Tropos MetroMesh Proven: Metro-Scale Wi-Fi in … MetroMesh Proven: Metro-Scale Wi-Fi in Chaska, MN...

Tropos MetroMesh Proven:Metro-Scale Wi-Fi in Chaska, MN

A Performance WhitepaperFebruary, 2005

Photo courtesy of NASA Image eXchange.Image use in no way implies endorsement by NASA of any of the products, services, or materials offered by Tropos Networks, Inc.

Proven MetroMesh Performance, Chaska.net

Metro-Scale Mesh Networking DefinedTM2

A Performance Whitepaper




Introduction

When the City of Chaska, Minnesota, decided to offer affordable broadband Internet service with speeds comparable to cable or DSL to its residents, how did they do it? They deployed a metro-scale Wi-Fi mesh network using Tropos Networks’ MetroMesh™ architecture. Using MetroMesh, chaska.net, Chaska’s city-owned ISP, created a broadband fixed and mobile Internet service covering a total of 18,000 residents at the low monthly subscription cost of $15.99. More than 20% of the homes passed in this community subscribed to the chaska.net service within the first month that it was offered. It is the largest and most intensively used metro-scale Wi-Fi mesh network for consumer Internet access. Serving more than 2,000 Wi-Fi subscribers over an area of 16 square miles, Chaska’s MetroMesh delivers mesh speeds in excess of 1Mbps (symmetrical).

Overall, as of the end of 2004, more than 125 cities worldwide have adopted the Tropos MetroMesh solution, a list that continues to grow rapidly. With the most and largest metro-scale Wi-Fi mesh networks deployed, Tropos is the clear leader in metro-scale Wi-Fi mesh.

In this paper, we use intimate knowledge of the Chaska network, including detailed measurements, to explore the unique benefits of MetroMesh:

• Ease and speed of deployment: Chaska’s city-wide MetroMesh broadband network was up and running within two weeks.

• Scaling of both coverage and performance: The Tropos MetroMesh architecture and its Predictive Wireless Routing Protocol (PWRP™) automatically create a metro-scale wireless mesh, eliminating all the protocol overhead and performance problems inherent to traditional routing and bridging approaches when the network grows.

• End-to-end throughput optimization: PWRP exploits dynamic path diversity to consistently select the highest performing end-to-end path through the wireless mesh, achieving >2x higher usable throughput over competing approaches that use fewest hops, signal-to-noise ratio (SNR) or spanning tree to make path selection decisions.

• Low latency: MetroMesh consistently delivers 1+Mbps throughput with less than 50ms latency across the mesh using paths that vary from 1 to 4 hops.

http://www.tropos.com/pdf/chaska_casestudy.pdf







• High performance under load: MetroMesh maintains high performance even under busy-hour traffic load.

• Use of standard clients: MetroMesh offers robust, high-speed connections to off-the-shelf Wi-Fi clients, fixed or mobile.1

Other Tropos papers present the compelling economics of the Chaska network created by MetroMesh2, the industryʼs only purpose built metro-scale mesh routing architecture.

Turning-On Broadband: Easy Deployment of MetroMesh in Chaska Just how easy is it to deploy a Tropos MetroMesh network? The first phase of the network was up within two weeks, MetroMesh routers taking, on average, under 15 minutes to install by a trade-level city employee with a bucket-truck. Subsequently, MetroMesh routers were simply added to the MetroMesh to create new links that enhance coverage and the network was deemed fully operational within three months. At this time, chaska.net began to bill subscribers. As shown on the accompanying map:

• Chaska’s MetroMesh network is composed of 250 Tropos MetroMesh routers mounted on lampposts covering 16 square miles of the city. Currently, this network serves more than 2,000 subscribers.

• Backhaul from the mesh to the Internet is provided

by 40 MetroMesh routers configured as gateways. The gateways connect to Chaska’s backbone network through either fiber or 5GHz 802.11a fixed line-of-sight wireless connections.

• Because the tree-covered terrain in Chaska is marked by hills and valleys, gateway locations were determined by the limited availability of line-of-sight to fixed wireless base stations and fiber drops to the city’s backbone network.

