Prepared by: Md. Jahangir Alam

Synchronization

The Objective of Network Synchronization

If the node clocks in a telecommunication network operate asynchronously then the transmit and receive rates of telecommunication systems in each node would be different to the other nodes. In this case causing data a errors commonly referred to as “slips”.

Slips per day = frequency difference x traffic frames/second x seconds/day (86400)

The object of network synchronization is therefore to avoid and to minimize slips. This can only be achieved by synchronizing all the node clocks, and hence all the telecommunication systems, to the nearly master clock.

Synchronization is a corner stone technology• Absolutely essential for continuous and high performance operation of the networkIt facilitates effective communication• Between different telecom system• Between different telecom operators• At interconnects between Networks

It enables mobile networks to work• Successful handover between Base Stations• Prevents cross-talks in mobile networks by synchronizing carrier frequency precisely• Provide QOS

SDH Clock Types

3 Types of clock :

G.811 Primary Reference Clock (PRC)

G.812 Synchronization Supply Unit (SSU)

G.813 Synchronous Equipment Timing Source (SETS)

Slips

Impact of Slips on QoS

Oscillator - Free-runningOscillator - LockedOscillator - Holdover

Jitter - defined by ITU-T G.810Jitter free clock (ideal)

jittered clock

phase-deviation

time

Wander - defined by ITU-T G.810

Primary Reference Clock (PRC).PRC is the network master clock building the basis for the long-term freq accuracy and stability in the network.

Two types of PRC have evolved:• Autonomous PRCs with cesium tubes incorporated in the PRC• Radio-controlled PRCs which use remote cesium tubes, e.g. in the satellites of the GPS navigation system.

Output interfaces:• 2,048MHz (ITU-T Rec. G.703)• 2,048Mbit/s (ITU-T Rec. G.703)

Basic properties:Free-run Accuracy: 1x10-11

Normative References:ITU-T Rec. G.811:

SSUs are the second level in the quality hierarchy of synchronization equipment. There are two types of SSUs, so called Local (lower quality) and Transit (higher Quality) clock. Important tasks of SSUs are:

• Selection of a reference signal coming from the PRC.• Supervision of the selected (active) reference signal.• Holdover the output when all reference input have failed.

Input interfaces:• 2048kHz (ITU-T G.703)• 2048kbit/s (ITU-T G.703) Output interfaces:• 2048kHz (ITU-T G.703)• 2048kbit/s (ITU-T G.703)

Basic properties:• Free run Accuracy: 1.6 x 10-8~ 4.6 x 10-6 /year• Holdover per day: 1.0 x 10-10 ~ 3.7 x 10-7 per day (for ageing)• Pull in Range: 0.016 ppm to 4.6 ppm.• Wander Filtering: 0.001 Hz to 0.003 Hz.

Normative References: ITU-T Rec. G.812:

Synchronization Supply Unit (SSU).

The SETS is not directly connected to input or output signals of the network element, but via the traffic interfaces or specific synchronisation interfaces.

Reference input signals:• T1, derived from SDH, STM-N [G.707] • T2, derived from PDH, 2048kbit/s [G.703/704]• T3, derived from 2048kHz [G.703] or derived from 2048kbit/s [G.703/704] Output signals:• T4, External reference signal 2048kHz [G.703] or 2048kbit/s [G.703/704]

Basic properties:• Free-run Accuracy: 4.6 x 10-6 • Holdover per day: 2 x 10-6/Day• Pull in Range: 4.6 ppm.• Wander Filtering: 1-10 Hz

Normative References: ITU-T Rec. G.813:

Synchronous Equipment Timing Source (SETS).

SDH Equipment Clock (SEC)

Synchronization Status Messaging (SSM).

S1 byte, Synchronization Status Message.

In STM-N signals capacity has been allocated in the Section Overhead (S1-Byte) for the exchange of synchronization-related messages between neighbor network elements.

The Purposes of synchronization status messaging are:

• Enable the selection of the best reference signal.• Avoid timing loops.• Possibility to mark signals that are not suitable as synchronous reference

Synchronization Status Messaging (SSM).

SDH Synchronization quality level

S1Byte, bit5-8 G.8xx Description Accuracy Q-level

0000 Quality unknown

0010 G.811 QL-PRC(highest quality)

1x10E-11(highest quality)

Q1

0100 G.812T (Transit) QL-SSUT 1x10E-9per day

Q2

1000 G.812L (Local) QL-SSUL 2x10E-8per day

Q3

1011 SETS (G.81s,G.813)

QL-SEC 4.6x10E-6(lowest quality)

Q4

1111 do not use forsynchronization

QL-DNU Q6

all other codes QL-INV (invalid)

Q5

Basic Clock Distribution.

SSUNode Clock

G.812

SSUNode Clock

G.812

SSUNode Clock

G.812

SSUNode Clock

G.812

According to G.803, with the master-slave method is appropriate for synchronization networks. The signalof a highly accurate and stable master clock is passedto all NEs via chains of slave clock.

In order to meet the performance requirements forinternational digital links (G.822) the master clock mustcomply to the standards for Primary reference clock

In large networks long chains of slave clocks may occur.

Long Chain of slave clocks can causes:

• Degradation of the synchronization signal due to: -jitter and wander -Clock noise• Increasing probability of an interruption in the chain

PRCG.811

SSUNode Clock

G.812

SSUNode Clock

G.812

Master

Slave

Synchronization Chain

PRC SETS SETS SSU SETS SETS SSU SETS

NE: 1 NE: n NE: 1NE: n

SSU: 1 SSU: 2

Maximum numbers according to G.803

- The limits are 15 NEs between SSUs,

- Maximum of 10 SSUs in a synchronization chain.

- The total number of cascaded clocks (NEs and SSUs) should not exceed 60.

NE: 1

Distribution Methods.

For the distribution of the PRC signal to the network elements two methods are currently being used:

PRCPRC

“Traditional Master Slave Network” “Distributed Master”

Clock Loops.

GPS

1

PRC

1

PRC

1

PRC

1

PRC

1

PRC

1

PRC 1

PRC

2

Don’t use

2

Don’t use

2

PRC

1

PRC

2

PRC

2

Don’t use

2

Don’t use

2

Don’t use

2

Don’t use

3

PRC

1

SEC

1

SEC

1

SEC

1

SEC

1

SEC 1

SEC

2

SEC

2

SEC

1

SEC

2

PRC

2

PRC

2

PRC

1

Don’t Use

2

Don’t use

2

PRC

2

PRC

2

PRC

1

Don’t Use

1

Don’t Use

1

Don’t use

1

Don’t use

1

Don’t use

2

Don’t Use

Standard Synchronization Architecture

Satellite

PRC

PRC

PRC

PRC

PRC

PRC

DHKJT

DHLKB

SYLLB

CGPTL

BSCOS

BOBET

Method of Distribution

P1

P2

P3

TimeSource 3600

• GPS Antenna.• Rubidium local

oscillator.

The Goal: Get PRC/G.811 reference at all nodes.Result: All network element internal oscillators elevated to the same level as the reference.

Antenna and TS 3600 installed at Certain site

Detail view: Output

1/488 ns= 2.049 MHz

Key Sync Standards

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