IEEE Standard 1588 defines the Precision Time Protocol (PTP) to synchronize clocks over packet networks. PTP is needed for applications that require precise timing such as Time Division Multiplexing (TDM) over IP. PTP uses network messages and timestamps to synchronize slave clocks to a master clock with nanosecond precision. PTP messages include sync, delay request, follow up, and delay response. Hardware time stamping is often required to achieve high precision with low delay and jitter. PTP is a cheaper and more scalable solution than alternatives like GPS or atomic clocks for synchronizing networks to within 50 parts per billion.

1. IEEE Standard 1588
Precision Timing Protocol - PTP
許勝倉@NCU 2008/March/08

2. Why Timing Sync
• Time Division Multiplexing - TDM
• Circuit Emulation over Packet - CESoP
• Time Division Multiplexing - TDMoIP
• Wireless Backhaul
• Building Integrated Timing Supplies - BITS

3. Clock Source to Sync
• Global Positioning System- GPS
• Network Time Protocol - NTP
• Adaptive clock Recovery - ACR
• Precision Timing Protocol - PTP
• Building Integrated Timing Supplies - BITS
• Clock from Loop
• Atom Clock Source

4. Timing Requirements
T1 Standard, Maximum time variation
• 8.4us over 900 seconds
• 18us over 24 hours
T1 Standard, Maximum time Interval error at sync
• 1us over 2000 seconds
• 2us over 100000 seconds
Mobile station frequency accuracy
• 50 Parts Per Billion (ppb)

5. Timing protocols
problems
• ACR - Expensive oscillate source
• GPS - Weak signal indoor
• NTP - Not meet 50ppb
• BITS - Price too high
• Loop Clock - Can’t over Packet Network
• Atom Clock Source - Price very high

6. Synchronizing TDM
Internet
BITS/SSU
PTP Master
T1/E1
Access Node
PSTN DS1
ADSL
PSTN Switch PTP Slave
Ethernet

7. How PTP work
Preamble SFD SRC MAC DST MAC Length/Type Data CRC
10101011 IP Header UDP Header PTP Message

8. Clock Network
Ordinary Clock (Best Master)
Master Clock Source
Master Master Master Slave Boundary Clock (Ethernet Switch)
Salve Salve Salve
Master Master Master Salve Boundary Clock (Ethernet Switch)
Salve Salve Ordinary Clock

9. PTP Messages Type
Message Value
SYNC 0
DELAY REQUEST 1
FOLLOW UP 2
DELAY RESPONSE 3
MANAGEMENT 4
Reserved 5-255

10. PTP in Software
• PTP define in IANA Multicast address
224.0.1.129-224.0.1.132
• Use UDP port 319 for Event message
• Use UDP port 320 for general message
• Packet size less than 300 bytes
• Header length less than 40 bytes

11. 0x0800 SRC IP 224.0.1.129 UDP
SYNC Port 319 PTP protocol
Message Origin TimeStemp in Origin TimeStemp in
PTP vsersion
Type Seconds Nano Seconds
DELAY REQUEST Port 319 PTP protocol
Message Origin TimeStemp in Origin TimeStemp in Estimated Master Estimated Master
PTP vsersion
Type Seconds Nano Seconds Variance Drift
FOLLOW UP Port 320 PTP protocol
Message Origin TimeStemp in Origin TimeStemp in
PTP vsersion
Type Seconds Nano Seconds
DELAY RESPONSE Port 320 PTP protocol
Message Delay Receipt in Delay Receipt in
PTP vsersion
Type seconds Nano Seconds

12. T1
Sync
T2
Follow-up
T3
Delay request
T4
Delay response
Sync T1
Follow-up
Sync
Follow-up
Master Network Slave

13. Offset
T1
Sync Delay
T2
Follow-up
T3
Delay
Delay request
Offset
T4
Delay response
Master Slave

14. Delay and Jitter
Master Slave
PTP PTP
Application
UDP UDP
IP IP
Delay and
Jitter
Protocol
Stack
Driver Driver
Driver
MAC MAC
MII
PHY PHY
Network
Delay and Jitter Network

15. Hardware for
Time Stamp
Microprocessor Ethernet MAC Ethernet PHY LAN/WAN
Time Stamping Unit
(TSU)
Precision time stamping in hardware

16. Open Source PTPD
http://ptpd.sf.net

17. Conclusion
• PTP require addition hardware to reduce
processor and software’s delay and jitter
for more precision clock.
• PTP is cheaper than Atom clock source.
• PTP could meet 50 ppb requirement of
TDM network.
• With PTP, TDM could over Packet Switch
Network without sync clock loss.

18. Reference
• IEEE Standard 1588-2002 Version 2
• IEEE 1588 Precision Timing Protocol, Jeremy Bennington, Symmetricom
• High Precision clock Synchronization according to IEEE 1588 Implementation
and Performance Issue, Hans Weibel, Zurich Univ.
• PTPD for Linux http://ptpd.sf.net
• IEEE Standard 802.1AS Draft