The 2P MAC for Rural WiFi Networks

Transcription

1 The 2P MAC for Rural WiFi Networks Bhaskaran Raman Department of CSE, IIT Kanpur Presentation at Cutting-Edge, 08 April 2005

2 Outline Why WiFi for Rural Internet? A Cost Analysis, Network Architecture Performance of the CSMA/CA MAC The 2P MAC Conclusions

3 < ;;;: 97 / < ;;;: 97 8/7 < ;;;: 97 8/7 $$$% $$- $$, LM $$* $$% $ Communication Revolution: Myth or Reality? Depends on who 5 / > =/ / / / you ask... K IJ HG F B E D B ACB > 75% of the world P ON N $$ + yet to see any $$ ) communication $$ ( ' $$ Cell-phone & $$ revolution in India: "! # restricted to the metros

4 What are the Barriers? Value Pricing of Cellular Technology Operations 1/3 People in developed Cost 1/3 1/3 Spectrum Few rs per minute economies, and metros are willing to pay this price because voice is a very high value application Cost amortized over seven years Equipment

5 WiFi: A Cost-Effective Technology Equipment: cost priced Open, inter-operable standard Competitive mass production Chip-sets: $25-30, Access- Points: $ , PCMCIA cards: $ Spectrum is free!!!

6 How to Use WiFi for Rural Internet? RF cable AP Antennae (high-gain directional) Point-to-point wireless link AP Antenna tower Ethernet cable (to hub/pc)

7 Digital Gangetic Plains Testbed Info kiosk Safipur BSNL tower Info kiosk MS3 Bithoor Mandhana BSNL tower 5.5 Km 23 Km Lodhar 2.5 Km IITK 23 Km Sawayajpur 16 Km 37 km Sarauhan IISTEM Banthar 18 Km Ajgain 22 Km Dhoura River Ganges 38.2 Km Kanpur-Lucknow Highway Lucknow End to end distance 85 Km Rajajipuram BSNL Tower Lucknow Airport Link Established Link experiments In progress Link experiments to be conducted Internet access station Mohanpur Amaur

8 Outline Why WiFi for Rural Internet? A Cost Analysis, Network Architecture Performance of the CSMA/CA MAC The 2P MAC Conclusions

9 What are the Costs? Antenna tower (30m) Rs. 70K devices Rs. 4K Tower cost is dominant Alright to have multiple radios per location in the network

10 Network Architecture Point-topoint links Radio1 To link-1 Link-1 Link-2 Link-3 Radio2 Ethernet hub Radio3 Landline node A generic mesh network Multiple radios per node One directional antenna per-link To link-2 To link-3

11 Outline Why WiFi for Rural Internet? A Cost Analysis, Network Architecture Performance of the CSMA/CA MAC The 2P MAC Conclusions

12 The CSMA/CA MAC Designed for heavy reuse (in the free spectrum) Carrier-Sense Multiple Access with Collision Avoidance Good for situations of random contention For example, several users in a room

13 % % % ) ) ) ( ( ( in a Multi-Hop Setting "$# + "$# * "$#! &' &' &' % % % Exposed node problem

14 $!#" Multiple Interfaces, Directional Antennae 90 o 0 o 180 o 270 o

15 $ $ The Exposed Interface Problem $ " "! " "!#"

16 Outline Why WiFi for Rural Internet? A Cost Analysis, Network Architecture Performance of the CSMA/CA MAC The 2P MAC Conclusions

17 SynOp: SynRx + SynTx Links at a node operating simultaneously, synchronously (on the same channel) Is this feasible? Yes, under certain conditions P R1 P R2 SL SIR reqd P R2 P R1 SL SIR reqd P R1 P R2 SL SIR reqd P R1 P R2 SL SIR reqd

18 SynOp Feasibility Write a set of linear equations Powers of transmission are variables Solve the linear equations Feasible ==> synop possible throughout the network Feasible for many practical cases

19 The 2P MAC Protocol 2-P: each node switches between SynRx and SynTx When a node is in SynRx, its neighbours are in SynTx, and vice versa B bits V 1 V 2 B bits SynRx + SynTx = 1 round Require a bipartite topology

20 2P vs CSMA/CA: UDP

21 2P vs CSMA/CA: TCP

22 Some Remarks on 2P 2-P can be implemented without tight global synchronization! Wait until end of transmission from all neighbours, switch immediately Wait until end of transmission to all neighbours, switch immediately Timeout mechanism to deal with packet losses Firmware, proprietary driver software (e.g. Atheros), or driver-level implementation possible Host-AP modifications tested for single-link Other issues: topology, TCP performance

23 Conclusions WiFi (802.11) is cost-effective But not performance effective Poor spectral efficiency Bad performance in mesh networks Performance can be partially fixed Do better scheduling than CSMA/CA Further issues: Performance of VoIP, Video 2P extension for a point-to-multipoint scenario