1. CS 414 - Spring 2012 CS 414 – Multimedia Systems Design Lecture 25 – Synchronization Issues Klara Nahrstedt Spring 2012

2. Administrative MP2 posted MP2 Deadline – April 7, Saturday, 5pm. CS 414 - Spring 2012

3. Internet Multimedia Protocol Stack CS 414 - Spring 2012 AAL3/4 IP Version 4, IP Version 6 UDP Media encaps (H.264, MPEG-4) RTP ATM/Fiber Optics Ethernet/WiFi TCP SIP RTSPRSVPRTCP AAL5 KERNEL APPLICATION Layer 4 (Transport) Layer 3 (Network) Layer 2 (Link/MAC) Layer 5 (Session) MPLS DCCP DASH HTTP Synchronization Service

4. Outline Synchronization in Distributed Environments Clock Synchronization Synchronization Reference Model  Media, Stream, Object, Specification Levels CS 414 - Spring 2012

5. Synchronization in Distributed Environments Information of synchronization must be transmitted with audio and video streams, so that receiver(s) can synchronize streams Sync information can be delivered before start of presentation (used by synthetic synchronization)  Advantage: simple implementation  Disadvantage: presentation delay Sync information can be delivered using separate sync channel - out-band (used by live synchronization)  Advantage: no additional presentation delay  Disadvantage: additional channel needed CS 414 - Spring 2012

6. Sync in Distributed Environments Sync information can be delivered using multiplexed data streams - in-band sync  Advantage: related sync information is delivered together with media units  Disadvantage: difficult to use for multiple sources CS 414 - Spring 2012

7. Location of Sync Operation Sync media objects by combining objects into new media object Sync operation placed at sink  Demand on bandwidth is larger because additional sync operations must be transported Sync operation placed at source  Demand on bandwidth smaller because streams are multiplexed according to sync requirements CS 414 - Spring 2012

8. Clock Synchronization Sync accuracy depends on clocks at source and sink nodes  T a = T av – Nl a – O a  T v = T av – Nl v – O v End-to-end delay  Nl a = EED a = T av -T a -O a  Nl v =EED v = T av -T v -O v  EED a = (T a1 -T a2 )/2 NTP (Network Time Protocol ) CS 414 - Spring 2012

9. AV Skew Calculation Select synchronization point in AV streams in terms of frame number for each stream Execute sync operation at sync points 1. Measure  Arrival time of sync audio packet Tsync A  Arrival time of sync video packet Tsync V 2. Calculate:  AVSkew = Tsync A – Tsync V 3. Alternative measure and calculate  Tsync A = Tsend A + EED A; Tsync V = Tsend V + EED V  EED = RTT/2 = (Trecv – Tsend)/2 CS 414 - Spring 2012

10. Network Time Protocol Protocol to sync clocks of computer systems over packet-switched, variable – latency data networks  Uses UDP port 123  Designed to resist effects of variable latency (jitter buffer)  Designed in 1985 by Dave Mills at U. Delaware  Can achieve accuracy of 200 µsec  Based on Marzullo Algorithm CS 414 - Spring 2012

11. Marzullo’s algorithm (1984) (intersection algorithm) Agreement protocol for estimating accurate time from a number of noisy time sources If we have estimates 10 2, 12 1, 11 1, then interval intersection is 11.5 0.5 If some intervals don’t intersect, consider intersection of majority of intervals CS 414 - Spring 2012 Source: wikipedia

12. Clock strata NTP uses hierarchical system of “clock strata” Stratum levels define distance from reference clock and exist to prevent cycles in hierarchy  Stratum 0 devices are atomic clocks, GPS clocks, radio clocks  Stratum1 computers attached to stratum0 devices Act as servers for timing requests from Stratum 2 servers via NTP  Stratum2 (similar to Stratum1, but they also have peering relation to other stratum2 servers CS 414 - Spring 2012

13. Precision Time Protocol (PTP) High Precision time protocol (original IEEE 1588-2002) for synchronization used in measurement and control systems residing on a local area network Accuracy in the sub-microseconds PTPv2 – IEEE 1588-2008 improves accuracy, precision and robustness PTP serves a niche not well served by NTP and GPS  it achieves accuracy beyond those attainable using NTP;  It is cheaper than GPS  It is accessible to receivers for which GPS signals are inaccessible. CS 414 - Spring 2012

14. PTP Details IEEE 1588 standards describe a hierarchical master- slave architecture for clock distribution  Ordinary clock – source for root timing reference, called ‘grandmaster’  Boundary clock – intermediate synchronization bridge from one network segment to another – used by synchronization master to sync clocks within a network segment PTP messages use IP-multicast over UDP (IPv4 or IPv6) Selection algorithm of best clock source uses properties such as quality, accuracy, variance, … CS 414 - Spring 2012

15. PTP Synchronization After selection of master source of time, clocks determine the offset between themselves and their master  Let t represent physical time, given slave device, the offset o(t) at time t is: o(t) = s(t) – m(t), m(t) is measured time at master at time t, and s(t) is measured time at local clock at time t.  Master periodically broadcasts the current time to other clocks (up to 10 measurements per second in 1588-2008) CS 414 - Spring 2012

16. PTP Synchronization CS 414 - Spring 2012 T1’ – T1 = o + d and T2’ – T2 = -o + d, then we find o = (T1’ - T1 – T2’ + T2)/2 The clock now knows offset o during the transactions and can correct itself by this agreement with the master clock.

