Wi232 vs Zigbee

7/29/2019 Wi232 vs Zigbee

1/29

Wi.232DTS vs. Zigbee

Comparing proprietary and standards based solutions

Written by: Steve Montgomery

Date: October 27, 2004

Revision: A

2004 Radiotronix Inc. All rights reserved


2/29

Table of Contents

1.

Introduction....................................................................................................1

2. What is WiSE technology?.........................................................................43. What is Zigbee / 802.15.4? ........................................................................8

3.1 PHY description......................................................................................83.2 MAC Description...................................................................................103.3 Summary ..............................................................................................12

4. Comparison: WiSE vs. Zigbee/802.15.4...............................................134.1 Module Cost .........................................................................................134.2 Range Performance .............................................................................154.3 Reliability ..............................................................................................17

4.3.1 Multipath........................................................................................174.3.2 Interference ...................................................................................20

4.4 Scalability .............................................................................................214.5 Battery Performance.............................................................................22

5. Summary .....................................................................................................236. References ..................................................................................................26


3/29

2004 Radiotronix Inc. All rights reserved


4/29

Wi.232DTS vs. Zigbee Comparing proprietary and standard based solutions

1. Introduction

Wireless sensor networking is one of the most exciting technology markets today

[4]. They say that over the next five to ten years, wireless sensors will have a

significant impact on almost all major industries as well as our home lives.

Broadly, this technology market includes application segments such as

automated meter reading, home automation, building automation, container

security/tracking, and many others.

Although products that span these application segments are diverse and different

in how they operate and what they do, their requirements from a wireless

communication technology are very similar. For example, these applications

generally require low data rates and are battery powered.

The main motivations for migrating these products to wireless communications

are three-fold:

1. Installation cost The cost of running wires in a typical buildingautomation project in an existing facility can be as high as 80% of the total

project cost [4].

2. Maintenance It is easier to configure a hot-water heater controller with a

hand-held remote than a keypad in the closet.

3. New markets Eliminating the wire opens new markets that were

previously unavailable to wired products.

2004 Radiotronix Inc, all rights reserved 1Last printed 11/2/2004 12:21 PM


5/29


According to [4], the market for RF modules for these applications is 6.2 million in

2004, growing to 465.2 million. However, there are limitations that will affect how

fast this market will develop:

Perception 73% of established OEMs and VARs say that installation and

lacking ease of use was the biggest concern in migrating to embedded

wireless technology. Notably, only 23% cited lack of interoperability as a

limitation [4].

Technology The reliability of embedded wireless enabled products being

used today is generally considered unacceptable. For the market to

develop, the issue of reliability must be solved and proven to the

satisfaction of the VAR , OEMs, and end-customers. This is both a

research and an education issue.

Customer development resources One consideration that seems to be

missing in all of the literature we reviewed on the market is the severely

restricted resources that OEMs can apply to new product development.

As a result of the recession starting in 2001, many companies have

significantly reduced engineering staffs. This translates into longer

development cycles because fewer people are working on new product

development. Many technology suppliers to this market space, including

Radiotronix and XEMICS, are noticing very long development cycles:

sometimes as long as 24 months. In fact, the research phase of projects

can take as long as 12 months. This will definitely impact the velocity of

market growth for at least the next two years.

Regardless of the limitations just discussed, it is apparent that there is a very realmarket opportunity for embedded wireless technology providers. This is

evidenced by the recent wellspring of technology companies in this sector.



6/29


In the wireless worlds of WiFi and BlueTooth, market growth was fueled by

standards development that ultimately brought down the cost of the technology

and ensured excellent value to the user. In that spirit, a number of companies

forged an alliance to create a wireless standard for the embedded wirelessmarket space, also called personal area networking (PAN); this standard is now

called Zigbee. The list of promoting members is prominent and includes

names like Honeywell, Phillips, Motorola, Freescale, Invensys, and many others.

