Data Communications Over PowerLines

7/30/2019 Data Communications Over PowerLines

1/6

Data CommunicationsData CommunicationsData CommunicationsData Communicationsover Power Linesover Power Linesover Power Linesover Power Lines

Since its introduction in the early 1990s, use of

the Internet has exploded. Access to the Internet

is fast becoming a need, not just a want, for most

homeowners. Although broadband access is now

available for most homes, distribution within the

household has remained a barrier to realization

of its full benefits. The home network connects

to the Internet through a central gateway making

the Web accessible at every connection point.

Information and peripheral devices can also be

shared across the network.

Three solutions can be considered for homenetworking: Ethernet and phone line, wireless,

and powerline networks.

Ethernet and phone line solutions provide fast,

reliable service but require snaking cable to each

connection. Network nodes must be identified

and placed during construction of new homes.

Considerable renovation is required to retrofit

older homes or to place additional nodes.

Wireless networks provide nodes everywhere.

They are ideal for hand-held or battery-operated

devices, but the addition of RF conversion

hardware makes this an inherently costlier

solution. Additionally, wireless networks suffer

from security concerns and competing standards.

Powerline networking uses power lines existing

in the home. Nodes are already available

throughout the household, making it a low-cost

solution. Each room in a residence possesses

one, two, or more outlets. Any device requiring

power will already be attached to the powerlinenetwork making it convenient and accessible to

low-tech users.

Home network users seek three major factors in

any solution: ease-of-use, low cost, and

ubiquitous node availability. Powerline

networking delivers all three. So, while it is true

that powerline networking faces some technical

hurdles, it remains a compelling choice.


2/6

Data Communications over Power Lines

2

Challenges

Several factors present technical challenges to

using power lines for data communication.

Cogencys HomePlug technology is able to

address these challenges through the unique

combination of OFDM, signal coding, and errorcorrection techniques.

Noise Sources on Power Line

The power line is admittedly a noisy

environment for data communications

(see Figure 1).

Noise sources include electronic and electro-

mechanical sources. Brush motors (found invacuums) are particularly noisy. Dimmer

switches, fluorescent and halogen lights create

impulse noise related to the 50 or 60 Hz power

cycle. Power supplies create harmonics related to

the switching frequency. Outside transmissions

such as impulsive noise, RF interference (short

wave and amateur radio), and RF pickup of other

bands can affect the quality of the channel on the

power line.

These noise sources interfere with reception of

data signals. At certain frequencies, the

amplitude of the data signal can fall far enoughbelow the noise floor to be lost.

Multipath

Multipath effects can distort the signal during

transmission. Reflections of the original (or data)

signal can arrive slightly ahead of or behind the

desired receive signal resulting in symbol error

(see Figure 2).

Devices on the powerline network transmit to

multiple stations simultaneously. Each station-

to-station communication presents a unique

channel profile. Noise and distortion effects can

result in a high rate of bit errors. Characteristics

of the devices present on the power line and theline itself contribute to the complexity of the

channel transfer function. The combination of

multipath distortion, complex wiring topology,

and line characteristics create an extremely

complex channel transfer function.

Wide Dynamic Range

Signal attenuation can also occur due to the

physical topology of the network (as shown in

Figure 3), varying termination impedances, loads

on the power line, and characteristics of the

transmission line itself resulting in a wide

dynamic range between any two nodes.Signal transmission between two outlets that are

close together (such as A to B) will often

experience little attenuation, but for nodes that

Figure 1

Figure 2

Figure 3


3/6


3

are far apart (such as C to F), the attenuation can

be significant.

Some attenuation is also experienced due to the

effects of 2-phase wiring. North American

homes typically have two phases of 110 Volt

wiring (the phases are used in tandem to achieve

the 220 Volts required for large appliances such

as a dryer or oven). Powerline data signals are

naturally coupled from one phase to the other

resulting in an attenuation that is generally less

than 10 dB.

Time-varying Conditions

All the effects discussed above vary with time

(see Figure 4). Noise sources differ as devices

are plugged into or removed from the line. Even

the character of one particular noise source can

be time-dependent (e.g. fluorescent or halogen

lights whose power function varies with time).

