Frequency and Time Synchronization In Packet Based … · • Drawback : hardware upgrades All...
Transcript of Frequency and Time Synchronization In Packet Based … · • Drawback : hardware upgrades All...
Cisco Confidential 1 © 2010 Cisco and/or its affiliates. All rights reserved.
Frequency and Time Synchronization In Packet Based Networks Peter Gaspar, Consulting System Engineer
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• Synchronization Problem Statement
• Overview of the Standardization Works
• Frequency Transfer: techniques and deployment Synchronous Ethernet Adaptive Clock Recovery
• Time Synchronization Two-Way Transfer Time Protocols
• Overview of IEEE Std 1588-2008 for Telecom
• Summary
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Problem Statement What and Why Do We Care About?
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Why and How are Packet Switched Networks Involved? • Transition from TDM to Ethernet networks.
• Connect consumers requiring Frequency and/or Time (F&T) synchronization.
• PSN is built with network elements that May have to support F&T distribution May be consumers of F&T
Aggregation
Subscriber Access
MSE
TDM / ATM
Ethernet
WiMAX
OLT xPON
xDSL DSLAM
M-CMTS
DVB-T/H 3GPP/2
DOCSIS
Backbone
Hub & Spoke or Ring
P P Internet
PE PE
MSA
PE
Peer ISP
Mesh P
TDM / ATM
P P
Identity Address Mgmt
Portal Subscriber Database
Monitoring Policy Definition
Billing
Service Exchange
VoD Content Network
TV SIP
Mobile user
Femto-cell
Mobile TV
Enterprise
Residential SoHO
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• Single domain vs. multiple domains Internet is a multi-domain network.
Wholesale Ethernet virtual link
• Frequency and time could use different distribution methods.
• Operators may provide synchronization services to their customers.
Aggregation
Subscriber Access
MSE
TDM / ATM
Ethernet
WiMAX
OLT xPON
xDSL
DSLAM
M-CMTS
DVB-T/H 3GPP/2
DOCSIS
Backbone
Hub & Spoke or Ring
P P
Internet
PE PE
MSA
PE
Peer ISP
Mesh P
TDM / ATM
P P
VoD
Content Network TV SIP
Mobile user
Femto-cell
Mobile TV
Enterprise
Residential SoHO
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• Frequency TDM interoperability and Co-existence: Circuit Emulation, TDM, MSAN (MGW) Access: Wireless Base Stations, PON, DSL
• Time and Phase alignment Wireless Base Stations SLA and Performance Measurements
BS : Base Station PON : Passive Optical Network DSL : Digital Subscriber Line SLA : Service Level Agreement
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• Inter-CO/LAN (WAN)
• Intra-CO, LAN
• Intra-node, -platform
External Integrated Time and Frequency Server
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The Leading Requirements Application Frequency Phase Alignment
Time Synchronization
TDM support (e.g. CES, SDH transformation), Access
PRC-traceability, jitter & wander limitations ITU-T G.8261/G.823/G.824/G.825
Mobile Base Stations
GSM, WCDMA and LTE FDD
Frequency assignment (fractional frequency accuracy) shall be better than • ± 50ppb (macrocells)
• ± 100ppb (micro- & pico-cells) • ± 250ppb (femtocells)
N/A (except for MBMS and SFN)
UMTS TDD Phase alignment between base stations must be < ±2.5µs
TD-SCDMA Phase alignment between base stations must be < ±3µs
CDMA2K Time alignment error should be less than 3 µs and shall be less than 10 µs
LTE TDD Phase alignment between base stations from ±0.5µs to ±50µs (service degradation)
WiMAX Mobile Shall be better than ± 15 ppb Phase alignment between base stations must be < ±1µs
DVB-S/H/T2 SFN TBD Cell synchronization accuracy for SFN support must be < ± 3µs
MB SFN Service Phase/time alignment between base stations requirement can vary but in order of µs
One-way delay and jitter Performance Measurement
To improve precision << 1 ms for 10 to 100µs measurement accuracy need ± 1 µs to ± 10µs ToD accuracy
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• Cost
• Limited utilization Locations Regulatory & Politics
• Reliability Geography Vulnerability
https://www.gsw2008.net/files/Civ%20Vulnerabilities_GSW2008.pdf
746th Test Squadron
Use of GPS (and GNSS alternatives) raises some concerns:
GPS : Global Positioning System
GNSS : Global Navigation Satellite System
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• As Replacement or Backup
• Alternative Radio Navigation LORAN-C ELORAN
• Atomic Clock Cheap Scale Atomic Clock Molecular Clock
• Network Clock Main topic of this session!