Technically, Zigbee is a protocol standard that defines network, security, and

application framework protocol software. Zigbee is designed to work on top of

the IEEE 802.15.4 PHY/MAC layer standard. The IEEE 802.15.4 standard was

ratified in May of 2003; to our knowledge the Zigbee standard is not at the time

of this writing ratified, though we understand that it is very close.

According to documentation widely available on the Zigbee website,

www.zigbee.org, the benefits that the Zigbee standard provides are:

Reliable and self-healing

Supports large number of nodes

Easy to deploy

Low cost

Long battery life

Secure

Global deployment

Additionally, the website claims that the standard has several benefits overproprietary solutions:

Product interoperability

Vendor independence

http://www.zigbee.org/http://www.zigbee.org/


7/29


Increased product innovation

Common platform reduces cost over creating new solution each

development cycle

There are drawbacks, as well, that are not mentioned on the website. We will

examine these in this report.

The market for RF modules in 2004 will be largely proprietary, about 95%, and by

2010 will become largely Zigbee, about 75% [4]. However, a more recent

market-centric recently released about the AMR and sub-metering market

predicts that in 2004, the market will be largely proprietary, about 98%, and in

2010 will still be largely proprietary, by a lesser margin, at about 75% [2]. This

illustrates a growing controversy over the ultimate acceptance of Zigbee and

the staying power of proprietary solutions.

The purpose of this report is to compare and contrast a Zigbee based solution

with our WiSE technology, evaluating specifications, costs, and performance.

Finally, we will summarize the data presented and attempt to draw some

conclusions about the comparisons.

2. What is WiSE technology?

A wireless serial engine (WiSE) combines a state-of-the-art RF transceiver

with a high-performance protocol controller, which contains a very optimized,

high performance protocol stack in a small IC-style package. WiSE RF

modules are designed to be complete solutions that can be used to create

wireless products in a matter of days, not months or years. Every WiSE

module is programmed at the factory with a unique 48-bit MAC address; the

Wi.232DTS modules support applications that TCP/IP and ARP.



8/29


The family of WiSE modules includes:

Wi232DTS(EUR) module shipping now

Wi232FHSS(EURHP) module shipping Q2 of 2005

Wi.MESH module in development

The family of modules address the variety of requirements found in embedded

wireless applications. For example, the Wi.232DTS module was designed

primarily for wire-replacement applications. Internally, it contains the PHY and

MAC layers of the ISO reference model. It was specifically designed for home

automation, building automation, mobile AMR, and wireless RS-232/422/485

applications. It supports point-to-point (P2P), point-to-multipoint (P2MP), and

multipoint-to-multipoint (MP2MP) networking applications. It does not contain a

link layer, so it is very flexible.

The Wi.232FHSS module is a 250mW FHSS module and is designed for long-

distance wire-replacement applications. It is targeted at the same markets as the



9/29


Wi.232DTS module; it is intended for applications that require longer range than

the Wi.232DTS module. It uses the same PHY and MAC layers as the

Wi.232DTS module; it also contains the LINK layer, allowing for more robust

communications through assured delivery.

The Wi.MESH module builds on the technologies developed for the

Wi.232DTS and Wi.232FHSS module, adding mesh-networking capabilities.

There are many different ways to implement mesh networking, and each

application has different requirements. For example, fixed AMR requires

excellent low-power performance and places little value on short latency. Home

lighting, in contrast, requires very low-latency and places little value on low power

performance. We are designing Wi.MESH specifically for the following

applications:

Fixed automated meter reading

Container security/tracking

These applications are not well suited to Zigbee, yet represent a large market

opportunity. Additionally, the requirements for these applications are very much

the same, allowing us to design a lean, focused solution that is optimized for low-

power, robust, scalable performance.

All of the WiSE modules are complete solutions. They provide a simple UART-

compatible interface, and operate transparent to the user application. All of the

WiSE modules have undedicated I/O pins and surplus resources; in some

cases we can actually embed the users application firmware into the module,eliminating the need for an external microcontroller all-together.

