The article is devoted to the General principles of construction of multifunctional cable TV systems, which are increasingly interested both users and operators in different regions of the country and in the implementation which first steps are taken.
The transition to digital television - actively solve the most important at present the scientific objective in the field of information and telecommunication. The user receives very noticeable new features: megaprogramming, interactivity, multi-functionality (telephony, data transmission, videoconferencing, access to Internet, multimedia and a variety of other services); improve the quality of TV reception. In addition, a more rational use of the radio spectrum, bandwidth channels, etc.
Among the ways to deliver TV programs to subscribers of a strong position, especially in large and medium-sized towns, took a cable TV system (CCTV). In the West in Sctv since the mid 90-ies became implemented interactivity. In the U.S. and Canada the potential to use interactive services in cable networks 1998 had about 11 % of the residents.
To create a modern info-telecommunications cable networks should be solved goal: to modernize the existing systems or build new networks on a single standard principles.
Currently in Russia the majority of TV sets connected to the system community reception of different generations: "antenna on the porch", a large system collective reception of television (CSKT) and cable TV system (CCTV). The first option is "antenna on the porch" is a predominantly passive network with directional couplers. With the necessity to enhance TV signal is applied one cascade channel or band amplifiers or UHF converters. XSLT and sctv was built to improve the quality of the TV receiving, in the systems used head stations with channel-by-channel processing signals and convert channels frequency: broadband backbone and house amplifiers with a bandwidth of 40 - 240 MHz without a return channel and frequency-independent trunk and subscriber taps splitters. The most active construction of such systems was carried out in 80-ies and continues now despite their limited capabilities in terms of increasing the number organized by TV channels, interactivity and use for solving the multifunctional purpose of the network.
To create linear structures cable network consumes more than 80 % investment, so very sad is the fact full the futility XCPT and CCTV built on that principle. Reconstruction of distribution networks of these systems with the aim of turning them in coaxial nodes (bushes) in any capacity for multifunctional systems impossible without a complete replacement of all components - coaxial cables, passive elements and amplifiers due to the fact that not only ensured the required the range of frequencies even when replacing passive and active network elements, but can not on existing networks to achieve the required (subject to the allocation of 40 - 50 analog TV channel) minimum level combinational interference the nominal level of the input signal on the subscriber point, etc. Thus, these distribution networks are doomed and building them today, not historically justified. To stop this process and make recommendations for expansion or network reconstruction in each case, apparently, the objective of the Association cable TV of Russia and the Ministry of communications of Russia.
Recently started the implementation of broadband sctv return channel (band 40 - 862 MHz in the forward direction and 5-30 MHz bandwidth return channel). The architecture of these networks is subject to the exclusive benefit of the cable television and it does not correspond with problems solved broadband rich, interactive networks, which are based on the use of fiber optic systems for transmitting analog and digital signals with high quality over long distances. The scope of services does not should be limited to the interests of the traditional volume of the TV and radio. Today is a clear need for such services, as the provision of paid channels of digital TV and TV-to-order (video on demand), multimedia, home shopping, security and fire alarm, the use of the network for dispatching systems engineering equipment residential neighborhoods, the provision of data transmission channels; organization of local and corporate computing networks; connecting subscribers to the Internet; digital telephony, etc.
It is likely that a simple replacement of part of the main coaxial network will not provide full use of the capabilities of fiber optic and drastically limiting the size and technical opportunities sctv. Therefore, adaptation of broadband in CCTV telecommunication network requires a review of the network architecture sctv (see figure).
