Mobile Backhaul Synchronization
The deployment of 4G LTE, small cells and HetNets to cater to an ever increasing demand for capacity and coverage is transforming the mobile network architecture. At the same time backhaul networks are migrating from TDM to packet based or Ethernet based infrastructure. While synchronization was traditionally delivered natively in TDM networks, new packet switched networks (PSNs) are asynchronous by nature and introduce inaccuracies, such as packet delay variation (PDV) and packet loss making synchronization one of the biggest challenges in the migration to Ethernet, IP and MPLS transport. Transporting quality sync is of critical importance. 4G LTE imposes demanding frequency stability and phase precision requirements to deliver efficient network operation and good quality of service.
The table below summarizes backhaul synchronization requirements for different 4G LTE radio access network (RAN) technologies.
|TDD||16 ppb||± 1.5 µs|
|MBSFN / eICIC etc.||16 ppb||± 1.32 µs|
|LTE-A (COMP)||16 ppb||± 0.5 µs|
Mobile Backhaul Edge Based Synchronization
GNSS based synchronization can meet the above requirements, but GNSS signals are not always available where small cells are deployed indoors, or in urban canyons (in dense urban environments). In addition, it is costly to deploy and operate. Therefore an alternate synchronization method is required to hold frequency and phase when GNSS signals are lost.
The two timing standards that satisfy the requirement are Synchronous Ethernet (SyncE) and IEEE 1588 PTP v2. However, SyncE can only deliver frequency synchronization at the physical layer for LTE-FDD. In contrast, IEEE 1588 PTP v2 can deliver both frequency and phase synchronization for the different LTE versions. SyncE also requires that all ports in the link be SyncE enabled, which is only economically viable in green field deployments. The brown field Ethernet switches will need upgrades to migrate to SyncE capability, which is a costly exercise. Moreover managing SyncE can significantly increase network Total Cost of Ownership (TCO).
IEEE 1588 PTP synchronization architecture with Grandmaster in the Mobile Core or Mobile Backhaul edge, full or partial support from the network switches and a carrier quality PTP slave in the small cell meets the synchronization requirements for Mobile Backhaul and the mobile network. Full support from the network implies that the all switches be PTP aware. The more viable and economically feasible model calls for partial timing support or Assisted Timing Partial Support Clock (APTSC).
The principal concept of the Assisted “Partial-Support” approaches delivery of timing (phase and frequency) in a wireless (LTE) environment by a combination of GNSS and PTP. The Primary Reference Timing Clock (PRTC) function is supplied by GNSS which sets the frequency and time, while the network between the slave device (small cell) and the upstream Grand Master (whether deeper in the network, or deployed closer to the edge) need not provide full on-path support.
For Mobile Backhaul, the following standards (MEF 22.1 through ITU-T G.826x) from Metro Ethernet Forum's "Implementation Agreement MEF 22.1 Mobile Backhaul Phase 2" have been reproduced with permission of the Metro Ethernet Forum.
Phase 1 of MEF 22.1 described a framework for using MEF 6.x Ethernet and MEF 8 CES services for Mobile Backhaul. Performance of synchronization architectures was limited to the case with TDM interfaces to base stations.. Phase 2 of MEF-22.1, currently in progress, adds enhancements for interoperable deployments of service as well as synchronization performance. Enhancements include the UNI Mode attribute, to lock SyncE to an EEC and to make sure inter operations of SyncE include a clock quality level (QL), synchronization as a class of service, and enhancements to the Class of Service mapping table. MEF-22.1 references ITU-T and IEEE standards for all relevant requirements.
Performance specifications for TDM networks and synchronization are contained within the ITU-
T G.81x and ITU-T G.82x Series of Recommendations. They specify frequency accuracy metrics of Maximum Time Interval Error (MTIE) and Time Deviation (TDEV) with limits in the order of nanoseconds, which correspond with the requirement to deliver ppb accuracy. There are different masks for the accuracy that a signal is expected to deliver in the ITU-T G.823 recommendation. For example, there are tighter limits for a PRC than there are for a Traffic Interface.
ITU-T G.781 describes a clock hierarchy deployment model as well as a clock selection process for TDM networks. This has been reprised by the ITU-T G.8264 recommendations, which covers the same aspects for an Ethernet synchronization network.
The development and evolution of the ITU-T G.82xx recommendations forms the core work of the ITU-T study group responsible for synchronization. This includes defining the ITU-T Telecom profile for Time of Day transfer and establishing performance metrics and limits. The series broadly covers the different requirements of providing synchronization over packet networks. ITU-T G.8261 defines SyncE network limits and describes the framework for deploying Synchronization in a packet network, including recommended pre deployment test Packet Synchronization over Carrier Ethernet Networks for MBH January 2012 cases. ITU-T G.8262 includes the performance requirements for synchronous EEC accuracy, in parallel to the ITU-T G.81x and ITU-T G.82x series mentioned above. ITU-T G.8264 adds the Ethernet Synchronization Messaging Channel (ESMC) protocol to manage the hierarchy and deployment of SyncE. ITU-T G.8265 and ITU-T G.8265.1 define the ITU-T Telecom Profile for frequency transfer (for example by using IEEE 1588v2).
Note: G.8263 includes performance requirements for slave clocks, has been approved after the MEF M22.1 document was released.
ITU-T G.8271.x set of standards cover the different requirements for transporting time and phase over packet networks. ITU-T G8271 defines network PDV requirements for time and phase while G.8272 includes performance requirements of a PRTC clock; G.8273.x includes performance requirements of a Grand Master (GM), Boundary Clock (BC) and Transparent Clock ( TC). G.8275.1 defines PTP Profile1 for Time and Phase transport for full on-path support (released standard), while G.8275.2 (not released yet) addresses PTP Profile 2 for assisted partial support.
Qulsar's Managed Timing Board (P64) Edge Master Clock provides a best of breed synchronization solution for IEEE 1588 embedded Grandmaster applications.
The Managed Timing Board (P64) provides high quality and reliable synchronization distribution over packet switched networks. The board provides full support for IEEE 1588 PTP Grandmaster functionality and multi-sync capability including GNSS and SyncE. This enables carrier class sync delivery to radio access networks.
It can be deployed as an Egdemaster at the network edge/aggregation point (examples: Ethernet Access Devices / NIDs) distributing synchronization to a small cell cluster over a packet based network or as a local Grandmaster situated inside a building, hooked up to a GNSS receiver providing synchronization to in-building small cells over the local LAN network.