sync theory - concepts tdm to ngn networks part 1
TRANSCRIPT
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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.
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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
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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
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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.
28Confidential © Copyright 2012
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
synchronization Profile
• Distributed Primary reference sources (PRS)
• ST2/G.812• filtering/holdover oscillators
• Remote management
• Distribution impaired by SONET payload pointers/created timing
islands
synchronization Profile
• 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
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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
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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
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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.
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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.
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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?
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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
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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
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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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Synchronous Ethernet
SyncE
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Synchronous Ethernet
• 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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Synchronous Ethernet
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
48Confidential © Copyright 2012
Synchronous Ethernet
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
Synchronous Ethernet
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