1 / 14

How 802.11 MAC interacts with Capacity of Ad-hoc Networks – Interference problem

How 802.11 MAC interacts with Capacity of Ad-hoc Networks – Interference problem. Capacity of Wireless Networks – Part 2 Page 1. 802.11 MAC Background. Use of 802.11 DCF (Distributed Coordination Funktion) access method used in ad-hoc mode four-way (RTS/CTS/Data/Ack) exchange

avi
Download Presentation

How 802.11 MAC interacts with Capacity of Ad-hoc Networks – Interference problem

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. How 802.11 MAC interacts with Capacity of Ad-hoc Networks –Interference problem Capacity of Wireless Networks – Part 2 Page 1

  2. 802.11 MAC Background • Use of 802.11 DCF (Distributed Coordination Funktion) • access method used in ad-hoc mode • four-way (RTS/CTS/Data/Ack) exchange • nearly CSMA/CA • binary exponential backoff scheme Capacity of Wireless Networks – Part 2 Page 2

  3. MAC Interactions • described by simulations with ns • tuned to model 2 Mbps data rate • transmission range 250 m • interfering range 550 m • only stationary nodes separated by 200 m • 5 runs lasting 300 sec. Capacity of Wireless Networks – Part 2 Page 3

  4. Single Cell Capacity • pattern: • 200 m² cell • nodes sends as fast as allowed • random destination • min. contention on 2-node-cell • overhead reduces data throughput-limit to1,7 Mbps Capacity of Wireless Networks – Part 2 Page 4

  5. Capacity of a Chain of Nodes (1) • Assumption:no interferences causedby non-neighbornodes (beyond 250 m) • channel utilization of ⅓ • However:with 550 m interfering range • expected channel utilization of ¼ Capacity of Wireless Networks – Part 2 Page 5

  6. Capacity of a Chain of Nodes (2) • Data flow formnode 1 to last node • max. throughput of1,7 Mbps at 2-node-chain • longer chains approach autilization of0,25 Mbps ≈ 1/7*1,7 Mbps Capacity of Wireless Networks – Part 2 Page 6

  7. Capacity of a Chain of Nodes (3) • How is the discrepancybetween ¼ and 1/7 caused? • achieving the max. through-put at 0,41 Mbps deliveredby controlled send rates • very close to1,7 Mbps* ¼ = 0,425 • peak rate isn't maintained by802.11, scheduling greedyad-hoc-forwarding senders Capacity of Wireless Networks – Part 2 Page 7

  8. Capacity of a Chain of Nodes (4) • Why fails 802.11 to achieve the optimum chain schedule? • node's ability to send is affected by its experienced competitions • a chain source injects more packets than subsequent nodes can forward • eventually dropped at forwarding nodes causes resends • decreasing throughput since it prevents transmissions of subsequent nodes • backoff window can dramatically increase Capacity of Wireless Networks – Part 2 Page 8

  9. Real Hardware Verification • 6 radios configured to mimic simulation parameters • matches fairly • no major errors • average difference only 6%(1500-byte packet) Capacity of Wireless Networks – Part 2 Page 9

  10. Capacity of a Regular Lattice Nettwork (1) • 200 m from its radio neighbors • every third chain of left scenario can operate withoutinter-chain interference • expected flow-throughput of ¼*⅓ • 1/12 * 1,7 Mbps = 0,14 Mbps (1500-byte packet) Capacity of Wireless Networks – Part 2 Page 10

  11. Capacity of a Regular Lattice Nettwork (2) • Per Flow Throughput settelsat about 0,1 Mbps • inefficiencies found inchain scenarios arestill present Capacity of Wireless Networks – Part 2 Page 11

  12. Cross Traffic in a Lattice (1) • vertical and horizontal flows • theoretical schedule: • one time cycle operating all verticals • and the horizontal in the next • assumed: each flow see half of its normal throughput • since there are twice as many flows, the overall capacity is the same • However, 802,11 may not schedule this efficently • caused by head-of-queue blocking Capacity of Wireless Networks – Part 2 Page 12

  13. One-hop Throughput under different Topologies • the sum total of data-bits send by all nodes per second • including forwarded data-bits • excluding non-successfullysink-arriving data • a constant factor decreasecross-traffic-one-hopcapacity to uncrossing ones ~ node number Capacity of Wireless Networks – Part 2 Page 13

  14. Random Traffic in Random Layout • Simulating realistic scenarios • non-uniformly placed nodes in a square • each node sends to randomly chosen sink • adjustable send rates to keep total drop rate < 20% • 3*lattice-node-density to guarantee conectivity • similar one-hop capacity to that of cross-traffic-nets • However: irregular placement leads to free areas -> lowers capacity • random destination -> tendential routing through center • resulting in a capacity limitation by the throughput of the center Capacity of Wireless Networks – Part 2 Page 14

More Related