USB Latency Explained

When we think of USB (Universal Serial Bus), speed often comes to mind, how quickly we can transfer files or charge our devices. However, another critical factor, often overlooked, is USB latency. Latency refers to the delay between when a command is sent and when the USB device processes it. While this might not seem significant in everyday usage, it plays a crucial role in certain applications like data streaming.

This article dives into what USB latency is, why it happens, and why it matters for various devices and applications.

What Is USB Latency?

USB latency is the time it takes for data to travel from the host (e.g., your computer) to the connected USB device and back. Unlike bandwidth, which measures how much data can flow in a given time, latency measures how quickly that data moves back and forth.

Latency is measured in milliseconds (ms) and, in most cases, is negligible. However, for devices that rely on fast communications, even slight delays can create noticeable performance issues.

Why Does USB Latency Occur?

USB latency stems from several factors:

  1. Protocol Overhead

    USB communication involves a host-controller architecture. Every data packet sent from the host needs to be processed, routed, and sometimes queued before reaching the device. This processing introduces latency.

  2. Data Transfer Mode

    USB supports different data transfer types, which inherently affect latency:

    • Bulk Transfer: Used for large data blocks, like file transfers. Latency is not a priority, but throughput is.

    • Interrupt Transfer: Designed for small, timely data transfers, such as keyboard and mouse inputs. Low latency is a priority here.

    • Isochronous Transfer: Used for time-sensitive data, like audio and video streaming. It minimises latency but sacrifices data integrity.

  3. Polling Frequency

    In some USB versions, devices must "poll" the host at specific intervals for new data. For instance, USB 2.0 polling occurs every 1ms by default, whereas USB 3.0 and later reduce this interval, improving latency.

  4. Device-Specific Factors

    Certain devices, like USB hubs, keyboards, or external hard drives, may introduce additional latency due to their internal processing or the number of devices they handle.

Why Does USB Latency Matter?

USB latency is rarely noticeable during routine tasks like transferring files or charging devices. However, it becomes in scenarios where timing and responsiveness are essential.

For example in data transfer when small data packets are sent devices must minimise latency to ensure high levels of data bandwidth are achieved. Delays here can lead to significant lower than advertised data bandwidth.

How USB Versions Impact Latency

Each version of USB technology improves on its predecessor, offering reduced latency and faster performance:

  • USB 2.0: Introduced a 1ms polling rate but suffered from higher latency due to its slower speeds (up to 480 Mbps).

  • USB 3.0 and 3.1: Improved latency by increasing polling frequency and data rates (up to 5 Gbps and 10 Gbps, respectively).

  • USB4: Reduces latency further with improved protocols and higher speeds (up to 40 Gbps), ideal for data streaming.

 

How to Minimise USB Latency

If you're experiencing USB related latency issues, here are some steps to address the problem:

  1. Upgrade Your USB Devices

    Using newer USB versions (e.g., USB 3.0 or USB4) can significantly reduce latency compared to older standards.

  2. Minimise hub depth

    Avoid nesting USB hubs to deeply as this can introduce additional delays.

  3. Check Device Drivers

    Outdated or improperly configured drivers can increase latency. Ensure your USB device drivers are up-to-date.

  4. Optimise Device Polling

    For devices with adjustable polling rates (e.g. mice), experiment with higher polling rates to improve responsiveness.

  5. Choose Isochronous Devices for Real-Time Applications

    If low latency is critical, ensure your device supports isochronous data transfer.

    Avoid Overloading Ports

    Connecting too many devices to a single USB controller can lead to increased latency. Use a connection strategy that balances latency when connected USB devices.

Latency Overall

While USB latency might not be noticeable in everyday tasks, it’s a crucial consideration to maximise data throughput and responsiveness. Understanding the factors that contribute to latency, and how to mitigate them, can help you optimise performance.