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Synchronisation in the 21st
Century Fixed Line Revolution - A Carriers Perspective
Mike Gilson
BT Exact – Next Generation Networks
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Agenda
• 21st Century fixed line revolution
– Do we need synchronisation
• Changing architecture
– Generation & distribution
– Some of the issues – The Layer problem
– Some measurements on Layer 2 & 3 solutions
• Concentration on Layer 1 / Layer 2
– Measurement & Technology Summary
– Network & resource convergence
• Possible high level scenarios
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Synchronisation - Why do we need it?
• The world is all packets – Sync not required!
• Life is not simple
– Migration of 20C to 21C
– Some applications require synchronisation
• What do we mean by synchronisation
– Commonly held to be frequency (timing)
– What about time?
• New applications
– Requirement for Time & Timing
• Embed the building blocks -> enable the future
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21C Rationale
• Convergence is gaining momentum
• Convergence needs an underlying infrastructure to deliver and support it
• Customers want more choice, flexibility and control
• Simplicity is key
Speed to market
Customer experience and empowerment
Cost transformation
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Simplified network
IP-MPLS-WDM
DSL
Fibre &
Copper
Copper
Agg Box
End
User
~5.5k
nodes
~100
nodes
Class 5
Call Server
Content
WWW
ISP
Multi-service access Converged core
Current thinking.
No implementation assurances
Simplification!
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Services required
• Who knows?
– Yet to be thought of!
• Our customers want
– simple & complete services…
– that enhance their lives…
– allow them to…
– carry out their business by…
– using services, connecting to
networks, seamlessly and simply!
– cost reduction
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Plug & Play
• Does it matter if it needs sync
– …to the sync industry yes…
– …to the user no…
– …they want to plug and play!
• The user doesn’t know or care if
the service requires sync…
• If sync is required we need to
provide it in the plug and play
connection.
– It may only be a small component in
the overall service!
– But have a big impact
Gibson Digital Electric Guitar
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Begin
Fibre
to the
PCP
~30,000 Multi-
Service
Access
Devices
~130 Metro
Routers
~20
Core
Routers
End
Customer
Data
Centre Logical
Nodes
Aggregation Service Edge Core
100+
Nodes 5,000+
Nodes
Reference & Distribution
Millions?
The Number Problem!
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Evolving Network Architecture
Apps
Apps Apps
Apps Apps
Apps CBR / TDM Switch
Router / Pkt Switch
Requirement for stability Apps Application requiring
stability
Apps Application no
Stability requirements
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Edge Stability
“Jam in the Doughnut!”
Jam in the centre Jam at the edge -
……..more jam required!
PRC
PRC PRC
PRC
PRC PRC
PRC
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Residential Space - RES ETH
Clock / Time matching
the listener buffers
Network Device #1 Device #2 Device #3
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Technology Choices & Trade Off
• If timing is required at the Application or Edge of the Network
– Choices based on various factors
• Reference generation
• Reference distribution
• Generation / Distribution balance
• Technology choices
– Satellite based systems
– Maintain an SDH / SONET Path
– Packet Based Solutions
– Synchronous Ethernet
– Propriety
Generation Application
Distribution
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GNSS - The Cost of Off Air
• Architecture (Performance) Vs Cost
• Larger systems
– CAPEX to Install and Ongoing OPEX
• Typically, 2% of installed base have issues…
– Interference
– Weather degrading install
– Roof rearrangements - Do you own the roof?
• The Street cab
– Engineer into the cab
– The same problems as on the roof – on a micro scale
– Consider the environment
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The GNSS Environment
The Weather
GSM / 3G
Mobile
Paging
Microwave
Engine
House
Exhaust fumes
Plant rooms
GPS!
“A mix of interference & environmental issues”
Offsite
interference
Issues!
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Core - Edge Reference Distribution
• TDM Technologies
– SDH / WDM (Well understood transport)
• Ethernet (carrier scale)
– Currently not synchronous to a network reference
• xDSL Technology
– Symmetrical (fairly well defined performance)
– Asymmetrical delivery creates challenges
• Optical Systems
– GPON etc
• Packet based technologies
– TDMoIP, CESoIP, SAToIP (bit rate limitation, load / delay)
– NTP, IEEE1588
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The Layer Problem
• “Science of the sensible”
– Clock signals and oscillators are
fundamentally analogue
– Why translate from a stream with a
given frequency to packets?