Multipath distortion effects vary as the channel

characteristics change due to load variations.

Outside sources of interference (such as RF) vary

with the time of day, the proximity of the

transmitters, and strength of the power source.

Meeting the Challenge

Cogency technology for powerline networking

includes a physical layer (PHY) and Medium

Access Control (MAC) layer. The PHY layer

implements the modulation techniques, the

coding, and basic packet formats. The PHY usespacket-based OFDM as the transmission

technique. The MAC uses a CSMA/CA protocol

to mediate access between multiple clients.

The MAC/PHY provides per-packet equalization

and efficient access to the shared powerline

medium. In addition, a priority resolution

signaling scheme enables latency-sensitive

applications such as VoIP and multi-player

gaming.

OFDM Technology

The Cogency MAC/PHY uses OFDMtechnology to carry the signal at a high data rate

with few bit errors. OFDM modulation generates

a set of tones in the frequency domain. The tones

are orthogonal to each other ensuring that there

is no inter-tone interference (i.e. the information

carried on any one tone is not affected by any

other tone).

Figure 4


4/6


4

Figure 5 illustrates the conversion process that

takes place at the transmitter. Forward Error

Correction (FEC) redundantly encodes the data

to compensate for harsh channel characteristics.

The encoded data is mapped onto a set of tones

which may be all available tones or a pre-agreed

upon subset. OFDM modulation, generated using

a fast Fourier transform (FFT) processor,

converts signals in the frequency domain to the

time domain. The inverse FFT, applied at the

transmitter, produces an OFDM symbol.

Intersymbol interference is a major complication

caused by multipath propagation. This is handled

through time domain processing. If a copy of the

signal arrives a significant fraction of one

OFDM-symbol-time late, symbol error can

occur. These multipath distortion effects can be

almost completely mitigated by adding a guard

time (cyclic prefix) to the OFDM symbol (as

shown in Figure 6).

The prefix is essentially a copy of the last few

microseconds of the symbol. The cyclic prefix

absorbs any multipath interference that occurs

when time-delayed reflections of the original

symbol arrive at the receiver. By ensuring that

the cyclic prefix is as long as the longest possible

delay variation, the integrity of the OFDM

symbol is preserved.

At the receiver, the reverse process takes place

(as shown in Figure 7). The cyclic prefix is

removed. An FFT is applied on each symbol,

converting it from the time domain to the

frequency domain.

Forward Error Correction

OFDM provides resistance to deep, narrow fades

by using many carriers. The loss of a few tones

can be compensated for with FEC coding which

redundantly encodes data across all active tones.

If some of the tones are not received due to noise

or other effects, the remaining carriers can be

used to recover the original signal. Automatic

channel adaptation allows the system to respond

to current conditions on the power line.

The tones are modulated using either differential

BPSK (76 bits per OFDM symbol) or QPSK

(152 bits per OFDM symbol). For harsh channels

or when channel adaptation has not been

performed, the payload data is sent using ROBO

(ROBust OFDM) mode. ROBO mode uses all

available tones with differential BPSK

Figure 5

Figure 6


5/6


5

modulation on each tone, as well as heavy error

correction and interleaving. ROBO mode is

useful for very harsh channels or when

establishing initial contact with another device to

negotiate the optimum communication scheme.

Convolutional or Reed-Solomon coding are used

for payload data. Convolutional coding rates of can be punctured to achieve a rate of . A

combination of coding rate and modulation is

used to adjust to varying channel conditions.

Product encoding is used for frame control fields

ensuring that all devices on the network can

detect and decode this information.

Channel Adaptation

The tone map indicates the set of tones to be

used for a particular communication between

two stations. The tone map to be used is

negotiated during channel adaptation. This is

performed when a station first joins the network,

periodically to ensure optimum throughput, or

when the quality of the channel varies. Channel

adaptation is also used to specify the modulation

or coding schemes for payload data. If

significant fading occurs, specific tones can be

dropped from the transmission. When an

acknowledgement is not received, the packets are

resent. This provides extra redundancy to guard

against effects such as in-band jammers orimpulsive noise.

