| Introduction
Synchronization in telecommunication networks plays a very important role:
the main reason is that
it has a great influence on the system performances
and the quality of services offered by the network operators to the users.
Synchronization is of fundamental importance at both the application level
(voice, audio, video and graphics) and at the transmission level
(packet, cell, symbol and bit). If the synchronization is not accurate enough and the network
can not store the informative units,
a certain amount of data is lost and the
throughput decreases. To avoid this problem, all the network entities need to be
synchronized with
a high accuracy.
In order to maintain the bit flow
integrity through a telecommunication network, an accurate synchronization
signal to
timing the bit reading is needed. In a wide area network the condition
of a perfect synchronism is not achievable because
of the phase fluctuations
along the transmissive vectors and in the network nodes.
High frequency phase
variations are referred as jitter, while low frequency ones are referred as
wander.
A frequency of 10 Hz is generally used to delineate wander from
jitter.
To avoid synchronization problems, accurate clocks are used.
There are two
kinds of clocks: autonomous clocks and slaved clocks. The first ones produce a
chrono-signal by themselves,
while the second ones are slaved to an input signal
and produce an output signal locked to the input one by means of a PLL.
If the
input signal fails they can generate an output in the holdover mode.
The clocks used in hierarchical networks may be divided into three different
groups according to
their features and the performances required by the
international normative: primary reference
clocks, synchronization supply
units and equipment clocks.
Primary reference clocks are the most important entities in a synchronization
network, because they
produce the synchronization signal directly or
indirectly used by all the other network clocks. This
signal degrades from
the higher levels to the lower ones: for this reason, it must provide
an
excellent long-term stability of 10-11/life. The most suitable clocks are
cesium standards or
hydrogen MASER.
The Synchronization Supply Units (SSU) consist of an oscillator and a PLL to
serve the output signal to an
input reference. This kind of clock must be
able to filter noise and phase discontinuities affecting
the reference
signal. Moreover, it has to maintain a stability level of 10-9/day, 10-8/day in
holdover
mode.
The most suitable clocks are rubidium standards or some high
performance quartz clocks.
Equipment clocks are generally constituted by quartz oscillators slaved to
the supply units: in fact,
in presence of a good reference, high performances
are not required (10-6/day).
This kind of clocks is very cheap in respect to
the other standards and provides a very good short-term stability.
SONET/ SDH (Synchronous Digital
Hierarchy)
Synchronous optical networking, is a method for communicating digital
information using lasers or light-emitting
diodes (LEDs) over optical fiber.
The method
was developed to replace the Plesiochronous
Digital Hierarchy (PDH) system for transporting large amounts of telephone
and
data traffic and to
allow for interoperability between equipment from different vendors.
Synchronous networking differs from PDH in that the exact rates that are
used to transport the data are
tightly synchronized across the
entire network, made possible by atomic clocks. This synchronization
system allows
entire inter-country networks to operate synchronously,
greatly reducing the amount of buffering required between elements in the
network.
The SDH network structure is based on the hierarchical strategy: the
reference signal is distributed from the primary clock (PRC) to the lower
layers.
PRC is the first layer clock, while in the intermediate network nodes
the SDH supply units (SSU) are used.
SSU are generally interconnected by
equipment clocks (SEC) chains of limited length to prevent from excessive signal
degradations.
As for Synchronization sources available to a SONET NE, these are:
- Local External Timing. This is generated by an atomic Cesium clock or a
satellite-derived clock, like GPS-Disciplined
- Rubidium, by a device located in the same central office as the SONET NE.
- Line-derived timing. A SONET NE can choose (or be configured) to derive its
timing from the line-level, by monitoring
- the S1 sync status bytes to ensure
quality.
- Holdover. As a last resort, in the absence of higher quality timing, a SONET
NE can go into "holdover" until higher quality
- external timing becomes available
again. In this mode, a SONET NE uses its own timing circuits to time the SONET
signal.
ATM (Asynchronous Transfer
Mode)
The ATM networks transfer information through cells of 53 byte and the bit
rates supported are
155.2 Mbit/s and 622.8 Mbit/s. ATM can transfer either
constant or variable bit rate services, but
because of the asynchronous
nature of the system, there are any problems in the AAL layer to the
receiver
in reconstructing the symbol rate of the source. In fact, arrival cell rate is
variable and
during the transfer through the network some cells may be lost
or affected by jitter and wander. For
this reason, an opportune dimensioning
of the traffic control parameters and a sort of low pass
filtering of the
cell interarrival time are needed.
There are, at least, three sources of jitter associated to the cell transfer
technology:
- Bursty nature of the traffic
- Statistical multiplexing in the network nodes
- Source jitter (a variable delay in filling up the payload of the cell)
In ATM connections the transmitter plays the role of the master and the
receiver the slave, bringing
to a point-to-point architecture.
There are three possible methods of synchronization of ATM traffic
streams:
- Immediate playout: consist in releasing the data to the
application as soon they enter the AAL
receiver.
- Adaptive playout: consist in buffering in the AAL receiver
the input traffic, in order to smooth
cell transmission jitter
- Synchronous residual timestamp: (SRTS), consist in
injecting appropriate Residual Timestamps in the transmitted data stream.
- A RTS
encodes the difference between the service clock frequency and a common clock
frequency.
Rubidium clocks can be used inside the cross-connect nodes of the ATM
network for distribution of synchronization in
the network infrastructure for
the long-term stability because of there good performances in
holdover
Periods.
Network time Servers, NTP
NTP is used, inside the Internet architecture, to synchronize hosts
and routers; a lot of services are
based on NTP, for example NFS, file
transfer and E-commerce. NTP architecture is quite similar to
the
hierarchical architecture adopted in the digital telephone networks: each level
is called stratum.
At the first level of the hierarchy, we find time servers
that are connected to a primary reference
clock, such as accurate GPS
receivers and atomic clocks.
The utilization of atomic clocks in this
architecture could be useful in timeservers combined to a
GPS receiver. A
unique primary signal for the synchronization could not be sufficient because
GPS
signal gives no continuity guarantees. Atomic clocks can also be used in
the 2nd stratum of the
hierarchy for the distribution of the network
synchronization
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