Overview
Definition
The Precision Time Protocol (PTP) is a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, which is also called IEEE 1588 (1588 for short). This protocol enables standard Ethernet terminal devices to synchronize time and clock frequency from a clock source. 1588 includes 1588v1 and 1588v2. 1588v1 provides sub-millisecond precision time synchronization, and 1588v2 provides sub-microsecond precision time synchronization. 1588v2 is a time synchronization protocol. 1588v1 initially applied to high-precision time synchronization between devices and now is also used for frequency synchronization. 1588v1 has been replaced by 1588v2. Unless otherwise stated, PTP described in this document refers to 1588v2.
Purpose
The evolution of mobile networks to IP networks requires high-precision network clock synchronization between data communication devices. On an IP radio access network (RAN), the clock frequencies of base stations must be synchronized in a certain accuracy. Otherwise, clock slip occurs or even calls are interrupted when mobile phones are switched from a base station to another. Some wireless standards require phase synchronization in addition to frequency synchronization. Table 5-1 describes clock phase synchronization requirements.
Wireless Technology |
Clock Frequency Accuracy Requirements |
Clock Phase Synchronization Requirements |
|---|---|---|
GSM |
0.05 ppm |
NA |
WCDMA |
0.05 ppm for a base station |
NA |
Pico 0.1 ppm |
NA |
|
TD-SCDMA |
0.05 ppm |
3 µs |
CDMA2000 |
0.05 ppm |
3 µs |
WiMax FDD |
0.05 ppm |
NA |
WiMax TDD |
0.05 ppm |
1 µs |
Clock synchronization requirements of base stations with different standards can be met by various technologies, including synchronous Ethernet, Network Time Protocol (NTP), and PTP. Traditionally, base stations are directly connected to the global positioning system (GPS) for time synchronization. This method requires antennas to be installed in each base station, increasing project implementation and maintenance costs. The sub-second precision provided by NTP and sub-millisecond precision provided by 1588v1 cannot meet requirements of base stations for time synchronization. Synchronous Ethernet can only synchronize the clock frequency. PTP can provide sub-microsecond precision time synchronization with the assistance of hardware, meeting time synchronization requirements of base stations.
Benefits
PTP brings the following benefits:- Provides sub-microsecond precision time synchronization for transmission devices.
- Reduces network construction and maintenance costs because no GPS receiver needs to be deployed or maintained for base stations.
- Protects national security because PTP allows clock synchronization without the GPS.
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