• Unless you have to…
• Building up from the duct
– Duct & fibre is a “given” & its stable
– Physical Layer – next stable point
– Adapting the frequency to a packet
stream
• Performance inheritance
Physical
Data Link
Network
Transport
OSI Stack
Physical
Data Link
Network
Transport
OSI Stack
Duct
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Layer 1 - Maintain The SDH Path
l ln
STM-n
WDM
Ethernet
•Requires a wavelength to maintain synchronisation
•May require for TDM / low latency services
•CAPEX & OPEX costs
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Layer 1 - Synchronous Ethernet
• Designed to robustly deliver synchronisation
– frequency / phase (Takes the best from SDH)
• Essentially looks like SDH / TDM timing
– Helps in the migration process – SDH transport to Ethernet Transport
– Will inter-work with native Ethernet
• Does not change basic Ethernet Standards
– Note: not native Ethernet / can not be supported over native Ethernet
• Requires hardware changes
– Ethernet Silicon requires control silicon
– message channel to support Sync Status Message (SSM)
• Changes some views on accepted functional modelling
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Layer 2 / Layer 3 - Packet Solutions
• Layer 2 – Running over native Ethernet
• IEEE1588 – Precise Time Protocol (PTP)
– Embed a 1588 solution within your network elements
– 1588 is more than a Protocol, requires hardware changes
– If you know frequency = very precise time
• Layer 3 – Running over native Ethernet
• CE TDM over Packet - flow combined with traffic
– Contention with traffic, Variable performance, Stabilisation period
• CE TDM over dedicated links
– Performance improvement – dedicated so traffic contention goes away
– Same issues as maintain the SDH path – CAPEX & OPEX
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Synchronisation Impact Summary Network Impairment
(Typical impairments that may occur)
G.8261
Synchronous
Ethernet - Layer 1
IEEE1588
PTP - Layer 2
Increased Channel Utilisation None Low impact
Packet Reordering None Low impact
Error Injection None Low (*) Not tested
Asymmetric Delay None Low impact
Delay Variation None High impact
Asymmetric Delay Variation None High Impact
Dropped packets None Low impact (*)
Recovery Time (e.g. link fail) Fast recovery Slow recovery (**)
Route Change None High impact (**) Not Tested
Power up recovery time Fast recovery Slow recovery
Note
- Summarises impact on stable synchronisation i.e. ITU-T G.811
- High impact = breach of all ITU-T G.823 Standards
- (*) or (**) related
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Technology Summary
Key Points Synch Ethernet
Layer 1
PTP – IEEE1588
Layer 2
Engine Cost $’s $’s
Inter-work with Native
Ethernet
Yes Yes
Operate over native
Ethernet
No Yes
New hardware required Yes Yes
Standardised Yes – ongoing
ITU-T G.8261 et al
Yes – ongoing
IEEE1588v2
Traffic Impairment Impact None Yes
Architecture Understood –
“SDH Like”
Requires work
Bandwidth Required None Yes - Minor
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High Level Architecture Solution
• Use appropriate solution in the appropriate place within architecture
• Frequency Recovery
– Layer (x) solutions competing - There maybe good reasons
– But not everywhere!
