Meinberg PTP270PEX, Manual

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3 Precision Time Protocol (PTP) / IEEE1588
Precision Time Protocol (PTP or IEEE 1588) is a time synchronization protocol that oers sub-microsecond ac-
curacy over a standard Ethernet connection. This accuracy can be achieved by adding a hardware timestamping
unit to the network ports that are used for PTP time synchronization. The timestamping unit captures the exact
time when a PTP synchronization packet is sent or received. These timestamps are then taken into account to
compensate for transfer delays introduced by the Ethernet network.
In PTP networks there is only one recognized active source of time, referred to as the Grandmaster Clock.
If two or more Grandmaster Clocks exist in a single network, an algorithm dened in the PTP standard is used to
determine which one is the best source of time. This Best Master Clock algorithm must be implemented on
every PTP/IEEE1588 compliant system to insure that all clients (Slave Clocks) will select the same Grandmas-
ter. The remaining deselected Grandmaster Clocks will step back and enter a passive mode, meaning that they
do not send synchronization packets as long as that is being done by the designated Grandmaster.
The existing network infrastructure components play a big role in a PTP network and directly inuence the
level of accuracy that can be achieved by the clients. Asymmetric network connections degrade the accuracy,
therefore classic layer 2 and 3 Ethernet switches with their store and forward technology are not suitable for
PTP networks and should be avoided. With activating the HQ-Filter (see chapter HQ-Filter) the Jitter can be
eliminated. Simple Ethernet hubs with xed pass-through times are not a problem. In large networks, special
switches with built-in PTP functionality help to maintain high accuracy even over several subnets and longer
distances. These components act as "Boundary Clocks" (BC) or "Transparent Clocks" (TC). They compensate
their internal packet processing times by using timestamping units on each port. When acting as a Boundary
Clock, they synchronize to the Grandmaster clock, and in turn act as a Master to the other subnets they are
connected to. When acting as a Transparent Clock, then the "residence time" of the Masters' Sync-Packet is
measured and added to the packet as a correction value. Internally the PTP timescale TAI (see chapter Timescale
in Global Parameters).
3.1 General Information
The internal PTP card acts as a network interface card (10/100MBit) with an integrated hardware time stamp
unit to obtain time stamps in PTP compatible networks. In conjunction with a single board computer running
the PTP protocol stack and a reference time source (PTP master only) the module is capable of building a PTP
Master or Slave system:
10/100MBit
LAN
Network Interface Card
PTP Time Stamp Unit
Single Board Computer
PTP Protocol Stack
Reference Time Source
GPS Receiver
PTP Master System only
USB
PPS 10MHz
PTP270PEX Date: 22nd November 2012 5