Full text of DOI:10.1109/35.846090

TOPICS IN LIGHTWAVE
Evolution of Optical Transport
Technologies: From SONET/SDH to WDM
Dirceu Cavendish, C&C Research Laboratories
ABSTRACT
technology on SONET/SDH equipment, address-
ing the various alternatives currently considered
for future all-optical WDM networks. Finally, we
sketch a roadmap for migration from SONET/
SDH to WDM networks.
It took roughly 10 years for the transport
network industry to migrate from PD H to
SONET. As this technology swap comes to an
end, WDM technology is dawning, promising to
revolutionize the network industry, with the pos-
sibility of transport bit rates above 10 Gb/s as
well as transparency to signal encodings. How-
ever, a new wave of equipment upgrade is
unlikely to happen as current SONET equip-
ment is just beginning to pay off for its large
investment. Thus, in years to come, SO NET
technology, the current standard for optical
fiber access, will have to make room for WDM
technology in a gradual way. On its part, WDM
equipment must be developed to be backward
compatible with SONET technology. This arti-
cle discusses the requirements and issues
involved in making WDM technology interoper-
able with SONET legacy equipment, as well as
the evolution path toward a transparent optical
transport network.
SONET/SDH TECHNOLOGY
SONET is a standard for optical communication,
providing framing, as well as a rate hierarchy and
optical parameters for interfaces ranging from 51
Mb/s (OC-1) up to 9.8 Gb/s (OC-192). Initially
developed by Bell Communications Research
(now Telcordia Technologies), it has been adopt-
ed as a standard by the American National Stan-
dards Institute (ANSI). A slightly different
version, SDH, has been adopted by the Interna-
tional Telecommunication Union — Telecommu-
nication Standardization Sector (ITU-T). Since
the discussions in this article are equally applica-
ble to SONET and SDH, we do not make a dis-
tinction between the two standards.
SONET was designed to provide a standard
access to the optical transmission medium. It uses
a specific frame format to carry data plus over-
head bytes. SONET channels are synchronous.
The synchronization of channels is supported by
pointers, which dictate the initial byte position of
each channel within the SONET frame. These
pointers are used to multiplex digital signals with-
in a single SONET frame efficiently.
SONET has four sublayers: path, line, sec-
tion, and physical. The path sublayer terminates
SONET connections, and is thus responsible
for monitoring and tracking the status/perfor-
mance of connections. The line sublayer is
responsible for multiplexing path-layer connec-
tions into a single link or fiber, connecting two
nodes. Thus, it is also known as the multiplex
section sublayer. The line sublayer is also
responsible for line protection in the event of a
failure. Each link is formed by various sections,
which are segments delimited by signal regener-
ators. The section sublayer, located below the
line sublayer, is present at each regenerator
and terminal in the network. The physical sub-
layer provides the transmission of the digital
signal over the fibers.
INTRODUCTION
Wavelength-division multiplexing (WD M)
promises to multiply the bandwidth capacity of
optical transmission medium many folds. The
principle behind it is the transmission of multi-
ple digital signals using several wavelengths so
that there is no interference among them. This
effectively allows us to tap the much greater
bandwidth capacity of optical fibers.
Digital transmission equipment currently being
deployed uses optical fibers to carry a single digi-
tal signal per fiber per propagation direction. The
most successful and widely spread technology is
synchronous optical network/synchronous digital
hierarchy (SONET/SDH). Most high-speed digital
backbones are SONET/SDH-based.
A natural source of concern is how the new
WDM technology is going to interoperate with
legacy SONET/SDH equipment. Is WDM tech-
nology likely to replace SONET/SDH technology
entirely? If so, what is the likely roadmap for
this transition to happen? This article tries to
foresee answers to these questions, reasoning in
terms of both technical arguments and the reali-
ty of the network industry. We first present a
quick tutorial on SONET/SDH and WDM tech-
nologies, presenting their main features. Then
we discuss the impact of the emerging WDM
Each of these sublayers has its own overhead
bytes in the SONET frame. Of particular impor-
tance for this article are bytes D1–D12, as well
as bytes K1 and K2. Bytes D1, D2, and D3 form
the section data communication channel (DCC),
Series Editors:
