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Path: utzoo!watmath!clyde!cbosgd!mark
From: mark@cbosgd.UUCP (Mark Horton)
Newsgroups: net.lan
Subject: Re: Baseband <--> Broadband gateways
Message-ID: <1828@cbosgd.UUCP>
Date: Sun, 16-Feb-86 20:16:32 EST
Article-I.D.: cbosgd.1828
Posted: Sun Feb 16 20:16:32 1986
Date-Received: Mon, 17-Feb-86 06:18:15 EST
References: <733@abic.UUCP> <440@ubvax.UUCP>
Organization: AT&T Bell Laboratories, Columbus, Oh
Lines: 48

In article <440@ubvax.UUCP> skip@ubvax.UUCP (Skip Addison Jr) writes:
>Bridges get involved at the network layer.  IEEE 802.3 & 802.4 only
>specify up through the data link layer.  What bridge you need depends
>on what network layer protocols you need.

Perhaps we're working with different definitions, but according to
my understanding of terms like "bridge", this is just plain wrong.

You're saying that 802.3 only goes up through the data link layer,
and at least one OSI person agrees with you, calling the 48 bit
Ethernet address a way of handling a "multipoint data link layer"
rather than a network layer.  Personally, I think if it looks like
a duck and quacks like a duck, it's a duck (or a good emulation),
even if it didn't hatch from an official duck egg.  I think Ethernet
and 802.3 support a network layer, and the IP or CLNS or IDP layer
above it is a sublayer of the same network layer above Ethernet.

But you say that bridges work above what 802.2 provides, which seems
to me that you're talking about the IP or CLNS et al layer.  To me,
this is not a bridge, it's a "gateway" or "packet router".

In my book, an Ethernet bridge operates at the Ethernet (802.2) layer.
It's much like a repeater with a buffer and some smarts about where to
route Ethernet level packets (based on its knowledge about which segment
the destination address is on.)  These bridges can connect various 802.2
networks together, such as 802.3 Ethernet to 802.4, and even hook similar
or different networks together with random virtual or physical circuits.
Such bridges are, as one of their features, totally transparent to whatever
protocols you may be running at the higher levels.

Gateways work at the same level as their peers on other nodes, so the
end to end protocols matter.  Bridges do not have peers on the other
side of the net, so they appear to not be there on the other end.
Gateways operate pretty much at one level, and are built into the
network architecture.  Bridges understand any lower levels they are
working with, so you can think of them as working at the next higher
level.  But this is still below the real next higher level; an Ethernet
bridge in effect works in a layer above Ethernet but below IP.

Incidently, is there a bridge between Ethernet (or 802.3) and Starlan
available or in the works?  Sure might be handy for cabling to run an
Ethernet backbone trunk with Starlan into the offices, or on the other
hand, to use Starlan for the building wiring (to reuse existing wiring)
but to plug in boxes with an Ethernet interface.  I'm thinking of something
inexpensive - almost like an adaptor, not a big expensive general purpose
bridge.

	Mark