Each MetroMesh router configured as a gateway injects, on average, 5Mbps of backhaul to the cluster, which is used by 2-20 other MetroMesh routers using node-node links formed by the MetroMesh. The network delivers burst speeds in the range of 500kbps-4 Mbps. Provisioned backhaul out of the







mesh exceeds 40kbps per provisioned subscriber on average. In Chaska, there are currently fewer than 10 provisioned subscribers per MetroMesh router. Each MetroMesh router can support a significantly larger number of users than is provisioned in Chaska.

Analogous to light from light bulbs, the fundamentals of RF wave propagation and physics show that providing for sufficient MetroMesh router density and, therefore, powerful signals3 permits the network to operate at high end-user throughputs. To enable quick and easy placement at the lowest possible density, Tropos MetroMesh routers incorporate the industryʼs best RF link budget, most flexible antenna pattern and best ease-of-installation features. Tropos MetroMesh routers transmit at the maximum EIRP permitted (36dBm in the U.S.; lower in some countries), and use only the 2.4GHz band for both communications between MetroMesh routers in the mesh and for client access. Only the 2.4GHz band is used because it offers the best propagation characteristics and least loss in the presence of foliage and other obstructions when compared to 5GHz solutions, which are best suited to strictly line-of-sight links.4 Tropos supports industry-leading receiver sensitivity of -98dBm at 1Mbps to maximize the range at which client devices can be used. The result is a total link budget of 141dB for inter-node mesh links and 123dB for between clients and MetroMesh routers (assuming an 18 dBm EIRP for client transmissions). The use of 7.4dBi omni-directional antennas with dual-diversity allows MetroMesh routers to maximize their visibility to other MetroMesh routers and clients in forming the mesh, ensuring rich path-diversity for a robust, high-performing mesh. The overall result is best client performance at lowest mesh router density in the industry.

The Heart of the Matter: MetroMesh and PWRPJust how scalable is Tropos MetroMesh? MetroMesh is the first and only deployed metro-scale Wi-Fi mesh architecture that has been architected from the ground up to deliver true broadband (>1Mbps) to standard Wi-Fi clients in coverage areas spanning entire metro-areas.

Although bridging and routing protocols developed for wired networks (e.g., spanning tree, distance vector and link state) have been adapted for use in small-scale Wi-Fi mesh networks, protocol limitations prevent them from scaling to large-size networks. This fact is confirmed by the absence of large-scale Wi-Fi mesh deployments using these approaches, despite the fact that products using them have been generally available for much more than a year. Adaptations of wired protocols do not scale because their growing control overhead5 consumes more and more of the network’s available bandwidth as the network grows, as well as their inability to cope with the effects of wireless interference and multi-path fading.

Using the patented PWRP, Tropos MetroMesh is the first and only architecture to overcome the challenges of metro-scalability6:







• PWRP delivers actual throughput of 1+Mbps in the mesh and TCP/IP session-persistent mobility for standard Wi-Fi clients.

• PWRP allows for easy deployment and unlimited scalability through a self-organizing, self-healing network in which control overhead consumes less than 5% of available bandwidth, regardless of network size.

• PWRP enables wired or fixed wireless backhaul to be injected into the network at any MetroMesh router to permit easy addition of capacity in line with subscriber growth.

• PWRP maximizes throughput available to end-users by exploiting path-diversity within the mesh to overcome the effects of multi-path fading and interference. This achieves >2x higher performance than competing routing and bridging approaches, an advantage that is magnified in larger networks because of PWRP’s constant, low protocol overhead.

A Better Choice: Path Diversity and Selection in a MetroMesh NetworkThe principle of a mesh is that each node can see many other neighboring nodes and form multiple links over which packets can be routed. These links are the foundation for the wireless mesh. Upon this foundation, the wireless meshing protocol constructs paths for data transmission.

Our measurements detected more than 2,000 links in Chaska’s mesh (each direction is considered separately). The adjacent graph is a histogram of number of neighbors for each link, with distinct data points for each hour of a 24 hour interval. This results in a total of over 45,000 link-hours shown in the graph. MetroMesh routers on each link typically see 10-30 neighbors. This path-diversity is needed since only the upper percentiles of available links are actually useful.

