Why Does USB Cable Length Matter?
Before we talk about how long a USB cable can be, it’s essential to understand the role of cable length in its performance.
USB is an interface that is used by computing devices to transfer data or power from one device to another. When two devices are connected via a USB cable, they use data packets to communicate with each other. But to ensure that the receiver gets each data packet, the source device waits for an acknowledgment signal from the destination before sending the next data packet.
However, the source device can only wait for a fixed time (PDF), usually in nanoseconds, before it considers the packet lost and tries again. So time is a critical factor in an efficient flow of data from one device to the other. And for how long a data packet has to travel before it reaches its receiver impacts that time.
As a result, the length of a USB cable becomes quite important. If it’s short, the data travel time from source to destination, also known as signal propagation delay, will be less. But if a USB cable is too long, the propagation delay will be long, and it can drastically reduce the data transfer rate.
Another factor that impacts the maximum length of a USB cable is the loss in signal strength or attenuation. When a data signal travels across a USB cable, it encounters resistance, and as a result, it loses some of its strength. The further data travels, the greater signal degrades. After a certain point, the signal strength is too low for the receiver to understand it.
So, USB cables can only be a certain length before they either have a long signal propagation delay or too much attenuation.
Maximum Length of a USB Cable
The maximum length of a USB cable depends on the USB generation it supports. USB-IF or USB Implementers Forum, the organization responsible for maintaining the USB standard, sets the specification for each USB generation.
The organization recommended the maximum cable length for the first two USB generations—USB 1.0 (3 meters) and USB 2.0 (5 meters)—but ever since, USB-IF no longer suggests a maximum cable length for any USB generation. Instead, it mandates the USB cables to meet specific performance requirements, including signal propagation delay and attenuation.
For the given performance requirements, the maximum practical length of the USB 3.2 Gen 1 (also known as USB 3.0, USB 3.1 Gen 1) cables is 2-3 meters. Similarly, USB 3.2 Gen 2 (a.k.a. USB 3.1, USB 3.1 Gen 2) and USB 3.2 Gen 2×2 cables can be a maximum of one meter long. And finally, USB 4 cables will be limited to a maximum of 0.8 meters or 31 inches.
Notably, USB-IF recommends slightly different practical lengths for USB 2.0 and USB 3.2 Gen 1 cables using the USB-C connector. This is most likely because USB-C cable specification includes higher power delivery requirements. For example, all USB-C cables must have a current rating of 3A or 5A, allowing them to deliver up to 240W of power.
The USB 3.2 Gen 1 Type-C cables can be up to two meters long, whereas the USB 2.0 Type-C cables can go as long as four meters.
One of the key reasons why the maximum USB cable length decreases with each new USB generation is the data transfer speeds. This is because, with every new generation, we get faster data transfer speeds. As a result, the USB cables have to have an even lower signal propagation delay, which can only be achieved by reducing the cable length.
It’s important to note here that these cable lengths are only valid for passive copper USB cables, the most common type of cables you’ll find on the market. Other types of cables can have more length while achieving the same speeds.
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Active and Optical USB Cables
Active or optical (or fiber optic) USB cables can be longer than passive copper cables. The active USB cables use electronic circuits to boost the data signal. As a result, they can compensate for attenuation and help data reach a longer distance. But they still have to deal with signal propagation delay. So the longer than standard active USB cables cannot offer all USB features, such as 4K video output.
In such cases, optical USB cables are a much better option. Unlike regular USB cables, optical cables use light to transfer data, which is considerably faster and less prone to attenuation.
Apart from active and optical cable, you can also use a self-powered USB hub, USB over Ethernet extenders, or USB over fiber extenders if you need a really long USB cable.
