Description

• Synchronization

  On a modern communications network, most telecommunications services require that the frequency offset or time difference between devices be within an acceptable range. To meet this requirement, network clock synchronization must be implemented. Network clock synchronization includes frequency synchronization and time synchronization.

  • Time synchronization

    Time synchronization, also called phase synchronization, means that both the frequency of and the time between signals remain consistent. In this case, the time offset between signals is always 0.

  • Frequency synchronization

    Frequency synchronization, also known as clock synchronization, refers to a strict relationship between signals based on a constant frequency offset or a constant phase offset, in which signals are sent or received at the same average rate at a valid instant. In this manner, all devices on the communications network operate at the same rate, with the phase difference between signals remaining at a fixed value.

  Figure 4-63 Time synchronization and frequency synchronization
  

  Figure 4-63 shows the difference between time synchronization and frequency synchronization. In time synchronization, Watch A and Watch B always keep the same time. In frequency synchronization, Watch A and Watch B keep different time, but the time difference between the two watches is a constant value, for example, 6 hours.

• IEEE 1588

  IEEE 1588, also called Precision Time Protocol (PTP), is defined by the Institute of Electrical and Electronics Engineers (IEEE) as a precision clock synchronization protocol for networked measurement and control systems.

  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 grown stronger. To satisfy this demand, 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.
  • The 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.

  IEEE 1588v2 is defined as a time synchronization protocol, which allows for high-precision time synchronization between devices. It is also used to implement frequency synchronization between devices.

• ITU-T G.8275.1

  ITU-T defines the precision time protocol telecom profile for phase/time synchronization with full timing support from the network, known as G.8275.1.

  G.8275.1 defines three types of clocks, including T-GM, T-BC and T-TSC. A bearer network device is configured as a 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.

• ITU-T G.8273.2

  ITU-T G.8273.2 defines various specifications, as shown in Table 1 ITU-T G.8273.2 specifications. The NetEngine 8000 F clock complies with ITU-T G.8275.3 and has the capability to meet the following specifications.

  Table 4-38 ITU-T G.8273.2 specifications

  Indicator	Class A	Class B	Class C
  Max|TE|	100 ns	70 ns	30 ns
  cTE	± 50 ns	± 20 ns	± 10 ns
  MTIE	± 40 ns	± 40 ns	± 10 ns
  TDEV	± 4 ns	± 4 ns	± 1 ns

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. In addition to frequency synchronization, some wireless standards require time synchronization. Table 4-39 lists the requirements of wireless standards for the accuracy of frequency and 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 microsecond 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 μs level. Consequently, high-precise time synchronization between measuring devices and measured devices is required, which I588v2 can provide.

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 carriers:

• 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-precision NQA-based unidirectional delay measurement is supported.

• Y.1731 and IP FPM are supported.

Concepts of G.8275.1

ITU-T defines the precision time protocol telecom profile for phase/time synchronization with full timing support from the network, known as G.8275.1, which is 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.

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 accuracy.

SMPTE-2059-2 is developed based on IEEE 1588. For details about the principles, networking, and related concepts of SMPTE-2059-2, see the IEEE 1588 protocol.