A question about Collision Domains

Okay hope this comes out well

lets just say i have the following as shown by the diagram

ok i understand that layer 2 and 3 devices create seperate collision domains based on each device attached from their ports.

So from this diagram there are 3 switches

The left most switch creates one collision domain (the hub and nodes below it)

The next switch creates another 2 collision domains for each node attatched to it

and the last switch has one node attached to it so theres another collision domain.

The part that confuses me is the top physical medium. As each switch is connected to this line does that mean that theres anotehr 3 collision domains here?

so 7 collision domains altogether is what i thought..
But apparently there is only 5, and im not too clear on this can anyone clarify why?

Connecting switches is usually a trunk link. And I don't believe a Trunk link is considered a collision Domain.

I count 4 collision domains...but I've been up for 22 hours and i'm a little zonked.... So i might change my mind in the morning.

cheers.

I can only assume the link connecting all three switches is logically a shared medium and hence a single collision domain - not sure how that translates physically though.

A switch does not create collision domains, it separates them, so only when you have a device that creates collision domains atached to a switch, you have a collision domain.

Basicaly

1 - a hub creates collision domains
2 - a switch separates collision domains and creates broadcast domains
3 - router separates broadcast domains

If you go back to the days of a physical bus topology, a single physical bus equalled one Ethernet segment equalled one collision domain. Now if you connected a second Ethernet segment to the end of the first one using a repeater you had two segments but one collision domain as a repeater by definition propogates collisions. Bringing this up to date a bit, a hub is basically a multi-port repeater so all the little point-to-point segments (really the network on each hub port consists of a logical Ethernet bus with only two devices - one at each end - wired in a physical star configuration) are all part of the same collision domain. Switches, on the other hand, don't propogate collisions so on a switched network each switch port and the little point-to-point network hanging off it (just two devices again) are their own collision domain. However you're unlikely to see any collisions there (why would you with only two devices?) unless you run the interfaces in half-duplex mode