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The distribution of the signals begins with the fibre optic network. fibre optic network serves two purposes: as an intermediate transmission mechanism to bring the cable signal from the headend to each of the local distribution centres (LDC), and as a point-to-point transmission mechanism to bring digital services from the headend to an end user who requests direct digital services.
Transmission using digital fibre optic technology is usually significantly more reliable than transmission using coaxial cable since fibre is not susceptible to electric or magnetic interference. Furthermore, the signal degradation in a fibre optic network is almost negligible compared to coaxial cable networks. It is this last benefit which makes fibre optic technology ideal as the "backbone" of the distribution system. In practice, the headend and the local distribution centres can be located anywhere in Hong Kong since a signal travelling from the headend to an LDC will be just as strong if it travels one hundred kilometres than if it only had to travel one kilometre. It should be noted that this benefit is not "free", the longer the path, the more regenerators (equipment which brings a weakened signal back to its original strength) are needed. As a comparison, if this backbone network were provided by a conventional coaxial cable system, a signal travelling such a long distance would be considerably weaker even with the use of regenerators.
Once a signal has arrived at the local distribution centre it is converted from its digital form to an analogue form which can be used directly by a television set. At this point it is distributed to buildings along a coaxial network referred to as the "horizontal network". The cable television signal is distributed in a tree and branch topology; that is, there is a main trunk cable which goes down all major roads. When the main trunk passes a housing block, a tap located in a joint box within the duct network, is put on the cable which allows a "branch" cable to split off of the main trunk to enter the housing block. [1] Along the main trunk, amplifiers are needed to strengthen signals which are weakened as they travel down the cable.
Once the cable has been brought in from the street to the main distribution frame (MDF) of the building, the final distribution of the cable television service is considered part of the "vertical" component of the network. Cable is brought up from the MDF to each of the floors of the buildings either in separate ducts, alongside water or gas pipes or in some cases within elevator shafts. As in the case of the horizontal component, if the path that the signal must travel is too long, amplifiers are needed to bring the signal back to its original strength. From the amplifiers, perhaps located on every other floor of a building, "customer feeders" (cabling from the amplifier to the customer's residence) must be installed. The final stage of bringing cable into a customer's home is the installation of a system outlet which allows the connection of a television to the cable.
Other terms used instead of tap include splitters and directional couplers.
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