LORAN : LOng Range Aid to Navigation
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Standardization Development Organizations Who’s doing what?
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• Frequency transfer Parallel (overlay) SDH/SONET network Radio Navigation (e.g., GPS, LORAN) PHY-layer mechanisms Packet-based solutions
• Time transfer (relative and absolute) Radio Navigation (e.g., GPS, LORAN…) Packet-based solutions
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SDO Techno Status Scope Market
ITU-T SG15 Q13
Synchronous Ethernet
G.8261(2008) G.8262(2007)+Amend.1
G.8264(2008) G.781 (2008)
PHY-layer frequency transfer
Service Provider (SP) Metro & Core
Ethernet
Packet-based timing
G.8261 (2006)
Multiple working items: profile, metrics,
modeling…
CES performance
Packet-based frequency, phase and time transfer
Service Provider (SP)
IEEE 1588 PTP
IEEE1588-2002 IEEE1588-2008
No “Telecom” profile
Precise time distribution
Enterprise: Time SP: Frequency,
phase and time ITU-T & IETF
802.1AS Based on PTP Ballot Precise time
distribution Residential
IETF NTP NTP
NTPv3 Standard NTPv4 (CY09)
Time distribution Internet
SP domain
TICTOC NTPv5 PTP Profile(s)
New WG (approved March 08)
Frequency and time transfer
Internet Specific SP areas
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IETF TICTOC
ITU-T Q13/15
IEEE1588-2008
(PTPv2) IEEE
802.1AS
IETF NTP
AVB Profile(s)
ATIS Telcordia
ProfiNet: IEC 61158 Type10 DeviceNet: IEC 62026-3
ControlNet: IEC 61158 Type2
IEC Profiles
Telecom Profile(s) On-going
IEEE 802.3 Timestamping
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Frequency Transfer Distribution of Frequency Reference
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• Physical layer options Ex: SONET/SDH, SDSL, GPON, Synchronous Ethernet Pros: “carrier-class”, well defined, guaranteed results Cons: node by node link bit timing, requires HW changes
• Packet-based options Ex: SAToP, CESoPSN, NTP, PTP (protocol of IEEE Std 1588) Pros: flexible, looks simple, some can do time as well Cons: the network and the network traffic, not so simple!
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• The task of network synchronization is to distribute the reference signal from the PRC to all network elements requiring synchronization.
• The method used for propagating the reference signal in the network is the master-slave method.
• Slave clock must be slaved to clock of higher (or equal) stability. hierarchical model
PRC : Primary Reference Clock
Source: ETSI EG 201 793 “Synchronization network engineering”
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• Synchronization equipments PRC (PRS) and SSU (BITS) do not belong to the Transport network.
• SEC (SDH/SONET Equipment Clock) belong to Transport network. They are embedded in Network Element : NE.
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• Synchronization information is transmitted through the network via synchronization network connections.
• Synchronization network connections are unidirectional and generally point-to-multipoint.
