Frequency and Time Synchronization In Packet Based Networks

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2 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 for Telecom Summary 2010 Cisco and/or its affiliates. All rights reserved. 2

Subscriber Mobile TV Mobile user DVB-T/H 3GPP/2 Access

multiple domains Internet is a multi-domain network.

ISP Residential SoHO xdsl xpon DSLAM OLT M-CMTS PE MS A

4 Subscriber Mobile TV Mobile user DVB-T/H 3GPP/2 Access WiMAX TDM / ATM 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. Femto-cell Ethernet Aggregation TDM / ATM Backbone Peer ISP Residential SoHO xdsl xpon DSLAM OLT M-CMTS PE MS A P Hub & Spoke or Ring P P PE MSE P P PE Mesh Content Network Internet Enterprise DOCSIS VoD TV SIP 2010 Cisco and/or its affiliates. All rights reserved. 5

5 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 2010 Cisco and/or its affiliates. All rights reserved. 6

7 The Leading Requirements Application TDM support (e.g. CES, SDH transformation), Access Mobile Base Stations WiMAX Mobile DVB-S/H/T2 SFN MB SFN Service GSM, WCDMA and LTE FDD UMTS TDD TD-SCDMA CDMA2K LTE TDD One-way delay and jitter Performance Measurement Frequency PRC-traceability, jitter & wander limitations ITU-T G.8261/G.823/G.824/G.825 Frequency assignment (fractional frequency accuracy) shall be better than ± 50ppb (macrocells) ± 100ppb (micro- & pico-cells) ± 250ppb (femtocells) Shall be better than ± 15 ppb TBD Phase Alignment Time Synchronization N/A (except for MBMS and SFN) Phase alignment between base stations must be < ±2.5µs Phase alignment between base stations must be < ±3µs Time alignment error should be less than 3 μs and shall be less than 10 μs Phase alignment between base stations from ±0.5µs to ±50µs (service degradation) Phase alignment between base stations must be < ±1µs Cell synchronization accuracy for SFN support must be < ± 3µs Phase/time alignment between base stations requirement can vary but in order of µs To improve precision << 1 ms for 10 to 100µs measurement accuracy need ± 1 µs to ± 10µs ToD accuracy 2010 Cisco and/or its affiliates. All rights reserved. 8

Use of GPS (and GNSS alternatives) raises some concerns:

Reliability Geography Vulnerability https://www.gsw2008.

pdf 746th Test Squadron GNSS : Global Navigation Satellite

8 Use of GPS (and GNSS alternatives) raises some concerns: Cost Limited utilization Locations Regulatory & Politics Reliability Geography Vulnerability rabilities_gsw2008.pdf 746th Test Squadron GNSS : Global Navigation Satellite System 2010 Cisco and/or its affiliates. All rights reserved. 9 GPS : Global Positioning System

9 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 2010 Cisco and/or its affiliates. All rights reserved. 10

11 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 2010 Cisco and/or its affiliates. All rights reserved. 12

12 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) G.8261 (2006) PHY-layer frequency transfer CES performance Service Provider (SP) Metro & Core Ethernet Packet-based timing Multiple working items: profile, metrics, modeling Packet-based frequency, phase and time transfer Service Provider (SP) IEEE 1588 PTP IEEE IEEE 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 TICTOC NTP NTPv5 PTP Profile(s) NTPv3 Standard NTPv4 (CY09) New WG (approved March 08) Time distribution Frequency and time transfer Internet SP domain Internet Specific SP areas 2010 Cisco and/or its affiliates. All rights reserved. 13

14 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! 2010 Cisco and/or its affiliates. All rights reserved. 16

15 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 Source: ETSI EG Synchronization network engineering PRC : Primary Reference Clock 2010 Cisco and/or its affiliates. All rights reserved. 17

16 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 Cisco and/or its affiliates. All rights reserved. 18

17 Synchronization information is transmitted through the network via synchronization network connections. Synchronization network connections are unidirectional and generally point-to-multipoint. Stratum 1 level CO Stratum 2 level NE (Stratum level 3) 2010 Cisco and/or its affiliates. All rights reserved. 19

18 PRC SSU SEC : Primary Reference Clock ( PRS) : Synchronization Supply Unit ( BITS) : SDH Equipment Clock Core Network Aggregation and Access Networks Source: ETSI EG Synchronization network engineering 2010 Cisco and/or its affiliates. All rights reserved. 20

20 NE s External Timing Input a.k.a. BITS IN NE s External Timing Output Figure 4-2. Recommended BITS Implementation with SONET Timing Distribution Source: Telcordia GR-436-CORE. Digital Network Synchronization Plan 2010 Cisco and/or its affiliates. All rights reserved. 22

