previous | start | next

Ethernet/802.3 Switches

An Ethernet switch has a similar function to a hub (see earlier) -- switches are sometimes called "switching hubs".
 
The difference is that a switch examines the MAC-level destination address of every frame it receives, and transfers it directly to the appropriate port, without other ports being involved in, or aware of, the communication. Many such transfers can occur simultaneously, which has the effect of increasing overall "system" throughput. Typical switches can also do full-duplex transfers, simultaneously transmitting and receiving frames.
 
Note:
A switch builds a table mapping source addresses to ports which it subsequently uses to make switching decisions. It's obvious that switches are significantly more complex than simple hubs, and this is reflected in their price. However, as with all technologies, the difference is rapidly diminishing.

 
Philsophical note: it's interesting that switches essentially remove the possibility of collisions. We should therefore imagine that network hardware is no longer executing the CSMA/CD protocol, because a host can send a frame at any time. This is ony partly true -- collisions can, in fact, still occur, and the important point is that an Ethernet switch can still interoperate with older, non-switching hardware. The retention of the same frame format is an important aspect of this interoperability.
 
Lecture 13: Multiaccess Networks Copyright © 2004 P.Scott, La Trobe University Bendigo.



previous | start | next