1. Wireless Mesh Networks
Unplanned Community Mesh Networks
A. Zubow
“Architecture and Evaluation of an Unplanned b Mesh Network”,
J. Bicket, D. Aguayo, S. Biswas, R. Morris, Mobicom 2005

2. Introduction Community wireless network
Share a few wired Internet connections
2 approaches to constructing community networks:
Multi-hop network
Nodes in chosen locations
Directional antennas
Requires well-coordination
Great coverage
Access point
Clients directly connected
Access points operate independently
Do not require much coordination
Small coverage

3. Introduction Ambitious vision for community networks
Operate without extensive planning or central management
Provide wide coverage and acceptable performance
Design decisions in this paper
Unconstrained node placement
Omni-directional antennas
Multi-hop routing
Optimization of routing for throughput in a slowly changing network
Risks
Small radio ranges, low-quality links (TCP), interference (inside + outside of the network)
Purpose of this paper
Evaluate unplanned mesh architecture (Roofnet case study)
Describe Roofnet’s end-to-end characteristics

4. Introduction What is Roofnet?
“Mesh networking" technology developed by MIT
Town-wide wireless network
Automatically calculates the best path and continuously monitors the network
Each Roofnet node is a small computer running Linux with an b card running in ad-hoc mode and a omni-directional antenna

5. Roofnet Design Deployment Hardware
Over an area of about four square kilometers in Cambridge, MA
Most nodes are located in buildings
3~4 story apartment buildings
8 nodes are in taller buildings
Each Roofnet node is hosted by a volunteer user (no planning)
Hardware
PC, omni-directional antenna, hard drive …
802.11b card
RTS/CTS disabled
Share the same b channel
Non-standard “pseudo-IBSS” mode
Similar to standard b IBSS (ad hoc)
Omit beacon and BSSID (network ID)

6. Roofnet Design Software and Auto-Configuration
Linux, routing software (Click toolkit), DHCP server, web server …
Automatically solves these problems
Allocating addresses
Finding a gateway between Roofnet and the Internet
Choosing a good multi-hop route to that gateway
Addressing
Roofnet carries IP packets inside its own header format and routing protocol
Assigns addresses automatically
Only meaningful inside Roofnet, not globally routable
The address of Roofnet nodes
Low 24 bits are the low 24 bits of the node’s Ethernet address
High 8 bits are an unused class-A IP address block
The address of hosts
Allocate x via DHCP and use NAT between the Ethernet and Roofnet

7. Roofnet Design Software and Auto-Configuration (Cont’d)
Gateways and Internet Access
A small fraction of Roofnet users will share their wired Internet access links
Nodes which can reach the Internet
Advertise itself to Roofnet as an Internet gateway
Acts as a NAT for connection from Roofnet to the Internet
Other nodes
Select the gateway which has the best route metric
Roofnet currently has four Internet gateways

8. Roofnet Design Routing Protocol Srcr Learning fresh link metrics
Find the highest throughput route between any pair of Roofnet nodes
Source-routes data packets like DSR
Maintains a partial database of link metrics
Learning fresh link metrics
Forward a packet
Flood to find a route
Overhear queries and responses
Finding a route to a gateway
Each Roofnet gateway periodically floods a dummy query
When a node receives a new query, it adds the link metric information
The node computes the best route
The node re-broadcasts the query
Send a notification to a failed packet’s source if the link condition is changed

9. Roofnet Design Routing Metric Bit-rate Selection
ETT (Estimated Transmission Time) metric
Srcr chooses routes with ETT
Predict the total amount of time it would take to send a data packet
Take into account link’s highest-throughput transmit bit-rate and delivery probability
Each Roofnet node sends periodic 1500-byte broadcasts
Bit-rate Selection
802.11b transmit bit-rates
1, 2, 5.5, 11 Mbits/s
SampleRate
Judge which bit-rate will provide the highest throughput
Base decisions on actual data transmission
Periodically sends a packet at some other bit-rate

10. Evaluation Method Multi-hop TCP Single-hop TCP Loss matrix
15 second one-way bulk TCP transfer between each pair of Roofnet nodes
Single-hop TCP
The direct radio link between each pair of routes
Loss matrix
The loss rate between each pair of nodes using 1500-byte broadcasts
Multi-hop density
TCP throughput between a fixed set of four nodes
Varying the number of Roofnet nodes that are participating in routing

