collision domain, utp cable and switch fabric - curious~

[SJ K]

Hi all,

I am very curious about how the internals of a switch break up collision domain.

1) I understand that for a UTP cable, there are RX and TX twisted pair, so there transfer and receive to the same host are on different path, hence there is no collision.

2) Hub is using a single shared media (wire) and hence every device attached to it is in the same collision domain as data transfer through the same medium.

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Q1) How about internal fabric of a switch ?   How does a switch breakup collision domain ?

I am thinking on a switch with 3 ports (a,b,c).  Each port is connected to a host (a,b,c).

Assuming Host B and C send to Host A at the same time, will they be traveling on the same path to A in the internals of the switch ?
Or each port has its own "line/fabric" to each of the other ports in the switch ?

E.G. port A has a 2 way fabric (rx,tx) to port B
        port A has another 2 way fabric (rx,tx) to port C

        port B has a 2 way fabric (rx,tx) to port A
        port B has a 2 way fabric (rx,tx) to port C

        port C has a 2 way fabric (rx,tx) to port A
        port C has a 2 way fabric (rx,tx) to port B

In this way, i can understand when B and C send data to A, they are traveling on the different "path" to A(hence no collision); and when both the data reaches port A, there might be some buffer in place which will store both "B" and "C" data 1st, and send "B" or "C" data through the cable to host A in sequence.

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If the above is wrong, then how does a switch breakup collision domain ?

Regards,
Noob

[Jon Marshall · Accepted solution]

Different switches have different internal architectures but basically yes each port has a path to every other port using a switch crossbar fabric to achieve this.

One of the key aspects of the switch fabric is arbitration which is the switches ability to realise when there could be a potential collision within the switch fabric and it uses buffers to avoid this collision.

But obviously switches have limited buffers so if enough traffic is being generated and the buffers overflow then packets have to be dropped to avoid collisions within the switch fabric.

Have a look at this link which goes into it in a lot more detail. There are also additional links at the bottom that cover other aspects of the internals of switches -

http://etherealmind.com/switch-fabrics-farbic-arbitration-and-buffers/

Jon

[SJ K]

Hi Jon,

Thank you.

I know this is not related but I am in a new domain in career where i really start to doubt my capabilities.

Your answer and response somehow assures me. I hope to be as good as you some day.

Regards,
Noob

[Jon Marshall]

I hope to be as good as you some day.

Well, thank you for the compliment, but I think you should aim higher than that :-)

Jon