As my good friend and WLAN expert, Keith Parsons, once pointed out, “Wi-Fi is 100 times more sensitive to other Wi-Fi than non-Wi-Fi interference!” Any Wi-Fi radio that hears another Wi-Fi radio on the same channel will defer and result in the medium contention overhead and delay.
As shown in Figure 1, despite a three-channel reuse pattern, APs on the same channel will hear each other and defer. For example, if AP-1 on channel 6 hears the preamble transmission of a nearby AP-2, also transmitting on channel 6, AP-1 will defer and cannot transmit at the same time. Likewise, all the clients associated to AP-1 must also defer transmission if they hear the preamble transmission of AP-2. All these deferrals create medium contention overhead and consume valuable airtime because you have two basic service sets on the same channel that can hear each other. Co-channel interference (CCI) is also often referred to as an overlapping basic service set (OBSS).
In reality, Wi-Fi clients are the primary cause of OBSS and CCI interference. As shown in Figure 2, if a client associated to AP-1 is transmitting on channel 11, it is possible that AP-2 (and any clients associated to AP-2) will hear the PHY preamble of the client and must defer any transmissions. Co-channel interference (CCI) is the top cause of needless airtime consumption that can be minimized with proper WLAN design best practices. What most people do not understand about CCI is the fact that clients are the number one cause of CCI. You should understand that CCI is not static and is always changing due to the mobility of client devices.
Because of the fact that only three channels are available in the 2.4 GHz band and because of CCI caused by clients, CCI is pretty much inevitable in the 2.4 GHz band. One strategy to reduce CCI in the 2.4 GHz band is to turn off a lot of the 2.4 GHz radios in dual-frequency access points and rely more of the coverage provided by the 5 GHz AP radios to meet density needs. While it is almost impossible to prevent CCI in the 2.4 GHz band, the airtime consumption that is a result of CCI can be minimized and possibly avoided with good 5 GHz WLAN design simply because there are more available channels.
Since CCI is such a big problem, why is the SD threshold for detecting an 802.11 preamble so sensitive? Probably the best answer is to minimize hidden nodes. In the early days of Wi-Fi, most WLANs were designed for wide coverage. A sensitive SD threshold helps to ensure that 802.11 stations hear each other, even at great distances. However, most modern-day WLANs are designed for user and device capacity as opposed to coverage. The sensitive SD threshold instead causes medium contention overhead because stations from overlapping basic service sets can hear each other.
Portions of this blog have been excerpted from the forthcoming 5thedition of Sybex Publishing’s Certified Wireless Network Administrator (CWNA) Study Guide: http://a.co/bXX3i9F