White Paper
Limiting IEEE 1588 Slave Clock Wander Caused by Packet Delay Variation
Michael Rupert, Principle System Design Engineer, Data Center Business Division

Abstract
This white paper compares two IEEE 1588 slave clock implementations for effectiveness in limiting wander
caused by packet delay variation (PDV). The implementations are:
An IEEE 1588 slave clock that includes a PDV filter in the servo loop



An IEEE 1588 slave clock that includes a low-pass filter in the servo loop

Introduction
IEEE 1588 defines the Precision Time Protocol (PTP) that enables accurate synchronization over packet
switched networks (PSN). For telecom frequency synchronization applications, PTP slave clocks must meet
ITU-T G.8261 network limits for clock wander.
The PTP telecom profile for frequency synchronization (ITU-T G.8265.1) assumes that PTP masters and PTP
slaves are used in PSNs that are not designed to support PTP. These PTP unaware networks can impose highpacket delay variation (PDV) on PTP packets. Because PDV is a cause of wander on PTP slave clocks, PTP
slave clock servos must be PDV tolerant if G.8261 wander limits are to be met.
This white paper compares two PTP slave clock implementations for effectiveness in limiting wander caused byPDV; the implementations are (1) a PTP slave clock that includes a PDV filter in the servo loop and (2) a PTP
slave clock that includes a low-pass filter in the servo loop. The PTP slave clock with the PDV filter performs
best with peak wander nearly 60 times lower than the PTP slave clock with a low-pass filter. Bothimplementations use the same local oscillator.
The Renesas PTP Clock Manager software includes a PTP clock servo with a PDV filter and other functionalitythat meets global ITU-T synchronization recommendations for telecom applications. The PTP Clock Manager isavailable under license from Renesas for use with Renesas timing products., and can be used with any IEEE
1588 protocol stack.

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Limiting IEEE 1588 Slave Clock Wander Caused by Packet Delay Variation White Paper

Contents
Abstract .................................................................................................................................................................. 1
Introduction ............................................................................................................................................................ 1
Contents ................................................................................................................................................................. 2
Definitions .............................................................................................................................................................. 3
Clock ................................................................................................................................................................ 3
Clock Signal ..................................................................................................................................................... 3
Jitter ................................................................................................................................................................. 3
Wander ............................................................................................................................................................ 3
MTIE ................................................................................................................................................................ 3
Synchronization Standards .................................................................................................................................. 3
IEEE 1588 - Precision Time Protocol .............................................................................................................. 3
ITU-T G.8261 - Timing and Synchronization Aspects in Packet Networks ..................................................... 3
ITU-T G.8265.1 - PTP Telecom Profile for Frequency Synchronization ......................................................... 3
ITU-T G.8275.2 - PTP Telecom Profile for Phase/Time Synchronization with Partial Timing Supportfrom the Network.............................................................................................................................................. 3
Basic Synchronization for a PTP Slave Clock .................................................................................................... 4Packet Delay Variation .......................................................................................................................................... 5
PDV Mitigation by the Network ............................................................................................................................ 5
Limiting PTP Slave Clock Wander with a PDV Filter .......................................................................................... 5
Limiting PTP Slave Clock Wander with a Low-Pass Filter ................................................................................ 6Conclusion ............................................................................................................................................................. 9Notes ....................................................................................................................................................................... 9
Revision History .................................................................................................................................................... 9

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Definitions
ClockIEEE 1588 defines a clock as follows: A device that can provide a measurement of the passage of time since a
defined epoch.

Clock Signal
IEEE 1588 defines a clock signal as follows: A physical signal that has periodic events. The periodic events markthe significant instants at which a time counter is incremented. The clock signal is characterized by its frequency
and phase.

Jitter
For telecom applications, phase noise with frequencies of 10Hz and above is considered jitter.

Wander
For telecom applications, phase noise with frequencies below 10Hz is considered wander.

MTIE
Maximum time interval error (MTIE) charts display the largest phase change accumulated by a clock during anyof a set of equal observation intervals within a measurement period. MTIE charts typically display MTIE on thevertical axis versus observation intervals on the horizontal axis. MTIE is often used to specify limits for wander
and phase transients. ITU-T G.8100 includes a formal definition of MTIE.

