Sync Theory - Concepts Tdm to Ngn Networks Part 1

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Confidential © Copyright 2012

Technical Presentation

2012

Basic Concepts

Terms and concepts

2Confidential © Copyright 2012

The objective of Synchronization

City

Town

City

Town

…to enable service providers to transport bits of

information within and across network and

national boundaries without losing any bits of

information.

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Synchronization Is Required In All Networks

All types of Network, TDM and NGN, need two synchronization

services:

• Distribution of Frequency

–Sending a regular clock signal across the network.

• Distribution of Time/Phase

–Sending Timestamps containing UTC traceable and relative Time-of-

Day information to NE

–Sending signals that will allow the oscillators to lock to a specific

phace


Phase Relationships

When comparing two signals there are three possible phase

relationships that may occur:

In Phase

Changing Phase

Phase offset

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Phase Accumulation

� Accumulating phase continuously in one direction is an indication of a frequency offset.

� The rate of phase change determines the magnitude of the frequency offset.

This is a frequency accuracy problem.


Phase Accumulation

Phase Accumulation

�This is a frequency stability problem.

�Accumulating phase in one direction then

reversing direction is an indication of a stability

problem.�The two signals may be of the same

frequency.

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Frequency Accuracy

1 ns

100 second observation period

FREQUENCY ACCURACY

is a long-term measurement based on the

AVERAGEphase accumulation over time.

Frequency offset = rTime/Time, (rT/T)

f offset = 1 ns/100 second = 1 x 10-11

10 ns/100 second = 1 x 10-10

100 ns/100 second = 1 x 10-9

1 us/100 second = 1 x 10-8


Frequency Stability - Jitter

How does jitter affect the network?

• High speed jitter may lead to bit-errors due to the inability of

digital equipment to sample the incoming bit-stream correctly.

• Jitter may lead to overflows or underflows of synchronizer and

de-synchronizer buffers

What is jitter?

Short-term variation of the significant instants of a digital signal

from their ideal positions in time.

Phase oscillations > 10 Hz

Measured in amplitude (UI) and frequency (Hz).What causes jitter?

Tuned-circuits in Repeaters used to recover

timing.

Removal of stuffing bits in the de-

multiplexing process.

The Solution!!!Filtering Clocks

like a

SSU

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Frequency Stability - Wander

What is wander?

• Long-term variation of the significant instants of a

digital signal from their ideal positions in time.

• Phase oscillations < 10 Hz

• Measured in amplitude (UI) and frequency (Hz).

How does wander affect the network?

DS1/E1 slip performance and jitter caused by pointer

adjustments for PDH signals carried on SONET/SDH.

Wander on an input reference signal can affect a

clocks holdover performance if it loses its reference

What causes wander?

Synchronized clocks – “clock breathing”

Temperature variation on transmission

media

SONET/SDH transported DS1/E1s

The Solution!!!Filtering Clocks

like a

SSU


Frequency Stability – Phase Transients

What is a phase transient?

• A phase transient is a sudden large excursion in

phase (with respect to surrounding phase

variations) of limited duration.

How do phase transients affect the

network?

• Clock alarms

• Holdover

• Data errors

What causes phase transients?

• Rearrangement activities in clocks

• Pointer adjustments for payload signals carried on

SONET/SDH

The Solution!!!

An intelligent SSU with input qualification, performance

monitoring, and phase build-out software algorithms.

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Oscillator Circuits

…but, because network

wander occurs so slowly, these same

clock circuits are not able to detect

wander. In fact, the circuit design

amplifies and passes wander to

downstream facilities.

Network Element

Most network elements are designed to

follow a common clock. In most cases

these clock circuits have the sophistication

to detect and filter jitter…

Network Element

Oscillators can be free running, acquiring

a reference, locked, or in holdover


MTIE – Maximum Time Interval to Error

• MTIE is a measurement of the largest phase movement in a defined

window of time

• MTIE is used to bound peak-to-peak phase movements and frequency

offsets at network interfaces

• MTIE measures phase movement and detects phase transients

• A true MTIE will have a defined start/stop time

• MTIE is usually measured in nanoseconds

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Locked

ST 1

Holdover

Signal lost

Precision as good as

element quality

Free Running

Free-running

Element Stratum Precision

Hydrogen

Maser- 1 E-15

Cesium ST 1 2E-12

Rubidium ST 2E 5E-11

Quartz ST 3 4.6E-6

A free running oscillator is the one that

has never been locked to a PRC or

reference and the precision at its output is

given by its element quality


Locked

ST 1Precision is almost

ST1 while following the

reference

Holdover

Free Running

Locked

An oscillator is locked when

it is following a higher quality

reference

Signal lost

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Locked

ST 1

Holdover

Lost of Reference

The precision will start

to degrade. The

degradation will depend

on the quality of the

element

Free Running

Holdover

When the locked oscillator

loses reference, it goes to a

state known as Holdover


Holdover

• Another feature of the sync trail which is designed to add to robustness is the

holdover mode of TSG/SSUs and NE internal clocks.