S. Dixit and P. J. Lin.
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0163-6804/00/$10.00 © 2000 IEEE
IEEE Communications Magazine • June 2000

which is a channel allocated for data communi-
cation between section entities, providing a 192
kb/s communication channel. The channel can
be used for alarms, maintenance, control, and
monitoring purposes. Bytes K1 and K2 provide
the automatic protection switch (APS) function,
which protects a line against fiber failures. Bytes
D4–D12 provide a 576 kb/s message channel for
alarms, maintenance, control, and monitoring of
the line.
niently transport constant bit rate applications,
for other emerging applications (e.g., multi-
plexed voice signals and variable bit rate
clients), the allocation of constant rate SONET
signals may be wasteful. This is especially true
for packet network clients, such as IP and cer-
tain asynchronous transfer mode (ATM) traffic
classes. These clients are bursty in nature;
hence, fixed rate signals would be inappropriate
to support them. Therefore, specific schemes
for packet transport over SONET have been
defined.
SONET clients
organize their
data into SONET
signals in various
ways. The
definition of how
the data bytes
are arranged
within the SONET
signal is
SONET FRAME STRUCTURE AND INTERFACES
Fram e Stru ctu re — SONET client signals are
encapsulated into the SONET frame in a byte-
interleaved format, with a basic frame time of
ATM o ve r SONET — ATM is a packet switch
technology in which 53-byte packets, called cells,
are switched across an ATM transport network.
The term asynchronous comes from the fact that
ATM does not assign fixed time slots to realize
information transfer between two endpoints, as
does synchronous transfer mode (STM). ATM
makes available an array of transfer services,
from constant bit rate (CBR) to variable bit rate
(VBR) to unspecified bit rate (UBR).
1
25 µs. The base signal is the synchronous trans-
port signal level 1 (STS-1). There are overhead
and payload data bytes in the 90-byte x 9-row
frame structure. The overhead bytes consist of 3
bytes/row. The payload bytes, also called the syn-
chronous payload envelope (SPE), consist of the
remaining 87 bytes x 9 rows. The resulting STS-1
line speed is 51.84 Mb/s. By squeezing multiple
frames into a 125 µs time period, higher SONET
signals are obtained. Rather than transmitting
multiple frames back to back, these higher-rate
signals use a slightly different frame structure,
called concatenated frames. A concatenated
higher-rate frame is formed by grouping all over-
head bytes of the various STS-1 frames together
in consecutive columns, and then adding the
payload columns of each frame afterward so that
the transport overhead bytes and payload bytes
be grouped.
important
because
equipment using
different methods
will fail to
ATM can run on top of several interfaces. In
particular, the ATM Forum has defined SONET
interfaces, which involve the mapping of cells
into an SPE. Cells are placed back to back, after
communicate.
4
3
the cell payload is scrambled by a 1 + X self-
synchronous scrambler. This scrambler is in
addition to the scrambler used in SONET. The
scrambling process is necessary to guarantee
that the SONET signal will have enough transi-
tions to allow line rate clock recovery at the
receiver.
In order to recover the cells, at the receiver
side ATM equipment relies on the ATM header
cyclic redundancy check (CR C). Namely, the
SPE is scanned, on a sliding 5-byte (ATM head-
er size) window, and CRCs are computed. When
a match occurs, synchronization is established,
and the scanning stops. The next CRC is checked
by jumping 53 bytes ahead, assuming back-to-
back cell placement. In case of mismatch, a new
synchronization scanning starts.
In t e rfa ce s — SONET is designed to operate
over a single-mode fiber physical medium. The
optical specification of SONET interfaces includes
the characteristics of the optical line, as well as
the parameters of the optical transmitters and
receivers. It also includes the spectral characteris-
tics of the signal, the pulse shape of the transmit-
ter, and the power levels involved at each
interface. These specifications ensure interoper-
ability between SONET equipment from different
vendors. Moreover, the power level definition of
various interfaces is important when budgeting
power against fiber lengths, as well as the number
and placement of regenerators in a SONET net-
work.
