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Packet-based and Network-Based
Synchronisation Solutions In Depth
Karim Traore
Senior systems architect
Symmetricom
ktraore@symmetricom.com
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Agenda
• Application requirements for clock
synchronization in 2G, 3G, LTE
• IEEE-1588 overview
• Synchronous Ethernet overview
• Advanced end-to-end clocking architectures
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Application requirements for clock
synchronization in 2G, 3G, LTE
3
Applications Requirements
(Air interface of Base Stations/NodeBs)
Applications Frequency Phase Time
±10µs(±3µs
CDMA2000 ±50 ppb
Preferred)(1)
GSM ±50 ppb
UMTS-FDD/WCDMA (FDD) ±50 ppb
UMTS-TDD/WCDMA (TDD) ±50 ppb ±2.5µs(2)
TD-SCDMA ±50 ppb ±3µs(2)
Not yet specified
LTE (FDD) ±50 ppb
for MBMS
LTE (TDD) ±50 ppb ±3µs(2)
Mobile WiMAX (802.16e/m)-FDD ±50 ppb
Mobile WiMAX (802.16e/m)-TDD
±50 ppb ±1µs(2)
• Note: (1)Time alignment error wrt UTC,(2) Phase alignment of neighboring base stations
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Synchronization Requirements
Frequency Phase Time
Synchronization Synchronization Synchronization
01:00:00 01:00:10
TA=1/fA TA=1/fA TA=1/fA
A A A
t
TB=1/fB TB=1/fB TB=1/fB
B B B
t
fA=fB fA=fB fA=fB 01:00:00 01:00:10
Aligning clocks with respect to Aligning clocks with respect to Aligning clocks with respect to time.
frequency phase The two clocks must utilize the
same epoch. Time synchronization
implicitly includes phase and
frequency synchronization
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IEEE 1588-2008 Overview
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IEEE 1588 Precise Time Protocol - PTP History and
Standards
• Developed by HP/Agilent Laboratories (John Eidson and al.) in
1990s
– Synchronization of test equipment
• Industrial Automation needs for high precision applications
(e.g., robotics, test & measurements)
– Sub-microsecond synchronization over Controlled Local Area Networks
– Independent of the physical layer
– Transfer frequency and time of the day
• IEEE 1588 PTP Specifications
– 1588 PTP 2002 (also called version 1) - approved in 2002
– 1588 PTP 2008 (also called version 2) - approved in 2008
• ITU-T Telecom Profile (under development)
– Set of required options, prohibited options, range and defaults value of
attributes
• ITU-T G.8261: Timing & Synchronization aspects in Packet
Network
– Performance testing framework
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IEEE 1588 PTP 2008
• Packet based synchronization mechanism
– UDP/IP layers messaging (multicast and unicast) over Ethernet
• Frequency, Phase and Time
– TDM synchronization and SyncE are Layer 1 mechanisms & support only
frequency
• Client/server model
– Master clock, slave clock (ordinary clock)
– Intermediary nodes may or may not support IEEE1588 PTP (unlike SyncE)
– On-path support mechanisms
• Boundary clock
– It acts as a slave clock at port that connects to the grandmaster, and as a master to all
other ports
– It isolates the “down stream” clocks from any delays and jitter within the switch/routers
• Transparent clock
– It measures residence time of PTP events
• Key performance aspect:
– Clock algorithm (vendor specific) – Not part of IEEE 1588 PTP standard
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IEEE 1588 PTP Protocol
Master Clock Slave Clock
Two options
• One step Announce Timestamps known
slave
• Two steps t1
Sync (t0 ,t1)
t0 estimated time
Follow_Up (t1) t1,t2 t1, t2 One step
t1 t1, t2 Two steps
Delay_Req t3 t1, t2 ,t3
t4
Delay_Resp(t4)
t4 t1, t2 ,t3,t4
Bandwidth requirements *
Master -> Salve: 101.5 Kbps
Slave->Master: 48 Kbps
*Sync update rate of 64 packets per second Offset =[(t2 - t1) - (t4 - t3) ]÷ 2
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Multicast vs Unicast Deployment –
Unicast Negotiation Sequence (Signaling Message)
Switch/Router Layer
Master Clock
Slave Clock
Time
Time
Lease Duration in Bytes (10-1000s for Telecom Profile)
Sync Interval (2X), where x = -7,-6,..0,1,..,,7
