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Synchronization of Networks and Applications: a Survey. Raffaele Noro - ICA Institute for computer Communications and Applications noro@epfl.ch icawww.epfl.ch. Outline. Part I - Synchronization features of communication systems Introduction general and technical aspects of synchronization

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Synchronization of Networks and Applications: a Survey


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    1. Synchronization of Networks and Applications: a Survey Raffaele Noro - ICA Institute for computer Communications and Applications noro@epfl.ch icawww.epfl.ch

    2. Outline Part I - Synchronization features of communication systems • Introduction • general and technical aspects of synchronization • PLL - a conventional synchronization algorithm, pros & cons • GPS - primary tool for time transfer and synchronization • Synchronized communication systems: architecture, protocol and algorithm • PDH, SDH, GSM, and UMTS - synchronization of networks • ATM Adaptation Layer and Network Time Protocol - synchronization of terminals • MPEG - synchronization of an application • Preliminary conclusion Part II - Our contribution: a new synchronization mechanism for packet networks • Synchronous applications for asynchronous networks • Synchronization with Least-square Linear Regression (LLR) • Analysis, implementation and performance • Benefits of LLR for applications • improving the accuracy and the response time of synchronization • Conclusion Synchronization of Networks and Applications: a Survey R. Noro

    3. Introductiongeneral aspects • Fundamental need of information consumed by humans • Definition of a synchronized system: ‘a system that maintains a process in step with another’ • Time scales must be the same for the two processes, and • Events on one time scale match with the events on the second time scale • @ many levels: e.g., transmission link (transmitter/receiver), network equipment, terminals • Synchronous system characterized by architecture, protocol and algorithm What synchronization looks like …. Synchronization of Networks and Applications: a Survey R. Noro

    4. Introductiontechnical aspects • Transmission: synchronous vs. asynchronous (circuit vs. packet) • Transfer mode: synchronous vs. asynchronous (STM vs. ATM) • Application: synchronous vs. asynchronous (video vs. still image) • Each level, if is needing synchronization, implements its own synch system • Most frequently, synchronous systems are superposed • Asynchronous systems can operate on top of synchronous systems • Much more difficult to operate synchronous systems on top of asynchronous ‘Classical’ ‘Easy’ ‘Difficult’ Terminal Synchronous application Asynchronous application Synchronous application Synchronous transfer mode Asynchronous transfer mode Asynchronous transfer mode Network Synchronous transmission Synchronous transmission Asynchronous transmission Synchronization of Networks and Applications: a Survey R. Noro

    5. Phase Locked Loop (PLL)principle Loop filter Reference phase signal Reconstructed phase signal Voltage Error Counter + Frequency • Simple, low-cost and accurate linear system • Designed for low-jitter environments, everlasting connections • Good jitter absorption requires slow convergence speed - Voltage controlled oscillator reconstructed reconstructed frequency phase reference reference Locking time time time Synchronization of Networks and Applications: a Survey R. Noro

    6. Global Positioning System (GPS)time transfer 4 atomic clocks • The GPS signal contains a timecode steered to UTC(USNO) • weeks number (10 bits, up to 1023) + # of 1.5 s periods within the week (19 bits, 1 week) 24 satellites Time transfer precision of 430 ns User Control Synchronization of Networks and Applications: a Survey R. Noro

    7. Synchronous and Plesiochronous Digital Hierarchy (SDH/PDH) - architecture User lines Multiplexer Concentrator Stratum 4 Central office • Designed for voice transport with strict synchronization requirements • Implements a Time Division Multiplex (TDM) hierarchy • maintain bitrate (frequency sync needed at all levels) • delineate TDM frames (phase sync needed at multiplex) • synch of Add and Drop Multiplexors (ADM) at SDH level Stratum 3 PDH equipment Lower synchronization accuracy Stratum 2 SDH ADM Backbone network Stratum 1 Sync flow Synchronization of Networks and Applications: a Survey R. Noro

    8. SDH/PDHprotocol Sync information Data T1/E1 64 kbps Slipping/Justification bits for mismatching of tributaries frequencies 125 ms • TDM consists of a periodic system in which the same pattern is repeated each 125 ms • T1/E1 signal contains sync information to maintain frequency synchronization between transmitter and receiver • T3/E3 multiplex compensate for rate mismatch of tributaries (justification bit). Aligns the frame to the reference phase of the network • At SDH level the frame synchronization is maintained by a pointer in the overhead field optional T3/E3 PDH mandatory SDH ADM STM-1 Sync to the common phase of the SDH SDH mandatory Synchronization of Networks and Applications: a Survey R. Noro

