Coaxial cable based access technology series: HFC Cable Network

The structure of the cable television network

  In order to adapt to the transmission of broadcast television signals, the existing cable TV transmission network basically adopts a hybrid fiber-coaxial (HFC) network based on mixed transmission of optical fibers and coaxial cables. The HFC network adopts optical fiber in the trunk transmission part and uses coaxial cable to access users in the service area of the user, as shown in the figure. The HFC network is usually composed of three parts: optical fiber trunk line, coaxial cable branch line, and user distribution network. The program signal from the cable television station first becomes optical signal transmission on the trunk line; after the user area is converted, the optical signal is converted into electrical signal. The distributor is distributed to the user via a coaxial cable. It differs from the early CATV coaxial cable network mainly in that optical signals are transmitted on the trunk by optical fibers. In the preceding stage, the electro-optical conversion needs to be completed, and the light-electricity conversion is completed after entering the user area.

The architecture of cable television networks (from wiki)

  The main features of the HFC network are: large transmission capacity and easy two-way transmission. In theory, a pair of optical fibers can simultaneously transmit 2000 television programs: good frequency characteristics, no need for equalization within the cable TV transmission bandwidth, and low transmission loss, which can be extended. The transmission distance of cable TV does not require relay amplification within 25 km; there will be no cross-talk between optical fibers and electromagnetic interference, ensuring the signal transmission quality.
  The HFC network is generally composed of a head-end, a fiber optic trunk line network, a cable distribution network, and a subscriber line. The open-end cable television signal, satellite television signal and dvd signal received by the front end are modulated and demodulated to form a radio frequency signal and mixed into one output. The splitter evenly distributes the signal to each main line transmitter, and then through the optical splitter, the optical cable The optical signal is transmitted to the optical node or to the front end. Each optical node is an optical receiver and then distributed to each user via extension amplifiers, distributors, and building amplifiers.

The two basic mode of HFC network transmission

Independent head-end model

  A cable television network in the same city or the same area shares a front end, and all signals are collected at the front end. The broadcast television signals collected at the total front-end are converted to optical signals (ie, e/o conversions) in the form of vestigial sideband-amplitude modulation (VSB-AM) through the optical transmitter. The star-tree optical fiber network transmits to each cell optical node. The optical receiver of each optical node restores the received signal to an RF electrical signal (ie, o/e conversion), and then distributes the signal via the coaxial cable. Send to each household to the user terminal. This mode is the most basic to the transmission mode of the cable TV HFC network.

Distribution hub (sub-head-end) mode

  This mode is based on the general front end of the city or regional cable television to the multiple front-end transmission mode. The total front-end signal is fed to each sub-head-end to the optical node via star-shaped radiation to a digital fiber network or analog fiber network. In the CATV broadband integrated information network, apart from the front-end, it is also capable of reverting local TV programs and self-hosted programs. It is also a digital optical fiber and analog fiber to optical distribution and switching center. This system adopts a star-like or star-tree structure to the transmission network, and this model is most commonly used in large-scale and wide-band cable TV integrated information networks.
  At present, the HFC network topology adopts a shape or a loop structure in the fiber trunk section, and the coaxial cable section of the branch line and the distribution network adopts a tree structure or a star structure. In the initial stage of the construction of cable television networks, the tree topology was adopted more often. However, relatively speaking, the advantage of star-shaped structure giant line special account is therefore the development trend of the HFC network structure. With the continuous development of cable television networks, the number of users covered by an optical node has also been gradually reduced from tens of thousands of households to 2,000 households with 5,000 households. The cable television network in most countries was originally designed to transmit unidirectional broadcast television signals. Since 2000, the cable television network has been developing in the direction of digitization and bidirectionalization. In order to meet new needs, an optical node covers 500 users. Households or 250 households are more suitable.

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