How does PWRP take advantage of path diversity available in the mesh? By predicting and then selecting those links and multi-hop paths that achieve the highest end-to-end packet success probability (PSP), PWRP consistently ensures high throughput across the network. Outdoor wireless links are prone to multi-path fading and interference, which induces







bit errors that necessitate retransmissions of data frames and thereby lower throughput. These effects are asymmetric and can change over timescales ranging from milliseconds to months. Throughput on a link can be directly related to packet success probability, measured as the percentage of broadcast frames that can be successfully transmitted to the receiver.7 A histogram of PSP for each of the links at Chaska is shown in the adjacent graph illustrating the wide diversity of packet success probability amongst the available links. Not all links will be robust, and only a small fraction of links have packet success probability above 90%. Due to the dynamic nature of wireless links, the best choice for routing traffic to and from the gateways is a decision that needs to be adapted constantly based on prevailing conditions. PWRP selects the best paths with the highest PSPs on each link, so that the end-to-end throughput from any MetroMesh router back to the gateway is maximized at any point in time.

Although signal-to-noise ratio (SNR) measured at the receiver is a commonly used metric to measure the quality of a link, average SNR turns out to be a very rough predictor of packet success probability for outdoor links, as shown in the adjacent graph8. An SNR of 20dBm could correspond to a PSP of anywhere between 0 (very poor) and 0.9 (very good).

Adaptations of wireline path selection approaches, including routing or spanning tree algorithms that minimize hop count or rely on RF signal strength-measurements to make path selection decisions, will fail to converge on a useful network topology and result in poor network throughput. The reason is simple – these path selection decisions are uncorrelated with throughput, resulting in sub-optimal performance.

http://www.pdos.lcs.mit.edu/papers/grid:mobicom03/paper.pdf

http://www.pdos.lcs.mit.edu/roofnet/sigcomm-talk.pdf







MetroMesh Delivers >1Mbps Client-Server ThroughputHow well does the MetroMesh architecture perform? Mesh throughput measured on the Chaska network (using open-source tools) exceeds 1Mbps in most cases as shown in the following graph. Throughput for each of the 190 MetroMesh routers not configured as a gateway in the Chaska network is plotted along with estimates of throughput derived from the PSP on each link of the selected multi-hop path. Note that that the 40 MetroMesh routers that are configured as gateways are directly connected to backhaul with no mesh hops and will experience even higher throughput than the MetroMesh routers plotted on this graph. Each MetroMesh router at Chaska serving Wi-Fi clients is continually evaluating its selection of the best path to transport client data to and from the Internet via provisioned backhaul. Due to variations of packet success probability (PSP) discussed in previous sections, relaying data across multiple links can at times result in higher throughput compared to an available direct single hop path suffering from low PSP.9 To show this, the adjacent graph plots the distribution of average hop-count (over one hour) for the selected path by each Tropos MetroMesh router taking one or more hops to reach backhaul in Chaska’s network. In clusters of up to 20 MetroMesh routers served by a single gateway, the hop-count ranges from one hop up to four, with some MetroMesh

routers switching between 1, 2, 3, or 4 hops as necessary to maintain high performance. Multiple studies10 establish that throughput along a multi-hop path with perfect links (100% packet success probability) is proportional to the inverse of the number of hops (or 1/n) and it does not decay exponentially (1/2**n) as other sources have contended.

http://www.pdos.lcs.mit.edu/papers/grid:mobicom01/paper.pdf







MetroMesh Performs During Peak UsageHow does MetroMesh perform under heavy usage? The Chaska MetroMesh network, used primarily for consumer Internet access, meets or exceeds service level requirements during peak periods. It follows traffic patterns typically observed in cable and DSL networks during busy hours. Traffic patterns graphed here exhibit the after-school/work peak (5pm), the lull at dinner time (7pm), and finally the peak in the hour prior to the nightly news (9pm) as is typical of residential Internet usage during these periods.

The next graph shows the effect of usage on the measured throughput. Measured throughput will naturally be higher when the network is less heavily loaded and vice-versa.