    9. Global System for Mobile comm. (GSM)architecture Reference TDMA frame • Full duplex • 2 separated 25 MHz bands for downlink and uplink • 124 FDMA channels (usable) at 200 kHz in each 25 MHz band • 8 TDMA slots in each 200 kHz channel Base Station is master for all MH TDMA frame as seen by MH to compensate propagation To/from the fixed telephone network BSC GMSC downlink uplink Mobile Host is slave of BS - frequency - slot - time adjusting Synchronization of Networks and Applications: a Survey R. Noro

    10. GSMprotocol User channels Common control channels • Two CCCH in downlink directions are used to synchronize the frequency and delineate the TDMA frames • One CCCH in uplink direction is used to compute the time advance for compensating the propagation time 1 2 downlink FCH, 148 zeros, 20 times per second perfect reconstruction of analog carrier SCH for frame delineation 64 bits+ 78 bits code uplink RACH for Time Advance 41 bits in a Guard Period 3 Common control channels Synchronization of Networks and Applications: a Survey R. Noro

    11. Zone 3 Suburban Universal Mobile Telecomm. System (UMTS)Architecture Zone 4 : Global Satellite Zone 2 Zone1 Neighborhood • Data rates up to 2 Mbps • Circuit and packet-switched services • Variable rate • Based on Code Division MultipleAccess (CDMA) • Asynchronous intercell operation supported • code, phase and frequency may change at each handover • Synchronization/tracking of the code used for despread of the received signal In-Building "Pico" Cell "Micro” Cell "Macro" Cell Frequency Spectrum 1850 1900 1950 2000 2050 2100 2150 2200 2250 MHz 15 20 60 30 15 60 30 Cell site #1 Cell site #0 Cell site #2 Scrambling code masked symbol T DECT slot UMTS TDD TD-CDMA UMTS FDD W-CDMA BS #0 UMTS TDD TD-CDMA BS #1 UMTS Satellite SW-CDMA Synchronization of Networks and Applications: a Survey R. Noro

    12. UMTSprotocol • Direct Sequence- CDMA: spread/despread principles • User signal modulated with a digital code of higher frequency (spectrum spreading) • Set of orthogonal codes • Each channel demodulated with its channel code • Each demodulated signal sees the remaining signals as noise Data pattern Data Code modulator Wideband modulator Code Data Wideband demod. Code modulator Modulated data Code generator Transmission Code generator Code synch/ tracking Must know the code sequence and must keep the synchronization Correlation • Delay Locked Loop (DLL) • conceptually similar to a PLL • use of correlation function instead of phase error Delay Synchronization of Networks and Applications: a Survey R. Noro

    13. Asynchronous Transfer Mode (ATM)architecture • Real time transfer capability of ATM (CBR and VBR) • Traffic contract at the UNI: the traffic described by: PCR, SCR, CDVT receives the network QoS described by: CLR, CTD, CDV • ATM= asynchronous: always more jitter than in circuit switched networks • statistical multiplexing • burst traffic • source jitter • Synchronization and jitter removal are terminal functionality • ATM Adaptation Layer (AAL) Traffic source Terminal Traffic contract Adaptation layer Adaptation layer ATM network ATM ATM ATM cells at the UNI Synchronization of Networks and Applications: a Survey R. Noro

    14. ATMprotocol Immediate playout (AAL5, AAL2) Asynch in practice, enforcing to have small network CDV • Three synchronization methods exist • Immediate playout (VBR and CBR) • Adaptive playout (CBR) • Synchronous Residual Timestamp (CBR) Buffer level controls the rate via the PLL Application Adaptive playout (AAL1) Data - Jitter removal - Source clock recovery - Data structure handling ATM adaptation layer AAL PDUs PLL Rate ATM layer Synchronous Residual Timestamp SRTS (AAL1) Overhead due to the SRTS field in the PDUs ATM cells SDH/SONET for transport OC-3/STM-1 A common network clock is needed for residual timestamping PLL Rate Synchronization of Networks and Applications: a Survey R. Noro

