FebruaMaximumPC 2008 02

r & d BREAKING DOWN TECH —PRESENT AND FUTURE

W

hen it comes to networks, Maximum

PC readers fall into two camps: Those

who already have one (and want to expand

it or extend its range), and those who wish

they had one. Whichever category you fall

into, power-line networking is fi nally becoming

a viable alternative to Cat5 and Wi-Fi.

Power-line networking takes advantage of

the unused bandwidth inside the copper wir-

ing used to distribute electrical power through-

out the home. Power-line adapters convert

data into a carrier-signal format, so it can be

transmitted from one device to another.

Since your house already has electrical

outlets, there’s no need to pull any new wires

or drill any new holes. And those existing

power cables are endowed with potential

bandwidth that’s much greater than what

today’s 802.11n Wi-Fi networks are capable of

delivering. (And when you think about it, most

Wi-Fi devices still depend on wires—even if

only to power them.)

In an ideal world, all you’d need to do to

assemble a network is plug your PC, DSL/

cable modem, router, and peripherals into

a wall jack and BAM! You’d have an instant

network. It’s not quite that easy, but the fact

that it works at all is remarkable.

POWER-LINE PIONEERS

In the United States, home electrical systems

operate on alternating current at a frequency

of 60Hz. Power-line networking uses much

higher frequencies—ranging from 2MHz to

30MHz—to carry data. Intelogis was the fi rst

company to offer power-line networking tech-

nology to consumers, but its Passport system

proved to be slow and extremely sensitive to

noise caused by appliances operating on the

same power lines.

The problem was Passport’s reliance

on frequency-shift keying (FSK) to encode

data carried on the network. FSK uses just

two frequencies to encode the data in a

binary system: one frequency for the 1s

and a second for the 0s. If a large appliance

or high-current

device (such as a

hair dryer) caused

an electrical surge

to step on either

of those frequen-

cies, the stream of

binary data would

be interrupted. The

device transmitting

the data would then

have to resend the

packets, causing

signifi cant conges-

tion on the network.

Two consor-

tiums are competing

to establish a de

facto power-line net-

working standard: the HomePlug Powerline

Association (whose members include Intel,

Motorola, and Texas Instruments) and the

Universal Powerline Association (or UPA, orga-

nized primarily by communications chipset

company DS2). A third group, the Consumer

Electronics Powerline Communication

Alliance, was established to promote the

coexistence of power-line networking prod-

ucts that use different technologies. (CEPCA’s

membership roster includes nearly every major

consumer-electronics manufacturer, including

Sony, Toshiba, and Philips.)

Although products based on DS2 chip-

sets have proven extremely effective (and the

company recently demonstrated a 400Mb/s

product), a merged specifi cation promulgated

by the HomePlug Powerline Association and

Panasonic won the most recent round of vot-

ing by the IEEE P1901 work group. Analysts

blame the slow growth of the power-line net-

working market on the lack of interoperability

between power-line networking products; if

HomePlug gets the IEEE’s seal of approval,

DS2 will have a diffi cult time bucking the trend.

The HomePlug Powerline Alliance’s base-

line specifi cation is derived from InTellon’s

PowerPacket system, which takes a very dif-

ferent approach to power-line networking than

Intelogis’s now-defunct Passport. Rather than

streaming data encoded to just two frequen-

cies, PowerPacket uses a spectrum rang-

ing from 4.3MHz to 20.9MHz. These bands

are organized into 84 “lanes” of traffi c using

orthogonal frequency-division multiplexing

(OFDM) with forward error correction. OFDM

uses a large number of closely spaced sub-

carriers, each of which is modulated using

quadrature amplitude modulation (QAM). QAM

conveys data by manipulating the amplitude

of two carrier waves.

Since PowerPacket can send data over

so many traffi c lanes, the transmitting device

will send redundant data on more than one

subcarrier. If a power spike or excessive

noise interrupts one lane of traffi c, the data

encoded in the other frequencies should still

get through. The transmitter also adds redun-

dant data to its messages, which is where

forward error correction comes into play. The

transmitting device sends a known preamble

at the start of each data packet. The receiving

device then compares that preamble to the

actual data received. If there’s a difference,

the receiver can either try to correct the error

or use one of the redundant streams. Garbled

data doesn’t need to be retransmitted unless

both these methods are unsuccessful.

The use of frequencies outside the range

of AC power explains why power-line network

devices and surge suppressors don’t play well

together: The latter interpret the data-carrying

frequencies generated by the former as electri-

cal spikes that must be tamped down.

HOMEPLUG AV

HomePlug 1.0 is limited to throughput of

14Mb/s, which means it’s inadequate for

streaming high-defi nition audio and video. The

improved Physical Layer (PHY) and Medium

Access Control (MAC) technologies in the

newer HomePlug AV and similar nonstandard

White Paper: Power-Line Networking

Once dismissed as hokum,

this LAN technology is fi nally

coming into its own.

BY LEE HAMRICK

58 MAXIMUMPC | FEB 08 | http://www.maximumpc.com

New power-line networking technology should soon enable consumers to build robust

data and A/V networks using their existing AC power lines.

THE PROMISE Networks with no new wires

Printer

Internet

Security Camera

Game Console Media Server Wireless

Speakers

VoIP Phone Consumer Electronics