Wireless Mesh Networks Unplanned Community Mesh Networks A. Zubow “Architecture and Evaluation of an Unplanned 802.11b Mesh Network”, J. Bicket, D. Aguayo, S. Biswas, R. Morris, Mobicom 2005

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

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

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 802.11b card running in ad-hoc mode and a omni-directional antenna

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 802.11b channel Non-standard “pseudo-IBSS” mode Similar to standard 802.11b IBSS (ad hoc) Omit beacon and BSSID (network ID)

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 192.168.1.x via DHCP and use NAT between the Ethernet and Roofnet

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

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

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

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

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):

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

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)

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 802.11 detects the losses with its ACK mechanism and resends the packets Links used by Srcr at the bit-rate chosen by SampleRate.

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

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

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

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

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%

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

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.

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:

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

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

Resources “Architecture and Evaluation of an Unplanned 802.11b Mesh Network”, J. Bicket, D. Aguayo, S. Biswas, R. Morris, Mobicom 2005