A look at 1588v2 and Synchronous Ethernet in mobile backhaul networks

Fig 1: Wireless Bandwidth Trends

Growth in Mobile Broadband Data Strains Backhaul Networks

As data delivery moves into a new paradigm of next-generation wireless networks that provide data, video, and voice services, one of the major challenges for service providers is provisioning enough mobile backhaul to fuel intense data demands.

Fig 2: Mobile Backhaul Network

Mobile backhaul (Fig 2) is the network for transporting mobile traffic between cell sites (BTS/NodeB's) and radio controllers (BSC/RNCs). Backhaul is one of the major contributors to the high costs of building out and running a mobile network - estimated to be approximately 25-30 percent of total operating expenses. As demands to support increased mobile data traffic grow, it is important that operators optimize their networks with the most cost-efficient backhaul techniques.

The Problem with TDM Backhaul

TDM circuits have historically inter-connected base stations to regional network controllers, which worked fine for voice-only systems or with low-bandwidth data traffic. However, the rapid growth in mobile broadband traffic has overloaded TDM circuits and providers are now unable to keep up with the uptick in wireless traffic growth.

Adding more TDM circuits to address this challenge is not a viable option since the recurring monthly costs for legacy backhaul technologies (PDH, ATM over PDH, and SONET/SDH) increase linearly with traffic. The relatively flat average revenue per user (ARPU) that an operator can charge for enhanced services prevents carriers from passing these increased expenses on to consumers. Operators are thus looking to move to packet-based backhaul techniques using IP and Ethernet to gain a lower cost per bit. Using Carrier Ethernet for wireless backhaul allows operators to support large bandwidth increases from cell sites, while keeping operational costs in check. Operators can significantly reduce their cost per connection by moving from TDM to Ethernet (Fig 3).

Fig 3: PDH vs. Ethernet: Annual Mobile Backhaul Service Charges per Connection

Making the Leap from TDM to IP/Ethernet Backhaul

The move from TDM- to Ethernet-based transport does not come without its challenges. TDM-circuits are well established at providing predicable quality of service (QoS),ultra-high reliability, and clock synchronization across the network. New technologies and enhancements to native Ethernet are required to achieve these same capabilities.

To address the QoS requirement, the Metro Ethernet Forum (MEF) has created the MEF 10.2 technical specification to define service attributes establishing traffic classes. This requires IEEE 802.1Q VLAN tagging providing use of the priority bits. Implementing QoS provides the ability to offer service level agreements (SLAs) - and service profiles. Service profiles can be specified according to different parameters, such required bandwidth, latency, frame delay variation, and frame loss ratio. Equipment in the network handles and prioritizes incoming traffic according to tags and associated profiles. It is important to test that a device or system can support Ethernet Virtual Circuits (EVCs) with defined service performance.

Unlike SONET, Ethernet has no "out of bound" control. To achieve the requisite "five nine's reliability," there is a whole subset of Ethernet now devoted to Operations Administration and Maintenance (OAM).OAM technology allows carriers to provide fault detection, verification, isolation, recovery and notification across Ethernet links, and end-to-end services. Functional, conformance, and interoperability testing of Ethernet OAM technology is required across network devices prior to deployment.

To formalize compliance to the standards and interoperability of Carrier Ethernet devices, the MEF has published series of certification standards referred to as MEF 9, 14, and 21.

QoS and OAM enhancements to Ethernet make it a viable technology for transporting services over the mobile backhaul. In a recent survey of Global Service Providers by Infonetics, 100% of service provider respondents claimed to be deploying IP/Ethernet backhaul in 2010. However, this will be a "phased" migration, as outlined by the MEF 22 Mobile Backhaul Implementation Agreement, and illustrated below.

Fig 4: Mobile backhaul migration

The first phase is a hybrid implementation where Carrier Ethernet is used for packet offload of data services, and TDM is retained for voice since it requires clock synchronization across the network for call set-up and handover. This approach is not an ideal solution as it forces carriers to maintain and pay for two separate networks. The ultimate goal is phase two, in which a single Carrier Ethernet network is used to backhaul all services. The Infonetics survey indicated that 65% of service providers plan to eventually move to a single IP/Ethernet backhaul. Before pursuing this final stage of migration, carriers must have confidence that timing over packet (ToP) technologies can satisfy strict clock synchronization requirements of wireless standards.