• Carrier scale Ethernet transport
– Synchronous Ethernet - used to recover good frequency
– May not be required at all points
– Push highest level of frequency stability as far to edge as possible
• Native Ethernet transport
– IEEE1588 can be used to transport frequency and time
– However, performance trade off (frequency) due to impairments
– May also impact recovery of time
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Convergence & Complementary
• Frequency and time are related – consider as a resource
• Networks
– Carrier - traditionally required frequency
– IT / computer networks – time
• Synchronous Ethernet
– Can provide the frequency base in carrier scale networks
• IEEE1588
– Time
– If you have good frequency at the end points - Time lock becomes quicker
– Can provide frequency base in native networks - limited
• Synchronous Ethernet & IEEE1588
– should be seen as complementary
– …in resolving the frequency and time solution
But these are converging
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Network & Resource Convergence
UTC
UTC UTC frequency
Network Network
UTC
UTC UTC frequency
Network Network
UTC
Network
UTC
Carrier IT Carrier IT Platform
Services Services Services Services Services
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Possible Scenarios
Core Edge
SyncE
Frequency
Frequency &
Time
device Apps
Copper
Fibre
Wireless
PTP over Native
Push to edge to trade off access impairment
Native Time & low
quality freq
SyncE
PTP Push into core
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Conclusions
• Simplify the evolving architecture
– Flexibility, cost base
– Embed the time & timing components
• Its not a choice of one technology over another
– Combination of technology
– Packet techniques do work but use appropriately
• If we are building networks fit for the 21st Century
– Yes build it to a cost…
• But
– Embed the key components in the base technology
– Understand that a few $’s additional cost may enable many future applications
– Should not accept a lower quality base performance
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Contact Details
Mike Gilson
BT Exact
Pp11, Orion Building 5
Adastral Park
Martlesham Heath,
Ipswich
Suffolk IP5 3RE
UK
Tel: +44 1473 609575
Email: [email protected]
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Sources of Network Reference
• Frequency and / or time generation
• The Primary Reference Clock (PRC)
– Source of frequency stability and increasingly time
• The obvious choices
– Caesium
– Disciplined off-air i.e. GPS, Galileo (2010 onwards?)
• Less obvious
– Low Frequency solutions – LORAN, MSF, DCF
– High stability oscillator on a chip e.g. Caesium
• Some sources have time embedded
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The Network Space
Network Network Network
Network #1 Network #2 Network #3
User /
Application
Space
User /
Application
Space
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Application Space
Network SAP SAP
Network #n Application Space
media media media
device
device
device
device
device
device
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Layer 1 Boundary
ETH = Ethernet ETH Layer
ETY = Ethernet ETY (Physical) Layer
Flows
ETH
ETY
Frame
Clock
Line Code
Clock recovered
from Line
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Hybrid OSI Stack
Application
Physical
TDM
Timing Flows
• Many timing flows now exist
• Physical (Network) timing flows
– Network Clock, Ethernet PHY
• e.g. point to point bit stream
• Service Timing flows
– point to point bit stream
• e.g. PDH / TDM stream,
• Message Flows
– Based on packets
• e.g. Time & Frequency dedicated packet based 1588
• RTP flows to enable voice?
• NTP flows / TOD flows
Hybrid OSI Stack
Application
Transport
Network
Data Link
Physical
Hybrid OSI Stack
Application
Transport
Network
Data Link
Physical
RTP
1588
Network Clock
Service Clock
Note: This is not a
real E2E Circuit
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Layer 2 / 3 - IEEE1588
• Still evolving
– Excellent performance achievable
• Performance
– Fixed delay (symmetric / asymmetric) - very good
– Varying Delay (symmetric / asymmetric) – degraded
– Stabilisation period
• Security
– Access to the packet flows
• Identify
– Correct place in the architecture
– Function (Frequency / Time or both)
IEEE1588 No Impairments Measurements
1.00E-09
1.00E-08
1.00E-07
1.00E-06
0.1 1 10 100 1000 10000
Observation Time (s)
MT
IE (
s) X-Over
Test Network
Test Network + Monitor
PRC (G811)
IEEE1588 Impairment Measurements
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E-05
1.00E-04
0.1 1 10 100 1000 10000
Observation Time (s)
MT
IE (
s) AsymDelay10ms5msFixed
SymDelay3-6ms∆1msVar.
PRC (G811)
G823 (Table 2)
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Layer 3 - CE TDM Transfer
• TDM cct over corporate LAN
with adaptive clock recovery
• Impact of
– varying PDV
– varying load
– These are unknowns!
• High levels of low frequency
wander
• Will impact performance
– Buffer slips
– Service impact
MTIE for Adaptive with No Errors
1.00E-07
1.00E-06
1.00E-05
1.00E-04
0.1 1 10 100 1000 10000
Observation Time (s)
MT
IE
G823 PDH Sync
G823 PDH Traffic
1 Frame (lab-office)
2 Frame (lab-office)
4 Frame (lab-office)