network collisions

I have taken care of a network for a long time, and since it has switches, collisions don't seem to be a problem. But I started studying to take the Network + exam, and I can't seem to visualize the whole concept of a collision domain.
So if you have a hub ( which i am assuming, they don't exist anymore, but in theory ) then all the computers attached to the hub are in a collision domain.
but if you remove the hub, and replace it with a switch, then each computer connected to the switch becomes it's own collision domain. is that meaning that from the switch, along the wire to the computer, that is considered a collision domain? and why even bother calling it a collision domain? is it even possible for the computer to have a collision with itself? doesn't the switch prevent collisions because it examines packets ( or would it be frames? ) and only sends the packet to the correct port that the destination computer resides on? i get the collisions thing when every computer can send at the same time, and then csma\cd kicks in, so on a switch is it still a thing that every computer can send at the same time? but if the switch is only sending to the destination computer, how could a collision occur?
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mcsweenSr. Network AdministratorCommented:
The reason hubs create collision domains of all systems attached vs switches that create a collision domain of everything attached to a single port is because a hub sends all packets out all ports where a switch is smart enough to only send packets to the port that needs them.

Collision domains aren't even spoken of in a switched network because they essentially don't exist.  A single host cannot have a collision with itself but is technically in a collision domain by itself.

It's a frame on the Network Access Layer (Ethernet, Token Ring, etc..)
It's a packet on the network layer (TCP/IP, IPX/SPX, NetBIOS, etc...)
JeffBeallAuthor Commented:
ok so my question

"is it even possible for the computer to have a collision with itself?"

is that it is NOT possible for a computer to have a collision with itself, because like you said

"A single host cannot have a collision with itself but is technically in a collision domain by itself."

so also like you said "technically" the computer is in a collision domain by itself - it's just a technicallity - it doesn't potentially have collisions with itself.
AkinsdNetwork AdministratorCommented:
Let me throw an analogy out there to visualize this.

Is it possible to have car collisions on a divided two-way street when all traffic is going the same direction?

Answer is obviously yes.

With hubs, what you have is 1 single collision domain, with switches, what you have is multiple collision domains depending on the number of ports on the switch. This reduces the chances of collision because all the traffic are not going through the same port.  

Don't visualize collision by the port or road but visualize it using packets that travel. It is the packets that collide. There is a higher probability of collision when traffic is congested.  Several hundreds of packets may be generated just by opening a web browser. There could be several destinations depending on the page opened, eg adverts, images, etc

Remember also that all the ports on the switch have a common exit point. The switch is just smart enough to manage the traffic and therefore drastically reduce collision by detecting. The only way to avoid collision eg in token ring, is to have devices send particular traffic one at a time. You'll agree that will be a very slow system.
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JeffBeallAuthor Commented:
ok, so for your anaolgy, would it be accurate to say, the packets are like the cars. and in a switch, the "road" is the port on the switch to the computer through the cat 5 or 6 cable. for full duplex, that road is 4 lanes represented by the 4 wires for sending, and the other 4 wires for receiving. so it is like a 4 lane divided highway. then, for instance, in the "lanes" for sending all the packets ( cars ) are flowing nicely, until some idiot cuts off another packet. Now the packets react by slowing down, and then another idiot that is texting and driving, doesn't see the slow down, and collides with a packet in front of him.
also, the switch would be like a traffic cop that can "see" the source and destination of the packets. So of example, the switch knows all packets coming from port A, need to go to location X, should go through switch port C, for example. so the switching part of a switch is that as the packets come out of port A the traffic cop puts detour sign up for those packets, which directs them through port C.
AkinsdNetwork AdministratorCommented:
I like your analogy........:)

Right on point!

I'm still trying to visualize the packet that's texting while

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JeffBeallAuthor Commented:
thank you for the help
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