    15. Network Time Protocol (NTP)architecture LAN Tertiary servers • Hierarchy is similar to the PDH/SDH case, but time servers are end-terminals • A server is also client of a selected, reliable server • Reliability depends on network load  Filtering mechanism to select the most reliable server • Accurate within a LAN • New generation of NTP foresees support for high speed, session-oriented streams Secondary servers Lower synchronization accuracy Primary servers Ordinary Internet links Accurate time source (GPS) Synchronization of Networks and Applications: a Survey R. Noro

    16. NTPprotocol • Authentication of the NTP message with DES • NTP uses port a specific UDP port Packet NTP Stack UDP Selection mechanism IP Synchronization of Networks and Applications: a Survey R. Noro

    17. Motion Picture Expert Group (MPEG)architecture The electron beam of TV must be in-sync with the video camera Residential user Stored material (films) Video server • Real MPEG systems implement their own synchronization system, regardless of the nature of transmission • MPEG receivers are always slaved to the server • The quality of synchronization depends on the jitter induced by the network Return channel Receiver/ decoder #1 DVD Distribution network Live material (TV) The decoder must synchronize to the server: - decoding purposes - generation of TV signal - audio and video sync Cable Satellite Terrestrial ATM STM Receiver/ decoder #N Synchronization of Networks and Applications: a Survey R. Noro

    18. MPEGprotocol Video signal Audio signal Receiver/ decoder • Consecutive Clock References are used to reconstruct the timebase from the jittered stream of Transport Packets • Total delay is not critical for the design of a decoder • Delay variation is critical: decoders are tolerant to a jitter of 4 ms  Network jitter should be minimized/controlled Decoding Clock reference Video Presentation TS Video data Audio Presentation TS Audio data Audio and video part contain Presentation TS referred to a common timebase A transport packet multiplexes audio and video, and carry a Clock Reference to reconstruct a common timebase Synchronization of Networks and Applications: a Survey R. Noro

    19. Observations and preliminary conclusion • The problem of synchronization arises in different contexts and at different levels • The solutions consist of architecture, protocol and algorithm • Architectures and protocols used for synchronization are ad-hoc and are the only two synchronization components that have evolved over time • The conventional algorithm for synchronization is PLL • PLL limitation is its slow convergence and the vulnerability to network jitter  Inappropriate to dynamic connections (e.g., TV zapping), to packet networks (e.g., Internet), especially with the rapidly increasing network speed: emerging challenges for research in synchronization Synchronization of Networks and Applications: a Survey R. Noro

    20. Synchronization over packet networksour contribution • Provide the protective synchronization interface between a synchronous and an asynchronous world • Design the efficient synchronization algorithm to cope with dynamic connections and larger network jitter • Least-square Linear Regression: the appropriate synchronization algorithm Synchronous applications • Voice services • Digital TV • Multimedia Protective interface Protective interface Asynchronous network • Bursty traffic • Statistical multiplexing • No network clock Packet network Synchronization of Networks and Applications: a Survey R. Noro

    21. Source clock recovery withLeast-square Linear Regression (LLR) • Model for the remote clock:ts= a ·tr+ b • Processing of clock samples with estimation of aand b • Three properties: • Simple implementation, efficient in removing jitter, short response time ts trm, tsm tr1, ts1 tr Synchronization of Networks and Applications: a Survey R. Noro

    22. LLR: frequency error e m K (short) 1 min 5 min LLR: an optimal performance Linear approximation of a LLR LLR vs. PLL: gain factor of ~100 Conv. rapidity k Decreasing e for PLL PLL 10000 s Input jitter: nin 100 s Output jitter: nout LLR • Residual jitter: nout= 4* nin / m * • Convergence rapidity: k<< m • rapidity* residual jitter  k* nout<< 4* nin PLL • rapidity* residual jitterk* nout> -log(e)* nin Decreasing e for LLR LLR 1 s e= e-4  1/60 Objective LLR: phase error Res. Jitter nout 1 ns 1 ms 1 s 1 ms nout 1 min 5 min * under the assumption of uncorrelated jitter PLL: phase error PLL: frequency error e nout K (long) 1 min 5 min 1 min 5 min Synchronization of Networks and Applications: a Survey R. Noro

