Switch Trafficking

Question

I'm running a simulation in riverbed modeler, and I've got two broadcasting domains each having one Ethernet advanced hub and 16 Ethernet advanced stations connected to it through 10BaseT links. Subsequently, another 10BaseT link has been adopted to connect together the hubs and switch whereby the switch would be in the middle of both hubs.

After executing the simulation the results are like this;

• Traffic forwarded by switch : 4 million bits/sec
• Traffic received by switch : 8 million bits/sec
• Point to point through switch to hub1 : 4 million bits/sec
• Point to point through switch to hub2 : 2 million bits/sec

I've studied all the possible factors such as collision count in collision domains, queueing delay for point to points, and the question is why switch behaves in this manner ? why the traffic being forwarded to hub1 varies from the traffic being forwarded to hub2 ? why the traffic on point to point throughput to hub 2 is 2 million bits/sec whereas the traffic on point to point throughput to hub 1 is 4 million bits/sec ? I've read all about the switch technologies, but I haven't found any relative topic to how does switch traffic in such cases ? any source or suggestion would be highly appreciated.
This scenario is in an office (100m x 100m).

Answer 1

I think you have used wrong terminology on your exposition. Switches do not isolate broadcast domains, that is done by routers. However, what switches do is isolating collision domains so in your picture you have two collision domains, one hub-1 and another one hub-2 both under the same broadcast domain. Having said that, my understanding is that “traffic received” on the switch includes the sensing of the collision domain so collisions get accounted on the amount of “received traffic per second”; these collisions sensed on the receiving are not forwarded and so less traffic per second on the “forwarded traffic per second”.