TICTOC S. Rodrigues
Internet-Draft Zarlink Semiconductor
Intended status: Informational K. Lindqvist
Expires: January 8, 2009 Netnod
July 7, 2008
TICTOC Requirement
draft-rodrigues-lindqvist-tictoc-probstat-00.txt
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Abstract
Distribution of high precision time and frequency over the Internet
and special purpose IP networks is becoming more and more needed as
IP networks replace legacy networks and applications and as new
applications with need for frequency and time are developed on the
Internet. The IETF formed the TICTOC workinggroup to address the
problem and perform an analysis on existing solutions and the needs.
This document summarises application needs, as described and agreed
on at an TICTOC interim meeting held in Paris from June 16 to 18,
2008.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Applications Requirements . . . . . . . . . . . . . . . . . . 3
3.1. Cellular Backhauling . . . . . . . . . . . . . . . . . . . 3
3.1.1. Cellular Backhaul Requirements . . . . . . . . . . . . 4
3.2. Circuit Emulation . . . . . . . . . . . . . . . . . . . . 6
3.2.1. Circuit Emulation requirements . . . . . . . . . . . . 7
3.3. Remote Telco . . . . . . . . . . . . . . . . . . . . . . . 9
3.3.1. Remote Telco requirements . . . . . . . . . . . . . . 9
3.4. Instrumentation and Measurement, Industrial
Automation, and Power . . . . . . . . . . . . . . . . . . 11
3.4.1. Instrumentation and Measurement, Industrial
Automation, and Power Requirements . . . . . . . . . . 11
3.5. Networking . . . . . . . . . . . . . . . . . . . . . . . . 13
3.5.1. Networking . . . . . . . . . . . . . . . . . . . . . . 13
3.6. ToD/ Internet . . . . . . . . . . . . . . . . . . . . . . 15
3.6.1. ToD/Internet . . . . . . . . . . . . . . . . . . . . . 15
3.7. Legal Time . . . . . . . . . . . . . . . . . . . . . . . . 17
3.7.1. Legal Time requirements . . . . . . . . . . . . . . . 17
3.8. Metrology . . . . . . . . . . . . . . . . . . . . . . . . 19
3.8.1. Metrology requirements . . . . . . . . . . . . . . . . 19
3.9. Sensor . . . . . . . . . . . . . . . . . . . . . . . . . . 21
3.9.1. Sensors requirements . . . . . . . . . . . . . . . . . 21
4. Network Dependencies . . . . . . . . . . . . . . . . . . . . . 23
5. Network Topology . . . . . . . . . . . . . . . . . . . . . . . 24
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
9. Informative References . . . . . . . . . . . . . . . . . . . . 24
Appendix A. Existing Time and Frequency Transfer Mechanisms . . . 25
A.1. Radio-based Timing Transfer Methods . . . . . . . . . . . 26
A.2. Dedicated Wire-based Timing Transfer Methods . . . . . . . 27
A.3. Transfer Using Packet Networks . . . . . . . . . . . . . . 27
A.3.1. NTP summary description . . . . . . . . . . . . . . . 28
A.3.2. IEEE1588 summary description . . . . . . . . . . . . . 28
Appendix B. Other Forums Working in this Problem Space . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30
Intellectual Property and Copyright Statements . . . . . . . . . . 32
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1. Introduction
There is an emerging need to distribute highly accurate time and
frequency information over IP and over MPLS packet switched networks
(PSNs). In this draft, the requirements for transporting accurate
time and/or frequency are addressed.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]
3. Applications Requirements
There are many applications that need synchronization. Some
applications only need frequency; for others a combination of
frequency and time of day or phase may be required. At the TICTOC
interim meeting, it was agreed that these applications be grouped
based on what was believed to be common requirements, and where the
requiremetns were distinct from each other. This section describes
these applications (or groups of applications) that was agreed on at
the TICTOC interim meeting.
3.1. Cellular Backhauling
Within Cellular backhauling, there are several applications that need
to be considered. Some of these applications only require frequency
information, others require time-of-day, and others require phase.
The cellular backhauling applications to be consiredered are:
o GSM
o Wimax
o LTE
o UMTS FDD
o UMTS TDD
o CDMA2000
o TD-SCDMA
Conventionally GSM and UMTS FDD base stations obtain this reference
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frequency by locking on to the E1/T1 that links them to the base
station controller. With the replacement of TDM links with Packet
Switched Networks (PSNs) such as Ethernet, IP or MPLS, this simple
method of providing a frequency reference is lost, and frequency
information must be made available in some other way.
The synchronization requirement is derived from the need for the
radio frequencies to be accurate. Radio spectrum is a limited and
valuable commodity that needs to be used as efficiently as possible.
In GSM, transmission frequencies are allocated to a given cellular
base station and its neighbors in such fashion as to ensure that they
do not interfere with each other. If the radio network designer
cannot rely on the accuracy of these frequencies, the spacing between
the frequencies used by neighboring sites must be increased, with
significant economic consequences.