17. Other Sync Issues Sync must be considered during object acquisition Sync must be considered during retrieval  Sync access to frames of stored video Sync must be considered during transport  If possible use isochronous protocols Sync must be considered at sink  Sync delivery to output devices Sync must consider support of functions such as pause, forward, rewind with different speeds, direct access, stop or repeat CS 414 - Spring 2012

18. Other Sync Issues Sync must be considered during object acquisition Sync must be considered during retrieval  Sync access to frames of stored video Sync must be considered during transport  If possible use isochronous protocols Sync must be considered at sink  Sync delivery to output devices Sync must consider support of functions such as pause, forward, rewind with different speeds, direct access, stop or repeat CS 414 - Spring 2012

19. Reference Models We need reference models to  Understand various requirements for multimedia sync  Identify and structure run-time mechanisms to support execution of sync  Identify interface between run-time mechanisms  Compare system solutions for multimedia sync CS 414 - Spring 2012

20. Synchronization Reference Model Sync model we will be evaluating in detail is according to Meyer, Effelsberg, Steinmetz:  Sync multimedia objects are classified according to Media level Stream level Object level Specification level CS 414 - Spring 2012

21. Media Level (1) Each application operates single continuous media streams composed of sequence of LDUs Each media stream responsible for intra- stream synchronization Assumption at this level: device independence Supported operations at this level:  read(devicehandle, LDU)  write(devicehandle, LDU) CS 414 - Spring 2012

22. Media Level (2) - Example window = open(“videodevice”); movie = open(“file”); while (not EOF (movie) ) { read(movie, &LDU); if (LDU.time == 20) printf(“Subtitle 1”); else if (LDU.time == 26) printf(“Subtitle2”); write(window, LDU); } close(window); close(movie); CS 414 - Spring 2012

23. Stream Level (1) Operates on continuous media streams and groups of streams Models inter-stream synchronization for need of parallel presentation Offers abstractions:  notion of streams,  timing parameters concerning QoS for intra-stream and inter-stream synchronization CS 414 - Spring 2012

24. Stream Level (2) Supports operations:  Start(stream), stop(stream), create-group(list-of- streams);  Start(group), stop(group);  Setcuepoint(stream/group, at, event); Classifies implementation according to  Support for distribution (end-to-end, local)  Support of type of guarantees (best effort, deterministic)  Support of types of supported streams (analog, digital) CS 414 - Spring 2012

25. Object Level (1) Operates on all types of media and hides differences between discrete and continuous media Offers abstractions:  Complete sync presentation Computes and executes complete presentation schedules that include presentation of non- continuous media objects and calls to stream level Does not handle intra-stream and inter-stream synchronization  (relies on media and stream levels) CS 414 - Spring 2012

26. Object Level (2) - Example MHEG – Multimedia Hypermedia Experts Group of ISO  Defines representation and encoding of multimedia and hypermedia objects (object-based declarative programming language)  Provides abstractions suited to real-time presentations implemented via multimedia synchronization functionalities  Provides abstracts for real-time exchange implemented with minimal buffering  Evaluates status of objects and performs actions (e.g., prepare, run, stop, destroy) For time-dependent streams – access to stream level For time-independent streams – direct access the object to present it Classification of this level according to (a) distribution capabilities, (b) type of presentation schedule, (c) schedule calculation CS 414 - Spring 2012

27. Specification Level (1) Open layer included in tools which allow to create sync specifications Examples:  Synchronization editors, document editors, authoring systems, conversion tools  Examples of such tools: multimedia document formatter that produces MHEG specifications Classification:  Interval-based spec  Time-axes based spec  Control flow-based spec  Event-based spec CS 414 - Spring 2012

28. Specification Level (2) - SMIL SMIL – World Wide Web Consortium’s Synchronized Multimedia Integration Language SMIL 2.0 and SMIL 3.0 (developed by CWI, PI: D. Bulterman) Features: Animations, Time Manipulations, Timing and Synchronization, Content Control, Meta-information, Media objects, Linking, Layout, Transitions http://www.w3.org/TR/SMIL3/ CS 414 - Spring 2012

29. SMIL CS 414 - Spring 2012 “Standards, SMIL 2.0, Examples and Comparisons”, D. Bulterman, Siemens Corporate research, Editor: P. Liu, 2002

30. Summary CS 414 - Spring 2012

31. Conclusion Carefully analyze what kind of synchronization is needed in your multimedia system and application Determine at which level you need synchronization Determine what the synchronization requirements should be based on prior experiments CS 414 - Spring 2012