Ref: 02880035
Title: Avoid Using Fan-Out Boxes with MultiConnect Repeaters
Date: 8/10/90

Copyright 3Com Corporation, 1991.  All rights reserved.

3Com does not support the use of fan-out boxes as transceivers for the
MultiConnect repeaters.  Following is an explanation of why repeaters and
fan-out boxes do not work well together:

SCENARIO:  A fan-out box is connected to a single transceiver.  The
transceiver connects to a coaxial segment.  The fan-out box has eight
AUI ports; ports 1 and 2 have DTEs on them and ports 3 and 4 have repeaters
attached.

When DTE 1 sends a packet on its AUI, the fan-out box distributes this
packet to ports 2, 3, and 4 and to the transciever on the coaxial segment.
Both repeaters and DTE2 are in receive mode.  At the end of DTE 1's
transmission, the transceiver sends a heartbeat to the fan-out box,
indicating that the transceiver is functioning properly.  The heartbeat
is not only sent to DTE 1, who is waiting for the signal, but also to
DTE 2 and both repeaters.  The last three are in receive mode and will
see these as collision pair-only carrier.  DTE 2 will either defer if a
transmission is scheduled, or ignore it completely.  However, the repeaters,
having been in receive mode on their ports, see the heartbeat as carrier and
dutifully forward the carrier to their other ports as a minimum-size
96-bit fragment.  The fragments are erroneous traffic and interfere with
the proper operation of the network system.  It is the result of erroneous
configuration of equipment, not erroneous operation of the equipment.

This problem might be fixed by replacing the transceiver with a non-heartbeat
type.  However, there are other ways the transceiver could behave regarding
heartbeat; for example, on a port-by-port basis or by jumperable options.
This discussion also does not include the possible collision behavior of
the fan-out box.

Notes:

*  There is no IEEE specification for a fan-out box.  Specifications for
fan-out boxes are vendor-specific.  Vendors can only claim IEEE compliancy
with the AUI connector specs and the signaling levels associated with the
AUI connector.

*  There are no IEEE specifications for the stimulus/response behavior of
a fan-out box.  Since the fan-out box is inserted into the network system,
its behavior is critical to system operation, particularly if attached
directly to a repeater.  Fan-out boxes were designed to allow multiple
DTEs to be added to a single transceiver, which would increase station
density on a single segment or density in a physical area.

Manufacturers do not recommend the direct attachment of a repeater to a
fan-out box.  The function and behavior of a repeater are quite different
from those of a DTE.  They have different requirements from the transceiver
standpoint.  Inserting or replacing the fan-out box in the repeater's path
requires behavior analysis on a case-by-case basis.  Since it is difficult
or impossible to obtain detailed behavior specifications for all the fan-out
boxes in the marketplace, 3Com recommends that fan-out boxes not be attached
to repeaters.
