Phone-line networking is one of several ways to connect the
computers in your home. If your computers are in different
rooms, then phone line networking could be a good solution for
Be sure to read the companion article How Home
Networking Works, which provides information about
configuring your computers, routers and firewalls, Ethernet
networking and sharing an Internet connection. There are also
companion articles about power-line
networking and wireless
networking. By the time you finish this series of
articles, you'll be able to choose a network technology that
suits your needs and then configure the whole thing!
In this edition of HowStuffWorks,
we'll talk about phone-line networking and the technology used
to make it happen. We'll also discuss the advantages and
disadvantages of using a phone-line network.
Phone-line Networking Phone-line networking
is easy to install, inexpensive and fast, and it doesn't
require any additional wiring.
Phone-line networking, most commonly referred to as
HomePNA, is based on the specifications developed by the Home
Phone Networking Alliance (HPNA). The HPNA is a consortium of
key networking technology companies that created a phone-line
standard for the networking industry. HPNA 1.0, the original
version of the standard, operated at a rather slow 1 megabit
per second (Mbps). The current specification, HPNA 2.0, is
based on technology developed by Broadcom and operates at a
faster 10 Mbps. Cards using HPNA 1.0 are still being sold, so
make sure that the cards you buy are HPNA 2.0!
Pros and Cons of Phone Line
Networking HomePNA has several distinct advantages:
It's easy to install.
It operates at a constant 10 Mbps, even when the phone
is in use.
It requires no additional networking equipment (such as
hubs or routers).
It supports up to 25 devices.
It is fast enough for bandwidth-intensive applications,
such as video.
It is compatible with other networking technologies.
It works on Macs and older PCs (in addition to Windows
HomePNA does have some drawbacks, though.
You need a phone jack close to each computer. Otherwise, you
will have to run phone extension cords or install new wiring.
Even though it operates at a very reasonable 10 Mbps, it is
still 10 times slower than fast Ethernet (100 Mbps).
Therefore, if you are going to be sending huge amounts of data
between your computers, you may want more speed. There is a
physical limit of 1,000 feet (304.8 m) of wiring between
devices, and the overall area of coverage should not exceed
10,000 square feet (929 m2).
Rarely (in fewer than 1 percent of U.S. homes), HomePNA will
not work on the existing wiring. And while this author did not
notice any interference with voice use, there have been
reports of voices sounding "funny" or of a lot of noise on the
phone once HomePNA is installed. Later, we will look at these
issues and possible solutions.
How HomePNA Works HomePNA uses a method
known as frequency-division multiplexing (FDM). FDM
puts computer data on separate frequencies from the voice
signals being carried by the phone line. FDM separates the
extra signal space on a typical phone line into distinct data
channels by splitting it into uniform chunks of bandwidth. To
better understand FDM, think of radio stations -- each station
sends its signal at a different frequency within the available
In HomePNA, voice and data travel on the same wires without
interfering with each other. In fact, a standard phone line
has enough room to support voice, a high-speed DSL modem and a
home phone-line network.
How Much Does It
Cost? HomePNA adapters come in two versions:
internal card (PCI) or USB. You can buy kits consisting of
HomePNA cards for two computers, an installation CD and all
the necessary cables for about $90 to $110. The actual cost of
implementing HomePNA depends primarily on the type of
interface you buy for each computer: PCI cards cost about $45
to $55, while USB adapters range from $75 to $85. If you plan
to use a laptop computer that does not have a USB port, you
can either buy a USB-to-PCMCIA adapter ($50) or get a parallel-port
If you need to know more about the costs of individual
systems, here are six leading manufacturers of HPNA equipment:
In the next section, we'll learn more about HPNA
Broadcom Technology Two custom chips
designed using the HPNA specifications were developed by
Broadcom and make up the core of the HomePNA card's
The small 4100 chip on the left acts as a transceiver
between the larger chip and the signal being received over the
phone line from another computer. It can send and receive
signals over 1,000 ft (305 m) on a typical phone line. Think
of it as an interpreter, translating the analog messages it
gets from the phone line into a digital format that the
PCI/MSI controller chip can understand. The 4100 does not try
to understand what it interprets, it just sends it along.
Since many phone lines in existing homes vary greatly in
length and signal quality, the larger 4210 controller chip on
the right has to be able to adapt to a variety of challenges.
Because the 4210 does this so well, the HPNA estimates that
more than 99 percent of U.S. homes have phone wiring that can
support HomePNA. Basically, it works this way: First, the
controller chip takes the unfiltered content it receives from
the smaller chip and strips away all the noise. It then takes
what is left and passes it on to the computer for processing.
Once the computer processes the information, it returns an
acknowledgment to the sending computer so it knows that the
data was received. This happens thousands of times each second
as the computers communicate.
In the next section, installing HomePNA and some of the
problems you might face.
How to Install HomePNA If you decide that
HomePNA is best for you, here are the basic steps:
Buy a kit, making sure you have a HomePNA card or
external adapter for each computer in your home.
Install the hardware (internal card or external
Plug the included cable into the hardware and into the
Install the software.
Troubleshooting There are
a few things you should keep in mind when you set up a HomePNA
network. First, most analog communication devices, such as
telephones and fax machines, create signal noise. Think of
signal noise as debris on a major highway. A little debris
probably won't affect traffic, but a lot of it could slow down
or even stop traffic in one or more lanes. If you install a
HomePNA network and your computers have trouble communicating,
try inserting a low-pass filter between any phones or fax
machines and their respective jacks. The low-pass filter will
block noise without impeding the performance of your fax or
phone. You can find these filters at most electronics stores.
Also, electrical fields generated by powered communication
devices, such as cordless phones or fax machines, can
introduce another type of signal noise. A different type of
low-pass filter, inserted between the electrical wall outlet
and the power cord for the device, can fix this problem.
The last potential issue is rare but much harder to fix. If
you have a very large home, or one that has been renovated
several times, you may have too much wiring between computers.
All of this wiring will weaken the signal, causing it to fade
out and lose strength. The result is that not enough of the
signal remains if and when it reaches the other computer for
that machine to process it. If this is the case, then you will
either have to move the computers closer together or redo the
wiring, at which point you may want to consider learning about
Many home computer users will find that connecting
computers through their phone lines is a good solution, but
there are still two other networking technologies to discuss:
power-line and wireless networks. Click on the title to go to
one of these articles for more information, or proceed to
either A Word
About Macs or The
Future of Home Networking.