The 2P MAC Protocol for WiFi Mesh Networks: Design and Evaluation

Transcription

1 The 2P MAC Protocol for WiFi Mesh Networks: Design and Evaluation Bhaskaran Raman Department of CSE, IIT Kanpur Kameswari Chebrolu Department of EE, IIT Kanpur 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 1

2 Outline Background SynOp: the basis of 2P The 2P MAC protocol Feasibility constraints Evaluation Conclusions Questions 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 2

3 Background WiFi (802.11) is a cost-effective solution for long-distance (broadband) wireless Examples Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 3

4 A WiFi Network in Djurslands, Denmark 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 4

5 The Ashwini Deployment (Planned) West Godavari, A.P., India Bhimavaram 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 5

6 Digital Gangetic Plains for last-hop access within a village Point-to- Point link Sep 02 Safipur 5 Km 17.3 Km 22.5 Km Mar 04 Rasoolabad Dec 02 Sarauhan 39 Km Mar 03 Rajajipuram/Lucknow Mandhana Lodhar Apr 02 Jun Km 2.3 Km 0.9 Km 12 Km 12 Km MS3 Jun 02 IITK Nov 02 Bithoor 23 Km 37 Km Land-line access point (close to high-population density area) Jun 03 Banthar 22 Km Dec 03 Sawayajpur End to end distance ~80 Km Not to scale River Ganges 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 6

7 Point-topoint links Network Model Link-1 Radio1 To link-1 Link-2 Link-3 Radio2 Ethernet hub Radio3 Landline node Point-to-point links To link-2 To link-3 Multiple interfaces (radios) per node One directional antenna per link Single channel operation 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 7

8 SynRx, SynTx, and Mix-Rx-Tx T1 A R1 A R1 A R1 T1 T1 N N N R2 T2 T2 R2 R2 T2 B B B (a) Syn-Rx (b) Syn-Tx (c) Mix-Rx-Tx Exposed interface problem within a node: CSMA/CA ( DCF) inherently allows only one link operation per node Problems: (a) Immediate ACK, (2) CS back-off 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 8

9 SynOp: SynTx + SynRx Experimental Verification Bithoor Tower ht. = 40m B Mandhana Tower ht. = 40m A =110 o 0.9km N 8km MSSS (MS3) Tower ht. = 30m 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 9

10 The 2P MAC Protocol Two phases: each node switches between SynRx and SynTx Topology has to be bipartite B C SynTx SynRx a) Links: A-->B, A-->D, C-->B, C-->D A SynTx SynRx D b) Links: B-->A, B-->C, D-->A, D-->C Note: diagram ignores system and propogation delays How to achieve 2P on off-the-shelf hardware? Can 2P work without tight time synchronization? Relation between 2P and network topology 2P performance versus CSMA/CA 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 10

11 Achieving SynOp Goal: bypass DCF to achieve SynOp Two offending factors: immed. ACKs, CSMA backoff Avoiding immediate ACKs: Use IBSS mode IP unicast to/from MAC broadcast Avoiding CSMA backoff: Make use of diversity antenna Change antsel_rx to the unconnected antenna before transmitting Unconnected RIGHT LEFT PCMCIA/ PCI card To external antenna SynTx: antsel_rx =RIGHT SynRx: antsel_rx =LEFT 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 11

12 2P on a Single P2P Link Send pkts until B bytes sent SynTx Init B bytes in each phase SynTx+SynRx = one round Marker packet acts as a token B bytes sent, send marker pkt Marker pkt recd., or timeout The two ends of the link are in loose-synchrony How do we handle: SynRx Wait for marker pkt Temporary loss of synchrony? Link recovery or initialization? 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 12

13 The 2P Timeout Mechanism N1 N2 SynTx SynRx SynTx SynRx timeout t0 t1 Time Note: diagram ignores system and propagation delays Timer started on entering SynRx Put on hold on starting to hear Link-resync takes only one round CRC errors of non-marker pkts immaterial 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 13

14 Bumping to Avoid Repeated Timeouts If SynTx phases coincide, repeated timeouts occur Use random delay bumping to avoid this N1 N2 SynTx SynRx SynTx SynRx timeout timeout b1 t2 b2 Time Note: diagram ignores system and propagation delays 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 14

15 Communication Across Interface-Neighbours Send pkts until B bytes sent B bytes sent, send marker pkt SynTx Marker pkt recd., or timeout SynRx Wait for marker pkt Init Recv. NOTIFs from all UP ifa-nbrs, or timeout Send NOTIF to all ifa-nbrs Waiting to switch Wait for NOTIFs NOTIF msgs to indicate end of SynRx Wait for NOTIF msgs from all ifanbrs before SynTx UP/DOWN state w.r.t. each ifa-nbr Communication through sharedmemory, or ethernet 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 15