Chaska’s MetroMesh performs well during high utilization as shown in the adjacent graph where measurements of throughput are shown both during peak usage (busy) period in blue and the early morning in purple. Due to utilization of the wireless links by customer traffic

during busy hour, throughput measured during off-peak hours will be necessarily higher, as observed here. Again, data is plotted only for the 190 MetroMesh routers not configured as gateways (i.e., that take one or more hops to backhaul). The data does not include the 40 MetroMesh routers that are configured as gateways and directly connect to backhaul

because they always offer the highest throughput, over 4Mbps. As a conclusion, even at peak usage hours, enough capacity remains to enable the network to maintain 1+Mbps performance across the majority of the MetroMesh routers.

Please note that 20 MetroMesh routers were added to the Chaska network after the measurements in this paper were taken, bringing the total number of MetroMesh routers deployed to 250.







Loud And Clear: Wi-Fi Clients Enjoy Strong Symmetric Signals With MetroMesh

While, as previously noted, SNR is at best a very rough predictor of packet success probability within the mesh, it is important as an indication of good client connectivity to the mesh. How strong are the signals of client devices connected to the MetroMesh? The MetroMesh in Chaska supports connectivity for standard Wi-Fi clients ranging from laptops to PDAs and bridges. Most of the client devices are mass-market 200mW Wi-Fi bridges costing under $100 each (retail). These wireless bridges were simply placed in a windowsill by residential customers, and do not require any special mounting or antenna sighting. Between 1,300 to 1,500 client devices are on the network at any time, and most clients have signal to spare even when operating at the highest 802.11b data rates (10-20dB SNR or greater). This can be seen in the adjacent graph showing the average SNR for client devices as measured by Tropos MetroMesh routers (uplink from the client device). Alternative broadband wireless solutions connecting to end-user devices can suffer from poor uplink since their RF link budget is subject to severe asymmetries. This is particularly true of 3G cellular solutions since they are deployed as overlays of voice networks beginning with insufficient cell density (typically 1 per square mile).11 Not so with Tropos, since MetroMesh enables the mesh router density required to maintain a client uplink capable of true broadband speeds. In addition, Tropos’ superior receive sensitivity serves to balance out the link from the lower-powered client CPE to create a more symmetric link budget.

http://www.tropos.com/pdf/price-performance.pdf







Evolving The VisionAt Tropos Networks, we recognized early on that true symmetric broadband serving both fixed and mobile users in a metropolitan area requires the use of densely placed nodes, enabled cost-effectively by a wireless mesh. MetroMesh takes advantage of path diversity offered by links in the wireless mesh, critical to delivering high performance. MetroMesh is the only solution proven to deliver high performance on a large scale. Open-standard Wi-Fi devices enjoy excess signal-to-noise ratio throughout the network coverage area ensuring robust performance down to the last link.

The result of years of refinement, Tropos MetroMesh is the easiest, most cost-effective way to deliver broadband across entire cities with speeds in excess of 1Mbps. Founded in 2000, Tropos has been continually optimizing the combination of mesh protocols, radio technologies and physical packaging to deliver the best in system performance, lowest total cost of ownership and simplest installation and operation. Our MetroMesh routers feature our patented Predictive Wireless Routing Protocol, which provides best throughput, lowest latency, most scalability, best security and most immunity to interference in the industry. Open-standard radio technology is at the beginning of dramatic increases in spectral efficiency as it transitions from 802.11b (DSSS) to 802.11g (OFDM) to 802.11n (MIMO) and 802.16 (WiMAX).

Due to forthcoming advances in protocols and network architecture, Tropos MetroMesh holds immense potential for achieving further advances in end-user speeds. Based on these projections, in the coming years MetroMesh will deliver end-user speeds in the metro-area that grow steadily for years to come.