The WiSE PHY layer is a high-performance 900 MHz WFSK transceiver based

on the field proven XEMICS XE1203 transceiver RFIC. All of the WiSE



10/29


products were developed in partnership with XEMICS. The Wi.232DTS module

has 114dB link budget at the maximum data rate, and 117dB link budget at the

minimum data rate. Customers have actually reported achieving 3+ miles range

outdoors, and several hundred feet indoors through walls and floors. TheWi.232FHSS module will increase this link budget to 124dB, nearly quadrupling

the range of the Wi.232DTS module in an outdoors, line-of-sight environment.

WiSE Module Specifications

Wi.232DTS Wi.232EUR Wi.232FHSS Wi.232EURHPAvailability Now Now Q2 2005 Q3 2005

Frequency 902-928 MHz 868-860 MHz 902-928 MHz 869.525Data Rate .3 - 152.34kbps .3 - 152.34kbps 152.34kbps 38.4kpbs# channels 32 15 25 1TX Power 0 12dBm 0 12dBm +24 dBm +24dBmRX Sensitivity -102 dBm DTS mode

-105 dBm LP Mode-102 dBm wide-105 dBm narrow

-102dBm -102dBm

Link Budget 114dB DTS mode117dB LP mode

114dB wide117dB narrow

126dB 126dB

Adjacentchannelrejection

40dB min. 40dB min. 40dB min. 40dB min.

The WiSE MAC layer is responsible for controlling access to the RF channel.

It accepts an un-encoded packet from the higher layers or directly from the users

application. The packet is then encoded using our proprietary DirectSPREAD

technique, and sent over the RF channel. The MAC layer controls access to the

RF channel by using carrier-sense-multiple-access with collision-avoidance

(CSMA-CA). This technique allows all modules on the channel to share the

channel cooperatively without the need for a master controller, making

installation very easy.

The Wi.232FHSS module implements the link layer, allowing addressable

communications and assured delivery. It uses the same CSMA-CA access

mechanism as the Wi.232DTS, so it does not require a master radio to control



11/29


the channel. The module can operate in an addressed or address-less mode. In

the addressed mode, every packet is addressed to a specific receiver. This

requires support from the users application, and the application must be aware

of the structure of the network, although the network itself is self-forming. Inaddress-less mode, the Wi.232FHSS module is transparent and operates like the

Wi.232DTS module. This mode is useful for point-to-point, streaming full duplex

applications like RS-232 wire replacement.

The WiSE modules are the only proprietary modules available today that use

CSMA-CA to control access to the RF channel. This advanced feature is

significant in that it increases network efficiency and eliminates or reduces the

number of collisions that occur in the network. This is also the access control

method used by Zigbee.

3. What is Zigbee / 802.15.4?

The IEEE 802.15.4 standard defines the PHY and MAC layers, which are used

by Zigbee. Detailed specifications can be found in [5].

3.1 PHY description

Three frequency bands are specified, though an implementation need only

operate on one of the three [5]. The bands are:

868 MHz for European applications

902-928 MHz for North American applications

2.450 GHz for world wide applications



12/29


In all bands, the modulation scheme is direct sequence spread spectrum. In the

868 and 902-928 MHz bands, the transmitter is modulated using BPSK. In the

2.450 GHz band, the transmitter is modulated using offset-QPSK, which is more

bandwidth efficient than BPSK.

Direct sequence spread spectrum is a technique that essentially spreads the

narrow band of data over a much broader bandwidth by using a pseudo-random

chipping sequence. This process provides gain at the receiver because of the

correlating effect of de-spreading the data. The amount of gain is determined by

the ratio of the chipping rate to the data rate. The higher the ratio, the higher the

gain. This gain also provides proportional rejection of on-channel interference.

As the wanted signal is correlated and de-spread, the interferer is spread,

increasing the level of the wanted signal and decreasing the level of the

interfering carrier. The amount of rejection is determined by the spreading gain.

