Comtech EF Data CDM-625A - Manual (Page 664)

CDM-625A Advanced Satellite Modem MN-CDM625A
Appendix K Revision 3
K–16
The Precision Time Protocol is used to synchronize clocks throughout a computer network.
Based on the IEEE 1588v2 (2008) standard, PTP achieves clock accuracy in the nanosecond range
– much more accurate than what is attainable by NTP (Network Time Protocol) – and it is also
used in network applications where GPS is either unaffordable or inaccessible.
Figure K-13 shows a typical network configuration. Here, the RNC/BSC (Radio Network
Controller/Base Station Controller) at the near-end side of the network and the BTS (Base
Transceiver Station) on the distant-end side of the network are the IEEE 1588v2-capable devices.
In this topology the RNC/BSC serves as the Grandmaster – the root timing reference that
transmits synchronization information to the clocks residing on its network segment – or slave
to the Grandmaster.
Comtech EF Data’s implementation of PTP in the CDM-625A operates over IP, without the
presence of the optional IP Packet Processor. To achieve high accuracy time and frequency
synchronization, PTP relies on hardware time stamping at the ingress and egress ports of the
network; therefore, all devices in the network must support PTP.
Without PTP implementation and support in the modem, it is very difficult to achieve PTP end-
to-end (RNC ► BTS) time synchronization in nanosecond range due to the presence in the
network of variable delay components such as QoS (Quality of Service) buffer, Rx/Tx FIFOs,
Satellite Doppler, etc.
The CDM-625A bridges PTP from LAN to WAN and vice versa, and uses 2-step synchronization
(i.e., it sends both Sync and Follow-up messages when acting as a master). When negotiating
with devices over the LAN interface, the modem uses UDP multicast PTP messages on UDP Port
319 for events, and UDP Port 320 for general packets. On the WAN interface, the modem uses
UDP Port 59319 for events, and UDP Port 59320 for general packets.
Some devices use the Announce message to negotiate which is the PTP master, a process known
as the Best Master Clock (BMC) algorithm. This algorithm allows PTP devices to vote on which
device has the best clock resolution. The CDM-625A will become a PTP master if there is no
Grandmaster device or the Grandmaster is not reachable.
You must configure the modem for the Grandmaster location:
• When the modem can reach the Grandmaster device only through its LAN interface, set
Grandmaster to LAN.
When Grandmaster is LAN, then the modem knows it must become a slave to the
Grandmaster on the LAN interface, and will set its PTP Clock Priority1 value to 255 (the
lowest).
• When the modem must synchronize with the Grandmaster over the WAN interface, set
Grandmaster to WAN.
When Grandmaster is WAN, the modem will set its PTP Clock Priority1 value to 1 (the highest)
to force slave devices that use the BMC algorithm to accept the modem as the PTP master.