Packet-level diversity - from theory to practice: an 802.11-based experimental investigation

Authors: Evangelos Vergetis, Eric Pierce, Marc Blanco, and Roch Guerin ´

Complete Citation

Vergetis, E., Pierce, E., Blanco, M., and Guérin, R. 2006. Packet-level diversity - from theory to practice: an 802.11-based experimental investigation. In Proceedings of the 12th Annual international Conference on Mobile Computing and Networking (Los Angeles, CA, USA, September 23 - 29, 2006). MobiCom? '06. ACM Press, New York, NY, 62-73. DOI= http://doi.acm.org/10.1145/1161089.1161098

Abstract

Packet-level diversity, or distributing packet transmissions over multiple, diverse channels, offers benefits in improving communication performance and robustness to channel variations. Previous works have analyzed and quantified those benefits, and developed transmission policies to realize them. However, translating those benefits into practice still faces numerous challenges from uncertainty in the adequacy of the channel models used to develop policies, to implementation dificulties in realizing the precise transmission schedules they mandate. This work is a first step in assessing what remains of those benefits once confronted with such practical challenges. Our investigation is carried out over an 802.11 testbed, where diversity is realized through the different frequency bands available for transmissions between hosts and access points. We use the testbed to evaluate the impact of transmission policies, channel characteristics, channel correlation, and various end-system constraints that affect our ability to precisely control transmissions timing. Our investigation reveals that in spite of the many gaps that exist between theory and practice, packet-level diversity still provides a simple solution to improve transmission performance and robustness across a broad range of configurations.

Annotations

This paper is a good reference paper on different methods of using channel diversity in order to improve packet delivery in the wireless medium. The authors also create a test-bed testing two popular methods of channel diversity (probabalistic and deterministic) under a few different scenarios. Additionally, this paper also addresses common issues that occur during actual implementation of said schemes, in order for the channel diversity to be of benefit.

Some Highlights:

• Different wireless cards will vary in the amount of time it takes to transmit a packet after it is received from the OS.
  • This means that alternating packets to two different wireless NICs will not neccessarily result in alternating patterns of packet transmission between the two mediums.
• To gain the benefits higher packet delivery of using multiple channels one of two situation must exist:
  • a single flow is transmitted across the various NICs in lock-step. Meaning that one packet on one NIC transmits, then the other NIC, then the other. Do not overlap transmissions.
  • multiple flows interleaved between the multiple channels.
• for a system with only one NIC, channel hopping is possible, but:
  • 80 us for channel switch
  • up to 25 - 40 ms delay for resynchronization purposes.

-- DavidSalyers - 15 Aug 2007