In the 2.450 GHz band, an 802.15.4 radio spreads the data using an 8 bit

chipping sequence. Actually, the chipping sequence is 32 bits, but the data

being spread is actually 4 bits, thus the 8:1 chipping ratio. The process gain in

dB is calculated by multiplying ten times the log of the chipping ratio; in this case

the gain is 9dB. Receiver sensitivity is specified at 85dBm; adjacent channel

rejection is 0dB minimum.

In the 868 and 902-928 MHz bands, an 802.15.4 radio spreads the data using a

15 bit chipping sequence. In this case, the chipping ratio is 15 and the spreading

gain is 12dB. Receiver sensitivity is specified at 92dBm; adjacent channel

rejection is 0dB minimum.



13/29


Zigbee/802.15.4 Specifications by Band

868 MHz 902-928 MHz 2.450 GHz

Data Rate 20 kbps 40 kbps 250kbps

# channels 1 10 16

TX Power -3dBm -3dBm -3dBm

RX Sensitivity -92dBm -92dBm -85dBm

Link Budget 89dB 89dB 82dB

Adjacent channel

rejection

0dB 0dB 0dB

Alternatechannel rejection

30dB 30dB 30dB

3.2 MAC Description

The 802.15.4 specification defines a very complicated MAC layer, and I will not

attempt to give a detailed explanation here.

802.15.4 defines two classes of implementations: full function devices (FFD) and

reduced function devices (RFD).

An FFD can operate in three modes serving as a PAN coordinator, a coordinator,

or a device. FFDs contain all of the features of 802.15.4 and can talk to both

RFDs and FFDs.

A PAN coordinator is the primary controller of the network, and it must be a FFD.

There can be only one PAN controller per network. A PAN controller is required



14/29


for an 802.15.4 network. A coordinator is a FFD that provides synchronization

services by transmitting beacons.

A RFD can operate only as a device. RFDs contain a subset of the features of802.15.4 and are intended to be high-volume, low cost devices. They can be

duty-cycled to reduce power consumption. RFD devices can talk only to FFDs.

This means that RFDs have no routing capability, so they must be on the

perimeters of a mesh network.

A device is a simple end-point. A device can be a RFD or FFD.

Conceptually, each network would have one FFD that acted as the PAN

coordinator and several more FFDs that formed the mesh network. The majority

of the nodes in the network would be low-cost RFDs. The number and position

of FFDs in the network would determine the coverage of the network.

The illustration on the right shows an

example Zigbee network configuration.

There is one PAN coordinator, six FFD

devices, and nine RFD devices. The

actual mesh network is formed by the F

devices and the PAN coordinator. The

RFD devices form a point to multip

network with FFD devices that are in

range.

FFD Device

RFD Device

PA N

Coordinator

1

2

3

4

6

10

13

1412

11

9

7

8

5

16

15

FD

oint

Node 8 is not connected to the network.

Although it is in range of nodes 7 and 9, it

cannot connect to them because all three

are RFD devices. An additional FFD



15/29


device would be required to connect node 8 to the network.

Therein lies an inherent limitation of the Zigbee model. The number of FFD

devices in the network determines the coverage area; the more FFD devices, thelarger the coverage area. It is probable, given the current 802.15.4 specification,

that a real-world application of Zigbee would require a high ratio of FFD

devices to RFD devices to attain the required coverage, which will adversely

affect the pricing model.

This also has implications in system deployment. The primary factor driving the

market need is lower installation cost [4]. Using the example just given, it is easy

to see how the installation will be complicated. If a device (node 8) is installed in

a location that is not in range of an FFD device, it will not be connected to the

network. The installer would then be required to place an additional FFD device

to serve as an intermediate router. This would have to be done by trial-and-error,

increasing both labor and materials cost.

If this all sounds complicated, that is because it is. The 802.15.4 specification

alone consumes 670 printed pages. A typical implementation requires nearly 32K

of flash, and that is just for the MAC layer [3].

The Zigbee specification is likely to be just as large and the software

implementation requires another 32K or more of flash memory.

