Ref: 13680015
Title: Remote NetBuilders and T-1 Support
Date: 08/17/90

Copyright 3Com Corporation, 1991.  All rights reserved.

The following features were added into the NetBuilder software (IB-3.1
and BR-1.1), and the current manuals and release notes do not cover
them well.  This is for U.S. T-1 lines; the interface can support speeds
up to T1C (3.154 Mbps).

T1Mode            The data stream is inverted (all 1s become 0s, and
                  all 0s become 1s) so Ones Density is guaranteed.  The
                  fastest speed is 1.536 Mbps, not 1.544 Mbps.  This is
                  because a frame bit is inserted every 193 bits and this
                  takes up 8 KB of bandwidth.

NoT1Mode          The data stream is not inverted so Ones Density is not
                  guaranteed (the DSU/CSU takes care of this).  The fastest
                  T1 speed is 1.344 rather than 1.536.

ITCMCompatible    Data is sent with a 24-bit preamble followed by an HDLC
                  flag for each packet.  The preamble is required because
                  the ITCM board uses two (send and receive) 82586 LAN
                  controllers that require the use of a preamble.

NoITCMCompatible  Data is sent with possibly two HDLC flags preceding the
                  packet (after idle) and WITHOUT a preamble.  We use HDLC
                  framing but do NOT use standard HDLC addressing or error
                  correction.

In all of the cases listed above, HDLC Zero-Insertion is used between flags
and an HDLC Flag (7E Hex) is present at the beginning and end of each packet.
If the data stream is inverted, the flag will be seen as a 81 Hex.

If you have an ITCM board at the other end of the line you must select
ITCMCompatible; do NOT set T1Mode at the same time.  If you set T1Mode
and ITCMCompatible the first time, the line will never come up.  If
you set T1Mode while the line is up, it will not affect the current traffic.
However, as soon as you change from T1Mode to NoT1Mode, the line will go
down and you will have to reset the IB/3 and NetBuilder at the same time
to recover the line.  (Resetting the T-1 path using ResetITCM command on
the IB/3 will not recover the line.)

To make a back-to-back connection between an IB/3 and a remote NetBuilder
using a null modem cable, the IB/3 should Provide Clock (be configured for
internal clocking) and the NetBuilder should Accept Clock (be configured for
external clocking) from the IB/3.

If you are making a back-to-back connection between two remote NetBuilders,
make one Provide Clock (internal) and another one Accept Clock (external).
Use a null modem cable.  Either NoT1Mode or T1Mode can be used, and both
NetBuilders should have the same configurations (baud rate, T1Mode or
NoT1Mode, and NoITCMCompatible).
