Technical data

www.westermo.com Theoretical and general applications 137
SNTP/NTP
RFC 2030 Simple Network Time Protocol (SNTP), RFC 1305 Network Time
Protocol (NTP) and P1588 are established protocols for time synchronised IP traffic.
SNTP is a subset of NTP. The SNTP/NTP server handles the system clock, which in
turn can be based on GPS or the internal clock. The time information is then distri-
buted either through unicast or multicast.
1. Updating via unicast, updating is initiated by the client after which the server returns
an answer. The time reference is added to all communication between the client
and server, this is to be able to calculate maximum accuracy.
2. Updating via multicast, the time is sent from the server to the group of clients
(multicast group) at defined intervals. It is not possible for the clients to calculate
the delay in the network.
Time stamping via applications
Most SNTP/NTP applications generate time stamping of data on the application
layer, accuracy is then dependent on the delay/jitter through the entire IP-stack.
Typical accuracy for this technique is one or two milliseconds.
Time stamping using Ethernet drivers
Accuracy can be significantly improved when time stamping is done using the
Ethernet Interrupt Service Routine, time stamping is then carried out when the data
is sent between the server and client.The request is generated from the client, the
accuracy is dependent in this case on the jitter in the interrupt handling on the serv-
er and client. Accuracy in this application varies from around 10 µs to about 100 µs.
Time stamping on the physical layer
The delay through the IP-stack can be eliminated if time stamping is carried out
on the physical layer, i.e. via hardware. In this case, time synchronisation can be
extremely accurate, better than 1 µs. This accuracy requires a direct connection
between the server and client, as further equipment would add to the delay. For this
reason the time server is integrated in the switch. In addition, there is the possibility to
synchronise the switch from the reference clock via GPS or from the internal oscillator.
Time stamping
via applications
Time stamping
using Ethernet drivers
Time stamping
on the physical layer
Server
Application
IP
MAC
Physical
TCP UDP
Application
IP
MAC
Physical
TCP UDP
Distributed
time
Distri-
buted
time
Inquiry
Client
Server
Application
IP
MAC
Physical
TCP UDP
Application
IP
MAC
Physical
TCP UDP
Inquiry
Client
Distri-
buted
time
Server
Application
IP
MAC
Physical
TCP UDP
Application
IP
MAC
Physical
TCP UDP
Inquiry
Client
B A C K