3.3 Summary

The important aspects of the 802.15.4 standard are listed below [5]:

82-89dB link budget



16/29


0 dB adjacent channel rejection

10 channels @ 900 MHz, 16 channels at 2.450 GHz MHz

40kbps @ 900 MHz, 250 kbps @ 2.450 GHz

RFD devices are not a part of the mesh network

Every network requires a PAN coordinator

The coverage area is determined by both the 802.15.4 link budget and the

number of FFD devices deployed.

4. Comparison: WiSE vs. Zigbee/802.15.4

The primary considerations important to OEMs evaluating different wireless

technologies are:

1. Cost of Solution

2. Range Performance

3. Reliability

4. Scalability

4.1 Module Cost

Price is perhaps the greatest driving factor behind the intense interest in the

Zigbee standard. It is believed that multiple vendors offering compatible silicon

will create a very competitive market that will ultimately lead to lower cost of the

overall solution.

Fundamentally, a Zigbee solution is comprised of an 802.15.4 compliant RF

transceiver IC, a microcontroller, and the Zigbee protocol software. Today,

these components are seperately offered by different companies, so vendor

alliances must be developed for each component.



17/29


There are several 802.15.4 transceiver integrated circuits which available today.

Already, competition (mostly between FreeScale and ChipCon) is driving the

price of these components downward to a sub $2.00 price point in quantity,which follows the claims of the Zigbee organization. A good example is the

CC2420 from Chipcon; it is a 2.4 GHz implementation of the 802.15.4

specification.

There are other costs, however, that must be included to understand the true

cost of an 802.15.4 solution; i.e. the microcontroller and the protocol stack.

First, the microcontroller both 802.15.4 and Zigbee are complicated

standards, and this complication drives the size and cost of the software stacks.

The smallest implementation of a Zigbee solution today requires at least 64K

of flash memory. Generally, 64K of flash program memory are found only on

high-end microcontrollers, which are very expensive.

That fact is reflected by the predicted that the average selling price for a

Zigbee module in 2004, will be $15 and will drop to $8 by 2010 [2]. In order to

realize that price drop, either the cost of flash memory will have to decrease at a

significantly faster rate than in the past, or the size of the Zigbee and 802.15.4

software stacks will have to be reduced by about 60%.

Projected Average Selling Price for Zigbee RF Modules [2]

2004 2005 2006 2007 2008 2009 2010

Zigbee $ 15 $ 14 $ 13 $ 11.5 $ 10.5 $ 9.5 $ 8

It is true that reduced function devices will require less program memory that full

function devices. However, the memory requirements are still significant and



18/29


there is a major drawback to using reduced function devices: they cannot route

packets therefore they cannot be a part of the network mesh. This has

implications on scalability, range performance, and robustness of the network,

which we will cover in the next few sections. The basic result is that if thenetwork is made up of mostly reduced function devices (for cost reasons), the

benefits of mesh networking will not be realized.

The Wi.232DTS module, in contrast, is available now. It is a self-contained, fully

tested solution that requires a single vendor relationship. The MAC layer

software requires less than 7K of flash, which is implemented in a very

inexpensive microcontroller, allowing us to reduce the cost of the module. Our

relationship with XEMICS gives us very favorable pricing for the RF transceiver.

The result is profound: Today, the Wi.232DTS module can be purchased in

quantity for under $10, which puts Radiotronix at least five years ahead of the

predicted pricing curve [2].

RF Module Price Comparison

Wi.232DTS (Actual cost) Zigbee (Projected cost) [2]

< $10 $ 15.00 (when available)

4.2 Range Performance

Comparing range performance is as simple as comparing the link budgets of the

two solutions.

The 802.15.4 specification requires a minimum link budget of 89dB. The

CC2420, a Chipcon implementation, exceeds the specification with a typical link

budget of 94dB [1]. Field evaluations have shown that the CC2420 is capable of



19/29


around 500 feet outdoors and line-of-sight. Indoors range of 10-20 meters, or 30-

60 feet can be expected.

The Wi.232DTS module, by contrast, has a typical link budget of 114dB; a full20dB better than the CC2420 based 802.15.4 solution [6].

