Method of Timing Distribution
The job of a PRTC is to continually sync with Universal Coordinated Time (UTC) and distribute it across the networks. It may use a combination of GPS/GNSS, Atomic Clock or Cesium Clock and timing distribution mechanisms to achieve this. There are several mechanisms to carry timing information throughout the networks that includes SyncE, BITS (Building Integrated Timing Supply), IRIG (Inter Range Instrumentation Group) time code type B (IRIG-B) and 1PPS (1 Pulse per Second). All these technologies are dedicated timing signals requiring a physical connection specifically for timing. One exception is that SyncE can coexist in a physical connection of a packetize network; in other words, same port of an ethernet switch can implement SyncE and transport packets for shared physical link.
Apart from dedicated timing signals, there are other packet base solutions for timing distribution as well, e.g., NTP (Network Transport Protocol) and PTP (Precision Time Protocol). Both protocols require no specific connection for timing and best suited for packetized networks. While NTP is a common time distribution protocol for computer networks and in existence for nearly three decades, PTP (IEEE 1588) is relatively new. It is defined by IEEE 1588 specification in 2002. Since it’s inception, PTP has gained increase attention due to the possibility of achieving sub nanosecond accuracy when used in conjunction with PRTC for primary clock source and SyncE to distribute Timing information.
The following table shows different methods of timing distribution and relative timing accuracy for each. A point to note here is that both NTP and PTP supports TOD, phase and frequency synchronization making them ideal for today’s packetized networks. However, NTP is less suitable for applications and network where sub nano seconds to microsecond accuracy is needed e.g. 5G TDD deployments such as CBRS, mmWave etc.
Table 1. Methods of Timing Distribution.
Note: A network with SyncE (Synchronous Ethernet) and PTP together can achieve sub nanosecond accuracy as evident in white rabbit experiment by CERN.
All time distribution methods should adhere to respective standards e.g. ANSI, Telcordia and ITU-T requirements for PRC (Primary Clock Source) or PRS (Primary Clock source) and time synchronization mechanisms. In a typical deployment, Stratum 1 level clock is considered as PRC/ PRS for the network. A Stratum 1 is part of clock hierarchy level defined by ANSI for which Stratum 0 is atomic clock that provides input to Stratum 1. Where atomic clock inputs are not available, the PRC/PRS may take input from GPS/GNSS or Cesium Clock and a combination thereof as required. At Stratum 2 level, time servers generally get time reference from Stratum 1. The sync plane design should consider respective ANSI and ITU-T standards together for optimal outcomes. It is also useful to define a sync plane that is backward compatible. The figure below shows a relative map between ANSI clock hierarchy and ITU-T recommendations for frequency plane and time/phase plane (e.g. ITU-T PTP profile).
Figure 1. Clock Hierarchy levels for timing source
Figure 2. Timing Distribution and applicable ITU-T standards in frequency and time/phase plane.
5G Splits and Sync Plane
Implementation of timing distribution is generally done in both frequency and time/phase planes planes due to underlying network requirements. Hence, understanding of the concept is important for 5G sync plane design as such network must be frequency and phased aligned. 5G as defined in 3GPP standards distinctly divides the network concept into two elements: RAN (Radio Access Network) and Packet Core. An obvious indication that packet network will be pervasive in 5G deployments. Even in the deployment of 4G, it is discernable that packetization and penetration of ethernet in fronthaul is increasingly becoming a reality. Packetization and ethernet technologies are fundamental conduit for flexible network configuration, virtualization and improved services.
In 5G deployments, RAN can be deployed in many split options and that is for good reasons: first, it allows easier decoupling of hardware and software. Secondly, network functions can be virtualized and processed in COTS (Common Off The Shelf) servers. These process of decoupling and virtualization significantly reduces CAPEX and OPEX and at the same time removes constraints of vendor locked systems. The concept of decoupling is not new, in fact cloud providers and data centers are benefiting from the implementation of this concept of “disaggregation” or “open networking”. For simplification, decoupling, disaggregation, open networking and whitebox terms are synonymous. For example, the concept of decoupling as in whitebox is implemented through DCSG (Disaggregated Cell Site Gateway), a project of TIP (Telecom Infrastructure Project) to provide telecom service providers a choice of vendor neutral networking solutions. The DCSG aims to replace CSR (Cell Site Router), a vendor locked product that help aggregate cell sites. It is an “open networking” initiative that takes into the benefit of decoupling and vendor neutral approach to help telecom service provider reducing their CAPEX and OPEX for fronthaul aggregation. Similarly, OpenRAN is a project of TIP that aims to decouple basestations or Base Band Unit (BBU). The BBU provides RF (Radio Frequency) processing services to cell towers. While DCSG provides whitebox solutions to replace CSR, OpenRAN initiative create standards for decoupling hardware and software for radio access networks. Both these projects help tremendously in 5G split options deployments.
5G specification allows fronthaul to be created in 8 different split options. Depending upon split options and fronthaul connectivity technologies, time sync requirements slightly differs. For example, for CPRI connectivity PTP is ideal whereas RoE (Radio over Ethernet) implementation requires both PTP and TSN are implemented in different segments of the network. The diagram below depicts four common split options for 5G deployments: option 1 (upgraded 4G), option 2 (5G standalone), Option 7 (dual connectivity) and Option 8 (vRAN/ORAN). For the sake of sync plane discussion, other options such option 3, option 4, option 5 and option 6 are not presented here.
Figure 3. 5G splits and Sync Plane requirement
Figure 4. Time Error Tolerance consideration for 5G mobile transport (Ref: ITU-T G.8271.1/Y.1366.1).
Figure 5. An example of Tier 1 5G option 7 sync plane deployment.
In this scenario, multiple RUs (Radio Units) are connected to DU (Distribution Unit) and DUs are directly connected to CU (Central Unit). The PTP grand master is connected to CU while DU implemented boundary clock. On the backend, PRTC-B and ePRTC coordinated synchronization is used for effective fault tolerance in case of a sync path failure. This deployment is a good example of how operator may choose to decide sync plane design bassed on their own time error budget calculation.
Similarly, deployments for 5G option 8 may requires specific consideration on sync plane since sync cluster span over different segment of network. Here, sync cluster may include vRU (virtual Radio Unit) and other VNF (Virtual Network Function) related to RAN functions. 5G option allows separation of the RF and PHY layers thus splitting sync cluster and extending it over to COTS server as VNF (please refer figure 6).
Figure 6. Typical Sync plane for OpenRAN or ORAN deployment
ITU-T recommendations for Time error tolerance should be defacto in 5G sync plane design whether in fronthaul or CBRS deployment. Placing a T-GM after one or two T-BC hop away is a great way to design 5G with ample room for time error budget. Given that the price of T-GM in lower client counts is much cheaper, deploying T-GM at CU after one or two hops is a viable and cost effective option. This type of design provides improved time error tolerance for fronthaul. If you are considering 5G sync plane design, please visit http://www.trimble.com/timing for product and solutions offered by Trimble.
Trimble offers an extremely cost effective T-GM, Antenna and GNSS timing module for edge making design of fronthaul sync plane much easier and cost effective.