• Holdover mode is entered by a slave clock whenever there are no acceptable

designated synchronization inputs.

• It is a strategy for preventing sudden jumps in frequency and phase when the

slave clock becomes isolated from the PRS

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PRC – PRS – G.811 – Stratum I

CESIUM

• Self contained

• No antenna required

• No time-of-day

GPS

• Satellite dependence

• Antenna required

• Time-of-day available


Slave or Node Clocks (SSU/SASE and SEC)

BITS

SASE

SSU

or

or

SSU2000

TimeProvider

•SSU – Synchronisation Supply Unit

•Defined by ITU-T G.812

•Normally highest quality clock in a node

•Has inputs, follows network clock or its own PRS

•Performance monitoring

•Contains clocks for holdover and sync outputs

•Hitless switching

•Intelligent algorithms, e.g. BesTime, SmartClock, filter jitter, attenuate wander

•SSM handling

•Management capabilities, remote and local

•SASE – Stand Alone Sync Equipment

•Same as SSU but stand-alone, defined by ETSI

•BITS – Building Integrated Timing Supply

•US equivalent of SASE

•SEC - SDH Equipment Clock

•The clock within an SDH NE is also considered as a slave clock. SEC tends to be a low cost and less

performing device when compared to an SSU.

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Traceability

• Traceability is one of the most important elements of network

synchronization.

• The traceable path from a PRS to a point where the clock signal is applied to

a network element (NE) is called a timing trail or sync trail

• The performance of the synchronous services supported by the network

depends to a large extent on the availability of PRS traceability


Achieving the Sync Integrity by….

• …distributing a highly accurate reference frequency source throughout the

Network, and to all network elements, by elevating the internal oscillators of

the various network elements to that reference.

Stratum

2

Stratum

2

Stratum

2

Stratum

2

Stratum

1 Stratum

3

Stratum

3Stratum

3Stratum

3E

Stratum

1

Stratum

3E

Stratum

3E

Stratum

1

Stratum

3E

Stratum

3EStratum

3E

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Sync Redundancy

• Redundancy is also an important aspect in maintaining robust sync trails.

• Equipment dedicated to sync distribution, and the designated physical

synchronization transport channels are duplicated to form active and hot-

standby pairs.

• TSGs and SSUs are also required to have internal redundancy for all functions

directly involved with timing signal regeneration and distribution.

• Synchronous NEs are usually required to have redundant internal clocks.


Asynchronous

Network ElementLow Speed Data

Network Element

Free-running

Clock Source

Free-running

Clock Source

Independent

Clocks

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Asynchronous Network Applications

– Asynchronous DS3/E3 transmission using free running clocks in the M13MUX.

– To accommodate for the frequency offsets between the signals being multiplexed the M13MUX

adds or deletes bits (called bit stuffing) on the DS1/E1 signals being multiplexed and de-

multiplexed.

M13MUX

~

M13MUX

~

Frame Length

125 usec

DS3 / E3

DS1/E1

DS1/E1


Plesiochronous

Network Element

Medium Speed Data

Network Element

Stratum 1G.811

Clock Source

Stratum 1G.811

Clock Source

Nearly the same

Clock

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Plesiochronous Network Applications

Confidential25

Office / Node #1 Office / Node #2

DS1 / E1 Trunks

External Clock In

Public switch

PRS

External Clock In

Public switch

PRS


Synchronous

Network Element Network Element

Stratum 1Clock Source

Same Clock

Clock

High Speed Data

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Synchronous Network Applications

Data & Clock

Data

Synchronous operation means all

nodes share a common clock.

ADM

ADM

ADM

ADM

SSU

ADM

SSU

PRS

ADM

Timing is not derived from

the optics at the master

node. This avoids a timing

loop.


