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Sangwook Ha (swha@stanford) Fouad A. Tobagi (tobagi@stanford)

Coexistence of Multiple Links in a Wireless Network: Interactions between the Physical Layer and the MAC Layer. Sangwook Ha (swha@stanford.edu) Fouad A. Tobagi (tobagi@stanford.edu) Part of Publication in Elsevier Journal Ad Hoc Networks 2007. STA 0. STA 1. d. data. data. AP 0. AP 1.

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Sangwook Ha (swha@stanford) Fouad A. Tobagi (tobagi@stanford)

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  1. Coexistence of Multiple Links in a Wireless Network:Interactions between the Physical Layer and the MAC Layer Sangwook Ha (swha@stanford.edu) Fouad A. Tobagi (tobagi@stanford.edu) Part of Publication in Elsevier Journal Ad Hoc Networks 2007 Wireless Mesh Networks (AIIT)

  2. STA0 STA1 d data data AP0 AP1 45° s A Simple Scenario:Video Streaming Even with this simple case there are many parameters regarding: Topology, Wireless Channel, Physical and MAC Layers, and Application Characteristics. Wireless Mesh Networks (AIIT)

  3. STA1 STA0 7 m AP1 AP0 s Video Throughput(Phy 12 Mbps, Video 8 Mbps, d = 7 m, ED = -95 dBm) AP1 → STA1 Throughput (Mbps) AP0 → STA0 Distance s between AP0 and AP1 (m) Wireless Mesh Networks (AIIT)

  4. Factors • Blocking –802.11 Carrier Sense Multiple Access prevents simultaneous packet transmissions from both APs • MAC layer behavior –Interframe Spacing depends on whether the last detected packet is received correctly or not • Interference –Packet reception corrupted due to simultaneous transmission (no blocking) Wireless Mesh Networks (AIIT)

  5. Extended Interframe Space802.11 MAC Protocol Normal frame exchange SIFS DIFS CW Data ACK Data Frame error EIFS SIFS DIFS CW Expected ACK Data Data SIFS, DIFS, EIFS: Interframe space / CW: Contention Window (random) EIFS is used to protect an eventual ACK transmitted by the intended receiver. Wireless Mesh Networks (AIIT)

  6. STA 0 STA 1 ACK AP 0 AP 1 SIFS DIFS+CW AP1 ACK STA1 EIFS AP0 EIFS Effect Channel is captured by AP1 more frequently. Wireless Mesh Networks (AIIT)

  7. STA 0 STA 1 data data AP 0 AP 1 AP0 STA0 below ED threshold AP1 Interference Effect Interference from AP1 causes high probability of error at STA0. Wireless Mesh Networks (AIIT)

  8. STA1 STA0 7 m AP0 AP1 s Impact of MAC Layer Behavior • EIFS at AP0 and AP1 results in unequal throughput for separations (s) of 5-30 m MAC layer behavior (EIFS) interference AP1 → STA1 Throughput (Mbps) no blocking (AP0-AP1) AP0 → STA0 partial coordination no coordination full coordination Distance s between AP0 and AP1 (m) • PHY Data Rate = 12 Mbps, Video Rate = 8 Mbps, d = 7 m, ED = -95 dBm Wireless Mesh Networks (AIIT)

  9. MAC layer behavior (EIFS) interference AP1 → STA1 Throughput (Mbps) no blocking (AP0-AP1) AP0 → STA0 partial coordination no coordination full coordination STA1 STA0 Distance s between AP0 and AP1 (m) 7 m AP0 AP1 s Impact of Interference • Interferenceat STA0 results in more errors for separations (s) of 20-25 m • Backoff at AP0 results in lower rate of transmissions • PHY Data Rate = 12 Mbps, Video Rate = 8 Mbps, d = 7 m, ED = -85 dBm Wireless Mesh Networks (AIIT)

  10. References Part of this talk published as: • F. A. Tobagi, A. K. Vyas, S. Ha, O. Awoniyi, “Interactions between the Physical Layer and Upper Layers in Wireless Networks,” in Elsevier Journal Ad Hoc Networks, Volume 5, Issue 8, Pages 1208-1219, November 2007. • F. A. Tobagi, A. K. Vyas, S. Ha, O. Awoniyi, “Interactions between the Physical Layer and Upper Layers in Wireless Networks, and Their Impact on Applications,” in Proc. Med-Hoc-Net 2006. The Fifth Annual Mediterranean Ad Hoc Networking Workshop, Pages 184-191, Lipari, Sicily (Italy), June 14-17, 2006. Wireless Mesh Networks (AIIT)

  11. Thank You !! Wireless Mesh Networks (AIIT)

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