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The need for Synchronisation in Telecommunications

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The need for some sort of synchronisation in telecommunications has existed almost as long as telecommunications itself. However synchronisation in the form dominant in the last 50 or so years arose from the introduction of Pulse Code Modulation (PCM) for transmission of voice telephony, and the use of digital switching techniques to establish voice circuits between subscribers as required. Martin Kingston explains.

*** Shared with Permission - ITP Journal Volume 10 | Part 1 - 2016 ***

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The need for Synchronisation in Telecommunications

  1. 1. THE JOURNAL TJ 10 MARTIN KINGSTON Volume 10 | Part 1 - 2016 THE NEED FOR SYNCHRONISATION IN TELECOMMUNICATIONS The need for some sort of synchronisation in telecommunications has existed almost as long as telecommunications itself. However synchronisation in the form dominant in the last 50 or so years arose from the introduction of Pulse Code Modulation (PCM) for transmission of voice telephony,and the use of digital switching techniques to establish voice circuits between subscribers as required. Martin Kingston explains. The principle of an early PCM voice telephony in which a digital circuit switch is used to route calls between subscribers is illustrated in Figure 1.The analogue voice signal from the subscriber on the left enters the local exchange and is sampled and digitised.The signal is sampled 8000 times per second,allowing audio frequencies up to about 3400Hz to be carried with relatively simple analogue anti-aliasing filters,and then digitised using an 8-bit non-linear code. This sampling rate of 8000 times per second,once every 125 microseconds,is the fundamental tick on which synchronisation in circuit-switched telecommunications is based.Each 8-bit sample isTime Division Multiplexed (TDM) into a frame containing samples from other subscribers,an 8-bit framing word and an 8-bit signalling word, this frame is serialised for transmission as a bit-stream to the central exchange.The frame is of fixed length allowing 30 samples, and along with a word each for framing and signalling that leads to a transmission bit rate of 32x8x8000 or 2048kbit/s,known as the Primary Rate. At the central exchange the bit-stream,along with those from other exchanges,is de- multiplexed and returned to a parallel form to be written into a particular location in the memory of the digital circuit switch.That location is then read for the sample to be serialised and multiplexed into a particular slot of another bit-stream for onward transmission.It is the combination of the ‘write to’ and‘read from’ particular memory locations that allows switching of calls between different subscribers.The bit- stream arrives at the destination subscriber’s local exchange,where it is finally converted to an analogue signal to be heard via the subscriber’s handset. For this technique 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. Approaches to synchronisation Three basic approaches to achieving synchronisation between elements are possible – independent,mutual and
  2. 2. 11 INFORM NETWORK DEVELOP hierarchical described below and illustrated in Figure 2. Independent –This might also be termed “no synchronisation”as each node uses its own internal frequency reference. This requires that each node’s reference is very accurate,such that the difference between the rates used by any two nodes is negligible.The difference has to be so small that sample drops or duplications happen very infrequently and this requires accuracies only achievable using atomic clock technologies. This would be very costly if used in every node. Mutual – Each node uses an average of the rates received from the nodes to which it is connected. Modelling and managing the behaviour of networks synchronised in this way is difficult,especially when nodes are connected in a mesh. It may be impossible to ensure that such an arrangement is stable. Hierarchical –A single master reference provides a frequency to the central nodes that is then distributed out to all connected nodes,more distant nodes receiving a reference from those closer to the master. Hybrids are also possible,and indeed a combination of hierarchical and independent approaches with independent groups of hierarchically synchronised nodes,became the dominant approach. Transmission and distribution of synchronisation Although an 8000 sample-per-second tick is the basis of the need for synchronisation,it wouldn’t be found as an interface or a signal for transmission.Rather than providing separate transmission for synchronisation, the Primary RateTDM bit-stream at 2048kbit/s was adopted (or for local transfer of synchronisation only,a bipolar signal of the same rate). Re-using theTDM bit-stream in this way does,however,create a challenge.Unlike a signal intended only for synchronisation,it will not have a wholly repetitive and predictable pattern of edges from which to recover the reference frequency.Because of this,short term variations (jitter) will occur and these will accumulate along a chain of nodes in the hierarchy (multiplexing into higher order bit-streams will also add jitter). In longer chains,this introduces