Did you know that every room in your home already has three or four
network ports? That's the message the HomePlug Powerline Alliance is
trying to get out. Since announcing the first release of their ac-line
networking specification in June, member companies, such as Netgear and
Phonex, have been busy building products that allow you to use the
ac-power lines in your home not only to power your networked PCs but
also to interconnect them (Reference 1,
1). The idea of using your home's ac-power lines to
exchange data between computers has been around for a while. The
difference this time is that technology has caught up to the task.
HomePlug is promising data rates as fast as 14 Mbps over your home's
power lines. Moore's Law, faster silicon, and sophisticated algorithms
are allowing engineers to overcome the technical obstacles that plagued
The rate at which customers subscribe to broadband services is not as
fast as broadband providers would like, but it's these consumers who are
most interested in home networking. There are several ways to connect
two or more computers together in a home. One of the cheapest and
easiest ways is to install a couple of inexpensive Ethernet cards into
your PCs (if they don't have them) and string some cable between them.
This method may be OK for the average EDN reader, but most
homeowners want a more sophisticated and unobtrusive option. People
building their own homes will probably spend the extra money to install
low-voltage or structured wiring consisting of Category 5 cable. But
that leaves millions of homeowners with two options: tear up walls to
install cable or use the "No New Wires" approach.
Because so few homes have structured wiring in their walls, vendors
of home-networking equipment are fond of the "No New Wires"
slogan. But not having to install new wiring to gain network access
throughout a home or building is a real advantage. Wi-Fi (Wireless-Fidelity)
owes a great deal of its success to peoples' reluctance to string new
wires throughout their homes or office buildings (Reference
2). In fact, Netgear's Home Networking Product Line Manager,
Debashis Pramanik, says that retail wireless-networking sales are
growing 50% per quarter, and 50% of those products are going into homes.
On the other side of that coin is the fact that retail prices for those
products have dropped significantly from only a year ago. HomeRF is the
other home-wireless-networking alternative, but its market share is
dropping. Cahners InStat/MDR reports that HomeRF shipments made up 45%
of all wireless nodes shipped in 2000. But in 2001 that figure dropped
to 30%, with Wi-Fi nodes making up nearly the entire remainder.
So why not just set up a Wi-Fi network in your home instead of
bothering with power-line networking? Good question. You'll be hard
pressed to find a home-networking vendor willing to say bad things about
Wi-Fi. One reason is that many of them sell Wi-Fi products. Another
reason is that Wi-Fi technology works and is popular with users. And a
third reason is that vendors feel it's important to first sell the idea
of home networking, then fight over who gets the biggest piece of the
pie. Most vendors pushing alternative home-networking technology say
that Wi-Fi and their "No New Wires" products are complementary
and both have their strengths. Their hope is that you will use a
combination of the two technologies—Wi-Fi for untethered network
access and HomePlug everywhere else.
Because home-networking vendors know they can't compete with the
mobility Wi-Fi offers, they point out that Wi-Fi networks rarely achieve
100% coverage in a home. Due to obstructions such as metal laths in
walls or interference from 2.4-GHz cordless phones, you will experience
diminished or nonexistent throughput between your PC and your access
point as you roam about your house. You can always add another access
point, but that option adds significant cost to your home network.
Wireless networking has another disadvantage, at least potentially. The
technology requires an RF section, which adds to the cost of the chip
set. You don't see this difference in cost today, though, because 802.11
chips have been shipping for more than four years, and their volumes are
much greater than for HomePlug chip sets. If HomePlug unit sales catch
up to wireless sales, HomePlug technology should cost less.
HomePlug advocates also claim that HomePlug networks outperform Wi-Fi
networks at greater distances. Also, 802.11b uses an optional 40-bit
encryption key, whereas HomePlug always uses a 56-bit key. Plus, you
have to be physically connected to the HomePlug network to intercept
transmissions. Another point is that HomePlug was designed for the home
market and is easier for users to set up.