There is an additional requirement derived from the need for smooth
handover when a mobile station crosses from one cell to another. If
the radio system designer can not guarantee that the preparations
required for handover occur in a few milliseconds, then they must
allow the mobile station to consume frequency resources
simultaneously in both cells in order to avoid service disruption.
The preparations required involve agreement between the mobile and
base stations on the new frequencies and time offsets; these
agreements can be accomplished quickly when the two base stations'
frequency references are the same to a high degree of accuracy.
3.1.1. Cellular Backhaul Requirements
The requirements for the Cellular Backhauling is summarized in the
table 1.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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Cellular Backhaul Requirements
+---------------+--------+--------+-------+-----+---------+---------+
| Requirements | GSM/UM | UMTS | Wimax | LTE | CDMA200 | TD-SCDM |
| | TS FDD | TDD | | | 0 | A |
+---------------+--------+--------+-------+-----+---------+---------+
| Time Type | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| Time | | | | | | |
| Resolution | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| Client's time | | | | | | |
| resolution | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| Freq | | | | | | |
| stability | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| freq accuracy | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| time/phase | | | | | | |
| stability | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| time/phase | | | | | | |
| accuracy | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| time/acquisit | | | | | | |
| ion time | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| service | | | | | | |
| jitter | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| service | | | | | | |
| wander | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| asymmetry | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| constrained | | | | | | |
| network | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| on-path | | | | | | |
| support | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| clients/serve | | | | | | |
| r | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| update rate | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
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| server | | | | | | |
| authenticatio | | | | | | |
| n | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| client | | | | | | |
| authenticatio | | | | | | |
| n | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| transaction | | | | | | |
| authenticatio | | | | | | |
| n | | | | | | |
| ------------- | ------ | ------ | ----- | --- | ------ | ------- |
| backwards | | | | | | |
| compatibility | | | | | | |
+---------------+--------+--------+-------+-----+---------+---------+
Table 1
3.2. Circuit Emulation
The PWE3 WG has produced three techniques for emulating traditional
low-rate (E1, T1, E3, T3) TDM services over PSNs, namely SAToP
[RFC4553], CESoPSN [RFC5086], and TDMoIP. The Network
Synchronization reference model and deployment scenarios for
emulation of TDM services have been described in [RFC4197], Section
4.3. The major technical challenge for TDM pseudowires is the
accuracy of its clock recovery.
TDM network standards for timing accuracy and stability are extremely
demanding. These requirements are not capriciously dictated by
standards bodies, rather they are critical to the proper functioning
of a high-speed TDM network. Consider a TDM receiver utilizing its
own clock when converting the physical signal back into a bit-stream.
If the receive clock runs at precisely the same rate as the source
clock, then the receiver need only determine the optimal sampling
phase. However, with any mismatch of clock rates, no matter how
small, bit slips will eventually occur. For example, if the receive
clock is slower than the source clock by one part per million (ppm),
then the receiver will output 999,999 bits for every 1,000,000 bits
sent, thus deleting one bit. Similarly, if the receive clock is
faster than the source clock by one part per billion (ppb), the
receiver will insert a spurious bit every billion bits. One bit slip
every million bits may seem acceptable at first glance, but
translates to a catastrophic two errors per second for a 2 Mb/s E1
signal. ITU-T recommendations permit a few bit slips per day for a
low-rate 64 kb/s channel, but strive to prohibit bit slips entirely
for higher-rate TDM signals.
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3.2.1. Circuit Emulation requirements
The requirements for the Circuit Emulation is summarized in the table
2.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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Circuit Emulation
+-----------------------------+-----------------------------+
| Requirements | Circuit Emulation |
+-----------------------------+-----------------------------+
| Time Type | |
| --------------------------- | --------------------------- |
| Time Resolution | |
| --------------------------- | --------------------------- |
| Client's time resolution | |
| --------------------------- | --------------------------- |
| Freq stability | |
| --------------------------- | --------------------------- |
| freq accuracy | |
| --------------------------- | --------------------------- |
| time/phase stability | |
| --------------------------- | --------------------------- |
| time/phase accuracy | |
| --------------------------- | --------------------------- |
| time/acquisition time | |
| --------------------------- | --------------------------- |
| service jitter | |
| --------------------------- | --------------------------- |
| service wander | |
| --------------------------- | --------------------------- |
| asymmetry | |
| --------------------------- | --------------------------- |
| constrained network | |
| --------------------------- | --------------------------- |
| on-path support | |
| --------------------------- | --------------------------- |
| clients/server | |
| --------------------------- | --------------------------- |
| update rate | |
| --------------------------- | --------------------------- |
| server authentication | |
| --------------------------- | --------------------------- |
| client authentication | |
| --------------------------- | --------------------------- |
| transaction authentication | |
| --------------------------- | --------------------------- |
| backwards compatibility | |
+-----------------------------+-----------------------------+
Table 2
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3.3. Remote Telco
Editor's note: need more info on this application.
