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SONET
SONET
Pocket Guide
revised version
Vol. 3
Please ask for:
Vol. 1
Pocket Guide
SDH
Fundamentals
and SDH Testing
Vol. 2
Pocket Guide
GSM
Fundamentals in
Mobile Radio Networks
Vol. 4
Pocket Guide
ATM
Fundamentals
and ATM Testing
Vol. 5
Pocket Guide
E1
The World of E1
Subject to change without notice
Nominal charge US $ 10 ± TP/EN/PG03/0201/AE repl. 1013 Pocket Guide for Synchronous Optical Networks ± Fundamentals and SONET Testing ±
Publisher:
Acterna Eningen GmbH
Postfach 12 62
72795 Eningen u. A.
Germany
E-mail: stephan.schultz@acterna.com
http://www.acterna.com
Author:
Stephan Schultz SONET pocket guide contents
The sun is made of copper
3
Why SONET?
6
SONET in terms of a layer model
9
What are the components
of a synchronous network
11
The STS-1 frame format
16
How are DSn and ATM signals
transported by SONET?
21
What is the difference between
SDH and SONET?
24
Pointer procedures
26
OC-12c contiguous concatenation
30
Transmission at higher hierarchy levels
32
Maintenance signals
32
Enhanced Remote Defect Indication (RDI) 36
Back-up network switching
37
Automatic protection switching
37
Linear protection
37
Ring protection
39
Synchronization
41
TMN in the SONET network
43
SONET measurement tasks
46
Sensor tests
49
APS response time measurement
49
ANSI/Bellcore performance analysis
51
Performance analysis
52
Tandem connection monitoring
53
Jitter measurements
54
Simulating pointer activity
61
Overview of current recommendations
relevant to SONET
62
SONET abbreviations
64
The sun is made
of copper
Nowadays, anyone making such a statement would likely be considered
quite mad, yet with these words, spoken back in 1861, Johann Philipp
Reis began something that has completely changed the world. This
nonsense message, just spoken by Reis into his new invention, was
clearly heard by the receiving party. The telephone was born. Despite
this, the first usable telephone (A.G. Bell, 1876: Patent for electrical and
magnetic transmission of sounds) was thought of as little more than a
toy.
Today, it would be difficult for us to imagine life without the telephone.
Worldwide, there are some 750 million telephone connections in use
and the number of Internet users has exploded in the last few years. By
the year 2000, according to a forecast from Nortel, there will be almost
475 million Internet users and the number of services provided will also
grow rapidly.
Right from the start, network providers have been faced with coping with
a steady increase in the number of users and hence in telephone traffic.
This has led to the development of various methods and technologies,
designed on the one hand to meet the demands of the market and on
the other hand to be as economical as possible.
With the advent of semiconductor circuits and the ever-increasing
demand for telephone capacity, a new transmission method known as
pulse code modulation (PCM) made its appearance in the 1960s.
3 PCM allows multiple use of a single line by means of digital time-domain
multiplexing. The analog telephone signal with a bandwidth of 3.1 kHz is
sampled, quantized and encoded and then transmitted at a bit rate of
64 kbit/s. A transmission rate of 1544 kbit/s results when 24 such coded
channels are collected together into a frame along with the necessary
signaling information. This so-called primary rate (ªT1º or ªDS1º) is used
in the US, Canada and Japan (see Fig. 1).
The growing demand for more bandwidth made more stages of
multiplexing necessary. The asynchronous hierarchy is the result. Slight
differences in timing mean that justification or stuffing is necessary
when forming the multiplexed signals. Inserting or dropping an
individual 64 kbit/s channel to or from a higher digital hierarchy requires
a considerable amount of complex multiplexer equipment.
Towards the end of the 1980s, a Synchronous Optical Network (SONET)
was introduced. This paved the way for a unified network structure on
a worldwide scale, resulting in a means of efficient and economical
network management for network providers. The networks can easily
be adapted to meet the ever-growing demand for ªbandwidth-hungryº
applications and services.