Synchronization Standards
IEEE 1588 - Precision Time Protocol
IEEE 15880 defines a protocol, the Precision Time Protocol (PTP), that enables accurate synchronization overpacket switched networks (PSN). IEEE 1588 provides for profiles that fit the needs of specific industries orapplications and support interoperability among devices using the same profile. IEEE 1588 does not specify
synchronization performance requirements, and it does not specify PTP slave clock architectures.

ITU-T G.8261 - Timing and Synchronization Aspects in Packet Networks
ITU-T G.82610 defines frequency synchronization aspects in packet networks including the network limits forwander at telecom synchronization interfaces. The network limits apply to synchronization interfaces whether
they are derived from the physical layer (for example, from synchronous Ethernet) or from PTP over a PSN.

ITU-T G.8265.1 - PTP Telecom Profile for Frequency Synchronization
ITU-T G.8265.10 defines the PTP telecom profile for frequency synchronization and specifies the required IEEE
1588 functions that ensure network element interoperability for the delivery of frequency. This profile addresses
the case where the PTP masters and slaves will be used in networks where there is no PTP support in any
intermediate node between the PTP master and the PTP slave.

ITU-T G.8275.2 - PTP Telecom Profile for Phase/Time Synchronization with Partial
Timing Support from the Network
ITU-T G.8275.20 defines the PTP telecom profile for phase/time synchronization with partial timing support fromthe network. This telecom profile is out of scope for this whitepaper, but it is mentioned because it covers
applications that can benefit from packet delay variation (PDV) filtering. Networks intended for G.8275.2 should
exhibit lower levels of PDV than networks intended for G.8265.1.

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Basic Synchronization for a PTP Slave Clock
Figure 1 shows the basic PTP timing message exchange between a PTP master and a PTP slave; also shown isa PTP slave clock implemented as a servo loop with a clock servo controller (clock servo). Under ideal
conditions, the clock servo can calculate the time offset of its local time stamp unit (TSU) versus the TSU of themaster using Equation 1. The time offset is digitally compensated by the clock servo; afterwards, the frequencyof the PTP slave clock signal is periodically adjusted to keep the time offset near zero. Therefore, the PTP slave
clock is synchronized and locked to the PTP master clock.
Equation 1.

𝑷𝑻𝑷 𝑺𝒍𝒂𝒗𝒆 𝑪𝒍𝒐𝒄𝒌 𝑻𝒊𝒎𝒆 𝑶𝒇𝒇𝒔𝒆𝒕 =(𝒕𝟐 −𝒕𝟏 )−(𝒕𝟒 −𝒕𝟑 )𝟐

PDV gives rise to phase noise, jitter/wander, on PTP slave clocks. Equation 1 is accurate if the packet delay(transmission time) of the Sync packet is equal to the packet delay of the associated Delay_Req packet 0. If thetwo packet delays differ, then the calculated time offset will include an error equal to half the difference. If theSync packet and Delay_Req packet delays change with respect to each other over time, then the PTP slave
clock signal will exhibit jitter/wander in response.

MasterClockTime

PTP Master ClockSlaveClockTime

PTP Slave Clockt1
Clock Signal

t3
t4

TSU = Time Stamp Unit

ClockCorrection

ServoLoopt2

TSU

Clock ServoController

t1, t2 , t3, t4TSU

Clock Signal

Clock
Synthesizer

The TSUs measure the passage of time bycounting cycles of a clock signal of known
nominal frequency.
t1, t2 , t3, t4 are measured by the TSUs and are
known to the PTP slave clock; they are used tocalculate the PTP slave clock time offset.

Figure 1. Basic Precision Time Protocol Timing Packet Exchange

Figure 2. PTP over a Packet Switched Network

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Packet Delay Variation
Figure 2 illustrates a PSN with multiple switching nodes. PTP packets traversing such a network are subject todelays at each switching node; the total packet delay varies from packet to packet depending on the path taken
through the network and on the direction of transmission (forward or reverse) 0.
A sequence of PTP packets transmitted over a PSN exhibit PDV that depends on the conditions encountered byeach packet in the switching nodes and in the overall network. A partial list of causes of PDV in a switching node
includes processing, statistical multiplexing, buffering, port contention, and dropped packets. A partial list ofcauses of PDV for a network includes route diversity and misordered packets.
G.8261 defines seventeen test-cases that model PSN conditions and associated PDV. These test cases areintended to assist in the acquisition of performance characteristics used to establish benchmarking results. Testcases 12 to 17 are intended for two-way protocols like IEEE 1588. The performance test topology for two-way
protocols includes a chain of ten switching nodes between the PTP master clock and the PTP slave clock withtraffic generators injecting interfering traffic in the forward and reverse directions.
Table 1. G.8261 Test-Cases TC-12 to TC-17
Test-Case

