In modern electronic devices, the Universal Serial Bus (USB) interface has become one of the most commonly used ways to connect external devices. Whether it is used for data transfer, charging, or peripheral access, the USB interface often plays the role of transit hub. USB interface transfer speed has been one of its important performance indicators. Very often, we buy USB devices advertised theoretical transfer speed seems very high, but in actual use, the speed is often not so fast. So why is there such a gap? In this article, we will use the conversion of bandwidth and speed as an entry point to delve into the difference between the theoretical and actual transmission speeds of USB to help you better understand this technical detail.

I. What is the Theoretical Transfer Speed of USB?

The theoretical transfer speed of USB is the maximum data transfer speed defined by the USB protocol standard under ideal conditions.There are several versions of USB, and the maximum theoretical transfer speed of each version keeps increasing. Below are the theoretical maximum transfer speeds for common USB versions:

• USB 1.1: Maximum transfer speed is 12 Mbps (Megabits per second).It was sufficient for low-speed devices of the time, such as mice and keyboards.
• USB 2.0: Maximum transfer speed is 480 Mbps. This is the standard still in use for many devices, such as regular USB flash drives and external hard drives.
• USB 3.2 Gen1 (USB 3.0, USB 3.1 Gen1): Maximum transfer speed of 5 Gbps (Gigabits per second) greatly enhances data transfer with support for large file transfers and HD video streaming.
• USB 3.2 Gen2 (USB 3.1 Gen2): Maximum transfer speed is 10 Gbps, which is double the speed of USB 3.0 and is suitable for devices with higher bandwidth requirements, such as 4K video transmission and mass storage.
• USB 3.2 Gen 2x2: Maximum transfer speed is 20 Gbps, supporting more high-performance peripherals such as high-speed SSDs and high-resolution displays.
• USB4: With a maximum transfer speed of 40 Gbps, it supports a wide range of high-bandwidth applications such as ultra-high-definition video streaming, external graphics cards and high-performance storage devices.

It should be noted that the theoretical transfer speed only represents the maximum transfer rate of the USB protocol under ideal conditions (e.g. ideal cable, interference-free environment).

II. Conversion of Bandwidth and Transmission Speeds?

When discussing USB speeds, the terms bandwidth and transfer speed are often seen. Simply put, bandwidth is the amount of data that can be transferred per unit of time, while transfer speed is the actual speed that can be achieved. They are closely related, but differentiated.

• Bandwidth: indicates the amount of data that the transmission channel can carry per second, usually in units of bit per second (bps). For example, USB 2.0 has a bandwidth of 480 Mbps, meaning it can transfer up to 480 megabits of data per second.
• Transmission speed: then it refers to the speed of the device when it is actually transmitting data. It is usually lower than the bandwidth because there are many factors that affect the transmission speed (e.g. protocol, device performance, etc.).

There is a simple conversion between bandwidth and transmission speed:

1 byte = 8 bits

So, a bandwidth of 480 Mbps corresponds to a theoretical transfer speed of about 60 MB/s . Assuming a bandwidth of 5 Gbps for USB 3.0, the conversion to bytes gives a transfer speed of: 

5 Gbps ÷ 8 = 625 MB/s

With these conversions, we can understand the performance of the USB interface more intuitively. For example, 10 Gbps theoretical bandwidth actually transfers at a speed close to 1.25 GB/s (gigabytes per second), which is great for transferring large amounts of high-definition video or high-capacity files.

USB standard	Bandwidth	Transmission speed
USB 1.1	12Mbps	1.5MB/s
USB 2.0	480Mbps	60MB/s
USB 3.2 Gen1 USB 3.0 USB 3.1 Gen1	5Gbps	625MB/s
USB 3.2 Gen2 USB 3.1 Gen2	10Gbps	1.25GB/s
USB 3.2 Gen 2x2	20Gbps	2.5GB/s
USB 4	40Gbps	5GB/s

 

III. Why is There a Gap Between Theoretical and Actual Speed?

According to the theoretical value of the bandwidth of USB and the actual test results, we can make some speed conversion and estimation. For example, suppose we use a USB 3.0 interface with a bandwidth of 5 Gbps, the theoretical maximum transfer speed is 625 MB/s.

However, due to a variety of factors, the actual test results are often lower than this value. Assuming we use an SSD for transmission, the actual speed may be closer to 500 MB/s, so the corresponding transfer rate is:

500 MB/s × 8 = 4 Gbps, which is significantly lower than the USB 3.0 5 Gbps bandwidth. This is because many factors (e.g. protocol overhead, storage device performance, etc.) affect transfer speeds in real-world environments.

Although the USB standard specifies a theoretical transfer speed, in reality, we often cannot achieve this speed. Actual transfer speeds are often affected by several factors:

1. Device performance: The performance of the device is the key factor affecting the actual speed. If your hard disk or storage device itself has a slow read/write speed, it will not be able to reach the theoretical transfer speed even if you are using a USB 3.0 or USB 3.2 interface. For example, an average mechanical hard disk usually only has a speed of about 150 MB/s, while an SSD can reach hundreds of MB/s or more.
2. Cable Quality: The quality of USB cables is also an important factor. Cheap or poor quality cables may not be able to carry the higher bandwidth, especially in USB 3.0 and higher versions, and the use of substandard cables may result in signal attenuation, affecting the actual transmission speed.
3. Host Hardware and Operating System: If your computer hardware is older, or the operating system and drivers are not optimized, it may also not be able to take full advantage of the USB port's maximum performance. For example, USB 3.0 speeds may be limited by your computer's motherboard, CPU processing power, or operating system support.
4. File Size and Transmission Type: During actual data transmission, the type and size of data transmitted also affects the speed. Sequential transfers (e.g. copying large files) can usually approach the theoretical maximum speed, while transfers of large numbers of small files usually have much lower actual speeds due to file fragmentation and additional protocol overhead.
5. Transmission Distance: The transmission distance of the USB also affects the speed, especially over long distances. The signal will decay as the distance increases, resulting in a decrease in speed. Typically, the maximum effective distance for USB transmission is 5 meters, beyond which performance may be affected.

IV. Conclusion

The difference between the theoretical and actual transfer speeds of USB interfaces stems from a variety of factors, including device performance, transmission media, system load, and protocol overhead. Although the USB protocol standard defines theoretical maximum transfer rates for each version, in practice we rarely reach these ideal values completely.

Understanding the conversion between bandwidth and transfer speed, and recognizing the gap between actual transfer speeds and theoretical values can help us choose and use USB devices more wisely and optimize the efficiency of data transfer. Choosing the right USB version, a quality cable and a high-performance device is critical if you want to get the best transfer experience.