Broadband ServicesBroadband ServicesSDH TransportDigital Network

1980 1990 2000 2010

synchronization Profile

• New requirements:

- Protection 1+1- Time of Day NTP/IEEE5888/UTI

- Remote management

• NGN transport technologies create even more timing islands

• Network monitoring, management, security


• Distributed Primary reference sources (PRS)

• ST2/G.812• filtering/holdover oscillators

• Remote management

• Distribution impaired by SONET payload pointers/created timing

islands


• Central Primary Reference Clock (PRC)

• Distribution over copper E1 trunks

• Local Node Clock phase following oscillators

• Non-redundant

• Protection 1:N

Application

Signaling

Transport

Transmission

Access Link

Voice

R2(PSTN)

TDMSwitching

SONET/SDH

2W loop

Voice+Data

SS7/IN(ISDN/B-ISDN)

TDM/ATMswitching

SONET/SDH

2W loop/dial-up

FLC

Voice+Data+Multimedia

SIGTRAN/MEGACO(ISDN/B-ISDN)

IP/ATMRouting

WDM/OXC/GbE

2W loop/xDSL

Cable/Ethernet/PON/FLC

Voice+Data+Multimedia

All IP Signalling(MEGACO/SIP/H.323

IP/MPLS(Routing,LDP)

WDM/OXC/NgSDH

All kinds of access types

Network Convergence

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Standards

ITU

G.803 Architecture of Transport Networks based on the SDH hierarchy

G.810 Definitions & Terminology for Synchronisation Networks

G.811 Timing Characteristics of Primary Reference Clocks (PRC)

G.812 Requirements of Slave Clocks suitable for use as Node Clocks in Synchronisation Networks; Clock Types I, II, III, IV, V and

VI

G.822 Controlled Slip rate objectives on an International Digital Connection

G.823 Jitter and Wander in Networks based on 2Mbit hierarchy

G.825 Jitter and Wander in Networks based on the SDH hierarchy

G.8251 The Control of Jitter and Wander within the Optical Transport Network

G.8261 Timing and Synchronisation Aspects in Packet Networks (G.pactiming)

G.8262 Timing characteristics of synchronous ethernet equipment slave clock (EEC)

G.8263 Timing Characteristics Of Packet Based Equipment Clocks (PEC) And Packet Based Service Clocks (PSC) (G.paclock-bis)

G.8264 Distribution of Timing Through Packet Networks

G.703 Physical interface characteristics of hierarchical and clock signals

G.704 Frame structures for the different hierarchical levels, e.g. 2Mbit/s.

G.709 Network node interface for the Optical Transport Network (OTN)

G.783 Characteristics of SDH equipment functional blocks

DEFINITIONS

ETSI

EN 300 462-1-1 Definitions & Terminology

EN 300 462-2-1 Synchronisation Network Architecture

EN 300 462-3-1 Control of jitter and wander in sync networks

EN 300 462-4-1 Timing characteristics of SASE type slave clocks for use within SDH & PDH networks

EN 300 462-5-1 Timing characteristics of SEC type slave clocks for use within SDH equipment

EN 300 462-6-1 Timing characteristics of Primary Reference Clocks

EN 300 462-7-1 Timing characteristics of slave clocks suitable for local node applications

EN 300 462-3: Network Limits

EG 201 793: Transmission and Multiplexing (TM) - Synchronization network engineering

ETS 417-6-1, ETS 300 147: SSM


Summary of Clock Specifications

CLOCK TYPES

ITU-T PRC

G.811

Type I

G.812

SASE for

SDH

Type II

G.812

Type III

G.812

Type IV

G.812Not defined

Type V

G.812

(Old Transit

Node)

Type VI

G.812

(Old Local

Node)

G.813

(SEC option

A)

ETSIEN 300 462-

6

EN 300 462-

4Not defined Not defined Not defined Not defined Not defined Not defined

EN 300 462-

5-1

North America

Stratum LevelStratum 1 Not defined Stratum 2 Stratum 3E Stratum 3 Stratum 4 Not defined Not defined

Accuracy 1x10-11 Not

applicable±1.6x10-8 4.6x10-6

1 year4.6x10-6 ±3.2x10-5 1x10-7 Not defined 4.6x10-6

Holdover

Stability

Not

applicable

2.7 x10-

9/day

(1)

1x10-10/day

(1)

1x10-8/day

(1)

3.7x10-7/day

(1)

Not

applicable1x10-9/day 2x10-8/day 2x10-6/day

Pull-in/ Hold-in

range

Not

applicable1x10-8 1.6x10-8 4.6x10-6 4.6x10-6 3.2x10-5 4.6x10-6

Wander

Filtering

Not

applicable0.003Hz 0.001Hz 0.001Hz

3Hz

0.1Hz

(SONET)

