PHY speed, also commonly referred to as PHY Data Rate, represents the maximum speed at which data packets are transmitted over a communication channel at the physical layer.
According to NetAlly, the PHY Data Rate is defined as the speed at which packets are transferred over a communications channel inclusive of headers, control, and management frames. This rate reflects the raw signaling speed possible across the physical medium, determined by the specific networking standard (like Wi-Fi 802.11ac or Ethernet) and current environmental conditions (like signal strength, interference).
Key Aspects of PHY Speed:
- Includes Overhead: PHY speed accounts for all data transmitted, including not only the actual user data but also the necessary overhead required for communication. This overhead includes:
- Packet headers
- Control frames
- Management frames
- Not Actual Data Transfer: It is crucial to understand that this is not the same as the actual amount of data frames being transferred. The useful data (or throughput) that applications see is typically much lower than the PHY speed. This is because throughput measures only the user payload data successfully transferred, excluding the various types of overhead and retransmissions.
- Theoretical Maximum: The PHY speed represents a theoretical or maximum signaling rate for the physical link under current conditions. It indicates the capability of the connection at the lowest level.
PHY Speed vs. Throughput
Understanding the difference between PHY speed and throughput is vital for evaluating network performance:
- PHY Speed: The speed at which all bits (data + overhead) are sent across the physical medium. It's the theoretical link speed.
- Throughput: The speed at which useful data is successfully delivered to the destination, after accounting for overhead, errors, and retransmissions.
Example: A Wi-Fi connection might report a PHY speed of 866 Mbps, but the actual internet download speed (throughput) measured by a speed test might only be 400-500 Mbps or less, due to protocol overhead, network congestion, and other factors.
In summary, PHY speed is the gross data rate of the physical layer connection, encompassing all transmitted bits, while throughput measures the net data rate available for applications.
