Overview

Wireless communication channels have notoriously time-varying characteristics, where the quality of received signals changes dramatically even over time durations lasting just milliseconds. The complex behavior of wireless signal propagation, particularly indoors and in presence of mobility, leads to bursty frame corruptions at the link-layer that can last over tens of milliseconds. The detrimental link-layer effects are manifested in the higher layers as packet losses and higher and more variable packet latencies that can severely impact the performance of many applications such as bulk file transfer, telephony, video streaming and other interactive wireless applications.

Through our experiments, we observe that wireless data transmissions are path-dependent and that channel conditions are weakly correlated between different paths in the wireless medium. Motivated by these observations, we design Divert, a system that employs path diversity techniques to reduce frame losses in wireless local area networks (WLAN).

In our current design, an operator of the Divert WLAN deploys wireless access points (AP) in spatially diverse locations to provide a set of alternate transmission paths between a client and the WLAN infrastructure. A Divert Monitor (DM) runs at each AP to monitor the channel condition of the wireless transmission path to a WLAN client and sends feedback to the Divert Controller (DC), which runs a fine-grained path selection algorithm. When the DC detects that the current transmission path is suffering from poor channel conditions, the DC quickly and with little overhead diverts subsequent frame transmissions to an alternate AP. The path selection is fine-grained because it can occur on a frame-by-frame basis to avoid potential burst losses in a path. Although not currently implemented in our system, Divert can also employ other techniques for coping with high frame loss rates such as packet fragmentation, varying packet sizes, forward error correction, and adjusting data transmission rates.

Some research challenges of Divert are:

• Reducing the overhead of switching transmission paths for a given client
• Designing an effective path selection algorithm that adapts to various environmental and mobility conditions
• Understanding how path diversity might interact with traffic load and affect the overall capacity of a WLAN system
• Identifying situations where Divert has the most/least impact on transmission performance
• Finding the best way to combine Divert with other loss reduction techniques (e.g., retransmissions, forward error correction, adjusting transmission rate) to minimize packet losses in the WLAN
• etc.

Current Status

We have implemented Divert using Linux PCs equipped with standard PRISM II-based 802.11b wireless interfaces. The prototype supports fine-grained path selection in the downlink direction and it works with standard, unmodified 802.11b clients that run in managed mode. Our experimental results show that Divert effectively reduces frame losses--especially for mobile clients--in realistic scenarios without consuming extra bandwidth in the wireless medium.

Our current plan is to implement fine-grained path selection in the uplink direction and also to compare the performance of Divert between 802.11a/b/g WLANs.

Papers

• Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks
  Allen Miu, Hari Balakrishnan, Can Emre Koksal
  Proc. of 11th ACM MobiCom, Cologne, Germany, August, 2005. (Best Paper Candidate)

• Divert: Fine-grained Path Selection for Wireless LANs
  Allen Miu, Godfrey Tan, Hari Balakrishnan, John Apostolopoulos
  Proc. of 2nd ACM MobiSys, Boston, MA, June 2004, pp. 203-216.

• Low-Latency Wireless Video Over 802.11 Networks Using Path Diversity
  Allen Miu, John Apostolopoulos, Wai-tian Tan, Mitchell Trott
  Proc. of IEEE ICME, Baltimore, MD, July 2003, vol. 2, pp. 441-444.

Talks

• Presentation at MobiCom 2005 (ppt, 1178kB)

• Presentation at MobiSys 2004 (ppt, 578kB)

• Presentation at ICME 2003 (ppt, 2.8MB)

Data Traces

Traces from our Mobisys 2004 paper.

Related Work

Please also see papers for additional useful references.

Tools and Drivers

• Wireless Tools for Linux - tools + a wealth of resources to get WLANs working on Linux
• Kismet - a general purpose sniffer app
• HostAP Driver - for Prism-based 802.11 chipsets
• Jun Sun's Mini-howto - on flashing Intersil Prism Chipsets
• Madwifi driver - for Atheros-based 802.11a/b/g chipsets
• TI acx100 driver - the acx100 chipset seems to support fairly reliable ACK sniffing!
• Criag's ACX100 HowTo - lots of up-to-date info to get acx100 up and running

Research Projects

• Mobile Communication Research at Microsoft - mesh networks, multi-radio unification protocol for 802.11, wake on wireless, and a bunch of other interesting wireless projects
• Christian Hoene's work - 802.11 measurements and traces
• Minho Shin's work - handoff latency and optimization in 802.11
• Micro Ad Hoc Networks - use ad hoc mode to communicate with a nearby client and bypass the AP whenever possible
• Mixed Mode WLAN - similar to Micro Ad Hoc Networks
• Pablo Rodriguez's work - parallel streaming and MAR commuter router infrastructure
• DIRAC - software router framework designed for wireless adaptation