No 1 – 10Hz

Phase

Transient

(Re-

arrangement)

Not

applicableMTIE < 1ms

MTIE <

150ns

MTIE < 150ns

Phase slope

885ns/s

MTIE < 1ms

Phase slope

61ms/s

Objective:

MTIE < 150ns

Phase slope

885ns/s

No

Requiremen

t

MTIE < 1ms

(1) Includes: (a) Initial frequency offsets; (b) Linear aging frequency drift rate, and (c) Temperature component, except that the temperature component is not applicable (NA) for the Type V and Type VI clocks

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Frequency Standards

Technology

Hydrogen Masers

Cesium Standards

GPS Receivers

Loran Receivers

CDMA Receivers

Rubidium Oscillators

Quartz Crystal Oscillators

Tuned Circuits

Stratum Level

STRATUM 1

STRATUM 2E

STRATUM 2

STRATUM 3E

STRATUM 3

STRATUM 4

1 X 10-15

1 X 10-5


Timing Requirements

Classical uses of NTP

NTP

Temporization

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Example of Network & Data Center

Equipment Requiring NTP

Base station timing, Billing, Location

Services

Wireless Basestations

CDR generationSS7

QoS Measurement Data

Event logs

CDRs

IP Traffic Monitoring Systems

VoIP Probes

IPTV Measurement Systems

Measurement and

Monitoring

Probes/Equipment

Measurements

Policy/QoS

Customer Prem Routers/switches, VoIP GatewayCustomer Premises

Measurements

Policy/QoS

IPTV Residential Gateway

IPTV STB/DVR

Billing

Radius/TACACS, AAA, Kerberos, SNMP

Media servers

VoIP Switches/gateways

Transmission Equipment – PON, DWDM, ROADM

Platforms

Routers/Switches/Access Gateways

Elements / Applications Requiring NTP

CDR generation

EMS, event logging, alarms etc

Call Logging, CDR Generation

EMS, event logging, alarms etcNetwork Elements

Initialization,Measurements, DRM

Access, Security, Accounting, CDR

generation

Call Logging, CDR Generation

Operational / Service Requirement

Databases/servers

Equipment Category


Circuit to Packet/Interworking Functions Need

Synchronization & Time Services

C2P, and I/F such as media

and signaling gateways, use

NTP for time stamping, and

event logging

• NGN Gateways require good synchronization, and time of day timestamping to ensure goodperformance, improve troubleshooting/diagnostics, and produce accurate IP/CDR.

• Time Synchronization is particularly critical when IP/Call Detail Record information is shared

between carriers.

• Billing discrepancies require time-consuming mediation and dispute settlement processes.

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Calls are Timed and Logged To Produce IP/Call

Detail Records

• Back office OSS/BSS logging, subscriber management, and database systems use NTP to create

IP/Call Detail Records.

• Aggregated IP/CDR include call initiation / termination timestamps, call duration, rating

information, a unique IP or Call Detail Record ID, and much more.


Logging & Billing Systems Are The Heartbeat of Carrier

Revenues

• IP/Call detail records are used to drive billing processes.

• NTP timestamps are therefore the source of an accurate invoice and eventually of revenue

statements.

• Minor errors in timestamping at the source can quickly cause expensive problems that are difficult to

diagnose.

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Carrier Class NTP Mitigates IP/CDR Reconciliation

Issues

• Remove variation in timestamping by using a Stratum 1 reference clock to generate NTP.

• Ensure all billing and logging servers and associated databases have the same deterministic time

reference to allow IP/CDR alignment.


FROM TDM NETWORKS TO PACKET NETWORKS

SYNCHRONIZATION FOR NEXT GENERATION

NETWORKS

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Synchronization Enables Your Network !

� Makes sellable services work

� Makes wireless mobility a reality

� Supports network migration to cost saving NGN solutions

� Enables throughput and performance technologies

… What you want

Inadequate synchronization manifests as:

� Repeated dropped calls (fixed & wireless)

� Switch resetting

� Poor bandwidth utilization

… and what to avoid

What does Sync do for you?


To deliver and recover the original signal :

� All the bits sent must be received…

� at the same frequency as the sampling frequency (source)…

� wireless transceivers must operate within narrow frequency bands… and

� the incoming bit rate to switches must match the outgoing rate

Digital Network

(SDH/SONET)

00 0 01 1 1 1 1 00 0 01 1 1 1 1 00 01 1 1 1 111 1 10 0 0 0 0 11 1 10 0 0 0 0 11 10 0 0 0 0

Why is Sync required?

