Do you ever wonder what happens on your wireless network when a mix of older and newer clients associate to an access point? Are the newer, faster clients hampered by older, slower clients? These are frequently asked questions when considering upgrading to 802.11n or adding 5 GHz capability to a network that started with 2.4 GHz only. The answer is found in band steering. Band steering is a radio management technique Meraki APs use to improve capacity, throughput, and the experience for users of crowded wireless networks.
Back when 802.11b was king
In the early days of wireless LAN, 802.11b was king, and its kingdom was the 2.4 GHz band. At a maximum rate of 11 Mbps, it reigned for a few years until 802.11g zoomed past at 54 Mbps. 802.11g became incredibly popular, and most of today’s mobile WiFi devices have at least an 802.11g radio inside. 802.11g is designed to co-exist with 802.11b, and their data rates are close enough that most people weren’t terribly concerned about the network performance when 802.11b and 802.11g clients both associated to a network.
802.11n takes the crown
Things are different now with 802.11n. Its data rates go to 150 Mbps for a single-stream, and these days, triple-stream MIMO cranks the speed up to 450 Mbps (per radio), such as in the Meraki MR24. Figure 1 shows the comparison of data rates between wireless LAN versions.
Additionally, 802.11n can use the 5 GHz band, which is nearly always less crowded and with less interference than the 2.4 GHz band. But it also works in 2.4 GHz, and 802.11n clients can happily associate there, in the mix with 802.11b and 802.11g clients. Table 1 shows the frequencies available to different types of wireless clients.
So what happens to the 802.11n clients when they are in 2.4 GHz alongside 802.11b and 802.11g clients? Are they limited to 54 Mbps or 11 Mbps? Can they operate at 150 Mbps and higher?
Waiting for slower clients
The answer depends on how the wireless network is configured. Older clients can’t understand transmissions sent at the faster rates that newer clients support, so the network can be configured to support both. For example, if an access point sends a transmission to an 802.11n client, the first part of the transmission, the header, should alert all clients that the contents are intended for an 802.11n client and the contents will be sent at a fast rate. 802.11b and 802.11g clients can then safely ignore the rest of the transmission. However, for this to work properly, the header needs to be sent at a rate understood by the slower clients. This means that 802.11n clients essentially have to wait a bit longer than they would if the header didn’t have to be sent at a slower rate.
Another penalty paid by 802.11n clients mixed in with 802.11b/g is that the mechanism used to determine channel availability and alert other clients of an upcoming transmission must follow the method supported by the older clients. This is a function of the medium access control (MAC) layer. 802.11n clients must send a short message that informs the older clients of the 802.11n transmission that will follow. This message, called the CTS-to-self message (clear to send to self), must be sent at the slower rates that the older clients can understand, or otherwise it’s of no use.
Band steering helps to use spectrum efficiently
So what can be done to get the maximum throughput out of 802.11n devices? One drastic technique would be to disable all 802.11b and 802.11g clients, but given the number of these devices still in users’ hands, it’s not a very practical solution. Instead, there is a more graceful approach: band steering.
Band steering sends 802.11n clients to the 5 GHz band, where 802.11b/g clients cannot go, and leaves the 802.11b/g clients in 2.4 GHz to operate at their slower rates. The 5 GHz 802.11n clients then operate at their full rate and the 802.11b/g clients aren’t affected down in 2.4 GHz.
Band steering works in the access point by directing 5 GHz-capable clients to that band. When the access point hears a request from a client to associate on both the 2.4 GHz and 5 GHz bands, it knows the client is capable of operation in 5 GHz. It steers the client by responding only to the 5 GHz association request and not the 2.4 GHz request. The client then associates in the 5 GHz band. See figure 2, below.
Of course, this only works if the access point has two radios inside: one for the 2.4 GHz band, and one for the 5 GHz band. The radios must be able to work concurrently, too. If you’re deploying wireless for a dense, bandwidth-intensive environment, it’s important to deploy access points with dual concurrent radios, or else band steering won’t be available.
Perhaps the most appealing aspect of operating in the 5 GHz band is the additional available spectrum compared to the 2.4 GHz band. 5 GHz channel availability may depend on the country of operation, but in the US it is common to have eight non-overlapping channels in 5 GHz, compared to three non-overlapping channels in 2.4 GHz.
The 5 GHz band generally has less interference than the 2.4 GHz band. This isn’t only because of WiFi clients – microwaves, Bluetooth devices, and many consumer products such as cordless phones all operate in the same 2.4 GHz band. This doesn’t mean the 5 GHz band is completely interference-free; however, in most environments it is not as crowded as the 2.4 GHz band.
The access point can steer the client to 5 GHz, but it doesn’t lock it out of 2.4 GHz if the client really wants to associate there. Sometimes, the signal conditions or signal strength might be better in 2.4 GHz than in 5 GHz, so it doesn’t make sense for the client to shift up to 5 GHz. In this case, the conditions may not allow the client to operate at full rate anyway, so there is little penalty if it associates in 2.4 GHz.
Band steering is generally quite helpful in assigning clients to their appropriate bands. Access points can use band steering when they have two (or more) radios and support both 2.4 GHz and 5 GHz bands simultaneously.
A more drastic measure is to disable 2.4 GHz operation entirely. This takes advantage of the additional capacity and reduced interference in 5GHz compared to 2.4 GHz, but legacy clients are not capable of using it.
Another technique to consider is to disable the slowest (legacy) 802.11b rates altogether. These are 1, 2, and 5.5 Mbps. The highest 802.11b rate, 11 Mbps, is still available for those clients, but they will not be able to use the slower rates.
Naturally, choosing band steering settings in Meraki wireless networks is as simple as selecting an option in the dashboard. Figure 3 shows this part of the dashboard.
The great news is that if you support only 2.4 GHz now, upgrading your wireless network to 802.11n and 5 GHz will open up significant capacity and will lead to a better experience for all the network users. If you already support 5 GHz in your network, you can use these tips to maximize your wireless network capacity and make the most of 802.11n and your spectrum.