Share this page : facebooktwitterlinkedinmailfacebooktwitterlinkedinmail

The APs would be connected to your access layer switches in a management or other isolated VLAN to keep the APs from having to process unnecessary packets.

Another consideration is how to deploy the APs per controller on the network if you will have more than one. A common deployment is known as a salt-and-pepper or checkerboard deployment. With salt-and-pepper, the idea is that you have every other AP on the floor joined to a different controller, as shown in Figure below. This physical deployment provides dynamic traffic load-balancing between the controllers and radio frequency (RF) resiliency. In the case of a single controller failure, only half of the APs on a particular floor would be lost; the remaining APs could still service the wireless clients.

Although salt-and-pepper sounds like a good idea in theory, it does have several drawbacks. With every other AP joined to a different controller, the number of inter-controller roaming events increases many-fold. As illustrated in Figure below, almost every time a client roams, it roams to an AP joined to the other controller. Even though inter-controller roaming is highly optimized when you configure it correctly, intracon-troller roaming is more efficient. The aggressive load-balancing feature of the controller, if enabled, works on a per-controller basis, so a salt-and-pepper deployment would essentially defeat the feature.

Figure 5-1


Dense AP deployment considerations


In the 2.4-GHz band, you have only three nonoverlapping channels: 1, 6, and 11. This limitation creates unique design constraints for a dense deployment because you need to mitigate co-channel interference while still providing adequate coverage for the users. As the number of clients increases, the majority of the co-channel interference will be caused by the clients sending and receiving packets on the same channel.

Although you can have up to 254 clients associated with a single AP (128 per radio if AP has both b/g and a radio), because wireless is half-duplex cisco recommends designing the network for no more than 40-50 users per AP in an auditorium-like environment. You can place the APs as close as to within 6 feet of each other. APs on the same channel should be as far apart from each other as you can manage.

When designing a dense AP deployment, Cisco recommends using the 5-GHz band. Unlike the 2.4-GHz range where you are limited to only three nonoverlapping channels, the 5-GHz band has 21 nonoverlapping channels. Having more channels to use can mean that no two APs have to use the same channel, which greatly reduces co-channel interference. If you needed to place more than 21 APs, you could really increase the physical distance between APs on the same channel.

Location Design Considerations

The Cisco Wireless Location Appliance and the new, more powerful Mobility Services Engine (MSE) aggregate client information from the APs on the controllers to track locations of wireless devices. A single Location Appliance can track up to 2500 individual devices, including legitimate wireless clients, rogue clients, rogue APs, and radio frequency identification (RFID) tags. The 3350 MSE can track up to 18,000 devices, 3,000 Monitor Mode APs, and its Mobile Intelligent Roaming software supports up to 2,000 registered devices simultaneously. Although the Location Appliance and MSE are outside the scope of this topic, it is important to understand the design considerations that a location-based deployment requires.

Cisco advertises accuracy within 30 feet (10 m) 90 percent of the time when using the Location Appliance. The more APs that hear a wireless device, the better the location accuracy is going to be. You want to ensure that no fewer than three APs, and preferably four or five, provide coverage to every area where device location is required.

Make sure these two important guidelines are adhered to, listed in order of priority:

1. Access points should surround the desired location.

2. One access point should be placed roughly every 50 to 70 linear feet (about 17 to 20 meters). This translates into one access point every 2500 to 5000 square feet (about 230 to 450 square meters).

You should place APs around the periphery of the environment to help locate devices close to outside walls. Staggering APs within the interior as well as the periphery greatly improve location accuracy, as illustrated:


Using directional antennas on the APs along the walls also helps by keeping the wireless signal within the building.

Note Devices must be detected at signals greater than -75 dBm for the controllers to forward information to the location appliance or MSE. No fewer than three access points should be able to detect any device at signals below -75 dBm.

As you already know, dense deployments pose unique design and RF considerations. Should you have both voice and data clients, a dense deployment of APs can actually hinder the voice clients because the phones hear too many APs and are not able to make good roaming decisions. To have a deployment that satisfies the requirements for good location accuracy and allows your wireless phones to operate optimally, you can place APs in Monitor mode throughout the network