Does a wireless-N (802.11n) network have poor performance when in b/g "mixed" mode?

Question

It seems to be an "old" wives tale that, when using a wireless-N router in "mixed mode" to support legacy 802.11b or 802.11g devices, the performance of 802.11n clients will suffer.

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Some places claim that when running in mixed-mode, all (some?) N clients run at G speeds. Others make the same claim, but say it only happens when a G client is connected.

Other places say N clients run faster, but still run about 30% slower than if the router were in N-only mode, even if there are no legacy B/G clients connected.

Still others claim there is no speed drop for N clients when running on a mixed-mode network. They say the only issue is that overall network throughput will be lower, because only one client can be transmitting at any time, so some of that transmission time must be shared with the legacy B/G clients running at lower speeds, reducing the overall throughput from what it would be if there were only N clients connected.

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So, which is it? Will running in mixed-mode slow down my network, even if there are no B/G clients? If I'm running N, will having another client connected at B/G slow me down substantially vs. if they were running N?

Answer 1

From Beginners Guide To Networks :

Q: Does the mere presence of an 802.11B device slow down an otherwise all-G or all-N network?

A: YES This is of course already well known, although the exact implications are often misunderstood. The presence of an 802.11B device on an G or N network causes the newer devices to have to resort to some kludgy behavior to make sure that the B devices don't transmit when the G/N devices are using the airwaves, and to make sure that both the B and G/N devices can see things like beacon packets.

The exact impact on throughput is hard to estimate generally, but it will NOT "slow the whole network to 802.11B" as is often stated. There is however a significant slowdown imposed by the mere presence of a B device, even when it is not active. We (Slim) did some testing of this a couple years ago and found that usually the throughput between the G devices dropped by 30-50% (eg from 20Mbps to 10Mbps), but not nearly as low as the speed of a B-only network (5Mbps in the same environment). The theoretical maximum throughput on 802.11g is 23 Mbps without any B devices associated, and 14Mbps with.

Q: Will 802.11G devices slow down an all-N network?

A: NO, except insofar as the air-time that they take when active will be at the G throughput level as opposed to the N level. I.e. the devices still each communicate at their optimal rate in each time slice.

Unlike in the 802.11B backward compatibility mode, G devices do not impose any performance-degrading behavior on N devices in order for them to be backward compatible. 802.11g devices are able to recognize the 802.11n preamble, and they play nicely in terms of knowing when one or the other is trying to transmit. The preamble tells which modulation scheme will be used, so the N devices can speak N, while G devices can speak G. They don't have to resort to "Esperanto" as with B in order to cooperate.

This means that when the G device is associated but not active, it has no impact at all. When the G devices are active they will consume air time roughly in proportion to the amount of data being transferred. This air time would of course be at the G rate as opposed to the N rate, so in the event that the airwaves are fully saturated (eg by a local file transfer), there would be some reduction in the total Mbps achievable by all devices collectively, but there is no penalty for having the G devices associated.

Confusingly, this appears to conflict with what is stated elsewhere - eg

• "Running a mix of draft 11n and 11b/g clients on the same draft 11n router will reduce speed somewhat for the draft 11n client but reduce the speed of the 11g clients by more than half." at SmallNetBuilder
• "In mixed mode, HT protection requires that 802.11n devices send a legacy preamble, followed by an HT preamble ... These HT protection mechanisms significantly reduce an 802.11n WLAN's throughput, but they are necessary to avoid collisions between older 802.11a/b/g devices and newer 802.11n devices." at TechTarget ANZ

Q: Is having a (draft) 802.11N access point advantageous, even if most or all clients on the network are 802.11G?

A: YES, primarily because 802.11N radios have the benefit of more sophisticated multipath reception capability. They can thereby extend the range and throughput available to G devices to some degree.

Answer 2

Generally not
For a b client, absolutely yes! When an 802.11b client connects, g and n networks fall back to legacy CTS to self mode because the g preamble is not compatible with b devices. The b devices won't recognize the g frames at all and might transmit over them! The CTS frames are being sent first to tell b nodes to stay quiet to prevent this. b is mostly gone today so the focus should be on g nodes and other forms of interference.

802.11 networks use the preamble at the beginning of frames to announce the type and speed of the higher speed data that follows. Even if data cannot be received, as long as the preamble is received the CSMA/CA channel sharing system can work.

When an n network is operating in 20MHz mode (not 40MHz HT mode), it is nothing more than an enhanced g network that supports 72mbps maximum speed (and multiples of that with multiple data streams) instead of the 54mbps maximum g speed. It uses the same PLCP frame header that g does, so there should not be any issue, unless the access point is poorly designed.

When an n network is operating in HT40 mode is when things get messy. Many n networks do not or should not operate in HT40 mode because there is so much interference from other near by networks that it actually makes it slower than 20MHz mode, or reduces the range so much that it is not practical to use. The HT preamble is not compatible with g devices. When a g device connects to a 40MHz n network, the whole network switches to what they call L-SIG TXOP Protection in the referenced white paper. It sends a g compatible preamble on the primary channel and then sends the HT preamble at the beginning of each frame. This slows things down but not that much.

A bigger issue that is not really addressed is interference from different wireless networks (BSSIDs). Different BSSIDs do receive each other's preambles and frames, so CSMA/CA channel sharing can work in this situation, as long as the both BSSIDs are using the same channel. Knowing that 802.11b/g/n channels overlap and that networks must be on the same channel for CSMA/CA to work is often not understood. The vast majority of interference issues are actually from neighboring networks.

What I am still not clear on is this: When an n only network is operating in HT mode say on channel 6, should other g only networks use channel 6? Will the n network switch to LSIG TXOP mode when a g only device is present but on a different BSSID? The HT40 n network on channel 6 with the second channel configured to be above also fully uses channel 10, so is the g compatible preamble also transmitted on channel 10, so that 20MHz networks can also use channel 10 with working CSMA/CA, or does the entire top of the band need to be unoccupied and reserved for the secondary channels of N networks operating on channel 6? From what I understand so far, the channel 10 data has no interference protection what so ever from other 20MHz networks using channel 10. The Atheros proprietary 108mbps hardware does check for interference on the second channel and reverts to single channel mode, but n doesn't do this.

Whitepaper that I found from someone else's answer: http://www.nle.com/literature/Airmagnet_impact_of_legacy_devices_on_80211n.pdf

Answer 3

Technically it can slow it down but in practice probably not. There's enough overhead that you likely won't notice the difference. What rate is your provider giving you? Probably not more than 11mbps anyway.