Data Packets

Each data packet carries a series of OFDM

symbols (as shown in Figure 8). The packet

consists of a start-of-frame delimiter, the

payload, and an end-of-frame delimiter. A

response delimiter is transmitted to indicate

whether or not the transmission was successfully

received.

The start-of-frame delimiter indicates that a

frame has begun, specifies the length of the

frame, and the index of the tone map to be used.

Delimiters consist of a preamble sequence

Figure 7

Figure 8


6/6


Headquarters North American Sales International Sales144 Front Street West, Suite 600 4695 MacArthur Court, 11

thFloor 362 Terry Fox Drive, Sui

Toronto, ON Newport Beach, CA Ottawa, ONCanada M5J 2L7 U.S.A 92660 Canada K2K 2P5Tel: 416-217-0250 Tel: 949-798-6100 Tel: 613-270-9300Fax: 416-217-0256 Fax: 949-798-5550 Fax: 613-270-9995

[email protected] [email protected] [email protected]

Copyright 2001 Cogency Semiconductor Inc. All rights reserved. The COGENCY name and logo and PIRANHA are trademarks of Cogency Semiconductor Inc. All other company and product names areademarks of their respective owners. Features, pricing, availability and specifications are subject to change without notice.

followed by a frame control field. The preamble

allows a receiver to reliably detect a packet and

acts as a symbol synchronization and gain

control reference. Frame control fields are highly

encoded to ensure decoding of media access

information by all devices on the network.

Data packets can be transmitted in two modes: to

all stations or to one specific station. When

sending a transmission to a single station, a

specific tone set must be used. Both the sending

and receiving station agree on an optimum tone

map which maximizes capacity. Instead of

negotiating a common tone map that applies to

each device, broadcast transmissions use all

tones. This results in lower throughput because

of the unique channel response between any two

devices. Note that data is encoded on all carriers

when transmitting frame control symbols.

Reliability

Transmissions sometimes fail because of

collisions with other transmissions or due to

severe noise on the line. The MAC/PHY

acknowledges receipt of unicast transmissions by

sending a response delimiter (ACK) to indicate a

successful transmission. A NACK signal is sent

to indicate that the packet was received but with

errors. The MAC/PHY uses Automatic Repeat

reQuest (ARQ) to guarantee reliability. Receipt

of a NACK (or no response) results in the packet

being resent.

Carrier Sense

As described earlier, each data packet carries a

start-of-frame and an end-of-frame delimiter.

The frame control field of the delimiters contains

information that assists with contention control.

By monitoring the frame delimiters, the

MAC/PHY can determine the state of the line or

carrier. This is known as carrier sense.

Channel Access

The end-of-frame delimiter also carries data

regarding the priority of transmissions. To

reduce collisions that occur with random access

to the channel, Cogency uses a Carrier Sense

Multiple Access with Collision Avoidance

(CSMA/CA) protocol enhanced with priority

signaling.

Prioritized access to the channel is achieved by

using the Priority Resolution Period (PRS0 and

PRS1 as shown in Figure 8). During this period,

all ready-to-transmit stations signal the priority

at which they intend to transmit allowing only

the highest priority transmissions to continue. A

slotted binary exponential backoff mechanism

used during the Contention State spreads the

time over which the remaining stations attempt

to transmit under busy conditions to reduce the

probability of collisions.

Get Plugged In

Using the powerline network as a data

transmission channel does present some

technical challenges: multipath distortion effects,

noise in the environment, RF interference, and

privacy concerns are formidable obstacles.

However, Cogencys HomePlug technology is

able to manage these concerns with a robust

solution that provides reliable data transmission

for the home networking environment. The

Cogency MAC/PHY adapts automatically to

changing conditions on the power line providing

a reliable channel under the noisiest conditions.OFDM technology manages multipath distortion

effects. Privacy management using 56-bit

encryption techniques provides privacy, while

priority contention control ensures timely access

for latency-sensitive applications.

Powerline presents a reliable, low-cost solution

for residential networking. Cogencys HomePlug

technology provides Ethernet-class data

networking to support VoIP, QoS, and streaming

media applications. Powerline networking turns

every AC outlet into a network port.

Documents

Data Communications Over PowerLines