We know that for every 6dB improvement in link budget, the range will increase

by a factor of two in an outdoors, line-of-sight environment [7]. Using that rule-of-

thumb, we can say that the Wi.232DTS module should operate at eight times the

range of the CC2420 based 802.15.4 solution, or 4000 feet. This tracks with

feedback from our customers; one customer reported that he was able to attain a

repeatable 3 miles range performance.

We also know that for every 14dB improvement in link budget, the range will

increase by a factor of two in an indoor, multi-floor environment [7]. Thus, the

Wi.232DTS indoor range should be 30-60 meters, or 90 to 180 feet. This is a

very pessimistic estimation. In fact, our customers are reporting results that are

three to four times better than that estimate.

Frequency also figures into the range performance. According to [7], the range

will be cut in half every time the frequency doubles. That means that for the

same link budget, a 900 MHz solution will operate at twice the range of a 2.4

GHz solution. 2.4 GHz has another property that makes it undesirable for many

applications: it is the resonant frequency of water. This fact has two implications.

First, microwave ovens are a broadband source of interference and the 802.15.4

specification provides very little rejection. Second, moisture will significantly

attenuate the signal.



20/29


If we factor that into the projected indoor range performance of the Wi.232DTS

module, we could expect 200-400 feet. Even as a pessimistic estimation, 200

feet of range is sufficient for home automation.

Range Performance Comparison

Zigbee Wi.232DTS Difference

Link Budget 94dB 114dB 20dB

Outdoor 500 feet 4000 feet +3500 feet

Indoor 40 feet 400 feet +360 feet

4.3 Reliability

Robustness and reliability are also key concerns. Two factors affect the reliability

of an embedded wireless link in the field [7]:

1. Multi-path fading

2. Interference

4.3.1 Multipath

Multi-path fading is caused when radio waves sent by the transmitter take

different paths, bouncing off of obstacles, and arrive at the receiver. Each of the

separate signals will have a different phase, causing summation or canceling of

energy at the receive antenna. There are receiver design techniques that can be

used to recover the energy from each of the signals, but neither the Wi.232DTS

module nor the Zigbee solution have that capability.



21/29


The only practical way to combat multi-path fading is to improve the link budget

of the radio. For example, using the single floor multi-path model given in [7], we

can estimate that the path loss is about 90dB for a 50-meter distance. In order to

ensure operation, we must add 15dB to account for the fading effect of localizednulls. Thus, we need 105dB link budget to go 50 meters in a 1 floor

building/house.

The following table shows the comparison between the Wi.232DTS module and

a Zigbee solution.

Comparison of multi-path performance betweenZigbee and Wi.232DTS

Zigbee Wi.232DTS

Required (dB) 105 105Actual (dB) 89 114

Surplus(deficit) (16.00) 9.00

Zigbee is 16dB short of the link budget required, assuming a 15dB fading

margin, to give the performance needed for a typical home or building

automation product. It is even worse for automated meter reading where the link

budget requirement is greater.

Zigbee can overcome this shortcoming through its application of mesh

networking. Since multiple paths exist in a mesh network, messages can be sentin several directions. If one path is lost for some reason (the refrigerator door

opens, for example), the message is likely to make it along another path.



22/29


However, many applications wont initially deploy enough nodes to make the

mesh networking effective. Products that have only two or three nodes wont

gain any advantage from mesh networking, and the limited link budget will

significant impact the overall performance in the field.

HVAC

ControlUnit

ZigbeeF

FD&PANCoordinator

ThermostatZigbee RFD

Effective

Range

Even though the nodes may be fixed, multi-path fading can still cause problems.

Consider the HVAC application shown above where the control unit is at one end

of the house and the thermostat at the other. At the time of installation, a path

loss of only 90dB or so may exist, so some Zigbee solutions may initially work.

In the example shown, the link budget is barely enough.

However, when something in the environment changes, like furniture being

added, the new environment may now exhibit as much as a 105dB path loss. In

that case, the Zigbee solution will not work.



