Tài liệu The Synchronisation of Telecommunications Network doc

Synchronisation between telecommunication systems : 1Synchronisation between telecommunication systems : 1 »Master-slave intra-network synchronisation Master System System clock data dat

The Synchronisation of Telecommunications Networks

I The Need for Synchronisation

II Characterizing Synchronisation Quality

III Synchronisation Distribution: General Principles

IV Synchronisation Distribution: SDH/SONET-based

Solution

1 Elements

2 Architecture

3 Synchronisation Status Message (SSM)

V Synchronisation Distribution: GPS-based and

MixedSolution

I The Need for Synchronisation

II Characterizing Synchronisation Quality

III Synchronisation Distribution: General Principles

IV Synchronisation Distribution: SDH/SONET-based

Solution

1 Elements

2 Architecture

3 Synchronisation Status Message (SSM)

V Synchronisation Distribution: GPS-based and

MixedSolution

VI Synchronisation Distribution: From Co-operating

Network

VII Summary on Standards

VIII How to Synchronize Mixed Technology

Networks

1 Mixed Technology Network Example

2 SDH and SONET Networks

3 The Public Switched Telephony Network

VI Synchronisation Distribution: From Co-operating

Network

VII Summary on Standards

VIII How to Synchronize Mixed Technology

Networks

1 Mixed Technology Network Example

2 SDH and SONET Networks

3 The Public Switched Telephony Network

6 GSM and UMTS-FDD Radio Access Networks

7 cdmaOne and cdma2000 Radio Access Networks

VIII How to Synchronize Mixed Technology

Networks (cont ’d)

4 ATM Networks

5 Optical Networks

6 GSM and UMTS-FDD Radio Access Networks

7 cdmaOne and cdma2000 Radio Access Networks

» ADM Add Drop Multiplexer

» ATM Asynchronous Transfer

» CBR Constant Bit Rate

» CDV Cell Delay Variation

» DNU Do Not Use

» DXC Digital Cross-Connect

» f frequency

» FIFO First-In First-Out

» GPS Global Positioning System

» GUI Graphical User Interface

» HSC High Stability Clock

» ADM Add Drop Multiplexer

» ATM Asynchronous Transfer

» CBR Constant Bit Rate

» CDV Cell Delay Variation

» DNU Do Not Use

» DXC Digital Cross-Connect

» f frequency

» FIFO First-In First-Out

» GPS Global Positioning System

» GUI Graphical User Interface

» HSC High Stability Clock

» Hz Hertz (cycles per second)

» PDH Plesiochronous Digital Hierarchy

» PEC Plesiochronous Equipment Clock

» PLL Phase Locked Loop

» Hz Hertz (cycles per second)

» PDH Plesiochronous Digital Hierarchy

» PEC Plesiochronous Equipment Clock

» PLL Phase Locked Loop

» PRC Primary Reference Clock

» ps/km/°C pico seconds per kilometre

per degree centigrade

» SEC Synchronous Equipment Clock

» SETG Synchronous Equipment Timing

Generator

» SETS Synchronous Equipment Timing

Source

» sin Sine function

» SOH Section OverHead

» PRC Primary Reference Clock

» ps/km/°C pico seconds per kilometre

per degree centigrade

» SEC Synchronous Equipment Clock

» SETG Synchronous Equipment Timing

Generator

» SETS Synchronous Equipment Timing

Source

» sin Sine function

» SOH Section OverHead

» SSM Synchronisation Status Message

» SSU Synchronisation Supply Unit

» STM-N Synchronous Transport Module

level N

» STS-N Synchronous Transport Signal

level N

» TF Transfer Function

» TIE Time Interval Error

» TS0 Time Slot Zero

» VBR Variable Bit Rate

» VCO Voltage Controlled Oscillator

» SSM Synchronisation Status Message

» SSU Synchronisation Supply Unit

» STM-N Synchronous Transport Module

level N

» STS-N Synchronous Transport Signal

level N

» TF Transfer Function

» TIE Time Interval Error

» TS0 Time Slot Zero

» VBR Variable Bit Rate

» VCO Voltage Controlled Oscillator

The Need for Synchronisation

Frequency synchronisation

System A

t

Clock signal of system A

Clock signal of system B

System B

TA = 1 / fA

TB = 1 / fB

fA = fB

Phase synchronisation

System A

t

Clock signal of system A

Clock signal of system B

System B

!

