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Technical basics
ELSA MicroLink Cable
47
Technical basics
This chapter is a short introduction into the technology used by your device. Network
professionals will find themselves just skimming these pages, but novices will find this
section to be very helpful for understanding the technical terms and processes.
Cable modem technology
The cable modem belongs to a new, promising generation of Internet access technology.
This device differs from conventional analog and ISDN modems in that it communicates
via the broadband radio and cable TV available in nearly all households, rather than the
usual telephone lines.
Downstream
:
Transfer of data
from the
provider to the
Internet user
This cable is well-suited for the transfer of large volumes of data. Up to now, the one-
way flow of data from the provider to the user (downstream) has been a problem for such
applications, however. Videotext uses this downstream data transfer: the provider, in
this case the television broadcaster, continuously transmits a selection of information in
the form of individual pages over the cable network.
Upstream
:
Transfer of data
from the
Internet user to
the provider
The user can then choose a page, by entering a number for example. However, the user
is restricted to the broadcaster's selection and cannot send data back (upstream).
Standards
Two standards get around this problem:
The first solution accepts the restriction of the cable network to downstream
transfers and handles the upstream using normal telephone lines. The
disadvantage is obvious: it requires an additional line on the telephone network
(through a normal modem, for example) subject to telephone connect charges.
A standard that was successfully applied in the USA equips the cable network with
suitable amplifiers and remote stations for the transfer of data back to the provider.
These remote stations are called headends or CMTSs (
C
able
M
odem
T
ermination
S
ystem). Connections using the MCNS standard (
M
ultimedia
C
able
N
etwork
S
ystem) no longer need an extra telephone connection. Access to the Internet is
thus no longer subject to time-related connect charges, in effect providing the user
with a permanent connection to the Internet.
Access
To clarify access to the Internet through the cable network (using a simplified
description), let's have a look at both sides of the connection. On one side we have the
user, who would like to connect to the Internet with a local network or an individual