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Revolutionizing Wireless LAN Deployment Economics with the Meru Networks Radio Switch
Deployment Challenges for Today's Wireless LAN Architectures
For the enterprise IT administrator tasked with deploying a wireless LAN (WLAN) of any scale, the significant investment in equipment, time and resources to properly plan and install the deployment is daunting. Regardless of whether the wireless LAN solution is 'heavy' or 'light' 1, the current generation of WLAN solutions rely on careful channel planning and time-consuming site surveys to utilize the limited spectrum in the most efficient method. Channel planning in particular plays a crucial role in the performance of the network, and is directly correlated to the complexity and time involved for deploying in a high density environment. However, as we will see, due to fundamental restrictions in the 802.11b and 802.11g bands, even the most carefully designed channel plans do not eliminate issues that severely impact a dense WLAN network requiring a completely new approach.
Understanding the Reasoning Behind Channel Planning
The 802.11 standards utilize the 2.4 GHz (802.11b and 802.11g) and the 5 GHz (802.11a) bands. Currently, the deployment choice for the majority of enterprises is 802.11b and 802.11g due to their compatibility and the near ubiquity of those standards in today's various client devices.
In the 2.4 GHz band, there are up to 112 channels available. Each channel offers 11 Mbps (802.11b) or 54 Mbps (802.11g) over-the-air data rates. All clients that are associated with the access point will share either 11 Mbps or 54 Mbps available in that channel. If there are numerous clients in a limited physical area, or the supported applications require significant bandwidth, adding capacity to the network is accomplished by using more access points. As wireless is a shared medium like Ethernet hubs, adding capacity to a WLAN network means that non-overlapping channels must be used. Otherwise, if the same channel is used by a second (or third or fourth) access point in the same physical area, then no additional capacity will be added. In the 2.4 GHz band, ensuring only non-overlapping channels are used when deploying a WLAN restricts the network designer to three (3) channels.
Factors Impacting Wireless LAN Deployment
With this limitation, designing a WLAN network for higher density usage becomes much more challenging. Several factors play into this. The first is spatial. Standard Access Points propagate their signals in all directions, as they are equipped with an omnidirectional antenna. Omni directional antennas direct the AP signals equally in all directions, both in the horizontal as well as the vertical plane. For multi-story buildings, this adds a third dimension to the challenge of preventing overlapping channels as the diagram below demonstrates. In fact, it is impossible to have non-overlapping access point coverage areas with only three channels.3 The second factor that complicates designing a high density wireless LAN network is the fact that over-the-air speed decreases as distance from the access point increases. That is, the highest data rates (11 Mbps or 54 Mbps) are only available at short ranges from the access point. Farther from the access point, data rates decrease as shown in the figure below.
Therefore, to maximize client throughput, access points must be spaced more closely together to ensure that clients attach at the higher data rates.
On the surface, decreasing the distance between access points seems like a simple answer to maintain high capacity for larger numbers of clients. However, access point RF propagation does not stop at the desired data rate. I.e. if a desired data rate of 24 Mbps is used to space access points to achieve a higher overall throughput, the access point signal will continue past the 24 Mbps radius through the 12, 6, 2 and 1 Mbps data rates. This means that even careful planning to avoid adjacent APs having the same channel will not avoid the problem of co-channel interference. In fact, higher desired WLAN network bandwidths significantly aggravate this problem as more and more APs on the same channel interfere with each other. The result of this is a decrease in network capacity and performance. Because an 802.11 WLAN is a shared medium, the impact of co-channel interference is increased by client collisions as the clients hear signals from the many APs and clients surrounding them.
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