Home Networking: Understanding Coaxial Cable
- Mar 5, 2004
Coaxial cables have been used to construct the cable TV distribution network. At the beginning, a cable TV distribution network was very simple and consisted of coaxial cables connecting an antenna on a hillside to many homes that did not get good reception using their rooftop antennas. To compensate for signal losses caused by branch splitting and cable attenuation, TV signals picked up by the hillside antenna are usually enhanced by a wide-band amplifier, whose bandwidth covers TV channels of interests, raising the signal level by 10 to 25 dB. The off-the-air TV channels of VHF (Very High Frequency) band cover frequencies from 54 to 72 MHz (channels 2, 3, and 4), 76 to 88 MHz (channels 5 and 6), and 174 to 216 MHz (channels 7 to 13). A wide-band amplifier might only cover a group of off-the-air TV channels and a number of wide-band amplifiers; signal splitters and combiners might be required to cover all TV channels of interests. There are also off-the-air TV channels allocated in the UHF (Ultra High Frequency) band of between 300 and 3000 MHz. UHF TV channel signals over a cable TV distribution network also need to be amplified and sometimes down-converted to an unused VHF TV channel.
Within a particular TV broadcast region, there are always some spare TV channels available. In fact, off-the-air TV programs in the same region are not allocated next to each other to avoid interference between adjacent channels. Exceptions are channels 4 and 5 and channels 6 and 7 because there are already frequency gaps between them. A TV channel signal usually has a little energy, at below 42 dB, leaked into its adjacent channels. According to the regulation, the carrier-to-noise or to adjacent channel interference ratio should be larger than 36 dB to maintain good-quality TV reception. Because of different transmission conditions, TV signals from different antenna sites of the same region can easily have a signal strength difference of more than 12 dB. That situation could result in a Signal-to-Noise Ratio (SNR) of less than 42 + 12 = 30 dB. Therefore, the leakage signal of an off-the-air TV channel with a relatively high signal level could disturb that of an adjacent TV channel if they occupy adjacent channels without any frequency gap in between. On the other hand, TV channels can be placed next to each other in a cable TV system as long as signal strengths of adjacent channels are adjusted to similar levels.
Because TV channels are located at high-frequency bands, coaxial cables are used to construct the cable TV distribution network. A coaxial cable consists of an inner copper conductor and an external aluminum wrap overlaid with another shield of copper or aluminum braid. There are also nylon form insulations between the inner conductor and the aluminum wrap and a plastic cover over the metal braid. The TV signal is carried over the inner conductor while the aluminum wrap and the metal braid are connected to electrical ground. The electromagnetic shielding effect of coaxial cables is pretty effective especially at high-frequency bands where TV channels are located. However, this shield is not perfect. There always exists some electromagnetic field potential between the earth ground, above which the electromagnetic wave propagates, and the electrical groundespecially when the coaxial cable is hanging high in the air supported by telephone poles. In other words, the external shield of a coaxial cable can pick up interferences from electromagnetic fields and generate ingress noise. Because the strength of background electromagnetic fields are finite and the shielding effect of coaxial cable is relatively effective, the desired SNR for a good transmission can be maintained by defining the TV signal strength at the household entrance point.
The cable TV distribution network is constructed following a tree and branch structure. The cable TV signal from a main hub is first brought to an optical node through a glass optical fiber where the cable TV signal is Amplitude Modulated (AM) to an optical carrier frequency. After demodulation using Optical-Electronic (O/E) devices in the optical node, the cable TV signal is carried over a few branches of coaxial cable network to every subscriber. The root of each tree and branch distribution network is at the optical node. The main branch of the distribution network consists of distribution coaxial cables. The common types of distribution coaxial cables are 500-F and 625-F. The distribution cable is connected to subscribers through a device called a Tap and drop coaxial cables. A Tap device is inserted by connecting one end of the cut distribution cable to its input port and the other end of the cable to its output port. The insertion of a Tap device will reduce the signal level on the distribution cable by a small amount. The captured signal is distributed to multiple drop cable ports on a Tap device. The common drop cable types are RG-6 and RG-59. Drop cables are also used for in-house TV signal wiring.
The in-house TV signal wiring has not been widely publicized as a transmission medium for home networking for several reasons. First, for most households where the coaxial cables are installed by cable TV companies as an extension to their cable TV network, there are usually not many cable connections, and the cable TV company owns the wiring infrastructure. For households where the coaxial cable TV wiring is built in and many cable connections are accessible, the available spectrum for home network is limited once the cable TV signal is brought into the in-house TV signal wiring infrastructure. Finally, signal losses between different in-house coaxial cable connections could be pretty high because of the use of splitters where signal loss between branches are engineered high to satisfy impedance match requirement and to minimize reflections. Despite these limitations or constraints, a high throughput home network can still be constructed over the in-house TV signal wiring within frequency gaps of cable TV channels using proper modulation methods of suitable signal strength.
3.1 In-House TV Wiring
3.1.1 In-House Wiring Configuration
In a new construction, coaxial cables are connected from a central location near the TV signal source, be it a cable TV or a satellite dish, to every room where an in-house TV wiring connection should be available. This configuration forms a star topology. A multiport splitter is located at the center of the star. Depending upon the number of rooms to be served, some times an amplifier is inserted between the video source and the multiport splitter to raise the signal level, compensating signal losses caused by branch splitting. Assuming that signal separation loss is 15 dB between two output ports of every splitter stage, the minimal signal separation loss is 15 dB between coaxial cable outlets.
For in-house wiring installed by cable TV companies on existing homes, splitters are usually randomly installed at the cable TV entrance point and some other convenient splitting points. This configuration forms a star daisy-chain topology similar to that of the in-house telephone wiring except a splitter is used at every cable branch point. The maximum signal separation loss for the general star daisy-chain follows the same rule of thumb described for the star topology.
3.1.2 Hybrid Fiber Coaxial and Tree Branch Distribution Plant
Figure 3.1 shows the general arrangement of the cable TV hybrid fiber coaxial and tree branch distribution network connecting all subscribers. TV signals are originated from the head end. Two-way services such as cable modem for Internet access and POTS are also terminated and managed at the head end. A dual optical ring is used to connect all head ends and hubs over the SONET and ATM infrastructure for delivering and exchanging digital programs. AM fiber links are extended to a number of fiber nodes. From each fiber node, a few coaxial trunk cables, each of which serves a few hundred subscribers, are further extended to neighborhoods. Subscribers are connected to trunk cables through drop coaxial cables. The configuration illustrates that there might be some possibilities of interference between neighboring cable TV subscribers if in-house wiring is used for a high throughput home network. However, the degree of interference is minimized by additional drop cable losses and the signal-splitting loss introduced inside the Tap devices.
Figure 3.1. A Cable TV Hybrid Fiber Coaxial and Tree Branch Distribution Network