Date : Tue, 14 Jul 2009 12:55:14 +0000 (GMT)
From : itonks@... (Ian Tonks)
Subject: Is it a bridge? Was: Master Ethernet upgrade
I think this discussion could run and run ;)
A couple of thoughts occur.
1. Despite the best efforts of Tanebaum and others to get some sort of structure
into the terminology of networking, it is still driven by the manufacturers
and their respective marketting departments. This means that words such as
bridge, router, gateway or others do not enjoy the same accuracy of meaning
that is found in every other sort of engineering.
2. Where some attempt is made to properly define the meaning of a networking
term, it is done with reference to the OSI model. With that fixed (I doubt
that's ever been anywhere near a marketting department, for good reason)
it gives half a chance of getting some definitions.
A bridge works at layer 2. It may or may not change the frame. If it does
not, that frame must make sense to both sides of the bridge. In this case
the bridge is a Transparent Bridge. If the bridge changes the frame it will
be because of a change in layer 2 protocols. This is usually driven by a
change in layer 1. For example, frames can be bridged between Ethernet and
Token Ring, with the frames modified as they pass through the bridge as the
layer 2 protocols are different. In this case, the device is a Translational
Bridge.
There are also Encapsulation Bridges. These work in pairs where frames have
to traverse a network that has a different set of layer 2 protocols but
will start and finish using the same layer 2 protocols. In this case the
frames are encapsulated between the bridges. This was done when carrying
Ethernet frame across a FDDI backbone for example. It is rarely done now,
best practice is to route traffic at layer 3 when traversing dissimilar networks.
Here's what Cisco have to say about it. http://www.cisco.com/en/US/tech/tk331/tk660/technologies_tech_note09186a0080093d4d.shtml
But, what's being proposed between Ethernet and Econet doesn't fit into
any of the above definitions, but I don't think I've seen a description of
what the Ethernet to Econet "bridge" actually does in a low level techie
way. Please correct me on this :) .
Looking at the description here of the function of an Econet bridge http://www.heyrick.co.uk/econet/bridge/whatdoes.html,
I think in OSI terms the device is functioning as a router as it is allowing
communications between two separate networks (in the OSI sense) with different
network addresses. While Econet doesn't pre-date OSI, it doesn't seem to
have figured in the design much :)
My money is on Gateway. It's about the least well defined term of the lot,
but is generally used to describe a system for passing data between significantly
different networks. For example, http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F6648%2F17837%2F00824469.pdf&authDecision=-203
discusses a data network to voice SS7 gateway.
Sorry for top-posting. The Yahoo web client is the best I can manage right now...
Ian
----- Original Message ----
From: Pete Turnbull <pete@...>
To: bbc-micro@...
Sent: Tuesday, 14 July, 2009 12:57:04
Subject: Re: [BBC-Micro] Master Ethernet upgrade
On 14/07/2009 09:14, Mark Usher wrote:
> That is also subjective to interpretation as to what criteria the network
> technologies are comparative.
There's pretty common agreement on that, and well-defined in Tanebaum,
Computer Networks, and other works....
> Principles of Digital Communication Systems and Computer Networks
> by Dr. K.V. Prasad
>
> Bridge:
> A bridge interconnects two LANs. If the two LANs use different protocols,
> the bridge does the necessary protocol conversion.
..where they use the same link-layer protocol. Econet and Ethernet do
not. Perhaps you'd also like to check the IEEE 802.1D standard.
--
Pete Peter Turnbull
Network Manager
University of York
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