Timing Distribution and Assurance in Mobile Backhaul Networks

By Brian Protiva February 19, 2014

The drive toward Long Term Evolution-Advanced (LTE-A) networks is steeping pressure on both mobile network operators and backhaul service providers—including incumbents and alternative access vendors—to define next-generation strategies for timing distribution and assurance thereof.

Performance-based timing and synchronization is critical in high-performance LTE and LTE-A networks, but that migration is no small undertaking. Ensuring success is not just a matter of increasing the density of radio base stations, for example. If the improvements in mobile bandwidth and user experience that LTE and LTE-A promise are to be reliably and consistently achieved, exact phase alignment—coupled with stringent assurance monitoring—is required among those base stations. And the target of 1msec phase accuracy or less can be achieved only by active timing distribution contribution from both the mobile network operator and backhaul service provider.

Still, there exists in the marketplace significant uncertainty with regard to what actually can be accomplished to embed and assure timing and synchronization in mobile infrastructures. The good news is that no longer must archaic legacy synchronous links be implemented to deliver timing. Instead, sophisticated timing and synchronization capabilities can be easily distributed end-to-end across networks, leveraging existing packet infrastructure.

IEEE 1588 “Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems” is a prime technology of climbing importance in this area. Published in 2008, Version 2 of the IEEE 1588 standard defines a protocol—Precision Time Protocol (PTP)—that is designed to enable precise synchronization of clocks in measurement and control systems for network communication, local computing and distributed objects. Synchronization accuracy and precision in the sub-microsecond range can be achieved system-wide with minimal network and local clock computing resources being required, as simple systems are installed and operated without management attention from users.

IEEE 1588v2 is especially valuable because it is a standards-based solution that is widely applicable across backhaul networks. In the migration to LTE and LTE-A, the mobile network operator and backhaul service provider alike must adopt effective timing, synchronization and assurance strategies. Backhaul networks typically have not been required to actively contribute to timing distribution and provide on-path support in order to achieve highly accurate phase alignment, and IEEE 1588v2 is key for helping backhaul providers cost-effectively and dependably succeed in this new role.

In addition, there have been recent developments in standards bodies that figure to impact capabilities for timing and synchronization. Assisted Partial Timing Support is one such example. Assisted Partial Timing Support—which leverages the Global Navigation Satellite System (GNSS)—operates over existing networks, including third-party access networks that may not have been architected to support PTP with full on-path support. In this way, the combination of IEEE 1588 PTP and GNSS in an Assisted Partial Timing Support architecture could emerge as key synergistic solution for timing and synchronization, with one technology providing backup to the other. The profile, architecture and clock specifications in Assisted Partial Timing Support are under definition in the International Telecommunication Union-Telecommunication (ITU-T), with consent planned for December 2014.

Furthermore, timing and synchronization performance must be monitored and assured. In the same way that mobile network operators and backhaul service providers must meet mutually agreed upon service-level agreements (SLAs) for the Ethernet data service performance, such SLAs are emerging as necessary for timing and synchronization distribution, too, in the drive toward LTE and LTE-A. To succeed, ensuring timing and synchronization performance must be embraced as a network-wide concept following the same principles of Ethernet service assurance. For example, timing performance must be continuously monitored, in service, and alarmed, as necessary.

Indeed, a variety of tools are needed for monitoring, verifying and testing timing and synchronization across infrastructures. Tools for clock accuracy enable measurement of the frequency and phase accuracy of clocks relative to a synchronization reference, which can be internal, external, recovered or originating from a GNNS signal. Tools for clock analysis are needed to ensure the frequency and phase accuracy of the PTP packet domain, even in environments where a synchronization reference is unavailable. And PTP network analysis, including monitoring and testing of the PTP communication path, also must be continually performed to satisfy contemporary assurance requirements.

The mobile industry’s migration toward LTE and LTE-A offerings supporting more users, more video and more connections tee up a host of requirements for mobile network operators and backhaul service providers alike: differentiation of multiple service classes by end-to-end Quality of Service (QoS) management; multi-point topologies; lowest latencies for key traffic types, enabled by strict priority forwarding; configuration and service management end-to-end across infrastructures, and—last but not of least importance—a next-generation approach to network synchronization and its assurance.

The choices that mobile network operators and backhaul service providers make in terms of timing distribution and assurance now will have considerable, long-term implications on critical competitive issues such as user experience and service differentiation moving forward.

Edited by Cassandra Tucker

CEO, ADVA Optical Networking

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