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

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