Albedo xGenius - Precedents: IP Synchronization with NTP; PTP Protocol Details

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User Guide
210
9.1.1. Precedents: IP Synchronization with NTP
The Network Time Protocol (NTP), is one of the oldest protocols still in use and it is
available in two flavours: the full version and Simple NTP (SNTP), a subset of NTP.
The latest version of NTP, version 4 (NTPv4) can usually maintain time to within 1-
20 ms using traditional software-interrupt based solutions over the public Internet and
can achieve accuracies of microseconds or better in LANs under ideal conditions. NTP
has been the most common and arguably the most popular synchronization solution,
because it performs well over LANs and WANs and at the same time it is inexpensive,
requiring very little hardware.
NTP should be able to deliver accuracy of 1-2 ms on a LAN and 1-20 ms on a WAN.
However, protocol performance is far from guaranteed largely because of variable
delays added by switches and routers.
9.1.2. PTP Protocol Details
PTP requires a central grandmaster clock and low-cost PTP slave clock sites. Master
and slave network devices are kept synchronized by the transmission of timestamps
transmitted within the PTP messages.
Depending on how many ports has a network clock, it is referred by the IEEE 1588
standard as an Ordinary Clock (single port device) or a Boundary Clock (multi port
device). The version 2 standard also defines the concept of Transparent Clocks that
improve timing accuracy when the protocol is run in network paths which contain
intermediate switches.
The normal execution of the PTP has two phases:
1. Master-Slave hierarchy establishment. Ordinary and boundary clocks decide
which port has the master or slave role in each link with the help of the Best Master
Clock (BMC) algorithm. The information required for operation of the BMC is sup-
Table 9.1: IEEE 1588v2 Device Description
Device Description
Ordinary Clock A single port device that can be a master or slave clock.
Boundary Clock A multi port device that can be a master or slave clock.
End-to-end Transpar-
ent Clock
A multi port device that is not a master or slave clock but a
bridge between the two. Forwards and corrects all PTP
messages.
Peer-to-peer Trans-
parent Clock
A multi port device that is not a master or slave clock but a
bridge between the two. Forwards and corrects Sync and
Follow-up messages only.
Management Node A device that configures and monitors clocks.