Ref: 02170008
Title: Basic Information on 3Com Broadband Products
Date: 4/26/91

Copyright 3Com Corporation, 1991.  All rights reserved.

3Com's broadband products use standard channels.  The designators and
frequencies are as follows:

    Channel     TX Freq.(MHz)                RX Freq.(MHz)
    -------     ---------------------        --------------
        1         4A' (71.75 - 77.75)        R  (264 - 270)
        2         5'  (77.75 - 83.75)        S  (270 - 276)
        3         6'  (83.75 - 89.75)        T  (276 - 282)
        4       FM1'  (89.75 - 95.75)        U  (282 - 288)
        5       FM2'  (95.75 - 101.75)       V  (288 - 294)
        6       FM3' (101.75 - 107.75)       W  (294 - 300)


The default assignment is channel 1 -- 4A' for Transmit (the Inbound Channel
as viewed from the CR/5 Channel Remodulator) and R for Receive (the Outbound
Channel as viewed from the CR/5) for a Translated single- or dual-cable
broadband network.  The offset between transmit and receive channels is
192.25 MHz; this value was chosen to minimize crosstalk due to harmonics
between frequencies.

For a non-translated (NT) broadband network, which must be a dual-cable
network (that is, with transmit and receive channels on different cables),
the same channel is used for both transmit and receive.  The frequencies
used are the "TX Freq." frequencies above.

3Com's broadband equipment transmits and receives data at 5Mbps.  Each of
the channels above are 6MHz channels, leaving 1MHz for guard bands to
prevent adjacent channel crosstalk or bleed-over and to allow the use of
adjacent channels for other transmissions.

The CR/5 (also known as the "headend" or "channel remodulator") performs
the following functions, in addition to allowing adjacent channels:

   *  Collision Detection - All collisions are detected by the CR/5.  The
CR/5 looks for framing errors or illegal bit transitions and, upon detecting
a collision, broadcasts a jamming signal to alert all RFM/5s.  (RFM-5s are
radio frequncy modems used to connect all devices except the headend to the
channel used to implement the logical 3Com broadband network).  This allows
for effective CSMA/CD operation.

  *  Automatic Signal Adjustment - CR/5 monitors inbound signals arriving
at the CR/5 and, when it detects a signal outside the user-defined limits,
commands the offending RFM/5 to make appropriate level adjustments "on
the fly."  (Limits are defined by thumbwheels on the CR/5.)

  *  Network Synchronization - Via carrier maintained by CR/5.  Because
there is a real carrier always present, "idle packets" are sent when the
CR/5 is not repeating packets (from inbound to outbound channel) to maintain
timing.  The network synchronization allows the CR/5 and RFM/5s to offer a
receiver dynamic range of 30dB.


With a headend for each network, six 3Com 5Mbps broadband networks can be
operated on a single cable.  To communicate from one broadband network to
another, you may use a "frequency translator" or a CB/1 (Channel Bridge/1,
an obsolete, unsupported 3Com product).  A frequency translator merely
repeats one pair of frequencies (TX and RX, or possibly just one of the two),
which effectively means that both TX freqencies and both RX frequencies are
all one network.  On the other hand, a Channel Bridge, treats the broadband
networks as two networks which are internetwork-bridged together.  Only
traffic between the two channel-pairs is forwarded, and collision domain
separation is maintained.

