Overview of 1588v2 , SMPTE-2059-2 and G.8275.1
Definition
Synchronization
This is the process of ensuring that the frequency offset or time difference between devices is kept within a reasonable range. In a modern communications network, most telecommunications services require network clock synchronization in order to function properly. Network clock synchronization includes time synchronization and frequency synchronization.
Time synchronization
Time synchronization, also called phase synchronization, means that both the frequency of and the time between signals remain constant. In this case, the time offset between signals is always 0.
Frequency synchronization
Frequency synchronization, also called clock synchronization, refers to a constant frequency offset or phase offset. In this case, signals are transmitted at a constant average rate during any given time period so that all the devices on the network can work at the same rate.
Figure 1 shows the differences between time synchronization and frequency synchronization. If Watch A and Watch B always have the same time, they are in time synchronization. If Watch A and Watch B have different time, but the time offset remains constant, for example, 6 hours, they are in frequency synchronization.
IEEE 1588
IEEE 1588 is defined by the Institute of Electrical and Electronics Engineers (IEEE) as Precision Clock Synchronization Protocol (PTP) for networked measurement and control systems. It is called the Precision Time Protocol (PTP) for short.
IEEE 1588v1, released in 2002, applies to industrial automation and tests and measurements fields. With the development of IP networks and the popularization of 3G networks, the demand for time synchronization on telecommunications networks has increased. To satisfy this need, IEEE drafted IEEE 1588v2 based on IEEE 1588v1 in June 2006, revised IEEE 1588v2 in 2007, and released IEEE 1588v2 at the end of 2008.
Targeted at telecommunications industry applications, IEEE 1588v2 improves on IEEE 1588v1 in the following aspects:- Encapsulation of Layer 2 and Layer 3 packets has been added.
- The transmission rate of Sync messages is increased.
- A transparent clock (TC) model has been developed.
- Hardware timestamp processing has been defined.
- Time-length-value (TLV) extension is used to enhance protocol features and functions.
1588v2 is a time synchronization protocol which allows for highly accurate time synchronization between devices. It is also used to implement frequency synchronization between devices.
ITU-T G.8275.1
ITU-T G.8275.1 defines the precision time protocol telecom profile for phase/time synchronization with full timing support from the network.
G.8275.1 defines three types of clocks, including T-GM, T-BC and T-TSC. A bearer network device is configured as the T-BC.
SMPTE-2059–2
SMPTE-2059-2 is an IEEE 1588-based standard that allows time synchronization of video devices over an IP network.
Purpose
Data communications networks do not require time or frequency synchronization and, therefore, routers on such networks do not need to support time or frequency synchronization. On IP radio access networks (RANs), time or frequency needs to be synchronized among base transceiver stations (BTSs). Therefore, routers on IP RANs are required to support time or frequency synchronization.
Frequency synchronization between BTSs on an IP RAN requires that frequencies between BTSs be synchronized to a certain level of accuracy; otherwise, calls may be dropped during mobile handoffs. Some wireless standards require both frequency and time synchronization. Table 1 shows the requirements of wireless standards for time synchronization and frequency accuracy.
Wireless Standards |
Requirement for Frequency Accuracy |
Requirement for Time Synchronization |
|---|---|---|
GSM |
0.05 ppm |
NA |
WCDMA |
0.05 ppm |
NA |
TD-SCDMA |
0.05 ppm |
3us |
CDMA2000 |
0.05 ppm |
3us |
WiMax FDD |
0.05 ppm |
NA |
WiMax TDD |
0.05 ppm |
1us |
LTE |
0.05 ppm |
In favor of time synchronization |
Different BTSs have different requirements for frequency synchronization. These requirements can be satisfied through physical clock synchronization (including external clock input, WAN clock input, and synchronous Ethernet clock input) and packet-based clock recovery.
Traditional packet-based clock recovery cannot meet the time synchronization requirement of BTSs. For example, NTP-based time synchronization is only accurate to within one second and 1588v1-based time synchronization is only accurate to within one millisecond. To meet time synchronization requirements, BTSs need to be connected directly to a global positioning system (GPS). This solution, however, has some disadvantages such as GPS installation and maintenance costs are high and communications may be vulnerable to security breaches because a GPS uses satellites from different countries.
1588v2, with hardware assistance, provides time synchronization accuracy to within one micro second to meet the time synchronization requirements of wireless networks. Thus, in comparison with a GPS, 1588v2 deployment is less costly and operates independently of GPS, making 1588v2 strategically significant.
In addition, operators are paying more attention to the operation and maintenance of networks, requiring routers to provide network quality analysis (NQA) to support high-precision delay measurement at the 100 us level. Consequently, high-precision time synchronization between measuring devices and measured devices is required. 1588v2 meets this requirement.
1588v2 packets are of the highest priority by default to avoid packet loss and keep clock precision.
Benefits
This feature brings the following benefits to operators:
Construction and maintenance costs for time synchronization on wireless networks are reduced.
Time synchronization and frequency synchronization on wireless networks are independent of GPS, providing a higher level of strategic security.
High-accuracy NQA-based unidirectional delay measurement is supported.
Y.1731 and IPFPM are supported.
Concepts of G.8275.1
ITU-T G.8275.1 defines the precision time protocol telecom profile for phase/time synchronization with full timing support from the network. G.8275.1 is defined as a time synchronization protocol.
A physical network can be logically divided into multiple clock domains. Each clock domain has its own independent synchronous time, with which clocks in the same domain synchronize.
A Telecom grandmaster (T-GM) can only be the master clock that provides time synchronization.
A Telecom-boundary clock (T-BC) has more than one G.8275.1 interface. One interface of the T-BC synchronizes time signals with an upstream clock, and the other interfaces distribute the time signals to downstream clocks.
- A Telecom-transparent clock (T-TC) has more than one G.8275.1 interface through which the T-TC forwards G.8275.1 packets, and corrects the packet transmission delay. A T-TC does not synchronize the time through any of these G.8275.1 interfaces.
A Telecom time slave clock (T-TSC) can only be the slave clock that synchronizes the time information of the upstream device.
The NetEngine 8000 F can function as the T-BC and T-TC only.
Concepts of SMPTE-2059-2
SMPTE-2059-2 is an IEEE 1588-based standard that allows time synchronization of video devices over an IP network.
The SMPTE-2059-2 protocol provides acceptable lock time, jitter, and precision.
SMPTE-2059-2 is developed based on IEEE 1588. For information about the principles, networking, and related concepts of SMPTE-2059-2, see the IEEE 1588 protocol.