Other Factors
Calls on a Wi-Fi network can can drop due to many other factors, some of which will create intermittent symptoms. A common problem is duplex mismatch, where one end of a link is configured for full duplex and the other end is configured for auto-negotiation (see my Netcraftsmen post, “
Auto-Negotiate Duplex or Not?”). It is best to configure the network for auto-negotiation, even though duplex mismatch can be detected by a network management system that alerts on frame check sequence and late collision errors.
Intermittent operation can also occur when a single AP is supporting too many clients, such as in an auditorium or public assembly location. The specific problems include overloaded AP radios that run out of bandwidth, overloaded DHCP address pools that prevent clients from getting an address, and uplink congestion at network aggregation points. Network monitoring provides visibility into these problems. Configure the NMS to report on the number of clients per AP, DHCP pool utilization, and interfaces that record a large number of packet drops or discards. Packet drop statistics are best reported using a Top-10 report and interface utilization is best shown using the
95th Percentile metric for each day, also sorted and filtered to show the Top-10 interfaces.
Wireless controller failures can also create interesting scenarios. In this case, the Wi-Fi network is designed with a primary and backup controller. The APs are distributed between the two controllers, and there are more APs than a single controller can handle. When a controller fails, is taken offline for software upgrade, or is isolated by a network failure, the affected APs attempt to register with the other controller. When the operational controller’s AP count reaches the license limit, additional APs will be left out. The first to re-register will win and the remainder will be Not Registered (yes, that’s the status description for one vendor). The Not Registered APs will seemingly be selected at random, depending on when they notice that the controller they were using is no longer available.
Your network management system should alert you to the controller’s failure. Hopefully, your Wi-Fi vendor will allow you to specify AP priority, allowing the controller to register only those APs that you consider important. But you’ll have had to take the time to decide which APs are important and which aren’t and apply the appropriate Wi-Fi controller configuration. In either case, any clients that were using the Not Registered APs will no longer have service, potentially affecting mobility. A better solution is to make sure that you have enough controllers to handle all APs, but that’s not always possible when working at the financial layer -- i.e., working within budget constraints that limit the number of controllers and licenses you can afford.
The final factor is the use of RF heat maps within the Wi-Fi manager to control AP radio transmit power. These maps rely on accurate AP location information to determine the distances between APs. If the maps are inaccurate or if the AP locations on those maps are incorrect, then the Wi-Fi management system won’t be able to optimize the RF environment. The resulting Wi-Fi coverage is unpredictable and can leave holes that impact mobile UC users.
Summary
Use a network management system to provide visibility into network problems that affect the wireless infrastructure. Careful AP placement and the proper configuration of the controllers is essential. Beware of using Layer 2 roaming in large networks, because a broadcast storm or spanning tree loop in one part of the network will affect the entire Layer 2 network.
Pay attention to detail when designing your Wi-Fi network and you’ll be rewarded with a system that seamlessly handles UC.