Network Conditions Modeled by Test-Case

TC-12"Static" packet load

TC-13

Sudden large, and persistent, changes in network load

TC-14

Slow change in network load over an extremely long (24 hour) period

TC-15

Temporary network outages and restoration for varying periods

TC-16

Temporary network congestion and restoration for varying periods

TC-17

Routing changes caused by failures in the network

Figure 4 and Figure 5 show packet delay measurements taken at the inputs to a PTP slave clock and a PTP
master clock, respectively, during a concatenated sequence of G.8261 test-cases. Traffic Model 2 and a packet
rate of sixteen packets per second were used, and the test sequence required approximately 34.7 hours tocomplete.
The vertical axis shows the time interval error (TIE) in nanoseconds above the floor delay measured for eachpacket, and the horizontal axis shows the time since the start of the test in days and hours. The charts illustrate
the delay conditions during each test-case and allow comparison of the simultaneous forward and reverse
delays during the test sequence.

PDV Mitigation by the Network
PDV can be mitigated in a PSN by deploying boundary clocks or transparent clocks at switching nodes; ideally,
this is done at every switching node that PTP packets cross. However, boundary clocks and transparent clocks
are out-of-scope for G.8265.1, and they are not used for the telecom profile for frequency synchronization.

Limiting PTP Slave Clock Wander with a PDV Filter
PTP slave clock wander due to PDV can be limited by including a PDV filter in the servo loop. The PDV filter
enables a PTP slave clock, using a low-cost temperature compensated crystal oscillator (TCXO), to generateclock signals that meet ITU-T network wander limits.
Figure 3 illustrates a PTP slave clock with a PDV filter in the servo loop. The PDV filter implements an array ofalgorithms that limit the effects of PDV on the frequency corrections that reach the clock synthesizer. The
wander measured on this PTP slave clock during the G.8261 test-cases is shown in Figure 7. The PTP slave

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clock with a PDV filter in the servo loop passes for every test case versus the G.8261 EEC-1 network wanderlimits.

PTP Slave Clock
G.8261 Performance Test Topology
for Testing Two-Way Protocols
Clock Servo
Controller withPDV Filter

t1, t2 , t3, t4

PTPMaster
Clock

PTP Packets

G.8261
Test-Cases

PTP Packets

ClockCorrection

MACTSU

ServoLoop

Clock
Synthesizer

Forward Direction
Reverse Direction
Clock SignalOutput

PDV = Packet Delay Variation

Figure 3. PTP Slave Clock with a PDV Filter in a G.8261 Test Environment

Limiting PTP Slave Clock Wander with a Low-Pass Filter
PTP slave clock wander due to PDV can be limited, to some extent, by including a low-pass filter in the servo
loop. But the PDV processes that cause the wander have distributions and mean delays that change over long
periods of time. For example, G.8261 test-case 13 models network phenomena that occur on a scale of hours
and test-case 14 models network phenomena that occur over an entire day. Effectively attenuating such wanderrequires an extremely low bandwidth filter.
Low-pass filtering of timing signals requires a stable local time base. TCXOs are often used in network clocks
with a bandwidth of 0.1Hz. For this reason, a TCXO and 0.1Hz PTP slave clock bandwidth were used for thesetests. Lower bandwidths are possible, but they require more stable and presumably higher cost oscillators.