3.3.1. Remote Telco requirements
The requirements for the Remote Telco is summarized in the table 3.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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Remote Telco
+-----------------------------+-----------------------------+
| Requirements | Remote Telco |
+-----------------------------+-----------------------------+
| Time Type | |
| --------------------------- | --------------------------- |
| Time Resolution | |
| --------------------------- | --------------------------- |
| Client's time resolution | |
| --------------------------- | --------------------------- |
| Freq stability | |
| --------------------------- | --------------------------- |
| freq accuracy | |
| --------------------------- | --------------------------- |
| time/phase stability | |
| --------------------------- | --------------------------- |
| time/phase accuracy | |
| --------------------------- | --------------------------- |
| time/acquisition time | |
| --------------------------- | --------------------------- |
| service jitter | |
| --------------------------- | --------------------------- |
| service wander | |
| --------------------------- | --------------------------- |
| asymmetry | |
| --------------------------- | --------------------------- |
| constrained network | |
| --------------------------- | --------------------------- |
| on-path support | |
| --------------------------- | --------------------------- |
| clients/server | |
| --------------------------- | --------------------------- |
| update rate | |
| --------------------------- | --------------------------- |
| server authentication | |
| --------------------------- | --------------------------- |
| client authentication | |
| --------------------------- | --------------------------- |
| transaction authentication | |
| --------------------------- | --------------------------- |
| backwards compatibility | |
+-----------------------------+-----------------------------+
Table 3
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3.4. Instrumentation and Measurement, Industrial Automation, and Power
In the test and measurement and industrial sector there is a desire
to move from special purpose communications infrastructure with
calibrated wiring run back to a centralize controller, to a
distributed system, in which instructions are distributed in advance
to be executed at a predetermined time, and in which measurements are
taken remotely and communicated back to a common point for later
correlation and analysis. Two examples of this tendency are
described below.
In the printing industry there is a need to control operations in
multi-stand printing machines. The paper travels through these
machines at a speed of nearly 100 km/h. At these speeds, co-
ordination error of 1 microsecond between operations taking place at
different positions in the machine produces a 0.03mm color offset,
which is visible to the naked eye and results in an unacceptable
degradation in quality.
In the electrical power industry there is a need to improve the
measurement of power flows in order to monitor and predict usage
patterns. One proposal is to extensively deploy synchro-phasors in
the power network and to correlate their output to determine demand.
These devices need to be able to determine the time of measurement
with an accuracy of 1 microsecond.
3.4.1. Instrumentation and Measurement, Industrial Automation, and
Power Requirements
The requirements for the Instrumentation and Measurement, Industrial
Automation, and Power Requirements are summarized in the table 4.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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Instrumentation and Measurement, Industrial Automation, and Power
Requirements Requirements
+----------------+--------------------------+-------------+---------+
| Requirements | Instrumentation/measurem | Industrial | Power |
| | ent | Automation | |
+----------------+--------------------------+-------------+---------+
| Time Type | | | |
| -------------- | ----------------------- | ----------- | ------- |
| Time | | | |
| Resolution | | | |
| -------------- | ----------------------- | ----------- | ------- |
| Client's time | | | |
| resolution | | | |
| -------------- | ----------------------- | ----------- | ------- |
| Freq stability | | | |
| -------------- | ----------------------- | ----------- | ------- |
| freq accuracy | | | |
| -------------- | ----------------------- | ----------- | ------- |
| time/phase | | | |
| stability | | | |
| -------------- | ----------------------- | ----------- | ------- |
| time/phase | | | |
| accuracy | | | |
| -------------- | ----------------------- | ----------- | ------- |
| time/acquisiti | | | |
| on time | | | |
| -------------- | ----------------------- | ----------- | ------- |
| service jitter | | | |
| -------------- | ----------------------- | ----------- | ------- |
| service wander | | | |
| -------------- | ----------------------- | ----------- | ------- |
| asymmetry | | | |
| -------------- | ----------------------- | ----------- | ------- |
| constrained | | | |
| network | | | |
| -------------- | ----------------------- | ----------- | ------- |
| on-path | | | |
| support | | | |
| -------------- | ----------------------- | ----------- | ------- |
| clients/server | | | |
| -------------- | ----------------------- | ----------- | ------- |
| update rate | | | |
| -------------- | ----------------------- | ----------- | ------- |
| server | | | |
| authentication | | | |
| -------------- | ----------------------- | ----------- | ------- |
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| client | | | |
| authentication | | | |
| -------------- | ----------------------- | ----------- | ------- |
| transaction | | | |
| authentication | | | |
| -------------- | ----------------------- | ----------- | ------- |
| backwards | | | |
| compatibility | | | |
+----------------+--------------------------+-------------+---------+
Table 4
3.5. Networking
Editor's note: need more info on this application.
