14.1 Co-Channel Contention vs Adjacent Channel Interference
Key Takeaways
- Co-channel contention (CCC) is 802.11 devices sharing one channel and deferring with CSMA/CA, which splits airtime even when frames still decode
- Adjacent or overlapping channel interference (ACI) corrupts frames because energy overlaps but stations cannot read each other's Duration fields or set NAV
- Non-802.11 sources such as microwave ovens, Bluetooth, radar, and analog cameras show up as frequency utilization on a spectrum analyzer, not as decodable MPDUs
- Mitigation is remove the source or change channel plan and reuse; raising transmit power often worsens CCC and ACI
- Strong RSSI with high retries is usually CCC, ACI, or non-Wi-Fi energy, not a coverage hole that needs more AP power
Harborline Logistics files the same ticket every peak shift: scanners show four bars, pings look fine, and the WMS still times out. The floor lead's instinct is to crank AP power. Objective 6.2 of CWNA-109 is why that instinct is usually wrong. This OpenExamPrep section teaches RF disruption from 802.11 devices and from non-802.11 devices, and it splits 802.11 disruption into two mechanisms you must name on sight: co-channel contention (CCC) versus adjacent / overlapping channel interference (ACI). Independent OpenExamPrep material here is study teaching from OpenExamPrep; it does not claim CWNP approval, partnership, or exact equivalence with CWNP training.
Chapter 13 asked whether coverage, roaming, and throughput met the design. This chapter asks what to do when the air is busy or dirty. Domain 6 weights RF validation and remediation at 20 percent of CWNA-109. You cannot treat every retry storm as "need more signal."
Contention is not the same as interference
English uses interference for anything that slows Wi-Fi. The exam does not. Contention is a MAC scheduling problem. Interference is a PHY corruption problem. Both raise retries and both lower goodput. The repair is different, so the diagnosis must be different.
IEEE 802.11 shares a channel with CSMA/CA. A station that can decode another station's preamble and Duration field treats the medium as busy, waits, and then contends. That is cooperation. Two BSS on channel 36 that hear each other are not smashing bits; they are taking turns. Throughput falls because each BSS gets a smaller slice of airtime, not because the constellation is unreadable.
Interference in the 6.2 sense is energy that prevents a correct decode. The receiver sees CRC/FCS failures, missing ACKs, and retries. The other transmitter is close enough in frequency to raise the noise floor or to collide in time, but not decodable as an 802.11 frame the local radio can parse. Without a parse, NAV never loads. Physical CCA may still energy-detect and defer, or it may not, depending on the interferer's duty cycle and the CCA thresholds. Either way you have lost the polite handshake that CCC still has.
If a stem says two APs are on the same channel and clients defer, think CCC. If it says channels overlap, frames corrupt, and stations cannot read each other's NAV, think ACI.
Co-channel contention (CCC)
Co-channel contention is what CWNP-style teaching used to lump under "co-channel interference" (CCI). The newer name is more honest: same-channel 802.11 neighbors mostly contend, they do not always interfere at the PHY.
Picture Harborline aisle 7. AP-A7 and AP-A8 both beacon 20 MHz channel 36. A forklift tablet associated to A7 can decode A8's preamble. CSMA/CA works across the overlapping BSS (OBSS):
- A8 starts a PPDU.
- The tablet's PHY recognizes an 802.11 preamble and holds CCA busy for the announced length.
- The MAC copies Duration into NAV if the frame is decoded far enough to read it.
- The tablet does not transmit into A8's burst.
- After SIFS/ACK and the next idle gap, both BSS draw backoff and share the remaining airtime.
The PHY of a successful frame is often clean. SNR can still look excellent because the intended AP is close. What you lose is airtime. Controller charts show high channel utilization, moderate-to-high retries when hidden edges or mixed PHYs still collide, and MCS that does not explain the application slowness. iperf through a quiet 02:00 cell is fine; the same cell at 14:00 with four OBSS neighbors is not.
CCC scales with how many 802.11 transmitters share the channel and how much they talk, not only with how many SSIDs you named. Neighboring tenants, mesh backhaul on the client channel, a high-density scan of Probe Requests, and a mis-set "auto" channel that parked eight Harborline APs on 36 are all CCC engines. 802.11 device proximity matters: two APs thirty meters apart on 36 with moderate power may share politely; the same two at ten meters with high EIRP hear each other constantly and serialize almost all traffic.
