Home Networking Basis: Transmission Environments and Wired/Wireless Protocols

The in-house coaxial cable wiring carries many channels of TV signals. The conventional TV VHF channels, channels 2 to 13, occupy frequencies from 54 to 216 MHz with frequency gaps in between for FM broadcast stations. The off-the-air UHF channels, channels 14 to 69, occupy frequencies from 470 to 806 MHz. Table 3.4 shows frequency arrangement for off-the-air TV channels.

This TV channel frequency arrangement is regulated by Part 73 (Radio Broadcast Services) of FCC rules and regulations, which is labeled as CFR title 47 [1]. Channel 37, occupying 608 to 614 MHz, is reserved exclusively for the radio astronomy service. Many off-the-air digital TV channels have been allocated in the UHF frequency band for the analog-to-digital transition period. Detailed information, including location and channel numbers, can also be found in the recent Part 73 document.

Cable TV channels share the same frequency arrangement as off-the-air TV only for channels 2 through 13. Immediately above the frequency of channel 13, cable TV channels occupy a continuous frequency band from 210 up to 552 MHz for channel numbers of 23 to 78. Cable TV channels 14 to 22 occupy a frequency band from 120 to 174 MHz merging to the frequency band of conventional TV channel 7. There is no conflict with the FM broadcast service because cable TV signals are contained within coaxial cables. This channel allocation plan is set forth in the EIA's ''Cable Television Channel Identification Plan, EIA IS 132, May 1994'' (EIA IS 132) [2]. Table 3.5 shows this channel frequency arrangement. Other regulation for the cable TV industry is provided by Part 76 (Cable Television Service) of FCC rules and regulations which is labeled as CFR title 47 [3].

Above channel 78 starting at 552 MHz, 33 digital cable channels can be allocated for a frequency band of up to 750 MHz. Digital cable channels can be used for cable modem as well as digital TV services. It is expected that the video carrier of a cable TV channel should have a signal strength of around 0 dBµV at an impedance of 75 ohms.

For delivering cable modem services, a reverse channel is required to send user command or information back to the head end. The frequency band below the conventional TV channel 2 has been allocated for reverse channels. There is one convention called sub-split where the reverse channel has a frequency band of 5 to 30 MHz. There is another convention called expanded sub-split where the reverse channel frequency band is from 5 to 42 MHz. Reverse channels in the frequency range between 25 and 37 MHz have been proposed by the IEEE 802.14 study group for a cable modem standard. The recommended reverse channel signal strengths are in the range of 5 to 55 dBµV or to 58 dBµV for sub-split or expanded sub-split, respectively.

The majority of the frequency band on existing in-house wiring has been occupied by cable TV channel allocations. CEBus [4] has recommended a dual coaxial cable wiring system to carry local video signals as well as data traffic. In case a second coaxial cable is not available, the frequency band between the upper edge of reverse channel at 42 MHz and the lower edge of TV channel 2 at 54 MHz can be used to establish a coaxial cable based home network. This coaxial cable based home network should not interfere with either TV channels or reverse channels by minimizing out-of-band energy at 60 dB below these TV or reverse channel signals.