Hello and welcome to my blog where I will be covering common topics and scenarios that I get asked about on a regular basis. Where appropriate, there will be a specific focus on the Australia and New Zealand market which is where I am based.
To begin, probably the most common topic is around optimising a wireless network. A few of these are well-known but hopefully there is something here that you find useful. With that said, when it comes to Wi-Fi design, here are my top 5 tips:
#1 - Use 5GHz
Wireless networks that are critical to the operation of the business or venue should be designed for ubiquitous 5GHz coverage. This is because:
- 2.4GHz has significantly fewer available non-overlapping channels which results in more co-channel interference and therefore a poorer user experience. In Australia and New Zealand, we have 3 non-overlapping 2.4GHz channels yet we have 23 non-overlapping 5GHz channels
- 2.4GHz tends to have more non-WiFi interference e.g. Bluetooth, microwave ovens etc.
My experience is that most deployments these days fall into this category. In addition to designing for 5GHz coverage, the BandSelect feature should be used to influence client devices that support both 2.4GHz and 5GHz to choose the 5GHz network.
#2 - Disable low data rates
Lower data rates, typically 11Mbps and below in 2.4GHz and 9Mbps and below in 5GHz should be disabled. This is due to the fact that 802.11 Management frames are sent at the lowest Mandatory data rate. Lower data rates use more time to send the same amount of data (see diagram) and thus increased channel utilisation.
- Leave 11Mbps enabled in 2.4GHz if 802.11b support is required.
- Keep in mind this reduces total cell size and this must be factored in to AP placement and survey.
#3 Reduce 2.4GHz coverage to match 5GHz
When designing for 5GHz, in most cases you will find that coverage in 2.4GHz has too much overlap which leads to higher co-channel interference. Additionally a large number of WiFi client devices choose a BSSID for association primarily on signal strength – leading to 5GHz-capable clients sticking to the crowded 2.4GHz frequency band (even if BandSelect is used). This is because 2.4GHz propagates further in free space due to its lower frequency (see diagram) as well as specific channels in 5GHz having a lower maximum transmit power (I will cover this in a future blog).
Therefore, it is often necessary to run 2.4GHz at a lower transmit power. On a Cisco network this can be dynamically adjusted through the Transmit Power Control (TPC) Power Threshold. This threshold sets the signal level used to evaluate the 3rd strongest neighbour when the TPC algorithm runs. Keeping in mind it is a negative integer, reducing it has the impact of weighting toward an overall power decrease and increasing it weights towards and overall power increase. Best practice would be to adjust it in increment/decrements of 2dB every 100 minutes (10 RRM intervals) until the signal strength at cell edge is similar to the 5GHz coverage.
In some cases, the 2.4GHz overlap is so extreme that some of the 2.4GHz radios should be turned off. This depends on the density of the APs and thus is specific to the particular deployment. This is made simple with the 2800 and 3800 Series APs with Flexible Radio Assignment which can move to dual 5GHz dynamically based on the coverage overlap of 2.4GHz.
#4 - Adjust TPC as needed for the environment
In certain circumstances it may be necessary to increase the TPC Power Threshold or set a minimum power level, particularly in 5GHz for reasons other than what is mentioned in #3.
Radio Resource Management (RRM) works based on neighbour messages sent from each AP every 60 seconds at the lowest data rate (1Mbps in 2.4GHz, 6Mbps in 5GHz) and at the highest power for the particular channel. This means that the power and channel calculations are based on AP-to-AP coverage. In certain environments, there could be situations where AP-to-AP coverage is significantly better than AP-to-Client coverage, leading to the TPC algorithm recommending a lower power than optimal. This is a very common scenario in high-ceiling deployments, particularly in warehouses that have the added affect of the racking/shelving. In these cases you can either set a minimum power for TPC or adjust the TPC Power Threshold.
#5 - Keep the number of SSIDs to a (practical) minimum
The number of SSIDs should be kept to a minimum. Each Access Point must broadcast a Beacon for each SSID that is advertised. This increases overhead and channel utilisation and can impact user experience. There is a nice 3rd party tool (see image) that can be used to visualise the impact of having additional SSIDs here - http://www.revolutionwifi.net/revolutionwifi/p/ssid-overhead-calculator.html). Instead of segmenting by SSID and impacting the RF performance, segment based on policy at the RADIUS server. That said, separate SSIDs will be required if different security methods are needed (e.g. 802.1X, PSK, Open).