IP over SONET — IP is another packet network
technology, ubiquitously used in computer com-
munication around the globe. Similar to ATM,
there are several interfaces over which IP proto-
cols run. IP routers’ first access to SONET net-
works used ATM as an intermediate layer,
through Internet Engineering Task Force (IETF)
RFC 1483 for IP encapsulation over ATM net-
works. However, for sake of efficiency, direct
access to SONET frames is more attractive for
IP. IP over SONET/SDH interface, described in
[1], consists of IP/PPP/HDLC over SONET. That
is, IP datagrams are encapsulated into Point-to-
Point Protocol (PPP) packets. PPP is a protocol
that provides link error control and initialization.
The PPP-encapsulated datagrams are then
framed, using high-level data link control
(HDLC), and finally mapped into the SONET
SPE. The H DLC framed datagrams are then
scrambled, and placed back to back into the SPE,
much the same way as ATM cells are arranged.
SONET CLIENTS
SONET clients organize their data into SONET
signals in various ways. The definition of how the
data bytes are arranged within the SONET signal
is important because equipment using different
methods will fail to communicate. For constant
bit rate client signals, the virtual tributary (VT) is
used to transport payloads of sub-STS-1 rates.
VT1.5 (1.728 Mb/s), VT2 (2.304 Mb/s), VT3
(
3.456 Mb/s), and VT6 (6.912 Mb/s) are defined.
These VTs are arranged in 3, 4, 6, and 12
columns of the SONET frame, respectively. VT
groups are used to carry VTs of various speeds,
as long as they all fit into the SONET frame. For
instance, a VT group may carry four VT1.5s,
three VT2s, two VT3s, and one VT6. Pointers in
the SONET frame allow for easy demultiplexing
of these sub-STS-1-rate signals.
SONET NETWORKS
SONET networks are typically organized as mul-
tiple interconnected rings. The reason for favor-
Although VTs and STS signals can conve-
IEEE Communications Magazine • June 2000
165

mechanisms currently in use: 1+ 1 and1:1. In
+ 1 protection, a SONET signal is transmitted
1
LTE
ADM
ADM
through two nonintersecting fiber paths from a
source to a destination. The destination decides
which signal to receive based on failure indica-
tions provided by the multiplex section sublayer.
In 1:1 protection, two nonintersecting paths are
also used, with the difference that the SONET
signal is transmitted only on one path, called the
working section (also fiber, or path), while the
other path is called the protection section. The
protection path, idle during normal operation,
may be used to transport unprotected traffic,
normally called extra traffic. The extra traffic is
to be stopped when the protection fiber is need-
ed by normal traffic. The process of stopping the
extra traffic is called traffic squelching.
ADM
Access
ring
OC3
ADM
ADM
ADM
DCS
Backbone
ring
BLSR
OC48
DCS
ADM
ADM
LTE
There are two types of SONET protection
rings: multiplex section (MS) dedicated protec-
tion rings, and MS shared protection rings. An
MS dedicated protection ring is constructed by
two counter-rotating rings, transmitting in oppo-
site directions. Only one direction carries normal
traffic, while the other is reserved for protection
of normal traffic. The dedicated protection ring
APS protocol has not been standardized at the
time of this writing. Shared protection rings, on
the other hand, are realized in two ways: two-
and four-fiber shared protection rings. Some of
these rings are best known as UPSRs (two fiber)
and BLSR /Xs (X = 2, two; and X = 4, four
fibers). The reader is referred to [2] for a more
comprehensive description of APS protocols in
their various forms.
ADM
ADM
Access
ring
OC12
ADM
ADM
ADM
ꢀ
Figure 1. A SONET network.
ing ring topology is mainly the APS feature pro-
vided by such rings, to be discussed shortly.
Obviously, point-to-point links also exist, termi-
nating SONET signals at line terminating equip-
ment (LTE). An example of a SONET network
is depicted in Fig. 1.
The figure shows several SONET rings, com-
posed mainly of add/drop multiplexers (ADMs).