MetroMesh technology is the first and only purpose-built wireless mesh approach scalable to metropolitan-areas with the following patents issued or pending:

• Method and Apparatus to Provide a Routing Protocol for Wireless Devices

• Method and System to Provide Increased Data Throughput in a Wireless Multi-Hop Network (mesh network using multiple-radios/channels)

• A Method of Wireless Accessing (Wi-Fi Roaming)• Selection of Routing Paths Based Upon Path

Quality of a Wireless Mesh Network• A Method and Apparatus for Monitoring and

Displaying Routing Metrics of a Network• Network Layer Emulation (Layer 2 emulation)• A Multi-Channel Mesh Network• Minimization of Channel Filters Within Access

Nodes• Channel Assignments Within A Mesh Network• Access Point Control of Client Roaming• Wireless Mesh Network Time Commit

Provisioning• A Method Of Subnet Roaming Within A Network• Determining Bi-Directional Path Quality Within A

Wireless Mesh Network







Endnotes1 Throughput to clients may vary depending on the specific type of Wi-Fi client used.

2 See “Metro-Scale Wi-Fi As City Service, chaska.net, Chaska, Minnesota,” http://www.tropos.com/pdf/chaska_casestudy.pdf. For a more general discussion of MetroMesh network economics, see “Price-Performance Comparison: 3G and Tropos MetroMesh Architecture,” http://www.tropos.com/pdf/price-performance.pdf and “Maximizing Profitability: Tropos Networks and the Wireless ISP, http://www.tropos.com/pdf/bus_case_whitpaper.pdf.

3 See “Price-Performance Comparison: 3G and Tropos MetroMesh Architecture,” http://www.tropos.com/pdf/price-performance.pdf

4 See S. Perras, L. Bouchard, “Fading characteristics of RF signals due to foliage in frequency bands from 2 to 60 GHz,” IEEE 5th International Symposium on Wireless Personal Multimedia Communications (WPMC ’02), October 2002

5 See the discussion of routing overhead in wireless mesh networks in “Routing in Communication Networks” by M. Steenstrup © 1995 Prentice-Hall

6 See “Metro-Scale Wi-Fi Using Tropos Networks MetroMesh Architecture,” http://www.tropos.com/pdf/tropos_metro-scale.pdf

7 See “A High Throughput Path Metric for Multi Hop Wireless Routing” (MIT Roofnet Group) http://www.pdos.lcs.mit.edu/papers/grid:mobicom03/paper.pdf

8 Also see “Link-level Measurements from an 802.11b Mesh Network,” http://www.pdos.lcs.mit.edu/roofnet/sigcomm-talk.pdf

9 “Performance of Multihop Wireless Networks: Shortest Path is Not Enough,” (MIT Roofnet Group)

10 For a discussion of multi-hop throughput see Figure 6 “Real hardware throughput along a chain of nodes” in “Capacity of Ad Hoc Wireless Networks” http://www.pdos.lcs.mit.edu/papers/grid:mobicom01/paper.pdf as well as “Analysis of TCP performance over mobile ad hoc networks,” Proceedings of the 5th annual ACM/IEEE international conference on Mobile Computing and Networking. The dependence on hops can be easily verified by a simple experiment placing multiple cells in close proximity to each other to create perfect links and then measuring throughput when routes are specified such that data traffic is routed in a chain across the cells.

11 See “Price-Performance Comparison: 3G and Tropos MetroMesh Architecture” http://www.tropos.com/pdf/price-performance.pdf

555 Del Rey Avenue • Sunnyvale, Ca 94085phone 408.331.6800 • fax 408.331.6801

www.tropos.com • [email protected]©2005 Tropos Networks, Inc. All rights reserved. Tropos Networks, MetroMesh and Tropos Control are trademarks of Tropos Networks, Inc. All other brand or product names are the trademarks or registered trademarks of their respective holder(s). Information contained herein is subject to change without notice.

The only warranties for Tropos products and services are set forth in the express warranty statements accompanying such products and services. Nothing herein should be construed as constituting an additional warranty. Tropos shall not be liable for technical or editorial errors or omissions contained herein.

http://www.tropos.com/pdf/chaska_casestudy.pdf


http://www.tropos.com/pdf/bus_case_whitpaper.pdf%20


http://www.tropos.com/pdf/tropos_metro-scale.pdf





Tropos MetroMesh Proven: Metro-Scale Wi-Fi in … MetroMesh Proven: Metro-Scale Wi-Fi in Chaska, MN...

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