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Appendix

Understanding WiFi Overheads

In ideal conditions, several factors contribute to optimal WiFi performance:

  • Clear line of sight between the access point (AP) and the device.
  • Distance between the AP and device ranging from 3 ft to 15 ft. Typically, being outside the dip in the RF antenna pattern occurs at around 3 ft.
  • A small number of active clients connected to the AP, usually between 5 to 10 clients sending traffic.
  • Good transmission (tx) and reception (rx) antennas on the client device.
  • All clients operating above the minimum basic rates, which are typically 12 Mbps or higher.
  • No interference on the AP channel.

Under these ideal conditions, the throughput on a single client can be expected to reach approximately 65% to 70% of the PHY (physical) rate, with a potential peak of up to 80% of the PHY rate. However, in real-life situations, factors such as overhead and contention can cause the throughput to fall between 33% to 50% of the client's PHY rate.

WiFi overhead is relatively substantial, which explains why we don't typically observe a PHY speed of 400 Mbps on a client with specifications like 2x2, 256 QAM, ⅚ coding, and short GI (MCS index of 9). In practice, the maximum achievable throughput is around 400 x 70% = 280 Mbps.

Overhead and Contention

TCP/IP and Ethernet Overhead: 

  • On wired Ethernet, there is approximately 5% overhead due to TCP/IP and Ethernet frames, resulting in a throughput of 95% of the physical (PHY) rate.
  • This overhead is similar on WiFi, excluding retransmissions.

Management frames: 

  • Management frames, such as 802.11 beacon frames, need to be transmitted at the slowest modulation to ensure all wireless devices on a given channel can receive and decode them. This means, devices that are furthest from the AP (which are already running at slowest speeds) can successfully receive and decode these frames. 
  • For example, beacon frames are sent at 1 Mbps on the 2.4GHz band and 6 Mbps on the 5GHz band.
  • More SSIDs on an access point (AP) cause increased management frame overhead.
  • The overhead can range from 3% to very high, depending on the specific circumstances. See this article for more details.

Half Duplex: 

  • Unlike Ethernet, WiFi does not have separate spectrums for download and upload.
  • Both download and upload traffic share a common channel, meaning that when a client is downloading, it is also transmitting acknowledgement (ACK) packets simultaneously.
  • This overhead can range from 2% to 4% relative to the client's activity.
  • For example, when a client is downloading a file at 150 Mbps, it is also uploading at 2 Mbps at the same time.

Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA): 

  • Because WiFi is a shared spectrum, devices (clients and access points) need to know when it is safe to transmit on a given channel. WiFi devices use Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA) to determine when it is safe to transmit on a given channel.
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  • To ensure fair access to the channel, devices waiting to transmit must wait for a random amount of time, dependent on channel utilization.
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  • This overhead can significantly increase if there are many devices attempting to transmit simultaneously. CSMA/CA works really well when there are not many devices wanting to transmit at the same time, which is how WiFi typically works. But this overhead can increase dramatically if there are too many devices trying to transmit at the same time and each client’s “random wait period” keeps adding up. 

WiFi Packet ACKs: 

  • Every WiFi packet sent must be acknowledged (ACK) to confirm receipt. 
  • A small timing window is appended to each packet, allowing the receiver time to transmit the ACK.

Collisions and Retransmissions: 

  • High channel utilization and collisions lead to packet loss and subsequent retransmissions when multiple devices want to transmit at the same time on a given channel.
  • Co-Channel, overlapping channel, and Adjacent channel interference in dense environments can also contribute to collisions.

Hidden Node Problem: 

  • In situations where two clients can hear the access point (AP) but not each other, simultaneous transmissions from both clients cause collisions on the AP's side, resulting in the loss of both transmissions.

Coexistence of 802.11n and 802.11ac, and wide channels:

  • When a wide channel and a 20MHz client operate on the same channel, there is an overhead in the form of a "request to send" (RTS) and "clear to send" (CTS) before each message, slowing down overall transmission.

It's important to remember that WiFi is a shared resource, and the transmission and reception rates are asymmetric based on various factors. In scenarios where different clients have different PHY rates, the channel usage is divided between them proportionally.