Wireless Communication : Wi-Fi, Bluetooth, IEEE · PDF fileWireless Communication : Wi-Fi,...

Wireless Communication : Wi-Fi, Bluetooth,

IEEE 802.15.4, DASH7

Helen Fornazier, Aurélien Martin, Scott Messner

16 march 2012

Abstract

This article has for objective to introduce the basic concepts of and tocompare di�erent wireless technologies applied to embedded systems. Itfocuses on Wi-Fi, Bluetooth, IEEE 802.15.4 and Dash7.

For each technology, this article covers multiplexing, topology, range,energy consumption, data rate, application, security and peculiarities.

At the end of the article, the developer should be able to choose thebest wireless technology for their own embedded application and have abasic notion as to how to integrate the technology into their system.

Contents

1 Introduction 3

2 Wi-Fi 32.1 Origins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Frequency Channels . . . . . . . . . . . . . . . . . . . . . . 42.3 Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . 42.4 Network Topology . . . . . . . . . . . . . . . . . . . . . . . 4

2.4.1 Infrastructure Topology (Point-to-Point or Point-to-Multipoint) . . . . . . . . . . . . . . . . . . . . . 4

2.4.2 Ad-Hoc Topology . . . . . . . . . . . . . . . . . . . . 52.5 Layers De�nitions . . . . . . . . . . . . . . . . . . . . . . . 52.6 Range, Power Consumption, Data Rate . . . . . . . . . . . 82.7 Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.8 Particularities and Embedded Applications . . . . . . . . . 8

2.8.1 Wi-Fi Con�guration Interface . . . . . . . . . . . . . 102.8.2 Embedded Software integration . . . . . . . . . . . . 102.8.3 Other Considerations . . . . . . . . . . . . . . . . . 102.8.4 Applications of Ad Hoc: Wi-Fi Direct . . . . . . . . 12

3 Bluetooth 123.1 Origins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2 Frequency Channels . . . . . . . . . . . . . . . . . . . . . . 123.3 Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . 13

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3.4 Network Topology . . . . . . . . . . . . . . . . . . . . . . . 133.4.1 Piconet Topology . . . . . . . . . . . . . . . . . . . . 133.4.2 Scatternet Topology . . . . . . . . . . . . . . . . . . 13

3.5 Layers De�nitions . . . . . . . . . . . . . . . . . . . . . . . 133.5.1 The Bluetooth Controller . . . . . . . . . . . . . . . 153.5.2 The Bluetooth Host . . . . . . . . . . . . . . . . . . 153.5.3 Host Controller Interface (HCI) . . . . . . . . . . . . 153.5.4 Generic Access Pro�le (GAP) . . . . . . . . . . . . . 15

3.6 Range, Power Consumption, Data Rate . . . . . . . . . . . 153.6.1 Bluetooth versions . . . . . . . . . . . . . . . . . . . 16

3.7 Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163.8 Particularities and Embedded Applications . . . . . . . . . 16

3.8.1 Sni�, Hold and Park mode . . . . . . . . . . . . . . 17

4 IEEE 805.15.4 174.1 Origins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.2 Frequency Channels . . . . . . . . . . . . . . . . . . . . . . 174.3 Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . 184.4 Network Topology . . . . . . . . . . . . . . . . . . . . . . . 18

4.4.1 Star Topology . . . . . . . . . . . . . . . . . . . . . . 184.4.2 Mesh Topology . . . . . . . . . . . . . . . . . . . . . 184.4.3 Beacon Enabled Mode . . . . . . . . . . . . . . . . . 184.4.4 Non-beacon Enabled Mode . . . . . . . . . . . . . . 19

4.5 ZigBee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.5.1 Origins . . . . . . . . . . . . . . . . . . . . . . . . . 194.5.2 Network Organisation . . . . . . . . . . . . . . . . . 19

4.6 Layers De�nitions . . . . . . . . . . . . . . . . . . . . . . . 194.6.1 Network Layer . . . . . . . . . . . . . . . . . . . . . 194.6.2 Aplication Layer . . . . . . . . . . . . . . . . . . . . 20

4.7 Range, Power Consumption, Data Rate . . . . . . . . . . . 204.8 Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204.9 Particularities and Embedded Applications . . . . . . . . . 20

5 Dash7 205.1 Origins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215.2 Frequency Channels . . . . . . . . . . . . . . . . . . . . . . 215.3 Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . 215.4 Network Topology . . . . . . . . . . . . . . . . . . . . . . . 225.5 Layers De�nitions . . . . . . . . . . . . . . . . . . . . . . . 225.6 Range, Power Consumption, Data Rate . . . . . . . . . . . 225.7 Security . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

6 CSMA/CA 23

7 Comparison table 23

8 Conclusion 23

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9 References 239.1 Wi-Fi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239.2 Bluetooth . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249.3 IEEE 802.15.4 . . . . . . . . . . . . . . . . . . . . . . . . . . 259.4 Dash7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1 Introduction

Wireless communication in embedded systems is a growing �eld. It can beused in a wide range of situations where mobility is essential and wires arenot practical. The key complications to choosing a wireless technology revolvearound the following requirements [ADA10]:

� Range

� Reliability

� Compliancy (Standards)

� Security

� Cost

� Power Consumption

� Transmission rate

� Network Architecture Envisioned

This article aims at helping in selecting the best available embedded wirelesstechnology for your devices amongst some of the most popular choices used inthe market today. The choices evaluated are Wi-Fi (802.11 a/b/g/n), Blue-Tooth, ZigBee (and 802.15.4), and Dash7.