CCC is still a problem. Voice MOS falls, scanner batches queue, and airtime fairness can only rearrange the pain. The repair is reuse design: more non-overlapping channels (5 GHz and 6 GHz help), lower AP power so cells do not hear as many co-channel neighbors, disable unused 2.4 GHz radios, move backhaul off the client channel, and stop adding APs on the same channel "for coverage" when RSSI is already −45 dBm.
Adjacent channel interference (ACI) and overlapping spectra
Adjacent / overlapping channel interference is the other 802.11-on-802.11 failure. The spectra overlap, so energy from BSS-B lands in BSS-A's receiver, but BSS-A cannot decode BSS-B's frames well enough to honor NAV.
Classic 2.4 GHz example: AP-A on channel 1 and AP-B on channel 3 or 4. Those 22 MHz-class DSSS/HR-DSSS and 20 MHz OFDM occupancy plots overlap. A station on channel 1 sees BSS-B as in-band noise. It does not reliably parse BSS-B's Duration. NAV stays zero. If energy detect does not hold the medium for the whole overlapping PPDU, the channel-1 station transmits on top of the overlapping burst. Both frames fail FCS. Retries explode. RSSI of the intended AP can still be strong because the AP is in the same aisle.
That is why 2.4 GHz enterprise design uses 1, 6, and 11 in the United States (and equivalent non-overlapping sets elsewhere). Channels 1 and 6 have enough separation that the energy in each other's passband is usually manageable. Channels 1 and 4 do not. "We used four channels so we could deploy more APs" is how Harborline's first warehouse overlay created ACI on purpose.
Width makes overlap even when the primary channel number looks unique:
| Plan | What overlaps | Exam reading |
|---|---|---|
| 2.4 GHz 20 MHz on 1, 6, 11 | Little overlap if filters and power are sane | Intended reuse set |
| 2.4 GHz 20 MHz on 1, 4, 8, 11 | Substantial spectral overlap | ACI among 802.11 BSS |
| 2.4 GHz 40 MHz | Occupies two 20 MHz blocks; almost always overlaps a neighbor | Usually a CCC+ACI tax indoors |
| 5 GHz 20 MHz on 36 and 40 | Adjacent 20 MHz channels; modest ACI at high power / close range | Prefer distance or lower power |
| 5 GHz 40 MHz on 38 next to 20 MHz on 40 | The 40 MHz BSS sits on top of the 20 MHz primary | Overlapping-channel interference |
| 5 GHz 80 MHz bonded next to another 80 MHz that shares any 20 MHz | Shared sub-channels | Treat as partial co-channel plus ACI |
ACI is PHY corruption. Protocol analyzers show retries, duplicate frames, and data frames without ACK. Spectrum views show 802.11-shaped bursts that are offset from your primary. WLAN scanners show a neighbor SSID on "the next channel." Raising power makes the overlapping energy stronger at the victim, so ACI gets worse.
802.11 proximity versus non-802.11 interferers
Objective 6.2 groups RF disruption from 802.11 devices with disruption from non-802.11 devices. The first family still speaks 802.11 (CCC or ACI). The second family does not.
Non-802.11 examples Harborline actually owns:
- Microwave ovens near 2.45 GHz. Broadband splatters across the middle of 2.4 GHz while the magnetron cycles. Duty cycle follows the cook timer, not CSMA/CA. Break-room APs die at lunch and recover at 13:05.
- Bluetooth (and BLE). Frequency-hopping across 2.4 GHz. Short hops look like speckles on a waterfall. Headsets and scanners in a supervisor's cup-holder raise retries without a second SSID on the scanner.
- Radar in 5 GHz DFS sub-bands. The AP must vacate; clients see a radio disappear, not a polite OBSS. Spectrum shows pulses that are not 802.11 preambles.
- Analog wireless cameras, poorly filtered video senders, some older cordless phones, and certain motion sensors. Continuous or high-duty energy. A protocol analyzer sees silence or garbage, not a BSSID.