a need for a filtering stage,usually realised in the form of a high quality oscillator and a long time- constant phase locked loop,to reduce these short term variations to an acceptable level. This filtering function is often needed where one of a small number of incoming transmission paths are to be used as the synchronisation source for a larger number of nodes.The requirements to select the source,to filter,and to provide multiple feeds are often met by one piece of equipment,a Synchronisation Supply Unit (SSU) shown in Figure 3. PDH multiplexing and synchronisation Increasing traffic demands increasing THE JOURNAL TJ THE NEED FOR SYNCHRONISATION IN TELECOMMUNICATIONS MARTIN KINGSTON Setting the scene for synchronisation Figure 1: Digital voice switching. Figure 2:Approaches to synchronisation Figure 3:An SSU featuring input ports (bottom); input selection,oscillators and output drivers (middle); and multiple outputs (top).
  3. 3. THE JOURNAL TJ 12 MARTIN KINGSTON capacity. More Primary Rate transmission paths could be added but it soon becomes more economical to transmit higher rates, and these higher rates are produced by multiplexing numbers of lower rate signals. The European hierarchy starts with Primary Rate bit-streams at 2048kbit/s (or 2Mbit/s) four of which are combined to form an 8Mbit/s multiplex.Similarly,four 8Mbit/s bit- streams are combined from a 34Mbit/s multiplex and four 34Mbit/s bit-streams to form a 140Mbit/s multiplex. It can be seen that the rates are approximate (they are names rather than specifications) but even so it is clear that 140Mbit/s is rather more than 4x4x4x2Mbit/s.There are some overheads for framing and management,but part of the“missing capacity”is due to the way that synchronisation is dealt with in this hierarchy of multiplexes. The multiplexing is asynchronous and does not assume that the Primary Rate bit- streams are synchronous with each-other,so it is known as the Plesiochronous Digital Hierarchy (PDH). This means the input bit streams may be running at a faster or slower rate than the multiplex frame and a mechanism is needed to cope with (or justify) the difference (as shown in Figure 4). This is done using justification positions within the multiplex frame; these may be bits from the input stream (when input rate is higher) or dummy bits (when input rate is the same or lower).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. SDH multiplexing and synchronisation Synchronous Digital Hierarchy (SDH) multiplexing has largely replaced PDH; the simple scalability of the structure makes further increments in capacity easier and the ability to access multiplexed elements independently controls cost and complexity of equipment (unlike PDH systems where a complete hierarchy of de-multiplexing and re-multiplexing is needed to access one Primary Rate element). SDH uses a byte-interleaved scheme to multiplex and cross-connect the payloads of SynchronousTransport Modules (STMs). However,it cannot be assumed that the payloads are synchronous with the STM-n frame,and even other STM-n 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 added to indicate the start of the payload within the STM-n frame (see Figure 5),allowing the payload to“float”within the SDH multiplex structure. A step change in timing between the client frame and the multiplex frame results in a change in the pointer value,and a frequency difference will result in a steady stream of changes in the pointer value. The timing steps introduced by a change in pointer value have to be multiples of eight bits,due to the byte interleaving scheme (unlike PDH multiplexing where one bit is possible).This causes higher jitter levels in de-multiplexed Primary Rate signals,but this can be minimised by ensuring SDH multiplexers are synchronised to a reference common with client signals.For this reason, SDH standards define a synchronisation architecture and require elements to have Synchronous Equipment Clocks and synchronisation functions to ensure a hierarchical distribution approach can be successfully implemented across all elements in an SDH network.SDH still uses a 125 microsecond frame,and so the synchronisation rate and interfaces are carried over from PDH. ‘Free ride’ to cellular base stations The emergence of IP telephony and soft- switching as a replacement for circuit switched telephony had been hailed as the beginning of the end of the need for synchronisation.However,a new user had quietly taken advantage of existing synchronisation infrastructure and techniques for a quite different purpose,and is now probably the dominant user of frequency synchronisation in telecommunications. Digital cellular base stations must control the frequency of their carriers and the rate of frame structures within certain limits so that a mobile device can successfully decode signals from different nearby base stations and seamlessly move between them during calls.Early base stations achieved this using high stability oscillators,but these are expensive and require regular adjustment. However,the transmission to these base stations was the same Primary Rate signal as used in conventional digital telephony and it wasn’t long before it was realised that the accurate frequency reference carried along Volume 10 | Part 1 - 2016 Figure 4: Principles of justification.