2 Wi-Fi

Wi-Fi is the common name used for the di�erent versions of the IEEE 802.11standard (a/b/g/n/i, etc.), which describes the technologies and protocols forachieving a Wireless Local Area Network (WLAN), a wireless bridge, or anadhoc network [WFD12][80207].

2.1 Origins

The standard IEEE 802.11 for WLAN networks was created in 1985.Wi-Fi suggests Wireless Fidelity, the term is used commercially since 1999

with the creation of Wi-Fi Alliance.Wi-Fi Alliance is responsible for the logo Wi-Fi which gives interoperability

certi�cations between the devices sporting the logo.

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122.467

12.412

22 MHz

ChannelCenter Frequency(GHz)

22.417

32.422

42.427

52.432

62.437

72.442

82.447

92.452

102.457

112.462

132.472

142.484

Figure 1: Wi-Fi channels in the 2.4 GHz band.Source : http://en.wikipedia.org/wiki/IEEE_802.11

2.2 Frequency Channels

In embedded systems, there are four main versions of the 802.11 standard thatare often used [GAI12]. 802.11a is one that operates in the 5GHz frequencyISM band. In terms of Compliancy and Reliability, this is useful when it isimportant to avoid interference with other RF devices which commonly operatein the 2.4 GHz ISM band (unlicensed RF band for industrial, scienti�c, andmedical devices). 802.11b/g operate in the 2.4 GHz band, but have faster datatransmission rates (11 Mbps and 54 Mbps respectively). 802.11n is the newestand most popular version, and can be con�gured to operate in either the 2.4GHz or 5 GHz frequency bands.

The 2.4 GHz band used by 802.11b/g is divided into channels separated from5 MHz each. Of the 13 (14) channels, there is an only set of 3 channels (4 incertain countries) which do not overlap.

2.3 Multiplexing

Wi-Fi uses OFDM (Orthogonal Frequency-Division Multiplexing) to perform afull-duplex communication in a single channel.

CSMA/CA (see section 6) is used as a mechanism to avoid collision betweenthe overlapping channels and other technologies that use the same ISM bandfrequency such as ZigBee and BlueTooth.

2.4 Network Topology

Depending on the desired application, Wi-Fi is commonly used in two di�erentnetwork architecture con�gurations (Note: di�erent network topologies can beimplemented at the application layer level) [80207].

2.4.1 Infrastructure Topology (Point-to-Point or Point-to-Multipoint)

A standard network architecture in Wi-Fi uses an access point (AP) and multipleclients which communicate on network through the latter.

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Figure 2: The Infrastructure topologies Basic Service Set (BSS) declares onededicated AP and multiple clients in any particular WLAN.Source : http://en.kioskea.net/contents/wi�/wi�modes.php3

2.4.2 Ad-Hoc Topology

The 802.11 standard allows also ad-hoc mode with Indepedant Basic ServiceSets (IBSS). This con�guration permits a node to operate as both an AP anda client.

Ad-hoc infrastructures are widely used in embedded wireless implementa-tions because they permit communication between two devices without a ded-icated access point. Implementing ad-hoc requires careful considerations de-pending on the power consumption requirements of the system.

2.5 Layers De�nitions

IEEE 802.11 de�nes Physical (PHY) and Data Link (LLC + MAC) layers forgeneric wireless connexions using electromagnetic waves. There are 3 distinctphysical layers, respectively based on DSSS, FHSS and infrared light.

The LLC is based on IEE 802.2, which allows a Wi-Fi network to join anynetwork implementing an IEEE standard. The MAC layer, though speci�c,is deliberately close to 802.3's one (terrestrial Ethernet). It uses CSMA/CA(see section 6). There are two access methods : DCS (Distributed Coordina-tion Functions), based on best e�ort, egalitarian (equal access of each actor tothe medium) for asynchronous data transport, and PCF (Point CoordinationFunction), based on polling, used for sensitive data and real time applications.

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Figure 3: An Extended Service Set de�nes the ability of two BSSs to be con-nected via their APs.Source : http://en.kioskea.net/contents/wi�/wi�modes.php3

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Figure 4: IBSS provides the capability for devices to interconnect by functioningdually as an AP and a client.Source : http://en.kioskea.net/contents/wi�/wi�modes.php3

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Protocol Release Frequency Modulation Max data rate Inner range

802.11a 1999 5 GHz OFDM 54 Mbps 35 m802.11b 1999 2.4 GHz DSSS 11 Mbps 35 m802.11g 2003 2.4 GHz OFDM/DSSS 54 Mbps 38 m802.11n 2009 2.4/5 GHz OFDM 150 Mbps 70 m

Table 1: Wi-Fi protocols overview.Source : http://en.wikipedia.org/wiki/IEEE_802.11

Wi-Fi is widely used under IP protocol.

2.6 Range, Power Consumption, Data Rate

Some important considerations - If data-transfer reliability and speed are impor-tant, 802.11n supports Spaced-time block coding (STBC) and Maximal RatioCombining (MRC). STBC provides redundancy by using multiple receivers toreceive messages on multiple channels. This reduces error-rate but consumesmore power. MRC similarly transmits messages on multiple channels.

source: http://en.wikipedia.org/wiki/IEEE_802.11

2.7 Security

Many commercially available embedded Wi-Fi modules come with the addedoption for con�guration in di�erent forms of security at the physical and trans-port levels. TKIP (or WPA) is considered to be su�cient protection for mostlow-security applications. WPA2 is considered to be the most secure. Other se-curity protocols for encryption of data can be added in the di�erent OSI layers,but WPA and WPA2 are available for basic encryption.