Stratum 1 level
Stratum 2 level
NE (Stratum level ≥ 3)
CO
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Core Network
Aggregation and Access Networks
PRC : Primary Reference Clock (≈ PRS) SSU : Synchronization Supply Unit (≈ BITS) SEC : SDH Equipment Clock
Source: ETSI EG 201 793 “Synchronization network engineering”
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Receiver for synchronization reference signal
Source: ETSI EG 201 793 “Synchronization network engineering”
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Figure 4-2. Recommended BITS Implementation with SONET Timing Distribution
NE’s External Timing Output
NE’s External Timing Input a.k.a. BITS IN
Source: Telcordia GR-436-CORE . Digital Network Synchronization Plan
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SSU/BITS
NE
PRC/PRS
SSU/BITS
NE NE NE NE NE
Intra-office
Intra-office
Intra-office
Inter-office Inter-office
BITS
NE
PRS
BITS
NE NE NE
PRS
Intra-office
Intra-office
NE NE
Inter-office
Intra-office
Inter-office
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• Some of these synchronized trail contain a communication channel, the Synchronization Status Message (SSM) transporting a quality identifier, the QL (quality level) value.
This is a 4-bit field in SDH/SONET frame overhead.
• Purpose: Traceability (and help in prevention of timing loops)
Stratum 1 level
Stratum 2 level
NE level
What clock quality do I get? Is that the best source I can use?
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SSM Allows Source Traceability
PRC synchronization network connection
SSU synchronization network connection SEC synchronization network connection
Representation of the PRC network connection
X
Fault Representation of the synchronization network connection in case of failure
Example of restoration of the synchronization
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• PHY-layer frequency transfer solution for IEEE802.3 links
• Well-known design rules and metrics Best fit for operators running SONET/SDH
• Fully specified at ITU-T Working Group 15 Question 13 For both 2.048 and 1.544 kbps hierarchies
• Expected to be fundamental to high quality time transfer
• Drawback : hardware upgrades All timing chain shall be SyncE capable.
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PLL
Synchronous Ethernet capable
Line Card
IEEE802.3 ± 100ppm
ITU-T G.8261 SyncE interface jitter & wander
ITU-T G.8262 (EEC): Synchronous Ethernet
Equipment Clock
ITU-T G.781: Clock Selection Process
Synchronous Ethernet capable
Line Card
Frequency distribution
traces
External timing interface inputs
External Equipment BITS/SSU)
External timing interface inputs
PRC-traceable signal from BITS/SSU
ITU-T G.8264 ESMC and SSM-QL
External timing interface outputs
Synchronous Ethernet capable Equipment
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• Ethernet Synchronization Messaging Channel Use OSSP from IEEE802.3ay (a revision to IEEE Std 802.3-2005)
• Key purpose: transmit SSM (QL) Outcome: Simple and efficient
But designed to support extensions
• Protocol model: Event-driven with TLVs
• Two message types Event message sent when QL value change Information message sent every second
• TLVs QL-TLV is currently the unique defined TLV. Other functions can be developed.
OSSP : Organization Specific Slow Protocol
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | Slow Protocols MAC Address | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | Slow Protocol MAC Addr (cont) | Source MAC Addr | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | Source MAC Address (continued) | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| |Slow Protocols Ethertype 0x8809| Subtype (10) | ITU-OUI Oct 1 | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | ITU-OUI Octets 2/3 (0x0019A7) | ITU Subtype (0x0001)* | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | Vers. |C| Reserved | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | Type: 0x01 | Length | Resvd | QL | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | Future TLV #n (extension TLV) | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | | | Padding or Reserved | | | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-| | FCS | |-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
* Allocated by TSB
IEEE 802.3 OSSP
ITU-T OUI Header
ESMC Header
QL-TLV
Future TLV Extension Payload
OSSP
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Assuring The Continuity at PHY Layer
• Extension or replacement of SDH/SONET synchronization chain
• Inherit from previous ITU-T (and Telcordia) recommendations
• Difference: frequency transfer path engineering will define the necessary upgrades.
Only the NE part of the engineered timing chain needs SyncE upgrades.
