Controller Based WLANs

Product and Software: This article applies to all Aruba controllers and ArubaOS versions.

Every virtual access point (VAP) appears as an independent AP to the client. The VAPs emulate the operations of a physical AP at the MAC level. All wireless management traffic that is transmitted by one physical AP also is transmitted by the VAP. For example, a physical AP can broadcast three SSIDs (using virtual APs). This AP also transmits the management traffic of three independent APs, one for each VAP it supports.

The actual bandwidth supported by an 802.11 AP is constant. (11 Mb/s for 802.11b, 54 Mb/s for 802.11g, and 54 Mb/s for 802.11a.) The actual bandwidth supported is independent of the number of the VAPs. The bandwidth available per 802.11 channel is fixed and the bandwidth required for management traffic requirement is on a per VAP basis. So the definition of multiple VAPs results in a proportional decrease in the data bandwidth.

This concept is further explained using this example:
Net = Net bandwidth available on an AP (11 Mb/s for a 802.11b AP and 54 Mb/s for a 802.11a AP)
Mgmt = Net bandwidth required per AP per SSID (per VAP)
VAP = Number of virtual APs configured
Data = Net data throughput available for data traffic
Data = Net - (Mgmt * VAP)

As can be seen, the data bandwidth decreases as the number of VAPs configured increases.

A large number of VAP definitions can result in very low data throughput, especially in an 80.211b/g environment. In extreme cases, on-the-air traffic congestion can result.

Note: The throughput of an 802.11 cell is not only affected by the traffic to and from the APs and stations in the WLAN, but also by the neighboring APs and stations whose transmit coverage area includes the 802.11 cell in question.

The effects of VAPs on the WLAN network largely depends on the 802.11 band used. This is attributed to the coverage area and the channels supported by the 802.11b/g versus the 802.11a band. 802.11a has a large number of channels to choose from and the coverage area is smaller (100 feet as compared to 250-300 feet for b/g). Smaller cell sizes and larger number of channels results in lower chances of having neighboring cells of the same channel whose traffic can result in interference in any given area. As a result with the 802.11a band, the effect of management traffic on the overall throughput is minimal when multiple VAPs are defined, as compared to the b/g band.