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.