WPS - Wi-Fi protected setup is a Wi-Fi con�guration and authenticationtechnique using push buttons for input entry. This has been proven susceptibleto brute-force attacks.

2.8 Particularities and Embedded Applications

Because embedded systems often have very speci�c constraints to meet, thereare some particularities which are generally followed. This section providessome common examples for implementing embedded Wi-Fi at di�erent levels ofcomplexity.

A WLAN Module is a full-featured Wi-Fi module provided for wireless com-munication abstraction. It most commonly requires a 3.3V power connectionand an external bus interface (SDIO, SPI, UART, USB, etc).

This type of WLAN Module is used more often with legacy, or existing em-bedded devices with an available serial port. These plug-in-play modules arecheap, but they require the UART connection (which it can sometimes be im-portant to leave available for debugging purposes and wired software updates).

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Figure 5: Wi-Fi module.Source : http://www.ept.ca/news/designing-embedded-systems-with-wi-�-connectivity/1000633756/

Figure 6: Easy serial-to-Wi-Fi package by GainSpan.Source : http://www.gainspan.com/products/GS1011_serial-to-wi-�_eval_kit.php

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Some other interesting Wi-Fi modules to consider at a reasonable price. Thecost can vary dramatically depending on features of reliability, range, powerconsumption, and Wi-Fi standard used.

LM006 NanoAdapter (uses USB) cost 25 eurosGS1011M GainSpan 802.11b Low Power Wi-Fi cost 30 euros. Provides im-

plemented TCP/IP Stack and security. Connection via UART/SPI/I2C/JTAG

2.8.1 Wi-Fi Con�guration Interface

Out-of-the-box Wi-Fi needs to be hardware calibrated and software con�gureddepending on the system and the desired operating parameters. Most self-containing Wi-Fi modules come pre-con�gured to avoid complexity in cost ofcalibration. Wi-Fi con�guration can also be done via external connection tothe embedded device (USB or Serial). The diagram shows below a Wi-Fi con�gmodule as part of the application layer.

The Wi-Fi host must be capable of running a Wi-Fi host driver (examplecomputer installed with a special con�guration software sends commands whichare received and interpreted by the Wi-Fi con�guration driver).

With RF tags and devices which cannot be con�gured via a connection in-terface, an external con�guration application would be used to con�gure thedevice via Wi-Fi (device would originally have a factory-reset state). Con�g-urable parameters may include the SSID, or the password.

Simple Network Management Protocol (SNMP) is a commonly used con-�guration protocol used for Wi-Fi devices that require an external, wirelesscon�guration interface.

2.8.2 Embedded Software integration

One may also choose to implement Wi-Fi with just the RF receiver. This allowsmore freedom, and a greater capacity to handle multi-socket communication,but it can be much more complex. Unlike a fully-integrated WLAN Module,the TCP/IP stack must be separately added for handling the underlying com-munication protocol layers comprising the Wi-Fi standard.

Choosing a TCP/IP stack Iwip seems to be the stack of choice by mostimplementers. It can be found here :

http://savannah.nongnu.org/projects/lwip/There are other open-source and non free versions.

2.8.3 Other Considerations

Power Consumption: Wi-Fi Power-save polling (PS-polling) A Wi-Fi module is said to be `active' when it is actively sending or receiving data.Though it varies depending on Wi-Fi modules and driving software, Wi-Fi canconsume anywhere between 30% and 60% of the total power consumed during�active� state in comparison to standby state.

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Figure 7: Software architecture of an embedded device with all networking andWLAN functionality resident in the WLAN module.Source : http://www.ept.ca/news/designing-embedded-systems-with-wi-�-connectivity/1000633756/

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Many Wi-Fi devices can be con�gured for variable power-save polling (PS-polling). This concept allows the Access Point to hold onto a queue of packetsuntil the client wakes up and polls for the pending data.

2.8.4 Applications of Ad Hoc: Wi-Fi Direct

Wi-Fi Direct allows connection between devices without a dedicated accesspoint. It has been implemented on recent versions of smartphones such as theSamsung Galaxy S. This is done by including software APIs which use the de-vice's Wi-Fi antenna as if it were an access point (SoftAP). This makes tetheringpossible among other things.

3 Bluetooth

Bluetooth technology is used in many devices such as mobile phones and per-sonal computers.

It is designed to be low cost, low range and low power supply.

3.1 Origins

Bluetooth was created by Ericsson in 1994 as a wireless alternative for RS232data cables.

Nowadays, Bluetooth is managed by Bluetooth Special Interest Group (SIG)and its speci�cation can be found on their site: www.bluetooth.org.

To be able to use Bluetooth for commercial use, you need to be a memberof SIG. Adopter members don't need to pay.

Bluetooth v1.2 was rati�ed as IEEE 802.15.1.


Bluetooth uses the unlicensed 2.4 GHz ISM Band (2400 - 2483.5 MHz), whichit shares with Wi-Fi and IEEE 802.15.4 as well.

Bluetooth de�nes 79 non-overlapping radio frequency channels, each spacedby 1MHz from the previous one, starting at 2402 MHz, with a lower guardband of 2 MHz and a upper guard band of 3.5 MHz in the 2.4 GHz ISM Band[BSIGF]. The frequency channel can be calculated: f=2402+k MHz, k=0,. . . ,78.3.2.1. Frequency-Hopping Spread Spectrum (FHSS)

As a mechanism to avoid collision between Wi-Fi, ZigBee, other Bluetoothnetworks, or other wireless communications using the same 2.4 GHz ISM Band,Bluetooth uses FHSS, which implements jumping from one channel to anotherfrom time to time.

