The Achilles heel of using Ethernet for mobile backhaul has always been timing, and the recent large-scale shift to Ethernet as mobile-backhaul interface of choice has forced a shift in network planners' timing strategies.
Carriers have historically relied on TDM's proven synchronization function for timing, as the "T" does stand for "time" after all. With TDM, you sort of get timing/synchronization for free, so there was little reason in the past to further develop timing protocols. With the arrival of packet traffic onto mobile devices, data was at first carried along with the voice on the TDM link, and eventually when data exceeded the capacity of the voice channel, along a separate parallel Ethernet path, with timing and voice still delivered by the TDM - a hybrid approach.
Three different industry dynamics are pressuring hybrid links.
- 1. As already mentioned, mobile bandwidth is growing faster than hybrid links can keep up.
- 2. While mobile bandwidth consumption is exploding, the cost/bit that carriers can charge is in freefall, creating cost pressures that favor pure Ethernet plays.
- 3. Mobile carriers long ago phased out analog phone support, and are in the process of phasing out 2G TDMA support, leaving many hybrid links without the clock distribution tree they depended upon.
The transition from hybrid to pure Ethernet mobile backhaul has been hastened by the listed pressures, becoming a catalyst to development of industry standard protocols to improve timing distribution over packet infrastructures.
One of the resulting innovations in this area has been IEEE 1588TM-2008, "Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems." Relevant to a host of applications including the Smart Grid, manufacturing and broadcasting, the Layer 2 Precision Time Protocol (PTP) has been an especially key breakthrough enabling the metamorphosis toward pure Ethernet.
IEEE 1588 PTP provides Ethernet with a standardized, end-to-end timing distribution mechanism that enables the recovery of frequency and time/phase synchronization signals in spite of the technology's inherent packet delay variation (PDV). This helps ensure that a carrier's pure Ethernet mobile backhaul infrastructure of equipment can synchronize in less than a microsecond across the network, enabling carrier-class performance for time-sensitive services, without significant administrative intervention and associated operational expenditures (OPEX).
IEEE 1588 PTP works by steadily trading timing packets between the master and various slave clocks in the servers and other devices across a network. Utilizing a system of "boundary" and "transparent" clocks (in the nomenclature of the standard), PTP allows path delay to be accurately measured in various segments across the infrastructure, Ethernet's inherent PDV to be offset and, in turn, the slaves and master clocks to be precisely synchronized to ensure carrier-class performance of time-sensitive services.
Continue to Part 3 >>
Jim Theodoras is President of the Board of Directors, Ethernet Alliance, and a columnist for FierceTelecom.