    23. LLR: a simple implementation Stsi tsi + ^2 a ts - - Mem. x +  x + Stsi2 • a and b updated at each cycle • Local clock tr in conjunction with a and b used to synchronize to ts- timekeeping function + x m b ^2 - - Mem. x +  Stsitri - x Mem. + x m x + Stri - - Mem. + tr Local clock Synchronization of Networks and Applications: a Survey R. Noro

    24. LLR: application to MPEG-2 transport Digital audio-video packet stream Synchronized system clock Digital audio-video packet stream Synch Buffer for de-jittering, equivalent to 100 ms System clock nout< 4 ms, e < 10 - 4 • Best effort model for the jitter • Initial frequency difference of 5 x 10 -4 MPEG transmitter MPEG receiver Clock references Jitter: nin~ 100 ms IP Network Synchronization with LLR is OK Synchronization with PLL fails LLR: phase error PLL: phase error 100 ms 100 ms -100 ms Buffer overflow/underflow -100 ms 0 min 5 min 10 min 0 min 5 min 10 min Synchronization of Networks and Applications: a Survey R. Noro

    25. Conclusion • In packet switched network, synchronization is a terminal equipment functionality • LLR is one efficient alternative to PLL for applications over packet networks • Efficient jitter removal • Short response time • Simple implementation • LLR has to be optimized for the specific service to be synchronized • MPEG-2 transport with ATM and with the Internet • Circuit Emulation Service over IP • Real-Time Variable Rate Stream transport over ATM Synchronization of Networks and Applications: a Survey R. Noro

    26. Sources • PLL • F.M. Gardner, Phaselock techniques, J.Wiley & sons ed., 1979 (all about analog PLLs) • IEEE Transactions on Communications, Special Issue on PLLs, Oct. 82 • GPS • W.Lewandowski et al., GPS: Primary Tool for Time Transfer, Proceedings of the IEEE, Jan. 99 • US Naval Observator NAVSTAR Global Positioning System, http://tycho.usno.navy.mil/gpsinfo.html • SDH/PDH • D.Minoli, Enterprise Networking, fractional T1 to SONET, Frame Relay to B-ISDN, Artech ed., 1993 • GSM • S.M.Redl, M.K.Weber, and M.W.Oliphant, An Introduction to GSM, Artech ed., 1995 • CDMA (and UMTS) • R.Prasad, CDMA for Wireless Personal Communications, Artech ed., 1996 • UMTS Forum, http://www.umts-forum.org/ • IMT-2000 Workshop, http://www.itu.int/imt/2-radio-dev/Workshop-97/index.htm • ATM • ATM Forum, http://www.atmforum.org/ • ITU-T I.363.1, B-ISDN ATM Adaptation Layer specification: Type 1 AAL, Aug. 1996 • NTP • Time Synchronization Server, http://www.eecis.udel.edu/~ntp/ • MPEG • ISO-IEC DIS 13818-1, Information Technology-Generic coding of moving pictures and associated audio information- Part 1: Systems, Nov. 1994 • LLR (and Synchronization over Packet Networks) • my homepage, http://icawww.epfl.ch/noro Synchronization of Networks and Applications: a Survey R. Noro

    27. Uncommented slides Synchronization of Networks and Applications: a Survey R. Noro

    28. LORAN-C • Loran-C was originally developed to provide radionavigation service for U.S. coastal waters. • Twenty-four U.S. Loran-C stations work in partnership with Canadian and Russian stations to provide coverage • As of September 30, 1997, 0300 UT, the OMEGA Navigation System terminated. • Sources Synchronization of Networks and Applications: a Survey R. Noro

    29. DVB system Synchronization of Networks and Applications: a Survey R. Noro

    30. DVB IRD Synchronization of Networks and Applications: a Survey R. Noro

    31. H.263 • Low bitrate video codec for videoconferencing applications across packet networks • low delay is an issue • jitter is less important • sync with audio is an issue • bandwidth reduction is an issue • Usage of PTS for lip sync Synchronization of Networks and Applications: a Survey R. Noro

    32. H.263 • Sampling clock is different of network clock: nominal value is 30 fps, but provision for higher or lower fps is made, so higher or lower bitrate • Transport is in H.221-ISDN recently RTP-IP Synchronization of Networks and Applications: a Survey R. Noro