Time synchronisation

System A

t

Time signal of system A

Time signal of system B

System B

14/01/00 08:34:56

14/01/00 08:34:57

14/01/00 08:34:55

14/01/00 08:34:55

14/01/00 08:34:56

14/01/00 08:34:57

Where do we need synchronisation?

Three examples

Where do we need synchronisation?

Three examples

»Public Switched Telephone Networks

»SONET and SDH transport networks

»Cellular mobile telecom networks

Public Switched Telephone Network:

Public Switched Telephone Network:

What is a slip?

underflows due to differences in timing

»A slip occurs when a buffer over- or

underflows due to differences in timing

Incoming data rate

Outgoing data rate

Slip

Incoming data rate

Some services affected by slips

» Uncompressed - only 5% of slips lead to clicks

» Compressed - a slip will cause an audible click

» A slip can wipe out several lines

» More slips can freeze frames for several seconds

» Voice

» Uncompressed - only 5% of slips lead to clicks

» Compressed - a slip will cause an audible click

» A slip can wipe out several lines

» More slips can freeze frames for several seconds

» Encrypted/compressed data protocol

Slip rate due to frequency deviation

» 10 -8 = 6.9 slips per day

» 10 -7 = 2.9 slips per hour

» 10 -6 = 28.8 slips per hour

»Slip rate = fractional freq dev / frame

» 10 -8 = 6.9 slips per day

» 10 -7 = 2.9 slips per hour

» 10 -6 = 28.8 slips per hour

A customer affected by slips:

SDH/SONET Transport Networks:

Nominally Synchronous Multiplexing

SDH/SONET Transport Networks:

Nominally Synchronous Multiplexing

Wander induced by pointer activity

SDH/SONET Transport Networks:

Nominally Synchronous Multiplexing

SDH/SONET Transport Networks:

Nominally Synchronous Multiplexing

with the SDH aggregates (pointer

technique)

incoming PDH tributary and the SDH

aggregates induce wander on the outgoing

PDH tributary (pointer adjustments!)

slips

»PDH tributaries need not be synchronous

with the SDH aggregates (pointer

technique)

»However, relative wander between the

incoming PDH tributary and the SDH

aggregates induce wander on the outgoing

PDH tributary (pointer adjustments!)

»If excessive, this tributary wander causes

slips

Cellular Mobile Telecom Networks

BTS

BTS

BTS

BTS

Successful handover requires synchronisation between

base transceiver stations (BTS)

Cellular Mobile Telecom Networks

Radio carrier frequencies must be synchronized precisely

in order to prevent cross-talk

Radio spectrum

Frequency

Characterizing Synchronisation Quality

Characterizing Synchronisation Quality

Definition of jitter : ITU-T Rec G.810

Definition of jitter : ITU-T Rec G.810

instants of a digital signal from their

reference positions in time

ÂThe short term variations of the significant

instants of a digital signal from their

reference positions in time

ÂGreater than 10Hz in modulation frequency

Ideal

Jittered

Definition of jitter : ITU-T Rec G.810

Definition of jitter : ITU-T Rec G.810

instants of a digital signal from their

reference positions in time

ÂThe short term variations of the significant

instants of a digital signal from their

reference positions in time

ÂGreater than 10Hz in modulation frequency

Ideal

Jittered

Definition of wander : ITU-T Rec G.810

Definition of wander : ITU-T Rec G.810

instants of a digital signal from their reference positions in time

·The long term variations of the significant

instants of a digital signal from their reference positions in time

·Less than 10Hz in modulation frequency

Ideal

Wandered

Tref(t) T(t)

t

x(t) = jitter + wander

Time Error or Phase-Time x(t)