Bluetooth implements an AFH mode (Adaptive Frequency-Hopping). Inthis mode, Bluetooth's devices listen to the channel and if it is occupied, thena new channel is chosen.

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3.3 Multiplexing

Bluetooth uses TDD (Time Division Duplex) to perform full-duplex communica-tion. The master (see section 3.4. Network Topology for master-slave structure)de�nes slots of time and allocates one slot per slave. The devices don't transmitat the same time, just in their respective allocated slot.

Therefore, two parallel Bluetooth networks at the same frequency channel(see 3.3. Frequency Channels) could potentially disturb each other. To avoidthis kind of interference, the protocol implements FHSS (frequency-hoppingspread spectrum, see 3.3.1. Frequency-Hopping Spread Spectrum for more de-tails).

Bluetooth uses CSMA/CA (see section 6 for CSMA/CA) to avoid collisionwhen using a shared channel with another wireless network.


Bluetooth is organized in master-slave mode. The topology supports 1 master,7 slaves and up to 255 slaves in parked mode (waiting to be awakened by themaster).

The master de�nes the clock and time slots for all nodes.A master can request to change his role with a slave, at which point the slave

becomes the master and the master becomes the slave. For example: a headphone starts the communication as a master, but it prefers to be the slave.

Two basic topologies are possible in Bluetooth : Piconet and ScatternetTopology.

3.4.1 Piconet Topology

In the piconet topology, the master connects to several slaves, but the slavesdon't communicate between them, even passing through the master.

Communication between the slaves can be performed in a higher softwarelayer, but the master is not naturaly like a router.

3.4.2 Scatternet Topology

A slave from one piconet can be master of another piconet. This is calledscatternet topology.


The Bluetooth layers can be divided in three sub-categories :

� The Bluetooth controller

� The Bluetooth host

� The Generic Access Pro�le (GAP).

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Figure 8: Piconet topology.

Figure 9: Scatternet topology.

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3.5.1 The Bluetooth Controller

The Bluetooth controller is divided into three main parts :Radio: responsible of sending information in the physical channel.LC (Link Control): Responsible for the transport layer, it encodes and de-

codes Bluetooth packets, manages the time schedule and takes care of AFHservices.

LM (Link Manager): Responsible for creating a logical link with an otherBluetooth device. It manages the discovery service.

The Bluetooth controller exposes to the host three basic types of services:Device control services: Allows modi�cations in the behaviour of the Blue-

tooth device.Transport control service: Controls channels and links.Data service: Carries data between devices.

3.5.2 The Bluetooth Host

SDP (Service Discovery Protocol): Responsible for managing the discovery ser-vice on the host's side.

L2CAP (Logical Link Control and Adaptive Protocol): responsible for man-aging the resource, the controller is assumed to have a limited bu�er and theapplications can be aware of this.

3.5.3 Host Controller Interface (HCI)

The HCI describes the interface protocol between host and controller.

3.5.4 Generic Access Pro�le (GAP)

The Bluetooth stack layer aims at providing full interoperability between layers,including layers at the top of the stack. The Generic Access Pro�les are a list ofmany other de�ned layers for a special kind of application. For a headset device(for example) it may support the Headset Pro�le (HSE).


The Bluetooth Radios are de�ned in 3 classes [BSIGT]:

Class 3:

� Range: 1 meters (max).

� Maximum Output Power: 1 mW.

Class 2: (most common)


� Maximum Output Power: 2.5 mW.

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Class 1: (for industrial uses).


� Maximum Output Power: 100mW.

3.6.1 Bluetooth versions

The Bluetooth wireless technology is basically divided in two di�erent systems:Basic Rate (BR) and Low Energy (LE).

The BR systems can include the Enhanced Data Rate (EDR) mode and aHigh Speed (HS) mode.

Pure BR systems (v1.2) are up to 721 Kbps [BSIGG]. BR/EDR (v2.0 andv2.1) o�ers the 2 Mbps (referred as π

4 -DQPSK) and 3 Mbps (referred as 8DPSK)modes and HS (v3.0) can reach 24 Kbps [BSIGS]. LE systems (v4.0) have lowerconsumption and lower data rates.

Versions 2.1 and 2.0 are backward compatible.The Bluetooth LE has an entirely new protocol stack compared to the stan-

dard protocols de�ned in v1.0 v2.0 and v3.0, previously named WiBree andBluetooth ULP (Ultra Low Power) [BVW12].

The wake up latency usually is of about 3 seconds [TWB10].

3.7 Security

In Bluetooth v2.0 and earlier, security is based on PIN code authentication :each device must enter the same PIN code. Limited input devices like headsetsusually have a default PIN code de�ned inside (0000 or 1234) [PMW12].

Bluetooth v2.1 is backward compatible and it implements the Secure SimplePairing (SSP), using public key cryptography. The goal is to simplify the pairingprocedure for the user and improve security mainly against Eavesdropping andMan-In-The-Middle attacks.

SSP has 4 pairing procedures [BSIGB]:Numeric Comparison: The user must con�rm that the number displayed on

one device is the same as on the other deviceJust Works: The user must con�rm the connection. Used in scenarios where

one of the devices is really limited like headsets with no display or keyboard.This pairing method o�ers no protection against Man-In-The-Middle attack.