11. Evaluation Basic Performance (Multi-hop TCP)
The routes with low hop-count have much higher throughput
Multi-hop routes suffer from inter-hop collisions
Theory (lossless links):

12. Evaluation Basic Performance (Multi-hop TCP)
TCP throughput to each node from its chosen gateway
Round-trip latencies for 84-byte ping packets to estimate interactive delay

13. Evaluation Link Quality and Distance (Single-hop TCP, Multi-hop TCP)
Most available links are between 500m and 1300m and 500 kbits/s
Srcr
Use almost all of the links faster than 2 Mbits/s and ignore majority of the links which are slower than that
Fast short hops are the best policy
Throughput/distance of all available links (left); only links used in some route (right)

14. Evaluation Link Quality and Distance (Multi-hop TCP, Loss matrix)
Median delivery probability is 0.8
1/4 links have loss rates of 50% or more
detects the losses with its ACK mechanism and resends the packets
Links used by Srcr at the bit-rate chosen by SampleRate.

15. Evaluation Effect of Density (Simulated from Single-hop TCP)
Mesh networks are only effective if the node density is sufficiently high
For each subset size n, a random set of n Roofnet nodes are selected. An estimate of the multi-hop throughput between every pair in the subset is computed, using only members of the subset as potential forwarders.
More than 1 kbytes/s

16. Evaluation Effect of Density (Simulated from Single-hop TCP)
Network only starts to approach all-pairs connectivity when there are more than 20 nodes, corresponding to a density of about five nodes per square kilometer.
A denser network offers a wider choice of short high-quality links though using them causes routes to have more hops

17. Evaluation Mesh Robustness (Loss matrix, Multi-hop TCP)
The number of potentially useful neighbors each node has
Neighbor is defined as a node to which the delivery probability is 40% or more
The majority of nodes use many neighbors
Roofnet makes good use of the mesh architecture in ordinary routing

18. Evaluation Mesh Robustness (Simulated from Single-hop TCP)
The extent to which the network is vulnerable to the loss of its most valuable links
The dozens of the best links must be eliminated before throughput is reduced by half

19. Evaluation Mesh Robustness (Multi-hop TCP)
The y axis shows the average throughput among four particular nodes
The best-connected two nodes are important for performance
Losing both decreases the average throughput by 43%

20. Evaluation Architectural Alternatives
Maximize the number of additional nodes with non-zero throughput to some gateway
Ties are broken by average throughput
Optimally chosen gateways
Randomly chosen gateways

21. Evaluation
Comparison of the two tables shows that careful gateway choice increases throughput for both multi-hop and single-hop routing.
For five or fewer gateways, even randomly chosen multi-hop gateways provide better performance than carefully chosen single-hop gateways.
For larger numbers of gateways, however, carefully chosen single-hop gateways are better than randomly-chosen multi-hop gateways.

22. Evaluation Inter-hop Interference (Multi-hop TCP, Single-hop TCP)
Concurrent transmissions on different hops of a route collide and cause packet loss
Each point on the graph represents one node pair. The y-value of the point shows the measured throughput between that pair of nodes. The x-value shows the throughput predicted along that route by Equation 1 and the single-hop TCP data-set.
Eq.1:

23. Network Use Measurements of user activity
One of the four Roofnet gateways monitors the packets forwarded between Roofnet and the Internet
In one 24-hour period
Average of 160kbits/s between Roofnet and the Internet
Data was 94%
48% of the data traffic was to or from nodes one hop form the gateway, 36% two hops
The gateway’s radio was busy for about 70% of the monitoring period
Almost all of the packets were TCP, less than 1% were UDP
30% of the total data transferred was P2P file sharing program

24. Conclusions The network’s architectures favors
Ease of deployment
Omni-directional antennas
Self-configuring software
Link-quality-aware multi-hop routing
Evaluation of network performance
Average throughput between nodes is 627kbits/s
Well served by just a few gateways whose position is determined by convenience
Multi-hop mesh increases both connectivity and throughput

25. Resources
“Architecture and Evaluation of an Unplanned b Mesh Network”, J. Bicket, D. Aguayo, S. Biswas, R. Morris, Mobicom 2005