3.5.1. Networking
The requirements for the Newtworking SLA and Network CDR are
summarized in the table 5.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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Newtworking SLA and Network CDR Requirements
+------------------------------+-------------------+--------------+
| Requirements | Newtworking SLA | Network CDR |
+------------------------------+-------------------+--------------+
| Time Type | | |
| ---------------------------- | ----------------- | ------------ |
| Time Resolution | | |
| ---------------------------- | ----------------- | ------------ |
| Client's time resolution | | |
| ---------------------------- | ----------------- | ------------ |
| Freq stability | | |
| ---------------------------- | ----------------- | ------------ |
| freq accuracy | | |
| ---------------------------- | ----------------- | ------------ |
| time/phase stability | | |
| ---------------------------- | ----------------- | ------------ |
| time/phase accuracy | | |
| ---------------------------- | ----------------- | ------------ |
| time/acquisition time | | |
| ---------------------------- | ----------------- | ------------ |
| service jitter | | |
| ---------------------------- | ----------------- | ------------ |
| service wander | | |
| ---------------------------- | ----------------- | ------------ |
| asymmetry | | |
| ---------------------------- | ----------------- | ------------ |
| constrained network | | |
| ---------------------------- | ----------------- | ------------ |
| on-path support | | |
| ---------------------------- | ----------------- | ------------ |
| clients/server | | |
| ---------------------------- | ----------------- | ------------ |
| update rate | | |
| ---------------------------- | ----------------- | ------------ |
| server authentication | | |
| ---------------------------- | ----------------- | ------------ |
| client authentication | | |
| ---------------------------- | ----------------- | ------------ |
| transaction authentication | | |
| ---------------------------- | ----------------- | ------------ |
| backwards compatibility | | |
+------------------------------+-------------------+--------------+
Table 5
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3.6. ToD/ Internet
General time distriubtion over the Internet or IP networks, is often
called Time of Day or Wall-clock. Most existing use cases are using
NTP over the Internet with low precision requirements. However, new
applications are arising that require higher precision rates than
what is currently available.
The users of Internet time
3.6.1. ToD/Internet
The requirements for the ToD/Internet is summarized in the table 6.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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ToD/Internet Requirements
+-----------------------------+-----------------------------+
| Requirements | ToD/Internet |
+-----------------------------+-----------------------------+
| Time Type | |
| --------------------------- | --------------------------- |
| Time Resolution | |
| --------------------------- | --------------------------- |
| Client's time resolution | |
| --------------------------- | --------------------------- |
| Freq stability | |
| --------------------------- | --------------------------- |
| freq accuracy | |
| --------------------------- | --------------------------- |
| time/phase stability | |
| --------------------------- | --------------------------- |
| time/phase accuracy | |
| --------------------------- | --------------------------- |
| time/acquisition time | |
| --------------------------- | --------------------------- |
| service jitter | |
| --------------------------- | --------------------------- |
| service wander | |
| --------------------------- | --------------------------- |
| asymmetry | |
| --------------------------- | --------------------------- |
| constrained network | |
| --------------------------- | --------------------------- |
| on-path support | |
| --------------------------- | --------------------------- |
| clients/server | |
| --------------------------- | --------------------------- |
| update rate | |
| --------------------------- | --------------------------- |
| server authentication | |
| --------------------------- | --------------------------- |
| client authentication | |
| --------------------------- | --------------------------- |
| transaction authentication | |
| --------------------------- | --------------------------- |
| backwards compatibility | |
+-----------------------------+-----------------------------+
Table 6
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3.7. Legal Time
With legal time is meant the cases where high precision wall-clock is
needed, just as in the ToD case, but with where the time source is
tracable to UTC in a secure manner, i.e through a certificate chain.
It's also important for the legal-time case that the certificate
chain is set-up so that it provides for an audit trail, where the ToD
provided at any given moment can be traced to a known source or
standard (i.e a national timescale or time laboratory). One typical
application that would benefit from high accuracy legal time is event
correlation in computer systems logs, and similar applications.
3.7.1. Legal Time requirements
The requirements for the Legal Time is summarized in the table 7.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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Legal Time requirements
+-----------------------------+-----------------------------+
| Requirements | Legal Time |
+-----------------------------+-----------------------------+
| Time Type | |
| --------------------------- | --------------------------- |
| Time Resolution | |
| --------------------------- | --------------------------- |
| Client's time resolution | |
| --------------------------- | --------------------------- |
| Freq stability | |
| --------------------------- | --------------------------- |
| freq accuracy | |
| --------------------------- | --------------------------- |
| time/phase stability | |
| --------------------------- | --------------------------- |
| time/phase accuracy | |
| --------------------------- | --------------------------- |
| time/acquisition time | |
| --------------------------- | --------------------------- |
| service jitter | |
| --------------------------- | --------------------------- |
| service wander | |
| --------------------------- | --------------------------- |
| asymmetry | |
| --------------------------- | --------------------------- |
| constrained network | |
| --------------------------- | --------------------------- |
| on-path support | |
| --------------------------- | --------------------------- |
| clients/server | |
| --------------------------- | --------------------------- |
| update rate | |
| --------------------------- | --------------------------- |
| server authentication | |
| --------------------------- | --------------------------- |
| client authentication | |
| --------------------------- | --------------------------- |
| transaction authentication | |
| --------------------------- | --------------------------- |
| backwards compatibility | |
+-----------------------------+-----------------------------+
Table 7
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3.8. Metrology
Metrology for time and frequency is today mostly using tailored
equipment and cabling for time/frequency transfer when doing
laboratory work. However, in the future, using IP over existing
networks in the laboratories would allow for greater flexibility and
reuse of existing infrastructure rather than building out more
special purpose infrastructure.
