Ref: 99980068
Title: RFM/5 Station Mgmt Interface and Automatic Level Control
Date: 8/1/86

Copyright 3Com Corporation, 1991.  All rights reserved.

Introduction

The RFM/5 is an intelligent microcomputer-controlled broadband
modem for use with the Bridge Communications Series 1 and Series/
200 Broadband Communications Servers.  In addition to configuring
system-specific Radio Frequency parameters, the station
management interface allows the user to investigate modem
transmit and receive levels.  Operation of the station management
interface requires the use of a terminal adapter available from
Bridge Communications.  This device breaks out the asyncrhonous
communications path between the modem and the server and converts
the signal voltage for use with a standard CRT.  In the future,
these commands will be available from the Bridge Communications
Server or Network Control Server User Interface.

The terminal used to communicate with the RFM/5 must be
configured for 9600 baud, 8 data bits, and NO parity.  It is
connected to the 25-pin D connected on the terminal adapter by
means of a standard one-to-one cable (the adapter looks like a
DCE device).  The adapter is installed at the RFM/5 in line with
the server cable and is powered by the server.  The adapter
should be installed with the server power off.

Once the terminal is connected and the adapter is intalled and
powered on, pressing RETURN should cause the modem prompt to
appear as follows:

RFM/5>

When the prompt is displayed, the RFM/5 is ready to accept
commands.

The Show Status Command

In addition to the current transmit and receive signal levels,
the show status command displays the configuration parameters of
the RFM/5.  The format and display of this command are as
follows:

sh status <CR>

.br;Model RFM/5
.br;Version R9 RFM5 Revision 01A
.br;Date of Manufacture - 08/86
.br;Hardware Revision - 01
.br;Training Program Revision - 1
.br;Transmit Channel - 1
.br;Receive Channel -1
.br;Nominal Transmit Level - +45 dBmV
.br;Minimum Transmit Level - +35 dBmV
.br;Maximum Transmit Level - +52 dBmV
.br;Current Transmit Level - +45 dBmV
.br;Current Receive Level - ~+02 dBmV

RFM/5>

The first five lines display general configuration information,
the model number, firmware revision, data of manufacture,
hardware revision, and training program revision.  Most of the
displays are self-explanatory, but the training program is used
during manufacturing to calculate and store modem-dependent
variables necessary to provide accurate signal level measurement
and control.

The next two lines show the current transmit and receive
channels.  These displays can be modified using the "set chan"
command.  The channel numbers and operating frequencies are
displayed as follows:

    Channel     Transmitted Frequency     Received Frequency

       1              4A'                         R
       2               5'                         S
       3               6'                         T
       4             FM1'                         U
       5             FM2'                         V
       6             FM3'                         W

       0       Special Programmable channel not yet implemented

The next three lines in the display show the transmitter level
control parameters.  The nominal value is set at power on and
should be the same as the broadband network outlet transmit level
design specification.  This can be thought of as the "Manual"
level control value or the level at which the RFM/5 should
transmit if the broadband network was perfectly aligned.  The
minimum and maximum values are limits that can be set to control
the amount of adjustment.  These limits allow the user to confine
the transmitter output to a range as narrow or wide as the user
application requires.  This prevents the automatic level control
features of the Bridge network from causing undesirable
interference to other services on the network in the case of a
broadband network malfunction.  These values can be modified by
using the "set notx", "set mitx", and "set matx" commands.

The last two lines show the current "real time" transmit and
receive signal levels.  At room temperature and nominal power
supply voltages, these readings are accurate to about +/- 1 dB.
The receiver has a range of -15 dBmV to +15 dBmV.  If the signal
is out of this range, the RFM/5 displays <-15 dBmv or >+15 dBmV
as appropriate.  For example, when the RFM/5 is not connected to
the broadband network, it will report the receive level as <-15
dBmV.

The Set Channel Command

The user can configure the RFM/5 to operate on any one of six
broadband channels by using the set channel command.  The valid
channels are 1, 2, 3, 4, 5, and 6 as shown in the chart above.
To change channels, enter the following command at the prompt:

set chan = X <CR>

where X is 1, 2, 3, 4, 5, or 6

The RFM/5 will then go off-line (you may be able to hear the
relay click) for two seconds, change operating frequencies and
return on-line.  The channel of operation, along with the other
configuration parameters, is stored in a non-volatile EEPROM
memory in the RFM/5 and will remain set until changed by the
user.

The Set Transmit Level Parameter Commands

Three similar commands are used to set the NOMINAL or power on
transmist level in addition to the MINIMUM and MAXIMUM control
limits.  These changes should only be made by the broadband
system manager since improper settings can cause unreliable
operation of the RFM/5 and the automatic transmit level control
features.  These values are factory set at the levels as shown in
the status display above and should be optimal for most broadband
networks.  These three commands are as follows:

.br;set notx = XX <CR> set nominal transmit level
.br;set mitx = XX <CR> set minimum transmit level
.br;set matx = XX <CR> set maximum transmit level

where XX = 21 to 52

The transmitter output is adjustable over a 32 dB range from +21
to + 52 dBmV.

Automatic Transmit Level Control

The automatic transmit level control feature of the RFM/5 works
in conjunction with the CR/5 (Channel Remodulator) at the
broadband network headend.  When the RFM/5 is initially powered
on or reset by the server, it sets its transmitter output to the
level stored by the "set notx" command.  Each time the RFM/5
transmits, its signal level is measured by a precision circuit in
the CR/5.  The CR/5 compares this level to a range of acceptable
values that is set by the broadband network manager on rear panel
switches.  If the level is out of the acceptable range, the CR/5
sets a contro bit to indicate that the RFM/5 level was too high
or too low.  As the incoming packet is being repeated by the CR/
5, it inserts an inband command sequence containing these control
bits in the repeated packet.

The RFM/5 uses the coincidence of reception of the command
sequence and the condition that it is transmitting to "address
itself" and pays attention to the level control bits.  The RFM/5
applies the "too high" and "too low" control bits to a counter.
When the RFM/5 has been repeatedly told that its transmit level
is out or range, it changes its level by 1 dB as long as the
level change does not cause the transmitter to operate beyond the
minimum or maximum limits.  Once a change has been made, RFM/5
disables its level control feature for 10 seconds to prevent
rapid changes.

Depending on how frequent the transmissions from a given server
are, the RFM/5 can change level no faster than 1 dB every 10
seconds or no slower than 1 dB every 3 minutes (assuming station
management packets are transmitted every 45 seconds).