Out of Band: Higher security method, di�erent types of implementationsexist. NFC system is an example.

Passkey Entry: The user must enter the PIN number displayed on one deviceinto the other.


In embedded systems applications the radio class 2 is most used, it is usuallyfound in mobile phones.

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One can �nd Bluetooth transceivers in the market starting at $7, the con-nection is usually made by UART interface.

3.8.1 Sni�, Hold and Park mode

Bluetooth devices have 3 modes to save power energy in transmission inactivestate : sni� mode, hold mode and parked mode [BSIGM].

� Sni� mode: the slave must listen during every master-to-slave time slot,it can be inactive in the others slots.

� Hold mode: In this mode, slave and master should agree on the durationthe slave remains in hold mode. When in hold mode, the slave is free todo other things like attending another its own piconet or entering powersave mode.

� Park state: The slave gives up his logical transport address and receivesa new address to be used in park state. The slave wakes up from time totime to synchronize with the master who sends a de�ned message (calledbeacon train) with a constant time period. The slave can be swapped toan active state by the master's beacon train.

4 IEEE 805.15.4

While Wi-Fi and Bluetooth are very power-greedy, and with the rise of em-bedded systems, the need has been heavily felt for more and more autonomousones, and thus for a more convenient communication protocol.

This protocol has been described by the IEEE 802.15.4 group. It is intendedfor wireless networks with small dimensions, small consumption and small cost(WPANs for Wireless Personnal Area Networks). It's thus perfectly adapted tothe embedded world.

Only a physical (PHY) and a medium access control (MAC) layer are de�ned,upper layers being left to the discretion of implementations.

4.1 Origins

IEEE 802.15.4 was released 12 May 2003, and has then been revised 7 June2006, 28 August 2007 and twice in 2009. [WIK02]


The original standard de�nes 3 frequency bands to use :2400-2483.5 MHz, 16 channels, available worldwide902-928 MHz : 10 channels until 2006, then 30, available in the USA868.0-868.6 MHz : 1 single channel, available in Europe

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The �rst one has to be shared with both Wi-Fi and Bluetooth, which causescollisions between networks making use of those protocols. common in envi-ronements where they cohabit. [I3E03] [I3E06]

4.3 Multiplexing

Multiplexing in IEEE 802.15.4 is managed using CSMA/CA, which is detailedin section 6 of the present document. Coordination can be provided throughthe use of beacon frames, see 4.4.3. [I3E03]


IEEE 802.25.4 networks are organized according to two types of topologies :star or mesh. Both involve a mandatory coordinator node, and any numberof FFD (Full Function Devices) and/or RFD (Reduced Function Devices), thelatter unable to connect to more than one other node (a FFD), but far lesspower-greedy thaxn the former. The coordinator must thus be a FFD. It canbe useful to note that modules usually implement only one of those solutions.[I3E03]

4.4.1 Star Topology

In star topology, each node is exclusively bounded to the coordinator of the net-work. Higher layers (for instance ZigBee network layer or Application layer) canallow routing so that any node could communicate with any other through thecoordinator. It is an unsafe method, since a failure of the coordinator endangersthe whole network, and only one link anchors each node in the network. Thismode is quite similar to a Bluetooth piconet, except in Bluetooth there cannotbe any communication between slaves.

4.4.2 Mesh Topology

It's a far more general topology, where each FFD is directly connected to eachother node at range. Here again, higher layers can allow each FFD to routecommunications. There are subcategories of meshes, such as tree networkswhere each node can only converse with its parent and children.

4.4.3 Beacon Enabled Mode

In beacon mode, time is divided in superframes delimited with beacons (specialMAC frames) and of prede�ned length. The superframe is itself partitioned in16 slots, which can either be contention-free or contention-access. A successionof contention-free slots at the end of the superframe constitutes the GuarantedTime Slots, to which a given node can subscribe to ensure reliability and lowlatency in its communications.

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4.4.4 Non-beacon Enabled Mode

In this mode, communications are asynchronous, though beacons can still berequested for some purposes. The general idea is that beacons keep your devicesawake, which is a waste of power if the e�ective communication load is light.Only the coordinator keeps constantly awake here. It stores pending messages,so that other nodes only have to poll it whenever they have to emit somethingor want to know whether they have mail. The coordinator can be plugged anddispose of unlimited power, while other nodes need to stay autonomous. (Forexample, in home automation, you can have your light bulb pumping power andany number of switches sending sparse signals to the former.)

4.5 ZigBee

Since IEEE 803.15.4 does only de�ne PHY and MAC layers, implementationsmust de�ne the higher ones That's what does ZigBee, a standard for WPANspecifying a Network and an Application Layer to complete the IEEE.

