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How Municipal WiFi Works

        Tech | Connectivity

Technology and Networks

Mesh

Most WiFi hot spots in coffee shops and other locations have a hub and spoke configuration. One radio (the hub) sends and receives data for several users (the spokes). The wireless router has a physical connection to the Internet -- a wire -- and it transmits data from multiple users through that wire.

Adding a wireless router to an existing wired connection is an easy, convenient way to provide wireless access on a small scale. Wireless routers are relatively inexpensive. Most allow people to choose from various sign-on and encryption options, providing a layer of security.

But if a wireless router goes down, there's not always another router nearby to pick up the slack. And on a large scale, like a whole city, using a physical wire to connect every wireless router to the Internet is expensive.

That's why most municipal wireless networks use a mesh rather than a hub and spoke. A mesh is a series of radio transmitters. Each transmitter is able to communicate with at least two others. They create a cloud of radio signals through the city. Signals travel from router to router through this cloud.

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In some networks, signals hop from one receiver to another until they reach a node that has a wired connection to the Internet. Other networks use backhaul nodes. These nodes do exactly what their name implies -- they gather up all the data from many transmitters and haul it back to the Internet by sending it to a router with a wired connection. Backhaul nodes are usually point-to-point or point-to-multipoint nodes. They can either connect one point to exactly one other, or they can connect one point to several points.

If you use your laptop to connect to the Internet in a mesh network, here's what happens:

Some networks use WiMAX transmitters for backhaul.
Some networks use WiMAX transmitters for backhaul.
Photo courtesy Intel
  1. Your computer detects the nearby network, and you sign on.
  2. The protocol that controls the mesh determines the best path for your data to follow. It plans the route that will make the fewest hops before reaching a wired connection or a backhaul node.
  3. Your data follows the path that the protocol sets. When your data reaches a node that has a wired connection, it travels over the Internet until it reaches its final destination.

If you're out and about in a city with public access, you can probably do this with no extra equipment. But if you're trying to access the network from home, you may need a stronger radio and possibly a directional antenna. Although signals from the city network are strong enough to make it into your home, the signal from your computer may not be strong enough to make it out again. Most service providers take this into account and provide the necessary equipment for free or for a fee, much like they do with DSL or cable modems.

This system has several advantages over the hubs and spokes of ordinary hot spots. First, since there are fewer wires, it's less expensive. If a few nodes fail, others in the mesh can compensate for it. In addition to being far less expensive than running high-speed cable to every location in a city, it's a lot faster to build.

Municipal networks use routers like these mounted on light poles throughout the city.
Municipal networks use routers like these mounted on light poles throughout the city.
Photo courtesy Tropos Network

When a city decides to built a wireless network, it generally issues a request for proposal (RFP). An RFP is simply a request for information from companies that are interested in building the network. While a city could theoretically build its own network, most choose to delegate that part of the process to a company that has experience in Internet and network technology.

Interested businesses respond to the RFP with a proposal that describes a plan for building and maintaining the network. The proposal includes everything from the number and type of radios to the final cost. The physical structure of the network has to take the size and layout of the city, tree cover, landscape and other factors into consideration. The proposal also includes who will end up owning, running and maintaining the network -- the city or the business.

In some of the earliest proposed networks, the cities themselves owned and controlled the networks. Businesses like ISPs and telecommunications companies objected to these plans. Their argument was that competition between municipalities and the private sector was unfair or even illegal.

Today, many existing and proposed networks follow one of the following four models:

  • The city owns the network, which is for city use only
  • The city owns the network, which is for city or public use
  • The city owns the network, and ISPs lease access to it, passing that access on to the public
  • A service provider owns and operates the network, providing access to the city, the public and even other service providers

The city reviews all of the RFPs, then decides which proposal to accept. EarthLink, for example, has been selected to build networks in Anaheim, California and Philadelphia, Pennsylvania and is a finalist in several other cities. EarthLink is also teaming up with Google to build a wireless network in San Francisco.

Exactly what the network ends up looking like depends on a few factors. The first is exactly what a city hopes to do with the network. A city-wide blanket of coverage that's open to everyone can look very different from a public safety network that will be open only to police officers and firefighters. (See "Wireless Applications" and "Public Safety" to learn more about what these networks can do.)

This Motorola node has radios for 2.4 GHz transmissions as well as 4.9 GHz public safety transmissions.
This Motorola node has radios for 2.4 GHz transmissions as well as 4.9 GHz public safety transmissions.
Photo courtesy Motorola

Different businesses' proposals can also vary widely depending on the hardware and protocols they use. EarthLink's projects combine mesh and point-to-multipoint networks. Most of its proposals incorporate radio transmitters on light poles throughout the city, which create the cloud of wireless signals. Radio antennas on tall buildings or towers also communicate to smaller antennas placed throughout the cloud. These point-to-multipoint antennas provide the backhaul, carrying the data from the cloud to the wired Internet.

Almost always, once a city has made a choice about who will build, run and maintain the network, the final step is a pilot program. A pilot program is like a preview or a test run of a smaller version of the network. It's generally a fraction of the size of the final project, and it lets the city to make sure the network is right for them.

Let's look at what a city can do with a wireless network once it's up and running.


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