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PHY rate (link connection) vs Data rate (transfer)

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802.11ax supports a maximum data rate of ~600 Mbps on a 80 Mhz channel for a single spatial Stream.

Does this mean I can achieve 600 Mbps of throughput in a data transfer between two devices ? Not really.

 

This article tries to describe in brief the 802.11 wireless communication components, explain the difference between PHY rate and Data rate and how show an example of Maximum Data rate for a given PHY rate.

 

PHY rate

"600 Mbps" is the Maximum PHY rate, what the specifications indicate and manufacturers must support. This rate is the overall capacity of the connection. As wireless communication is a shared media (contrary of cable RJ45 Ethernet), a not negligeable part (up to 30%) of the connection bandwidths is used in control and signaling traffic.

The PHY rate depends on a lot of factors like :

  • wireless environment, how many devices (station and AP) are connected and transmitting in the same channel ? Is the wireless channel crowded ? Is the device far ?
  • what capabilities are supported by the devices ? does it support 4 or 3 spatial streams ? which modulations ?
  • Are the device up-to-date to avoid bugs and interoperability issues ?

In this website mcsindexyou can find all PHY rate supported for the standard 802.11n.

 

Anatomy of a 802.11 packet

 

802.11 frame802.11 frame

 

As you can see in the image, a lot of the frame size is used in control data for Wireless communication. Data Rate is just only the UDP data which is called Payload, the effective data we want to transfer.

 

Data Rate

 

The data rate is the effective throughput we reach during a data transfer, for example, a download or a speedtest will indicate this rate.

For a given PHY rate, the data rate depend himself of several factors like :

  • which Transport Protocol are used (TCP or UDP)
  • what are the Operating System configurations for such transport protocols (buffer size)

Rule of thumb in good condition we reach 50% of PHY rate. If we are connected with a PHY rate of 600Mbps, we could expect to reach 300Mbps in Data rate.

 

Example

 

Samsung S10 connected to WLAN Internet Box 2 (IB2)

 

PHY link 

 

phy ratephy rate

 

Data Rate 

 

data ratedata rate

 

In conlusion, even if my PHY rate (WiFi Speed on photo) is great of 975Mbps, I reach only ~300 Mbps of TCP throughput between iperf3 App on my smartphone to a server behind IB2.

This test was local (no server used on internet, which bring a lot of external factors variables) and in my specific wireless conditions (which can change from location to location, customer to customer)

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Freundliche Grüsse, Dominik
Cordial salids, Dominik
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Cordiali saluti, Dominik

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