PTP and SyncE on Dell Next-Generation Servers
Download PDFFri, 03 Mar 2023 19:57:24 -0000
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Summary
The telecom industry is on a journey of transformation, making pitstops to disaggregate hardware and software, virtualize networks, and introduce cloudification across RAN and core domains. The introduction of 5G and ORAN has accelerated the transformation, and we now see telecom becoming a universal phenomenon and touching all aspects of life.
This telecom evolution opened a number of opportunities for CSPs to diversify their revenue streams, but it also introduced stringent technological implementations. To support higher bandwidth and mMIMO technologies in new-generation systems, solution development teams were faced with strict requirements of latency and synchronization.
In this tech note, we discuss synchronization systems in 5G and ORAN fronthaul interfaces, and next-generation Dell PowerEdge support for synchronization standards.
Note: This document may contain language from third-party content that is not under Dell Technologies’ control and is not consistent with current guidelines for Dell Technologies’ own content. When such third-party content is updated by the relevant third parties, this document will be revised accordingly.
Network synchronization
Telecom networks have always required proper and accurate synchronization for handover between different cell sites, reducing interference and increasing performance at cell edge. 2G, 3G, and 4G networks all required a certain level of synchronization, but 5G requires timing in the range of nanoseconds. This enables features such as beamforming and time-division duplexing to function accurately.
Telecom systems generally work based on the following synchronization methods:
- Frequency synchronization: Frequency synchronization ensures that the frequency of the local clock at the 5G/4G cell site is the same as the PRTC. The quality of frequency synchronization is measured in parts per billion (ppb) by the difference between the actual and desired frequency.
- Time synchronization: Time synchronization refers to the distribution of time across clocks in a network. Time synchronization is one way of achieving phase synchronization.
- Phase synchronization: Phase synchronization takes care of the timing signals occurring at the same time. It ensures that the rising edge of each clock input occurs at the same time. The quality of phase synchronization is measured in microseconds (ms) or nanoseconds (ns) by the difference between the timing of the signals.
The following table lists the telecom requirements for the frequency and phase specifications:
Table 1. Telecom technologies with standard units
Telecom technology | Frequency air interface/network | Phase/time air interface/network |
GSM | 16 ppb/50 pbb | - |
LTE-FDD | 16 ppb/50 ppb | - |
LTE-TDD | 16 ppb/50 ppb | 1500 ns |
LTE-A | 16 ppb/50 ppb | 500 ns |
5G | 16 ppb/50 ppb | 65 ns |
Methods of synchronization transmission
Different standards apply to the transmission of the synchronization signals, as outlined in the following table:
Table 2. Synchronization methods for distribution standards
Synchronization distribution standard | Time synchronization | Frequency synchronization | Phase synchronization |
PTP (IEEE 1588) | Yes | Yes | Yes |
SyncE | No | Yes | No |
GNSS | Yes | Yes | Yes |
NTP | Yes | Yes | Yes |
Fronthaul synchronization requirements for 5G in ORAN
O-RAN ALLIANCE has defined four synchronization topologies— LLS-C1 , LLS-C2 , LLS-C3 and LLS-C4—to address different deployment topologies in telecom networks. The following figure shows a typical synchronization flow diagram with synchronization from PRTC flowing to end cell sites:
Figure 1. ORAN synchronization overview
PRTC: Primary Reference Time Clock
GNSS: Global Navigation Satellite System
T-GM: Telecom Grand Master Clock
Stay tuned for another tech note from Dell with more details about synchronization technologies.
In telecom systems, synchronization is delivered by various mechanisms:
- Synchronization signals are delivered from GNSS directly to each node in the network. This system is accurate, but it becomes quite costly.
- Synchronization signals are delivered from a centralized PRTC/T-GM in-band over a transport network.
- Synchronization signals are delivered by a dedicated synchronization transport network that is separate from the transport network used to carry user and control signals.
The transport network for carrying synchronization can be either the backhaul network used to carry traffic or a dedicated network for transporting synchronization signals.
In 5G and ORAN, gNBs need frequency, phase, and time synchronization. The following two protocols are used for transporting synchronization signals over a packet-based network:
- Precision Time Protocol (PTP)
- Synchronous Ethernet (SyncE)
The same packed-based transport network can be used to carry users and control plane traffic.
