Ref: 11640091
Title: Functions of the MAC Layer in 3Com's Token Ring Adapters
Date: 12/13/90

Copyright 3Com Corporation, 1991.  All rights reserved.

Token ring has one major difference from Ethernet:  the hardware that
supports token ring must have a MAC software support layer.  The MAC
software decides whether or not an open onto the ring is OK, and whether or
not the traffic on the wire is OK.  It also participates in ring recovery if
a problem occurs.

3Com's token ring adapters use the TI chip set, and the MAC layer is
contained in the on-board adapter PROMs.  For detailed information on the TI
chip set, read the Texas Instrument TMS 380 Adapter Chip Set User's Guide
(SPWU001D) and TMS 380 Adapter Chip Set User's Guide Supplement (SPWU003).

In the 3+ share environment, the 3+ adapter driver is not aware of any MAC
layer activity.  It may see the end result of a failure at the MAC layer
due to an error return from the TI chip set for a requested operation.  For
example, suppose the driver requests the adapter to open onto the ring at
RUNMINDS time.  The TI chip set goes through the various steps to open, but
decides it cannot, so it closes the ring and returns an error to the driver,
which the driver returns to RUNMINDS.

The adapter is sensitive to several things at open time.  When the adapter
opens onto the ring, it cannot handle inbound traffic.  This is not
normally a problem, unless there are a lot of broadcast frames on the wire
at open time.  This problem is documented in the TI chip set manual.

When the TI chip set opens, it sends two requests for information from the
ring parameter server.  If no reponse is received, the adapter will set
up the default configuration.  This is only a problem if the ring
parameter server goes down, and if the default settings are not compatible
with the rest of the ring.  The ring parameter server should have an IBM
adapter in it with IBM's ring parameter server software.

The MAC layer performs active monitor, standby monitor, and a 7-second poll
as part of its ring maintenance functions.  The adapter driver is not aware
of these functions and does not participate in them.  The ring parameter
server function is not automatically passed if it fails.  It requires
operator intervention to correct the fault and reboot the server.

