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 ***

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. 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. 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. 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)
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