Cisco AP Channel Utilization

One thing that keeps me awake at night is the whole concept of Co-channel interference in WiFi networks (well, that and acid reflux) . I'm always concerned by it's presence in a Wireless LAN, how to detect it, how to avoid it and how to measure its effect. In this article I share a tip I picked up from a recent Cisco webinar to determine the general level of utilization of an AP's channel. I also look at how this may give us a clue about the level of CCI in a Cisco wireless network.

Background

Co-channel interference (CCI), co-channel contention (CCC)...whatever you want to call it (I'm growing quite fond of the term "Co-channel Chatter"), is a blight in many WiFi networks. In all but the most isolated and carefully designed of wireless LANs, it lurks, waiting to disrupt the efficient operation of our WiFi network.

As the number of WiFi networks rises, AP channel-widths increase (with newer WiFi standards) and the population of WiFi devices explodes, the growing spectre of CCI/CCC  increases steadily over time. In simple terms, if you deploy a wireless access point on a particular channel, it contends for access to the channel with any other access points (and clients) assigned to the same channel that are within "hearing range" of the AP. (Here's a link to a very useful article that gives more background on this subject).

In an ideal world, each AP we deploy on a WLAN is assigned to a unique channel and is isolated sufficiently (from an RF perspective) so that it cannot "hear" any other APs on the same channel. This will ensure that they are at a sufficiently low level that they not will impinge on the AP's clear channel assessment (CCA) process. In this ideal scenario, our AP will not have to contend with any other APs for access to the RF channel. Therefore, it will be able to service the needs of its associated clients with all of the bandwidth (air-time) that is theoretically available, within the limitations of the client and AP standard support.

However, the real world is rarely like this. All too often, APs from neighboring networks or even APs on our own network will operate on the same channel that a particular AP may be using. APs on the same channel may "hear" each other and will have to contend for (i.e. share) access to the RF channel that they are all using. This limits the potential data throughput that may be offered by each AP.

(Note: this is something of a simplified explanation, as there are additional CCC effects from WLAN clients. However this will suffice for the point I am trying to demonstrate.)

Channel Utilization Statistics

For a while, I have been troubled by the question: "how does the world (well, channel) look from an AP's point of view?" If an AP is assigned to a channel, how busy does that channel look from its perspective? I'm not talking about data the AP may itself be passing over the channel, but the level of traffic it perceives, including "chatter" from other APs and clients on the same channel.

Understanding how busy the channel is by considering all sources tells us how often the AP has an opportunity to use the channel. It may also give us a indication of how much co-channel contention (CCI/CCC) our AP may be exposed to. Once we know how busy a channel is, we can decide whether using this channel is viable for our AP.

In the Cisco AireOS solution (traditional 5508/2504 controller solution), each AP has a set of statistics available from the WLC GUI that show the AP's channel utilization. The screen shot below shows the AP channel statistics ("Load Statistics") available. Theses are available from GUI option:

Monitor > Access Points > Radios > 802.11x/y/z  >  AP Name

Figure 1 - AP Channel Utilization Stats 

The statistics available are: Rx Utilization, Tx Utilization and Channel Utilization. The statistic we're interested in is: "Channel Utilization".

The "Rx Utilization" and "Tx Utilization" figure indicate the level of traffic (%) sent and received by the AP itself.

Channel Utilization indicates how much 802.11 traffic the AP can "hear" on its channel, from all sources. The statistic is a percentage figure (15% in the example shown). This will include all 802.11 frames that the AP can hear from all APs and clients in the vicinity. It indicates the amount of time that the AP considers the channel to be busy.

High Channel Utilization

This is incredibly useful. For one thing, we can see when our channel is "full". Once we hit 100%, no amount of additional APs is going to improve things and allow us to accommodate more traffic - all air-time (i.e. opportunities to transmit) is being used up. The utilization may be caused by our own AP, by nearby clients or by nearby APs using the same channel. Whatever the reason, having the ability to detect a highly utilized channel means that we can come up with a strategy to try to fix the situation.

A typical concern in new WLANs is the effect of APs in our own WLAN network that are using the same channel. We have a finite number of channels to use, so channel re-use is inevitable in all but the simplest of networks. Although we may have planned our network channels carefully, just how "clear" or "available" is the channel we have chosen for each AP? The channel utilization statistic give us an excellent view of this aspect of a channel, from the AP's viewpoint. If we choose a channel and the channel utilization statistic is low, we're probably in pretty good shape.

