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Assumptions for
for Sources
o
f
of T
ime
Time
Synchronization Error in IEEE
802.1AS
Rev 03
Geoffrey M. Garner
SAMSUNG Electronics (Consultant)
IEEE 802.1 AVB TG
2007.05.29
gmgarner@comcast.net
1
Introduction
This presentation
p
rovides
provides a
s
ummary
summary of
assumptions
pertaining
t
o
to s
ources
sources o
f
of
error in 802.1AS time synchronization
A network that satisfies these assumptions will be capable of meeting the desired
time synchronization accuracy of 1 μs over a maximum of 7 hops
It i i
s t
n
d
en
d
e th
th t
a
ft
, a er discussi
d
on an
diti
e
ng, these assumptions will b
i
e cop d
e t
o
the master list of AVB assumptions [1]
This work was requested in the April 30, 2007 AVB timing call, after an initial
discussion of sources of error based on [2]
SAMSUNG Electronics
IEEE 802.1 AVB May 2007
2
Assumptions Relevant to AVB Time Synch
Network d
iameter
diameter
Maximum diameter of any spanning tree of the network is 7 hops
•This includes end stations
–See avb-pannell-assum
p
ptions-0507-v5
p
for details and examples
p
Local oscillator quality
±100 ppm or better free-run accuracy
Rate for 100 Mbit/s Ethernet is nominally 25 MHz
Rate for GbE is nominally 25 MHz in some cases and nominally 125 MHz in some
cases
PTP clock quality
E d
n
i
-po t
n ti
h
me sync
i
ron
ti
za
f
on accuracy or st
d
ea y-st t
a
t
e opera i
tion i 1
s μ
b
s or
t
e t
tter
over 7 hops
•i.e., any 2 PTP clocks separated by at most 7 hops differ by no more than 1 μs
End-point time
p
synchronization accuracy during GM chan
g
ges is TBD
g
SAMSUNG Electronics
IEEE 802.1 AVB May 2007
3
Assumptions Relevant to AVB Time Synch
Assumptions on
error
s
ources
sources p
resent
present in
network
to
,
meet
the
above
t
ime
time
synchronization requirement for PTP clocks
Maximum frquency drift rate of local oscillator ≤ 1 ppm/s (this assumption, combined
with maximum frequency offset of ±100 ppm, results in maximum time
synchronization error
due to
this effect
of < 1 ns
ns (
see
(see [2]))
[2]))
Effect of frequency measurement granularity is negligible
•e.g., if 32 bits is used to express the measured frequency offset, the maximum
frequency error due to this effect is 2.3 × 10-10
SAMSUNG Electronics
IEEE 802.1 AVB May 2007
4
Assumptions Relevant to AVB Time Synch (Cont.)
Assumptions on
error
s
ources
sources p
resent
present in
network
to
,
meet
the
above
t
ime
time
synchronization requirement for PTP clocks (Cont.)
Effect of PHY latency asymmetry and phase measurement granularity for 100
Mbit/s Ethernet
•Any PHY latency asymmetry can be known as part of the design and
compensated for to within 18% of the maximum allowable PHY latency
•This means that of the allowable PHY latency asymmetry of IEEE 802.3 for
100BASE-X (
table
(table 24
24-3, plus
plus additional
16
ns; s
ee
see [2])
of 476
ns
per
hop, the
maximum remaining uncertainty after compensation is 86 ns/hop, or 602 ns for 7
hops
•The cumulative time synchronization error due to phase measurement
granularity over 7 hops is 280 ns (40 ns allowance per hop)
–This assumes that the variation of this error is sufficiently fast that, with a Sync interval
between 10 ms and 100 ms, the effect of this variation can be reduced by endpoint
filtering
•All the above error components, taken together, leave a margin relative to the
total 1 μs of approximately 111 ns (11%)
–i.e., (1000 ns) – (602 ns) – (280 ns) – (7 ns ) = 111 ns
SAMSUNG Electronics
IEEE 802.1 AVB May 2007
5
Assumptions Relevant to AVB Time Synch (Cont.)
Assumptions on
error
s
ources
sources p
resent
present in
network
to
,
meet
the
above
t
ime
time
synchronization requirement for PTP clocks (Cont.)
Effect of PHY latency asymmetry and phase measurement granularity for GbE,
assuming a 25 MHz nominal frequency for the local oscillator
•Any PHY latency asymmetry can be known as part of the design and
compensated for to within 25% of the maximum allowable PHY latency
•This means that of the allowable PHY latency asymmetry of IEEE 802.3 for
100BASE-X (
table
(table 40
40-14, plus additional
16
ns; s
ee
see [2])
of 344 ns
per
hop, the
maximum remaining uncertainty after compensation is 86 ns/hop, or 602 ns for 7
hops
•The cumulative time synchronization error due to phase measurement
granularity over 7 hops is 280 ns (40 ns allowance per hop)
–This assumes that the variation of this error is sufficiently fast that, with a Sync interval
between 10 ms and 100 ms, the effect of this variation can be reduced by endpoint
filtering
•All the above error components, taken together, leave a margin relative to the
total 1 μs of approximately 111 ns (11%)
–i.e., (1000 ns) – (602 ns) – (280 ns) – (7 ns ) = 111 ns
SAMSUNG Electronics
IEEE 802.1 AVB May 2007
6
Assumptions Relevant to AVB Time Synch (Cont.)
Assumptions on
error
s
ources
sources p
resent
present in
network
to
,
meet
the
above
t
ime
time
synchronization requirement for PTP clocks (Cont.)
Effect of PHY latency asymmetry and phase measurement granularity for GbE,
assuming a 125 MHz nominal frequency for the local oscillator
•Any PHY latency asymmetry can be known as part of the design and
compensated for to within 35% of the maximum allowable PHY latency
•This means that of the allowable PHY latency asymmetry of IEEE 802.3 for
100BASE-X (
table
(table 40
40-14, plus additional
16
ns; s
ee
see [2])
of 344 ns
per
hop, the
maximum remaining uncertainty after compensation is 120 ns/hop, or 840 ns for
7 hops
•The cumulative time synchronization error due to phase measurement
granularity over 7 hops is 56 ns (8 ns allowance per hop)
–This assumes that the variation of this error is sufficiently fast that, with a Sync interval
between 10 ms and 100 ms, the effect of this variation can be reduced by endpoint
filtering
•All the above error components, taken together, leave a margin relative to the
total 1 μs of approximately 97 ns (10%)
–i.e., (1000 ns) – (840 ns) – (56 ns) – (7 ns ) = 97 ns
SAMSUNG Electronics
IEEE 802.1 AVB May 2007
7
References
1. Don P
annell
Pannell Audio/Video
,
Bridging
(AVB)
Assumptions
IEEE
,
802 1
. AVB
Conference Call, April 18, 2007 (available at
http://www.ieee802.org/1/files/public/docs2007/avb-pannell-assumptions-
0407-v4.pdf).
2. Geoff
ff
M
rey . G
S
arner, ources of Ti
S
me ynchronizati
ti
E
on
i
rror
I
n EEE
IEEE
802.1AS, April 29, 2007 (available at
http://www.ieee802.org/1/files/public/docs2007/as-garner-error-sources-
time-synch-0407.pdf).
SAMSUNG Electronics
IEEE 802.1 AVB May 2007
8
