Ref: 99980066
Title: Broadband Technology
Date: 8/1/86

Copyright 3Com Corporation, 1991.  All rights reserved.

Our experience is that baseband networks fit well in most data
processing, engineering, or office automation environments.
However, although a single baseband network can span a small
campus, it may be best suited for operation within a single
building.  Also, with current technology, baseband networks can
handle only data traffic.  Broadband networks work most
effectively in large configurations, where economies of scale can
justify the higher cost of cable plan and maintenance.  A
broadband network can cover a large campus of buildings, or even
a good-sized city.  In addition, broadband networks can carry
multiple channels and subchannels of voice, video, and data.
Standard broadband cable has a high immunity to noise and can
come in heavy-duty versions especially designed for outdoor or
underground environments.

For many applications, hybrid broadband and baseband
configurations can be used to take advantage of both
technologies.  The broadband network is used as a trunk to carry
data among several local baseband subnetworks; voice/video and
additional data traffic is also supported directly on the
broadband itself.  Such a system minimizes the number of direct
connections on the broadband portion of the network.

.h1;CATV AND BROADBAND

Broadband communications has evolved from the CATV (Community
Antenna Television) industry.  In suburban areas where TV
reception is poor, large antennas are built to achieve the best
possible reception of the signal.  The signal is then distributed
to subscribers in the community via broadband cables.  These
cables support bandwidths of over 400 MHz.  The bandwidth is
divided into 6 MHz intervals, each capable of carrying a standard
color TV channel (channels 4, 5, 7, etc., on your home TV).  Due
to the wide area coverage, in-line amplifiers are generally
needed as the signal gets attenuated over distance.

Broadband LANs use the same concept.  They support data, video,
and voice much like CATV.  Whereas communication on baseband LANs
is digital, it is analog on broadband LANs.  Digital data is
"modulated" to an analog form by a Radio Frequency (RF) modem
before being sent over the broadband channel.  At the receiving
end, a similar RF modem performs the "demodulation" to recover
the data in its orginal form.

Broadband communication is unidirectional due to the nature of
the distribution and amplifying hardware.  This works fine for
CATV applicaitons because TV signals go only one way to the
subscribers.  However, to allow for two-way data communiations in
broadband LANs, different  frequency channels are used for
transmitting (defined as the "reverse" or "inbound" channel which
carries signals from the modem to the "headend) and for receiving
(the "forward", or "outbound" channel, which carries signals from
the headend to the modem).  A frequency translator located at the
cable headend, where the main trunk cable originiates, performs
the frequency upshifting.  The inbound channels (the lower
frequency channels) and the outbound channels are separated by a
neutral band in the middle.  There are multiple formats for
setting the boundaries of these bands.  Subsplit, midsplit, and
highsplit are some of the most commonly used formats:

    Format   Inbound Frequency Band   Outbound Frequency Band
   Subsplit       5 - 30 MHz               54 - 400 MHz
   Mid-split      5 - 116 MHz             168 - 400 MHz
   Highsplit      5 - 174 MHz             232 - 400 MHz

Bridge's Broadband products are compatible with either midsplit
or highsplit systems.  The "offset", the frequency difference
between the outbound and the corresponding inbound channel, is
192.25 MHz.

Bridge provides a channel remodulator for its broadband system.
Unlike some systems using a frequency translator, the user does
not have to reserve a guard band, a frequency zone where no
signal can be carried, between adjacent channels.  This way, a
user can concurrently run a Bridge network on channel 4A' and
video on channel 5' on the same cable.  Some broadband systems
use a frequency translator that translates all signals on a bit
level.  Bridge uses a channel remodulator that also regenerates
packets, thus filtering out any noise that was picked up in the
inbound path.  Each operational channel on the broadband system
requires either a remoldulator or a translator.  In a typical
broadband installation, a location called the headend consists of
several racks of remodulators, translators, power supplies,
cables, and other miscellaneous components.

Alternatively, instead of using different frequency channels for
transmit and receive, two cables can be used to achieve two-way
communications.  Stations transmit on one cable and receive on
the other.  Very often the "two" cables are physically the same
cable looped around at the headend.  In a dual-cable scheme, a
station transmits and receives on the same channel.  A variation
of this is the dual-translated cable scheme where the outbound
and inbound channels are on separate cables but are again offset
as in a single-cable system.  Bridge Broadband can run on a
single-cable of dual-translated cable system.

