Aviat Networks addresses considerations with respect to the effectiveness and implementation of synchronisation technologies
Synchronisation is creating quite a stir in the mobile backhaul industry as operators are wrestling with a variety of synchronisation technology options including Synchronous Ethernet (SyncE) and Precision Time Protocol (PTP) a.k.a IEEE 1588v2. Microwave is the most dominant technology globally for mobile backhaul, and will continue to be so, as it has rapidly evolved to support hybrid and packet microwave networking options. The combination of these two technologies poses some interesting questions, such as, which synchronisation options are effective over a microwave backhaul? Are there any differences or unique considerations when it comes to the implementation of synchronisation over a packet microwave network?
Network synchronisation choices are available
As mobile operators are now moving to “All-IP”, the clash between asynchronous and synchronous networking is nowhere more heated than in the mobile backhaul arena. Networks are undergoing a dramatic migration to support a host of bandwidth hungry data and video services, while needing to maintain traditional mobile voice services. Timing is especially critical to mobile voice quality and overall performance, since synchronisation degradation can result in dropped calls, speech clipping, or the inability to hand off calls when mobile users are travelling between cell sites.
Mobile base stations all rely on some means of synchronisation, and timing signals transported on the mobile backhaul are critical to maintain and deliver quality of service. Synchronisation to a primary reference clock in a mobile network can be based on frequency, time or phase; while frequency synchronisation is the most common in mobile backhaul, time and/or phase synchronisation is also necessary for CDMA, LTE TDD, LTE MBMS, Mobile WiMAX and other applications.
Fortunately, there are several methods available today to provide synchronisation across a packet-based mobile backhaul infrastructure, but this means that mobile operators have to make tough decisions on which one or ones to adopt, moving forward. There are also unique microwave backhaul characteristics that need to be considered when delivering synchronisation and migrating to packet.
Unique microwave backhaul characteristics
For topologies that utilise microwave in all or a portion of the backhaul, these are some key considerations that need to be taken into account for support of synchronisation.
No “rip and replace” with microwave
When migrating from legacy T1/E1 to fibre, copper wire is decommissioned, literally “ripped out”, and replaced with the new physical media. This is extremely disruptive to the network and to customers, and very costly - completely contrary to the goals for migrating to Ethernet in the first place to reduce cost for burgeoning mobile video and data user traffic. Advanced microwave systems provide the flexibility ton upgrade from TDM, to Hybrid, or all Packet, via a simple software reconfiguration and additional capacity modules, while continuing to utilise the same physical media – a radio path, typically over a licensed RF channel.
Having this flexibility with microwave provides a simple means to maintain synchronisation for some operators. By reserving a small amount of bandwidth for TDM control and synchronisation traffic, the TDM timing distribution is kept in place while the rest of the network can evolve to packet transport. Network disruption is eliminated with no need for a ‘rip and replace’, and upgrade costs are kept under control.
Radio path variability and availability
Fibre, wireline and microwave all support binary availability - they can be either up or down and optional link or ring protection mechanisms may be put in place to increase availability as needed. However, microwave networks have a distinct advantage in that they can be engineered to deal with non-binary operational states as a result of radio path fading, mostly due to weather or environmental related impairments. Under these conditions, advanced microwave systems are engineered to operate under multiple states of network availability (e.g. 99.999 per cent, 99.99 per cent etc.) and can invoke various mechanisms to respond to changes in the state of availability.
As synchronisation traffic is critical to the operation of the mobile network, it is important that it receives the highest prioritisation under all conditions of microwave transport and/or ACM state changes. An advanced microwave system provides efficient mechanisms for dealing with radio path variability and the resulting available throughput, and can ensure that timing traffic is prioritised under all conditions.
