Digital Equipment ChannelWorks DECTV-AA, Руководство по установке, Страница: 33 / 106

Cable TV Network Overview 2-3
Single Cable Plants
Single cable plants divide the flow of signals by frequencies allowing for bidirectional
communications in a single cable. A block of frequencies are used for forward trans-
missions (going out from the headend) and a block of frequencies are used for return
transmission (coming in to the headend). The cable TV equipment is located at the
headend.
There are three common types of frequency allocation for single cable plants:
8. Sub-split (most cable TV systems)
Return frequencies = 5 to 30 MHz (4, 6 MHz channels)
Forward frequencies = 54 to 550 MHz (83, 6 MHz channels)
9. Mid-split (most popular for MAN environment)
Return frequencies = 5 to 116 MHz (18, 6 MHz channels)
Forward frequencies = 168 to 550 MHz (63, 6 MHz channels)
10. High-split (relatively new)
Return frequencies = 5 to 174 MHz (28, 6 MHz channels)
Forward frequencies = 232 to 550 MHz (53, 6 MHz channels)
The ChannelWorks Bridge can operate on any of the three types of frequency allocated
single cable plants with the appropriate diplex filter attached.
Dual Cable Plants
A dual cable plant is really one cable looped at the headend. The standard return and
forward frequencies for dual cable plants are 40 to 450 MHz (68, 6 MHz channels).
A diplex filter is not required when connecting the ChannelWorks Bridge to a dual
cable plant where the return and forward frequencies are the same.
Topologies
The topology of the cable TV network describes the physical layout of the cabling run-
ning to nodes or stations on the network. Popular network topologies are described as
follows:
• Tree — Resembles a tree. The root would be the headend. The trunk would be the
main cable. And the branches would be the feeder cables off of the main cable
(trunk).
• Star — In the star topology, all communications must pass through the central con-
troller for routing to the receiving stations. This topology communicates as though
it is point to point connectivity.