Download Distributing Phase—with Sub-Microsecond Accuracy—Unaffected

Distributing Phase—with Sub-Microsecond
Accuracy—Unaffected by PDV
ITSF 2008 Munich
Kenneth Hann
Principal Engineer
Copyright Tellabs Inc
Phase Synchronization overview
“t b
“to
boldly
ldl sync …””
Agenda:
g
1. Background information on Phase synchronization
2. Goal – Ubiquitous availability of Phase
3. One example application needing Phase
4. Distributing Phase – ideally common for
> Local distribution
> Node distribution
> Network distribution
5. PPS over Ethernet
6 Phase synchronous Ethernet
6.
7. Service phase and Network phase
8 Extras (leftovers)
8.
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November 10, 2008
Background
Frequency Time and Phase
Frequency,
ƒ Frequency
q
y
> TAI International Atomic Time (Temps Atomique International)
> weighted average from about 300 atomic clocks in national laboratories worldwide.
> Caesium ticks at
9,192,631,770 hertz
ƒ Time
> Count of ticks from an epoc [ + leap seconds + …]
> Based upon TAI
> E.g. SAT 01NOV2008 HELSINKI FI MUNICH DE 0920 Local time (UTC+3)
ƒ Phase
> Count of frequency over a period of time
> Pulse Per Second is most common measure of phase
> The “LSB of TIME”
> I.e. In the case of IEEE1588
Seconds
Nano Seconds
3B
00
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November 10, 2008
9A
00
C9
00
FF
00
Pulse per second (PPS) Signal Interfacing
Pulse-per-second
“Alice is trying to find the PPS signal connector”.
from Professor Dave Mills webpage on NTP
Now PPS connectors are not often seen
Especially on backplanes there're far, far between.
Dr.Zeuss
euss
ƒ PPS support can also be hard to find on:
> IP/MPLS switches,
> Telecoms measurement equipment,
> NPUs, ASICS etc…
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November 10, 2008
Phase synchronization – who needs it?
ƒ Lets have a q
quick overview of the transport
p
network…
> Frequency synchronization – long distributed via transport network
> How could phase be delivered by the transport network?
ƒ Applications:
> One Way delay (e.g. for SLA monitoring )
> Mobile – MBMS,
MBMS TDD,
TDD Single frequency,
frequency CDMA,
CDMA WIMAX networks
networks…
ƒ GPS is not always available/ desirable/ practical
ƒ “I want my own phase” (service phase) supplied over the network.
(Synchronous Residual Time Stamp for Phase)
Question - Can PPS at 1us accuracy be delivered ubiquitously across a transport
network without being impacted by PDV?
5
November 10, 2008
”Evolution” of Transport network synchronization
(from internal operation to a service)
Every network consists of nodes connected by links
Links are synchronous (SDH, PDH, Ethernet, Base-band modems…)
1. Let Clocks appear and synchronize the nodes!
2. Packet rebellion - Down with the clocks!
3. Client outcry – the clocks are back (G.826x)
4.
4 Clocks transformed – support for Phase
Synchronization has become a service of the transport network!
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November 10, 2008
Synchronization flow
Processing options within a node
Sync flow
Sync flow
1. Terminate Flow; Synchronize node; Regenerate flow
2. Process on-the-fly
on the fly E.g. IEEE1588 with Transparent clock.
3. Transparently forwarded to egress (as data)
Requirement for 1us means the sync. flow is processed in EVERY node
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November 10, 2008
Example: accurate OWD measurement Node A to Node B
Target accuracy (+/
(+/- 100usecs)
Remaining Problems (no solution):
Could Use GPS…
Could
use 1588 Clients
1 Asymmetrical routing
1.
But no PPS inputs!
Yes
IF
2. PDV dragon the Node (Agents) can be phase synchronized?
Accuracy +/- 5 milliseconds – oops!
