The Wi-Fi is slow is a common complaint in the network world. Poor performance on a Wi-Fi network has many different causes, some of which include:
- Access Points using legacy data rates such as 1 and 2 Mbps
- Too many devices transmitting on the same channel
- Accesso Points transmitting too high of power level
- A high retry rate of transmissions
Let’s analyze with more details each one of these problems!
Access Points using legacy data rates such as 1 and 2 Mbps
Newer Wi-Fi devices are continuously being designed and delivered to be faster than before. But older devices that are considered “slow” by today’s standards are still out there, and many Wi-Fi networks by default still support them. This means the network’s access points (AP from now on) support connections at slow legacy data rates such as 1 and 2 Mbps. This, in turn, can cause Wi-Fi devices that are far from the AP to connect at very low rates, and very old devices to connect at very low rates. Such connections can slow things down for everyone on the network.
When troubleshooting a slow performance issue, or when optimizing a Wi-Fi network for best performance, one of the first and simplest things you can do is check the data rates that the APs support. When you select the network you are testing, you need to view the list of APs on that network, and check the supported rates. These rates are in Mbps and include 1, 2, 4, 5.5, 6, 9, 11, 12, 18, 24, 36, 48, 54 Mbps. These rates include the basic rates (which a client device must support in order to connect) and extended supported rates.
To improve the performance of the network for all devices, consider disabling the lower rates on your APs. This must be done carefully to ensure that devices that must connect are still supported. For example, some environments must support 802.11b devices which require 1 Mbps connection rates. But if really old devices are not required to be supported, you can set the lowest rates at 11 or even 24 Mbps. How low depends also on AP density. If the APs are relatively far apart, set the minimum rate to be no higher than 11 Mbps so that devices on the edge of coverage can still connect. If the APs are dense and are in close proximity to each other, you can set a minimum rate to be higher, such as at 24 Mbps.
Too many devices transmitting on the same channel
One common cause is the fact that airtime is shared among all Wi-Fi devices on the same channel in the same area. If too many devices are trying to transmit on the same channel, performance will slow down for everyone. Having too many access points on the same channel in the same area is known as co-channel interference (CCI), and you want to reduce this to optimize performance.
However, in the 2.4 GHz Wi-Fi band, there is another problem source known as adjacent channel interference (ACI). Adjacent channels in the 2.4 GHz band overlap with each other, so that traffic on one channel can interfere with traffic on a nearby channel. A best practice in 2.4 GHz is to use only channels 1, 6, and 11 because these channels are far enough apart that they do not overlap. But if an access point is on channel 2, 3, 4, 5, 7, 8, 9 or 10, it will overlap and interfere with an AP on channel 6. This is adjacent channel interference, and can cause more performance degradation than co-channel interference. It is better to have 3 APs on channel 6 than to have one each on channels 4, 6, and 8, for example.
Access points transmitting too high of power level
Your network customer/user is complaining that they cannot connect to the Wi-Fi network. You go on-site to consult with this user and to understand why. At their specific location, you see that their Wi-Fi client is showing 4 bars from the network to which they are trying to connect. If you are really serious, you might even pull out a signal meter on your phone app to see the actual signal level of that network in terms of dBm. It all looks good. So why no connection?
One of the network’s access points (AP) may actually be transmitting at way too high of power level, resulting in the 4 bars and high dBm measurements at the user’s location. This will also cause the user’s client to try to connect to that specific AP. However, does that AP hear the user’s client? Wi-Fi communication is a two-way street, and ideally both the user’s client and the AP and should be transmitting at similar power levels. Commonly, the user’s client should see the strongest signal level on the network from the AP that is closest to them. If you measure the user client’s signal level at the location of that AP, you should see a similar signal level. If you see a signal level that is way too low (e.g., less than -80 dBm) or none at all, there is your problem!
A high retry rate of transmissions
Good Wi-Fi connections and high performance depend on a well-designed network. This means strong coverage, minimum interference, minimum airtime contention and good network capacity. When conditions go south, Wi-Fi devices will transmit but their transmissions are not received.
When a Wi-Fi device, either an access point or a client, transmits a frame of data, it must receive an acknowledgement from the receiver. If it does not, it will re-transmit that frame of data. This is known as a retry. The retry rate is the percentage of total frames transmitted that are retried A high retry rate means that much of the precious airtime is used (or wasted) by duplicate transmissions because the conditions are not allowing a transmitted frame to be properly received and acknowledged.
While there is no set standard on a good vs bad retry rate, if the measured retry rate of the connection is more than 15%, you should start looking at other issues like poor SNR, too many APs on a channel, too many clients on a channel, or high channel utilization.
For more information on Troubleshooting 802.11ac Wi-Fi, you can watch our recorded webinar on Brighttalk.