You can do data networking over your existing phone lines or over the power lines in your home. These protocols share the existing wiring in your home and coexist peacefully with the current uses of your wiring.
While this may seem daunting, it happens all of the time in Cable Television. One cable with carries over 100 channels of information. Each channel is modulated to use a different frequency range on the cable and so they do not interfere with one another. Your television simply must tune to the right frequency to extract the desired signal.
Phone line and power-line wiring use the same approach. When sending data, they modulate it up to a high frequency and send it across the cable. The receiving network cards are tuned to the frequency so they get the signal. At the same time, your telephone responds to a much lower frequency so the high-frequency data passes right by. Similarly in power line networking, your hairdryer is "tuned" to 60Hz and so the high frequency data is completely ignored by the hair dryer.
Now of course, there are very significant technical details to make these protocols work reliably under a wide range of network conditions and wiring configurations. It has taken many years of research into both technologies to arrive at well-engineered standards which work reliably.
Early on, there were a number of competing "standards" for this type of technology, forcing you to purchase all of your equipment from one vendor. Thankfully in recent years, most of the vendors have chosen to conform to standards which makes the equipment from different vendors interoperable. Standards are very critical to networking equipment because they protect your investment over time. A standard means that if you purchase your network cards from one vendor and a home gateway from another vendor they will work together.
This approach makes it quite easy to add another phone extension or wire a new jack. It is also why you can hear a conversation when you pick up any phone. On the other hand, if you have two phone numbers for your home, they will be wired on separate pairs. Most phone jacks are wired with at least four wires (two pairs) and are capable of supporting two phone numbers in one jack if wired properly. Because phone line networking depends on the wires being phyiscally connected, all of your phone line network equipment should connect to "line 1" or "line 2". Unless you do something special, you will be using "line 1" for your phone line networking. If, for some strange reason, you have a reason to use two separate phone lines for phone line networking, you will need some sort of hardware or software gateway to make them appear as a single network.
To use the phone line for data networking, you must install a network adapter which supports the phone-line protocols such as the Intel AnyPoint Home Network Card. These cards install in the exact same manner as an Ethernet card, but their connector is a telephone line connector. Some of the adapters will connect to your computer using a USB port and others will be built-in PCI cards. You use a standard telephone cable to connect the adapter to the telephone outlet.
If you want to use a phone and computer at the same location, you can use a simple phone-line jack doubler. Some of the phone line network equipment provides a "loop through" telephone jack so you don't have to run a wire all the way back to the phone jack at the wall to connect the telephone, modem, or fax machine.
You will install the proper device drivers for the network equipment following the manufacturer instructions. Once the computers are rebooted, you should have basic network connectivity between the computers with the ability to share files and printers. At this point, there is absolutely no relationship between your phones and your computers. It is like you have a basic two-station Ethernet network - except for the fact that you did not have to run any new wires.
Sharing an Internet connection in a phone line networking environment is not really that different from sharing an Internet eonnection in a standard Ethernet environment, you have several options:
If you are sharing a connection using a modem, the connection may seem a bit strange at first. You will actually have to make two connections to the phone jack. One for the phone line networking and another for the modem.
HomePNA choses a frequency that is high enough that it does not interfere with the frequencies used by Digital Subscriber Loop (DSL) Internet service. It is possible for standard telephone service (POTS), DSL, and HomePNA to coexist on the same wiring without interference.
The HomePNA protocol essentially is an Ethernet-style protocol with adaptations to the physical, electrical, and modulation requirements necessary to operate on telephone lines. There are several advantages to this approach. First, many higher level network protocols, TCP/IP, Microsoft File and Print Sharing, and Novell Netware, all are highly adapted to operate well in an Ethernet environment. Because of Ethernet's beginnings as a single wire ptorocol, Ethernet protocols are well adapted for the phone line wiring environment. A second advantage to using the Ethernet protocol as the basis for HomePNA is that it is possible to develop a bridge which transparently converts from one physical media (phone line) to another physical media (10BaseT - or twisted pair Ethernet).
One of the flaws of early phone-line networking was that it was difficult to use a mix of standard Ethernet equipment and phone line networking equipment. Now vendors have developed a relatively inexpensive bridge which has a HomePNA port and an Ethernet port on the back. These bridge units tie the two networks together to make them appear as a single network. With this bridge, you can use Ethernet (even 100Mb/sec Ethernet) in your "main computer room" and connect the rest of the house using phone line networking. This way you can have the best of both worlds.
The advantages of phone line networking include:
The disadvantages and limitations of phone line networking include:
With the standardization of the phone-line network protocols, and the strong support from a wide range of vendors, phone line networking is a good bet for exsiting homes which you do not want to rewire. But make sure that you look closely at what it will take to share your Internet connection or interoperate with an existing Ethernet network.
The primary advantages of power line networking are:
Phone line networking is becoming a relatively mature market with the release of HomePNA 2.0 with the accompanying support from the hardware vendors. By adding a HomePNA to Ethernet bridge, you can create a nicely integrated home network using both technologies where appropriate.
Power line networking has some definite advantages over phone line networking, but power line networking has had a much slower acceptance in the market place. However, there may be some applications for which power line networking is the only practical solution.