QOS (quality of service) is another issue. A lot of people will use
their home networks for more than sharing an Internet connection or
printer. More and more people are using their PCs to store and play
digital music. Some of these users will want to stream digital content
to other rooms of their homes. To do that task well, a home-network
protocol must support QOS, so that priority is given to streaming audio
or video data over other data. The current 802.11 standard does not
support QOS, but the IEEE 802.11 Task Group E recently adopted
QOS-enhancement proposals made by the 1394 Trade Association's Wireless
Working Group. In fact, some see the IEEE 1394 standard as an
alternative for multimedia home networks. The 1394a standard has a
cable-length limitation, but the next-generation 1394b standard has
greater flexibility (Reference 3).
HomePlug and most other home-networking protocols support QOS.
One such protocol is HomePNA (Home Phoneline Networking Alliance),
which uses the phone wiring in your home rather the power lines (see sidebar
"HomePNA is on the line"). The biggest drawback to HomePNA is
that the average house in the United States has fewer than five phone
jacks. On the other hand, most homes have 10 times as many ac outlets.
In Europe, the average number of phone jacks per house is less than two.
Another obstacle to HomePNA is lack of consumer education. It's not
intuitive to a lot of consumers that you can simultaneously use your
phone wiring as a computer network and make phone calls. HomePlug may
face similar resistance when salespeople try to convince potential
customers that it's OK to plug a network card into a 120V wall outlet.
The analysts at Cahners InStat/MDR believe HomePlug will overcome
such resistance. The group is predicting that the
home-power-line-networking market will grow from less than $18 million
in 2001 to nearly $190 million in 2002. Hoping to grab a share of that
market is HomePlug competitor Inari (see sidebar
"Outlet competition"). The advantage HomePlug has at the
moment is the backing of 90 member companies, multiple silicon suppliers,
and the HomePlug logo, promising that networking gear from different
vendors will interoperate after you plug it in. The Consumer Electronics
Association's R-7.3 is a rival power-line standard that will be based on
technology from Inari, Itran, or nSine. The association plans to
complete trials of the three power-line-networking technologies by April.
Thirteen companies founded HomePlug as a nonprofit industry
association in March 2000. Their goal was to create an open
specification for home-power-line networking and create demand for their
products. The group created an MRD (marketing-requirements document)
describing goals that HomePlug technology should meet before its release
to the market. The MRD includes regulatory, interoperability,
performance, noninterference, reliability, scalability, diagnostics, and
maintenance requirements. Performance requirements include
"10BaseT-like rates" for multinode file transfers and QOS
support for multiple voice-over-IP calls. The MRD also addresses
coexistence with other power-line-networking technologies, such as X-10,
CEBus, and LonWorks. HomePlug members also want strong built-in privacy
HomePlug reviewed 10 proposals before selecting Intellon's
PowerPacket technology as the basis for the HomePlug specification. The
group spent more than a year comparing Intellon's proposal against the
MRD. After making modifications, the PowerPacket technology evolved into
the HomePlug 1.0 specification, which the alliance announced in June
2001. The specification is available to members. Version 1.0.1 is the
most recent update and includes errata to clean up the initial release.
Before releasing the specification, HomePlug conducted field trials in
more than 500 homes in the United States and in Canada. It tested nearly
10,000 outlet-to-outlet paths against the MRD requirements. HomePlug
testers hoped to see a throughput of at least 5 Mbps between 80% of all
outlet pairs within a home and a minimum of 1.5 Mbps in 98% of outlet
pairs. The field trials' results showed 80% of outlet pairs getting at
least 4.8 Mbps of throughput and 1.4 Mbps in 98% of all outlet-to-outlet
With field trials complete and the specification released, HomePlug
members are concentrating on getting products out the door and not
worrying, for the moment, about HomePlug 2.0. The alliance is satisfied
with HomePlug's performance and feels that it compares favorably with
Wi-Fi's. Both organizations have several members who hope to create
demand for their respective products by offering consumers compatible
Sending millions of bits per second over common house wiring requires
sophisticated algorithms running on fast silicon. House wiring is a
hostile environment for high data rates. Brush motors in hair dryers and
kitchen appliances are a significant source of interference. Turning
appliances on and off, using dimmer switches, and using halogen lights
injects noise spikes into the transmission line. Each branch off the
main breaker panel acts as a stub, causing multipath interference. Plus,
the whole network of house wiring acts as an antenna, picking up RF
interference from radio transmitters.