Traditionally architecture CCTV high capacity (five thousand and more subscribers) has a tree structure. For coaxial (especially unidirectional networks she gave the best ratio price-quality. However for an interactive network such a construction limits the possibility of creating reverse channel. In hybrid networks (fiber - coaxial) each piece to some extent imposes some specific requirements to each other fiber to the quality of the signal delivered by the coaxial network and, accordingly, Vice versa. When designing CCTV calculation of parameters was reduced to determining the optimal level of the signal at the output of the subscriber connecting device and the output level to consistently enabled amplifiers, which was only limited by the noise value. To enable CCTV part of a hybrid fibre-coaxial network is necessary to make recalculation taking into account intermodulation distortion (IMD) second (CSO - Composite Second Order) and third (CTB - Composite Triple Beat). Depending on the parameters of fiber optic network, coaxial preceding, to the last recalculation may be charged additional requirements not only by changing the output levels of amplifiers and limiting the number of amplification but also by some rearrangement of house distribution network to increase the minimum levels of signals on the subscriber taps splitters.
Most likely, such changes will be within the possibilities of reconstruction system and will not require its global reconstruction, i.e. significant capital expenditures. However, when the design and construction of interactive broadband CCTV need to be considered (and world practice it confirmed) that at this stage the most cost-effective option building telecommunications networks, providing subscriber access multifunctional tasks, are hybrid structures using fiber-optic and coaxial cables.
In the foreseeable period of the telecommunications network used, in particular, for the transmission of TV signals must have the structure and use of the transmission system, compatible with traditional analog and more widely implemented digital. TV signals in broadband networks is a huge, disparate on the width of the range of other funds of the band, and delivery of these signals to subscribers in digital form is also the most difficult. We must assume that in the decade of the hybrid network will be the dominant structure, with will be used to convert digital TV signals to analog for group subscribers and applied to individual user terminals.
The process of creating a network subscriber access (first level) on the basis fiber coaxial distribution networks can be accelerated only if the following principal conditions:
- development of normative-technical base, which allows to start mass designing networks for hybrid technology:
- universal build transport information bus formed take into account the transfer of existing and future TV programs;
- operational conversion interactive broadband CCTV in coaxial nodes, providing technical possibilities of modern telecommunications networks.
The fulfillment of these conditions will avoid investment in obsolete or hopeless technology.
The main level access from the hub to the node coaxial network (second level) is based, like the previous one, on the basis of FOL. but unlike transport can be not only digital but also analog.
The third level is coaxial submaxillary and independent distribution network, including subscriber coaxial cables to the subscriber terminal. This network covers from a few hundred to a thousand or more subscribers.
The first two levels are based, as noted, on the basis of FOL, which has many advantages, which are repeatedly written in the journal. Note only that they allow you to organize several dozen TV channels. Even when transmitting digital TV with speeds of 2.5 and 10 Gbit/s potential the possibility of at least an order of magnitude higher when using modulated light waves of the same length. But today the industry produces devices wave division multiplexing, which allows to dramatically increase the number of threads signals.
Spectral seal allows you to increase the flow rate of digital signals, and for analog TV signals such sealing is the new quality. Today there are two types of transmission of such signals: the first one fiber is used for organization one channel of the second transmission spectrum containing a certain the number of TV channels. Spectral same seal allows you to optimize the FOL and in that and in other cases. The independence of the signals available in one the optical waveguide toward each other on a wave of the same length and in the same direction on different wavelengths, provides unique opportunities in the implementation of new architectures networks, including for interactive networks. Are not used here an opto-and electro-optical converters and optical couplers, allows you to branch off a certain percentage of optical power and insert the optical signal in the total flow, i.e. implemented bidirectional tree structure: from the trunk to the branches is transmitted TV and any other threads, but in reverse the direction of narrowband signals SMS service, voice signals from various sensors, etc.
The third advantage is the flexibility to rebuild the network from analog to digital.
In several cities of Russia already work is underway to create an information cable network with integration of services.
Make a conclusion based on the above. For any telecommunication system the most costly and time-consuming its plot is the network. Therefore, at the stage the transition to a fully digital distribution network should be built the maximum possible use of traditional structures I. significantly, allowing some time to use both digital and analog of the transmission system.
Authors: S. Dmitriev, K. Kukka, B. Exler, Moscow