Day3 MAC Time Protocol

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DOCSIS 3.1
MAC SUBLAYER
DTP – DOCSIS Time Protocol
John Chapman
Fellow, CTO Cable BU
Cisco Corporation
© 2014 CableLabs®. All rights reserved.
DTP Introduction
What is DTP trying to achieve?
• Goal
– To provide precise frequency and time to an external
system that is connected to the network port of a DOCSIS
CM.
– Running timing protocols over the top of DOCSIS adds
timing error due to upstream scheduling jitter and
asymmetrical delay.
– DTP allows conversion to and from other timing protocols.
• Application
– Wireless/cellular backhaul of pico, femto & macrocells over
DOCSIS.
– Any service requiring precision timing.
© 2014 CableLabs®. All rights reserved.
Time Protocols
A Brief History
© 2014 CableLabs®. All rights reserved.
Timing Protocols
A brief history
Name
Standard
Typical
Accuracy
Signaling
Server - Client
Comments
Time
Protocol
RFC 868
1983
seconds
Req time
Send time
Used for ToD in DOCSIS
Extremely basic.
NTP v4
RFC 5905
1985-2010
1 – 100 ms
 Delay Req
 Delay Rsp
Depends upon network topology.
OTT system.
32/64/128 bit timestamp
PTP v2
IEEE 1588
2002-2008
1 – 100 us
 Sync
 Delay Req
 Delay Rsp
Modifies nodes in networks to
achieve precision.
64 bit timestamp.
SyncE
G.8261
0 ppm delta
ESMC
Locked to source clock.
Initially HW only. SONET-like.
1 PPS
Evolving
Not a true protocol
(yet)
HW Pulse marks a describable
event.
© 2014 CableLabs®. All rights reserved.
Synchronous Ethernet
A hardware centric design
Frequency Synchronization
Ethernet
Network
OC-X
STM-X
SONET/SDH
Network
Hybrid
Hybrid
EEC
T1
Engineered
Timing Path
• Initially for frequency only.
– Follows a Stratum clock hierarchy.
• Extended with ESMC (Ethernet Synchronization Message Channel)
– SyncE/ESMC are intended to replace SONET/SSM
© 2014 CableLabs®. All rights reserved.
IEEE-1588 Precision Timing Protocol (PTP)
A software centric design with hardware support
• PTP is a message based SYNC and two-way time transfer (TWTT)
protocol for synchronizing distributed nodes in time and frequency.
• PTP defines a hardware and software method for passing clocking
through a network node.
– Timestamps are placed in an Ethernet packet and replaced or corrected as
they pass through a node.
– If a network node can be made to look like a constant delay, and since the
network link is usually constant delay, then the overall network can have
predictable and precise timing coupled through it.
© 2014 CableLabs®. All rights reserved.
PTP Clock Types
Definitions for all network scenarios
• PTP defines various clock types
GM:
Grandmaster Clock
GM is the ultimate source of time in a
PTP domain
Clocking is terminated (Slave) and
regenerated (Master) at a network
node.
BC:
Boundary Clock
TC:
Transparent Clock
Clocking passes through a network
node and receives a correction factor.
OC:
Ordinary Clock
End node clock
© 2014 CableLabs®. All rights reserved.
Frequency Synchronization
Time Synchronization
c & sp
n
y
s y_re
a
del
sync &
delay_resp
S
SyncE
SyncE
OC
GPS
GM
S
M
de
M
la
y_
re
q
Non-Participant
Nodes
M
Non-Participant
Nodes
sync +
corrA
TC
TC
TC
GM : Grand Master
BC : Boundary Clock
OC : Ordinary Clock
TC : Transparent Clock
M : Master port
S : Slave port
a y_
syn c
_req
delay +A
rrB
+ co
delay_req
del
BC
res
p
sync +
corrA+B
TC
TC
TC
delay_req
+ corrB
S
OC
delay_req
© 2014in
CableLabs®.
All rights reserved.
Note: TC is often used end-to-end
a network.
DTP Network Protocol Support
DOCSIS 3.1 can interface with 1588 and SyncE
IEEE-1588 (PTP)
• CMTS/CM can be a Grandmaster Clock (GC)
– Frequency and time derived from CMTS. PTP originated in CM.
• CMTS/CM can be a Boundary Clock (BC)
– CMTS syncs to PTP. CM syncs to CMTS. BC is regenerated
• CMTS/CM can be a Transparent Clock (TC)
– CMTS and CM update correction fields in PTP messages
Synchronous Ethernet
• CM derives Ethernet Frequency from OFDM Baud clock.
© 2014 CableLabs®. All rights reserved.
