1. CS 414 Indian Institute of Technology, Bombay CS 414 Long-distance, Outdoor 802.11 Wireless Networks

2. CS 414 Indian Institute of Technology, Bombay 802.11 ● Originally designed as wireless replacement for wired LANs – Indoor, office, enterprise environments – Mostly used in infrastructure mode – Last-hop wireless access ● Typical range – 802.11a: 10m to 100m – 802.11b & 802.11g: 30m to 100 m

3. CS 414 Indian Institute of Technology, Bombay Ad-hoc/ IBSS Networks ● Multi-hop networks ● No central control – Wireless backbone ● Popular for – Community networks – Campus networks – Rural access networks

4. CS 414 Indian Institute of Technology, Bombay MIT RoofNet

5. CS 414 Indian Institute of Technology, Bombay Digital Divide Source: Turning 802.11 inside-out (2003) by Pravin Bhagwat, Bhaskaran Raman, Dheeraj Sanghi In HotNets-II ● How to improve communication infrastructure?

6. CS 414 Indian Institute of Technology, Bombay Case for Long Distance 802.11 ● Mature technology – Widespread deployment & acceptance – Open standard (& driver implementations) – 802.11 hardware prices lowest ● compared to wired, cellular technology (?) ● 802.11 for low-cost long-distance access

7. CS 414 Indian Institute of Technology, Bombay Source: Turning 802.11 inside-out (2003) by Pravin Bhagwat, Bhaskaran Raman, Dheeraj Sanghi In HotNets-II

8. CS 414 Indian Institute of Technology, Bombay DGP Testbed ● Rural setting (Flat terrain) ● Wirelss access network ● Last-hop AP-based wireless access ● Point-to-point wireless links ● Antennas – directional and omni-directional – towers, masts...

9. CS 414 Indian Institute of Technology, Bombay

10. Issues in Long-distance 802.11 ● PHY – Path Loss Models ● How well does Free Space Path Loss fit? – Multipath effects over long distance – Power

11. CS 414 Indian Institute of Technology, Bombay 802.11 MAC Issues ● MAC designed for indoor operation ● Propagation delay is not negligible – For 15 km, progation delay is 50us – Default ACK timeout and slot times not applicable ● Slot Times (typically 20 us) tailed for indoors ● Hidden node mitigation has high overhead – RTS/CTS: 100/150 us overhead for a 1000 byte packet aat 11 Mbps (~700 us) ● No arbitrary contention – TDMA-like MAC may be better

12. CS 414 Indian Institute of Technology, Bombay Radiation Pattern Directional Antenna

13. CS 414 Indian Institute of Technology, Bombay Implication of Multiple Interfaces ● Is Syn-Rx, Syn-Tx, Mix-Tx-Rx possible? – CSMA lets only one link operate in (b) – Intereference in (c) and (a)

14. CS 414 Indian Institute of Technology, Bombay Mix-Tx-Rx ● T1's transmission interferes with R2's reception ● Not feasible with sidelobes

15. CS 414 Indian Institute of Technology, Bombay Syn-Rx ● T1 is interference at R2 ● T2 is interference at R1 ● Feasible if SIR greater than a threshold – ~10 dB (for.11b)

16. CS 414 Indian Institute of Technology, Bombay Syn-Tx ● Interference possible at A and B ● Satisfy same angle separation constraint ● Traditional CSMA will backoff with Syn-Tx

17. CS 414 Indian Institute of Technology, Bombay

18. SynOp: SynRx, SynTx Feasible Not Feasible

19. CS 414 Indian Institute of Technology, Bombay 2P: MAC for Long distance Links ● Shortcomings of traditional MAC – RTS/CTS overhead – Propagation delay timings not accounted – Backoff during Syn-Tx – ACK results in switch to Mix-Tx-Rx ● Motivates a TDMA-style MAC – 2P (two-phase)

20. CS 414 Indian Institute of Technology, Bombay 2P: MAC for Long distance Links ● Each node switches between SynRx and SynTx ● Defined timings (slots) for each phase – Round: SynRx phase + SynTx phase ● Condition on network topology? – Bipartite

21. CS 414 Indian Institute of Technology, Bombay 2P Issues ● Node synchronization ● Bipartite topology requirement ● Performance comparison with CSMA

22. CS 414 Indian Institute of Technology, Bombay 2P State Diagram for each Link ● Marker packet acts as synchronization reference ● How to handle marker loss?

23. CS 414 Indian Institute of Technology, Bombay 2P Timeout Mechanism ● Timer start in entering Syn-Rx ● Timeout after fixed interval, switch mode ● Resync in one-round

24. CS 414 Indian Institute of Technology, Bombay Bipartite Topology ● Design network as a tree – trivially bipartite – no multi-path routing

25. CS 414 Indian Institute of Technology, Bombay

30. 2P Limitations

31. CS 414 Indian Institute of Technology, Bombay 2P Limitations ● Fixed slot sizes – A -> B: x%, B->A: (1-x)% – What if traffic pattern changes? ● Cannot easily adapt, need to recode! – Link capacity under-utilized ● Tree-based setup – Is this always desirable/optimal?

32. CS 414 Indian Institute of Technology, Bombay 2P Limitations ● Fork Topology ● What should be slot size if only A sending? ● If A & B sending? ● If A, B & C sending? ● If A,B, C &D sending?

33. CS 414 Indian Institute of Technology, Bombay 2P Limitations ● Overlapping Transmissions – A->C: 20 ms – A->B: 6 ms – B->D: ?? ● After 6 ms? ● After 20 ms (in 2P)

34. CS 414 Indian Institute of Technology, Bombay Addressing 2P Limitations ● JazzyMac – Adaptive slots – Generalized Topologies – Basic idea (on board)