Cisco Cisco Nexus 7000 Series Network Analysis Module (NAM-NX1) White Paper
Wireless LAN Design Guide for High Density
Client Environments in Higher Education
13 © 2011 Cisco and/or its affiliates. All rights reserved.
What is Co-Channel Interference and Why is it Important in High-density WLANs?
CCI is a critical concept to understand when it comes to understanding the behavior and performance of 802.11 WLANs. It is a
phenomenon where transmissions from one 802.11 device bleed into the receive range of other 802.11 devices on the same
channel, causing interference and reducing the available spectrum and resulting performance. CCI can cause channel access
delays as well as collisions in transmissions that corrupt frames in transit. Figure 8 illustrates how APs on the same channel
interfere with each other.
phenomenon where transmissions from one 802.11 device bleed into the receive range of other 802.11 devices on the same
channel, causing interference and reducing the available spectrum and resulting performance. CCI can cause channel access
delays as well as collisions in transmissions that corrupt frames in transit. Figure 8 illustrates how APs on the same channel
interfere with each other.
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802,11 networks are contention based and rely on Clear Channel Assessment (CCA) mechanisms to judge the medium state
(if busy we wait, when free we transmit). In the example above, this client’s performance is being impacted because it can hear
both APs. To this client, the two AP cells are coupled or acting as one super cell. For the uplink, both APs’ transmissions will be
seen as a busy channel by the client and the client will simply wait for an opportunity to transmit. Worse yet, on the downlink,
transmissions from either AP will potentially collide and retries will increase the contention for the medium and continue to drive
the data rates down overall. The effects of CCI are not limited to just the AP cell. In a high-density environment, the clients
themselves will have the effect of increasing the overall cell size.
(if busy we wait, when free we transmit). In the example above, this client’s performance is being impacted because it can hear
both APs. To this client, the two AP cells are coupled or acting as one super cell. For the uplink, both APs’ transmissions will be
seen as a busy channel by the client and the client will simply wait for an opportunity to transmit. Worse yet, on the downlink,
transmissions from either AP will potentially collide and retries will increase the contention for the medium and continue to drive
the data rates down overall. The effects of CCI are not limited to just the AP cell. In a high-density environment, the clients
themselves will have the effect of increasing the overall cell size.
CCA is based on a receive threshold that evaluates the carrier for activity. It is generally a good practice to consider -85 decibels
per milliwatt (dBm) as that threshold. Figure 9 shows a coverage model based on data rates. Higher data rates do not propagate
as far. If the distances look long in this model, it is because it was calculated using an outdoor open space model rather than an
indoor model which assumes attenuating factors in the environment. There are not many walls between the APs and clients in
most high-density deployments.
per milliwatt (dBm) as that threshold. Figure 9 shows a coverage model based on data rates. Higher data rates do not propagate
as far. If the distances look long in this model, it is because it was calculated using an outdoor open space model rather than an
indoor model which assumes attenuating factors in the environment. There are not many walls between the APs and clients in
most high-density deployments.
Basic CCI – AP’s on the same Channel interfere with one another