May 22

Written by: Jason Shannon
5/22/2008 9:00 AM

In light of the recent announcements by EarthLink to discontinue operations of municipal Wi-Fi networks throughout the U.S., I thought it would be appropriate to share some of my insights from a technical perspective of Municipal Wi-Fi 1.0. I've personally been involved in the design and optimization of more than a dozen active municipal Wi-Fi networks, and experience suggests that a dense, urban- scale municipal Wi-Fi network will likely fail to provide a universally available, technically viable, low-cost alternative to existing broadband services. This isn't to say that Wi-Fi doesn't have its place in the broadband ecosystem. In fact, it has become obvious that Wi-Fi will be a fundamental means of access for some time. With this in mind, it is important to know the limits of the technology.

So, what really were the technical issues with Municipal Wi-Fi 1.0? I've made a list of the six issues that are at the top of my list:

It is challenging to establish a reliable connection to an outdoor Wi-Fi access point using the low power client devices inherent in handheld, laptops and indoor customer premise equipment (CPEs). This challenge is due to the constrained power budgets associated with Wi-Fi radio equipment and the power output and emission mask limits established by the FCC. The typical uplink gain for municipal Wi-Fi systems is very low and does not withstand heavy loss factors due to exterior wall penetration or significant obstruction between transmitters.

An especially demanding use-case for municipal Wi-Fi where this problem is common is multi-dwelling units (MDUs), where the penetration through many densely constructed walls is required for communication between transmitters. Another example of where constrained link budgets limit the ability to establish reliable Wi-Fi connections is in high rise or mid rise buildings where: (1) construction is dense and highly absorbent of the 2.4GHz electromagnetic signal, (2) exterior metallic reflective windows reflect the electromagnetic signal, and (3) the higher elevation floors (often above the 4th or 5th floor) are outside of the vertical radiation pattern of the omni directional antenna.

We find that Wi-Fi is most reliable when client devices are within the main radiating lobe of the antenna and only minimal obstructions exist between the transmitters. A universally available service based on this operating assumption would require a massive node density in an urban environment, and would require a significant infrastructure of both outdoor and indoor mounted Wi-Fi access points. As we'll discuss in the following paragraphs, this massive node density presents other challenges outside of the obvious cost of infrastructure and operations.

The 2.4GHz spectrum used by Wi-Fi is an unlicensed frequency band, and is susceptible to interference from other radiators in the 2.4GHz band as defined by the FCC’s Part 15 regulations. An important factor in considering the availability of a Wi-Fi system is that transceivers are always susceptible from outside, non-cooperative interference. This interference will exhibit itself as complete loss of connection, or RF "jamming", or as significant degradation of data throughput. The intensity of the interference is dependent upon factors such as the power output of the interfering device, the proximity to the interfering device, and the baseline signal to noise ratio (SNR) of the Wi-Fi signal.

A dense, urban scale deployment of Wi-Fi transmitters is often susceptible to performance degradation due to the issue of media sharing. Wi-Fi systems operate across a total of 3 non-overlapping channels in the US and North America. In situations where a dense deployment of transmitters is needed, nodes operating on a given channel will have to share that channel with neighbor transmitters. This issue is known as "mutual node visibility" and will cause system capacity to decrease by up to 1/n the number of mutually visible neighbors on the same channel, where n is the number of mutually visible nodes.

Municipal Wi-Fi systems require an infrastructure underlay to provide and distribute capacity throughout a network. Traditionally we find that this additional network tier leverages terrestrial fiber or some other unlicensed microwave frequency such as 5GHz. Both of these options present challenges. A terrestrial fiber infrastructure is often financially prohibitive to provide the number of physical connections required to support a dense, urban scale municipal Wi-Fi network. The use of 5GHz for capacity distribution requires the arduous process of identifying and acquiring key high sites throughout the City. The density of buildings and height differential in a downtown corridor often lead to situations where it is extremely difficult to identify viable high sites from a radio frequency (RF) propagation perspective. This is exacerbated by the fact that 5GHz is inherently a line of sight (or near line of sight) wavelength.

Many of the challenges noted above are specific to the common architecture used for municipal Wi-Fi 1.0, but many are inherent in the constraints of the technology itself. As next-generation, 2.0 models emerge, engineering approaches and deployment models (e.g. increased use of multi-radio solutions, organic ad-hoc networks and so on) may address some of these challenges, while at the same time introducing new ones that architects and engineers will have to resolve.

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