Fractional Frequency Deviation y(t)

y(t) =

where

y(t) =

where

ν(t) = actual frequency of the signal

νNOM = specified nominal frequency

ν(t) - νNOM

νNOM

Synchronisation Distribution:

General Principles

Synchronisation Distribution:

General Principles

Synchronisation between telecommunication systems : 1

Synchronisation between telecommunication systems : 1

»Master-slave intra-network synchronisation

Master System

System

clock

data data + clock

transmission

link

Synchronisation between telecommunication systems : 2

Synchronisation between telecommunication systems : 2

own clock to the incoming signal

» The incoming signal contains both the clock

and data information

systems have the same transmit and

receive rates

» There are no slips

»The slave system continually adjusts its

own clock to the incoming signal

» The incoming signal contains both the clock

and data information

»Therefore both the master and slave

systems have the same transmit and

receive rates

» There are no slips

Synchronisation between telecommunication systems : 3

Synchronisation between telecommunication systems : 3

»Inter-network synchronisation

Network A System

System

atomic clock

data data

atomic clock transmission

link

Synchronisation between telecommunication systems : 4

Synchronisation between telecommunication systems : 4

atomic clocks

free running frequency

» There is little difference between the transmit

and receive rates at both ends

» Totally acceptable for inter-national and

»Each system is synchronised by separate

atomic clocks

»The atomic clocks have nearly the same

free running frequency

» There is little difference between the transmit

and receive rates at both ends

»The slip rate is only one in every 72 days

» Totally acceptable for inter-national and

Physical synchronisation network : 1

Physical synchronisation network : 1

master-slave chains

PRC

SSU

Synchronisation Supply Unit

SDH

Equipment Clock

PRC = Primary

SEC

Physical synchronisation

network : 2

Physical synchronisation

network : 2

direct connection to the master network

clock

are synchronised in chains or trees

» Each system clock is the master clock of the

subordinate system clocks slaved to it

» The chains can be very long or very short

»Not every system in the network can have a

direct connection to the master network

clock

»Therefore the telecommunication systems

are synchronised in chains or trees

» Each system clock is the master clock of the

subordinate system clocks slaved to it

» The chains can be very long or very short

Physical synchronisation network : 3

Physical synchronisation network : 3

to the SSU

SEC

Synchronisation Distribution (SD)

Trails

Synchronisation Distribution (SD)

Trails

SAME as the head-end, ie PRC, SSU or SEC

synchronisation network

»The clock frequency along a SD trail is the

SAME as the head-end, ie PRC, SSU or SEC

»SD trails can be very long or very short

»There can be hundreds of SD trails in a

synchronisation network

Causes of jitter

circuits in transmission network elements

equipment clocks

»Phase-noise generated by clock recovery

circuits in transmission network elements

»Phase-noise generated by low-quality

equipment clocks

Causes of wander

phase-noise in network element clocks

and night) modify the propagation delays in

transmission cables

½Temperature variations induce low-frequency

phase-noise in network element clocks

½Temperature variations (e.g between day

and night) modify the propagation delays in

transmission cables

Jitter and wander control

predefined limits called Network Limits

following jitter and wander components:

Á Jitter and wander in the spectral domain above ≈

1 mHz

Á Wander in the spectral domain below ≈ 1 mHz

ÁJitter and wander must be kept below

predefined limits called Network Limits

ÁTwo distinct techniques are used for the

following jitter and wander components:

Á Jitter and wander in the spectral domain above ≈

1 mHz

Á Wander in the spectral domain below ≈ 1 mHz

Jitter and wander filtering

of jitter and wander in the spectral domain

clocks (SSUs) at intervals on the SD trails

ÁRequired to prevent excessive accumulation

of jitter and wander in the spectral domain

above ≈ 1 mHz

ÁUse very narrow bandwidth (≈ 1 mHz) slave

clocks (SSUs) at intervals on the SD trails

PLL transfer function (TF)

» TF = A plot of : Amplitude of output jitter (frequency)

Amplitude of input jitter (frequency)

» TF = A plot of : Amplitude of output jitter (frequency)

Amplitude of input jitter (frequency)