3.8.1. Metrology requirements
The requirements for the Metrology is summarized in the table 8.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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Metrology requirements
+-----------------------------+-----------------------------+
| Requirements | Metrology |
+-----------------------------+-----------------------------+
| Time Type | |
| --------------------------- | --------------------------- |
| Time Resolution | |
| --------------------------- | --------------------------- |
| Client's time resolution | |
| --------------------------- | --------------------------- |
| Freq stability | |
| --------------------------- | --------------------------- |
| freq accuracy | |
| --------------------------- | --------------------------- |
| time/phase stability | |
| --------------------------- | --------------------------- |
| time/phase accuracy | |
| --------------------------- | --------------------------- |
| time/acquisition time | |
| --------------------------- | --------------------------- |
| service jitter | |
| --------------------------- | --------------------------- |
| service wander | |
| --------------------------- | --------------------------- |
| asymmetry | |
| --------------------------- | --------------------------- |
| constrained network | |
| --------------------------- | --------------------------- |
| on-path support | |
| --------------------------- | --------------------------- |
| clients/server | |
| --------------------------- | --------------------------- |
| update rate | |
| --------------------------- | --------------------------- |
| server authentication | |
| --------------------------- | --------------------------- |
| client authentication | |
| --------------------------- | --------------------------- |
| transaction authentication | |
| --------------------------- | --------------------------- |
| backwards compatibility | |
+-----------------------------+-----------------------------+
Table 8
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3.9. Sensor
More generally, there is growing interest in clock synchronization in
massively parallel sensor networks. Advances in wireless
communications have enabled the development of low power miniature
sensors that collect and disseminate data from their immediate
environment. Although each sensor has limited processing power,
through distributed processing the network becomes capable of
performing various tasks of data fusion, but only assuming a common
time base can be established.
3.9.1. Sensors requirements
The requirements for the Sensor is summarized in the table 9.
Editor's note: This table was discussed at the interim meeting, but
it has not been agreed on, therefore is left in blank. The
requirements listed in the table and the definitions of the terms
need to be addressed as they have not been agreed on.
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Sensor requirements
+-----------------------------+-----------------------------+
| Requirements | Sensor |
+-----------------------------+-----------------------------+
| Time Type | |
| --------------------------- | --------------------------- |
| Time Resolution | |
| --------------------------- | --------------------------- |
| Client's time resolution | |
| --------------------------- | --------------------------- |
| Freq stability | |
| --------------------------- | --------------------------- |
| freq accuracy | |
| --------------------------- | --------------------------- |
| time/phase stability | |
| --------------------------- | --------------------------- |
| time/phase accuracy | |
| --------------------------- | --------------------------- |
| time/acquisition time | |
| --------------------------- | --------------------------- |
| service jitter | |
| --------------------------- | --------------------------- |
| service wander | |
| --------------------------- | --------------------------- |
| asymmetry | |
| --------------------------- | --------------------------- |
| constrained network | |
| --------------------------- | --------------------------- |
| on-path support | |
| --------------------------- | --------------------------- |
| clients/server | |
| --------------------------- | --------------------------- |
| update rate | |
| --------------------------- | --------------------------- |
| server authentication | |
| --------------------------- | --------------------------- |
| client authentication | |
| --------------------------- | --------------------------- |
| transaction authentication | |
| --------------------------- | --------------------------- |
| backwards compatibility | |
+-----------------------------+-----------------------------+
Table 9
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4. Network Dependencies
When using packet networks to transfer timing, packet delay
variation, propagation asymmetry, and maximum permissible packet rate
all have a significant bearing on the accuracy with which the client
is able to determine absolute time. Thus the network environment has
a large bearing on the quality of time that can be delivered.
Timing distribution is highly sensitive to packet delay variation,
and can thus can deteriorate under congestion conditions.
Furthermore the disciplining of the client's oscillator (the sole
component of frequency transfer, and a critical component of time
transfer) is a function that should not be disrupted. When the
service is disrupted the client needs to go into "holdover" mode, and
its accuracy will consequently be degraded. Depending on the
relative quality of the client's clock and the required quality after
disciplining, a relatively high packet rate may be required.
