5G introduced architectural changes that require synchronization. Depending on the location and network site, these timing requirements require different PTP profiles and PTP capacities.
As critical infrastructure such as communications, utilities, transportation, and defense transition from 4G to 5G, one might assume that these essential services will universally adopt the ITU-T G.8275.1 Precision Time Protocol (PTP) profile for network time synchronization. After all, PTP incorporates PTP Boundary Clocks (BCs) with superior quality compared to 4G. However, this trend may underestimate the finer grained nature of 5G mobile synchronization.
5G’s TDD (Time Division Duplex) introduces new relative and absolute phase requirements, as well as Open RAN, where the Baseband Unit (BBU) functionality is split into Radio Unit (RU), Distributed Unit (DU) and Centralized Unit (CU). As a result, the telecommunications industry has standardized two PTP profiles, ITU-T G.8275.1 and ITU-T G.8275.2, to address PTP-enabled and non-PTP-enabled networks, respectively.
Synchronization issues
Operators understand the implications of having a backhaul network. Depending on the type of network, Packet Delay Variation (PDV) can have a significant impact on synchronization. In many countries, PTP was deployed in 4G as a backup synchronization mechanism, with Global Navigation Satellite System (GNSS) being the primary synchronization source. To avoid situations where GNSS failures would result in loss of phase services, the idea of using PTP flows to connect Primary Reference Time Clocks (PRTCs) at the edge to a centralized core clock was developed. This was adopted by ITU-T as G.8273.4 – Assisted Partial Timing Support (APTS).
Figure 1. A typical synchronization architecture for mobile 4G uses grandmaster clocks and the G.8275.2 PTP profile.
In this architecture, the time error of the PTP input is calibrated using a local edge PRTC GNSS, which has the same reference (Coordinated Universal Time, UTC) as the upstream GNSS. The incoming PTP flow can essentially be seen as a proxy GNSS signal from the core with traceability to UTC.
Figure 1 It illustrates a typical 4G synchronization scenario where a PTP Grandmaster provides services to a 4G eNodeB over the backhaul using the PTP unicast G.8275.2 profile.
As mobile networks become increasingly complex due to the decentralization of Open RAN, 5G requires a new synchronization architecture. Figure 2 It highlights the key elements of 5G architecture.
In addition to the general 5G architecture, operators must also consider backhaul, with decentralization introducing fronthaul and midhaul networks.
Figure 2. In an Open RAN 5G network architecture, BBUs are distributed, adding more devices to the network and making synchronization more complex.
From a synchronization perspective, the fronthaul will be the central network point serving the 5G RU or 5G base station. Figure 3 We demonstrate how a fronthaul network can serve 5G base stations (gNodeBs) using the G.8275.1 multicast profile, with PTP being the primary synchronization mechanism in this scenario.
Key considerations when implementing 5G include the end-to-end timing budget (±1.5 µsec) and the relative time accuracy of 130 nsec/260 nsec between adjacent Ru (see Figure 3).
Figure 3. A typical synchronization architecture for 5G where the fronthaul serves the gNodeBs and the backhaul also serves the 4G eNodeBs.
On the other hand, the ITU-T G.8275.2 profile is at Layer 3, unicast. It does not require on-path support in all network elements. The PTP protocol traverses these network elements as high priority traffic. In this use case, the network requires support for a large PTP client capacity from the PTP Grandmaster. Typically, the number of clients is more than 100 and in some cases can reach thousands.
Fronthaul Profile
From a synchronization perspective, the fronthaul operates based on the time source from GNSS signals, and Assisted Partial Timing Support (APTS) protects the fronthaul in situations where GNSS signals are unavailable or intermittent.
Fronthaul is typically found in large cities or metropolitan areas with many base stations. A nearby PTP Grandmaster serves the base stations. In this situation, the network uses a profile based on G.8275.1, a PTP profile defined specifically for the telecommunications industry with network elements incorporating modern boundary clocks. G.8275.1 uses a multicast mode that does not require a lot of capacity.
To date, PTP provides frequency synchronization outside of metropolitan areas. Grandmaster clocks deployed in these locations primarily serve older FDD radio systems. These clocks are increasingly part of the mix of older and newer radio environments introduced by the transition to 5G.
Many operators are transitioning their frequency-focused grandmasters to newer generation IEEE 1588 PTP grandmasters that support 5G requirements with better time and phase accuracy. These clocks also offer more features and more PTP ports than previous generations. The new grandmasters need to connect to many more devices, including older radios, cell towers, and other PTP grandmasters.
These backhaul sites and grandmasters typically use the ITU-T G.8275.2 profile, which runs at the Internet Protocol (IP) layer. The telecommunications industry is focused on enabling the migration from legacy environments to new architectures and devices. Existing legacy signaling systems such as Synchronization Supply Units (SSUs) and Primary Reference Clocks (PRCs) are not going away and will need to be integrated into new architectures focused on 5G and PTP. Another aspect to consider besides capacity is the ability to integrate systems located at sites far from the grandmaster.
The transition to 5G
Carriers adding 5G mobile services can leverage and build on existing synchronization investments.
Typically, large telecommunications operators install PTP Grandmasters in central offices supporting wired broadband and wireless mobility, which gives rise to four typical use cases:
- Operators use dedicated Primary Reference Sources (PRS) which are common in North America. In these cases, operators are often replacing their legacy PRS systems and migrating to a new generation of Grandmasters that can function as a PRS or an enhanced PRS (ePRS).
- Operators will migrate from legacy PRTC Grandmasters to a more modern platform that offers more connectivity options and advanced APTS capabilities, as well as frequency synchronization for cell site backhaul (thousands of clients) using PTP G.8275.2.
- Carriers will deploy new PRTC Grandmasters for 5G fronthaul using PTP G.8275.1.
- Operators will transition their existing synchronization systems to more modern and resilient PTP grandmasters that meet a strict 30ns accuracy against UTC and a 14-day holdover at selected sites.
These installations protect investments: over time, operators will evolve their existing synchronization infrastructure and leverage new technologies to serve 5G sites.
Other Market Trends
Apart from fronthaul and backhaul considerations for selecting timing profiles and capacity requirements, some countries or operators may not own some or all of the infrastructure for their deployment.
In North America, carriers commonly lease backhaul lines from third parties. However, these leased lines do not always meet the carrier’s time and phase performance requirements. Mobile carriers cannot always rely on the backhaul links and may not have the means to monitor the synchronization quality provided by third-party leased line providers.
To serve mobile operators and ensure high accuracy given the stringent timing requirements of the 5G architecture, leased line backhaul providers are upgrading their network elements with boundary clocks to provide operators with highly accurate time and phase.
New entrants such as satellite providers and cable operators are adding mobile to their portfolios and relying on third parties to provide precise time over leased architectures.
Traditional wireline providers often lease their wireline infrastructure to mobile operators and emerging mobile operators. Leased line providers may need to upgrade their infrastructure to provide accurate time and phase to mobile operators. Mobile operators can run either G.8275.1 or G.8275.2 on a dedicated backhaul layer. Operators leasing lines must ensure that the third-party provider can guarantee a certain level of time accuracy.
One size doesn’t fit all
Mobile operators deploying 5G architectures or launching 5G services have standards-based options that can be deployed in their fronthaul and backhaul networks, enabling different PTP profiles and different PTP capacity levels depending on region, network transport, and integration requirements.
