Architecture and Evaluation of an Unplanned 802.11b

Mesh Network

John Bicket, Daniel Aguayo, Sanjit Biswas, and Robert Morris

MIT Computer Science and Artificial Intelligence Lab

Presented by Anuradha KadamFebruary 6, 2007

Outline

Introduction Roofnet Design Evaluation Network Use Conclusion

Introduction

Community wireless networks: Multi-hop network with nodes in chosen locations

and directional antennas Require well-coordinated groups with technical

expertise, result in good connectivity and throughput Hot-spot access points to which clients directly

connect Do not require much coordination to deploy and

operate, not as much coverage per wired connection. Best characteristics of both network types.

Introduction

Unconstrained node placement Omni-directional antennas Multi-hop routing Optimization of routing for throughput Roofnet

Roofnet Design

37 nodes spread over four square km

Each node hosted by a volunteer

Most buildings are 3 or 4 story

Hardware

Node: PC, an 802.11b card and roof-mounted omni-directional antenna

PC’s ethernet port provides Internet service to user

802.11b card based on Intersil Prism 2.5 chip-set

RTS/CTS disabled, pseudo-IBSS mode

Software and Auto-configuration

Each node: Linux, routing software implemented in Click, a DHCP server, a web server

Software is pre-installed. Node acts as like a cable or DSL modem User connects PC or laptop to the node’s ethernet

interface Node automatically configures user’s computer via

DHCP Lists itself as default IP router

Addressing

Roofnet carries IP packets inside its own header format and routing protocol

Node: chooses address whose low 24 bits are low 24 bits of node’s Ethernet address and high 8 bits are an unused class-A IP address.

Same address at both the Roofnet and IP layers These addresses are meaningful only inside

Roofnet Allocates addresses from 192.168.1.x to users NAT between Ethernet and Roofnet

Gateways and Internet Access

Each node on startup asks for an IP address as a DHCP client.

If it succeeds, the node advertises itself as an Internet gateway.

Gateway acts as NAT for connections from Roofnet to the Internet.

Node selects gateway to which it has the best route metric.

Four Internet gateways

Routing Protocol

Srcr - find highest throughput route between pair of nodes

Omnidirectional antennas give choice of links Dynamic source-routing (DSR) Each node maintains partial database of link

metrics Dijkstra’s algorithm

Routing Protocol

Link metric learning: Node includes link’s current metric in packet’s

source route DSR-style flooded query Overheard queries and responses

Routing Protocol

Combination of link-state and DSR-style on demand querying

Roofnet gateway floods dummy query Node sends data to a gateway – gateway

learns about links back to the node Nodes do not need to send flooded queries

Routing Protocol

Flooded queries often do not follow best route

Srcr solution – compute best route from database

Link conditions change leading to change in best route

Notification of failed link sent back to source New metric information sent to source

Routing Protocol

Source re-runs Dijkstra’s algorithm Better metric information:

Sources learn through dummy queries from gateways or

Unsolicited link metric information about nearby links

Routing Metric

Srcr uses Estimated Transmission Time (ETT) metric

ETT predicts total amount of time needed to send data packet along a route

Srcr chooses route with lowest ETT

Routing Metric

“Srcr predicts that a link’s highest-throughout bit-rate is the bit-rate with the highest product of delivery probability and bit-rate.”

1500-byte periodic broadcasts at each available 802.11 bit rate

Periodic minimum-size broadcasts at 1Mbps

Routing Metric

“ETT metric for a link is the expected time to send a 1500 byte packet at that link’s highest throughput bit-rate.”

ETT metric for a route is the sum of the ETTs for the route’s links.

t = 1 / Σi 1/ti

t = route’s end-to-end throughput

ti = throughput of route’s hop

Bit-Rate Selection

SampleRate: Roofnet’s algorithm to choose among 802.11b transmit bit rates.

Adjusts bit-rate as it sends data packets over a link Adjusts choice more accurately and quickly than

ETT Bases choice on actual data transmission v/s on

periodic broadcast probes Sends packets at bit-rate which currently provides

highest throughput

Evaluation

Method Basic Performance Link Quality and Distance Effect of density Mesh Robustness Architectural Alternatives Inter-hop Interference

Method

Multi-hop TCP data set 84-byte pings once per second for 10 seconds Route established and latency measured Throughput = number of bytes delivered to

receiving application 10% pairs: no working routes

Single-hop TCP data set Measure throughput on direct radio link between

pair of nodes

Method

Loss matrix data set Measure loss rate between pair of nodes 1500-byte broadcasts at each 802.11b bit-rate

Multi-hop density data set Measure throughput between fixed set of four nodes Vary number of nodes participating in routing

Some of the analyses involve simulated route throughput calculated from the single-hop TCP.

Basic Performance

Average throughput is 627 kbits/sec

Basic Performance

TCP throughput to each node from its chosen gateway

Link Quality and Distance

Srcr favors short links of a few hundred meters.

Fast, short hops are the best policy

Link Quality and Distance

Median = 0.8 Single-hop route with

40% loss can deliver more data than a two-hop route with perfect links.

Effect of density

“Mesh networks are effective only if the node density is sufficiently high.”

Simulate different size subsets of Roofnet Estimate multi-hop throughput between pairs

in the subset

Effect of density

Mesh RobustnessMost nodes have many neighbors Majority of nodes use many neighbors

Roofnet makes good use of the mesh architecture in ordinary routing

Mesh Robustness

Extent to which network is vulnerable to loss of its most valuable links

Dozens of the best links must be eliminated before throughput is reduced by half.

Mesh Robustness

Effect on throughput of cumulatively eliminating the best-connected nodes.

Best two nodes are important for performance.

Architectural Alternatives: Optimal Choice

Comparison with single-hop (access point) network.

Single-hop: 5 gateways to cover all nodes

Multi-hop forwarding provides higher average throughput

Sequence of short high quality links

Architectural Alternatives: Random Choice

If Roofnet were single-hop, 25 gateways would be required to cover all nodes.

Multi-hop routing improves connectivity and throughput.

Careful gateway choice improves throughput for both multi-hop and single-hop routing.

Inter-hop Interference

Measured multi-hop throughput lower than expected.

Concurrent transmissions on different hops collide and cause packet loss.

Inter-hop Interference

802.11 RTS/CTS mechanism: prevent collisions

RTS/CTS does not improve performance

Network Use

Measurements of user activity on Roofnet One of the four gateways monitored packets

forwarded between Roofnet and the Internet. In one 24-hr period: Average of 160 kbits/sec Gateway’s radio busy 70% of the monitoring

period Less than 1% UDP. Rest were TCP. 16 Roofnet nodes accessed the Internet

Conclusion

Ease of deployment 37 nodes in one year with little administrative

or installation effort Average throughput = 627 kbits/sec Position of internet gateways determined by

convenience Multi-hop mesh increases both connectivity

and throughput

Questions?