Signal attenuation is another problem. Long runs between outlets are
one cause, but the common surge-suppressor power strip often contains a
filter to block high frequencies—the very ones HomePlug uses to carry
data. And most houses in the United States take power from both sides of
the neighborhood distribution transformer's secondary windings, creating
two 120V phases and one 240V phase. Power-line signals must go through
this winding if you use an outlet on one phase and a second outlet on
the other phase. The secondary winding acts as a lowpass filter,
attenuating the signal. All these factors create a unique, often-complex,
time-varying, transfer function for each outlet-to-outlet channel in a
Plugging away at the
HomePlug technology overcomes these obstacles using a combination of
approaches. The PHY uses OFDM (orthogonal-frequency-division
multiplexing) to transmit on as many as 84 carriers in the 4.5- to
21-MHz band. The PHY also applies concatenated Viterbi and Reed-Solomon
FEC (forward error correction) with interleaving to the payload data to
ensure the receiver can recover the data even if parts of the bit stream
are lost. Next, the PHY maps the encoded data onto a set of channels
that the transmitter and receiver previously agree upon. An inverse FFT
processor modulates each bit stream to create each channel's waveform—in
effect, converting the signals from the frequency domain to the time
domain. HomePlug typically uses DQPSK (differential
quadrature-phase-shift-keying) modulation, in which the current symbol
is encoded as the difference in phase between it and the previous symbol.
Using all carriers, DQPSK delivers a raw bit rate of about 20 Mbps,
which yields just less than 14 Mbps to the MAC (media-access controller).
To recover the bit stream, the receiving PHY demodulator applies a
forward FFT to the received waveform, converting the symbols back into
the frequency domain. The PHY reconstructs the original data payload
from the symbols modulated onto the carriers.
Before a transmitter sends data to a receiver, the two nodes agree on
what carriers to use based on the characteristics of the channel between
them. Deselecting "bad" carriers helps prevent the loss of
data that would otherwise be transmitted on those carriers. During
transmission, if a few of the carriers encounter interference, the
receiver can still recover the lost data thanks to the FEC that the
transmitter applies. HomePlug hardware can also adapt to changing
channel characteristics by switching between DQPSK and DBPSK (differential
binary-phase-shift keying). DQPSK yields the highest throughput at 152
bits per symbol. The more robust DBPSK symbols each carry 76 bits. A
third adaptation technique varies the convolutional code rate between ½
and ¾. When two nodes communicate for the first time or when the
channel between them is especially harsh, the transmitter uses ROBO (robust-OFDM)
mode, which uses DBPSK on all available carriers.
The PHY is also responsible for the basic HomePlug frame format (Figure
2). A frame consists of a start-of-frame delimiter, a data
payload, and an end-of-frame delimiter. The start- and end-of-frame
delimiters begin with a preamble followed by a 25-bit frame-control
field. The preamble is a pattern of bits chosen so that all receivers
can reliably detect it under the most demanding conditions. The PHY
encodes the frame-control field using a Turbo Product Code to ensure
that receivers can decode the MAC information it contains (for example,
packet lengths and status). A destination node acknowledges a unicast
transmission by sending a response delimiter. ACK signifies that the
destination successfully received the packet. FAIL indicates that the
receiver was unable to process the packet. And NACK means that the
packet was too corrupted for the receiver to understand. The PHY sends
delimiters using all available carriers, whereas the payload uses a
predetermined carrier subset.
The HomePlug MAC uses a CSMA/CA (carrier-sense
multiple-access-with-collision avoidance) protocol similar to the one
that 802.11 uses. The protocol listens to the channel before
transmitting. If the node detects no other transmission, it waits a
random amount of time before beginning its transmission. HomePlug also
uses virtual carrier sense to avoid collisions. When the MAC sees a
frame on the channel, it reads the payload-length in the start-of-frame
delimiter and determines how long the channel will be in use. The node
can postpone transmitting by setting a timer to the length of time it
will take the frame to complete. HomePlug also adds other features to
the MAC to support priority, provide fairness, and control latency.
HomePlug also adds a priority scheme to its protocol to enhance QOS.
