Home Toys Article
- February 2005 -
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Running 10BaseT Ethernet over existing
but unused coax cable.
Both the receive and transmit signals are combined in the hybrid before being sent to the balun to be impedance matched for transmission on the coax cable. A second identical device at the other end of the cable converts the impedance from 75 back to 100 ohms.
The cable companies have, over the years, installed a lot of coaxial cable that is no longer in use. In some cases, the household decides to forgo television. In other installations they go to satellite and even wireless broadband devices. Then you have those who have been caught up in changing. technology.
For instance, just a few years ago, many places in the US required two cables to decode all the channels. Cable A for the lower channels and cable B for the higher ones. Then the cable companies up-graded the infrastructure, running fiber to the neighborhood followed by coax to the home.
Another source of unused cables is households that have installed or plan to install some version of DSL to bring video over the telephone line. Along with the video they get data, typically 10BaseT Ethernet.
About now something should be bothering you. 10BaseT Ethernet was designed to use two pairs of what is called twisted pair or telephone wiring, while coax is a single wire surrounded by an insulator and sheath conductor. We found the transmit and receive signals could coexist on the same pair, allowing a balun to convert the signal to coax. The same coexistence is done in 4 pair bidirectional 1000BaseT.
The wire with the insulator and outer conductor forms a transmission line that exhibits a number of characteristics, most which we do not need for our analysis. We are, however, interested in the impedance of the line, which is 75 ohms.
Twisted pair nowadays is usually four pair, all within a common sheath. It is often referred to as Cat5 wiring. The current standard (TIA/EIA 568 B2) is for category 5e or better. Wire more than 2 years old but less than 12 years old is likely a category 3, 4 or 5 wire if in a commercial building, otherwise it is, most likely, non-rated POTS wiring. No category 4 and little category 3 are currently being installed. Twisted pair wiring is all categories 5 or better, with the current minimum in the standard being category 5e. All the UTP is designed to be 100 ohms impedance.
So, if our device is to convert between a 100 ohm balanced line signal and a 75 ohm unbalanced signal, it should be able to send the signal from the balanced line to the unbalanced line (Cat5 UTP to RG6 coax) and vise-versa. Such a device is called a balun (balun is the concatenation of the words balanced and unbalanced as modem is the concatenation of the words modulate and demodulate). This conversion satisfies the cabling requirements, but unfortunately, not the Ethernet protocol requirements.
10BaseT Ethernet uses one pair for transmission and the other for receiving. Fortunately, there is a device called a hybrid (a splitter/combiner) that can split the energy on a line into two equal streams. (or combine from two to one). There is a minor technical issue to be resolved in the design (to make sure that all the energy is used or dissipated so that it cannot disrupt the system).
Thus, both the receive and transmit signals are combined in the hybrid before being sent to the balun to be impedance matched for transmission on the coax cable. A second identical device at the other end of the cable converts the impedance from 75 back to 100 ohms. The hybrid then splits the signal to activate the two pairs. The receivers transmit pair ignores the signal while the receive pairs signal is normally processed.
These devices are on the market and available on the www.etslan.com website.