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TC-16

TC-17

Packet Delays, Above the Floor Delay
Measured at the PTP Slave Clock(Forward Direction)

TC-12

TC-15

TC-13

TC-14

Figure 4. Packet Delay vs Time, Forward Direction, G.8261 Test Cases TC-12 to TC-17

TC-16

TC-17

Packet Delays, Above the Floor Delay
Measured at the PTP Master Clock(Reverse Direction)

TC-12

TC-15

TC-13

TC-14

Figure 5. Packet Delay vs Time, Reverse Direction, G.8261 Test-Cases TC-12 to TC-17

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Figure 6 shows a PTP slave clock with a low-pass filter in the servo loop. Equation 1 is used to calculate timeoffset corrections. The time offset corrections are filtered by a low-pass filter that outputs frequency corrections
for the clock synthesizer. The bandwidth of this servo loop is tuned to 0.1Hz. The wander measured on this PTP
slave clock during the G.8261 test-cases is shown in Figure 7. The PTP slave clock with a low-pass filter in theservo loop fails for every test case versus the G.8261 EEC-1 network wander limits.

PTP Slave Clock
G.8261 Performance Test Topology
for Testing Two-Way Protocols
t1, t2 , t3, t4PTPMasterClockPTP Packets

G.8261
Test-Cases

PTP Packets

Clock Servo
Controller withLP Filter
ClockCorrection

MACTSU

ServoLoop

Forward Direction

Clock
Synthesizer

Reverse Direction
Clock Signal
OutputLP = Low-Pass

Figure 6. PTP Slave Clock with a Low-Pass Filter in a G.8261 Test Environment

PTP Slave Clock Wander vs G.8261 EEC-1 Wander Limit
MTIE Results from G.8261 Test-Cases
Low-Pass Filter in the
Servo Loop
(0.1Hz Loop Bandwidth)

PDV Filter in the
Servo Loop

Figure 7. PTP Slave Clock Wander vs G.8261 EEC-1 Wander Limit

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Limiting IEEE 1588 Slave Clock Wander Caused by Packet Delay Variation White PaperConclusion
The PTP slave clock with a PDV filter in the servo loop complies with G.8261 EEC-1 network wander limits whentested versus ITU-T G.8261 test-cases 12 to 17. Whereas, the PTP slave with a low-pass filter in the servo loop
does not comply with G.8261 EEC-1 network wander limits when tested versus ITU-T G.8261 test-cases 12 to17. The highest wander measured for the PTP slave clock with a PDV filter had MTIE of 0.7μs from test-case 16.
The highest wander measured for the PTP slave clock with a low-pass filter had MTIE of 41.9μs from test-case
13. Both PTP slave clock implementations used the same TCXO as the local oscillator.
For the low-pass filter implementation, a tighter bandwidth can be used at the cost of a more stable local
oscillator. The tighter bandwidth would reduce the MTIE for shorter observation intervals. However, to reduce theMTIE for the 10000s and 100000s observation intervals would require a very stable and possibly cost prohibitive
local oscillator.
A PTP slave clock with a PDV filter in the servo loop, together with a stable and low-cost TCXO, is effective forlimiting wander due to PDV and is compliant with G.8261 network limits for wander at a telecom frequency
synchronization interface.
The Renesas PTP Clock Manager software includes a PTP clock servo with a PDV filter and other functionalitythat meets global ITU-T synchronization recommendations for telecom applications. The PTP Clock Manager isavailable under license from Renesas for use with Renesas timing products, and can be used with any IEEE
1588 protocol stack.

Notes1Recommendation ITU-T G.810 (1996), Definitions and Terminology for Synchronization Networks

2

IEEE Std 1588-2019, Standard for a Precision Clock Synchronization Protocol for Networked Measurement andControl Systems

3

Recommendation ITU-T G.8261/Y.1361 (2019), Amendment 1 (03/2020), Timing and Synchronization Aspects inPacket Networks

4

Recommendation ITU-T G.8265.1/Y.1365.1 (2014), Amendment 1 (08/2019), Precision Time Protocol Telecom
Profile for Frequency Synchronization

5

Recommendation ITU-T G.8275.2/Y.1369.2 (2020), Precision Time Protocol Telecom Profile for Phase/Time
Synchronization with Partial Timing Support from the Network.

6In addition, the frequency of the PTP slave clock must be stable from the arrival time of the Sync packet until thetransmission time of the Delay_Req packet.

7

Networks can be designed to ensure deterministic transmission times for PTP traffic; such networks are outside thescope of this whitepaper.

Revision History
Revision

Date

1.1

Feb.1.21

Updated diagrams and text to improve consistency with related documents.

1.0

Nov.25.20

Initial release.

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Description

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