By Eric Colard, Microchip Technology

Flexible PTP Profiles Ease the Transition to 5G

As critical infrastructure such as telecommunications, utilities, transportation, and defense migrate from 4G to 5G, you might assume that these essential services universally adopt the ITU-T G.8275.1 Precision Time Protocol (PTP) profile for time synchronizing their networks. After all, PTP embeds a superior quality PTP boundary clock (BC) compared to 4G. This tendency, however, could understate that 5G mobile synchronization has become more granular.

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Time division duplex (TDD) in 5G brings new phase requirements, both relative and absolute. It also introduces Open RAN, with baseband unit (BBU) functions disaggregated into Radio Units (RUs), Distributed Units (DUs), and Centralized Units (CUs). As a result, the telecom industry has standardized two PTP profiles, ITU-T G.8275.1 and ITU-T G.8275.2, to address PTP-aware and PTP-unaware networks, respectively.

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Synchronization Issues

Operators understand the implications of having a backhaul network. Depending on the type of network, packet-delay variation (PDV) can have a major impact on synchronization. In many countries, PTP was deployed in 4G as a backup synchronization mechanism, while Global Navigation Satellite System (GNSS) was the primary synchronization source. To avoid a situation where a GNSS failure leads to the loss of phase services, the idea of connecting the edge primary reference time clock (PRTC) to the centralized core clock using a PTP flow was developed. It was adopted by the ITU-T as G.8273.4 — Assisted Partial Timing Support (APTS).

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In this architecture, the PTP input is calibrated for time error using the local-edge PRTC GNSS. This GNSS has the same reference (Universal Coordinated Time, UTC) as the upstream GNSS. You can consider the incoming PTP flow as effectively a proxy GNSS signal from the core with traceability to UTC.

Figure 1 shows a typical 4G synchronization scenario where a PTP grandmaster serves 4G eNodeBs over a backhaul using PTP unicast G.8275.2 profile.

5G needs a new synchronization architecture because the mobile network has become increasingly complex due to the Open RAN disaggregation. Figure 2 shows the key elements of a 5G architecture.

Operators need to consider the backhaul in addition to a typical 5G architecture. Furthermore, disaggregation introduces fronthaul and mid-haul networks.

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From a synchronization standpoint, the fronthaul becomes the focal network point for serving 5G RUs or 5G base stations. Figure 3 shows how a fronthaul network serves 5G base stations (gNodeBs) using G.8275.1 multicast profile. In this scenario, PTP becomes the primary synchronization mechanism.

Important considerations when implementing 5G include the end-to-end timing budget (±1.5 µsec) and the 130 nsec/260 nsec relative-time accuracy between adjacent Rus, as shown in Figure 3.

ITU-T G.8275.2 profile, on the other hand, resides at layer 3, unicast. It doesn’t require on-path support capability on all the network elements. The PTP protocol flows through those network elements as high-priority traffic. In this use case, the network needs large PTP client capacity support from the PTP grandmaster, typically over one hundred clients and up to several thousand in some cases.

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Fronthaul Profile

Fronthaul, from a synchronization standpoint, operates from a source of time from a GNSS signal. Assisted Partial Timing Support (APTS) protects it in situations when the GNSS signal is unavailable or intermittent.

Fronthaul typically resides in large cities and metro areas that contain many base stations. PTP grandmasters located nearby serve the base station. In this situation, the network uses a profile based on G.8275.1, a PTP profile defined specifically for the telecom industry with network elements that embed a modern boundary clock. G.8275.1 uses multicasting mode, which doesn’t require a lot of capacity.

To date, PTP provides frequency synchronization outside of metro areas. Grandmaster clocks deployed at these locations serve mainly older FDD radio systems. Increasingly, these clocks are part of a mix of older radios and new environments brought to the deployment by the move to 5G.

Many operators are migrating frequency-focused grandmasters to newer generations of IEEE 1588 PTP grandmasters that support 5G requirements through better time and phase accuracy. These clocks also provide additional capabilities and more PTP ports than prior generations. The new grandmasters must connect to many more devices, including older radios, cell towers, and other PTP grandmasters.

These backhaul sites and grandmasters typically utilize the ITU-T G.8275.2 profile, which runs at the internet protocol (IP) layer. The telecom industry focuses on enabling migrations of legacy environments towards newer architectures and devices. Existing legacy signal systems such as Synchronization Supply Units (SSU) and Primary Reference Clocks (PRC) are not going away and need integration into the newer architectures focused on 5G and PTP. Another aspect to consider beyond capacity is the ability to integrate systems located at sites distant from the grandmasters.

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Moving Into 5G

Operators adding 5G mobile services can leverage existing synchronization investments and build upon them. Typically, large operators will install PTP grandmasters in central offices that support wireline broadband and wireless mobility. This leads to four typical use cases:

- Operators use a dedicated Primary Reference Source (PRS), which is common in North America. In those instances, operators will often replace the legacy PRS systems     and migrate to a newer generation grandmaster that can function as a PRS or enhanced PRS (ePRS).

- Operators migrate from a traditional PRTC grandmaster to a more modern platform. This provides more connectivity options and advanced APTS capabilities as well as     frequency synchronization for cell site backhaul (thousands of clients) using PTP G.8275.2.

- Operators will deploy new PRTC grandmasters for 5G fronthaul using PTP G.8275.1.

- Operators migrate existing synchronization systems to more modern and resilient PTP grandmasters that meet stringent 30 ns accuracy to UTC, as well as 14 days    holdover in selected sites. These installations preserve investments. Over time, operators leverage newer technologies to serve 5G sites through an evolution of existing    synchronization infrastructure.

These installations preserve investments. Over time, operators leverage newer technologies to serve 5G sites through an evolution of existing synchronization infrastructure.

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Other Market Dynamics

Aside from the fronthaul and backhaul considerations for choosing a timing profile and capacity requirements, some countries or operators may not own the infrastructure for part or all their deployments.

In North America, operators commonly lease backhaul lines from third parties. These leased lines, however, don’t always meet the operator’s time and phase performance requirements. Mobile operators can’t always rely on the backhaul links and may lack the means to monitor the synchronization quality that third-party leased-line providers deliver.

To serve mobile operators and ensure high accuracy given the stringent timing requirements for 5G architectures, leased line backhaul providers are upgrading their network elements with boundary clocks to deliver highly accurate time and phase to operators.

New entrants such as satellite providers or cable operators are adding mobile to their portfolio. They also rely on third parties to deliver precise time over the leased architecture.

Legacy wireline providers often lease their wireline infrastructure to mobile operators and new mobile entrants. Leased-line providers may need to upgrade their infrastructure to serve mobile operators with accurate time and phase. Mobile operators can then run either G.8275.1 or G.8275.2 over the leased backhaul layer. Operators leasing lines should make sure third-party providers can guarantee a level of time accuracy.

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No One-Size-Fits-All

A mobile operator deploying a 5G architecture or launching a 5G service has options based on standards that can be deployed at the fronthaul network and the backhaul network. This will lead to various PTP profiles as well as various PTP capacity levels depending on the region, network transport, and integration requirements.

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