23/29


HVACControlUnit

ZigbeeF

FD&PANCoordinator

ThermostatZigbee RFD

Effec

tiveR

ange

In this application, Zigbee operates as a simple point-to-multipoint network. Nobenefits are gained by mesh networking and the ultimate range is determined

solely by the link budget. Its link budget is not sufficient to meet the needs of the

application.

The Wi.232DTS module can operate in a point-to-multipoint network as well. It

does have sufficient link budget to meet the needs of the application, so it would

make a better choice.

4.3.2 Interference

There are several types of interference that affect the performance of an

embedded wireless link:

1. Adjacent channel interference

2. Out of band interference

3. In channel interference

Adjacent channel interference is usually caused by the co-location of two

networks that are operating on adjacent channels. Energy from each channel



24/29


will bleed into the adjacent channel, causing interference. Receivers are

designed to reject adjacent channel interference. The ability to reject adjacent

channel interference is directly related to the reliability of co-located networks.

The 802.15.4 specification requires only 0dB rejection. Thus, if a signal received

on an adjacent channel is equal to or less than the wanted signal, the receiver

will operate perfectly. If, however, the adjacent signal is greater than the wanted

signal, the receiver will not be able to reliably receive the wanted signal.

The Wi.232DTS module has 40dB of adjacent channel rejection. An adjacent

channel signal that is up to 40dB more than the wanted signal will not interfere

with the receiver.

Out of band interference is caused by strong transmitters that flood the front-end

of the receiver. For example, a common problem with poorly designed 900 MHz

solutions is that they are rendered useless in the presence of a local cellular

tower. Three design criteria affect the ability of a receiver to reject out-of-band

interference: front-end filtering, 3-dB compression point for the LNA, and IP3 for

the receiver chain. Both the Wi.232DTS module and the 802.15.4-based

modules are comparable in performance on these specifications, and both

perform well.

A transmitter or RF source emitting energy at the frequency that the receiver is

currently tuned to causes in-channel interference. The 802.15.4-based modules

will give slightly better performance (5-8dB) in this regard due to spreading gain

derived from direct sequence spread spectrum modulation.

4.4 Scalability


Scalability of a wireless solution is very important. While Zigbee seems

focused on upward scalability; we contend that downward scalability is also

important.


25/29


It is true that there are applications that will deploy large numbers of nodes, and

the ability of a technology to handle that scale is important. But it is also true that

a large number of applications will deploy only a few nodes, at least initially; thetechnology must be able to handle this scale as well.

The Wi.232DTS module has very good performance at the physical level, so it

can operate well in either case.

Zigbee, however, relies on mesh networking to achieve reliable performance,

which in turn requires larger numbers of FFD devices to work. In applications

where only a few devices will be deployed, Zigbee will give very short-range

performance, limiting its suitability for many applications.

4.5 Battery Performance

A FFD device, under the current Zigbee concept, is generally line powered

because it cannot be duty-cycled the way a RFD can [3]. Therefore, a battery-powered device must be a RFD, which means it wont be able to route

messages.

There is the dilemma. FFD devices cannot be battery powered but can route

messages and form a mesh network. RFD devices cannot route messages, but

can be battery powered.


To understand how this is a problem, consider the applications of fixed

automated meter reading and container security/tracking. In both applications,

all of the nodes need to be powered by batteries and all of the nodes need to

route messages to form a mesh network, extending the range of the network

beyond one node, thereby reducing the infrastructure cost required to monitor the


26/29


network (i.e. more nodes per reader). With Zigbee, there is no way to meet

both requirements. Neither an FFD nor and RFD can do both.

In contrast, the WiSE technology, and specifically Wi.MESH, is designed tosupport the needs of battery powered embedded networks. There are not

separate definitions of node functionality; all nodes have the full functionality of

the specification. A Wi.MESH node, for example, uses a sophisticated

rendezvous mechanism to maintain local synchronization of nodes, allowing

them to sleep most of the time, wake up, transmit data, and go back to sleep.

Using this mechanism, each node will be able to route packets for neighbors, a

key requirements for fixed automated meter reading and container tracking.

