Synchronisation in telecommunications largely arose from the introduction of 64kbit/s digital switching and transmission of voice telephony. For digital switching to work, it is critical that the sampling, coding, multiplexing and switching all occur at exactly the same rate. If samples arrive at the switch more often than they can be written then at some point a sample must be thrown away; and if samples arrive less often then at some point the same sample will be repeated. Either of these will result in an audible disturbance. Hence each piece of equipment in each exchange must run at the same rate (or frequency), and from this arises the fundamental need for synchronisation.
Rather than providing separate infrastructure for distributing synchronisation, the Primary Rate bit-stream at 2048kbit/s was adopted. However these bit-streams do not have a wholly repetitive and predictable pattern of edges from which to recover the reference frequency. Such short-term variations (or jitter) in frequency that accumulate along a chain of nodes in the hierarchy are smoothed out by means of a high quality oscillator and a long time-constant phase locked loop.
Increasing traffic demanded more capacity which was met, in part, by multiplexing the Primary Rate bit-streams into higher and higher bit rate transmission paths. However, these Primary Rate signals may not be synchronous with each other and, to cater for the differences, the old Plesiochronous Digital Hierarchy (PDH), used a technique known as justification whereby dummy bits are either added (or not) to equalise the bit rates. This approach means that the synchronisation borne by a Primary Rate input signal is carried transparently and independently in the multiplex (although some jitter may be introduced by the justification).
Synchronous Digital Hierarchy (SDH) multiplexing largely replaced PDH. SDH uses a byte-interleaved scheme to multiplex and cross-connect the payloads of the SDH signals. However, it cannot be assumed that the payloads are synchronous with the overall SDH frame, and even other SDH frames may not be synchronous (for example they may originate within another operators network). To cope with this, SDH has a justification method in which a pointer is included in the overheads to indicate the start of the payload within the frame, allowing the payload to ‘float’ within the SDH multiplex structure. SDH still uses a 125 microsecond frame, and so the synchronisation rate and interfaces are carried over from PDH.
The emergence of IP telephony and softswitching had been hailed as the beginning of the end of the need for synchronisation. However, cellular mobile networks had quietly been taking advantage of existing synchronisation infrastructure for a quite different purpose, and is now probably the dominant user of frequency synchronisation.
Digital cellular base stations have tight limits on the frequency of their carriers and frame repetition rates in order that mobile devices can successfully decode signals from different nearby base stations and seamlessly move between them during calls. Early base stations achieved this using costly high stability oscillators. However, the transmission to these base stations uses the same Primary Rate signal as described above and this is used as an accurate frequency reference. The signals do suffer from jitter but this can be smoothed out by a relatively inexpensive oscillator locked to the incoming transmission.
More recently, base stations began using Ethernet-based transmission; it is mandatory for 4G and now common for 3G and 2G as well. Therefore the delivery of synchronisation was incorporated into the Ethernet standard to create Synchronous Ethernet (SyncE). An alternative to SyncE is Precision Time Protocol (PTP), a method for transferring time over packet networks. PTP has some advantages over SyncE in that it can be implemented on pre-existing Ethernet networks but is sensitive to packet delay variation.
The LTE-Advanced base station air interfaces will require alignment in time (with submicrosecond accuracy) as well as frequency so that mobile devices can sort out the signals from different base stations. Time synchronisation can be achieved via the transmission or from an off-air source such as GPS. None of the available methods is without challenge and much of current work on synchronisation is focused on solving these.