4.5.1 Origins

In 2002, seeing that neither Wi-Fi nor Bluetooth could �t some of their needsfor embedded systems (namely, autonomy), a number of industrial companiesformed the consortium called ZigBee Alliance, aimed at providing standardsfor low cost / low consumption wireless communications. Then, with the birthof IEEE 802.15.4 group, both teams have worked closely to specify the entireprotocol stack, so that, with the IEEE being released in May 2003, a �rstZigBee speci�cation could be rati�ed 14 December 2004 and go out 13 June2005. A �rst revision saw the light in 2006, then ZigBee PRO, an enhancedstack o�ering advanced features such as multicasting and many-to-one routingbut more RAM-greedy than its humbler counterpart, in 2007. [WIK03]

4.5.2 Network Organisation

The topology of a ZigBee network can be any one of those o�ered by IEEE802.25.4, namely star, tree or mesh. Devices are similarly typed, but beardi�erent names : a FFD becomes a ZR (ZigBee Router), a RFD a ZED (ZigBeeEnd Device), and the coordinator is called a ZC (ZigBee Coordinator). [ZIG08]


4.6.1 Network Layer

ZigBee Routers and the ZigBee Coordinator are given routing capacities, andcan discover neighbours and routes to those neighbours. This is performedthrough the AODV (Ad-hoc On Demand Vector) protocol, as follows : a routerequest is broadcasted to all neighbours, which froward it to all their neighbours,and so on until the searched for device receives the request, and unicasts its routeanswer to the source via the lowest cost path. [ZIG08]

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4.6.2 Aplication Layer

The application Layer is the one which the end user is faced with. It de�nesZDOs (ZigBee Device Objects), whose roles comprise keeping the role playedby the device in the network (namely ZC, ZR or ZED) and managing devicediscovery, requests to join a network, security and more. The Application Sup-port Sublayer stores and maintains binding tables as a database. ZigBee alsode�nes device pro�les, so that any application could ideally dispose of a protocolperfectly adapted to its needs and constraints. [ZIG08]


ZigBee communications can reach up to 500m, with a data rate of up to 250kbs, for a typical power consumption of 125 to 400 µW. As ZigBee is based onIEEE 802.15.4, there is no wake-up signal, but slots for sleep or activity, or inasynchronous mode, devices sleeping anytime they have nothing to say, with anever-vigilant coordinator. [WIK03]

4.8 Security

Based on facilities provided by IEEE 802.15.4, ZigBee o�ers means of securingcommunications, such as use of cryptographic cyphering frames with symmetrickeys, frame integrity control, sequential freshness control and ACLs (AccessControl Lists). Unsecured mode allows none, ACL mode allows ACLs only andSecured mode allows all. [MAS07]


ZigBee therefore seems a good technology to put to use when you want yourdevices to communicate sparsely (no video streaming !) while consuming fewpower for a low cost. To use a ZigBee module with a microcontroller, you need toconnect it to a UART. There are other, optional pins to use, including a numberof analog inputs / digital IOs and a PWM output indicating the strength of thesignal which you can directly connect to a LED pin for observation purposes.

While the o�cial ZigBee stack's implementation access is exclusive to sub-scribing members of the ZigBee Alliance (who pay the annual fee and must useit in their devices), you can make your own implementation of the whole stack,but it has to be approved by the Alliance before being used in a commercialpurpose.

5 Dash7

Dash7 is a wireless technology created to have low power consumption and lowlatency response. It is known as a Wireless Sensor Network (WSN)

Dash7 applies BLAST concept [JPN09] :

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B: Bursty, abrupt data transfer, no support for stream data (i.e. video oraudio).

L: Light-data, limited packet size, small amount of exchanged data.AS: ASynchronous, no hand-shaking between devices. Command-response

oriented.T: Transitive, network can be changing, not a �xed structure.

5.1 Origins

The standard ISO 18000-7 (Dash7) was rati�ed in 2004 as a Wireless SensorNetwork (WSN) and modi�ed in 2008.

In 2009 the U.S. Department of Defense signed a $429 million contract forDASH7 devices. And in March of the same year the Dash7 Alliance was created[D7W12][DAW12].

Dash7 Alliance is an association which gives interoperability certi�cationsfor devices. Dash7 Alliance is similar to Wi-Fi Alliance.

Dash7 has a mode 1 (rati�ed as ISO 18000-7) and mode 2 [D7212] which iscompatible with mode 1.

Mode 2 is not an o�cial ISO standard yet. This document will focus onDash7 mode 1 ISO 18000-7:2004.


Dash7 mode 1 uses just one frequency channel centered on the 433,92 MHz ISMBand and bandwidth of 500 kHz [ISO4P]. There is thus no con�ict with Wi-Fi,Bluetooth and ZigBee signals.

Dash7 takes advantage of his low frequency band to save device power andpenetrate water, concrete and walls [WDB12].

Dash7 mode 2 has 8 frequency channels and bandwidth of 216kHz per chan-nel [D7AM2].

5.3 Multiplexing

Multiplexing is performed by collision arbitration [ISO4T] in a broadcast mes-sage, point-to-point messages are synchronous (collision may happen with an-other network only) and collisions are seen as a damaged package.

The interrogator (or master) asks the tags (slaves) to collect their Tag's IDsby broadcast and it sets a window size (WS, in number of slots). Each slot ofthe window lasts a predetermined period of time and tags must transmit justduring these slots, the slot during which to transmit is chosen randomly bythe tag. In case of collision between tags, they choose another random slot totransmit (Slotted Aloha method).

After WS slots the interrogator sends a sleep signal to the tags who answeredthe �rst collection round, and then it starts again another collection round untilthere is no more answer.

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The network is based on master-slave mode, also called interrogator and tag.Dash7 mode 2 is event-driven and an event can trigger a tag-initiated com-

munication or a tag-to-tag communication which is not possible in Dash7 mode1.

Event-drivenness brings a better battery use compared to mode 1 where thenetwork polls in the background.

The interrogator can set the Owner ID for each tag to separate networkgroups.


The ISO 18000-7:2004 de�nes physical and data link layers [ISO4D].The physical layer de�nes the modulation used in air communications and

band frequencies.The data link layer de�nes the communications protocol, data header, com-

mands, data, collision treatment, broadcast communication and point-to-pointcommunication.

