1. IEEE 1588 Conference and Plug-fest Winterthur, October 10-12, 2005
Using PTP for Synchronizing Legacy Networks
Heiko Gerstung, Martin Burnicki, Udo Maltzahn Meinberg Radio Clocks

2. Synchronizing Legacy Networks Overview
Motivation
Todays Solutions
Differences between PTP and NTP
Solutions
Combined PTP GMC/NTP Time Server with GPS
Using PTP to synchronize a “slave” Time Server with a GPS based Time Server
Comparing NTP and PTP Clients

3. Synchronizing Legacy Networks Motivation
Time Synchronization over Network Links with an accuracy of <1ms
Use an already existing stable time source for PTP networks
Provide a high-precision time using standard network infrastructure (standard cable systems)
Maximize the distance between GPS antenna location and time server location (e.g. underground installations)

4. Synchronizing Legacy Networks Today‘s Solutions
Network Time Synchronization:
Network Time Protocol (NTP)
Simple Network Time Protocol (SNTP)

5. Synchronizing Legacy Networks Today‘s Solutions
Single Node Synchronization: using dedicated hardware references
GPS
Longwave Radio
Frequency Standards
High Quality Oscillators
IRIG

6. Synchronizing Legacy Networks Main Differences between PTP and NTP
Network Infrastructure NTP: LAN/WAN,Unicast,Broadcast,Multicast PTP: LAN,Multicast
Time Sources NTP: multiple servers, weighted average PTP: One single GMC at a time
Accuracy NTP: under 1ms possible, 1-10ms typical in LAN, <100ms over the Internet PTP: Sub-Microsecond
Security NTP: Symmetric Keys, Public Keys PTP: Work In Progress

7. Synchronizing Legacy Networks Main Differences between PTP and NTP
Different Target Audiences:
PTP
Applications with high demands on accuracy
Relative synchronization
Dedicated Networks
NTP
Applications with low/medium demands on accuracy
Synchronization using absolute time (UTC)
Corporate LAN/WAN and Internet

8. Synchronizing Legacy Networks Applications
Time Server talking both PTP and NTP  both NTP and PTP clients use the same time source  reducing equipment
Using PTP for synchronizing NTP servers  using PTP as a „time backbone“ for NTP infrastructures  reduce network related time error by using PTP instead of NTP for synchronization between two NTP servers

9. Synchronizing Legacy Networks Solutions

10. Synchronizing Legacy Networks Time Server talking both PTP and NTP
both NTP and PTP clients use the same time source, e.g. GPS, IRIG, Long Wave radio…
all NTP and PTP clients are using comparable time stamps
Eliminates the need to purchase, install, configure and maintain two different systems for each client community

11. Synchronizing Legacy Networks Combined PTP GMC/NTP Time Server

12. Synchronizing Legacy Networks Combined PTP GMC/NTP Time Server

13. Synchronizing Legacy Networks Combined PTP GMC/NTP Time Server
Design Overview:
The 50 Mhz clock of the TSU is derived from the internal Oscillator of the GPS receiver
This oscillator is constantly disciplined using the GPS signal
At start-up the absolute time of the TSU is set to the GPS time
The TSU has an additional input with which the nanosecond part can be zeroed. The PPS output of the GPS receiver is connected to this pin and is used to reset the ns part once the internal oscillator of the GPS receiver has warmed up
By using this mechanism, the timestamp offset from the GPS PPS is <20 ns

14. Synchronizing Legacy Networks Applications

15. Synchronizing Legacy Networks Using PTP for Synchronizing NTP Servers
Combined PTP GMC/NTP servers use PTP to achieve a highly accurate inter-server synchronization („time backbone“) and NTP to provide synchronization to „leaf nodes“ (which are only compatible with SNTP/NTP)
Provide special purpose outputs (10Mhz, PPS, IRIG, E1/T1) without the need to install GPS receivers

16. Synchronizing Legacy Networks Using PTP for synchronizing NTP Servers

17. Synchronizing Legacy Networks Using PTP for Synchronizing NTP Servers
Design Overview:
At start-up the system waits until it is synchronized by PTP (PTP_SLAVE state has been reached)
Now the system clock is set to the time of the TSU
Afterwards, NTP is started. The NTP daemon has been modified with an additional refclock driver
NTP reads the TSU‘s time by using a kernel device driver
The TSU looks like a hardware reference clock to the NTP daemon

18. Synchronizing Legacy Networks Comparing NTP and PTP Clients

19. Synchronizing Legacy Networks Thank You
Questions?