Detection splits on what you can decode:
| What you measure | CCC (same channel 802.11) | ACI (overlapping 802.11) | Non-802.11 energy |
|---|---|---|---|
| WLAN scanner SSIDs | Neighbors on your channel | Neighbors on nearby channels | Often none |
| Protocol analyzer | Valid MPDUs, NAV, OBSS, retry bit | Your frames retry; neighbor frames may not decode | Few or no 802.11 frames during the hit |
| Spectrum analyzer | 802.11-shaped occupancy on your primary | 802.11-shaped occupancy offset | Microwave smear, Bluetooth hops, CW, radar pulses |
| Airtime / channel utilization | High, and it is mostly Wi-Fi airtime | High retries; utilization may look "busy" from energy detect | Frequency utilization without matching Wi-Fi airtime |
| RSSI to serving AP | Often strong | Often strong | Often strong |
Airtime utilization (Wi-Fi busy time from decoded or CCA-busy 802.11) versus frequency utilization (RF energy in the band regardless of PHY) is the 6.2 skill. High Wi-Fi airtime plus many BSSIDs on 36 is CCC. High energy on the spectrum with an empty SSID list is a microwave or a camera. High retries plus a neighbor on channel 4 is ACI.
Mitigation: remove the source or change channel usage
The objective's remediation language is blunt: remove the interfering source or change channel usage. Translate that into tickets:
- Remove or relocate the microwave, camera, Bluetooth HID pile, or the extra AP a vendor wheeled in on channel 3.
- Replan channels so 2.4 GHz is 1/6/11 only, 5 GHz primaries do not overlap bonded widths, and DFS channels are used only where radar is acceptable.
- Reduce cell size (power, antennas, 2.4 GHz disable) so co-channel APs stop hearing every peer. That cuts CCC. It is the opposite of "turn it up."
- Move clients to 5 GHz or 6 GHz when the interferer is stuck in 2.4 GHz.
- Shield or replace analog cameras; Ethernet or 5 GHz radios beat a 2.4 GHz analog sender.
Do not "fix" strong-RSSI retry storms by raising AP power. Extra EIRP grows the contention domain, deepens ACI into the next channel, and still does nothing to a microwave. The trap the exam loves: strong RSSI + high retries looks like a weak client to a helpdesk that only reads bars. Bars measure intended-signal strength, not noise, not airtime, and not overlap.
Worked Harborline aisle 7
Ticket: "Aisle 7 scanners at −48 dBm, 35 percent retries, WMS timeouts 10:00–16:00. Please increase power."
Walk the diagnosis, not the power slider. Scanner capture shows three other Harborline BSSIDs on channel 36 and a tenant AP on channel 38 (40 MHz). Spectrum at lunch also shows a 2.4 GHz smear from the nearby break-room oven; aisle 7's 5 GHz radio is the one in the ticket, so the oven is a distractor unless the scanners are stuck on 2.4 GHz. Channel utilization on 36 is 62 percent with only a handful of local clients. That is CCC from OBSS, with a possible ACI contribution from the 40 MHz neighbor. Repair: move two Harborline APs off 36, shrink 2.4 GHz, ask the tenant to use a non-overlapping 20 MHz primary, and leave power alone. After the change, RSSI is still about −50 dBm and retries fall under 8 percent. The application follows retries, not bars.
On the exam
- Same channel + deferral + airtime sharing = CCC, not "the PHY is always corrupted."
- Overlapping channel + cannot decode NAV + FCS failures = ACI.
- Microwave, Bluetooth, radar, cameras = non-802.11; use spectrum signatures and frequency utilization.
- Mitigation is remove or rechannel; more power is the trap when RSSI is already strong.
Key Takeaways
- CCC is CSMA/CA airtime sharing among 802.11 devices on one channel.
- ACI is overlapping spectra that corrupt frames because NAV cannot be set.
- Non-802.11 interferers are found in spectrum, not in SSID lists.
- Strong RSSI with retries points to contention or interference, not a coverage hole.
Two Harborline APs and their clients operate on the same 20 MHz channel, decode each other's preambles, and defer using CSMA/CA. Application throughput falls at shift change even though SNR to the serving AP stays high. What is the primary RF mechanism?
Harborline places a 2.4 GHz AP on channel 1 and a neighbor AP on channel 4. Clients show strong RSSI to their own AP, high FCS failures, and the radios cannot read each other's Duration fields to load NAV. Which description matches this disruption?
A Harborline break-room WLAN shows excellent RSSI, a clean SSID list on channel 6, and a spectrum waterfall with broadband energy near 2.45 GHz that appears only while the microwave runs. Channel utilization from decoded Wi-Fi stays modest. What should the administrator do first?