Westermo U200 Operator manal - Manual (Page 38)

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- Peer-to-Peer (P2P) propagation delay measurement
E2E according to IEEE1588 version 2 is based on the same principle for propagation delay
measurement and compensation as for IEEE1588 version 1. That means the Slaves are
responsible the measurement and compensation for propagation delay between the
respective Slave and the Grand Master.
P2P according to IEEE1588 version 2 is based on a different principle. IEEE1588 Grand
Masters, Transparent Clocks or Slaves that support P2P perform propagation delay
measurements on each network link, and the device is responsible for propagation delay
compensation related to SYNC and DEL_REQ packets that are forwarded on the respective
link. The benefit of the P2P method is first of all related to potential network topology changes
that will represent new network paths between the Grand Masters and the Slaves. Thus, P2P
support on the Transparent Clocks (i.e. the Ethernet switches and routers with IEEE1588
version 2 support) is very useful since these IEEE1588 devices handle the network topology
changes.
The T200 IEEE1588 version P2P implementation can be utilized by IEEE1588 Grand Masters
and Slaves that either is based on IEEE1588 version 1 or IEEE1588 version 2 E2E since the
T200 uses P2P on the network links between the T200’s and on those network links used
versus end nodes that support T200 IEEE1588 version P2P.
12.4 SNTP/NTP time server
The SNTP/NTP is based on unicast communication. I.e. the NTP/NTP client sends a
SNTP/NTP request, and the T200 SNTP/NTP server sends a corresponding SNTP/NTP
reply. The SNTP/NTP replay packet contains both the receive time stamp of the SNTP/NTP
request packet and the transmit time stamp of the reply packet. I.e. the T2 and T3 time
stamps (see figure above).
Note: the SNTP/NTP T3 accuracy is as good as the corresponding T3 time stamp of IEEE
1588 implementation. This is achieved by using the Westermo OnTime IPR principle for
deterministic access of an Ethernet packet to an Ethernet drop link.
12.5 SNTP/NTP time client or PTP Slave and GPS emulation
The switch can also operate as an SNTP/NTP time client or PTP Slave. The switch will then
perform time updates versus a SNTP/NTP servers or a PTP grandmaster in the network in
order to update the local clock of the T200 switch. The SNTP/NTP time servers or PTP
Grand Master used for time updates are automatically detected. The SNTP/NTP server IP
address must be defined on the SNTP client since the time updates are based on unicast.
The switch will perform an IGMP join to the PTP MC group in case the switch is enabled as a
PTP Slave.
The switch can also act as a serial time server, GPS emulation, on the serial interface since
this switch is configured as a SNTP client or PTP Slave. This means that serial enabled
devices can be connected to the T200 serial interface and receive time updates on this
interface. Two different formats can be generated:
- UTC string
- NMEA, GP RMC report
Time updates is sent every seconds.