Most of the automated meter reading applications deployed in 2004 were mobile,

though it is predicted that a larger number will be fixed by 2010 [2]. The

Wi.232DTS module, which is available today, is a perfect solution for mobile

automated meter reading applications, and supports future upgrade to fixed

application.

5. Summary

In this report, we examined the technical attributes of the WiSE and Zigbee

based solutions. A comparison was made of cost, range performance,

robustness, and scalability.

The Wi.232DTS module is available now. It can be purchased for under $10 in

large production quantities. It has a very good link budget; 114dB. A solution

based on the Wi.232DTS is scalable; it will work equally well with two end-points

or two hundred. It is transparent and simple to use. The OEM/VAR is only

required to form a supplier relationship with one company. The technology is

proven and in use today.



27/29


The Zigbee solution will be available soon. It will not hit the $10 price point

until 2009 [2]. The 802.15.4 radio specification has a very poor link budget;

89dB. A Zigbee based solution is not scalable; it will not work reliably with onlytwo end-points separated by the length of a house. It is complicated and

requires a significant learning curve from the engineer and significant resources

from the protocol controller. The customer must form three supplier

relationships; the chip vendor, the software vendor, and the microcontroller

vendor. The technology is unproven.

A Wi.232DTS solution will have 3 to 8 times the range performance of a

Zigbee solution. It will cost less than a Zigbee solution. It is available now,

and is more appropriate for most embedded wireless applications, including

home automation, building automation, HVAC, automated meter reading,

SCADA, etc.

In addition to cost, reliability, and scalability, Zigbee purports to offer other

advantages over proprietary solutions:

Interoperability

Vendor independence

Common platform

There are three separate frequency bands specified for Zigbee. If one

manufacturer of heating controls chooses the 900 MHz band, and another

chooses the 2.4 GHz band, the products will not operate together. Additionally, itis likely that IC vendors will add proprietary features to their 802.15.4

implementations in an effort to differentiate their product; if the OEM uses these

proprietary features, the benefit of interoperability will be negated.



28/29


In the end, the only way to guarantee interoperability using Zigbee is to design

only 2.4GHz products using only Zigbee standard features. However, the 900

MHz band for North America and the 868 MHz band for Europe are technically

superior and would probably be the first choice of OEMs designing products forthose countries. In that case, Zigbee offers no advantage to the WiSE

modules available from Radiotronix.

The concept of proprietary features also negates the possibility of vendor

independence. For example, the CC2420 from Chipcon exceeds the receiver

sensitivity required by the 802.15.4 specification by 8dB. Additionally, the

CC2420 transceiver implements a good portion of the 802.15.4 MAC functionality

on the chip. These are features that are not available from other manufacturers.

So if an OEM chooses the CC2420 to take advantage of these features, the

OEM is tied to a single vendor.

In conclusion, we believe that the WiSE family of embedded wireless modules

from Radiotronix offer a lower cost, higher performance alternative to Zigbee

solutions. Furthermore we believe that the purported advantages Zigbee

offers over proprietary solutions, interoperability and vendor independence, will

not be realized because of the various implementations that are possible under

the Zigbee specification.



29/29


6. References

[1] Chipcon, CC2420 2.4 GHz IEEE 802.15.4 RF Transceiver Data Sheet

[2] On World, October 2004, Wireless AMR and submetering: A market

dynamics study on fixed wireless technologies

[3] Electronic Design, January 2004, The Zigbee buzz is growing: New low-

power wireless standard opens powerful possibilities

[4] On World, March 2004, Wireless Sensor Networks: Mass Market

Opportunities

[5] IEEE, October 2003, 802.15.4 Part 15.4: Wireless Medium Access control

(MAC) & Physical (PHY) Layer Specifications for Low Rate Wireless Personal

Area Networks

[6] Radiotronix, Wi.232DTS Users Manual

[7] Radiotronix, October 2004, Wireless 101: Embedded Wireless Link

Performance

Documents

Wi232 vs Zigbee