Transparent versus Boundary Clocks (PTP) inBroadcast Environments

by Renaud Lavoie

Introduction
This document describes the difference between transparent clock and boundary clock, and explains why boundary clock scales better
than transparent clock. The first section defines key terms, including precision time protocol (PTP) functionality, ST 2059-1 and ST 2059-2,
transparent clock and boundary clock. The next section shows the IP broadcast center clocking scheme and relevant PTP standards.

PTP TIME
IEEE-1588/PTP
Messages
UDP
IP NETWORKIP MEDIA ENDPOINT

IP MEDIA ENDPOINTSLAVE VIDEO SPGPTP TIMEPTP TIMEPTP TIMEGENLOCKGENLOCKGENLOCKVideo Ref

Time CodeVideo Ref

Audio Ref

Time CodeBlackDARSTime CodeLEGACY SDI EQUIPMENTFigure 1. Video and Time Reference in an IP Media Facility (courtesy of Michel Proulx)

1. Definition
The PTP is defined by the specification IEEE-1588. It is an algorithm
that synchronizes the peripherals on the network with a common
time reference. Before PTP, there was the network time protocol(NTP) which allowed systems to be synchronized within a few
milliseconds of coordinated universal time (UTC). While theprovided precision proved to be enough for basic transfer of timeof day information, it is not enough for real-time applications
such as audio and video. To improve accuracy, thus enabling realtime applications to utilize the Ethernet network, the Institute ofElectrical and Electronics Engineers (IEEE) defined PTP.
The method is simple. A high precision clock, the master, will sendout PTP sync messages using the User Datagram Protocol (UDP).
Slaves will then receive the sync messages with the master time (t1).
If hardware timestamping is not provided by the master, a Follow_Riedel Communications GmbH & Co. KG | Uellendahler Str. 353 | 42109 Wuppertal | Germany

Up message will be sent out to provide the time at which the initialsync message was sent out (t1). The slave stores the time at which it
receives the Sync message (t2). After reception of the Sync message,
the slave sends out a Delay_Request to the master clock (t3). The
master finally answers with a Delay_Response message (t4).
Having these four timestamps, the slave device can estimate thepropagation delay and calculate its own offset from the master. Thefollowing formulas are used in the slave to establish the time at themaster:

Delay = [(t2-t1) + (t4 -t3)]/2
Offset = (t2 - t1) - Delay

Equation 1: PTP Delay and Offset calculation

2020-04 | Subject to change without notice. Errors and omissions excepted.

1.1 PTP functionality

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﻿The following image shows the low-level PTP algorithm

Master PTP ClockSlave Device

t1
Sync (~t1 )Followt2

Up (t )
1Delayt4

Delay

1.3 Boundary Clock
The boundary clock ready switches possess a built-in PTP masterclock. The switch will be the master for the endpoint devices
attached to it. For stability, the switch will be a slave to another
PTP master clock. In this scenario, the PTP master in the switch willcommunicate to a limited number of slaves. This way the boundary
clock method ensures that PTP masters are not over solicited,
and this will greatly improve the accuracy of the PTP time and thesystem scalability. The following picture shows a boundary clocksystem.

t3

Master PTP ClockstReque
Slave Clock

Response (t4)Basic Calculation
Delay = [(t2-t1) + (t4 -t3)]/2Offset = (t2 - t1) - Delay

IP Switch TOR

Transfer clock

Master Clock

Figure 2: PTP Algorithm

The transparent clock will relay the master’s PTP Sync, Follow_Up
and Delay_Resp messages to all of the Riedel IP SFPs and willtransfer PTP Request_Delay messages from all the SFPs to its
master. The PTP master usually can answer to a limited number ofslaves. When this number becomes too large, the master starts tobe over solicited by the number of messages.

MuoN
Encapsulator ST 2110
De-Encapsulator ST 2110

Software-defined
SFP+FusioN
ST 2110Figure 4. Boundary clock system

1.4 Timing Signal Generation in IP MediaThe network gets more congested, and packet scheduling acrossthe network can add delays, which in turn cause inaccuracy in timesynchronization (PTP messages have different delays that are notcompensated for). The transparent clock adjusts the PTP messages
to remove the delays of its own packet processing, and thus
compensates delays in PTP messaging.

Once the slave has iteratively computed its time difference with its
master, it is synchronized within sub-microsecond accuracy of UTC.
But time synchronization is just the first step. The second step is touse precise time to derive the timing reference signals needed by ST
2110 audio/video devices.

In a spine-leaf architecture, the top of rack switches can adjust thedelay inside the PTP packets (by adding a correction field in PTPmessage) to ensure it is viewed as transparently as possible.

The Society of Motion Picture and Television Engineers (SMPTE)
decided to create the ST 2059-1 and ST 2059-2 standards. The ST
2059-1 defined a method of deriving phase aligned audio and video
sync from PTP. The ST 2059-2 defined a profile of IEEE-1588 suitable
to audio and video requirements.
From the PTP ticks count, slaves accurately find the vertical andthe horizontal pulses and align its video to the video reference. It
is also possible to extract the audio clocks and finally to generate
the Digital Audio Reference (DARS) if required. The following image
shows the process of re-creating the clock for synchronization ofsignals.

Master PTP Clock

IP Switch TOR
PTP message are adjusted for switch delay
MuoN
Encapsulator ST2110
De-Encapsulator ST2110

Software defined
SFP+FusioNST2110

Figure 3. Transparent clock system

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2020-04 | Subject to change without notice. Errors and omissions excepted.

1.2 Transparent Clockwww.riedel.net

﻿

PTP ENGINEGENLOCKIP MEDIA ENDPOINTPTP ENGINE

Audio
Ref

ST 2059 Time
Epoch of day

Accurate
Clock

ST 2059 Time
Epoch of day

Synthesis

CompareSynthesis

Compare4.43 MHz
3.58 MHzGENLOCK

H, V
Ref

Accurate
Clock

48 KHzVPhaseH

Video Reference Generator

Video Reference Generator

Black Reference

DARS

Time CodeFigure 5. Timing signals generation in IP Media devices (courtesy of Michel Proulx)

2. IP Broadcast Environment
Clocking

Finally, endpoints such as the MuoN and FusioN gateways will
work in both transparent clock and boundary clock systems. Butfor scalability and accuracy, the boundary clock system should be
implemented over the transparent clock.
For more information about our products. Please contact us atinfo@riedel.net or sales@riedel.net
Riedel, MediorNet, MuoN, FusioN, and VirtU are trademarks ofRiedel Communications GmbH & Co KG. All other products orservices mentioned herein are the property of their respectiveowners.

2020-04 | Subject to change without notice. Errors and omissions excepted.

In an SDI broadcast environment, timing is crucial. It is no different
in an IP broadcast environment; timing is important to synchronize
endpoints. PTP was already used by various industries, so it was an
obvious choice to be reused for broadcast.
From our industry’s perspective, PTP is mandatory and therefore
is now defined in the ST 2110-10 standard for audio, video andmetadata transport. To be compliant, ST 2110 endpoint (devices)
should be PTP / ST 2059-1 and ST 2059-2 compliant.

3. Conclusion

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