Determine bottleneck for the Wi-Fi speed and match the Ethernet speed

Question

The speed on my Wi-Fi is substantially less than the speed on the Ethernet.

Through mtr, I don't see any substantial packet loss between the laptop to the router.

I want to be able to diagnose the speed bottlenecks in the router and try to fix those.

I know I can choose the best practices such as "Use the 5GHz band, use 802.11n, choose 40MHz-wide channels" etc.

Is there a step by step guide to identify exactly what improves the performance by how much, ideally through some prior diagnosis rather than change it and look for the changes, approach?

Answer 1

First, know that the ceiling on the best PHY rate (Physical-layer rate: the basic signaling rate before protocol overhead) you can get is limited by how the capabilities of the client and AP overlap. Do they both support 802.11n or 802.11ac? Do they both support 40MHz- or 80MHz-wide channels? Do they both support 2 or 3 spatial streams? Do they both support short guard intervals? Do they both support all the relevant MCSes?

Example: If your AP is 2.4GHz-only and supports "N150" by doing 1 spatial stream and 40MHz-wide channels in 2.4GHz, and your client is dual-band and supports "N300" by doing 2 spatial streams and 40MHz-wide channels in 5GHz, but limits itself to 20MHz-wide channels in 2.4 GHz to leave room for Bluetooth, then the best they can do together is 1 spatial stream on a 20MHz-wide channel, for a max PHY rate of 72.2Mbps (which isn't that much better than 802.11g's old 54Mbps max PHY rate).

• Channel Width: Doubling channel width slightly more than doubles your PHY rate.

  Example: In 2 spatial stream 802.11n with short guard intervals, going from 20MHz- to 40MHz-wide channels improves your max potential PHY rate from 144.4 to 300Mbps.

• Spatial streams: Going from 1 spatial stream to 2 or 3 provides a straight doubling or tripling of your PHY rate.

  Example: In 802.11n with 40MHz-wide channels and short guard intervals:

  • 1 spacial stream has a max potential PHY rate of 150Mbps
  • 2 spatial streams can do 300Mbps
  • 3 spatial streams can do 450Mbps

• Guard Interval: A short guard interval is about 10% faster than a long one.

  Example: In 802.11n with 40MHz-wide channels and 2 spatial streams:

  • Long GI can get 270Mbps
  • Short GI can get 300Mbps

• MCS: (Modulation and Coding Scheme) Varies widely. Going from one MCS to the next higher MCS is just an 11% improvement in some cases, or as much as a 200% improvement in other cases. Moving from the slowest to the fastest MCS for a given number of spatial streams, channel width, and guard interval can be a 13.3x (1,333.3%) jump.

  Example: In 802.11n with 40MHz-wide channels, 2 spatial streams, and short GI:

  • MCS 8 gets 30Mbps
  • MCS 9 gets 60Mbps (200% improvement over MCS 8)
  • MCS 14 gets 270Mbps
  • MCS 15 gets 300Mbps ( 11% improvement over MCS 14)

  Note that what MCS your equipment can sustain has everything to do with signal quality (signal strength, signal-to-noise ratio). And that has everything to do with how clean (interference-free) the channel is, how close together the devices are, how good their antennas and radios (especially amplifiers) are, etc.

• Frame Aggregation: (A-MPDU, A-MSDU). This improves how efficiently your devices can use the PHY rate they're getting. Without it, your TCP/IPv4 throughput will typically be slightly less than 50% of your PHY rate. With it, you may see your efficiency go up to 80%, but I'm usually happy if it's above 70%.

  Example: 802.11g didn't have frame aggregation built into the standard, and the most TCP throughput you'd see when using 802.11g's 54Mbps max PHY rate was about 25Mbps. 802.11n introduced A-MPDU and A-MSDU, and much 802.11n gear supports A-MPDU aggregation (A-MSDU is much less popular). With 300Mbps 802.11n (40MHz-wide channels, 2 spatial streams, short guard intervals, MCS 15) and A-MPDU, you can see TCP throughput of over 200Mbps.

• Everything else is just tweaky fiddly stuff, where the right settings depend greatly on your radio environment and what your traffic patterns look like, and even experienced 802.11 engineers don't usually bother tweaking.

  This includes:

  • legacy protection modes
  • beacon/DTIM/listen intervals
  • multicast/broadcast rates
  • management frame rates
  • basic rates
  • supported rates
  • RTS/CTS threshold
  • Fragmentation threshold
  • etc., etc.

For more information on how channel widths, spatial streams, guard intervals, and MCSes affect data rates, refer to the table that is MCSIndex.com. This table is basically taken right out of the 802.11n and 802.11ac standards.

Answer 2

Here's a list of real-world speeds for various protocols. As you can see, you can typically only expect about 6MB/sec using wireless-N, though if you have a high-end AP and WiFi card with multiple antennas that can do 300Mbps or more, you may able to get somewhat higher transfer rates.

Compare that with ubiquitous gigabit Ethernet getting you 120MB/sec and you can see that wired connections are preferred for maximum speed.