As packet-switched networks (PSNs) are asynchronous by nature, they introduce inaccuracies such as packet delay variation (PDV) and packet loss. As a result, synchronization – a crucial factor in cellular networks – has become the biggest challenge in the migration to packet-switched networks. While clocking characteristics is transmitted natively in TDM networks, synchronization over packet necessitates a dedicated and robust method to distribute accurate timing information to all network elements.
The evolution of synchronization over packet solutions has involved several approaches, resulting in the development of various methods and standards, the most notable of which include the following:
ACR is a method in which the clock is distributed over the PSN as an inband TDM stream and regenerated using the packets’ time-of-arrival information, independently of the physical layer. The clock stream format is a standard pseudowire flow, simplifying interoperability with third-party equipment. In addition, bandwidth consumption can be minimized by using a multicast pseudowire for clock distribution. Today, pseudowire gateways incorporating high-performance adaptive clock recovery mechanisms are already deployed and meet stringent GSM/UMTS requirements.
IEEE 1588, also known as Precision Time Protocol, is a frequency and time of day distribution protocol, which is based on timestamp information exchange in a master-slave hierarchy, whereby the timing information is originated at a Grandmaster clock function that is usually traceable to a Primary Reference Clock (PRC) or Coordinated Universal Time (UTC). Similar to NTP (network time protocol), it nonetheless offers better accuracy, with HW-based timestamping support and fractional nanosecond precision. IEEE 1588 defines the packet format for timing distribution but does not specify the actual clock recovery algorithm – the critical element in network synchronization. Although it can be implemented end-to-end, support of 1588 by intermediate network elements (“boundary clocks” and “transparent clocks”) ensures better performance. Current deployments are limited as the second version of the IEEE 1588 standard has been only recently ratified.
NTP is a widely deployed IETF standard for distributing time of day in wide area networks (usually for public Internet). It uses a hierarchical system of "clock strata", whereby the stratum levels define the distance from the master reference clock and, consequently, the associated accuracy. NTPv4 is an enhancement of the NTP protocol, providing an accuracy level of milliseconds. In order to achieve the exacting accuracy required for real-time services, a high-precision (and costly) oscillator is required at the customer premises, or base stations.
Synchronous Ethernet, defined in ITU-T standards G.8261, G.8262 and G.8264, uses the Ethernet physical layer to accurately distribute frequency, using clock mechanisms similar to those of SDH/SONET. Unlike timing distribution in emulation services, where clocking information is carried in the same flow as the data payload, in Synchronous Ethernet the bits clock of the Ethernet physical layer is disciplined to a PRC, regardless of the higher layer transmission protocols used. As SyncE is a link-by-link frequency distribution scheme, it requires the entire clock distribution path (i.e. all the network nodes involved) to be Sync-E compliant.
NTR is a highly accurate standardized method for frequency distribution in DSL-based Last Mile segments. A network reference clock (i.e. a service clock) is distributed from the DSLAM to the CPE by mapping its clock information to the DSL modem transmission. Depending on the specific DSL technology, this is achieved by either directly locking the DSL symbol clock to the reference clock or by mapping to the DSL frame phase difference bits information between the reference clock and the DSL free-running symbol clock. The advantages of NTR lie in its high level of accuracy and in the fact that it eliminates the need for advanced synchronization hardware in the DSL modem/IAD, thereby reducing the overall cost of the solution.
Other methods for synchronization over packet include GPS and hybrid topologies involving a separate E1/T1 link for synchronization purposes. Next generation networks, however, are most likely to utilize some sort of combination between several methodologies. For example, NTR can be used between a DSL gateway and the local DSLAM, which can employ ACR, SyncE or 1588v2 towards the packet network.
TICTOC is an IETF Working Group responsible for the development of the next generation of solutions for distribution of time and frequency over IP and MPLS networks in order to serve a broad range of applications. These solutions are targeted at attaining higher performance than presently available using NTP. Unlike IEEE 1588, which focuses on Ethernet networks and often requires enhancements, such as transparent clocks, TICTOC address existing IP and MPLS IP networks.
Synchronization over packet is an integral element of cellular backhaul over packet transport. Any of the schemes listed above can be used in such an environment, provided that the following key requirements are met:
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The clock recovery mechanisms must comply with G.823/G.824 Traffic Interface specifications, using G.8261-defined reference networks and testing scenarios
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Frequency accuracy, when locked to a PRC (stratum 1) or SSU (stratum 2) clock, must be better than 16 ppb (parts per billion)
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Frequency accuracy when disconnected from clock source (holdover) should comply with Stratum 3E holdover requirements, i.e. not drift more than 1 ppb per 24 hours.
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