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Introduction
Voice
(and fax) service over cable networks is known as cable-based Internet Protocol
(IP) telephony. Cable based IP telephony holds the promise of simplified and consolidated
communication services provided by a single carrier at a lower cost than consumers
currently to pay to separate Internet, television and telephony service providers.
Cable operators have already worked through the technical challenges of providing
Internet service and optimizing the existing bandwidth in their cable plants to
deliver high speed Internet access. Now, cable operators have turned their efforts
to the delivery of integrated Internet and voice service using that same cable
spectrum. Cable based IP telephony falls under
the broad umbrella of voice over IP (VoIP), meaning that many of the challenges
that telecom carriers facing cable operators are the same challenges that telecom
carriers face as they work to deliver voice over ATM (VoATM) and frame-relay networks.
However, ATM and frame-relay services are targeted primarily at the enterprise,
a decision driven by economics and the need for service providers to recoup their
initial investments in a reasonable amount of time. Cable, on the other hand,
is targeted primarily at home. Unlike most businesses, the overwhelming majority
of homes in the United States is passed by cable, reducing the required up-front
infrastructure investment significantly. Cable
is not without competition in the consumer market, for digital subscriber line
(xDSL) has emerged as the leading alternative to broadband cable. However, cable
operators are well positioned to capitalize on the convergence trend if they are
able to overcome the remaining technical hurdles and deliver telephony service
that is comparable to the public switched telephone system.
In
the case of cable TV, each television signal is given a 6-megahertz (MHz, millions
of cycles per second) channel on the cable. The coaxial cable used to carry cable
television can carry hundreds of megahertz of signals -- all the channels we could
want to watch and more.
In a cable TV
system, signals from the various channels are each given a 6-MHz slice of the
cable's available bandwidth and then sent down the cable to your house. In some
systems, coaxial cable is the only medium used for distributing signals. In other
systems, fibre-optic cable goes from the cable company to different neighborhoods
or areas. Then the fiber is terminated and the signals move onto coaxial cable
for distribution to individual houses.
When a cable company offers Internet
access over the cable, Internet information can use the same cables because the
cable modem system puts downstream data -- data sent from the Internet to an individual
computer -- into a 6-MHz channel. On the cable, the data looks just like a TV
channel. So Internet downstream data takes up the same amount of cable space as
any single channel of programming. Upstream data -- information sent from an individual
back to the Internet -- requires even less of the cable's bandwidth, just 2 MHz,
since the assumption is that most people download far more information than they
upload.
Putting both upstream and downstream
data on the cable television system requires two types of equipment: a cable modem
on the customer end and a cable modem termination system (CMTS) at the cable provider's
end. Between these two types of equipment, all the computer networking, security
and management of Internet access over cable television is put into place.
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