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11/01/2007Getting Up to Speed, Part 2: Vertical Assets and Network Design
Additional Reading: Getting Up to Speed, Part 1: Optimizing Wi-Fi Mesh Node Communication
One of the most significant challenges in designing and deploying a Wi-Fi network is identifying appropriate vertical assets for network nodes and backhaul. Vertical assets include buildings, light poles, towers, and terrain suitable for mounting wireless gear and establishing clear line of sight (LOS).
Establishing a clear LOS between radios and potential subscriber facilities is critical. The frequency of radio waves that the 802.11a/b/g standards operate in is highly susceptible to interference by terrain and vegetation. Additionally, the materials used in construction also affect the ability for the signal to penetrate a facility.
There are two general rules of thumb: - In an outdoor network the signal will lose 50% of your signal for every 3 meters of tree canopy the signal passes through.
- When your signal moves through a facility wall lose 20% of your signal for each wall to signal must penetrate.
Tree canopy is one of the most difficult elements to deal with in an outdoor wireless network. (Note: In particular pine trees are really good at absorbing wireless signals) In areas of the country where vegetation exceeds the height of local street lights and buildings it is a challenge to establish a clear line of sight (LOS) between wireless devices.
The type of building construction in the local market also has an impact on the ability to provide customer service. For example, in the Southwest, buildings are constructed of wood framing, covered with Styrofoam, and an outer layer of metal lath to support the stucco finish. This grounded metal lath is particularly effective at blocking radio waves. (Note: If you're interested in learning more about this phenomenon reference Google “faraday cage.”) Additionally, metal roofing, metal-oxide window tinting at steel framing can also interfere with the ability of radio waves to penetrate a facility.
The selection of vertical assets is critical to establishing clear line of sight. Overcoming obstacles is essential to establishing good link budgets between radio devices. There are three general approaches to deploying Wi-Fi networks in use today. - Point-to-multipoint networks
- Mesh networks
- Hybrid networks
In the point-to-multipoint of our environment, you can only have central towers that communicate directly to client subscriber devices. Subscriber devices will require a clear line of sight to the central tower. The central tower is where the data passes from the radio network to the physical cabled network. (Generally, the tower has an effective range 1 to 2 miles between subscriber device and tower). If we are using a single radio mesh, establishing a clear LOS from the tower to one in three nodes is a costly engineering challenge.
In a pure mesh network, nodes within the network pass data from node to node to a central backhaul location with the data are transferred to the physical network. The line of sight issues are generally restricted to node to node (500 to 1,000 feet) in a pure meshing environment than nodes that have multiple radios dedicated to passing data in and out of each node — as well as radios dedicated to serving subscriber devices. Establishing LOS at street level can be challenging but is generally easier than the point-to-multipoint environment. To provide a robust pure mesh, you need to have two radios (on different channels) dedicated to the mesh and additional radio for client connections.
And hybrid networks use a mix of point-to-multipoint networks to inject capacity to nodes that usually have a limited ability to mesh. Generally the ability to go multiple hops in a single radio mesh is restricted when you have a single radio backhaul. The introduction of this second layer to the network is required to facilitate fewer hops to the wired network. Several networks are using WiMAX as a backhaul or capacity injection layer. WiMAX has better signal penetration of clutter than the 802.11a/b/g backhaul. However, fiber is the most popular backhaul option in currently deployed networks.
We have yet to see a node equipment provider who is including a WiMAX radio in the node. Generally you need to have a WiMAX client devise on the pole with the node. I believe it is just a matter of time before we see this convergence of devises. A WiMAX network with WiFI at the edge is logical direction for deployment to take where fiber back haul is not available to support a multi-radio mesh.
I say this because increasingly the applications cities want to run on the network include video from hundreds of locations. The introduction of video drives the need to have a larger amount of bandwidth available to move the video traffic both on the mesh and in the back haul.
Ultimately, the applications need to be front and center in the development and planning of municipal networks. The design your city needs will select depends on the latency, jitter and bandwidth you will require. As you consider building a network do your homework and get and experienced partner to help.
Ryan McCaigue is President of Ivy League Consulting LLC. Previously, Ryan worked for Earthlink Municipal Wireless Networks, planning and building some of the largest wireless networks in the US, including Anaheim, San Francisco and Houston. He can be reached at (480) 213-7240.
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