Signaling Type
-b0 announce request
-00 sync request
-90 delay_resp(onse) request
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Multicast vs Unicast Deployment – Unicast Announce
Message
Master Clock Slave Clock
Switch/Router Layer
t1
t2
Flags information
t3
t4
Unicast/Multicast
1-step or 2-step
Time
Time
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MEF 22 Class of Service (CoS)
• MEF 22 provides service modeling and mapping
guidelines
– Guidelines for number of CoS classes
– Bundling traffic types into limited number of CoS classes
• The MBH IA specifies QoS requests to enable service
class differentiation
Service Class Example of Generic Traffic Classes mapping into CoS
Name 4 CoS Model 3 CoS Model 2 CoS Model
Very High (H+) Synchronization - -
High (H) Conversational, Conversational and Conversational and Synchronization,
Signaling and Control Synchronization, Signaling and Control,
Signaling and Control Streaming
Medium (M) Streaming Streaming -
Low (L) Interactive and Interactive and Interactive and
Background Background Background
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Synchronous Ethernet Overview
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Synchronous Ethernet
History and Standards
• Migration of carrier networks from SONET/SDH-centric to
Ethernet-centric infrastructure
– Derived from timing distribution over SONET/SDH standards
• Initial driving forces: BT and FT/Orange - 2005
– FT demonstrated test results confirming viability of approach
– BT provided method for handling SSMs
• Synchronization Requirements
– Must be directly frequency synchronized by an SSU/BITS or line-
timed
• Based on well established SONET/SDH synchronization
model
• ITU-T G.813 clock model
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Synchronous Ethernet (SyncE)
• Developed by ITU-T standard (G.8261/G.8262/G.8264), SG15 -
Question 13
– No impact to IEEE 802.3 standard
• Physical layer technology
– Based on well established SONET/SDH synchronization model (ITU-T
G.813 clock model)
• Transfer frequency
– SyncE does not support phase or time synchronization
• Standardization : ITU, SG15, Question 13
– ITU-T G.8261 – Timing and Synchronization aspects in packet network
• Defines timing and synchronization elements of packet networks
– ITU-T G.8262 – Timing characteristics of Synchronous Equipment Slave
Clock (EEC)
• Defines clock (PLL) performance characteristics such as wander, jitter, phase
transients, clock bandwidth, frequency accuracy, holdover
– ITU-T G.8264 – Distribution of timing through packet networks
• Specifies the SSM transport channel, namely the Ethernet Synchronization
Messaging Channel (ESMC) protocol behavior and message format
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How SyncE is Different
Node without SyncE
• Ethernet without SyncE (e.g. 1000Base-T)
Slave Master
– Rx clock extracted from line (line timed)
– Tx clock generated from free-running XO PHY PHY
• Clock within 100 ppm (125 MHz+/- 0.01 percent)
– No relationship between Rx and Tx PHY PHY
– Each Ethernet interface (uni-dir or bi-dir) has own
Line cards Line cards
independent timing Backplane
Node with SyncE
• Ethernet with SyncE (e.g. 1000Base-T)
SyncE
SyncE Timing
Timing
Card
Card
– Rx clock extracted from line & transferred to timing engine
– Tx clock generated from the timing card PHY EPHY
• Clock within 4.6 ppm when free-running
– Tx clock uses traceable to PRC/PRS PHY PHY
– Note SyncE clock input may replace XO or may be in Line cards Line cards
addition to normal XO input Backplane
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SyncE Synchronization Plane
► Native Ethernet recovers the Rx clock, but does not propagate the
clock downstream
► SyncE Tx clock is sourced from a traceable timing source, instead of
XO
► The equipment (system) clock to be used as the Tx clock
► Point-to-point (link-by-link) technology
► All links in the chain must support propagation of synchronization
Primary Reference
Clock (BITS/SSU)