4 Fig. 1: Summary of plesio-
chronous transmission rates
5 Why SONET?
Following the introduction of PCM technology in the 1960s, communi-
cations networks were gradually converted to digital technology over
the next two decades. To cope with the demand for ever higher bit
rates, a complex multiplex hierarchy evolved. The bit rates include the
standard multiplex rates of 1.5 Mbit/s and 45 Mbit/s. In many other
parts of the world, however, a different multiplex hierarchy evolved
based on a primary rate of 2 Mbit/s (often called the ªE1º). Because of
these very different developments, gateways between one network and
another were very difficult and expensive to implement.
The late 1980s saw the initial field trials for SONET (Synchronous
Optical NETwork) technology. SONET takes advantage of technological
advances in the areas of semiconductors and fiber optics and is
superior to asynchronous systems in many ways. The benefits for net-
work providers are as follows:
1. High transmission rates
Transmission rates of up to 10 Gbit/s are standardized in SONET
systems. SONET is therefore the most suitable technology for back-
bones, which can be considered the ªsuperhighwaysº of today's
telecommunications networks.
2. Simplified add
Compared with pre-SONET systems, it is much easier to drop and
insert low-bit rate channels from or into the high-speed bit streams in
SONET. It is no longer necessary to demultiplex and then remultiplex
6 the entire asynchronous mux structure, a complex and costly proce-
dure at best.
3. High availability and capacity matching
With SONET, network providers can react quickly and easily to the
requirements of their customers. For example, leased lines can be
switched in a matter of minutes. The network provider can use
standardized network elements that can be controlled and monitored
from a central location by means of a telecommunications manage-
ment network (TMN).
4. Reliability
Modern SONET networks include various automatic back-up and
repair mechanisms to cope with system faults. Failure of a link or a
network element does not lead to failure of the entire network, which
could be a financial disaster for the network provider. These back-up
connections are also monitored by a management system.
5. Future-proof platform for new services
Right now, SONET is the ideal platform for services ranging from
POTS, ISDN and mobile radio through to data communications (LAN,
WAN, etc.), and it is able to handle new, upcoming services such as
video on demand and digital video broadcasting via ATM.
6. Interconnection
SONET makes it much easier to set up gateways between different
network providers and to SDH systems. SONET interfaces are
7 globally standardized, making it possible to combine network
elements from different manufacturers into a network. The result is a
reduction in equipment costs compared with pre-SONET.
The driving force behind the growth in SONET networks is the rising
demand for bandwidth, higher quality of service and reliability on the
one hand, coupled with pressure to reduce costs in an increasingly
competitive environment on the other hand.
What will the future of communications networks look like? One trend is
toward ever higher bit rates, e.g. OC-768 (TDM time division multi-
plexing). The other possibility is to use dense wavelength division multi-
plexing (DWDM). DWDM allows single-mode fibers to be used to trans-
mit multiple channels. Several different wavelengths acting as carriers
for the digital signals are transmitted down the fiber simultaneously. A
combination of TDM and DWDM is the solution that currently offers the
best prospects. It unites the excellent scalability of SONET with broad-
band (and hence inexpensive) transmission of signals using DWDM.
The performance of optical networks will improve greatly in future,
eventually leading to an all-optical network. In such networks, optical-
electrical conversion of the signals can be avoided entirely. The DWDM
networks will handle SONET functions, such as automatic protection
switching.
Considering the ISO-OSI layer model, this means that, in future, there
will be an additional ªphotonic layerº below the ªSONET layerº (see
figure 4).
8 SONET in terms of
the layer model
Telecommunications technologies are generally illustrated using so-
called layer models. SONET can also be depicted in this way.
SONET networks are subdivided into various layers that are directly
related to the network topology. Each layer of the SONET network has
its own overhead information.
The lowest layer is the physical layer, which represents the transmission
medium. This is usually a fiber link or occasionall