16

I have this router:
https://www.asus.com/Networking/RTN12LX/specifications/
It supports speeds up to 300 Mbps. However all my clients connected to it show a Link Speed of 72 Mbps. The clients are all modern devices like OnePlus 3, etc.
Even the SpeedTest results are more on wired connections connected to the same router (~60 Mbps) and nearly halved on wireless connections (~25 Mbps) even when standing next to the router!

What is going on here?

  • 4
    Regarding your low throughput (not low connection speed)... try disabling encryption (for a test only)... I have seen ASUS WiFi access points with horrifically poor/lumpy throughput due to exhausting their available entropy (required for encryption). – Attie Jul 7 '17 at 12:39
  • Depending on the router too, you may have to enable a higher link speed for the 2.4g or 5g bands. – Kaizerwolf Jul 7 '17 at 12:41
  • This router is pretty old. It doesn't have 5 GHz band. How do I enable higher link speed? – Mayank Jul 7 '17 at 12:42
  • 2
    @Mayank - You cannot. – Ramhound Jul 7 '17 at 14:56
  • 2
31

The maximum Wi-Fi link speed (PHY rate) a given client can get when connecting to a given AP is the intersection of which speed-increasing factors both devices support (the overlap in the venn diagram of what they support).

Smartphones — even top-of-the-line models — often don't have the power and thermal budget and physical space required to support all of the speed-increasing factors that a top-of-the-line AP, laptop, or desktop Wi-Fi card can support. I've never seen a smartphone that supports 3 spatial streams or more, even though 3x3 APs and laptops started shipping in 2009.

So if one device supports 2 spatial streams but only 20MHz-wide channels, and the other device supports up to 80MHz-wide channels but only 1 spatial stream, the subset that they both support is 1 spatial stream and 20MHz-wide channels.

With 1 spatial stream and 20MHz wide channels and only 802.11n modulation schemes, the max PHY rate you can get is 72.2 Mbps. It seems likely that this is the situation you're in.

The only piece I'm unsure about is your OnePlus 3. It's hard to find reliable information about how many spatial streams it supports. It uses a Qualcomm Snapdragon 820 SoC, and at least one variant of the 820 supports two spatial streams. It could be that your OnePlus 3 uses a different variant that only supports a single spatial stream, or it could be that OnePlus decided to save battery and space by only enabling support for a single spatial stream even though the chip they were using supported 2 (it's very common for system designers to choose not to fully exploit every possible feature of a chip they're using, when the tradeoffs like saving battery and size are good enough).

  • Since client devices don't communicate directly but only through the AP, is the exact intersection really important? I.e. won't the AP use 2 streams when sending data to device A, and 40 MHz channel when sending to device B? (I'm not sure which parameters are always forced to those of the "weakest" client, and which remain per-client...) – grawity Jul 7 '17 at 15:15
  • According to the OnePlus forums, OnePlus 3 doesn't support MIMO. – Mayank Jul 7 '17 at 15:21
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    @grawity I think you may have misinterpreted something I wrote. The max PHY rate a particular client can get to/from a particular AP is the intersection of what capabilities that client and that AP support. It has nothing to do with what other clients of the network support. None of the main performance parameters are ever forced to the level of the weakest client (that's a pervasive Wi-Fi myth that has never been true). – Spiff Jul 7 '17 at 16:24
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    @Spiff: Collision sense does constraint the preamble of every transmission to the common subset of the capabilities of all clients. Only the preamble, though, the rest of the transmission can use the fancier modulation. – Ben Voigt Jul 7 '17 at 17:56
  • @BenVoigt Right, that's one of a handful of minor tweaks that modern devices have to do when legacy clients are present. Those tweaks pale in comparison to the main performance parameters such as channel width, spatial streams, and MCSes, which are never forced into some kind of mythical lowest-common-denominator mode. – Spiff Jul 7 '17 at 19:15
9

The router might support a 300 Mbps link speed but your client devices don't.

The advertised maximum requires 40 MHz channel width (twice the capacity of regular 20 MHz channels) and two simultaneous MIMO streams (twice the capacity of a single stream).

If your client device supports neither MIMO nor 40 MHz channels, naturally you'll only get a quarter of that 300 Mbps.

(Actually it's very common for Wi-Fi access points themselves to start in "20 MHz only" mode unless told otherwise – wide channels are fine on 5 GHz, but there's just very little space for them in 2.4 GHz band which already only has ~60 MHz worth of space.)

  • My AP is set to 20/40 MHz. As I said these client devices are modern. So they should support higher speeds. – Mayank Jul 7 '17 at 12:47
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    Because of how much the 2.4 GHz WiFi band is used, you cannot get 40 MHz channels on 2.4 GHz unless you’re living in a cabin in the woods. The spec demands devices shift down to 20 MHz when other networks are detected within its channel. – Daniel B Jul 7 '17 at 12:50
  • "As I said these client devices are modern." - They can't be that modern since they are not 802.1ac devices. – Ramhound Jul 7 '17 at 14:56
  • @ramhound The device supports AC but not MIMO. – Mayank Jul 7 '17 at 15:22
  • @Maynank - 802.11ac is a 5 Ghz only standard, "This router is pretty old. It doesn't have 5 GHz band.", so you router can't be a 802.11ac router. – Ramhound Jul 7 '17 at 16:09
7

In 802.11n you have a lot of rates (so called MCSes). AP and client sta trying to choose the best rate. Here is full list. https://en.wikipedia.org/wiki/IEEE_802.11n-2009#Data_rates

72 means you are using MCS # 7 because of:

  • You have 1 spatial stream (one physcial antenna on device)
  • You use best modulation possible (thats because no serious interference)
  • You use 20Mhz channel. Thats because there are a lot of other wifi networks so you do not have enough space for 40Mhz channel. When using 2.4 it is almost never possible to use 40Mhz unless you live in woods). To use wide channel try to move to 5Ghz but your AP and client should support it and 5 works bad on long distance.
  • You use 400ns (short) guard internal because you do not have serious delays on your line.

So, to increase bandwidth you need client with 2 antennas. You would not be able to get 300 (because you can't use 40Mhz in 2.4) but 144 is possible.

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