Cable Networks
To better understand the nature
of Internet access over cable, it is helpful to review how the cable network
infrastructure works. The overall structure or topology of existing residential
cable TV networks follows the tree and branch architecture. In each community,
a head end (the originating point for cable TV signals) is installed to
receive satellite and traditional over-the-air broadcast television signals.
These signals are then carried to subscribe rs' homes over coaxial cable
that runs from the head end throughout the commu nity.
Each 6 MHz TV channel is transmitted
in analog form over adifferent 6 MHz of enclosed spectrum on the cable.
Multiple channels are sent over the same cable using Frequency Division
Multiplexing (FDM). Because different channels are sent at different frequency
offsets (e.g. 54-60 MHz, 60-66 MHz, etc.), this form of transmission is
referred to as broadband. (In contrast, baseband transmissions all take
place in the base band starting at 0 Hz.) The speed of this access depends
on the equipment used to modulate digital computer information on to cable's
analog TV channels. Such equipment provides bandwidths ranging from 500
Kbps to over 10 Mbps.
To achieve geographical coverage
of the community, the cables emanating from the head end are branched into
multiple cables. When the cable is physically split, a portion of the signal
power is split off to send down the branch. The signal content, however,
is not split: the same set of TV channels reach every subscriber in the
community. The network thus follows a logical bus architecture. With this
architecture, all channels reach every subscriber all the time, whether
or not the subscriber's TV is on. Just as an ordinary television includes
a tuner to select the over-the-air channel the viewer wishes to watch,
the subscriber's cable equipment includes a tuner to select among all the
channels received over the cable.
Because the signals decrease in
strength as they travel several miles through the cable to subscribers'
homes, amplifiers have to be deployed throughout the plant to restore the
signal power. The more times the cable is split and the longer the cable,
the more amplifiers are needed in the plant.
Two Way Transmission Television,
as its critics have long noted, is a one-way street: programs are broadcast
to viewers, but no mechanism is built in for viewers to talk back to producers,
or to send their own video (or Internet) signals into the network. This
fact is reflected in typical residential cable TV network implementations,
which transmit signals only in the direction from the head end to the subscriber
(referred to as the downstream direction).
Many of new interactive services
require transmission from the subscriber as well, with bandwidth requirements
for such upstream transmission varying tremendously depending on the service.
Pay-per-view movies could be ordered over the network with only a few low-bandwidth
clicks of the remote control (or mouse), while telephony would require
a larger (but still relatively small) 4 KHz of bandwidth. Internet applications
span a range of upstream
requirements, from low-bandwidth
email to high-bandwidth video sent from a home-based World Wide Web server.
- Enabling upstream transmission
requires three types of technical changes to the network:
- Spectrum must be allocated for
the signals traveling in the upstream direction. The range of spectrum
from 5 to 42 MHz is typically dedicated to upstream transmission. This
range generally provides a maximum of 4 usable upstream channels.
- Amplifiers must include circuitry
(diplex filters) to separate the upstream and downstream signals and amplify
each direction separately, in the right frequency range. Rather than upgrading
existing amplifiers in their tree and branch networks, most network providers
have elected to incorporate upstream capability into the new amplifiers
they need to purchase to pass the higher-bandwidth signals of hybrid fiber
and coax networks.
Downstream transmission from the
head end is broadcast: the same signal is sent on all the wires. In contrast,
upstream transmission is inherently personalcast: each subscriber is trying
to place a different signal onto the network. When going up the tree these
different signals must eventually share the same piece of transmission
spectrum. Some form of access method is needed to arbitrate which signal
is actually carried. Access methods come in different varieties, such as
Time or Frequency Division Multiple Access (TDMA or FDMA), Carrier Sense
Multiple Access (CSMA), etc. Which method is appropriate depends on the
expected application.
Upstream transmission also presents
a technical challenge because multiple noise sources invariably accompany
the multiple signal sources. Careful plant engineering is often required
for the upstream signal to arrive in recognizable form.
Cable Modems
The cable modem is a key component
of Internet access which allows the user's PC to connect with the cable
network and the Internet beyond. Cable modems in production or under development
can be grouped into three broad categories:
- Devices which route IP (Internet
Protocol) messages exclusively, the Motorola product being a good example.
- Devices capable of supporting IP
and other network protocols such as Ethernet. Current vendors include LANCity
and Zenith.
- Devices which implement the ATM
(Asynchronous Transfer Mode) cell and signaling formats. These products
are further down the road.
For consumers, the most immediate
requirement is the Internet, but businesses are certainly interested in
the point-to-point Ethernet capabilities of cable modems.
Cable Modem Issues
The salient issues surrounding a
cable company's selection of a cable modem are as follows:
Network management - Cable Operators
must be able to remotely monitor and manage the functionality of customers'
modems on the network to ensure network reliability. Data layer encryption
- this will provide a level of network security beyond what the end user
installs on his/her own PC. Cable modem operations - the cable modem should
be conveniently co-located with the user's PC and should have various operational
features like power level control for transmission, the ability to support
more than one PC on a Local Area Network, and the ability to detect its
location in the network. Telephony return option - for smaller cable operators
who haven't upgraded their cable network to full two way capability the
option exists to use the telephone as the return path for Internet traffic.
Here, the customer still gets the full benefit of high speed Internet reception
over cable, but uses the telephone line for upstream data. Price - cable
modem cost must be comparable to that of a high end telephone modem: i.e.
$300 - $400. Cable modem rental as a portion of the monthly Internet over
cable service cost will likely be an option for the customers.
Cable Modem Vendors
Several cable operators have announced
large orders for Motorola CyberSurfer modems, which are emerging as an
early favorite among large cable companies. Other major vendors include
Zenith, LANCity, Hybrid Intel/AT&T, Hewlet Packard, Nortel, Scientific
Atlanta and General Instrument.
For additional information please
check out:
