1. James Hou, Benjamin Chang, Dae-Ki Cho, Mario Gerla BodyNets 2009 April 1, 2009

2. Introduction

3. What is ZigBee? 802.15.4 based standard for wireless personal area networks (WPANs) Low cost, low power wireless mesh networking standard Uses the industrial, medical, and scientific radio (ISM) bands Designed to be simpler and cheaper than other WPANs such as Bluetooth

4. 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) Ventilators Infusion Pumps

5. What’s the problem? 802.11 and ZigBee share the same ISM band 802.11 CSMA doesn’t accept ZigBee transmissions Dynamic channel switching not supported Bottom line – ZigBee transmissions can be lost as a result of local WIFI interference with no resolution scenario

6. Controversy Two 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.

7. How Does ZigBee Promote Coexistence? Multiple Channels 16 non-overlapping channels 5 MHz apart Data Rate Able to transmit up to 250 Kbps. DSSS Uses narrow band signals CSMA

8. Scenario

9. WiFi-ZigBee Interference Experiments Setup 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

10. 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 nodes Interference is also affected by distance. WiFi Interference% of Packets Lost in ZigBee Light2.56 Mbps17.88% Heavy8 Mbps87.18%

11. Proposed solution

12. Channel Switching ZigBee switches automatically between multiple frequencies to avoid interference. Pros: Out of proposed solutions, easiest to implement. Cons: May require hardware modification If 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

13. 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.

14. 802.11 Periodic Blocking with RTS/CTS 3 rd 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 3 rd party device. Cons: Some claim windows devices ignore RTS/CTS Requires special hardware to force an RTS/CTS May inadvertently block ZigBee and 802.11 networks

15. Developing the Blocker Created 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?

16. Experiment

17. CTS Jammer WIFI interferer (Sender) WIFI interferer (Receiver) ZigBee Transmitter

18. Experiment Results with Periodic Blocker

19. Using our Periodic CTS Blocker However, 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

20. Controlled blocking with RTS/CTS Implement a device that uses both WIFI and ZigBee. When a send is requested, the device will issue a CTS followed by a ZigBee transmission This will block WIFI for a short duration, enough time to get a ZigBee transmission out.

21. Experiment Setup for Controlled Blocking Solution WIFI interferer (Sender) WIFI interferer (Receiver) Hybrid ZigBee Client/ WiFi Blocker ZigBee Receiver

22. Experiment Results with Controlled Blocking Solution

23. Future work

24. 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

25. Thank you Key References: 1. Improving WLAN Performance with RTS/CTS: http://www.wi- fiplanet.com/tutorials/article.php/1445641http://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=YMLHY4EV LHV2YQSNDLQCKHSCJUNN2JVN?printableArticle=true 5. Avoiding RF Interference Between WiFi and Zigbee: http://www.xbow.com/Products/Product_pdf_files/Wireless_pdf/ZigBeea ndWiFiInterference.pdf 6. WLAN Interference to IEEE802.5.4: http://www.z- wavealliance.org/modules/iaCM-DocMan/?docId=53&mode=DEhttp://www.z- wavealliance.org/modules/iaCM-DocMan/?docId=53&mode=DE

26. Questions?