At the end of a transmission, the HomePlug network enters a
priority-resolution period in which nodes transmit the highest priority
level of their queued frames. The priority bits (PRS1 and PRS0) are
encoded so that all nodes can determine the level of the highest
priority frame requesting access. Nodes with frames having the highest
priority randomly select a transmission slot within a contention window
and start counting down to their slot number. If a transmission begins
before a node has counted down to its slot number, the node stops
counting and listens. The node resumes counting after the transmission
ends. Because each node chooses its slot number independently of other
nodes, two or more nodes may select the first slot, causing a collision.
The PRS bits provide four levels of priority, with the highest
priority restricted to time-critical packets, such as streaming audio
data. HomePlug also breaks up packets with long transmission times to
prevent them from tying up the network. Packet segmentation allows
higher priority packets to jump in between the segments of a long packet.
A node can transmit all segments of a long packet back-to-back if no
other higher priority packets are queued. A contention-free access
scheme can also improve QOS by giving a node uninterrupted access when
transmitting a limited number of frames to different destinations.
HomePlug addresses security by creating a logical network in your
home based on a password and a 56-bit DES (Data Encryption Standard) key.
Although power-line networks don't broadcast their data to the world
like a wireless network, data can travel to other homes connected to the
same power transformer. In the United States, one transformer usually
connects less than six homes, somewhat mitigating the problem. HomePlug
has tweaked the transmitter power level so that the signal reaching
another residence is low enough to make eavesdropping difficult but
strong enough to ensure the nodes in your home network can hear one
HomePlug tries to be a good neighbor by avoiding frequencies used by
other power-line technologies. The technology also limits its power
spectral density around the amateur-radio bands by inserting 30-dB
notches in the 4.5- to 21-MHz HomePlug frequency range.
So far, a handful of vendors have announced HomePlug-compatible chip
sets. It's not surprising that Intellon's INT5130 is the first chip set
to receive HomePlug 1.0 certification, because Intellon's PowerPacket
technology forms the basis of the HomePlug specification (Figure
3). The 5130 is an integrated MAC/PHY transceiver packaged
in a 144-pin LQFP. The part uses 0.25-micron CMOS and 5V-tolerant 3.3V
I/O. The 100-MHz core runs off 2.5V. You can configure the transceiver's
host interface as either an eight-wire GPSI (general-purpose serial
interface) or an IEEE 802.3u MII (media-independent interface). The
INT5130 also gives you the option of using an external EEPROM for
configuration and control information, allowing you to use standard
Ethernet device drivers.
The analog half of the chip set is the 64-pin INT1000 power-line
converter available in a LQFP. The INT1000 contains a 10-bit ADC and
DAC, which provide 54 and 45 dB of dynamic SNR, respectively. The chip
set began shipping last year and costs $23 (1000). You'll need some
discrete components, including filters, a line driver, and an isolation
transformer, adding $3 to $6 to the bill of materials. Intellon also
offers reference designs for Ethernet (RD5130-ETH) and USB 1.1 (RD5130-USB)
to shorten development schedules. You can also purchase the EK5130-PCI
evaluation kit, which is ready to plug into the wall.
Cogency Semiconductor recently announced a HomePlug chip set (Reference
4). The Piranha chip set consists of the CS1100 MAC/PHY and
Analog Devices' AD9875 mixed-signal front end (Figure
4). The CS1100 has a 1.8V, 150-MHz core fabricated in 0.18-micron
CMOS. Its I/Os operate at 3.3V, and the chip comes in a 100-pin LQFP.
The CS1100 includes an ARM-based MAC, and you can upgrade its firmware
in the field. Interfacing to the Piranha MAC is similar to using
Intellon's chip. The CS1100 has an IEEE 802.3u MII interface, which you
can configure as an 8-bit general-purpose CPU control bus. You can also
connect an optional EEPROM for code storage.
The AD9875 has a 10-bit ADC and DAC and provides –6 to +36 dB of
programmable gain control. The chip operates from 3.3V and comes
packaged in a 48-pin LQFP. The Piranha chip set sells for $23 (25,000)
with volume production expected in the first quarter of 2002. Cogency
also offers an evaluation kit with two evaluation boards, drivers, and
software tools for $5000.
