James Hou, Benjamin Chang, Dae-Ki Cho, Mario Gerla

BodyNets 2009April 1, 2009

Introduction

What is ZigBee?802.15.4 based standard for wireless

personal area networks (WPANs)Low cost, low power wireless mesh

networking standardUses the industrial, medical, and scientific

radio (ISM) bandsDesigned to be simpler and cheaper than

other WPANs such as Bluetooth

Why use ZigBee?Targeted for RF applications which require:

low data rate low-cost low-power (for longer battery life)mesh-networking

ZigBee used in several medical applications:Electrocardiogram (ECG)Arterial blood pressure (ABP)Central veinous pressure (CVP)VentilatorsInfusion Pumps

What’s the problem?802.11 and ZigBee share the same ISM band802.11 CSMA doesn’t accept ZigBee

transmissionsDynamic channel switching not supportedBottom line – ZigBee transmissions can be

lost as a result of local WIFI interference with no resolution scenario

ControversyTwo papers claim opposing results

ZigBee and Wireless Radio Frequency Coexistence ZigBee White Paper - June 2007

“Even in the presence of a surprising amount of interference, ZigBee devices continue to communicate effectively.”

“ZigBee contains a great many features that are designed to promote coexistence and robust operation in the face of interference.”

WLAN Interference to IEEE802.15.4 – Zensys White Paper

Concludes that WLAN causes significant interference against IEEE802.15.4 and thereby also ZigBee devices.

How Does ZigBee Promote Coexistence?Multiple Channels

16 non-overlapping channels

5 MHz apartData Rate

Able to transmit up to 250 Kbps.

DSSSUses narrow band

signalsCSMA

Scenario

WiFi-ZigBee Interference ExperimentsSetup a test scenario to verify that ZigBee

devices can experience severe packet loss as a result of 802.11 traffic

ZigBee Client A ZigBee Client B

WiFi-ZigBee Interference Experiment Results

Found that a single WiFi device can cause significant loss in Zigbee networks.

Could be further influenced by a greater number of nodesInterference is also affected by distance.

WiFi Interference % of Packets Lost in ZigBee

Light 2.56 Mbps 17.88%

Heavy 8 Mbps 87.18%

Proposed solution

Channel SwitchingZigBee switches automatically between multiple

frequencies to avoid interference.Pros:

Out of proposed solutions, easiest to implement.Cons:

May require hardware modificationIf all ZigBee devices used one channel, the single

channel could become congested.This solution will only work in tightly controlled

environments and will not work in densely populated dynamic 802.11 environments

Channel Switching could add significant complexity and as a result, cost to these devices.

Instead of modifying the ZigBee stack, somehow have surrounding 802.11 devices cooperate with ZigBee devices.

Solution: devise an external device that will periodically block 802.11 devices to allow ZigBee devices to communicate.

802.11 Periodic Blocking with RTS/CTS3rd party system could send CTS packets periodically to

802.11 devices with a long transmission duration.

Pros:Would block out 802.11 devices for a

predictable set period of time.No need to change 802.11 or ZigBee stack.External 3rd party device.

Cons:Some claim windows devices ignore RTS/CTSRequires special hardware to force an

RTS/CTSMay inadvertently block ZigBee and 802.11

networks

Developing the BlockerCreated an application on an Ubuntu machine

designed to transmit CTS messages According to WiFi standard, most WiFi devices will shut off

their radio for the duration that is set in an incoming CTS packet

Used the Lorcon library to craft CTS frames and inject or broadcast these packets to the local WLAN

Able to successfully block WiFi devices for a set duration

Verified through WiFi monitor (WildPackets AiroPeek) that most WiFi devices follow desired behavior

Can periodically blocking WiFi devices actually help reduce the interference between ZigBee devices?

Experiment

CTS Jammer

WIFI interferer(Sender)

WIFI interferer(Receiver)

ZigBee TransmitterZigBee Transmitter

Experiment Results with Periodic Blocker

Figure 1. Experiment Results with Periodic Blocker

0102030405060708090

100

Ad-Hoc Infrastructure

% o

f p

acke

ts r

ecei

ved

WiFi Interference Only

With Periodic Blocking Solution

Using our Periodic CTS BlockerHowever, in several

additional experiments, we found that the jammer only improved ZigBee performance when WiFi used high data rates(700KB/s+)

Jammer decreased performance when WiFi was transmitting at a rate lower than 300KB/s

Analysis of Periodic Blocking Solution Under Different Loads

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

% o

f p

acke

t lo

ss

WIFI 87% 17%

WIFI + Periodic Blocker 40% 35%

Heavy WiFi Traffic (8Mbps) Medium WiFi Traffic (2.56 Mbps)

Controlled blocking with RTS/CTSImplement a device that uses both WIFI and

ZigBee.When a send is requested, the device will issue

a CTS followed by a ZigBee transmissionThis will block WIFI for a short duration,

enough time to get a ZigBee transmission out.

-20

-15

-10

-5

0

5

10

15

20

25

Time

Tra

nsm

itte

r P

ow

er (

dB

m)

Blocker Signal ZigBee Signal

Experiment Setup for Controlled Blocking Solution

WIFI interferer(Sender)

WIFI interferer(Receiver)

HybridZigBee Client/WiFi Blocker

ZigBee Receiver

Experiment Results with Controlled Blocking Solution

Figure 2. Experiment Results with Proposed Solutions

0102030405060708090

100

Ad-Hoc Infrastructure

% o

f p

acke

ts r

ecei

ved

WiFi Interference Only

With Periodic Blocking Solution

With Hybrid ZigBee/WiFiControlled Blocking Solution

Future work

Gateway Will passively determine ZigBee transmission

intervals Once intervals are determined, jam 802.11 for just

those intervals Will incorporate an 802.11 chip Will support multiple ZigBee devices Ideally located on/near the ZigBee receiver Will need to support devices joining and leaving Allows current ZigBee devices to remain unchanged

while solving the interference issue

Thank you Key References:

1. Improving WLAN Performance with RTS/CTS: http://www.wi-fiplanet.com/tutorials/article.php/1445641

2. Crossbow Tech. (2005) Avoiding RF interference between WiFi and Zigbee, Crossbow Technical Report.

3. IEEE 802.15.4: a wireless communication technology for large-scale ubiquitous computing applications. In Proc of . Conference on Mobile and Ubiquitous Systems (CSMU 2006), Guimarães, June 29-30, 2006

4. Avoiding Interference in the 2.4-GHz ISM Band: http://www.wirelessnetdesignline.com/60401206;jsessionid=YMLHY4EVLHV2YQSNDLQCKHSCJUNN2JVN?printableArticle=true

5. Avoiding RF Interference Between WiFi and Zigbee: http://www.xbow.com/Products/Product_pdf_files/Wireless_pdf/ZigBeeandWiFiInterference.pdf

6. WLAN Interference to IEEE802.5.4: http://www.z-wavealliance.org/modules/iaCM-DocMan/?docId=53&mode=DE

Questions?