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Where is Sync Required?

All originating/terminating service interfaces must be at the same frequency

� ITU-T G.823/5 define the minimum

requirements for synchronization

� Synchronization need is independent of

the transport

� The same frequency must be maintained

throughout the network

� TDM Circuit Emulation/PWE requires

synchronous interfaces (ITU-T G.8261),

and

� Synchronous Ethernet requires

frequency sources & filtering


How Is Synchronization Delivered?

�Primary Reference Clock(s) provide the frequency reference

� The TDM network transports the clock between offices

�All TDM elements have the same frequency

� The clock quality degrades as it passes through the network, and an

SSU removes the jitter & wander

� Frequency dependant services derive timing (service clock) from the

transport clock

Primary Reference Clock

PRC Referenced

Timing Out

ADM ADM

Sync Flow

To NE’s in

the local Office

Sync Flow Sync Flow

BITS/SSU

ADM

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Drivers For Change

IP addresses the transport

economics, but does not distribute

synchronization:

�Circuit interfaces are

synchronous (require frequency

reference)

�Mobile base station’s need

synchronization for mobility &

spectral efficiency

� Synchronization (and QoS) must be

engineered into the system

How do we provide synchronization

to support the seamless migration?

Sync

Sync

Timing Island


Sync Delivery Strategies

Synchronization Strategies

E1/SDH Hybrid

Shorter term strategy based on use of legacy systems (higher

OPEX). Bandwidth & 4G/LTE limit long term suitability

Adaptive Clock Recovery

A vendor specific book-end solution used to support TDMoIP

services. ACR methods are being superseded by IEEE 1588

GPS Radio at Base Stations

Good performance, supporting wide range of applications. Cost

and autonomy define deployment adoption

Synchronous Ethernet

An point-to-point solution that depends on an uninterrupted SyncE

switch path

IEEE 1588-2008

A versatile standards based solution with flexibility, low cost, and

high rate of adoption by NEM vendors

E1/SDH

ACR

SyncE

1588-v2

1PPS

2048kHz

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SyncE



• Schema that transports frequency at the Ethernet physical layer

• Higher layers including IP are asynchronous

• Point to Point scheme similar to SONET/SDH

• Remote timing traceable to upstream PRC

• Quality of frequency is deterministic

• ITU-T G.8261, G.8262 and G.8264 define Synchronous Ethernet

Frequency Transported by SYNC-E PHY

SyncE Switch

Ethernet Frames

T1/E1

Service

CENTRAL OFFICE

PRCSyncE

Switch

SyncE

Switch

SyncE

Switch

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How is SyncE different from normal Ethernet?

Existing Ethernet PHY (Physical Layer)

• Rx uses the incoming line time

• Tx uses the built-in 100 ppm clock

• No relationship between the Rx & Tx

SyncE PHY (Physical Layer)

• Rx disciplines the internal oscillator (4.6ppm)

• Tx uses traceable clock, creating point-point scheme

• 125 MHz (GigE) coded into transmission

SyncE Switches support:

• External Sync inputs

• Line timing mode

• Reference outputs (derived from Ethernet time)

4.6 ppm

TXTX

RXRX

SyncE Switch

100 ppm

TXTX

RXRX

Asynchronous Switch

TX

RX

E1/T1

Ext.Sync

SyncE Switch

TX

TX

E1/T1

Sync Ref.

Line

Timing



SyncE and asynchronous switches cannot be mixed …

• Data is transported but frequency traceability is interrupted

Frequency distribution based on:

• PRC as source, and SSU to filter accumulated jitter & wander

TX

RX

TXTX

RXRX

E1/T1

Ext.Sync

Inaccurate

100 ppm

TX

RX

Accurate

SyncE Switch Asynchronous

Switch

100 ppm referenced

frequency source

PRC

Sync Flow Sync Flow

Sync Flow

SSU

SyncE

Switch

SyncE

SwitchE1

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20

201


Synchronization Guide:

• Follows SDH timing guide defined in ITU-T

G.803

• N is the upper limit of a guide

• Practically N is between 5-10

for SDH. The same can be

expected for SyncE

EEC (Synchronous Ethernet

equipment slave clock)

K=2

K=3

Co-located PRC & SSU

K=1

1

20

1

1

20

1

20

1

20

PRC

PRC

PRC

SSU

SSU

SSU

SSU

Rule 1

K < 10 SSUs

N < 60 EECs

Rule 2

N < 20 EECs

SWITCH

ADM

Sync Theory - Concepts Tdm to Ngn Networks Part 1

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