In many wireless LAN deployments, access to the network from inside elevators is often necessary. For example, nurses and doctors using wireless VoIP phones must be able to continue communicating over the phones while traveling between floors. If you have requirements for providing Wi-Fi coverage inside elevators, you’ll need to understand the issues and implement a well-tested solution.
Many elevator shafts are surrounded by metal, which offers a high degree of attenuation to Wi-Fi signals. Even elevators that appear to be constructed with wood may still have metal frames, which significantly attenuate radio waves. Of course the problem with high attenuation is that signal levels coming in and out of the elevator are relatively weak, requiring careful positioning of access points to ensure that signal coverage is adequate.
Another problem is with roaming as the wireless client devices in the elevator must handoff to different access points as the elevator moves between the floors. Two or three story buildings usually have relatively slow moving elevators, and they don’t go very far. As a result, roaming is usually not an issue for smaller buildings.
You’ll likely experience more roaming problems in taller buildings, where elevators service many floors. A thirty story building, for instance, may have an elevator that travels much faster, with speeds similar to automobiles racing along a highway. In this case, the relatively high speed results in roaming-related issues.
For example, a person actively using a wireless application may enter an elevator on the 1st floor and punch the button for the 23rd floor. As the elevator zooms out of range of the initial access point (maybe around the 3rd floor), the wireless client device begins to handoff to an intermediate access point as the elevator makes its way up to the 23rd floor. If the handoff takes too long, the elevator will pass the intermediate access point before it has time to successfully complete the re-associating process. In fact, the client device probably won’t complete a handoff to another access point until the elevator comes to a stop at the 23rd floor. Of course this causes a disruption in the flow of information between the client device and the network, which causes VoIP calls to drop and other applications to fail.
In order to counter the attenuation problems with elevators, it’s usually best to install an access point near the elevator door on each floor (or possibly every other floor). You should complete specific testing when performing the site survey, though, to confirm the optimum spacing of access points.
Initially perform the following steps to better understand the attenuation problem and gain insight on where you’ll need to install access points:
1. Determine the minimum SNR level for acceptable signal coverage based on the target client device.
2. Power up an access point just outside the elevator door at the elevator landing on the 1st floor.
3. Measure the SNR level inside the elevator with the door shut while the elevator is parked at each floor (including the 1st floor), and as the elevator moves between the floors. Be sure to take notes so that you don’t forget the values.
4. Analyze the SNR data, and determine where signal coverage is above and below the minimum SNR. Acceptable coverage is where the SNR value is equal to or greater than the minimum SNR.
5. Determine the optimum placement of access points. For example, if the access point on the ground floor provides acceptable coverage inside the elevator parked on the 2nd floor (but not the 3rd floor), you can probably install access points at the landings of every other floor, such as floors 1, 3, 5, etc. If the coverage at the 2nd floor is not adequate (but okay on the 1st floor and at least the first half way up to the 2nd floor), then you should install an access point on the landing of each floor.
There’s a rare possibility that an access point installed at the landing of a particular floor will not provide adequate signal coverage inside the elevator while parked at that same floor with the elevator door closed. If this occurs, then consider using a “leaky coax” approach for providing signal coverage. This involves installing specially-designed cabling (coax cable with holes) along the interior of the entire elevator shaft. This may seem like an easy fix for the attenuation (and possibly roaming) problem, but there are many issues.
For example, the leaky coax approach may cause performance troubles on the network, especially for VoIP applications. The long coax cable acts as an antenna for a single access point, which offers a single collision domain throughout a large portion of the facility. This limits the number of users who can utilize the network.
In addition, many companies and organizations will not allow the installation of cable inside the elevator shaft due to safety regulations. This approach also requires specially-equipped installation crews (there are not many who can scale elevator shafts) and substantial coordination with facility owners. As a result, it’s probably best to stick with the conventional approach of using access points if possible.
Don’t forget to address potential roaming issues. As with the attenuation problem, perform tests to determine the magnitude of the roaming problem as the basis for considering possible solutions. Once you have access points installed in a manner that satisfies signal coverage criteria, make use of the target applications while traveling on the elevator from the bottom floor to the top floor. If this results in unacceptable disruptions in communications, then consider implementing roaming enhancement software that provides session persistence from companies such as NetMotion and Bluesocket.