Between commands, the interrogator can ask by broadcast for each tag's IDs(at once) in its RF communication range, set a tag's owner ID by point-to-pointcommunication, verify a tag's battery status (if it is less than 20%) and set aconnection password.

Dash7 Alliance supports the development of Open Tag, an open-sourceproject that implements the Mode 2's communication stack [ODA12][OWS12].


The ISO 1800-7:2004 sets transmission rate to 27.7 kbps. Dash7 mode 2 has amax throughput of 250 kbps [WDT12]. Wake up time is about 2,5 seconds.

The range can vary from 10 meters to 10 kilometers. The power consumptionis less than 1 mW.

5.7 Security

The interrogator can encrypt the connection between the tag by setting a pass-word [ISO4S]. To allow a new password, the tag must be unlocked �rst with theold password. Tags are unlocked by default.

Locked tags answer a point-to-point communication only if the message isencrypted with this password, but broadcast messages are not a�ected by lockedor unlocked status.

Dash7 mode 2 can use 128-bit AES, public key.

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Wi-Fi BlueTooth ZigBee Dash7 mode 1

Operating frequency 2.4, 5 GHz ISM Band 2.4 (worldwide), 902(USA), 868 (Europe) GHz ISM Band 868 MHz (Europe), 900-928MHz (USA), 2.4GHz (worldwide) 433 MHzISM BandChannels 14 79 16 (worldwide),10 (USA),1 (Europe) 1Data rate 11, 54, 540 Mbps 3 Mbps 250 Kbps 28 kbps

Power supply ~4,7mW /324,71mW ~2,5mW ~300 / 600 µW < 1 µWRange ~75m / 125m ~10m ~10m / 75~ ~10m / ~10km

Multiplexing OFDM - CSMA TDD - CSMA CSMA Slotted ALOHANetwork topology Star, ad-hoc Piconet, sScatternet Star, mesh Master-slave

Security WPA, WEP, TKIP PIN Code, Public key Security WPA, WEP, TKIP PIN Code, Public key Symmetric cryptography + ACLs 128-bit AES, public keySignal susceptibility Reference More sensitive Less sensitive Far less sensitive

Speci�cation IEEE 802.11 IEEE 802.15.1 (v1.2) IEEE 802.15.4 ISO 18000-7

6 CSMA/CA

CSMA (Carrier Sense Multiple Access) is a multiplexing method based, as itsname implies, on listening to signals by sensing carriers.

In CSMA, the device regularly samples the medium, and waits for it to beclear before beginning to emit. That's just the algorithm you're implementingwhenever you are in a regular conversation.

CSMA/CA (with Collision Avoidance) introduces a random Delay betweeneach test, as a way to decrease medium utilization and over all to narrow chancesof collision when several devices are waiting simultaneously.

7 Comparison table

8 Conclusion

We expect this article has been su�cient to give you the keys to choose wiselybetween those wireless communication protocols for use in your own embed-ded applications. In a nutshell : Wi-Fi for heavy communications, BlueToothfor short distances, point-to-point such communications consuming less power,ZigBee for light and power-savvy applications, Dash7 for even lighter and eventhriftier ones.

9 References

9.1 Wi-Fi

[ADA10] Adams, Lew. Easy Provisioning with GainSpan Embedded Wi-FiTechnology. GainSpan Corporation. 21 Oct 2010. accessed 15 Mar 2012.http://www.slideshare.net/gainspan/easy-provisioning-with-gainspan-embedded-Wi-Fi-technology

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[ERG12] Ergen, Mustafa. IEEE 802.11 Overview. UC Berkeley. http://wow.eecs.berkeley.edu/ergen/docs/IEEE-802.11overview.ppt accessed 16 Mar 2012.

[LWO12] http://www.lesswatts.org/tips/wireless.php. LessWatts.org. ac-cessed 15 Mar 2012.

[WFD12] Wi-Fi Direct. Wikipedia. http://en.wikipedia.org/wiki/Wi-Fi_Direct.accessed 14 Mar 2012.

[WAK12] Wake On LAN. Wikipedia. http://en.wikipedia.org/wiki/Wake-on-LAN. accessed 15 Mar 2012.

[80207] IEEE 802.11. Wikipedia. http://en.wikipedia.org/wiki/IEEE_802.11.accessed 15 Mar 2012.

[MAG09] Magee, Owen. Digi International. EE Times. 28 Mars 2009.http://www.eetimes.com/design/embedded/4026972/Sorting-through-the-embedded-Wi-Fi-confusion-item-1?pageNumber=1. accessed 15 Mar 2012.

[I3E07] IEEE Standard for Information technology � Telecommunicationsand information exchange between systems � Local and metropolitan area net-works � Speci�c requirements; Part 11: Wireless LAN Medium Access Control(MAC) and Physical Layer (PHY) Speci�cations. IEEE Computer Society. 3Park Avenue, New York, NY. 12 June 2007.

[GAI12] GainSpan Resource Library. GainSpan Corporation. https://www.gainspan.com/products/resource_library.php.accessed 15 Mar 2012.

[COM] Les modes de fonctionnement du Wi-Fi (802.11 ou Wi-Fi). comment-camarch.net. http://www.commentcamarche.net/contents/wi�/wi�modes.php3#gonext.accessed 15 Mar 2012.

