2. May 17, 20002 Isochrony and Synchronization Geert Knapen Philips ITCL-USA

3. May 17, 20003 Agenda w What is the problem? w Synchronization w Implementation in USB2.0 w Conclusions

4. May 17, 20004 Focus on Synchronization Isochrony Implies... w Data transport requirements w Synchronization w Latency considerations w Buffer requirements w Host responsibilities w Data error detection w…w…w…w…

5. May 17, 20005 The Synchronization Problem w Establish data connection between source and sink – In general, data is synchronous in nature at both ends – Source and sink potentially run off independent sample clocks w Source and sink have their private stream characteristics – Sampling rate – Synchronization type – Data format

6. May 17, 20006 The Synchronization Problem w USB is used to interconnect both devices – No support for synchronous streams – Isochronous is next best thing – Source sampling rate gets lost w To establish connection, proper conversions must take place – From synchronous to isochronous – Sampling rate conversion (SRC) – Synchronization – Data format translation Continued

7. May 17, 20007 nn+1n+2n+3n+4n+5 Sample Clock Data (Micro)Frame Time 012301230123012301234 Packetizing From Synchronous to Isochronous on USB

8. May 17, 20008 nn+5 Sample Clock Frame Time n+1n+2n+3n+4 Back to Synchronous ? ?

9. May 17, 20009 Agenda w What is the problem? w Synchronization w Implementation in USB2.0 w Conclusions

10. May 17, 200010 Synchronization w Provides the solution for the missing sample clock w Source sample clock is reconstructed at the sink side w Different synchronization types needed, depending on source and sink nature – Asynchronous Sample clock locked to external or free-running internal clock Sample clock locked to external or free-running internal clock – Synchronous Sample clock slaved to SOF Sample clock slaved to SOF – Adaptive Sample clock derived from average desired data rate Sample clock derived from average desired data rate

11. May 17, 200011 Synchronization w Requires additional data rate information exchange between source and sink – Feedback Rate info sent against the direction of the data flow Rate info sent against the direction of the data flow – Feedforward Rate info sent in the direction of the data flow Rate info sent in the direction of the data flow w Rate info can be conveyed explicitly or implicitly – Explicit A dedicated isochronous pipe is used A dedicated isochronous pipe is used – Implicit Rate info is extracted from the data stream or derived from SOF Rate info is extracted from the data stream or derived from SOF Continued

12. May 17, 200012 Asynchronous Synchronous Adaptive Source w Free runs w Uses implicit feedforward w Locked to SOF w Uses implicit feedback w Locked to sink w Uses explicit feedback w Free runs w Uses implicit feedforward w Locked to SOF w Uses implicit feedback w Locked to sink w Uses explicit feedback Sink w Free runs w Uses explicit feedback w Locked to SOF w Uses implicit feedforward w Locked to data flow w Uses implicit feedforward w Free runs w Uses explicit feedback w Locked to SOF w Uses implicit feedforward w Locked to data flow w Uses implicit feedforward Synchronization Characteristics

13. May 17, 200013 Agenda w What is the problem? w Synchronization w Implementation in USB2.0 w Conclusions

14. May 17, 200014 Implementation in USB2.0 w For Adaptive source and Asynchronous sink – Explicit isochronous feedback pipe is necessary – Feedback information and format defined – Association mechanism between data endpoint(s) and feedback endpoint w Device reports its synchronization capabilities – Endpoint descriptor fields w Clock Mastering capability has been removed

15. May 17, 200015 Feedback Rate Info F f w Goal is to measure sample clock F s – Accuracy better than 1 Hz w F f indicates the desired sampling clock F s referenced to USBs notion of time, ( )SOF w Expressed in number of samples per USB ( )frame – Can be fractional number – Measuring F f can take multiple ( )frames w Used by adaptive source and generated by asynchronous sink

16. May 17, 200016 F f Format for Full Speed w Integer part needs 10 bits to encode up to 1023 one-byte samples per frame w Fraction needs 10 bits to resolve sampling frequency to < 1 Hz within a 1 kHz frame rate w Lower 4 bits are reserved for future use 0231314IntegerFraction34RFU

17. May 17, 200017 IntegerFractionRFURFU0281516232931 F f Format for High Speed w Integer part needs 12 bits to encode up to 3072 one-byte samples per frame w Fraction needs 13 bits to resolve sampling frequency to < 1 Hz within a 8 kHz frame rate w Lower 3 bits and upper 4 bits are RFU

18. May 17, 200018 Association D/FB Endpoint w Based on Endpoint Number matching (EPN) – Data EP(s) and feedback EP always have opposite direction – Feedback EP must have same EPN as first data EP – Additional data EPs must have ascending EPNs (not necessarily contiguous) – Data EP uses first feedback EP with EPN number < its own EPN

19. May 17, 200019 Endpoint Descriptor w Bits 1..0: Transfer Type – 01: Isochronous w Bits 3..2: Synchronization Type – 00: No Synchronization – 01: Asynchronous – 10: Adaptive – 11: Synchronous w Bits 5..4: Usage Type – 00: Data endpoint – 01: Feedback endpoint – 10: Implicit feedback data endpoint – 11: Reserved bmAttributes field New!New!

20. May 17, 200020 Implicit Feedback Data EP w Bi-directional isochronous endpoint pair that uses the same sample clock w Two possible cases: – Asynch sink/source EP pair Source EP can serve as feedback EP for sink EP Source EP can serve as feedback EP for sink EP – Adaptive source/sink EP pair Sink EP can serve as feedback EP for source EP Sink EP can serve as feedback EP for source EP w Remarks – USB2.0 spec does not define how feedback info is retrieved – Multiple data EPs can use same implicit feedback data EP

21. May 17, 200021 Endpoint Descriptor w For isochronous endpoints, defines bus access period as 2 (bInterval-1) ( )frames – bInterval in the range [1,16] w Usually set to 1 for data endpoints w Indicates how often new feedback info ( F f ) is available for feedback endpoints bInterval Field

22. May 17, 200022 Agenda w What is the problem? w Synchronization w Implementation in USB2.0 w Conclusions

23. May 17, 200023 Conclusions w Descriptors now contain all necessary information – Support for bi-directional EP pairs w Explicit feedback data and format is well defined w USB2.0 synchronization mechanisms allow for high-quality synchronous source to sink connections w All other requirements for high-quality isochrony must be in place