Synchronization of Networks and Applications: a Survey Raffaele Noro - ICA Institute for computer...

Synchronization of Networks and Applications: a Survey

Raffaele Noro - ICA

Institute for computer Communications and Applications

[email protected]

icawww.epfl.ch

Synchronization of Networks and Applications: a SurveyR. Noro 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


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 ….


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

Synchronoustransmission

Synchronoustransmission

Asynchronoustransmission

Synchronoustransfer mode

Asynchronoustransfer mode

Asynchronoustransfer mode

‘Classical’ ‘Easy’ ‘Difficult’

Synchronousapplication

Terminal

Network

Asynchronousapplication

Synchronousapplication


Phase Locked Loop (PLL)principle

Simple, low-cost and accurate linear system Designed for low-jitter environments, everlasting connections Good jitter absorption requires slow convergence speed

+Loopfilter

Counter

Voltagecontrolledoscillator

Referencephase signal

Reconstructedphase signal

-

Error Voltage

Frequency

time

frequencyreconstructed

reference

time

phasereconstructed

referenceLocking time


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)

Global Positioning System (GPS)time transfer

24 satellites

4 atomic clocks

User

Time transferprecision of 430 ns

Control


Synchronous and Plesiochronous Digital Hierarchy (SDH/PDH) - architecture

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

Lo

we

r sy

nch

ron

izat

ion

acc

ura

cy

User lines

Concentrator

Centraloffice

Multiplexer

Stratum 1

Stratum 2

Stratum 3

Stratum 4

SDH ADM

PDHequipment

Error Slip rate

Stratum 4 10-3 15/min

Stratum 3 10-6 130/hour

Stratum 2 10-8 10/day

Stratum 1 10-11 2/year

Backbonenetwork

Sync flow


SDH/PDHprotocol

TDM consists of a periodic system in which the same pattern is repeated each 125 s 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

64 kbpsT1/E1

T3/E3

STM-1

Data

Sync information

Slipping/Justification bitsfor mismatching of tributariesfrequencies

SDH ADM

Sync to the commonphase of the SDH

PDH

SDH

125 s

optional

mandatory

mandatory


Global System for Mobile comm. (GSM)architecture

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

BSC

Base Stationis master forall MH

Mobile Hostis slave of BS- frequency- slot- time adjusting

ReferenceTDMA frame

TDMA frame as seen by MHto compensate propagation

To/from the fixed telephone network

GMSCdownlink

uplink


GSMprotocol

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

User channels

Common control channels

FCH, 148 zeros, 20 times per secondperfect reconstruction of analog carrier

SCH for frame delineation 64 bits+ 78 bits code

3 Common control channels

uplink RACH for Time Advance 41 bits in a Guard Period


Satellite

"Macro" Cell "Micro” Cell

Zone 2Neighborhood Zone1

In-Building

"Pico" Cell

Zone 4 : Global

Zone 3Suburban

1850 1900 1950 2000 2050 2100 2150 2200 2250

15 20 60 30 15 60 30

DECT

UMTSTDD

TD-CDMA

UMTSFDD

W-CDMA

UMTSSatellite

SW-CDMA

UMTSTDD

TD-CDMA

MHz

Universal Mobile Telecomm. System (UMTS)Architecture

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

Cell site #0

Cell site #1

Cell site #2

BS #0

Scrambling code masked symbolT

slot

BS #1

Frequency Spectrum


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

Codemodulator

Widebandmodulator

Widebanddemod.

Codemodulator

Codesynch/

tracking

Codegenerator

Codegenerator

Data

Data

Must know the code sequence

and must keep the synchronization

Datapattern

Code

Modulateddata

Transmission

Delay Locked Loop (DLL) conceptually similar to a PLL use of correlation function instead of phase error

Delay

Correlation


ATMATM

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)

Adaptation layer

ATMnetwork

Adaptation layer

Trafficsource

Trafficcontract

Terminal

ATM cellsat the UNI


PLL

ATMprotocol

Three synchronization methods exist Immediate playout (VBR and CBR) Adaptive playout (CBR) Synchronous Residual Timestamp (CBR)