Packet delay variation can to some extent be addressed by traffic
engineering, thus time transfer within a constrained network
environment might reasonably be expected to deliver a higher quality
time service than can be achieved between two arbitrary hosts
connected to the Internet. Greater gains can probably be obtained by
deploying equipment that incorporates IEEE 1588 style on-the-fly
packet timestamp correction (or any other form of on-path support),
or follow-up message mechanisms that report the packet storage and
forward delays to the client. However one can only be sure that such
techniques are available along the entire path in a well-controlled
environment. Therefor, time transfer protocols should not assume the
availability of on path support, but utilise it where available.
The packet rate between the time-server and its client also has a
bearing on the quality of the time transfer, because at a higher rate
the smart filter has a better chance of extracting the "good"
packets. How the packet rate relates to the accuracy is dependent on
the filter algorithm in use. In a controlled environment it is
possible to ensure that there is adequate bandwidth, and that the
server is not overloaded. In such an environment the onus moves from
protecting the server from overload, to ensuring that the server can
satisfy the needs of all of the clients.
Congested and overloaded paths might influence the quality of timing
transfer. In a constrained network environment, it's assumed that a
service provider will ensure that packet delivery is done in
according to the timing transfer needs of the network operator.
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5. Network Topology
Editor's note: This section needs to be discussed.
6. Security Considerations
Time and frequency services are a significant element of network
infrastructure, and are critical for certain emerging applications.
Hence time and frequency transfer services MUST be protected from
being compromised, and for some of the applications described above
such as legal time, the ability to provide and audit trail to the
timing source. One possible threat is a false time or frequency
server being accepted instead of a true one, thus enabling an
attacker to alter the time and frequency service provided. Other
possible scenarios are to be able to distinguish between trusted
clients and non-trusted clients when providing service.
Any protection mechanism must be designed in such a way that it does
not degrade the quality of the time transfer. Such a mechanism
SHOULD also be relatively lightweight, as client restrictions often
dictate a low processing and memory footprint, and because the server
may have extensive fan-out.
The following authentication mechanism need to be considered:
1. of server by client (depending on the application)
2. of client by server (depending on the application)
3. transactions (depending on the application)
7. IANA Considerations
No IANA actions are required as a result of the publication of this
document.
8. Acknowledgements
The authors wish to thank Stewart Bryant, Yaakov Stein, Karen
O'Donoghue and Laurent Montini for input on this draft.
9. Informative References
[1588] IEEE, "1588-2002 Standard for A Precision Clock
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Synchronization Protocol for Networked Measurement
and Control Systems".
[G8261] ITU-T, "Recommendation G.8261/Y.1361 - Timing and
synchronization aspects in packet networks",
April 2008.
[G8262] ITU-T, "Recommendation G.8262/Y.1362 - Timing
Characteristics of Synchronous Ethernet Equipment Slave
Clock (EEC)", August 2007.
[G8264] ITU-T, "Recommendation G.8264/Y.1364 - Distribution of
timing through packet networks", 2008.
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305, March 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4197] Riegel, M., "Requirements for Edge-to-Edge Emulation of
Time Division Multiplexed (TDM) Circuits over Packet
Switching Networks", RFC 4197, October 2005.
[RFC4553] Vainshtein, A. and YJ. Stein, "Structure-Agnostic Time
Division Multiplexing (TDM) over Packet (SAToP)",
RFC 4553, June 2006.
[RFC5086] Vainshtein, A., Sasson, I., Metz, E., Frost, T., and P.
Pate, "Structure-Aware Time Division Multiplexed (TDM)
Circuit Emulation Service over Packet Switched Network
(CESoPSN)", RFC 5086, December 2007.
Appendix A. Existing Time and Frequency Transfer Mechanisms
In this section we will discuss existing mechanisms for transfer of
time information, frequency information, or both. It should be noted
that a sufficiently accurate time transfer service may be used to
derive an accurate frequency transfer. Indeed, this is exactly what
happens in a GPS disciplined frequency standard. On the other hand,
an accurate frequency transfer service, while itself unable to
transfer absolute time, is usually used to support and improve the
performance of the time transfer service. Indeed, implementations of
NTP or IEEE 1588 clients can be considered to consist of two phases.
First, a local oscillator is locked to the server's frequency using
incoming information from the incoming packets, and then the local
time set based on the server's time and the propagation latency. By
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maintaining a local frequency source, the client requires relatively
infrequent updates, and can continue functioning during short periods
of network outage. Moreover, it can be shown that this method
results in significantly better time transfer accuracy than methods
that do not discipline a local clock.
Time transfer mechanisms can be divided into three classes. The
first class consists of mechanisms that use radio frequency
transport, while the second mechanism uses dedicated "wires" (which
for our purposes include optical fibers). The third, which will be
our main focus, exploits a Packet Switched Network for transfer of
timing information. Advantages and disadvantages of these three
methods are discussed in the following subsections.
