Voice over wireless LAN deployment requires constant maintenance

If you have successfully implemented voice over wireless LAN (VoWLAN) and believe that your work is complete, you will get an unpleasant surprise. Unlike a wired network, a wireless network is susceptible to changes external to the network itself, which may affect coverage and call quality. Even though you have made no changes to your network, call quality can degrade.

Monitor signal strength

You will need to measure signal strength and coverage throughout the facility on a regular basis. During the initial deployment, APs were placed and signal quality was measured throughout the facility. AP power levels were adjusted to provide adequate signal strength. But changes in the layout of the area can require you to change power levels, move an AP or even add APs. Adding a file cabinet or even moving a cabinet in one of the cubicles can cause a dead spot. To detect changes of this type, you will need to make periodic measurements and compare them against the measurements made during initial deployment.

Tools such as spectrum analysers and specialised VoWLAN measurement tools were used during the initial deployment. If a network integrator performed this work, you will need to bring the integrator back regularly to repeat the measurements or allocate the time and resources necessary to buy this equipment and train staff on how to use it.

Rogue APs

Rogue APs have long been recognised as a security risk for WLANs, independent of whether VoWLAN is implemented or not. Employees may decide to provide themselves with improved wireless access by buying and connecting retail APs to the wired network outlet in their offices. An AP with no security configured creates an open gateway to both the wired and wireless LAN.

The advent of IEEE 802.11n and the availability of retail 802.11n APs increases the level of risk. IEEE 802.11n can operate at either 2.4 GHz or 5 GHz. If your network operates at 2.4 GHz, and you scan only at that frequency, you will miss an AP operating at 5 GHz.

External interference

External interference not present when the network was initially deployed may cause call quality degradation. Microwave ovens, Bluetooth devices and wireless phones operating at 2.4 GHz can interfere with IEEE 802.11b and 802.11g networks.

A new or modified network in an adjacent area may also create interference. If your facility shares a building with others, you will probably not have any warning that your neighbours are adding or modifying a network. IEEE 802.11n can create an increased chance of interference. The new standard uses multiple antennas to provide much longer range than IEEE 802.11b or 802.11g. A neighbour may have been too far away to cause a problem when using one of the older standards. When they upgrade to the new standard, the longer signal reach may begin to intrude on your network.

A nearby 802.11n network operating at 5.0 GHz will not interfere with your 2.4 GHz 802.11b or g network, but interference can result if your neighbour has chosen to operate at 2.4 GHz. A periodic sweep of your facility and comparison to earlier readings will detect new or increased sources of interference.

Network load

The most obvious source of increased VoWLAN load is additional staff. More people using wireless phones mean more traffic. You will need to monitor traffic levels and take action before increased load and resultant poor quality become noticeable. You may need to add or move APs to deal with the increased traffic.

Even if overall usage remains the same, poor call quality may occur at some locations. If the office area is rearranged and employees sit in different locations, heavy users may now be bunched in one area. You may have made the necessary adjustments to deal with moved walls and furniture, but each AP can handle a limited number of concurrent calls. You may need to add or rearrange APs so that traffic is spread over more APs.

Voice does not require high bandwidth, but it does require low latency and jitter. This is achieved by using either the IEEE 802.11e WLAN prioritisation standard or a vendor's proprietary prioritisation method. Since email and Web access can tolerate latency and jitter, voice packets are given higher priority than email and Web access.

Email and Web access should not interfere with call quality, but the increasing use of video can present a problem. Video also requires low latency and jitter. Configuring high priority for both voice and video can mean poor performance for both. If dividing available bandwidth is not a solution, consider increasing bandwidth to accommodate both voice and video.

IEEE 802.11n

IEEE 802.11n was cited earlier as a possible source of interference, but it also offers a solution to contention between voice and video. The 802.11n standard is still not final, but products based on Draft 2 have been available since early this year. Work is proceeding rapidly toward final completion of the standard.

Initial products were aimed at the home user market, but Cisco, Meru Networks and SMC have recently released products aimed at the enterprise market. Though none of these products has achieved the theoretical maximum of 300 Mbps, all greatly exceed the throughput of 802.11g. If you are currently experiencing poor performance resulting from network congestion, consider moving to 802.11n now. Otherwise, wait for final completion of the standard, but monitor reports on new product releases.

As VoWLAN is deployed more widely, both improved network products and diagnostic tools will make it easier to deploy. For now, though, a VoWLAN deployment requires constant vigilance to maintain the level of call quality that users expect.