Conexant offers the CX11647 HomePlug 1.0 PHY that you can connect to
an embedded MAC through its IEEE 802.3u MII. The MII is also
configurable as a GPSI. The device operates on both 2.5 and 3.3V and
comes in a 144-pin LQFP. Conexant expects to ship the CX11647 in the
first quarter of 2002 for $35 (1000). You can use Ubicom's IP2022
Internet processor to control a HomePlug PHY, such as the CX11647,
through the processor's GPSI port. The IP2022 has a 120-MIPS RISC
processor with a 64-kbyte flash memory, a 16-kbyte program/data RAM, and
a 4-kbyte data RAM.
Once your design is working, you need to make sure your device
complies with FCC regulations. Part 15 of the FCC's rules classifies a
power-line device as an intentional radiator operated as a
carrier-current system. Such devices have restrictions on the amount of
interference they create for other devices, such as ham radios. The FCC
requires that you make measurements at three installation sites before
the FCC will certify your device.
To earn the HomePlug logo for your product, you must complete a
self-certification process. Part of the process requires you to
participate in plugfests, in which you test your equipment with other
vendor's products. HomePlug conducted the first plugfest in February.
The second part of the certification process requires you to complete a
formal checklist of conformance statements. As of today, HomePlug has no
independent lab testing products for compliance. The certification
process will improve in phases, progressing from self-certification to
establishing an independent certification lab.
Until then, concentrate on testing as many error-handling conditions
within your MAC as possible. This process can be difficult when you work
with equipment designed to function correctly instead of intentionally
creating errors. Spend some time learning how to extract bit settings in
frames captured on the network. This effort will pay off when you
diagnose bugs. Also, pay attention to your transmitter's signal quality
to ensure good performance and reduce interference. Good receiver
performance is also important. A sensitive receiver with good immunity
to large narrowband interference will go a long way toward improving
your product's throughput.
Using home power lines for data networking has been a goal for years.
There's a lot of appeal for a product that connects to a network by
simply plugging into a wall outlet. Using sophisticated algorithms
running on powerful silicon enables engineers to design this type of
equipment for the first time. HomePlug offers one approach to building
power-line-networking gear. Inari and other companies offer other
approaches. It's fair to say that HomePlug has the marketing edge at the
moment. Whether the alliance prevails is anyone's guess—especially
when no one yet knows whether power-line networking itself will survive
as a product category.
- Schweber, Bill, "Specification
allows ac-line line wiring to do double duty," EDN,
July 19, 2001, pg 24.
- Vrana, Greg, "Wireless
Ethernet: serving the public," EDN, May 24,
2001, pg 36.
- Vrana, Greg, "Two
PC buses go for round two," EDN, Jan 18,
2001, pg 113.
- Vrana, Greg, "Chip
set touts HomePlug 1.0 compliance," EDN, Jan
10, 2002, pg 22.
HomePNA (Home Phoneline Networking Alliance) uses
your house's phone wiring, instead of ac power lines,
for home networking. Several companies interested in
home networking created the alliance in 1998, and today
it has more than 120 member companies. In the fall of
1998, HomePNA released its 1.0 Specification, which
supported a 1-Mbps throughput. In December 1999, HomePNA
released the current 10-Mbps 2.0 Specification, which
maintains compatibility with the 1-Mbps specification.
The alliance announced at the Fall 2001 Comdex
convention in Las Vegas that it has released a
marketing-requirements document for a 100-Mbps HomePNA
3.0 Specification. Expect to see the specification
released at the end of 2002.
Like HomePlug and Inari power-line technology,
HomePNA supports quality of service for streaming-data
applications. HomePNA 2.0 data rates can exceed 10 Mbps.
Broadcom's iLine32 HomePNA 2.0 chip set can transmit as
fast as 32 Mbps. The technology uses frequency-diverse
QAM (quadrature-amplitude modulation) in the 4- to
10-MHz band. Companies such as Ucentric believe that a
peak bandwidth of 30 Mbps and $20 silicon will make
multimedia applications in the home practical.
HomePNA may have a head start over HomePlug in terms
of certification. HomePNA claims to have so far
certified around 100 products. The organization has a
rigid certification process in place and requires you to
participate in its plugfests, which are held every three
months. Upon successful certification, HomePNA
authorizes you to use the official HomePNA seal of
Security may be a weak point for HomePNA. The
specification does not require encryption, leaving your
privacy at the mercy of Windows' networking software and,
of course, good ol' signal attenuation.
