Defining Wi-Fi: CCI (Co-Channel Interference) also called CCC (Co-Channel Contention)

This series of blogs (Defining Wi-Fi) will likely stretch to infinity. The blogs will focus on defining terms related to Wi-Fi at a level between the dictionary and a concise encyclopedia, but not quite matching either. Hopefully, the community finds them helpful over time.

NOTE: Entry created August 24, 2016.

Co-Channel Interference (CCI) or Co-Channel Contention (CCC), which is the more apt name, but not in the standard, is an important factor to consider in WLAN design. Co-Channel Interference is that which occurs when a device (station) participating in one Basic Service Set (BSS) must defer access to the medium while a device from a different service set (either an infrastructure or independent BSS) is using the medium. This behavior is normal and is the intentional result of the 802.11 specifications. The behavior is driven by standard Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) algorithms defined in the 802.11 protocol.

For further understanding, consider the scenario where a laptop (STA1) is connected to an AP on channel 1 (AP1). Another AP (AP2)  is on channel 1 at some distance and another laptop STA2) is connected to that remote AP. Even if the two APs are not required to defer to each others’ frames (because the signal level is too low), the two laptops must defer to each others’ frames if they can hear each other at a sufficient signal level.

CCI

That is, the two laptops are transmitting on channel 1 and they are within sufficient range of each other and, therefore, they must contend with each other for access to the medium, resulting in CCI. Additionally, both laptops may transmit a strong enough signal to cause both APs to defer even though they have chosen to associate to only one of the APs based on superior signal strength. Also, both APs may transmit a strong enough signal to cause both laptops to defer even though they are associated to only one of the APs.

To be clear, it is common for APs to create CCI with each other. The point of using this example is to eradicate, from the start, the common myth that CCI is just about APs. CCI is created by any 802.11 device operating on the same channel with sufficient received signal strength at another device on the same channel.

Now, because CCI is not like other RF interference, a modern movement to call it Co-Channel Contention (CCC) has started. In my opinion, this is not a bad thing. CCC brings more clarity to the picture. CCI is about contention and not traditional interference.

What we commonly call interference is a non-Wi-Fi signal or a Wi-Fi signal from another channel that corrupts the frames on the channel on which a device is operating. That is, with other types of interference, unlike contention, the Wi-Fi client may gain access to the medium and begin transmitting a frame while the non-Wi-Fi (or other channel Wi-Fi) is not communicating such that the transmitting Wi-Fi device sees a clear channel. During the frame transmission, the other transmitter may begin transmission as well, without acknowledgement of current energy on the channel, and cause corruption of the Wi-Fi frame. This is not the same as CCI.

Excessive CCI results in very poor performance on the WLAN. With too many devices on a given channel, whether in a single BSS or from multiple service sets, the capacity of the channel is quickly consumed and performance per device is greatly diminished. For this reason, CCI must be carefully considered during WLAN design.

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