ITU-T G.8262 (EEC) Node
BITS/SSU
SONET/SDH PHY SyncE
BITS/SSU PRC/PRS BITS/SSU
PHY SyncE
ITU-T G.8262 (EEC) Node
ITU-T G.8262 (EEC) Node
ITU-T G.8262 (EEC) Node
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• Three key steps: Generation: from signal to packet Transfer: packet transmission over packet network(s) Recovery: from packet to signal
Reference Clock Recovered
Clock PSN
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• ITU-T Recommendation G.8261 (2008) Adaptive Clock Recovery Definition
“In this case the timing recovery process is based on the (inter-) arrival time of the packets (e.g., timestamps or CES packets). The information carried by the packets could be used to support this operation. Two-way or one-way protocols can be used.”
ACR Protocol / Method One-Way Two-Way Timestamp
CES (SAToP, CESoPSN) X
IETF NTP (X) X X
IEEE Std 1588-2008 PTP X X X
IETF RTP X X X
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Independent Timing Stream
TDM TDM
IWF IWF
Recovered TDM timing based on the adaptive clock recovery
ACR Packet Stream Reference
Clock
TDM PW bit stream
Clocking method a.k.a. “out-of-band” (here, used for CES clocking)
TDM TDM IWF &
PEC
IWF &
PEC
ACR Packet Stream
TDM PW bit stream
PEC
Reference Clock
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Source: Diagram from “Time Domain Representation of Oscillator Performance”,
Marc A. Weiss, Ph.D. NIST
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• Frequency Accuracy ≤ ±50ppb at base station radio interface (specified) Turns into ≤ ± 16ppb at base station traffic interface (not specified*)
• Frequency Stability For T1, it shall comply to G.824 traffic mask (specification; 3GPP Rel8) Sometimes* G.824 synchronization mask preferred
* Note: real requirements are variable as they are dependent on base station clock servo.
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• Phase measurement Measure signal under test against a reference signal
• Phase deviation plot TIE : Time Interval Error
• Analysis MTIE TDEV
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Step 1 : Phase Measurements
• At a certain signal threshold, time stamp the edges of timing signal.
• Signal edges are the significant instants.
• PHY-layer signals have high frequency (e.g., 1544 kHz)
-0.1 -0.2
+0.1
-0.2
+0.1
Signal
Ref.
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Step 2 : Phase Deviation
• Phase deviation or TIE (Time Interval Error)
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Step 3: Analysis • Analysis cover different aspects of the
Clock (oscillator) e.g. in free-running or holdover mode
Signal
• Primary used measurement analysis are: Phase (TIE) Frequency (fractional frequency offset) Frequency accuracy MTIE TDEV
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Signal with jitter and wander present
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Jitter: Filter out low-frequency components with high-pass filter
Frequency Jitter range 10 Hz
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Wander: Filter out high-frequency components with low-pass filter
Frequency Wander range 10 Hz
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• Both MTIE and TDEV are measures of wander over ranges of values. From very short-term wander to long-term wander
• MTIE and TDEV analysis shows comparison to standard requirements. Defined by ATIS/ANSI, Telcordia/Bellcore, ETSI & ITU-T E.g., ITU-T G.824, ANSI T1.101 or Telcordia GR-253-CORE
• MTIE is a peak detector: simple peak-to-peak analysis.
• TDEV is a highly averaged “rms”-type of calculation.
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Frequency Accuracy (Frequency Offset)
ITU-T G.823 Traffic Interface (MRTIE mask)
ITU-T G.823 Synchronization Interface (MTIE mask)
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• Physical layer signals can be characterized.
• Recommendations exist for node clock and interface limits.
• Synchronous Ethernet Equipment Clock (EEC) inherits from SONET NE clock specifications.
• The performance of SyncE-capable NE and SyncE interface are fully specified and metrics exist.
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• How to guarantee the packet-based recovered clock quality?
PSN
Reference Clock
Recovered Clock
Slave/ Client
Master/ Server ?
OK
Packet Delay Variation is key impairment factor for timing.
DS1 DS1
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• TIE is still a valid measurement for characterizing the packet-based servo (slave).
