Co-channel interference

Co-channel interference

By CWNP On 01/15/2008 - 25 Comments

While 802.11a/b/g stations connect to APs at data rates anywhere from 54 Mbps down to 1 Mbps, when the network is properly designed, data rates are kept as high as possible.  It's important to consider, however, that just because the slowest client might be connected at 12 Mbps in a cell, that doesn't mean that the RF signal just stops right there at that client.  On the contrary, the signal keeps going well past where a user might connect at 1 Mbps.  Even at this great distance from the AP, the RF energy is strong enough to cause clients to defer transmissions due to "busy" clear channel assessments.  This distance might be hundreds of feet indoors, depending on the environment.  In addition to the energy emitted by AP transmissions, one must consider the energy emitted by client transmissions as well.  Clients move away from APs while transmitting, and thus cause co-channel interference at a much greater range even than the AP can cause. 

Consider a scenario where a client is connected to an AP at 36 Mbps at a distance of 50 feet.  The AP has its own interference range that may extend to 200 feet (4 times the current connection distance), and the station's inteference range may be 250 feet (200 + 50) from the AP if transmitting at the same output power as the AP.  If the client moves to 100 feet from the AP and connects at 6 Mbps, its interference range may be 200 feet around itself, but that will mean 300 feet from the AP with which it's connected.  

Given this scenario, it's obvious that with recommended AP spacing of 50-70 feet and 15-20% cell overlap that co-channel interference will be a tremendous throughput thief across an entire WLAN.  In addition to a significant loss of throughput, increased jitter and delay will be evident in VoWLAN phones.  Although WLAN QoS prioritizes WLAN traffic, this occurs after the CCA and therefore prioritization does not overcome the jitter and delay introduced by CCA.  While follows most manufacturer's guidelines on AP spacing and cell overlap, an AP on channel 1 (for example) will almost always still interfere with many other APs on channel 1.  The same will hold true for APs on channel 6 and 11.  

If you're thinking that deployment in the 5 GHz UNII bands will let you escape this problem, think again.  While 5 GHz signals have less effective range than 2.4 GHz signals, APs are spaced closer together to meet the recommended cell overlap.  This causes exactly the same problem in 5 GHz deployments.  By now you must be wondering, "What's the answer to this problem?"  I will let Cisco Systems give you a hint as to where the answer does and does not lie:

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"It is not an effective strategy to reduce the overlap in order to reduce co-channel interference.  As users satisfaction can be greatly affected by poor roaming performance.  In contrast, call capacity can be addressed in planning and design."
                                Cisco Systems
                                VoWLAN Design Guide v4.1
                                December 28, 2007
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My opinion of how to address this problem starts with choosing the right system architecture for the application being deployed.  It doesn't lie in proper cell planning.

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