I recently installed a new wireless network which required a fairly dense deployment of access points. Co-channel interference was unavoidable in some areas and had to be minimized as far as possible. In some areas of the network, the Channel Utilization statistic revealed that the channel used by some APs was being heavily utilized (around 60%). The question I had was: "Is this due to client traffic, or the effect of co-channel interference from other APs nearby?" By waiting until the end of day, I saw that the channel utilization did, in fact, fall down to much lower levels once all users (and their clients) had gone home for the day. The majority of the channel utilization was down to real client traffic, rather than CCI  from nearby APs (which was a relief).

Lab Test

To demonstrate the effect of CCI on the Channel Utilization statistic, I ran up some APs in my home lab and placed them on the same channel to deliberately cause AP CCI. I used the 2.4GHz band, created several SSIDs and configured the AP radios to support rates down to 1Mbps to chew up some air-time. By varying the number of APs on the channel and the number of SSIDs being broadcast, we can see the effect on the channel utilization from the point of view of our test AP.

Here are the results:

1. The first test had 3 SSIDS, with 1 AP enabled. I also had a neighbor's home network using the same channel which was adding to the channel utilization observed. The channel used (channel 11) was running at around 16% utilization:

Figure 2 - 1AP, 3SSIDs

2. Next I added another AP running the same 3 SSIDs - so now we had 2 lab APs on the same channel. Again, it ran on channel 11 to create more CCI/CCC via the beacons being broadcast by the APs. This increased the channel utilization to 24%:

Figure 3 - 2 APs, 3SSIDs

3. Next, I added a third AP, again running the same 3 SSIDs on channel 11. This bumped the channel utilization up to 33% with the additional beacon traffic.

Figure 4 - 3 AP, 3 SSIDs

4. Finally, I doubled the number of SSIDs to 6 across all 3 APs. This again bumped up the beacon traffic and increased utilization to 54%:

Figure 5 - 3 APs, 6 SSIDs

Note that in all cases, only new SSIDs and APs were being added to the equation - little or no client traffic was being generated to cause this effect. This was due to the CCI/CCC being added by the beacons being broadcast by the APs on the same channel.

Although in the real world, not all channel utilization is caused by AP CCC/CCI, channel utilization can be a useful indication, particularly if you compare levels in-hours and out-of-hours (when client numbers are very low). 

(Note: channel utilization statistics update around every minute or so - they are not dynamic)

CLI Utilization Stats

If you're a 'CLI' sort of person, you may prefer to look at this information from the CLI of your WLC. You can pull the same information using the commands:

show ap auto-rf 802.11b <ap name> 

or

show ap auto-rf 802.11a <ap name>

You'll get an output similar to the output shown below when you run the command.The channel utilization information is highlighted below in the example using red text:

(Note: channel utilization stats update around every minute or so - they are not dynamic)