Oscillators and timing interfaces
• How can the PSN behavior be characterized? Algorithms use minTDEV value Need sufficient numbers of minimal latency packets Packet Delay Variation (PDV) as metric?
• First approach is to reuse known tools to PDV analysis/measurement. Some can be applied to PDV as to TIE.
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10 Switches, 40% Load
10 Switches, 80% Load
minTDEV
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• One metric would not be sufficient characterizing the various possible conditions.
Reference Clock
Recovered Clock
Classification (metric)
Master/ Server
PSN
Common, generic PSN metrics for timing performance
characterization?
Today, very close relationship between metric (packet classification) and implementation specific algorithm.
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• Protocol parameters
• Influenced by : the PSN design, the HW & SW NE configuration, the traffic.
• Master implementation
PSN
Reference Clock
Recovered Clock
Slave/ Client Master/
Server
PSN Metrics
? ?
Slave implementation
minTDEV used in algorithms, but still not adopted as metric
Even with (still to be agreed) metrics, other parameters will remain critical.
?
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1. PHY-layer Synchronization Distribution guarantees the quality.
2. Packet-based Synchronization Distribution provides the flexibility.
3. Mixing the option for getting best of both solutions.
PRC/PRS Thru BITS/SSU
EEC
EEC
EEC
EEC
Consumer
Non-capable PHY Layer Synchronization Network
SEC
PHY-layer method e.g., SDH/SONET, SyncE
Packet-based method (ACR)
PHY-layer Freq Transfer
PHY-layer Freq Transfer
PHY-layer Freq Transfer
e.g. SyncE
PHY-layer Freq Transfer
e.g. SyncE
SyncE consumer
Packet-based
consumer
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Time Synchronization What Specific Challenges Does the
Time Distribution Introduce?
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• Transmitting time reference can be absolute (from national standards) or relative (bounded timekeeping system).
• Time synchronization is one way achieving phase synchronization. Phase alignment does not mandate giving a time value.
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• This is not phase locking which is often a result of a PLL in a physical timing transfer.
Phase locking implies frequency synchronization and allows phase offset.
• The term phase synchronization (or phase alignment) implies that all associated nodes have access to a reference timing signal whose significant events occur at the same instant (within the relevant phase accuracy requirement).
Figure xxx/G.8266 – Phase Synchronization
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Target from ±1µs to tens of µs (alignment between BS)
Target from ≤ ±0.5µs to tens of µs (from common reference)
Time Source
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• Strictly speaking, the term synchronization applies to alignment of time and the term syntonization applies to alignment of frequency.
• The master/server and slave/client clocks each have their own time-base and own wall-clock and the intent is to make the slave/client “equal” to the master/server.
• The notion of frequency synchronization (or syntonization) is making the time-bases “equal”, allowing a fixed (probably unknown) offset in the wall-clocks. The notion of time synchronization is making the wall-clocks “equal”.
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NTP vs. PTP Message Exchange
NTP
PTP
As part of time recovery, there’s always a frequency recovery process.
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• Forward and backward delays and delay variations are not identical.
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• Each Node and Link can introduce asymmetry.
• There are various sources of asymmetry.
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• Link Link delays and asymmetry Asymmetric (upstream/downstream) link techniques Physical layer clock
• Node Different link speed (forward / reverse) Node design LC design Enabled features
• Network Traffic path inconsistency Interface speed change
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Summary and Introduction to IEEE Std 1588 • Basis of all packet time transfer protocols (NTP, IEEE1588) is the two
way time transfer mechanism.
• TWTT consists of a time transfer mechanism and a time delay “radar”.
• Assumes path symmetry and path consistency.
• IEEE1588 incorporates some in-network correction mechanisms to improve the quality of the transfer.
• IEEE1588 has the concept of asymmetry correction. But the correction values are not dynamically measured - they need to be statically configured.