9.2 Bluetooth

Channel division information:[BSIGF] Bluetooth Special Interest Group, Core Version 4.0, Vol 2, Part A

- FREQUENCY BANDS AND CHANNEL ARRANGEMENT.Range and power supply information:[BSIGT] Bluetooth Special Interest Group, Core Version 4.0, Vol 2, Part A

- TRANSMITTER CHARACTERISTICS.Data rate information:[BSIGG] Bluetooth Special Interest Group, Core Version 4.0, Vol 1, Part A

- GENERAL DESCRIPTION.Data rate information for Enhanced Data Rate mode:[BSIGS] Bluetooth Special Interest Group, Core Version 4.0, Vol 2, Part A

- SCOPE.Bluetooth Low Power: [BVW12] Bluetooth v4.0. Wikipedia. http://en.wikipedia.org/wiki/Bluetooth#Bluetooth_v4.0

accessed 13 Mar 2012.Bluetooth wake-up delay: [TWB10] The comparison of Wi-Fi, Bluetooth

and ZigBee. Sena Blog. http://www.sena.com/blog/?p=359 25 Feb 2010.Bluetooth Security:[BSIGB] Bluetooth Special Interest Group, Core Version 4.0, Vol 1, Part

A - BR/EDR SECURE SIMPLE PAIRING. [PMW12] Paring Mechanisms.

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Wikipedia. http://en.wikipedia.org/wiki/Bluetooth#Pairing_mechanisms ac-cessed 13 Mar 2012.

Bluetooth Sni�, Hold and Park mode:[BSIGM] Bluetooth Special Interest Group, Core Version 4.0, Vol 2, Part B

- SNIFF MODE, HOLD MODE, PARK STATE.

9.3 IEEE 802.15.4

[DUS07] Stevanovic Dusan, http://www.cse.yorku.ca/~dusan/Zigbee-Standard-Talk.pdf, 20 June 2007

[I3E03] The Institute of Electrical and Electronics Engineers, Inc., IEEE Std802.15.4TM-2003, Part 5 - General Description, 1 October 2003

[I3E06] The Institute of Electrical and Electronics Engineers, Inc., IEEEStd 802.15.4TM-2006 (Revision of IEEE Std 802.15.4-2003), Part 5 - GeneralDescription, 8 Septemeber 2006

[JEN06] Jennic, IEEE 802.15.4 Wireless Networks - User Guide Revision 1.1,6 October 2006

[MAS07] Masica Ken, Recommended Practices Guide For Securing ZigBeeWireless Networks in Process Control System Environments Draft, April 2007

[SIN04] Sinem Coleri Ergen, ZigBee/IEEE 802.15.4 Summary, September10, 2004

[WIK02] http://en.wikipedia.org/wiki/IEEE_802.15.4, 23 February 2012[WIK03] http://en.wikipedia.org/wiki/ZigBee, 5 March 2012[ZIG08] ZigBee Alliance, ZiggBee Speci�cation, Section 1.1 - Protocol De-

scription, January 17, 2008

9.4 Dash7

BLAST Reference :[JPN09] JP Norair, Introduction to DASH7 Technologies, 1st Edition. 16

Mar 2009.Dash7 Origins:[D7W12] Dash7. Wikipedia. http://en.wikipedia.org/wiki/DASH7 accessed

15 march 2012.[DAW12] Dash7 Alliance. Wikipedia. http://en.wikipedia.org/wiki/DASH7_Alliance

accessed 15 march 2012.Dash7 Mode 2: [D7212] Dash7 Alliance. DASH7Mode 2 FAQ. http://www.dash7.org/index.php?option=com_content&view=article&id=162&Itemid=209

accessed 15 Mar 2012.Frequency channel information and wake up information:[ISO4P] ISO/IEC, Internation Standard ISO/IEC 18000-7:2004 - Physical

Layer.Signal penetration in walls, water and concrete: [WDB12] Dash7 Alliance.

Why is DASH7 Technology Better?. http://www.dash7.org/index.php?option=com_content&view=article&id=11&Itemid=13accessed 15 Mar 2012.

Frequency channel information in Dash7 mode 2[D7AM2] Dash7 Alliance, DASH7 Mode 2 Webinar presentation.

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Collision information[ISO4T] ISO/IEC, Internation Standard ISO/IEC 18000-7:2004 - Tag Col-

lection and Collision Arbitration.Data link layer information:[ISO4D] ISO/IEC, Internation Standard ISO/IEC 18000-7:2004 - Data Link

Layer.Range, power supply and security information: [WDT12] Dash7 Alliance.

What is DASH7 Technology?. http://www.dash7.org/index.php?option=com_content&view=article&id=9&Itemid=11accessed 15 Mar 2012.

Security Information:[ISO4S] ISO/IEC, Internation Standard ISO/IEC 18000-7:2004 - Security -

Set Password.CC430 Dash7 module: [D7M12] SAN RAMON, Calif. DASH7 Alliance and

Texas Instruments Join Forces to Bring Wireless Networking to the Mass Mar-ket. http://dash7.org/index.php?option=com_content&view=article&id=126%3Adash7-alliance-and-texas-instruments-join-forces-to-bring-wireless-networking-to-the-mass-market&catid=14%3Apress-releases&Itemid=190 accessed 15 Mar 2012. [CC412]Texas Instruments. CC430F5133 Description. http://www.ti.com/product/cc430f5133accessed 15 Mar 2012.

OpenTag library: [ODA12] Dash7 Alliance. Opentag. http://www.dash7.org/index.php?option=com_content&view=article&id=130&Itemid=193accessed 15 Mar 2012. [OWS12] OpenTagWeb Site. http://sourceforge.net/projects/opentag/accessed 15 Mar 2012.

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