DTP
DOCSIS Time Protocol
IEEE-1588, SyncE, NTP and DOCSIS
as an Integrated System
© 2014 CableLabs®. All rights reserved.
DTP Scope
CMTS
Clock
CM
Clock
DTP
IEEE-1588
IEEE-1588
SYNC-E
SYNC-E
DTI
NTP
NTP
GPS
•
•
CMTS synchronizes DOCSIS
to a network source.
DTP adds support to DOCSIS
to support protocol
conversion.
•
•
CM generates precision
timing using DOCSIS timing
to support network protocols
DOCSIS latency and
asymmetry are measured and
compensated for by DTP.
© 2014 CableLabs®. All rights reserved.
Timestamp - The Concept of EPOCH
" PTP typically uses the same epoch as Unix time
(Midnight, 1 January 1970).
Whereas Unix time is based on Coordinated
Universal Time (UTC) and is subject to leap
seconds, PTP is based on International Atomic
Time (TAI) and moves forward monotonically.
The PTP grandmaster communicates the current
offset between UTC and TAI so that UTC can be
computed from the received PTP time.”
- Wikipedia
•
•
•
The first thing that is needed is a
more accurate timestamp.
EPOCH anchors a timestamp to
time of day.
DOCSIS 3.1:
– Uses a 64 bit timestamp
– Uses EPOCH of Midnight, 1 Jan 1970
– Uses TAI monotonic counting method
•
This allows for easier conversion
between D3.1 and IEEE-1588.
© 2014 CableLabs®. All rights reserved.
DOCSIS 3.1 Timestamp
D3.0 timestamp is contained in the D3.1 timestamp
8 bytes
23 bits
32 bits
Epoch
112 years
D3.0 Timestamp
7 min
97.66 ns
10.24 MHz
5 bits
4 bits
÷ 20
÷ 16
4.88 ns
204.8 MHz
with roll-over
305 ps
204.8 MHz x 16
10.24 MHz x 320
Comparison
Size
Precision
Max Time
DOCSIS 3.0
32 bits
97.6562 ns
7 min
DOCSIS 3.1
64 bits
305 ps
112 years
IEEE-1588
64 bits
1 ns
584 years
NTP
64 bits
232 ps
136 years
© 2014 CableLabs®. All rights reserved.
True Ranging Offset
•
•
•
•
DTP defines a True Ranging Offset (TRO)
The TRO is the measured ranging offset of the CM between two defined reference
points.
1. This value will be the same for all CMs
2. This value equals the round trip delay of the HFC plant.
The TRO is measured at the CM between:
A. the time the first bit of a packet is transmitted in the upstream from the CM,
and
B. the time the first bit of a packet is expected to arrive at the CMTS (MAP entry)
Since the measurement is done when an upstream packet is transmitted, (after
buffering), all upstream scheduling jitter is eliminated.
© 2014 CableLabs®. All rights reserved.
PTP and DTP
System View
CMTS
CMTS
Clock
CM
CM
Clock
DTP
PTP
NSI/DTI
PHY
Buffering
Switching
Buffering
DOCSIS
PHY
HFC
Plant
DOCSIS
PHY
Buffering
Switching
Buffering
CMCI
PHY
PTP
Legend:
green = fixed delay path
blue = variable delay path
• IEEE-1588 (PTP) runs external to CMTS/CM
• DOCSIS DTP runs internal to CMTS/CM
• Timing is hardware coupled to the device edges
Slide 15
© 2014 CableLabs®. All rights reserved.
IEEE-1588 Syncing
The tricks of the trade
• All timing protocols use a variant of TWTT
– Two-Way Time Transfer
• A timestamp is sent from A to B
• Messaging is used to determine the network
delay and asymmetry.
• Timestamp B is corrected to match timestamp
A.
• DTP has this information built into DOCSIS
Slide 16
© 2014 CableLabs®. All rights reserved.
DTP True Ranging Offset
TWTT is achieved by measuring the result of DOCSIS Ranging
• The TRO is the measured ranging offset of the CM between two defined
reference points.
– TRO is a measured (or derived) value that is different than the actual
implemented ranging offset a CM might use in its communication with
the CMTS.
• The value of TRO is the equivalent to the round trip delay of the combined
downstream and upstream propagation delays of the HFC plant, the
CMTS and CM PHY paths.
• Since the measurement is done between the downstream clock path and
when an upstream packet is transmitted (after buffering), all jitter and
delay from internal packet queues are eliminated from the measurement.
Slide 17
© 2014 CableLabs®. All rights reserved.
