Protocol-Layer Packets

• Communication between a host controller root port and a SuperSpeed device uses Protocol-Layer packets.
• All packets (with one exception) are 16 bytes in size and are referred to as Header packets.
• The DATA Header packet may include a Data Packet Payload (DPP):
  • Payload size: 1–1024 bytes

USB 2.0 Token/Data/Handshake Shortcomings

USB 2.0 protocols suffer from four main limitations:

1. Inefficient three-packet sequence (Token → Data → Handshake)
2. Shared broadcast bus, increasing power consumption
3. Polled flow control, relying heavily on NAK packets
4. Limited error handling, with only three retries before software intervention
   SuperSpeed protocols address these issues as outlined below.

1. Inefficient Token/Data/Handshake Sequence

USB 2.0

• A typical transaction requires three packets:
  • Token → DATA → ACK
• (Isochronous transfers omit the handshake packet.)

SuperSpeed (SS)

• Uses a two-packet transaction:
  • DATA Header packet (includes token information)
  • Data Packet Payload (DPP)
• This reduces protocol overhead and improves efficiency.

Key Benefit:
Lower latency and reduced packet overhead per transaction.

2. Broadcast Bus and Power Consumption

USB 2.0

• Uses a shared broadcast bus:
  • Packets are sent to all devices of the same speed.
  • Each device must decode the address to determine relevance.
• Both data and handshake packets may be broadcast, increasing unnecessary link activity and power usage.

SuperSpeed

• Uses unicast transactions:
  • Packets are sent only to the target device.
  • Only links along the path from the root port to the device are activated.
• Routing information is embedded in packets and used by SS hubs to forward traffic to the correct downstream port.

Key Benefit:
Significant reduction in power consumption and unnecessary link activity.

3. Polled Flow Control

USB 2.0

• Relies on repeated polling:
  • Endpoints are frequently polled to check readiness.
  • Often results in repeated NAK responses, wasting bus bandwidth.
• Affects most endpoint types (except Isochronous).

SuperSpeed

• Uses a poll-once protocol:
  • Host accesses an endpoint once.
  • If not ready, the device responds with NRDY (Not Ready).
  • The host waits until the device sends ERDY (Endpoint Ready) before retrying.
• In some cases, devices can send ERDY without being polled again.

Key Benefit:
Eliminates excessive retries and dramatically improves bus efficiency.

4. Error Handling and Reporting

USB 2.0

• Performs packet error checks.
• Allows up to three retries per transaction.
• If all retries fail, the error is reported to software.

SuperSpeed

• Retains the same error-checking mechanism and three-retry limit.
• Applies these checks end-to-end at the protocol layer.

Key Benefit:
Maintains robustness while integrating with improved SS protocol efficiency.

Data Bursting

• SuperSpeed normally requires each DATA payload to be acknowledged by an ACK Header packet.
• Data Bursting allows:
  • Multiple DATA payloads to be transmitted before receiving the first ACK.
• Burst limits are defined by the Max Burst value in the Endpoint Companion Descriptor.

Specifications:

• Maximum burst size: 16
• Maximum payload per DATA packet: 1024 bytes
• Maximum burst transfer size: 16 KB

Key Benefit:
Reduced latency and improved throughput.

Bulk Streaming

• Bulk Streaming is an optional feature available only for bulk endpoints.
• Designed to manage multiple data streams efficiently.

Standard Bulk Endpoints

• One endpoint buffer maps to one main memory buffer.

Bulk Streaming

• Expands buffer support to nearly 64k buffers.
• Uses a 16-bit Stream ID (in header packets) to:
  • Select a specific endpoint buffer
  • Map it to a corresponding main memory buffer on the host

Key Benefit:
Supports high-throughput, parallel data streams with reduced software overhead.