Stringent synchronization in Transport layer: Paving the way for 5G network

As 5G networks develop, it becomes obvious that the quantum leap in capabilities of radio network also brings significant shift in system-wide architectural requirements. In this context, transport network, connecting 5G radio to the core of the network, needs to evolve in several directions:

· Significantly higher capacity and scalability

· Support for varied deployment models

· Sophisticated slicing capabilities

· Precise, varied, and robust timing and synchronization

· Low and controllable latency characteristics, to the point of determinism

These requirements effectively define the new mobile transport network, one purpose-built to support mature 5G networks, capable of supporting major new revenue-generating use cases, such as IoT, AR/VR streaming content and gaming, critical communications, and autonomous driving. In the context of system-wide performance, 5G transport network that effectively delivers capabilities in line with the above requirements represents a necessary element of operator’s 5G network development strategy.

The Importance of Synchronization in 5G Networks

Network synchronization is a very specialized topic that has seen its relevance to network operators come and go over time as technology trends have changed. In the era of synchronous TDM (SDH and SONET) networks, synchronization was critical, but in recent times the availability of “good enough” synchronization for Ethernet-based transport has pushed the topic to more of a niche in many network operators’ networks. The need for a step change in synchronization performance in 5G networks is reversing this trend, bringing synchronization back into the top group of challenges that need to be addressed within transport networks.

The new Phase 2 5G services, especially ultra-reliable low-latency communications (uRLLC) services, will drive significant changes into overall mobile network architecture, as well as into the mobile transport network that connects the cell tower to core processing resources. These architectural changes include lower latency through multi-access edge compute (MEC), new network slicing capabilities, and better synchronization performance to support new 5G RAN functionality like carrier aggregation (CA) and previous 4G/LTE-A functionality that is now being rolled out in 4G/5G networks, such as coordinated multipoint (CoMP).

Transport network requirements

Figure below illustrates the synchronization requirements in the end-to-end transport network. Timing budgets allow for 1.1 µs time-of-day difference from the core of the network to the access edge, and about 400 ns from the access edge to the radio. This gives a total budget of 1.5 µs absolute time-of-day difference between radio nodes, which meets the timing requirements of most radio applications today. For example, Carrier Aggregation and Mobile Broadcast require the absolute time-of-day difference between nodes of 3 to 5 µs.

It’s important to note, however, that some coordination features like Coordinated Multipoint (CoMP) demand much tighter timing. This is typically on the order of 260–350 ns time-of-day difference relative to the neighboring radio nodes. OTDOA, used for positioning for emergency services, requires relative timing as tight as 100 ns! In all instances, accurate, stable, and reliable timing is absolutely essential.

Figure below indicates how the three different time and phase sync distribution options overlay onto the transport network. Option 1 shows GNSS at the individual radio sites with no network backup. Option 2 shows GNSS at the access edge sites distributed to the radio nodes using FTS. It includes backup timing from a reference clock in the core using APTS. Option 3 highlights strategic placement of GNSS receivers in the access network and the radio edge. FTS-based distribution to all nodes provides redundant clocking for all radio sites. This option can also be backed up with a reference clock elsewhere in the network and distributed using either APTS or FTS. In all cases, frequency sync distribution can be achieved using SyncE/SyncµW.

Types of Synchronization?

What architectural changes are required in transport network to embark on 5G network and what is the solution?

In the simplest form, mobile networks require good synchronization in the radio access network (RAN) to ensure that devices can connect seamlessly to the cell tower and to enable smooth handover from one cell to another without the user or connected device noticing any drop or interference in the connection performance. All cell sites require good synchronization back to a centralized primary reference time clock (PRTC) so that essentially all the RAN portions of the network are in synchronization with each other, as shown in below Figure.

Stringent timing and synchronization requirements are a necessity for precision 5G transport. Moreover, due to increased complexity of 5G transport deployments compared to previous mobile technology generations, operators will need more robust and redundant timing sources going forward. Most of today’s network elements use GNSS (global navigation satellite system) clock information as a synchronization source; the use of these systems will continue, but will increasingly include multi-band capability, to improve accuracy and resiliency of these systems. Additionally, ITU-T has defined a new set of enhanced network clocks (enhanced primary reference clocks (ePRCs) and enhanced primary reference time clocks (ePRTCs)), designed to enable greater timing accuracy with PTP and SyncE services. Combination of GNSS receivers with network-based clocks will likely be necessary for most 5G transport networks, for resilience, accuracy, and support of indoor.

Big changes ahead for IP mobile transport

Mobile networks have effectively been all-IP since the Evolved Packet Core was introduced for LTE. This means that all interfaces in the 4G/LTE/LTE-A architecture are based on IP protocols. Given that an immense number of end-user devices use IP as their communication technology, it is easy to see why centering the transport network around the IP networking paradigm is such a good choice. IP-based mobile transport provides a solid foundation for end-to-end, cost-efficient control of network resources, along with operational transparency to all IP services that run in the network.

While new network interfaces in 5G will continue to be all-IP, 5G also brings new demands that can’t be satisfied with existing mobile transport networks and architectures.

To address these demands, a next-generation IP mobile transport network must provide:

• Extreme bandwidth with terabit-capacity products that support up to 100 GE link speeds, delivered in a variety of compact form factors (some of them temperature hardened for outdoor applications)
• Low latency to facilitate the evolution to packet-based fronthaul and enable cost-efficient use of the same network for fronthaul, midhaul, and backhaul
• Enhanced QoS to allow service providers to customize and optimize services according to demanding service-level agreement (SLA) requirements for a diverse range of new applications
• Dynamic interconnectivity to support large-scale deployments of Multi-access Edge Computing and mobile cloud data centers
• Improved synchronization to support the stringent timing, frequency and phase synchronization requirements of 5G, and provide robust and flexible timing options (GNSS, SyncE, IEEE1588v2, BITS) for the most demanding services and network technologies
• Built-in security that can protect the 5G network’s much larger attack surface by surgically filtering out harmful traffic at the perimeter
• Programmability that supports software-defined networking (SDN) control and automation and facilitates the use of techniques such as network slicing.

Solution with with Nokia IP Anyhaul:

Synchronization Delivery Mechanisms with AnritsuFMT1000A Tester to Transport network:

Finally synchronization is fundamental to the performance of a cellular network and the services it offers. Both 3G and 4G cellular technology required frequency synchronization, primarily to prevent interference when cells overlap. But with the introduction of 5G technology, we’ve reached a new level in terms of TDD phase and frame synchronization. Validation testing is essential to meet stricter synchronization requirements and to ensure quality of service. Understanding the timing and synchronization requirements of the RAN network as it evolves is critical to ensure RAN performance and stability, and the transport network has an important role to play.

Thank you!!

Monowar Hossain

HOD, Microwave Unit (Planning & Operation)

VEON, Bangladesh

Mobile:+8801962424691

E-mail:monowar.hossain@banglalink.net

Originally published at https://www.linkedin.com.