21 Stratum 1 level What clock quality do I get? Is that the best source I can use? Stratum 2 level NE level 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) 2010 Cisco and/or its affiliates. All rights reserved. 24

22 SSM Allows Source Traceability Representation of the PRC network connection Fault X Representation of the synchronization network connection in case of failure Example of restoration of the synchronization PRC synchronization network connection SSU synchronization network connection SEC synchronization network connection 2010 Cisco and/or its affiliates. All rights reserved. 25

23 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 and kbps hierarchies Expected to be fundamental to high quality time transfer Drawback : hardware upgrades All timing chain shall be SyncE capable Cisco and/or its affiliates. All rights reserved. 26

24 External timing interface outputs External Equipment BITS/SSU) ITU-T G.8262 (EEC): Synchronous Ethernet Equipment Clock ITU-T G.781: Clock Selection Process PRC-traceable signal from BITS/SSU External timing interface inputs ITU-T G.8261 SyncE interface jitter & wander IEEE802.3 ± 100ppm External timing interface inputs Frequency distribution traces PLL Synchronous Ethernet capable Line Card Synchronous Ethernet capable Line Card ITU-T G.8264 ESMC and SSM-QL Synchronous Ethernet capable Equipment 2010 Cisco and/or its affiliates. All rights reserved. 27

25 Ethernet Synchronization Messaging Channel Use OSSP from IEEE802.3ay (a revision to IEEE Std ) Key purpose: transmit SSM (QL) Outcome: Simple and efficient But designed to support extensions Protocol model: Event-driven with TLVs Two message types TLVs Event message sent when QL value change Information message sent every second QL-TLV is currently the unique defined TLV. Other functions can be developed. OSSP : Organization Specific Slow Protocol 2010 Cisco and/or its affiliates. All rights reserved. 28

26 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 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 IEEE OSSP ITU-T OUI Header ESMC Header QL-TLV Future TLV Extension Payload OSSP * Allocated by TSB 2010 Cisco and/or its affiliates. All rights reserved. 29

27 Assuring The Continuity at PHY Layer BITS/SSU PRC/PRS BITS/SSU BITS/SSU SONET/SDH PHY SyncE PHY SyncE ITU-T G.8262 (EEC) Node ITU-T G.8262 (EEC) Node ITU-T G.8262 (EEC) Node ITU-T G.8262 (EEC) Node 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 Cisco and/or its affiliates. All rights reserved. 30

28 Reference Clock PSN Recovered Clock Three key steps: Generation: from signal to packet Transfer: packet transmission over packet network(s) Recovery: from packet to signal 2010 Cisco and/or its affiliates. All rights reserved. 31

29 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 PTP X X X IETF RTP X X X 2010 Cisco and/or its affiliates. All rights reserved. 32

30 Independent Timing Stream IWF TDM PW bit stream IWF TDM TDM Reference Clock ACR Packet Stream Reference Clock Recovered TDM timing based on the adaptive clock recovery PEC ACR Packet Stream TDM IWF & PEC TDM PW bit stream IWF & PEC TDM Clocking method a.k.a. out-of-band (here, used for CES clocking) 2010 Cisco and/or its affiliates. All rights reserved. 33

Source: Diagram from Time Domain Representation of Oscillator Performance,

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.

32 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 Cisco and/or its affiliates. All rights reserved. 35

34 Step 1 : Phase Measurements Ref. Signal 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) 2010 Cisco and/or its affiliates. All rights reserved. 37

36 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 2010 Cisco and/or its affiliates. All rights reserved. 39

Signal with jitter and wander present 2010

Jitter: Filter out low-frequency components with high-pass filter 10 Hz

40 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 Cisco and/or its affiliates. All rights reserved. 43

42 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 Cisco and/or its affiliates. All rights reserved. 45

43 How to guarantee the packet-based recovered clock quality? OK Reference Clock DS1 DS1 Recovered Clock PSN Master/ Server? Slave/ Client Packet Delay Variation is key impairment factor for timing Cisco and/or its affiliates. All rights reserved. 46

44 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 Cisco and/or its affiliates. All rights reserved. 47

46 mintdev used in algorithms, but still not adopted as metric Even with (still to be agreed) metrics, other parameters will remain critical. Reference Clock PSN Metrics PSN Recovered Clock Master/ Server Master implementation Protocol parameters??? Influenced by : the PSN design, the HW & SW NE configuration, the traffic. Slave/ Client Slave implementation 2010 Cisco and/or its affiliates. All rights reserved. 50