HomePNA silicon includes AMD's HomePNA 1.0 Am79C978A
controller and Am79C901A HomePNA PHY. Conexant's CX24611
is a HomePNA 2.0-compatible PHY.
HomePlug may have an edge in marketing, but it's not
the only game in town. Inari is proposing its technology
to the Consumer Electronics Association R-7.3 Committee
as the basis for the new Electronics Industries Alliance
power-line data-networking specification. Novell spun
off its power-line-networking group in 1997, forming
Intelogis, the developer of the PassPort Plug-In Network
product. In 1999, the spin-off switched its focus to
being a fabless silicon provider and changed its name to
Inari claims its technology better addresses the
issues of scalability, affordability, and QOS (quality
of service) than HomePlug. Inari offers a family of
chips that range from 2 to 12 Mbps with pricing
proportional to performance. By offering a chip set that
offers midlevel throughput at 2 Mbps, Inari believes it
is making power-line technology more affordable than
HomePlug. In effect, you pay only for the bandwidth you
need. Inari also claims that its outlet coverage is
better than 99%. HomePlug claims 98% coverage.
Inari also stresses that its QOS is superior, calling
HomePlug's QOS only a "best-effort" attempt to
provide bandwidth to time-sensitive data. Inari uses a
two-phase protocol on its networks. When a node wishes
to use the network, it senses datagrams and randomly
backs off if it detects one using datagram-sensing
multiple access. Once a node asserts itself as the
Active Powerline Exchange Protocol Manager, the network
uses centralized token passing to eliminate collisions
and multinode contention. Inari and others in the
industry see this protocol as a better approach to
addressing QOS than HomePlug's.
Inari uses a multiple-carrier system with phase-shift
modulation similar to HomePlug. But to reduce cost,
Inari reduces the number of channels a particular
transceiver processes based on its rated throughput. For
higher throughput applications, Inari offers
transceivers that implement more channels. Starting with
the 2-Mbps IPL0202 Powerline Network Controller, you can
add your own front end, comprising a DAC, a power amp,
and a receiver. The IPL0202 chip set has a built-in
full-speed USB port, sells for $12.50 (100,000), and is
shipping. Inari plans to offer the $15 (100,000) IPL0401
4-Mbps network controller in the second quarter of 2002.
The IPL0401 will have an MII (media-independent
interface) and generic-memory-bus interface and an
eight-channel communications processor. Inari plans to
offer a 12-Mbps network controller in the third quarter
of 2002. The IPL1201 will support 24 channels and
rate-adaptive data speeds. Interfaces will include MII,
generic memory bus, and PCI. The IPL1201 will cost $28
(100,000). According to Inari, power-line networking
adds $3 to $4 worth of components to an entry-level
design plus the power-line chip set. So, for example,
adding Inari's 2-Mbps chip set to your set-top box costs
an additional $15. Adding 12-Mbps power-line networking
capability increases your bill-of-materials costs by
Scalability can significantly affect your design's
cost. Not only can entry-level designs use a cheaper
network controller, they make the other parts in your
bill of materials less critical. High-end designs using
a full-speed controller need more expensive parts. Keep
this fact in mind when you consider your product's
requirements. With HomePlug, one size fits all. For
example, if you're building a voice-over-IP product, you
probably don't need the full 14-Mbps performance of
HomePlug chip sets and their corresponding costs.
Inari also likes to point out that its technology's
security is more robust than HomePlug's. Inari uses the
recently announced 128-bit AES (Advanced Encryption
encryption scheme, whereas HomePlug uses the 56-bit DES
(Data-Encryption Standard) system. According to Inari,
if a computer took one second to break the DES, it would
take that same computer about 1.5 trillion years to
break the AES.
At January's Consumer Electronics Show in Las Vegas,
Inari demonstrated several vendors' Inari-based products
interoperating on a power-line network. Inari also
announced its Power Developer Program. Program members
will have early access to Inari's development tools and
products, access to reference designs and enhanced
support, and "fast-track" access to "Inari-enabled"
For help with building networking gear based on Inari
and other power-line technology, Lugh Networks offers
design and engineering services. Lugh offers a reference
design of an Ethernet/USB access point built around the
Inari IPL0202 (Figure