How It Works: Phone-Line Network

Phone-line network: a technology that uses existing phone wiring to connect multiple computers for sharing files, printers, or Internet access.

You want to share a single Internet account between two computers, but your PCs are at opposite ends of the house. The solution is literally a phone call away--use your existing phone wiring to network them. With phone-line networks, you can:

• Connect up to 25 computers using the wires already snaking through your walls.
  
  
• Transfer data and talk on the phone at the same time, over the same wire.
  
  
• Share printers and other peripherals, sending data at rates up to 10 megabits per second.
  
  

All local area networks--wired or wireless, phone-line or ethernet--depend on adapters to send and receive data. In a phone-line network, the adapter breaks the digital information generated by your computer into packets that contain the data and the address of the computer you want to send the packets to. It then converts the packets into analog waves that travel along the phone wires. The adapter includes a jack for a standard phone-wire plug, called an RJ-11 plug. You plug one end of the wire into the adapter and the other into the wall.

To access the same line that you use for phone calls, the adapter uses frequency division multiplexing, a process that splits the available bandwidth into sections. And there's plenty of bandwidth to go around: When you make a call, your voice travels over one small section of the bandwidth. With a phone-line network, data from your PC travels over a different, much larger section. As a result, you can transfer data between PCs and still talk on the phone. In fact, you can use the same phone line for voice, networking, and a digital subscriber line Internet connection, since DSL signals travel over yet another, unused section of bandwidth.

PCs connected to a phone-line network generally are arranged in a peer-to-peer topology: You don't connect through a central hub, as you would in an ethernet setup. With peer-to-peer architecture, a data packet passes from one PC to the next until all have read it, but the packet is accepted only by the PC for which it is intended.

New Standards, More Speed

An industry group called the Home Phoneline Networking Alliance developed standards that dictate how data travels across phone lines, and at what speeds. The HomePNA 1.0 specification dictates 1-mbps transfer speeds; products using that version have been available since 1998. Released in October 1999, HomePNA 2.0 boosts the speed to 10 mbps. The new version uses a technique known as Frequency Diverse Quadrature Amplitude Modulation, which increases speed and reliability by sending duplicate versions of the signal over the wire.

Not all homes have adequate wiring to handle HomePNA 2.0's speed. In the United States, 99 percent of all homes can run 1-mbps phone-line networks, according to HomePNA research, while about 80 percent can use 10-mbps models; the different wire configuration and low-quality wires used in older homes may interfere with transfers. Additionally, PCs on the network can have no more than 500 feet of wire between them. Most homes fall under that limit, although small-business offices may not.