Understanding USB Packet Formats and Types (V2.0 and V3.2)

The USB, or Universal Serial Bus, is an interface akin to RS232 and RS485, but it offers higher data rates across various distances. USB facilitates peer-to-peer communication, where the communication primarily occurs between a host and a peripheral device, rather than directly between two hosts or two peripherals.

The USB standard encompasses specifications for cables, connectors, and protocols. It outlines the necessary requirements for connection, communication, and power supply (PS) between PCs and peripheral devices. Several USB versions exist, including v1.x, v2.x, and v3.x, each supporting different data rates: 60 Mbps, 625 Mbps, and 1.25-2.5 Gbps, respectively.

USB 2.0 is also known as High Speed USB, USB 3.0 as SuperSpeed USB, and USB 3.2 as SuperSpeedPlus USB. These standards typically support cable lengths up to 5 meters. Beyond this limit, USB hubs are required to expand connectivity. Let’s explore the USB packet formats, types, and structures, including Token packets, Data packets, SOF (Start of Frame) packets, and Handshake packets, as defined by V2.0 and V3.2.

USB Packets as per V2.0

• All USB packets begin with a “SYNC” field and end with an “EOP” (End Of Packet) sequence. The SYNC field, also known as the preamble, ensures synchronization between the device and the host.
• PID (Packet ID): This field identifies the type of packet being transmitted. It consists of 4 bits, but to ensure correct reception, these 4 bits are complemented and repeated. Therefore, the total PID size is 8 bits. PID = { PID0, PID1, PID2, PID3, PID0', nPID1', nPID2', nPID3 }
• ADDR: This 7-bit field specifies the device to which the packet is designated.
• ENDP (Endpoint Field): This 4-bit field allows for 16 possible endpoints.
• CRC (Cyclic Redundancy Check): A 5-bit field used to detect erroneous packets.
• EOP: End of Packet marker.

USB (Universal Serial Bus) is a standardized communication protocol commonly used in computer systems and electronic devices for connecting peripherals. USB communication involves the exchange of information through a series of packets. Each USB transaction consists of several packet types, including Token packets, Data packets, Start of Frame (SOF) packets, and Handshake packets.

Let’s understand the functions of each of these packet types as defined in USB 2.0.

USB Token Packet

• Function: Token packets initiate and manage USB transactions. They convey information about the transaction type (e.g., read or write), the target device’s address, and the endpoint within the device.
• Packet Structure: Token packets contain fields such as Sync, PID (Packet ID), ADDR (device address), ENDP (endpoint number), and CRC16 (Cyclic Redundancy Check for error detection).

USB Data Packet

• Function: Data packets carry the actual payload of information being transferred between the host and the device. The payload can include data from the host to the device or vice versa, depending on the transaction type.
• Packet Structure: Data packets include Sync, PID, DATA (payload), and CRC16. The payload size can vary depending on the endpoint’s specifications.

USB Handshake Packet

• Function: Handshake packets acknowledge the successful receipt of data or indicate errors in the communication. They confirm the completion of a data transfer or report any issues encountered during the process.
• Packet Structure: Handshake packets include Sync, PID (ACK, NAK, or STALL), and CRC16. The PID field specifies the type of handshake response.

USB SOF Packet

• Function: SOF packets are used to keep devices synchronized in time. They are sent periodically to mark the beginning of each frame, helping devices maintain proper timing for communication.
• Packet Structure: SOF packets contain Sync, PID (usually SOF), and Frame Number. The Frame Number field helps devices track the current frame.

Conclusion

In summary, these packet types work together to facilitate communication between USB devices and the host. Token packets initiate transactions, Data packets transfer information, SOF packets maintain synchronization, and Handshake packets confirm the success or failure of data transfers. The USB protocol ensures reliable and efficient communication by employing these different packet types in a structured and standardized manner.

USB Packet Types as per V3.2

The following figure depicts USB packet types as per the USB 3.2 Standard. It includes packets such as LMP (Link Management Packet), TP (Transaction packet), DP (Data packet), and ITS (Isochronous Timestamp) packet.

A “TYPE” field (5 bits in size) is present in all packet types to specify how the packet should be interpreted. CRC-16 or CRC-32 is used at the end of these packet types for error detection.

Link Management Packet (LMP) Format

LMPs are used for link initialization, training, and management. They help establish and maintain a reliable communication link between USB devices.

The figure illustrates the structure of the LMP packet type. The “TYPE” field is set to “00000b.” The Subtype field (4 bits in size) indicates the following:

Transaction Packet (TP) Format

Transaction packets are fundamental to USB communication and consist of types such as IN, OUT, SETUP, and PING. They initiate and control the transfer of data between USB hosts and devices. The figure shows the structure of the TP packet type. The “TYPE” field is set to “00100b”.

Data Packet (DP) Format

The Data Packet Header is used in USB 3.2 to provide additional information about the data being transferred, including the number of data bytes, the sequence number, and other parameters.

The figure shows the structure of the DP packet type. The “TYPE” field is set to “01000b”.

Isochronous Timestamp Packet (ITP) Format

The Isochronous Timestamp Packet (ITP) is used in isochronous transfers to provide timing information. Isochronous transfers are used for real-time data, such as audio and video streaming.

The figure shows the structure of the ITS packet type. The “TYPE” field is set to “01100b”.

Conclusion

In summary, Link Management Packets (LMP) are employed for link initialization and management, ensuring a reliable communication link. Transaction Packets serve as the backbone of USB communication, initiating and controlling data transfers between hosts and devices. Data Packet Headers provide crucial information about data transfers, enhancing efficiency and reliability. Isochronous Timestamp Packets play a specialized role in isochronous transfers, ensuring accurate timing information for real-time data such as audio and video streams.

Together, these packets facilitate a standardized and efficient USB communication protocol, addressing link establishment, data transfer control, data payload management, and real-time data synchronization.