17 July 2011 |
The increasing demand on network operators to supply richer service offerings at more competitive rates has forced a convergence of network technologies to packet-based methods.
This has caused a paradigm shift in the availability of physical-layer synchronisation moving away from the circuit switched world to that of connectionless operation. The requirements for stable time and timing still exist for many applications, and synchronisation can help wireless operation as base stations can be synchronised in order to hand off calls between base stations, minimise dropped calls and ensure proper billing.
What are the different synchronisation requirements for different types of networks?
SONET and SDH networks are based on having a synchronous architecture, meaning that all data signals are synchronised and clocked using virtually the same clock throughout. This ensures that all ports that carry data do so at the same frequency or with very little offset and therefore network throughput is deterministic and fixed for specific transport rate.
However, as the network is shifting to packet-based architecture, operators are facing new challenges from a synchronisation perspective. Ethernet is an asynchronous technology where each Ethernet port has its own independent clock circuit and oscillator. Because each port is clock independent, frequency offsets between interconnected ports can be relatively high. To help solve this issue, Ethernet devices typically implement buffers that store traffic and can then mitigate the effect of offsets between two ports.
What are the various types of synchronisation?
Frequency synchronisation is typically a physical synchronisation where the output clocks between devices is synchronised. When two devices are frequency synchronised, they generate the same number of bits over an integration period; typically one second. When they are not frequency synchronised, one device will generate more bits per second than the other, which can cause overflow and eventually bit errors or traffic loss.
Phase synchronisation refers to the simultaneous variation of clocks between devices. When phase synchronised, the two devices will shift at exactly the same time from one clock pulse to the other.
Other than frequency synchronisation and phase synchronisation, telecoms networks require time synchronisation.
What protocols are being used for synchronising packet-based networks?
Standards have been introduced for Ethernet-based networks that introduce synchronisation capabilities for frequency and time of day distribution.
Time of day synchronisation can be achieved with protocols such as Network Time Protocol (NTP) which ensures that clients are correctly updated with time of day information based on a standard universal time source. NTP and its different versions distribute time and day information periodically to clients, such as personal computers and network devices, while ensuring corrections for geographic locations.
Synchronous Ethernet on the other hand ensures frequency synchronisation at the physical level. Ethernet SyncE achieves frequency by timing the output bit clocks from highly accurate stratum 1 traceable clock signals, in a fashion similar to traditional TDM and SONET/SDH synchronisation.
However, the phase synchronisation is a major requirement for newer mobile technologies and has only been addressed recently via the introduction of the IEEE1588v2 Precise Time Protocol (PTP) standard. This packet-based synchronisation mechanism combines frequency and phase synchronisation at sub-microsecond levels, with ToD distribution capabilities via the efficient mechanism of packet exchanges. This process ensures that edge clients are all frequency and phase aligned to a reference clock through apacket distribution.
Are there any problems with this technology?
NTP only provides time of day distribution and does not address frequency and phase requirements, which is a major limitation in its application for telecoms sync. Also it only offers accuracy in the millisecond range, which is completely inadequate for the technologies presently requiring synchronisation.
SyncE is a synchronisation technology based on Layer 1, and therefore requires that all ports on the synchronised path be enabled for SyncE. Any node that is non SyncE-enabled on the path will automatically break the synchronisation from this node. This is an issue for network providers that have a multitude of Ethernet ports between the primary synchronisation unit and the edge device that needs synchronisation, as all the ports must be SyncE enabled to synchronise to the edge of the network.
The major weakness of PTP is also due to its packet nature, as the synchronisation packets used by PTP are forwarded in the network between grand master and hosts, which are subject to all network events such as frame delay (latency), frame delay variation (packet jitter) and frame loss. Even with the best practice of applying high priority to synchronisation flows, these synchronisation packets will still experience congestion and possible routing and forwarding issues such as out-of-sequence and route flaps.
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