SyncE SyncE SyncE
Timing Timing Timing
Card Card Card
Master Slave Master Slave Master
Data Data
PHY PHY PHY PHY PHY PHY
Clock Clock
PHY PHY PHY PHY PHY PHY
Line cards Line cards Line cards Line cards Line cards Line cards
Backplane Backplane Backplane
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Ethernet Synchronization Messaging Channel (ESMC)
• Transport of Synchronization Status Message
(SSM) defined in ITU-T G.707 & G.781
– SONET/SDH: S1 Line overhead bits
• Indication of quality level (QL) of clock
driving sync chain
OSI Model
– Used to control/maintain/restore sync chain Application
– Timing loops, bad quality signals etc Presentation
Session
• IEEE 802.3ay Organization Specific Slow Transport
Protocol (OSSP) Network
– Extendable using TLV formats
Data Link
LLC
– PRC traceability information ESMC
MAC
– Processing in each network element (on reception and
Physical
generation)
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Ethernet Synchronization Messaging Channel (ESMC) –
ITU-T OSSP
Octet Size ESMC Field
number
1-6 6 bytes DA =01-80-C2-00-00-02 (hex)
7-12 6 bytes Source Address
13-14 2 bytes Slow Protocol Ethertype = 88-09 (hex)
Clock Quality Level Information
15 1 bytes Slow Protocol Subtype = 0A (hex) (PRC, SSU, EEC, DoNotUse)
16-18 3 bytes ITU-OUI = 00-19-A7 (hex)
19-20 2 bytes ITU Subtype
Size SSM QL TLV
21 4 bits Version 8 bits Type: 0x01(SSM)
1 bit Event flag 16 bits Length: 0x04
4 bits 0 (unused)
3 bits Reserved
4 bits SSM code
22-24 3 bytes Reserved
25-28 4 bytes SSM QL TLV
29-1532 32-1486 bytes Future TLV and padding
Last 4 4 bytes FCS
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Ethernet Interfaces eligible to SyncE
Examples
PHY Description IEEE Coding Synchronous
802.3 Ethernet
Clause capable
10BASE-F 10Mb fiber 15 NRZ, intermittent No
10BASE-FP 10Mb fiber, star 16 NRZ, intermittent No
10BASE-T 10Mb TP copper 14 intermittent No
100BASE-BX10 100Mb bidi fiber 58, 66 4B/5B Yes
100BASE-FX 100Mb fiber 24, 26 4B/5B Yes
100BASE-LX10 100Mb fiber 58, 66 4B/5B Yes
100BASE-T2 100Mb TP copper 32 PAM-5 No
100BASE-T4 100Mb TP copper 23 8B6T No
100BASE-TX 100Mb TP copper 24, 25 MLT-3 Yes
1000BASE-CX 1Gb twinax 39 8B/10B Yes
1000BASE-KX 1Gb backplane 70 8B/10B Yes
1000BASE-LX 1Gb fiber 38 8B/10B Yes
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Advanced end-to-end clocking
architectures
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Native Ethernet/MPLS Access Network
Mobile Backhaul Owned by Service Provider
Aggregation
Point/Hub 1588 PTP
Edge GM
SyncE Node
SyncE BSC
SyncE
Ethernet Edge
SyncE
Node
IP/MPLS SyncE Ethernet RNC
1588 Router over
PTPSlave SyncE
SyncE DWDM
SGSN
Edge
Node
SyncE
SyncE
Edge BITS
Node Clock
Base Station Site MTSO(RNC)
Access network Aggregation network
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Native Ethernet Access Network
Mobile Backhaul Not Owned by Service Provider
1588 PTP
Aggregation GM
Point/Hub
Edge
Node
BSC
Ethernet
Ethernet RNC
1588 over
PTPSlave
DWDM
SGSN
Edge
Node
Edge
Node
Base Station Site MTSO(RNC)
Access network Aggregation network
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Summary
IEEE 1588 PTP Synchronous Ethernet
• Frequency & Phase transfer • Frequency transfer only
• Packet layer technology • Physical layer technology
• End-to-end technology • Point-to-point technology
• Performance affected by layer 2 &3 • Unaffected by layer 2 &3 impairments
impairments (e.g., PDV) • No multi-operator support yet
• Multi-operators support • Hop-by-hop deployment
• End-to-end deployment • Early stage of deployment
• Deployment by one tier 1 wireless • Feel of trust (robust technology)
carrier in one country – Conservative choice
– Pre-deployment trials & extensive testing
• Brownfield considerations
by carriers & vendors
• Note: 1588 PTP addresses just the protocol
aspects: what is critical is the clock
algorithm, which is not standardized
Hybrid cases: PTP used for phase and SyncE used for frequency on same network
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