77
Figure 32
Basic work flow
The synchronization process is as follows:
1.
Device A sends Device B an NTP message, which is timestamped when it leaves Device A.
The time stamp is 10:00:00 am (T1).
2.
When this NTP message arrives at Device B, Device B adds a timestamp showing the time
when the message arrived at Device B. The timestamp is 11:00:01 am (T2).
3.
When the NTP message leaves Device B, Device B adds a timestamp showing the time when
the message left Device B. The timestamp is 11:00:02 am (T3).
4.
When Device A receives the NTP message, the local time of Device A is 10:00:03 am (T4).
Up to now, Device A can calculate the following parameters based on the timestamps:
•
The roundtrip delay of the NTP message: Delay = (T4 – T1) – (T3 – T2) = 2 seconds.
•
Time difference between Device A and Device B: Offset = ((T2 – T1) + (T3 – T4)) /2 = 1 hour.
Based on these parameters, Device A can be synchronized to Device B.
This is only a rough description of the work mechanism of NTP. For more information, see the related
protocols and standards.
NTP architecture
NTP uses stratums 1 to 16 to define clock accuracy, as shown in
. A lower stratum value
represents higher accuracy. Clocks at stratums 1 through 15 are in synchronized state, and clocks at
stratum 16 are not synchronized.
IP network
Device B
Device A
10:00:00 am
11:00:01 am
10:00:00 am
NTP message
NTP message
Device A receives the NTP message at 10:00:03 am
1.
3.
2.
4.
NTP message
10:00:00 am
11:00:01 am
11:00:02 am
NTP message
10:00:00 am
11:00:01 am
11:00:02 am
