Ericsson Review No. 1, 2001
33
signal to smaller local distribution ampli-
fiers. The fiber is then terminated and the
signal is converted back into electrical sig-
nals that are sent over coaxial cable into the
neighborhood (Figure 2). This scheme of
using a mixed fiber and coaxial cable distri-
bution scheme is called a hybrid fiber-coax
(HFC) system. The amplifiers in this part of
the network must be bidirectional.
In the past, the cable plant typically dis-
tributed signals from the head-end system
to the customer premises using coaxial
cable. Today, however, nearly all operators,
including multiservice operators (MSO),
have upgraded or are in the process of up-
grading their plants to hybrid fiber-coax
systems. These systems provide clean, high-
quality signals to the neighborhood with-
out replacing all of the cable to the cus-
tomers’ home.
To introduce data into the system, the IP
infrastructure must be overlaid on existing
systems. Connections to the Internet back-
bone via concentrators and routers are pro-
vided via high-speed optical connections,
typically OC-8 and higher-speed connec-
tions. These connections are brought into
the head-end hub and distributed via routers
and optical links to various regional distri-
bution hubs in the MSO network. The IP
network is tied into a special adapter called
a cable modem termination system (CMTS),
which consists of one or more cable modem
line cards (CMLC). CMLCs convert the IP
data stream into downstream (to the home)
and upstream (from the home) RF signals.
The downstream signals are sent through an
up-converter, which puts them on a specif-
ic channel and combines them with the
other standard TV signals.
The upstream signals are collected from
the subscribers. In a properly designed sys-
tem, there are several upstream channels for
each downstream channel. This is because
the upstream data is transmitted at a slow-
er rate than the downstream data. The traf-
fic must be engineered to provide adequate
service to users. Voice, streaming video, and
gaming services use the greatest amounts of
bandwidth. The downstream data rate—ap-
proximately 30 to 40 Mbit/s—can be shared
by some 500 to 2,000 users. The upstream
data rate is approximately 8 Mbit/s per chan-
nel. Proxy servers and data cache servers are
also employed at the local hub to improve
system performance.
Cable service providers in North Ameri-
ca and Europe have different transmission
requirements for channel width, upstream
bandwidth, downstream bandwidth, chan-
nel center frequencies, and power limits per
channel. After the data-over-cable service
interface specification (DOCSIS) had been
successfully certified in North America and
Europe, cable operators adapted it and most
of the specifications set by DOCSIS for Eu-
rope. Euro-DOCSIS was formed to address
European cable operators, Internet service
providers (ISP), and end-user needs (for a
(20 pages)
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