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The PTP 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 I-13 shows a typical network configuration. Here, the Radio Network Controller/Base
Station Controller RNC/BSC at the near-end side of the network and the Base Transceiver
Station (BTS) 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-425 Advanced Satellite Modem 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 Quality of Service (QoS) buffer, Rx/Tx FIFOs, Satellite
Doppler, etc.
The CDM-425 Advanced Satellite Modem bridges PTP from LAN to WAN and vice versa, and
uses a 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-425 Advanced Satellite Modem 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.
Appendix I
CDM-425 Advanced Satellite Modem
I–17
Revision 0
MN-CDM-425
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