A broadband network resembles a tree in topology.  At the root is
the headend location; the cable plant branches out from the
headend in various directions.  Couplers are placed to allow
cables to branch out and amplifiers are placed to ensure that
more distant cable segments have enough signal strength.
Stations are connected to main cable via taps and drop cables.
This free-form topology makes broadband very attractive for
certain applications.

.h1;Access Method

Bridge's Broadband products, though similar to IEEE 802.4, do not
use token passing.  They use an Ethernet-like Carrier Sense
Multiple Access/Collision Detection (CSMA/CD) access method.  The
Bridge approach is superior to other CSMA/CD broadband products
in that it performs collision detection at the headend and thus
guarantees consistent collision detection.  On collision, the
Bridge Channel Remodulator(CR/5) sends a jamming signal to the
entire network to make sure all stations "see" the collision.
This scheme is very effective and significantly improves
performance by reducing retransmission due to CRC errors.

.h1;Baseband vs. Broadband

Baseband employs a single high-speed digital channel (10M bps in
the case of Ethernet) and is used primarily for data
communications.  The broadband cable medium, which can carry
signals at up to 500 MHz, is divided into many 6 MHz channels,
each of which can carry a video signal or one of more data
subchannels at speeds up to 5M bps.  Data traffic is always
modulated to analog form before transmission on the broadband
network.

In baseband, transceivers are used to send data packets to and
receive packets from the network, and also to detect collisions.
In broadband, RF modems are used instead to translate digital
data to analog form (and vice versa) suitable for transmission on
the broadband network.  Frequency-agile modems are modems that
can be configured via software or switches to operate on more
than one frequency channel.

Baseband Ethernet uses a bus topology in which signals are
broadcast in both directions.  Broadband networks, however, are
unidirectional and more commonly resemble a tree.  The base of
the tree is where the headend is located.  The network branches
out through trunk cables, distribution cables, and drop cables to
users' equipment.

.h1;Cabling

The impedance for Ethernet coaxial cable is 50 Ohms and for
broadbands is 75 Ohms.  Ethernet cable is always 0.5 inch in
diameter and broadband cables vary between 0.25 and 1 inch.
Broadband cable can be ordered in a variety of types including
PVC jacketed, jacket/flooded, and armored.  Jack/flooded means
that a gel is placed between the aluminum outer conductor and the
PVC jacket. It is used primarily for underground burial to guard
against cracks.  Armoring the cable with steel jackets prevents
kinks from developing in the cable.

The Bridge Broadband products run on any kind of broadband
coaxial cable.  These cables can be divided into the following
three types:

1.  The trunk cable transports signals between amplifiers.  This
type of cable is usually used to interconnect buildings.  It
ranges from 0.412 to 1.0 inch in diameter.

2.  Distribution or feeder cables are used to connect the trunk
cable to the vicinity of the office.  These are primarily for
indoor use and are typically 0.5 inch in diameter.

3.  Drop cables link the feeder cable to the individual LAN
outlet.  They range from RG-11 and RG-6 to RG-59.

In general, the larger the cable, the lower the attenuation and
the longer the cable can run.  Any of these cables can be
jacketed or unjacketed, or installed with or without conduits,
depending on the insulation material, the environment, and local
building codes.

.h1;Design and Maintenance

Broadband cable plant design is more complicated than Ethernet
design.  Attenuation due to the cables, couplers, and taps must
be accounted for in detail to correctly place and set amplifiers
for optimum performance of the network.  In order to accommodate
minor changes and avoid completely redesigning the network,
future expansion plans must be carefully considered and
incorporated into the original network design.

Cable plant layout and installation are difficult because
broadband cables can be quite heavy and stiff.  These networks
generally span multiple buildings, so cables have to go
underground of on poles.  As a result, broadband cable plant
installation usually outstrips the capabilties of the average in-
house facilities department.  Fortunately, because of the
maturity of the CATV industry, there are many professional
broadband subcontractors who can do excellent design and
installation work.  The CATV industry's high-volume activity
also lowers the cost of the physical cable and amplifiers.

Outdoor cable plants are susceptible to termperature and humidity
variations or other environmental factors.  These factors impact
on performance can vary from minor to severe.  It is therefore
necessary to perform frequent maintenance checks and keep the
network tuned; requirements often cited as one of the major
drawbacks of broadband LANs.  Bridge Broadband is designed
specifically to eliminate much of this maintenance.  The Bridge
CR/5 monitors each incoming data packet for its signal level and
sends control codes back to the intelligent modem to inform it to
adjust its transmitter, if necessary.  This is done continuously
and completely automatic.  On the forward direction, the modems
continuously monitor outbound signal levels.  Servers can read
this information and then use it for fine-tuning the network.



