Delivering timing accuracy in 5G networks
How IEEE 1588 PTP has evolved to meet the stringent time synchronization requirements of 5G
In mobile networks, time synchronization has always been important. Each new generation of mobile networks has driven the need for increased precision and accuracy in synchronization standards and solutions. Nevertheless, 5G is different. 5G time synchronization requirements are the most demanding seen to date and have elevated the importance of time synchronization and the need for more accurate solutions.
It is 5G requirements to support Ultra-Reliable Low-Latency Communications (URLLC) services, which are driving the need for greater timing accuracy. The 5G vision of new revenue generating services like autonomous vehicle connectivity and eHealth services are simply not possible without highly accurate and reliable time synchronization.
Even services like massive Industrial Internet of Things (IIoT) and other industrial automation services require reliable time synchronization.
5G is an entirely packet-based network from the core to the antenna. The challenge for 5G networks is how to provide ultra-reliable and highly accurate time synchronization over the 5G packet network. The answer is IEEE 1588 Precision Time Protocol (PTP).
PTP profiles for 5G networks
The IEEE 1588 PTP standards provide a wealth of options and the basis for highly reliable and accurate time synchronization solutions. However, the time synchronization needs of specific applications in different industries can vary quite significantly. These specific needs are defined in separate PTP profile standards, often in collaboration with other industry standards organizations. These profiles provide guidance on how to implement PTP for the specific needs of that application. There are over a dozen profiles for applications ranging from electricity distribution networks to audio/video broadcast, industrial automation and in-vehicle communication.
Figure 1: PTP Profiles
For 5G, both IEEE and ITU-T provide relevant profiles that can be used to design ultra-reliable and highly accurate time synchronization solutions. These can be seen in Figure 1 (below).
Figure 2: G.8275.1 PTP Telecom Profile
The ITU-T standards include two important telecom profiles for PTP, namely G.8275.1 and G.8275.2. G.8275.1 provides the most accurate solution, but is not always practical to implement. G.8275.2 provides the next best option that can meet some of the real-world challenges that confront modern mobile carriers. We will examine these two profiles in more detail below.
In addition, IEEE also provides a couple of important PTP standards that are relevant to mobile networks. IEEE 802.1AS provides a “generalized” PTP (gPTP) profile that is a part of the Ethernet Time-Sensitive Networking (TSN) set of standards. As the name suggests, gPTP defines a PTP profile that can be used for a wide variety of applications in an attempt to reduce PTP profile proliferation.
For 5G fronthaul networks, the Ethernet TSN set of standards includes a telecom profile that also provides specifications for time synchronization. Based on ITU-T G.8271.1, the IEEE 802.1CM standard specifies fronthaul maximum time alignment errors for different classes of applications that are driving the need for more accurate boundary clocks.
ITU-T PTP Telecom Profiles
ITU-T has defined three PTP profiles for telecom applications described in specifications G.8265.1, G.8275.1 and G.8275.2:
- 8265.1 was first used to deliver accurate frequency synchronization, but not time synchronization, to mobile base-stations
- 8275.1 is designed to deliver both highly accurate frequency synchronization, phase synchronization and Time of Day (ToD) with support for accurate frequency synchronization from the network physical layer
- 8275.2 is designed to deliver accurate frequency synchronization, phase synchronization and ToD with only partial support from the network where non-PTP nodes are also in the network path
G.8265.1 provides frequency synchronization in Frequency Division Duplex (FDD) mobile networks where phase synchronization is not as critical. Only PTP ordinary clocks are defined with transport of unicast PTP packets over IPv4/IPv6 networks. The PTP performance is dependent on the network implementation.
For applications that need frequency synchronization over IPv4/IPv6, it is recommended to use the G.8275.2 profile going forward.
For 5G networks based on Time Division Duplex (TDD), G.8275.1 is the recommended profile to be used, but requires that all switching nodes in the network can operate as PTP Telecom Boundary Clocks (T-BCs). This means that each device has to support its own high-performance oscillator (e.g. an OXCO) and timing logic that will allow the termination and regeneration of PTP packets. It also means that each device must support Synchronous Ethernet or a similar physical layer frequency synchronization input that can meet requirements. Tests have shown that it is very difficult to meet end-to-end time error requirements without a physical layer synchronization input.
There are many circumstances where the requirements of G.8275.1 cannot be met. This can be because parts of the network include switches and routers that cannot support PTP or Synchronous Ethernet input. In these circumstances, G.8275.2 can be used to provide the best time synchronization possible.