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Tags: Access and Aggregation, Optical
October 3, 2019

Tick, Tock…Is Sync the Ticking Timebomb in Your Network?

By Jon Baldry
Director, Metro Marketing


The Importance of Synchronization in a 5G Network

I’m sure it hasn’t escaped anyone reading this blog’s attention that 5G is coming along rather rapidly now, and if it hasn’t started to hit a network near you already then it probably will soon. Of course, 5G is a massive network investment that will take many years to roll out fully and for the highly publicized next generation of advanced mobile services to become widely available. I shan’t repeat all the background here, but if you aren’t familiar with the phased introduction of 5G services and the impact of this phased approach on the underlying optical transport network, then check out this article in The Fast Mode.

The Importance of Sync in 5G Phase 2

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. Back in the day of synchronous time-division multiplexing (SDH and SONET), network sync was critical, but in recent times the availability of “good enough” sync for Ethernet-based transport has pushed sync to be more of a niche topic in many operators’ networks.

The need for a step change in sync performance in 5G networks, driven by new radio access network (RAN) capabilities, such as carrier aggregation (CA), with a correspondingly higher sync specification within the RAN, is now reversing this trend. This is bringing sync back into the top group of challenges that need to be addressed within transport networks.

The very brief synopsis of the article mentioned above is that 5G will evolve over time with a phased introduction of new 5G capabilities. Initial 5G networks will essentially be higher-speed 4G networks offering faster versions of existing services, and as standards mature and new network infrastructure is rolled out, we’ll eventually see the much-hyped new 5G services that utilize the higher-performance characteristics of 5G.

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 CA and previous 4G functionality that is now being rolled out in 4G/5G networks, such as coordinated multipoint (CoMP).

It’s All in the Details

5G synchronization specifications define the quality of the sync around three main parameters – frequency, phase, and time of day. Depending on how these standards are implemented, mobile operators are looking at a range of 1588v2-based phase sync specifications for the transport element of their networks, with only 1100 nanoseconds of absolute phase error at the cell site and as low as only 60 ns of relative phase error between adjacent cells.

In addition, frequency error must improve significantly from 4G to 5G, and specifically, SyncE performance must now adhere to the newest enhanced 5G SyncE requirements and the G.8262.1 enhanced Ethernet equipment slave clock (eEEC) specification in particular, which specifies a demanding fourfold decrease in wander generation. Wander is a measurement of how the frequency drifts over time.

This high level of sync performance is required before any Phase 2 RAN capabilities and corresponding services that specify this level of sync performance can be turned up in a 5G network – it isn’t something that can be engineered into the network further down the road.

Sync Delivery Strategies

Delivering sync of this high quality can be achieved in a number of ways. Historically, some global regions were highly reliant on a Global Navigation Satellite System (GNSS), such as the U.S.’s Global Positioning System (GPS), while other regions have used the transport network to deliver sync, either in-band with actual backhaul traffic, out-of-band using external sync transport solutions, or a combination of the two.

Many GNSS-based operators still plan to use GNSS for their primary 5G sync, although an increasing number of them are looking at network-based sync as a backup for the GNSS signal in case of unintentional or intentional GNSS blocking by individuals  or even major blocking attacks from foreign nations. These operators are also evaluating network-based sync as a means of delivering sync to locations that might be more challenging for GNSS, such as subways or small cells deep inside buildings.

The large number of mobile network operators that primarily use network-based sync and those GNSS-based operators who are looking at network-based backup sync all need to consider how they can achieve 5G-quality sync distribution within the transport network without busting the bank.

High-quality network-based synchronization can be achieved by deploying specialized, and typically expensive, out-of-band synchronization equipment that provides the necessary grandmaster and boundary clocks and transmits a sync signal that is separate from the data plane.

Of course, some of this equipment, especially the grandmaster clocks, is always needed, but the key to managing the delicate balance between sync performance and network economics is utilizing a range of sync tools to minimize the need for any additional equipment beyond the transport network. Those tools include:

  • High-performance sync in all IP routing elements. Routers should have excellent sync transfer from port to port, GNSS/GPS receiver options, and 1588v2 support with Transparent and Boundary Clocks
  • High-performance packet-optical transport that is capable of transporting sync between the core, RAN, and IP nodes within the demanding 5G specifications and that wherever possible utilizes SyncE and 1588v2 to support sync within in-band traffic to minimize cost, especially at the edge of the network where endpoints proliferate
  • Coordinated end-to-end sync management across all layers, from optics up to IP

Delivering this range of capabilities takes a deep understanding of network synchronization and a heritage in both IP and optical within mobile operator networks. Network operators across the globe are refocusing on synchronization in preparation for Phase 2 5G services that will require advanced RAN capabilities and the corresponding demanding synchronization performance.

Those operators should assess how to achieve this synchronization performance most economically by talking to both specialized synchronization companies and optical networking vendors that can provide the right tools to support synchronization within the transport network data plane.

Anyone in the New York area who is interested in network synchronization is welcome to join Infinera at the Light Reading 5G Transport & The Edge event on October 10, 2019. I’ll be participating on a panel entitled “A Closer Look at Timing and Synchronization in High-Performance 5G Networks,” and my colleague Tim Doiron is also a panelist on the “Redefining the Access Network: Examining RAN Architecture and the Implications for 5G Transport” panel.