physics

USB communications

USB communications is a physics topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand USB communications rather than just read about it. In short: This article provides information about the communications aspects of Universal Serial Bus (USB): Signaling, Protocols, Transactions. USB is an industry-standard used to specify cables, connectors, and protocols that are used for communication between electronic devices.

Key takeaways

USB communications belongs to physics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect USB communications to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of USB communications from memory before moving on to harder problems.

Reference excerpt

This article provides information about the communications aspects of Universal Serial Bus (USB): Signaling, Protocols, Transactions. USB is an industry-standard used to specify cables, connectors, and protocols that are used for communication between electronic devices. USB ports and cables are used to connect hardware such as printers, scanners, keyboards, mice, flash drives, external hard drives, joysticks, cameras, monitors, and more to computers of all kinds. USB also supports signaling rates from 1.5 Mbit/s (Low speed) to USB 80Gbps (USB4 Gen 4) depending on the version of the standard. The article explains how USB devices transmit and receive data using electrical signals over the physical layer, how they identify themselves and negotiate parameters such as speed and power with the host or other devices using standard protocols such as USB Device Framework and USB Power Delivery, and how they exchange data using packets of different types and formats such as token, data, handshake, and special packets.

Signaling (USB PHY)

Signaling rate (transmission rate)

The maximum signaling rate in USB 2.0 is 480 Mbit/s (60 MB/s) per controller and is shared amongst all attached devices. Some personal computer chipset manufacturers overcome this bottleneck by providing multiple USB 2.0 controllers within the southbridge. In practice and including USB protocol overhead, data rates of 320 Mbit/s (38 MB/s) are sustainable over a high-speed bulk endpoint. Throughput can be affected by additional bottlenecks, such as a hard disk drive as seen in routine testing performed by CNet, where write operations to typical high-speed hard drives sustain rates of 25–30 MB/s, and read operations at 30–42 MB/s; this is 70% of the total available bus bandwidth. For USB 3.0, typical write speed is 70–90 MB/s, while read speed is 90–110 MB/s. Mask tests, also known as eye diagram tests, are used to determine the quality of a signal in the time domain. They are defined in the referenced document as part of the electrical test description for the high speed (HS) mode at 480 Mbit/s. According to a USB-IF chairman, "at least 10 to 15 percent of the stated peak 60 MB/s (480 Mbit/s) of Hi-speed USB goes to overhead—the communication protocol between the card and the peripheral. Overhead is a component of all connectivity standards". Tables illustrating the transfer limits are shown in Chapter 5 of the USB spec. For isochronous devices like audio streams, the bandwidth is constant and reserved exclusively for a given device. The bus bandwidth therefore only has an effect on the number of channels that can be sent at a time, not the speed or latency of the transmission.

Low speed (LS) rate of 1.5 Mbit/s is defined by USB 1.0. It is very similar to full-bandwidth operation except each bit takes 8 times as long to transmit. It is intended primarily to save cost in low-bandwidth human interface devices (HID) such as keyboards, mice, and joysticks. Full speed (FS) rate of 12 Mbit/s is the basic USB signaling rate defined by USB 1.0. All USB hubs can operate at this rate. High speed (HS) rate of 480 Mbit/s was introduced in 2001 by USB 2.0. High-speed devices must also be capable of falling back to full-speed as well, making high-speed devices backward compatible with USB 1.1 hosts. Connectors are identical for USB 2.0 and USB 1.x. SuperSpeed (SS) rate of 5.0 Gbit/s. The written USB 3.0 specification was released by Intel and its partners in August 2008. The first USB 3.0 controller chips were sampled by NEC in May 2009, and the first products using the USB 3.0 specification arrived in January 2010. USB 3.0 connectors are generally backward compatible, but include new wiring and full-duplex operation. SuperSpeed+ (SS+) rate of 10 Gbit/s is defined by USB 3.1, and 20 Gbit/s using two lanes is defined by USB 3.2.

Framing The host controller divides bus time into 1 ms frames when using low speed (1.5 Mbit/s) and full speed (12 Mbit/s), or 125 μs microframes when using high speed (480 Mbit/s), during which several transactions may take place.

Electrical specification USB signals are transmitted using differential signaling on a twisted-pair data cable with 90 Ω ± 15% characteristic impedance.

Low speed (LS) and Full speed (FS) modes use a single data pair, labelled D+ and D−, in half-duplex. Transmitted signal levels are 0.0–0.3 V for logical low, and 2.8–3.6 V for logical high level. The signal lines are not terminated. High speed (HS) mode uses the same wire pair, but with different electrical conventions. Lower signal voltages of −10 to 10 mV for low and 360 to 440 mV for logical high level, and termination of 45 Ω to ground or 90 Ω differential to match the data cable impedance. SuperSpeed (SS) adds two additional pairs of shielded twisted wire (and new, mostly compatible expanded connectors). These are dedicated to full-duplex SuperSpeed operation. The half-duplex lines are still used for configuration. Logic levels are defined not in terms of voltage levels but in terms of voltage transitions. The voltage varies between ±500 mV (±20%) during a transition in normal mode and between ±200 - 600 mV (depending on the transmitter) in low power mode. SuperSpeed+ (SS+) uses increased signaling rate (Gen 2×1 mode) and/or the additional lane in the Type-C connector (Gen 1×2 and Gen 2×2 mode). A USB connection is always between a host or hub at the A connector end, and a device or hub's upstream port at the other end.

Signaling state The host includes 15 kΩ pull-down resistors on each data line. When no device is connected, this pulls both data lines low into the so-called single-ended zero state (SE0 in the USB documentation), and indicates a reset or disconnection.

Line transition state The following terminology is used to assist in the technical discussion regarding USB PHY signaling.

The idle line state is when the device is connected to the host with a pull-up on either D+ (for full speed USB 1.x) or D− (for low speed USB 1.x), with transmitter output on both host and device is set to high impedance (hi-Z) (disconnected output). A USB device pulls one of the data lines high with a 1.5 kΩ resistor. This overpowers one of the 15 kΩ pull-down resistors in the host and leaves the data lines in an idle state called J. For USB 1.x, the choice of data line indicates what signal rates the device is capable of: full-bandwidth devices pull D+ high, low-bandwidth devices pull D− high. The K state has opposite polarity to the J state.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with USB communications

Start with the simplest possible case. Write down what USB communications claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to USB communications before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about USB communications ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of USB communications

In research: USB communications appears in physics research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses USB communications in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: USB communications is common in secondary-school and first-year university syllabi. It links to neighbouring topics Communication interfaces, Computer buses, Physical layer protocols, so understanding it makes those chapters shorter.
In everyday life: Look for USB communications outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study USB communications in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what USB communications means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain USB communications out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is USB communications in simple terms?

Why does USB communications matter?

Because it connects several physics ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study USB communications?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on USB communications.