You've got to be pulling my chain... (Ethernet)

From: Jack Peacock <peacock_at_simconv.com>
Date: Fri Apr 10 17:12:02 1998

> From: allisonp_at_world.std.com [mailto:allisonp_at_world.std.com]
>
> 802.. eithernet was always 10mbit/s. The 1.5mhz stuff was
> arcnet. They
> are similar in that they are both bus topology using CSMA/CD
> arbitration.
>
Not quite. Arcnet was not a CSMA/CD but token passing, more in common
with Token Ring rather than Ethernet. Arcnet used a bus, but a token
was passed along to each node and that node sent a packet only when it
got the token. Whenever a new node came online the rest of the nodes
would reconfigure and start the token over again. Collisions only
occured when new nodes came on line the first time. IIRC it was
Datapoint that first came up with Arcnet for their multi-user terminals.

For a small number of static nodes the Arcnet throughput was actually
quite good. Whereas ethernet is probabilistic for access, token passing
is deterministic in that you are guaranteed a slice of bandwidth to
every node. At a minimum you always had roughly 1/n of available
bandwidth, where n was the number of nodes. Arcnet degraded in a linear
fashion as nodes were added, but Ethernet degradation can approach
geometric rates, as anyone knows when they see that collision light on
the UTP hub go on solid.

Also, Arcnet was 2.5Mbps. The original version used coax (not Ethernet
thinwire! I think it was RG-92?) into hubs, later versions went to
twisted pair. There was a 20Mbps follow-up from SMC (the primary chip
maker for Arcnet) but it never caught on.

Arcnet was a common networking standard for S-100 systems in the early
to mid 80s, using MP/M and CP/NET from Digital Research. I think it was
one of the first network transports supported by NetWare too.
                Jack Peacock
Received on Fri Apr 10 1998 - 17:12:02 BST