47 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. SyncE consumer SEC Packetbased consumer PHY-layer Freq Transfer e.g. SyncE EEC PHY-layer Freq Transfer e.g. SyncE PHY-layer method e.g., SDH/SONET, SyncE Consumer EEC PHY-layer Freq Transfer EEC PHY-layer Freq Transfer EEC Non-capable PHY Layer Synchronization Network Packet-based method (ACR) BITS/SSU PRC/PRS Thru BITS/SSU 2010 Cisco and/or its affiliates. All rights reserved. 51

49 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 Cisco and/or its affiliates. All rights reserved. 53

50 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). Reference timing signal to system A Reference timing signal to system B System A B System B timing signal recovered by system A timing signal recovered by system B t t Figure xxx/g.8266 Phase Synchronization 2010 Cisco and/or its affiliates. All rights reserved. 54

53 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 timebase 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 Cisco and/or its affiliates. All rights reserved. 57

54 NTP vs. PTP Message Exchange As part of time recovery, there s always a frequency recovery process. Usual unidirectional ACR protocol t-ms t 1 Master time Sync PTP Slave time Timestamps known by slave t 2 t 2 Follow_Up t 1, t 2 t-sm t 4 Delay_Req NTP t 3 t 1, t 2, t 3 Delay_Resp t 1, t 2, t 3, t Cisco and/or its affiliates. All rights reserved. 58

57 Link Node Link delays and asymmetry Asymmetric (upstream/downstream) link techniques Physical layer clock Different link speed (forward / reverse) Node design LC design Enabled features Network Traffic path inconsistency Interface speed change 2010 Cisco and/or its affiliates. All rights reserved. 61

58 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 Cisco and/or its affiliates. All rights reserved. 62

60 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), 2010 Cisco and/or its affiliates. All rights reserved. 64

61 MASTER Master time = T M SLAVE Slave time = T S MS_Delay t 1 SYNC Timestamps known by slave t 2 t 1, t 2 t 3 SM_Delay Delay_Req t 1, t 2, t 3 t 4 Delay_Resp t 1, t 2, t 3, t Cisco and/or its affiliates. All rights reserved. 65

62 MASTER SLAVE µp MAC/PHY MAC/PHY µp Need to inject the timestamp into the payload at the time the packet gets out. t 1 t 1 SYNC t 2 t 2 Timestamps known by slave t 1, t 2 Delay_REQ t 3 t 3 t 1, t 2, t 3 t 4 t 4 Delay_RESP t 1, t 2, t 3, t 4 Hardware assistance necessary to prevent insertion of errors or inaccuracies Cisco and/or its affiliates. All rights reserved. 66

63 MASTER SLAVE µp MAC/PHY MAC/PHY µp Two-step clock mode Vs. One-step (a.k.a. on-the-fly ) clock mode t 1 t 4 SYNC() Follow_Up(t 1 ) Delay_REQ() t 2 t 3 Timestamps known by slave t 2 t 1, t 2 t 1, t 2, t 3 Delay_RESP(t 4 ) t 1, t 2, t 3, t Cisco and/or its affiliates. All rights reserved. 67

64 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 2010 Cisco and/or its affiliates. All rights reserved. 68

65 BC and TC aims correcting delay variation into intermediate nodes between OCs. Can correct link asymmetry if known. Recovered Clock Ordinary Slave TC BC Ordinary Master Ref. Clock Transparent Clock Boundary Clock 2010 Cisco and/or its affiliates. All rights reserved. 69

66 Can help on scalability when using unicast. Equivalent to NTP Stratum (>1) Server UTC Node by node: BC slave function is critical Recovered Clock Ordinary Slave Ordinary Master Ref. Clock BC BC Boundary Clock Boundary Clock 2010 Cisco and/or its affiliates. All rights reserved. 70

67 TC calculates Residence Time (forward / reverse intra node delays). TC are supposed to be transparent but: One-step clock issue Recovered Clock Ordinary Slave TC TC Ordinary Master Ref. Clock Transparent Clock Transparent Clock 2010 Cisco and/or its affiliates. All rights reserved. 71

68 If IEEE 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 2010 Cisco and/or its affiliates. All rights reserved. 72

69 Recovered Clock Ordinary Slave TC TC BC BC Ordinary Master Ref. Clock Wholesale Boundary Clock Who owns the master? Who owns the slaves? Who owns the intermediate nodes? 2010 Cisco and/or its affiliates. All rights reserved. 73

70 How to guarantee the recovered clock quality? Objective: accuracy and stability from reference Recovered Clock Slave/ Client?? TC PSN BC Master/ Server Ref. Clock??? 2010 Cisco and/or its affiliates. All rights reserved. 74

71 IEEE Std 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 Cisco and/or its affiliates. All rights reserved. 75

73 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 2010 Cisco and/or its affiliates. All rights reserved. 77

74 Thank you.

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