision. And a half-duplex connection means collision with the switch port. It does not change the collision domain at all. In the case of hub, full-duplex will be downgraded to half-duplex anyway. So, the duplex mode does not matter either...

TehToG

Full duplex means data can be sent and received at the same time. Think about how data works over a physical media.

On a cat5 cable you use two wires to create a circuit and by varying the voltage between high and low we get 0 and 1s (this is how our bits are coded). In order to send and receive we need two pairs. (one for send, one for receive)

A hub is just a big circuit. Now if we ever share the same circuit for more than one voltage source we can potentially get a collision if both devices try to use the media at once.

A switch allows multiple devices to be connected without the devices being on the same circuit. Therefore no collisions occur. In order to send and receive at the same time you NEED separate circuits otherwise the send and receive would be working on the same circuit and cause a collision.


Simply put, Full duplex mode REQUIRES that the Rx/Tx pair have their own collision domain.