Wi-Fi networks can be very tricky to properly design and configure, especially in the small, crowded 2.4 GHz frequency band. In addition to interference from neighboring wireless networks, capacity issues arise when there are a high number of users on the network or a high density in a certain area.
In the early days of Wi-Fi, there weren’t that many Wi-Fi users or devices in the world. Today, the situation is much different. Private offices and buildings that have a wireless network may provide access to one, two, or even more Wi-Fi devices per worker and then maybe provide access for guests as well. More and more people are looking for Wi-Fi connectivity, especially at public venues -- on their laptops, smartphones and tablets -- to help conserve cellular data usage.
1. Design for throughput and capacity
When there weren’t many Wi-Fi users, you could design wireless networks pretty much based on coverage. You could perform a RF site survey and find the optimum locations for access points to ensure they provided adequate coverage. Now you should also design for throughput and capacity.
When designing a wireless network you should evaluate the Wi-Fi client devices that will be using it and how they’ll utilize it. Then you can do some calculations to figure estimated throughput and access points needed to support them, while also accounting for future growth and changes.
With 802.11b/g/n in the 2.4GHz band, there are only three non-overlapping channels. Thus co-channel interference becomes an issue when you bunch more than three access points in close proximity. Ideally, you don’t want an access point to hear any other access point on the same or overlapping channel. Though the 802.11 standards have mechanisms in place to deal with interference like this, co-channel interference will decrease performance.
2. Think about airtime
In areas where there is a high density of Wi-Fi users, like in public venues, you may find that the three 2.4GHz channels aren’t enough. However before overlapping channels and causing co-channel interference there are some techniques you might be able to utilize to increase capacity with the access points you already have.
Remember, wireless networks are all about airtime. Wi-Fi clients must contend for airtime with the access points as only one device, whether an access point or client, can transmit at any one time on a given channel. The higher the throughput and speeds data is transferred, the less airtime that is required, and generally the more clients that can connect and utilize the wireless access.
There are many settings you can configure to help boost performance and trim airtime.
3. Utilize 5GHz band steering
To help alleviate the crowded 2.4 GHz band, try to get Wi-Fi users onto the larger, less congested 5GHz band. Consider using dual-band 802.11n or 802.11ac access points that support band steering. When supported and enabled on the access points, dual-band clients will be guided or forced onto 5GHz instead of just leaving it up to the user or device to decide which band to connect to.
Most access points implement this type of functionality by responding only to the probe and association requests in the 5-GHz band when it has seen the same client with a probe/association request in the 2.4-GHz band. Thus once the access point knows a client is dual-band capable, it only allows connections from the client in the 5-GHz band.
In the 5-GHz band you have many more channels, and more Wi-Fi devices these days are supporting this band. However, do understand that 5GHz generally has less range due to the higher frequency. Thus you may have to do more Wi-Fi surveying to design for good 5GHz coverage.
If 5-GHz coverage isn’t up to par, consider configuring any band steering thresholds supported by the access point. Some allow you to set a minimum signal level a client must have before band steering will be used or a number of missed probe/association requests to 5GHz from the client before allowing connections on 2.4GHz.
4. Only use WPA2 security
Although both WPA and WPA2 security versions will work with 802.11n and 802.11ac, the data rates are limited to 54Mbps with WPA. You should select WPA2 only for the security on the private SSID(s) to allow maximum throughput when using the newer wireless standards. Any legacy clients not supporting the newer security should be upgraded.
5. Limit the amount of virtual SSIDs
When creating additional SSIDs, keep in mind that each one increases the overall overhead of the wireless network. Each SSID will generate additional beacons, probes, and other management traffic, taking up more airtime, even if the SSID isn’t being used. So consider limiting the number of virtual wireless networks; perhaps one for private access and another for public access. If needed, you can further segregate private access levels via dynamic VLAN assignment using 802.1X authentication for instance.
6. Disable lower data rates
Consider disabling the lower data rates to force packets, including those for management, to be sent via higher data rates and to ensure clients connect at higher data rates. This also encourages clients to automatically roam to better access points quicker rather than staying connected to an access point until the last second like they may normally do.
If you still have legacy 802.11b clients on the network you should really consider upgrading/replacing them, but you could still disable the lowest data rates (1M, 2M, and 5.5Mbps) and leave the highest (11Mbps) enabled.
If you don’t have any 802.11b clients, consider disabling all data rates at and below 11Mbps.
You’ll likely still need to support 802.11g clients, but if your Wi-Fi coverage is good enough you may also be able to disable some of the lower 802.11g data rates: 12M, 18M, 24M, 36M, and 48Mbps.
7. Configure proper channel-widths
On access points that support channels larger than the legacy 20MHz, you likely want to disable the Auto 20/40 MHz selection for 2.4GHz and only use the 20-MHz channels. In this band, it’s only possible to have one non-overlapping 40-MHz-wide channel. Thus larger channels are only really useful for areas where only one access point or channel will be used, including any neighboring networks.
For 5GHz, however, you may be able to use larger channel-widths since there is more frequency spectrum. Just ensure the bonded channels will not cause co-channel interference with yours or neighboring networks.
8. Transmission times
Shortening packet sizes or transmission times can help increase performance as well. Here are a few settings you may want to enable:
Short Preamble Length to shorten header information on packets, if you don’t have any 802.11b clients
Short Slot Time to decrease the amount of time a device waits after a collision before retransmitting a packet.
Short Guard Interval to lessen the time between transmitted symbols.
Frame Aggregation to allow multiple frames to be sent in a single transmission, but keep in mind that it can cause compatibility issues with iPads and iPhones.
9. Limit broadcast traffic
Broadcast traffic can also slow down the overall throughput of a wireless network, thus consider these two techniques to decrease broadcast traffic:
Enable wireless client isolation to prevent Wi-Fi devices from broadcasting to each other, if the user-to-user communication isn’t required. The Wi-Fi devices will still be able to communicate to wired clients, but not directly with wireless clients.
Separate the LAN and WLAN broadcast domains to cut down on the amount of broadcast traffic on the WLAN side.
10. Adjust the beacon interval
As mentioned earlier, each access point will broadcast a beacon packet for each individual SSID, which contains the basic information about the wireless network. The default interval rate at which beacon packets are sent over the airwaves is usually 100ms.
Increasing the interval rate will decrease the amount of beacons and the airtime they take up, but that can also cause other unwanted side effects. Typically, the smaller the interval, the quicker the clients will connect to and roam between the access points. The bigger the interval, the longer it takes clients to connect/roam and the longer delay for clients sending/receiving data that have power save mode enabled.
11. Adjust the fragmentation and RTS thresholds
Lowering the fragmentation and Request to Send (RTS) thresholds can help increase performance on wireless networks with a large number (at least over 5%) of collisions and/or interference.
If it appears you have a hidden node issue where clients are far apart and can’t hear each other but both can hear the access point, then start with reducing the RTS threshold. Perhaps start with threshold of around 500 bytes.
If hidden nodes don’t appear to be an issue, start with reducing the fragmentation threshold. Perhaps start with threshold of around 1,000 bytes.
12. Additional site surveys
Keep in mind, reducing these thresholds can also slow the network if not truly needed. I recommend making slight changes and then performing testing to ensure you’re seeing an improvement.
In addition to tweaking these settings, you may want perform additional RF site surveys if capacity issues still arise. You may find adjusting access point transmit levels and the access point locations can help make cell sizes smaller, enabling you to put more access points into an area. Also look into other network configurations that could affect capacity, for instance an adequate DHCP range.