Time & Clock Flashcards
What is Offset (context in error for time)
Time difference between respective microticks of two clocks, measured in the number of microticks of the reference clock z.
What is accuracy (context in error for time)
offset of clock k with respect to reference clock z
Network Time Protocol (NTP) Summary
- Standard for clock synchronisation in variable-latency networks
- Synchonisation to within a few milliseconds of UTC (usually within tens of milliseconds)
- Server-client architecture
- Structured in strata
Sources for Latency (4)
- Speed of signal traveling across a medium
- Conversion time in the transceivers
- Quality of the network router switches and interfaces
- Software/driver/firmware stack to send/receive a network packet
Precision Time Protocol (PTP) Schema
- T1…’s timestamp of the Sync transmission
- T′1…’s timestamp of the Sync reception
- T2…’s timestamp of the Delay_Req transmission
- T′2…’s timestamp of the Delay_Req reception
The master send the Sync message without its timestamp. Why does it send the Follow_Up message containing the timestamp of the Sync message and not only the Sync message?
Generic hardware and software stacks that implement PTP don’t have the ability to add a precise timestamp to a message. Therefore, the Follow_Up message is sent by the master to convey the captured timestamp of transmitted Sync message
How do we define which clock becomes the master
We compare announce messages to determine which clock is the most accurate using the best master clock algorithm (BMCA)
How does the BMCA work?
We compare their properties in the following order to determine the best master clock
- Priority1
- Class
- Accuaracy
- Variance
- Priority2
- Identity
How do we define which clock becomes the master?
We compare announce messages to determine which clock is the most accurate using the best master clock algorithm (BMCA)