PPS
1588 GMC
(GPS)
1588
PPS
(GPS)
PPS
NTP
1588
NTP
network
1588
Monitoring Client
Monitoring Agent
Node A
8
(GPS)
Node B
November 10, 2008
Phase synchronization across a network…
Actually starts in the first node
ƒ Not application specific
> Clock for PLT
> Clock for X & Y…
Node Phase Clock
PPS
S (I/O)
Card #n
Card #1
NTP
client
(sw)
Node
1588
client
Ethernet
9
…
PLT
(SAA)
Measure
Ethernet
November 10, 2008
ƒ Add node phase clock
> Similar to frequency
> Minimal cost delta
Packet Loop Tester
Service Assurance Agent
IEEE1588 is just a
(small) part of the puzzle
Let’s Synchronize our watches…
I e main points to date…
I.e.
date
ƒ Client Applications exist that require phase synchronization.
ƒ Market interest in using transport network provide a synchronization service
ƒ State of readiness in “datacoms” boxes is low, but Telecoms NGN…
ƒ Nodes require:
> Phase input output ports (PPS);
> Phase transfer within node
> Phase clock – which is the basis for…
> Synchronization flow processing
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November 10, 2008
Let’s look at a transport network
Nodes + Links = Topology
ƒ “End-to-End” Versus “link-by-link”
> Packet Delay Variation – depends on bit-rate, queuing, shaping, QOS…
> Accurate p
phase synchronization
y
requires
q
p
processing
g in nodes
ƒ Properties of links
> Links
are
usuallyy glass
sa
e usua
g ass fibre
be
> Probably carrying Ethernet directly (includes also WDM)
Remember - Transport nodes simple (Client nodes can be more) complex
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November 10, 2008
IEEE1588 overlay for link-by link phase synchronization
Transport Network
Master
Slave
ƒ Discovery – for a point-to-point link?
ƒ Best Master Clock selection – multiple masters?
ƒ Addressing – to find the other end?
ƒ Topology – another one?
ƒ Options – for protocol, protocol encapsulations, Timescales, message types….
Simpler solution needed for Transport networks!
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November 10, 2008
Comparison between IEEE1588 and PSE
For synchronizing a transport network
Phase Synchronous Ethernet
IEEE 1588
Topology
Follows existing (SSM) topology
New topology
Addressing
Implicit – same as for Frequency
(delta is 0)
IP uncast/multicast or
Mac unicast/multicast
Message size
4 byte timestamp
9 message types;
y
minimum 54 bytes
Encapsulation
TLV of existing ESMC (SSM)
PTP/UDP/IP/Ethernet or
PTP/Ethernet
M
Management
t
D lt for
Delta
f phase
h
“On
“O or Off”
PTP managementt messages
(clock Id; clock domain;
Port Id; clock selection
algorithm; timescale …))
Modelled on
WAN (Synchronous Ethernet)
LAN
Physical interface
(for phase input/output)
PPS
PPS
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November 10, 2008
How to transmit “PPS” over Ethernet
ƒ Classic 4 timestamps
p approach
pp
> Principle as per NTP, IEEE1588 etc.