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IEEE Std 1588-2008 for Telecom Challenges of IEEE 1588-2008 applied
in Service Provider networks
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• A set of event messages consisting of:
- Sync - Delay_Req - Pdelay_Req - Pdelay_Resp
• A set of general messages consisting of:
- Follow_Up - Delay_Resp - Pdelay_Resp_Follow_Up - Announce - Management - Signaling
Transmission modes: either unicast or multicast (can be mixed)
Encapsulations: L2 Ethernet, IPv4, IPv6 (others possible)
Multiple possible values or range of values, TLVs (possible extensions), …
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MASTER SLAVE
Delay_Resp
t1
t3
t4
Timestamps known by slave
t1, t2, t3, t4
SM_Delay
Master time = TM Slave time = TS
t2
t1, t2, t3
t1, t2
SYNC
Delay_Req
MS_Delay
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SYNC
MASTER SLAVE
Delay_REQ
Delay_RESP
MAC/PHY MAC/PHY µP µP
Hardware assistance necessary to prevent insertion of errors or inaccuracies.
t1
t2
t3
t4
t4
t1
t2
t3
Need to inject the timestamp into the payload at the time the packet gets out.
Timestamps known by slave
t1, t2, t3, t4
t1, t2, t3
t1, t2
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SYNC()
MASTER SLAVE
Delay_REQ()
Delay_RESP(t4)
MAC/PHY MAC/PHY µP µP
Timestamps known by slave
Follow_Up(t1)
t1
t2
t4
t3
Two-step clock mode Vs. One-step (a.k.a. “on-the-fly”) clock mode
t1, t2, t3, t4
t1, t2, t3
t1, t2
t2
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• Five basic types of PTP devices (“clocks”) Ordinary clock (master or slave) Boundary clock (“master and slave”) End-to-end Transparent clock Peer-to-peer Transparent clock Management node
• All five types implement one or more aspects of the PTP protocol
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• BC and TC aims correcting delay variation into intermediate nodes between OCs.
• Can correct link asymmetry if known.
Ref. Clock
Recovered Clock
Ordinary Slave
Ordinary Master
TC BC
Transparent Clock
Boundary Clock
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• Can help on scalability when using unicast.
• Equivalent to NTP Stratum (>1) Server UTC
• Node by node: BC slave function is critical
Ref. Clock
Recovered Clock
Ordinary Slave
Ordinary Master
BC
Boundary Clock
BC
Boundary Clock
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• TC calculates Residence Time (forward / reverse intra node delays).
• TC are supposed to be transparent but: One-step clock issue
Ref. Clock
Recovered Clock
Ordinary Slave
Ordinary Master
Transparent Clock
Transparent Clock
TC TC
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• If IEEE 1588-2008 is not planned node to node, with every node IEEE 1588 aware and in unique domain…
• Multiple interface types IEEE 802.3, ITU-T G.709, …
• Multiple interface frequencies 10GE, 100GE, STM64, STM192…
• Multiple encapsulations Ethernet, IP MPLS, MPLS-TP, PBB-TE…
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Ref. Clock
Recovered Clock
Ordinary Slave
Ordinary Master
TC BC
Wholesale Boundary Clock
TC BC
• Who owns the master?
• Who owns the slaves?
• Who owns the intermediate nodes?
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• How to guarantee the recovered clock quality?
PSN
Ref. Clock
Recovered Clock
Slave/ Client
Master/ Server
? ?
?
TC
? ?
BC
Objective: accuracy and stability from reference
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• IEEE Std 1588-2008 is actually a “toolbox” !
What does “support of IEEE 1588” really mean ?
• IEEE Std 1588 itself is not sufficient for telecom operator operations. Node characterization, modeling, performance, metrics…
• For phase & time support, it is expected any telecom standardization would take time.
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Summary
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• Timing is a new service many networks shall have to support.
• Different solutions are necessary to cover disparate requirements, network designs and conditions.
Physical layer solutions required to upgrade routers and switches. Packet-based solutions are more flexible but less deterministic.
• Whatever the timing protocol, it must deal with the same network constraints.
• Each network is different
• Synchronization Experts are welcome to enter the packet based networks and assist with the designs