PTP
PTP, defined by the IEEE 1588 standard, was developed to provide accurate distribution of time and frequency over a packet-based network. A PTP synchronization system is composed of PTP-aware devices and non-PTP-aware devices.
The following table describes the clock types in PTP:
Table 3. Types of clocks in PTP
Clock type | Definition | Usage |
Telecom grandmaster (T-GM ) | The master clock at the start of a PTP domain. It is typically located at the core network. | At the beginning of the network to provide timing signals to network. |
Telecom boundary clock (T-BC) | Clock that can act both as a slave and master clock. It takes the sync signal from the master, adjusts for the delay, and generates a new master synchronization signal to pass downstream to the next device. | When the synchronization signal needs to travel through multiple nodes across a long distance. |
Telecom transparent clock (T-TC) | Clock that timestamps a synchronization packet message and sends it forward to the secondary device. It enables the secondary device to calculate the delay of the network. | For scenarios where timing signals are passing through switches. |
Telecom time slave clock (T-TSC) | The end device that receives the synchronization signal. | In telecom, the end node that receives the synchronization signals. |
The following figure illustrates how various types of clocks in PTP interact with each other:
Figure 2. Types of clocks in PTP
In 5G and ORAN, PTP generally works with two types of timing profiles, G8275.1 and G8275.2 . As shown in the following figure, G8725.1 is the profile where all devices are PTP-aware devices, and G8275.2 is the profile where are all devices can be, but might not be, PTP-aware devices. Figure 3. G8275.1 and G8275.2 timing profiles
Why do ORAN and 5G need two PTP profiles? One reason is the use case and implementation perspective of the CSP, as outlined in the following table:
Table 4. PTP telecom profiles
PTP Telecom Profile | Description | Usage |
G8275.1 (full timing support) |
|
|
G8275.2 (partial timing support) |
|
|
SyncE
SyncE is a synchronization technology that enables the transfer of synchronization signals at the physical layer. It is used to provide accurate and stable frequency synchronization between the different components of a network architecture. Over the fronthaul interface in ORAN and 5G, both SyncE and PTP are used together to provide nanosecond-level synchronization accuracy. SyncE can deliver frequency synchronization, but it cannot deliver phase and time synchronization. It functions independently of the network load and supports the transfer of sync signals where all the interfaces on the intermediate path must support SyncE.
Why fronthaul synchronization requires both PTP and SyncE
In ORAN, for topology architectures LLS-C1, LLS-C2, and LLS-C3, both SyncE and PTP are used on the fronthaul interface between DU and RU or in the mid-haul interface between CU and DU. When PTP itself can cater to frequency, time, and phase synchronization, why do we need SyncE along with PTP?
The answer is that using both PTP and SyncE delivers these advantages:
- Higher accuracy: SyncE is a physical layer protocol that can deliver a stable and consistent frequency. Combining the stability of SyncE for frequency synchronization with the precision of PTP for time and phase synchronization provides for a very accurate clock.
- High availability: When a PTP clock fails, the node can fall back to the SyncE frequency clock and advance the time more accurately and for longer, rather than relying only upon the internal oscillator. Also, when the PTP clock source fails, locking to a new PTP source can occur quickly when SyncE is already in use. Without SyncE, PTP can take more time to identify the actual time and lock to a new clock source.
Implementation of PTP and SyncE in next-generation PowerEdge servers for telecom applications
Next-generation Dell PowerEdge servers come with Intel NIC cards such as Westport Channel and Logan Beach. All these NIC cards are timing aware and can be used to provide synchronization to downstream nodes. Because these servers can be positioned both as CU and DU, and support LLS-C1 , LLS-C2, and LLS-C3 deployment, support of SyncE and PTP makes these servers an apt choice for RAM and edge deployments.
Conclusion
Dell Technologies continues to provide best-in-class features and specifications for its constantly evolving PowerEdge server portfolio for telecom. The PowerEdge XR8000 (430 mm depth) and XR5610 (463 mm depth) provide scalability and flexibility, with the latest technologies for PCIe, storage, memory, I/O, and even node-chassis infrastructure in a dense (SA1) form factor. With support for PTP and SyncE technologies, these next-generation PowerEdge servers provide essential infrastructure support at the edge.