16 Topology Constraints 2P has two main constraints: Topology should be bipartite Power constraints Write a set of linear equations with variables P i SIR >= SIR reqd Simple set of heuristics for topology formation a j a i Overall gain from a i to a j = (Gain of a i ' s Tx in a j ' s dirn) (Gain of a j ' s Rx in a i ' s dirn)= Gain at angle Gain at angle 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 16

17 Evaluation of 2P Topology formation Simulation studies Implementation 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 17

18 Evaluation of Topology Creation Aspects of interest: How well does the algorithm scale? How much head-room in SIR reqd is possible? Evaluation: Using parts of the map of Durg district, Chattisgarh, India Using random topologies 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 18

19 Topology Creation on Durg District Four clusters of villages Q i (i=1..4) 31, 32, 32, and 32 villages each # links formed Q1 Q2 Q3 Q SIR_reqd (db) SIR reqd of 18-20dB easily possible 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 19

20 The Topology on Q 1 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 20

21 Simulation-based Evaluation ns-2 modification Parameters: Q 1 's 31-node topology used UDP or TCP traffic Packet size: 1400 bytes UDP: saturating CBR traffic (every 2ms) TCP: NewReno used Simulated time duration: 10sec 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 21

22 Saturation Throughput (UDP) The difference is due to SynOp 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 22

23 TCP Performance Very poor interaction between CSMA and TCP 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 23

24 Implementation-based Evaluation Implementation using HostAP v0.2.4, Linux 2.4 (also works on Linux 2.6) 2P on a single link: 6.1Mbps Less than the max. possible 6.5Mbps Overhead in antsel_rx, marker pkt, CW min being 32 2P performance on a pair of links: A <--> N1, N2 <--> B, UDP traffic Avg (SD) thrpt at A (Mbps) Avg (SD) thrpt at N1 (Mbps) Avg (SD) thrpt at N2 (Mbps) Avg (SD) thrpt at B (Mbps) 2P 2.70 (0.31) 2.06 (0.24) 2.81 (0.15) 2.81 (0.10) CSMA 2.07 (0.13) 1.13 (0.22) 1.90 (0.15) 3.11 (0.14) 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 24

25 Concluding Remarks Future directions: Can be extended to P2MP scenarios as well Provided the antenna is suitable Topology creation is an interesting aspect of study 2P good for mesh networks Reuse of spectrum for max. throughput Applicable in a wide-range of deployments Campus network, community network Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur

26 Parameters in 2P Phase duration: B bytes Large B implies lower % overhead, but higher latency For B=10KB, 6% overhead, 13ms latency For B=4.5KB, 11% overhead, 6ms latency Timeout: Lower bound: one phase duration Simulation: 1.25 times the phase duration Implementation: 25ms (kernel jitter ~ 10ms) Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur

27 Some Remarks on 2P Dummy bytes sent when no IP data Power consumption not a major concern Embedded platform ~ 4-6W at least radio ~ W only Unequal phase durations possible But not really useful for more than a single hop network RF leakages: not too many interfaces can be placed close to each other Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur

28 Power Constraints Denote by P i, the txpower at antenna A i Each tx acts as interference to all other tx Write a set of linear equations with variables Pi SIR >= SIR reqd Probably should have some head-room too Feasibility of a solution to this implies that the topology is 2P-compatible Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur a j a i Overall gain from a i to a j = (Gain of a i ' s Tx in a j ' s dirn) (Gain of a j ' s Rx in a i ' s dirn)= Gain at angle Gain at angle

29 Tree topology: Topology Formation Trivially bipartite Only one landline ==> tree is natural Only a tree is active at any time Heuristics: H1: use short links H2: avoid short angles between links H3: minimize the number of hops Mimic a natural deployment pattern Nodes close to landline connected first, then the next level Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur

30 Topology Creation on Random Scenarios SIR_reqd=14dB SIR_reqd=16dB SIR_reqd=18dB # links formed Topology number SIR reqd of 16-18dB mostly possible for up to node topologies Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur

31 Simulation-based Evaluation TeNs: Channel interference, grey regions, multiple interface support, directional antennas Further extensions: Populating the ARP table appropriately 24dBi directional antenna support MAC modifications: air propagation delay, ACK timeout LLC: sliding-window protocol Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur

32 Single Channel Operation b has only three independent channels a has twelve independent channels Four are meant for outdoor use Why only a single channel for the mesh? Mitigation of RF pollution The mesh may not be 3-edge-colourable If the frequency is licensed, more channels could imply more cost 31 Aug 2005 Bhaskaran Raman, Kameswari Chebrolu, Indian Institute of Technology, Kanpur 32