  4. 4. 13 INFORM NETWORK DEVELOP with this signal could be used by the base station. The cellular systems requirements are actually significantly less demanding, requiring only that adjacent radio interface bursts (which may be from different base stations) are within +/-50 parts per billion. The incoming transmission may be expected to have accuracy several orders of magnitude better than this in the long term, since it will have inherited the performance required for a circuit switched network.In the short term there will be some variation (jitter) introduced by multiplexing along the path but a relatively inexpensive oscillator locked to the incoming transmission will smooth these out and work as well or better than the independent oscillators used before. A de-facto standard developed whereby a transmission feed with a long term frequency accuracy of 15 parts per billion and jitter within the bounds set for a PDH transmission interface is acceptable for base station synchronisation. Primary Rate transmission continued to be used for base stations from 2G (TDM) through to 3G (AsynchronousTransfer Mode,but still on PDH transmission). More recently,base stations began to adopt packet transmission based on Ethernet; it is mandatory for 4G and now common for 3G and 2G as well.This created a challenge for synchronisation which has been addressed in two quite different ways. Firstly,delivering a frequency on the transmission physical layer was a clear, fundamentally good solution,and so synchronisation based on the transmission bit rate was incorporated into the Ethernet standard to create Synchronous Ethernet and architectures,similar to SDH,developed for distribution of synchronisation through Ethernet equipment [1]. With no need for the bit stuffing or pointer multiplexing methods used in PDH and SDH,Synchronous Ethernet transmission will in general introduce significantly lower levels of jitter. Secondly PrecisionTime Protocol,a method for transferring time over packet networks developed with measurement and automation in mind,was adapted to deliver a frequency reference to base stations over packet transmission.This method has some advantages over Synchronous Ethernet in that it can be implemented on pre-existing Ethernet transmission networks,but also the disadvantage of being sensitive to packet delay variation that may result with changes in traffic.Much of the development of PrecisionTime Protocol has been focused on methods to deal with this challenge. Time synchronisation for cellular base stations Some cellular base station air interface technologies require more than just frequency synchronisation to operate correctly. They require alignment in time as well.This has been the case for some time where aTime Division Duplex scheme is being used with the same spectrum serving both uplink and downlink,but new technologies in LongTerm Evolution (LTE)-Advanced1 have brought particularly strict requirements. The LTE-Advanced technologies include the LTE version of Multimedia Broadcast Multicast Service for broadcast,the LTE version of Inter- Cell Interference Coordination for interference coordination,and Coordinated Multi Point for increased capacity,all of which involve signals arriving at a user’s device from multiple base stations at the same time on the same frequency.These multiple sources can be sorted out by the device only if they have tightly co-ordinated times of arrival and this is how the requirement for time synchronisation at the base station arises. The co-ordination requirement can be as tight as a few microseconds at the device and, therefore,once differences in path length from different base stations are included,it may be necessary to time-synchronise base stations with sub-microsecond accuracy.The delivery of this time synchronisation (often called phase synchronisation since it is only the alignment of short term events that is important) can be achieved in several ways, via the transmission or from an off-air source like Global Positioning System for example. None of the available methods is without challenge and much of current work on synchronisation is focused on solving these. ABOUT THE AUTHOR Martin Kingston Principal Designer,EE Martin has over 20 years experience in communications covering broadcast,fixed,mobile and internet services, and the transmission and transport technologies that underpin them.He has been pursuing transport network convergence for the last 10 years.He is active in the field of synchronisation in next generation transport networks and has delivered transport convergence solutions using DWDM,SDH,ATM,MPLS,Ethernet and IP technologies and turned the stack on its head with Pseudowire technology. THE JOURNAL TJ THE NEED FOR SYNCHRONISATION IN TELECOMMUNICATIONS Figure 5: SDHVirtual Container. 1 LTEAdvanced is a major enhancement of the LongTerm Evolution (LTE) mobile standard. REFERENCES 1. Hann,K.,and Jobert,S. Synchronisation and time distribution in modern telecommunications networks.The Journal of the Institute ofTelecommunications Professionals, Vol 10(1).Mar 2016 (this issue) ITPINSIGHT CALL Want to talk to the author? To discuss this article and its content, join in the ITP Insight Call on 25 April, 2016. To book onto the call visit: https://www.theitp.org/calendar/

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