Wander buffering on input ports

bandwidth of the SSUs cannot be

attenuated

across the synchronisation network

»Wander in the spectral domain below the

bandwidth of the SSUs cannot be

attenuated

ÁThis low frequency wander accumulates

across the synchronisation network

ÁBuffer stores on traffic input ports must be

able to absorb at least 18 µs of wander

Input wander greater than 18 µ s

Input wander greater than 18 µ s

the buffer, then the buffer over- or underflows,

thus causing slips

»The size of the buffer in telecommunications

systems is usually just slightly larger than 18µs

»If the input wander is greater than the size of

the buffer, then the buffer over- or underflows,

thus causing slips

»To prevent slips the level of wander in the

network must be kept below 18 µs

Synchronisation Distribution:

SDH-Based Solution

Synchronisation Distribution:

SDH-Based Solution

1 Elements

Clocks and Links

(SSU)

» If it is an independent piece of equipment, then

it is called a SASE (Stand-Alone Synchronisation Equipment)

»Primary Reference Clock (PRC)

»Node Clock or Synchronisation Supply Unit

(SSU)

» If it is an independent piece of equipment, then

it is called a SASE (Stand-Alone

Synchronisation Equipment)

Primary Reference Clock (PRC)

network with a frequency accuracy of

< 1 x 10-11

»Master clock used to synchronise the entire

network with a frequency accuracy of

< 1 x 10-11

»Based on atomic Cesium clocks

PRC Implementation

atomic Cesium clocks

remote atomic Cesium clocks (e.g Global

Positioning System - GPS)

»Autonomous equipment with one or several

atomic Cesium clocks

»Radio-controlled clock synchronized to

remote atomic Cesium clocks (e.g Global

Positioning System - GPS)

»A combination of the above

Node Clock or Synchronisation

Supply Unit (SSU)

Node Clock or Synchronisation

Supply Unit (SSU)

Reference Selector

Output Interface

Output Interface

Output Interface

Jitter/Wander Low-Pass Filter

Holdover Memory

Node Clock or Synchronisation

Supply Unit (SSU)

Node Clock or Synchronisation

Supply Unit (SSU)

» priority table, or

» SSM signaling and priority table.

the input via narrow-band (mHz) low-pass

filtering

the last phase & frequency as good as it

can (holdover mode)

»Selects an input reference signal based on

» priority table, or

» SSM signaling and priority table.

»Attenuates the jitter and wander present at

the input via narrow-band (mHz) low-pass

filtering

»If all reference signals are lost, maintains

the last phase & frequency as good as it

can (holdover mode)

• SSU Type

• Application

Frequency accuracy

Holdover frequency

departure BandwithType I

2048 kbit/s based N/A 5E-10 + t x 2E-10 / day 3 mHz

Type II

1544 kbit/s based 1.6E-8 1E-10 + t x 1E-10 / day 1 mHz

Type III

1544 kbit/s based 4.6E-6 1E-9 + t x 1E-9 / day 1 mHz

Node Clock or Synchronisation

Supply Unit (SSU)

Node Clock or Synchronisation

Supply Unit (SSU)

SDH network element ’s synchronisation function SDH network element ’s

synchronisation function

STM-N

timing output (2 MHz or 1.5 Mbit/s, or

Selector B

NE internal timing

Synchronous Equipment Timing Source (SETS)

SDH SEC features

» STM-N aggregates and tributaries

» 2 Mbit/s tributaries

» 2 MHz, 2 or 1.5 Mbit/s (non traffic) timing inputs

» a priority table, that is user definable

» Synchronisation Status Message (SSM) on the

STM-N and 2 Mbit/s interfaces

» All STM-N aggregates and tributaries

»

ÀInput synchronisation signals are :

» STM-N aggregates and tributaries

» 2 Mbit/s tributaries

» 2 MHz, 2 or 1.5 Mbit/s (non traffic) timing inputs

ÀInput selection is determined by :

» a priority table, that is user definable

» Synchronisation Status Message (SSM) on the

STM-N and 2 Mbit/s interfaces

ÀOutput synchronisation signals are :

» All STM-N aggregates and tributaries

»