DTP Timing Model
Generic Model for CMTS, CM, and HFC Plant
DTP Calibration System
CMTS
HFC Plant
CM
CMTS
Clock
ts-cmts-in
t-cmtsds-i
CM
Clock
t-cmtsds-o
t-cmtsds-p
t-hfcds-o
t-hfcds-p
t-cmds-o
t-cmds-p
t-cmtsus-o
t-cmtsus-p
t-hfcus-o
t-hfcus-p
t-cmus-o
t-cmus-p
t-cmds-i
t-cmadj
ts-cm-out
t-tro
• The accuracy of the system depends upon the accuracy of
the parameters in the model.
Slide 18
© 2014 CableLabs®. All rights reserved.
PTP
Timestamp
PTP
Timestamp
Sync &
Conversion
(t-cmts-ds-i = 0)
Sync &
Conversion
(t-cm-adj = 1800)
(t-cm-ds-i = 0)
CMTS
Clock
CMTS
PHY
CM
PHY
CM
Clock
DOCSIS RANGING, DTP Signaling
2000
t-cmtst-cmtsds-o
ds-p
500
25
SYNC
t-hfcds-p
750
t-hfcds-o
0
t-cmds-o
25
500
3800
4000
500
750
0
25
5800
6100
RNG
REQ
20
t-cmtsus-o
8800
2000
2200
25
MAP
(Tx=7000)
7000
200
t-cmds-p
30
t-cm
-us-p
750
t-hfcus-p
50
t-hfcus-o
30
t-cmus-p
500
20
4000
4300
t-cmus-o
5200
7000
True Ranging Offset
(TRO) Example
TRO Round Trip Delay
Measured Between Reference Points
500
✔ known
+ 25
✔ characterized
+ 750
? to be measured
+ 25
✔ characterized
+ 500
✔ known
+ 50
✔ characterized
7000 + 800
? to be measured
measured
transmit
- 4300
+ 50
✔ characterized
time
====== ====
True
2700
=
Ranging
Offset
2700
MAP
entry
Offset needed = 2000 – 200 = 1800
© 2014 CableLabs®. All rights reserved.
CMTS
Timetamp
Reference
CMTS
PHY
CM
PHY
CM
Timestamp
Reference
Calculating Latency
DOCSIS RANGING
2000
500
25
Ti/2
TRO = known delays + unknown delays
200
SYNC
3800
TDS-HFC
25
TDS-CM
4000
500
CMTS
CM
750
TDS-CMTS
25
500
HFC
Plant
2000
Ti/2
MAP (Tx=7000)
2200
2700 – (500+25+25+500+50+50) = 1550
4000
DS HFC Plant Latency
= 1550 / 2 * (known plant asymmetry) = 750
750
25
5800
6100
RNG REQ
500
4300
50
800
50
7000
TUS-HFC
TDS-CM
5200
TUS-CMTS
8800
Where:
- All values are in arbitrary time units for sake of example.
7000
True
Ranging
Offset
Total DS Latency
= 500+25+750+25+500 = 1800
Corrected CM Timestamp
= 7000 + 1800 = 8800
© 2014 CableLabs®. All rights reserved.
DTP Signaling
CMTS is DTP Master
• Used for generic CM with CMTS centric design. Larger scale.
• Note the extra INFO message. Unique.
Slide 21
© 2014 CableLabs®. All rights reserved.
DTP Signaling
CM is DTP Master
• Used for higher end CM centric designs
• CMTS can still override specific parameters
Slide 22
© 2014 CableLabs®. All rights reserved.
Multi-System
Designing across systems
CMTS 1
T-cmts-error
Timing
Protocol
Source
HFC 1
T-hfc-error
CM 1
Timing Protocol
T-cm-error
T-source-skew
T-cm-cm-skew
T-cmts-error
CMTS 2
T-hfc-error
HFC 2
T-cm-error
CM 2
Timing Protocol
• 1588 timing needs
consistency
across systems.
• DTP defines five
levels of
performance
between two
systems.
Slide 23
© 2014 CableLabs®. All rights reserved.
DTP Recap
• CMTS synchronizes DOCSIS to a network source.
– DTI, IEEE-1588v2, etc
• DTP manages DOCSIS latency and asymmetry.
• CM generates precision timing using DOCSIS timing
• Accuracy is dependent upon accurate modeling of
CMTS, CM, and HFC plant.
– Target accuracy of better than a few μseconds
© 2014 CableLabs®. All rights reserved.
DTP Summary
It’s about time.
• The DOCSIS system is already based
upon highly precise timing.
– DTP leverages this asset.
• Rather than run NTP or PTP over-thetop, the DOCSIS system can be
modified to generate or correct these
timing protocols with a very high
degree of precision.
© 2014 CableLabs®. All rights reserved.
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