Number Of Slots.................................. 2

AP Name.......................................... AP2600

MAC Address...................................... 44:2b:03:9a:xx:xx

  Slot ID........................................ 0

  Radio Type..................................... RADIO_TYPE_80211b/g

  Sub-band Type.................................. All

  Noise Information

    Noise Profile................................ PASSED

    Channel 1....................................  -95 dBm

    Channel 2....................................  -94 dBm

    Channel 3....................................  -76 dBm

    Channel 4....................................  -85 dBm

    Channel 5....................................  -93 dBm

    Channel 6....................................  -78 dBm

    Channel 7....................................  -88 dBm

    Channel 8....................................  -91 dBm

    Channel 9....................................  -92 dBm

    Channel 10...................................  -93 dBm

    Channel 11...................................  -93 dBm

    Channel 12...................................  -92 dBm

    Channel 13...................................  -92 dBm

  Interference Information

    Interference Profile......................... PASSED

    Channel 1....................................  -77 dBm @  7 % busy

    Channel 2.................................... -128 dBm @  0 % busy

    Channel 3.................................... -128 dBm @  0 % busy

    Channel 4.................................... -128 dBm @  0 % busy

    Channel 5.................................... -128 dBm @  0 % busy

    Channel 6....................................  -83 dBm @  2 % busy

    Channel 7....................................  -81 dBm @  7 % busy

    Channel 8.................................... -128 dBm @  0 % busy

    Channel 9.................................... -128 dBm @  0 % busy

    Channel 10................................... -128 dBm @  0 % busy

    Channel 11................................... -128 dBm @  0 % busy

    Channel 12................................... -128 dBm @  0 % busy

    Channel 13................................... -128 dBm @  0 % busy

  Load Information

    Load Profile................................. PASSED

    Receive Utilization.......................... 0 %

    Transmit Utilization......................... 4 %

    Channel Utilization.......................... 66 %

    Attached Clients............................. 0 clients

WLCCA

Another great way to assess channel utilization is to use the WLC Configuration Analysis (WLCCA) tool from Cisco.

Briefly, the tool is a Windows application  available for free from Cisco and provides an analysis of a WLC configuration. You simply load in the output of the show 'run-config' command from the WLC CLI. You can get it from : https://supportforums.cisco.com/document/7711/wlc-config-analyzer (access request required)

It does plenty more than showing AP channel utilization, but it provides a nice report showing both 2.4 and 5GHz channel utilization per AP. Here are the results from my lab:

Figure 6 - WLLC AP Channel Utilization

Prime Infrastructure

One other way of viewing AP channel utilization is to use the Cisco Prime Infrastructure management application. 

If you have set up building and floor maps of your wireless LAN within CPI, one of the available maps views for access points  is channel utilization. I don't have a graphic to add in to this article, but I'm sure you will be able to find it with a little experimentation.

How Do We Fix High Utilization?

Although my fixation with CCI/CCC has been the main focus of this article, high channel utilization will not necessarily be caused by CCI/CCC.

Here are a number of suggestions for sources of high channel utilization. By attending to one or more of these possible causes, we may cut down the levels of utilization observed, and improve the performance of our WLAN:

1. WLAN support for low basic rate speeds: if your AP radios are configured to support lower, legacy basic-rate connection speeds, then much of the management traffic (e.g. beacons) that is exchanged will use the lowest available connection speeds. This causes the traffic to occupy more of the available air-time and takes longer to transmit (increasing channel utilization).
   
   Support for lower speeds also provides wireless clients the opportunity to rate shift to use lower connection speeds as they move away from an AP and consume more air-time (increasing channel utilization).
2. Channel planning: have a look at your WLAN channel planning. Do you have APs on the same channel that are perhaps contending to use the same channel (i.e. CCC/CCI)? This will be most obvious once users have gone home for the day and channel utilization levels remain high. Try adjusting channel allocations to avoid APs in close proximity using the same channel.
3. Cell size adjustment: you might also try adjusting  AP transmit power to reduce AP cell sizes (and hence how well APs can "hear" each other). One caveat of this approach is that it may affect the coverage experienced by clients in a negative way. You need to be very careful with this approach, or you may get calls about users who suddenly can't connect in a particular area. You might need to get the survey gear out for this one.
4. Neighboring networks: high channel utilization may be caused by access points (or their clients) on neighboring WiFi networks. Check you rogue AP lists and see if they are operating on the same channel as your own WLAN APs.You may need to change your AP channel to avoid the neighboring network.
5. Client traffic levels: it may be that a high level of channel utilization is simply due to a high level of client traffic. This might be caused by the number of clients connecting to an AP, or perhaps the type of traffic that some clients are imposing on the network (e.g. HD video). This would need careful analysis, with perhaps some attempts to more evenly distribute clients across APs or perhaps rate-limit specific applications.  

Summary

In this article, we've taken a brief stroll through the Channel Utilization statistic available for Cisco APs. Hopefully it has provided some useful information to assess how busy an AP's channel appears from a channel availability perspective. 

We've seen that it shows us the air-time utilization of a channel that an AP may be using. This can be very useful to understand whether a channel is appropriate for use. If it's too busy, we may be well-advised to select an alternative. 

We also looked at how we might get an indication of CCC/CCI by comparing utilization levels in and out of hours. 

We explored the various methods we can use to look at utilization figures, which included: the WLC GUI, WLC CLI, WLCCA and Prime Infrastructure.

Finally, we took a quick look at possible causes of high channel utilization and possible remediation of the issue.