ELSA manual Cable modem technology, Standards, Access, Technical basics

Technical basics 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

Downstream:

Transfer of data from the provider to the Internet user

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.

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:

KThe 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.

KA 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 (Cable Modem Termination System). Connections using the MCNS standard (Multimedia Cable Network System) 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

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