ToP Technologies Enable all-Ethernet Backhaul

IEEE specification 1588 and ITU-T Synchronous Ethernet (SyncE) are ToP technologies that serve to synchronize clock frequency across devices in the Ethernet backhaul network and improve clock accuracy to satisfy the timing requirements of supporting mobile voice subscribers (i.e., achieve the desired +-50ppb when synchronized to a primary reference clock source).

SyncE is a technology that achieves frequency synchronization across Ethernet network devices. Synchronous Ethernet offers two major changes over traditional Ethernet to make it suitable for clock distribution:

* A mandated clock accuracy of 4.6ppm

* The ESMC protocol (described in ITU-T G.8264) for clock selection, distribution, management, traceability, and failover. (Requires priority marking of ESMC packets.)

The basic operation (Fig 5) of SyncE interfaces is to derive the frequency from the received bit stream and pass that information up to the system clock.

Fig 5: Operation of Synchronous Ethernet

The IEEE 1588 standard specifies the Precision Timing Protocol (PTP), for network synchronization. IEEE 1588 differs from SyncE in 2 fundamental ways:

* In addition to frequency synchronization, it achieves time-of-day (ToD) synchronization (an accurate value of the current absolute time) to achieve phase alignment which is required for multi-channel communications.

* It is a purely packet-based solution, with the actual clock values being passed inside the payloads of special packets dedicated to that task.

IEEE-1588 establishes a Master-Slave hierarchy of clocks in a network, where each Slave synchronizes to a Master clock that acts as the primary time source. High-priority synchronization packets are exchanged between the Master and Slave so the slave can continually adjust its own oscillator. 

Version 2 of the IEEE 1588 Precision Time Protocol (IEEE 1588v2) introduced the concept of 'Boundary Clocks' and 'Transparent Clocks" (Fig 6) that serve to further improve system and network scalability and the accuracy of clock synchronization.

Fig 6: 1588v2 Boundary and Transparent Clocks

New Test Tools Accelerate Deployment

With industry standards in place, SyncE and IEEE 1588v2 technologies have been widely accepted in industry and are being delivered in Ethernet chip sets today. However, "industry standards" do not guarantee that the technology implementation on different network devices will interoperate since granular details, such as specific field values, are left up to each equipment vendor's discretion. SyncE and 1588v2 interoperability testing between different vendor devices prior to deployment is critical to ensure network-wide clock synchronization. Phase and frequency synchronization with 1588v2 and SyncE was a key focus of the European Advanced Networking Test Center (EANTC) interoperability showcase at Carrier Ethernet World Congress September 20-23, 2011 in Warsaw, Poland. Test equipment played a critical role in identifying, troubleshooting, and resolving communication barriers between the different vendor equipment in the event.

Furthermore, the functionality, performance and stability of 1588 can be compromised under network stress and heavy loadm which can result in missed call handovers, clock frequency drift, and even network downtime. Functional, performance and stress testing of 1588 under 'real world' conditions is required prior to deployment.

Service providers can use newly released test tools that support IEEE 1588v2 emulation, along with multi-service traffic generation, to simulate entire mobile backhaul environments in a lab setting - allowing them to test the quality and scalability of mobile voice services over an IP/Ethernet infrastructure before making the transition from TDM. The first public demonstration of 1588v2 performance validation was featured at Mobile World Congress (Feb 14-17, 2011). Ixia test equipment was used to benchmark the performance of Alcatel-Lucent's 7705 Service Aggregation Router, verify its ability to maintain clock synchronization, and traffic forwarding under traffic load and highly scaled network conditions.

Fig 7: Industry-first 1588v2 Performance Demonstration at Mobile World Congress 2011

The public demonstration proved that network equipment can satisfy the requirements necessary to offer mobile voice services on packet backhaul networks. A video and case study of the demonstration is available at this link:  www.ixiacom.com/1588test

As service providers migrate from synchronous TDM based networks to non-synchronous Ethernet/IP, they require a cost effective mechanism that propagates timing in order to support the existing synchronous infrastructure - especially in mobile backhaul networks. Test equipment is playing a critical role in building service providers' confidence to make the leap to an all-Ethernet backhaul that can accommodate the continued growth of mobile subscribers and traffic - without breaking their budgets.

About the Author

Tara Van Unen is a Sr. Manager, Market Development for Ixia. She specializes in developing Ixia's strategic marketing plans for routing, switching and broadband technologies