ƒ extremely simple
> A Single 4 byte value exchange
ƒ Runs over existing Ethernet protocol
(E.g. Ethernet Synchronization Message Channel)
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November 10, 2008
Classic 4 timestamps – optimized two-way algorithm
4 bytes per direction (per second)
Slave
Synchronous
y
Ethernet
Master
Timestamp
=t1
t1
t2-t1
t2
t3
t4
Timestamp
=t2-t1+t3
t2-t1+t3-t4
Clock offset = (t2-t1+t3-t4)/2
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November 10, 2008
Phase Synchronous Ethernet PSE
Identical topology to Synchronous Ethernet
Phase Distribution
(ESMC)
PRC+
Clock Distribution
(Synchronous link)
Pulse Per Second
(PPS)
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November 10, 2008
Service phase – transported over Network phase as a difference
IEEE1588 used in Client network (transparent to transport network)
Service Phase
(1588)
Client
Network
PRC
Service
Phase (PPS)
Service
S
i Phase
Ph
(1588)
Client
Phase Synchronous
Ethernet Transport
network
Client
Client
Differential Time of Day
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11/10/2008
Service Phase
(1588)
Service Phase
(1588)
Service phase – transported over Network phase as a difference
IEEE1588 used in Client network (transparent to transport network)
Correction Field
1588 v2 Sync message
ToD
Ps
ToD
Client
#1
Ps
X - Pn1
00
00
Pn
00
X
Client #2
Ps
ToD
00
00
PSE
00
00
X–Pn1+Pn2
00
Pn
Ps Service Phase
Pn Network Phase
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11/10/2008
Differential Time of Day (Diff ToD)
ƒ Same approach
pp
as SRTS uses for frequency
q
y
> (carries service clock as a difference to Network clock)
ƒ IEEE1588 features can be put to good use…
especially the Correction field
> Adds the phase difference of the Service phase to the Network phase.
ƒ Advantages
> Simple for transport network
> Flexible for Client network
> Clear demarcation of transport and Client networks
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November 10, 2008
Summary
1.
PSE provides 1PPS network phase alignment across transport infrastructure
>
No PDV dragons,
>
No Asymmetric synchronization topologies
>
Very Small delta to existing frequency Synchronization Management
2
2.
Each End user can have autonomous Service phase and ToD
ToD.
>
ToD layer management requires no transport operator involvement
>
Ongoing Proposal to ITU-T SG15 Q13 (Telecom Profile Differential ToD method)
3.
IEEE1588 is an integral part of Diff ToD.
Phase Synchronous Ethernet + Diff ToD form the simplest mechanism for offering
a synchronization service via a telecoms network
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November 10, 2008
Thank You
Kenneth Hann
Principal Engineer
Copyright Tellabs Inc
TSE Management Interface –
Info Model specification
ƒ Add e.g.
g the following
g attribute to the ethrSyncTTPBid
y
MO:
ethrSyncTTPBid::ethTseMode
Description:
Time Synchronous Ethernet (TSE) operating mode.
Syntax:
TseMode
0 = disabled
(= default)
1 = master
2 = slave
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November 10, 2008
Copyright Tellabs Inc
Other PPS over Ethernet Implementation techniques.
Ethernet is synchronous - one off those
th
8ns
8 spaced
d pulses
l
is
i the
th PPS
1.
The ”pulse event” can be sent at the specified GPS pulse time
>
>
>
2.
The ”pulse
pulse event
event” is sent at or after the specified GPS pulse time
>
3.
Clock phase jump. (See experimental results)
by scheduling the link such that the line is kept clear at the transmit instance (flow control the
Mac).
Transmitting a known symbol,
symbol or symbol sequence
(IEEE 803.2 hasn’t defined a PPS symbol)
correction value allows correction, by the number of phy clock cycles the pulse is delayed.
Offset (in phy clock cycles) until next second
>
sent continuously, say in IFG
All Require Media Delay compensation (Round Trip Time) measurement
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November 10, 2008
Experimental accuracy achieved with phase pulse detection
DLL shifter
0,1710030000
Moving Average
Average,wnd
wnd size 16,sour
16 sour col.B
col B
Actual
forecast
0,1710020000
Phase detector
0,1710010000
0 1710000000
0,1710000000
0,1709990000
0,1709980000
Phase pulses to Tx DLL
0,1709970000
1
24 47 70 93 116 139 162 185 208 231
30ns accuracy (after averaging) using FPGA boards
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November 10, 2008
Measurement options using Round Trip Time
1. Direct Round
Round-Trip
Trip Time
2. Round-trip Trip with “prepare”
(e g 802
(e.g.
802.3ah
3ah intrusive loopback)
3. Round-trip Time with “residence”
Master
Slave
Prepare for RTT
T=0
T=0
T
0
TRTT = T
TRTT = T - T Residence
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November 10, 2008
T Residence
T Residence