On Fri, 21 Mar 2003, Gareth Williams wrote: ... > I somehow overlooked that DSL definitely != ethernet. doh *whacks forehead* ...
To contribute a little bit to the confusion, that would not be a problem for a bridge. As long as it is working on the data link layer on both sides, it could still be a bridge, but then it needs some interworking functionality to match the two network standards. > One more question, if I may - I thought bridges were switched (at > the ethernet layer). Yes. And strictly spoken, in TCP/IP terminology, it is the data link layer. > You imply that a bridge is simply a 2 port hub That would be wrong. > ... bridge's intended function was different from a hub... ie. > hub -> join end terminals together > bridge -> join networks together A bit more precisely, one might say - A hub establishes a network segment (like the old coax cable did) by joining hosts. - A bridge joins network segments, and it may perform packet filtering on the data link layer. > identical, no? So... is then "hub" just another word for "bridge" > (ie. they are essentially the same device) ? No, definitely not! A hub in the traditional sense (not modern stuff like switch hubs) is only a signal regeneration and distribution device. It does not look at the semantics of whatever layer. All traffic from all input ports is copied to all other output ports. If you connect too many hosts to one segment, network throughput will break down, because Ethernet uses a CSMA/CD medium access. To build up larger networks, you have to keep local traffic local. That is what bridges can do. A bridge receives and transmits protocol data units (PDUs) on the data link layer, which are called Ethernet frames in Ethernet networks. Hence a bridge cares about the Ethernet headers, and the beauty of this is that self-learning bridges can learn the network topology around them from the source address from headers of packets they receive on their ports. From the information gathered, they can build up a table allowing them to forward frames only to the port where the actual destination is connected, thus reducing traffic load on all the connected segments. Routers are still a different story, one layer further up, on the network layer. With the hierarchy of IP addresses corresponding in many parts to the actual physical network topology, a router can easily identify which IP packets have to stay within the local network, and which have to go outside. The price for this is that processing on the relatively high network layer has to be done, which traditionally was a task completed by an embedded processor. More modern approaches join the desired functionality of routers with the high speed offered by switches. But this leads too far... You will get more info on the different approaches from manufacturers like Allied Telesyn here in Christchurch, Cisco, 3com, etc. Cheers, Helmut. +----------------+ | Helmut Walle | | [EMAIL PROTECTED] | | 03 - 388 39 54 | +----------------+