A.1. Radio-based Timing Transfer Methods
The transfer of time by radio transmission is one of the oldest
methods available, and is still the most accurate wide area method.
In particular, there are two navigation in wide use that can be used
for time transfer, The LOng RAnge Navigation (LORAN) terrestrial
radio system, and the Global Navigation Satellite System (GNSS). In
both cases the user needs to be able to receive the transmitted
signal, requiring access to a suitable antenna. In certain
situations, e.g. basement communications rooms and urban canyons, the
required signal may not be receivable.
Radio systems have high accuracy, far better than what we will later
see can be achieved by existing PSN technologies. However coverage
is limited; eLORAN for example only covers North America, and GPS
does not have good coverage near the poles.
Although civilian use is sanctioned, the GPS was developed and is
operated by the U.S. Department of Defense as a military system. For
this reason there are political concerns that rules out its use in
certain countries. The European Union is working on an alternative
system called Galileo, which will be run as a commercial enterprise.
In addition, GPS has some well-documented multi-hour outages, and is
considered vulnerable to jamming. One major PTT also reports that
they see a 2% per year failure rate for the antenna/receiver/
clock-out chain.
While a radio-based timing service may be acceptable for some sites,
it is frequently impractical to use on a per equipment basis. Hence,
some form of local timing distribution is usually also required.
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A.2. Dedicated Wire-based Timing Transfer Methods
The use of dedicated networks in the wide area does not scale well.
Such services were available in the past, but for reasons of cost and
accuracy have been superseded by GPS based solutions.
In the local area, one new technique is emerging as a mechanism for
time transport, namely DOCSIS Timing Interface(DTI). DTI was
designed by DOCSIS for the distribution of time in a cable head-end
in support of media access control. Time transfer is packet-based
over a multi-stage hub and spoke dedicated network. It uses a single
twisted-pair in half-duplex to eliminate inaccuracies due to the
length differences between the pairs in a multi-pair cable.
The DTI approach is applicable for special applications, but the need
for a dedicated network imposes significant drawbacks for the general
time transfer case.
Synchronous Ethernet is a technique that has recently been approved
by ITU-T, it provides frequency distribution over Ethernet links.
Modern dedicated-media full-duplex Ethernet, in both copper and
optical physical layer variants, transmits continuously. One can
thus elect to derive the physical layer transmitter clock from a high
quality frequency reference, instead of the conventional 100 ppm
crystal-derived transmitter rate. The receiver at the other end of
the link automatically locks onto the physical layer clock of the
received signal, and thus itself gain access to a highly accurate and
stable frequency reference. Then, in TDM fashion, this receiver
could lock the transmission clock of its other ports to this
frequency reference. Apart from some necessary higher layer packet
based configuration and OAM operations to transport synchronization
status messaging, the solution is entirely physical layer, and has no
impact on higher layers.
At first sight it would seem that the only application of synchronous
Ethernet was in frequency transfer (it has no intrinsic time transfer
mechanism). However, the quality of packet-based time transfer
mechanism can be considerably enhanced if used in conjunction with
synchronous Ethernet as a frequency reference.
A.3. Transfer Using Packet Networks
When using a PSN to transfer timing, a server sends timing
information in the form of packets to one or multiple clients. When
there are multiple clients, the timing packets may be multicast.
Software/hardware in the client recovers the frequency and/or time of
the server based on the packet arrival time and the packet contents.
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There are two well-known protocols capable of running over a general-
purpose packet network, NTP [RFC1305], and IEEE 1588 [1588]. NTP is
the product of the IETF, and is currently undergoing revision to
version 4. IEEE 1588 (a product of the IEEE Test and Measurement
community) is specified in a limited first version, and the second
version (1588v2)was approved recently.
It is important that NTP, IEEE-1588 or any other future packet based
time transfer mechanism do not break each other if they run in the
same network.
A.3.1. NTP summary description
NTP is widely deployed, but existing implementations deliver accuracy
on the order of 10 milliseconds. This accuracy is not adequate for
the applications described above. Current NTP suffers from the fact
that it was designed to operate over the Internet, and the routers
and switches make no special concessions to NTP for preservation of
time transfer accuracy. Furthermore, typical update rates are low
and can not be significantly increased due to scalability issues in
the server. In addition most NTP time servers and time receivers
have a relatively unsophisticated implementation that further
degrades the final time quality. However, proprietary NTP
implementations that use other algorithms and update-rates have
proved that NTP packet formats can be used for higher accuracy.
A.3.2. IEEE1588 summary description
IEEE 1588v1 was a time transfer protocol that exclusively used a
fairly crude multicast technique. It is widely anticipated that wide
scale deployment of IEEE1588 will be based on 1588v2. The
information exchange component of IEEE 1588 is a protocol known as
Precision Time Protocol (PTP).