Application

ATM adaptationlayer

ATM layer

SDH/SONETfor transport

Data

AAL PDUs

ATM cells

OC-3/STM-1

- Jitter removal- Source clock recovery- Data structure handling

Immediate playout (AAL5, AAL2)

Adaptive playout (AAL1)

Rate

Buffer level controls the rate via the PLL

Asynch in practice, enforcing to have small network CDV

Synchronous Residual TimestampSRTS (AAL1)

Rate

Overhead due to the SRTS field in the PDUs

PLLA common network clock is needed for residual timestamping


Network Time Protocol (NTP)architecture

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

Lo

wer

syn

chro

niz

atio

n a

ccu

racy

Primaryservers

Secondaryservers

Tertiaryservers

Accuratetime source

(GPS)

LAN

OrdinaryInternet

links


NTPprotocol

Authentication of the NTP message with DES NTP uses port a specific UDP port

IP

UDP

NTP

Stack

Packet

Selection mechanism


Motion Picture Expert Group (MPEG)architecture

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

Distributionnetwork

DVD

Residentialuser

Receiver/decoder #1

Videoserver Return

channel

Stored material(films)

Live material(TV)

CableSatellite

TerrestrialATMSTM

The electron beam of TVmust be in-sync with thevideo camera

Receiver/decoder #N

The decoder mustsynchronize to the server:- decoding purposes- generation of TV signal- audio and video sync


MPEGprotocol

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

Video data Audio dataVideoPresentation TS

AudioPresentation TS

Clockreference

Decoding

Video signal Audio signal

Audio and video part containPresentation TS referred to a

common timebaseA transport packet multiplexes audio and

video, and carry a Clock Reference toreconstruct a common timebase

Receiver/decoder


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 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

Asynchronous network Bursty traffic Statistical multiplexing No network clock

Protectiveinterface

Protectiveinterface

Packetnetwork

Synchronous applications Voice services Digital TV Multimedia


Model for the remote clock: ts= a ·tr+ b Processing of clock samples with estimation of a and b

Three properties: Simple implementation, efficient in removing jitter, short response time

Source clock recovery withLeast-square Linear Regression (LLR)

btat

ttttm

tttttb

ttttm

ttma

rs

rsrs

srrss

rsrs

ss

iiii

iiiii

iiii

ii

ˆˆˆ

ˆ

ˆ

2

22

tr

ts

tr1, ts1

trm, tsm


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()* nin

LLR: an optimal performance

m

rr

rrr

i

i

iii

dtt

aaa

dttt

*

* Decreasing for PLL

= e-4 1/60LLR

PLL

1 ns 1 s 1 ms 1 s Res. Jitternout

Conv. rapidityk

1 s

100 s

10000 s

Linear approximation of a LLR

nout

LLR: phase error

1 min 5 min

K (short)

m

LLR: frequency error

1 min 5 min

Input jitter: nin

Output jitter: nout

PLL: frequency error

K (long)

1 min 5 min

PLL: phase error

1 min 5 min

nout

LLR vs. PLL: gain factor of ~100

Decreasing for LLR

Objective

* under the assumption of uncorrelated jitter


tr

tsits

tsi

tsi

2

tsitri

tri

a

b

+

LLR: a simple implementation

Mem.

Mem.

^2

x

Mem.+

+

+

Mem.x

x

x

^2

x m

x m

+

+

+

x +

Localclock

-

-

-

-

-

-

-

Operators LLR PLL

Multiplications 10 4

Sums 7 4

a and b updated at each cycle

Local clock tr in conjunction with

a and b used to synchronize to ts - timekeeping function


5 min 10 min0 min

-100 ms

100 ms

LLR: application to MPEG-2 transport

Best effort model for the jitter Initial frequency difference of 5 x 10 -4

IP Network

Digital audio-videopacket stream

Digital audio-videopacket stream

Synchronizedsystem clock

SynchSystem clock

Clock references

Buffer for de-jittering,equivalent to 100 ms

Jitter: nin~ 100 ms

nout< 4 ms, < 10 - 4

MPEGtransmitter

MPEGreceiver

-100 ms

100 ms

5 min 10 min0 min

Bufferoverflow/underflow

Synchronization with LLR is OK Synchronization with PLL fails

LLR: phase error PLL: phase error


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


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


Uncommented slides


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


DVB system


DVB IRD


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


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

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