WiFi Mesh and WiMAX Technologies - the basics
There are primarily two technologies being used in municipal wireless networks: WiFi mesh and WiMax. A key point is that they are not necessarily mutually exclusive, but can be combined into a single infrastructure to take advantage of the best characteristics of both.
Quick Tutorial on WiFi Mesh
WiFi mesh takes advantage of the hundreds of millions of low-cost wireless Ethernet 802.11 clients already in circulation around the world – mostly in laptops but increasingly in other devices such as cellphones and so on. This technology should also facilitate relatively low-cost infrastructure equipment, and operators can use unlicensed spectrum for free.
WiFi Wireless Mesh Access Points (MAPs), or mesh nodes, are scattered over the coverage area, and are interconnected into a mesh using the same WiFi technology as do the end-user clients in connecting to the MAPs themselves. This WiFi mesh eliminates the need for an additional wired or wireless backhaul network to connect the MAPs to the service-provider wired network, and provides a workaround to any obstacles preventing line-of-sight wireless connection to an MAP. This makes MAP siting less critical and more flexible, and, of course, a mesh topology makes the network inherently redundant to a certain amount of node failure.
In principle, these characteristics mean that a WiFi mesh should be fairly flexible and straightforward to design, install, and operate, and the minimal backhaul helps to keep operational expenditures down. It is possible for closed user groups, such as public-safety agencies, to use their client devices to participate in the mesh and thereby extend coverage, and sometimes capacity, on an ad hoc basis.
Early mesh architectures were designed using the single-radio concept which resulted in a negative impact in the industry due to low performance and coverage issues. In the single radio solution the same Access Point handles both client access and the forwarding of traffic through the mesh.
This runs into what is called the 1/N problem (request Solving the Multihop Dilemma white paper) – the greater the number of node hops that have to be made through the network, the less bandwidth is available for node access, since the network is running just a single channel on a single frequency, and this is shared between the two functions. In practice, this means that three hops is about the maximum feasible in a mesh that uses standard 802.11 protocols. As the number of hops increases when the number of nodes increases, the maximum size of mesh clusters is probably in the region of six or seven nodes. In real life, even three hops may not be viable in some situations.
Strix Systems innovated multi-radio mesh networking with the Access/One Outdoor Wireless System and has since dominated the market worldwide with the greatest marketshare worldwide. |
Typically, between 20 to 60 WiFi MAPs would be needed per square mile, depending on environmental factors, bandwidth and applications requirements, and so on. In real city deployments, the weakest links have been under-powered MAPs and the transmitter on the end-user's laptop or other WiFi enabled device. Putting additional MAPs closer together can provide greater coverage potentially removing the need upgrade laptops, for example, with higher-power USB or card-based WiFi adapters. However, for closed user groups (such as for public safety) it is possible to use special higher-gain devices and therefore more widely spaced MAPs.
An interesting if not important attribute of WiFi mesh in a practical example is that it can usually be combined with solar power to minimize power costs and to allow wireless to be deployed where it is needed, rather than where power happens to be available, giving more effective coverage. Among many examples of the application for solar power systems for WiFi is the Department of Homeland Security (DHS) project for North County Transit District Railway deployment which utilizes Strix Systems equipment.
Quick Tutorial on WiMax
WiMax is fundamentally a cell-type technology defined by the IEEE 802.16 Layers 1 and 2, which the WiMax Forum has added some upper-layer work. WiMax introduces adaptive modulation and deterministic QOS into the 802.16d (or, more formally, 802.16-2004 or ETSI HiperMAN) which is intended for fixed applications, however 802.16e version (or mobile WiMax), published in 2006, introduces features such as scaleable Orthogonal Frequency Division Multiple Access (OFDMA), subchannelization, and Multiple-Input/Multiple-Output (MIMO) antenna technology. For carriers, WiMax is the preferred mass-market solution that allows (if implemented) for QoS to be delivered on a subscriber basis.
The plans for WiMax deployments have been overstated and in some cases completely foiled, but there remains momentum for the technology and it will continue to be deployed as a higher performance 3G service (some call 4G). It may also be deployed as a moderate performance DSL replacement technology for generalized populations with no wired connectivity. 802.16 technologies, not necessarily WiMax certified, may also be used in a point-to-point configuration to backhaul WiFi mesh traffic, for example, and this is roughly equivalent to a 5GHz bridge that would be used today.
As a licensed spectrum technology, WiMax is expensive. It is designed for a generalized wireless applications synergistic for cellular service upgrades and a generalized class of users. In contrast, WiFi broadband today offers a proven base of users who continue to thrive on broadband WiFi wireless access and the benefits of greater throughput, lower latency, higher resilience and demanding mobility applications. Products such as Strix Systems Access/One devices go even further by offering a mixture of RF technologies in a single enclusure as well as technologies specifically for public safety applications. Strix high-power DSRC-C 4.9 GHz for public safety is leading the market and adds to the integration flexibility of Strix Access/One architecture.
Integration and Coexistence of WiMax and WiFi Mesh
The strategy for a city-wide wireless network is that a carrier will deploy WiMax, probably using 802.16e for general overall coverage and then tie into deployed WiFi Mesh areas (small to large zones) and WiMax picocells or other kinds of technologies to add capacity where needed.
The argument remains that a carrier may deploy WiMax more cost effectively than Mesh, however, the WiMax network may only offer good cellular-like service, yet barely approach the broadband performance that the public and government agencies require. That being said, in a generic city-wide installation more WiFi MAPs are required to attain broadband performance, however, utilizing Strix Access/One, the number of wired backhaul locations is reduced to approximately 1/10th the number of backhaul locations compared to any other MAP product on the market.
In the industry there was a consideration that 802.16d could provide the high-speed backhaul for WiFi mesh deployments. While the it is plausible that WiMax could provide the backhaul, it is less likely today that 802.16d is the answer due to real life deployments of the technology and the observed performance.
Today existing WiFi technologies provide higher performance than what we've seen with WiMax. Additional innovations in WiFi, such as 802.11n are proving to provide 100Mbps and higher performance. While coverage may not equal the claimed range of 802.16d, the performance is not capable of matching Strix industry-leading performance.
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