IEEE 1588v2 can be considered to consist of several components:
1. A configuration and control protocol
2. A time transfer protocol
3. A time correction protocol
4. Physical mapping
The configuration and control protocol is based on the multicast
approach of IEEE1588 v1 (multicast IP with recommended TTL=1, UDP,
IEEE1588 payload with equipment identifier in the payload). The
rationale for this approach was that the equipment needed to be "plug
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and play" (no configuration), was required to map to physical media
other than Ethernet, and had to have a very low memory and processor
footprint. IEEE1588 v2 includes Unicast messages.
The time transfer protocol is a standard two-way time transfer
approach used in other packet-based approaches. Like all such
approaches it is subject to inaccuracies due to variable store and
forward delays in the packet switches, and due to the assumption of
symmetric propagation delays. For IEEE1588 v2, the time transfer
packets (in both directions) may be operated in a multicast or
unicast mode.
The time correction protocol is used to correct for propagation,
store and forward delays in the packet switches. This again may be
operated multicast or unicast. This mechanism requires some level of
hop-by-hop hardware support. This mechanism may also be considered a
concept in its own right and may be adapted to enhance other packet
time transfer protocols such as NTP.
The base 1588 specification describes how the PTP operates over the
Ethernet/IP/UDP protocol stack. IEEE1588 v2 includes annexes that
describe PTP operation over pure layer 2 Ethernet, and over a number
of specialist media.
The mappings of interest for telecommunications are PTP over UDP/IP,
PTP over MPLS , and perhaps PTP over Ethernet, all in unicast mode
only. Issues of a suitable control management and OAM environment
for these applications are largely in abeyance, as are considerations
about the exact nature of the network environment.
It is also worth noting the existence of a second IEEE effort, IEEE
802.1AS. This group is specifying the protocol and procedures to
ensure synchronization across Bridged and Virtual Bridged Local Area
Networks for time sensitive applications such as audio and video.
For these LAN media the transmission delays are assumed to be fixed
and symmetrical. IEEE 802.1AS specifies the use of IEEE 1588
specifications where applicable in the context of IEEE Standards
802.1D and 802.1Q. Synchronization to an externally provided timing
signal (e.g., a recognized timing standard such as UTC or TAI) is not
part of this standard but is not precluded. IEEE 802.1AS will
specify how stations attached to bridged LANs to meet the respective
jitter, wander, and time synchronization requirements for time-
sensitive applications.
Appendix B. Other Forums Working in this Problem Space
The NTP WG is the IETF group working on time distribution, but is
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presently only documenting NTPv4 and is not working on new algorithms
or protocols. It is expected that many participants of the NTP WG
will participate in the TICTOC effort.
The PWE3 WG has discussed frequency distribution for the TDM PW
application, however it is not chartered to develop protocols for
this purpose. It is expected that participants of the PWE3 WG who
were active in the TDM PW discussions will participate in the TICTOC
effort.
The IEEE 1588 approved a new version of the IEEE 1588 protocol that
will run over more types of PSNs. The protocol to be specified
contains elements that will be of use in an IETF environment, but is
unlikely to be regarded as being a complete, robust solution in such
an environment. If the IEEE 1588 structure is deemed to be a
suitable platform, then the IETF could contribute an Internet
profile, including a complete distributed systems environment
suitable for our purposes. Alternatively, the IETF could perhaps
borrow some of the delay correction mechanisms and incorporate them
into a development of a new version of NTP.
In addition, IEEE 802.1AS is working on Timing and Synchronization
for Time-Sensitive Applications in Bridged Local Area Networks,
basing itself on the IEEE 1588 standard.
ITU-T SG15 Question 13 has produced Recommendation G.8261 "Timing and
synchronization aspects in packet networks" [G8261]. This
Recommendation defines requirements for various scenarios, outlines
the functionality of frequency distribution elements, and provides
measurement guidelines. It does not specify algorithms to be used
for attaining the performance needed. ITU-T has also consented
G.8262 "Timing Characteristics of Synchronous Ethernet Equipment
Slave Clock (EEC)" [G8262], and G.8264 "Distribution of timing
through packet networks" [G8264]. G.8262 specifies the requirements
for Synchronous Ethernet clocks and G.8264 defines the protocol for
Synchronization Status Message (SSM) for Synchronous Ethernet. To
date the ITU-T has focused on Ethernet infrastructure, but this is
likely to extend to an MPLS environment. Two new work items,
G.paclock.bis and G.pacmod.bis extend the work, and in particular,
G.pacmod.bis intends to introduce time transfer. The scope for
G.paclock.bis is to define the requirements for packet-based clocks.
This is an area where the IETF, with its expertise in IP and MPLS
networks, may co-operate with the ITU.
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Authors' Addresses
Silvana Rodrigues
Zarlink Semiconductor
400 March Road
Ottawa K2K 3H4
Canada
Phone: +1 613 2707258
Email: silvana.rodrigues@zarlink.com
Kurti Erik Lindqvist
Netnod
Bellmansgatan 30
Stockholm S-118 47
Sweden
Phone: +46 708 30 60 01
Email: kurtis@kurtis.pp.se
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