Understand the USB interface and how it is used to transmit data

Types and Methods of Data Transmission (IGCSE 2.1)

Learning Objectives

• Identify the main categories of data transmission (serial/parallel, simplex‑half‑duplex‑full‑duplex, packet‑switching vs circuit‑switching).
• Explain the purpose, structure and typical applications of the Universal Serial Bus (USB) interface.
• List the USB connector families, power limits and the devices that normally use each type.
• Describe how data is transmitted over USB – signalling, packet format, transfer modes and error‑detection methods.
• Compare the performance of the different USB versions and relate it to real‑world tasks.
• Recall basic encryption concepts that can be applied to data sent over USB.

1. Overview of Transmission Methods

The Cambridge syllabus expects you to know the following categories. The table summarises the definition, why the method is used and a typical example.

USB is a **serial, full‑duplex, packet‑switched** interface. It uses a single differential pair (D+ / D‑) for data, so many devices can share one bus while the host controls the timing of each transfer.

2. What Is USB?

The Universal Serial Bus (USB) is a standard, hot‑plugable interface that connects peripheral devices to a host computer.

• Bidirectional serial data on a differential pair.
• Power delivery – 5 V, up to 500 mA (USB 2.0), 900 mA (USB 3.0) and up to 5 A with USB‑PD on USB‑C.
• Automatic device detection (enumeration) and configuration.
• Wide range of devices – keyboards, mice, storage, audio/video, chargers, etc.

3. USB Connector Families

4. USB Versions and Data Rates

5. How USB Transmits Data

5.1 Signalling

• USB uses **differential signalling** on the D+ and D‑ lines. A voltage difference represents a logical ‘1’ or ‘0’.
• Differential signalling reduces electromagnetic interference and permits higher speeds.

5.2 Packet Structure

Every transfer is wrapped in a packet. The basic fields are:

+-----------+-----+----------------+----------+------+
| Preamble  | PID | Addr/Endpoint  | Payload  | CRC  |
+-----------+-----+----------------+----------+------+

5.3 Transfer Types (Modes)

• Control Transfer – Device configuration, command & status. Always starts with a SETUP token.
• Isochronous Transfer – Guarantees a steady data rate (e.g., audio or video streams). Errors are tolerated; no retransmission.
• Bulk Transfer – Large, non‑time‑critical data (e.g., file copies). Uses CRC and automatic retransmission.
• Interrupt Transfer – Small, infrequent packets that need low latency (e.g., keyboards, mice).

5.4 Error‑Detection (IGCSE 2.2)

• Parity checking – An early method (odd/even) used in some serial standards; not used in modern USB but worth knowing for the syllabus.
• Checksum / CRC – Every packet ends with a CRC field. The receiver recomputes the CRC; a mismatch signals an error.
• Automatic Repeat reQuest (ARQ) – For Control, Bulk and Interrupt transfers the host or device resends a packet if the CRC check fails (handshakes NAK/STALL).

5.5 Simple Throughput Estimate (IGCSE 2.3)

For exam‑style calculations you can use a straightforward approximation:

Usable throughput ≈ Raw bit rate × (1 – Overhead)

• Typical protocol overhead for USB 2.0 is about 15 %.
• Thus, usable speed ≈ 480 Mbps × 0.85 ≈ 410 Mbps (≈ 51 MB s⁻¹).
• For USB 3.0, overhead is roughly 10 % → 5 Gbps × 0.90 ≈ 4.5 Gbps (≈ 560 MB s⁻¹).

These figures help you decide which version is needed for a given task (e.g., copying a 2 GB video file: USB 2.0 ≈ 30 s, USB 3.0 ≈ 3 s).

6. USB Communication Process (Simplified)

1. Connection detection – Host senses a pull‑up resistor on D+ (Full‑Speed) or D‑ (Low‑Speed).
2. Enumeration
   • Host assigns a unique 7‑bit address.
   • Host reads the device, configuration, interface and endpoint descriptors.
3. Driver loading – Based on the class descriptor (e.g., HID, Mass Storage) the operating system loads the appropriate driver.
4. Data transfer – Host and device exchange packets using the appropriate transfer type (Control, Isochronous, Bulk, Interrupt).
5. Disconnection – Removal of the pull‑up resistor signals a disconnect; the host frees the address.

7. Encryption and USB (IGCSE 2.3)

USB itself does not define encryption, but data sent over USB can be encrypted at higher protocol layers:

• USB flash drives may contain hardware that encrypts stored data with AES‑256.
• File‑transfer protocols (e.g., SFTP) can run over a USB‑connected network adapter, providing end‑to‑end encryption.
• When discussing “encryption” in the syllabus, note that it is a separate security measure that can be applied to any data, including that carried by USB.

8. Advantages of USB for Data Transmission

• Single, standardised connector reduces cable clutter.
• Integrated power delivery removes the need for separate adapters.
• Hot‑plug capability allows devices to be added or removed without restarting the computer.
• Backwards compatibility protects existing hardware investments.
• Full‑duplex, packet‑switching provides efficient, high‑speed communication.

9. Common Applications

• External storage – flash drives, SSD enclosures.
• Input devices – keyboards, mice, game controllers.
• Audio/Video peripherals – webcams, microphones, external sound cards.
• Charging and powering mobile devices.
• Data‑acquisition tools, debuggers and programming adapters.

10. Quick Revision Questions

1. What are the two main signalling lines in a USB cable and why is differential signalling used?
2. Match each USB transfer type (Control, Isochronous, Bulk, Interrupt) with a typical device example.
3. Estimate the usable throughput of a USB 2.0 connection if the protocol overhead is 15 %.
4. Explain why USB‑C is called a “reversible” connector and list two benefits of this design.
5. Identify the four fields of a USB packet and state the purpose of the CRC field.
6. Give one example of an error‑detection method used in USB and briefly describe how it works.
7. State one way encryption can be applied to data transferred over USB.