A cable distribution network is understood to be a broadband cable network or CATV network. In terms of concept, these are unidirectional distribution networks for supplying city districts, municipalities, regions or residential areas. Cable distribution networks can be divided into four classes according to the number of subscribers. The class with the largest number of subscribers typically has 10,000 subscribers, with a maximum of 100,000. The following subscriber classes each have a power of ten fewer subscribers, so that the smallest network class is 10 subscribers and a maximum of 100 subscribers can be supplied.
The frequency range of the cable networks for the hink channel to the subscriber includes VHF with Band I from 47 MHz to 68 MHz, VHF from 80 MHz to 108 MHz, VHF Band III from 174 MHz to 230 MHz, UHF Band IV from 470 MHz to 606 MHz and Band V from 606 MHz to 862 MHz. In front of Band III is the lower special channel band( USB) between 125 MHz and 174 MHz, behind Band III is the upper special channel band(OSB) between 230 MHz and 300 MHz followed by the hyperband between 302 MHz and 446 MHz. For optical transmission links, the frequency range extends up to 1 GHz.
Cable distribution networks consist of distribution points and amplifiers. Due to the coaxial technology used, the network has a relatively high attenuation, which makes the use of amplifiers absolutely necessary. From the distribution points, the networks are structured in a star shape to the amplifiers, from which a linear or tree-shaped structure follows to the subscriber lines
From unidirectional to bidirectional networks
In the early years, cable distribution networks were used for the transmission of analog TV and radio; later, digital TV( DVB) and digital audio broadcasting( DAB) were added. On the one hand, they expanded the range of programs, and on the other, they improved reception quality.
The increasing range of programs due to more and more new television and radio stations, as well as the need for additional, interactive distribution services, such as teleshopping, video-on-demand, pay-per-view, telelearning, online banking, the multimedia applications, and the use of the online services on the Internet, has resulted in a structural change in the cable distribution network, namely the restructuring of a unidirectional system into a bidirectional one for triple play
Various standards have been developed for these applications, bidirectional transmission over cable networks, such as DOCSIS, EuroDOCSIS and DVB-RCC. These services naturally place different demands on the transmission technology. As a result, the purely unidirectional cable distribution networks with assigned frequency bands and frequency channels must be converted to bidirectional networks. A return path channel must be available to the subscriber so that he can operate interactively via the cable distribution network.
Frequency band division for interactive services
The lower frequency range of cable distribution networks up to 65 MHz has been divided into two frequency bands: One frequency band is for upstream and ranges from 4 MHz to 10 MHz. It is reserved for network monitoring and management. The second frequency band can be used as an upstream and downstream frequency band. It ranges from 10 MHz to 65 MHz and is reserved for interactive data transfer and telephone applications. The frequency band reserved specifically for downstream is in the UHF Band IV range at approximately 530 MHz to 600 MHz.
To communicate in cable distribution networks with central equipment such as video servers, telephone PBXs or other servers, it makes sense to use internationally standardized interfaces. This has the advantage that transparent data transmission is possible with the standardized interfaces. For the interfaces, the PCM hierarchy has been chosen because data rates up to 34 Mbit/s can be converted directly into SDH data rates. This is particularly important because city networks are built exclusively in SDH hierarchy or in ATM technology, thus enabling direct connection to city networks or to wide area networks.