ADMs are SONET devices which perform low-
rate signal grooming into the high-speed SONET
signals used in the rings. The rings can be of var-
ious speeds, such as OC-3 and OC-12. Depend-
ing on the APS provided, they can be
unidirectional path-switched rings (UPSRs) and
bidirectional line-switched rings (BLSRs), which
in turn can make use of a two (BLSR/2) or four
WDM TECHNOLOGY
WDM is a technology that enables various opti-
cal signals to be transmitted by a single fiber. Its
principle is essentially the same as frequency-
division multiplexing (FDM). That is, several sig-
nals are transmitted using different carriers,
occupying nonoverlapping parts of a frequency
spectrum. In the case of WDM, the spectrum
band used is in the region of 1300 or 1500 nm,
which are two wavelength windows at which
optical fibers have very low signal loss.
Initially, each window was used to transmit a
single digital signal. With the advance of optical
components, such as distributed feedback (DFB)
lasers, erbium-doped fiber amplifiers (EDFAs),
and photodetectors, it was soon realized that
each transmitting window could in fact be used
by several optical signals, each occupying a small
fraction of the total wavelength window avail-
able. In fact, the number of optical signals multi-
plexed within a window is limited only by the
precision of these components. With current
technology, over 100 optical channels can be
multiplexed into a single fiber. The technology
was then named dense WDM (DWDM).
(
(
BLSR/4) fiber link span. Digital cross-connect
DCS) devices are used to connect rings togeth-
er. A D CS cross-connects low-speed signals
across rings, providing multiplexing/demultiplex-
ing and switching functions.
THE AUTOMATIC PROTECTION SWITCH
Due to the large bandwidth capacity of fiber
optic medium, operations, administration, main-
tenance, and provisioning (O AM&P) is an
important factor. Network administrators must
be able to configure paths, monitor their utiliza-
tion and performance, and take adequate action
when necessary. The technology, on the other
hand, must provide the means for executing
these functions. Thus, some of the SONET/SDH
overhead bytes are dedicated to the support of
these functions. APS the protocol used for pro-
tection of SONET networks against fiber and
node failures, is thus one of the most valued fea-
tures of SONET/SDH equipment. APS provides
protection for fiber cuts by automatically redi-
recting traffic affected by the failure to alterna-
tive routes. There are two types of protection
DWDM’s main advantage is its potential to
cost effectively increase the optical fiber band-
width many folds. The large network of fibers in
existence around the world can suddenly have
their capacity multiplied manifold, without the
need to lay new fibers, an expensive process.
Obviously, new DWDM equipment must be con-
nected to these fibers. Also, optical regenerators
might be needed.
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IEEE Communications Magazine • June 2000

The number and frequency of wavelengths to
be used is being standardized by the ITU-T. The
wavelength set used is important not only for
interoperability, but also to avoid destructive
interference between optical signals.
Tx
Tx
Rx
Rx
WDM COMPONENTS
Power
amplifier
Line
Pre
amplifier amplifier
WDM components are based on various optics
principles, outside the scope of this article. Fig-
ure 2 depicts a single WDM link. DFB lasers are
used as transmitters, one for each wavelength .
An optical multiplexer combines these signals
into the transmission fiber. Optical amplifiers
are used to pump the optical signal power up to
compensate for system losses. On the receiver
side, optical demultiplexers separate each wave-
length, to be delivered to optical receivers at the
end of the optical link.
Optical
mux
Optical
demux
ꢀ Figure 2. A WDM link.
The speed of the lightpath is determined by the
technology of the optical components (lasers,
optical amplifiers, etc.). Speeds on the order of
O C-48 (2488.32 Mb/s) and O C-192 (9953.28
Mb/s) are currently achievable.
Optical signals are added to the system by
optical ADMs (OADMs). These optical devices
are equivalent to digital ADMs, grooming and
splitting optical signals along the transmission
path. O AD Ms are usually made of arrayed-
waveguide gratings(AWG), although other opti-
cal technologies, such as fiber Bragg gratings,
have also been used.
A key WD M component is the optical
switch. This device is capable of switching opti-
cal signals from a given input port to a given
output port. It is the equivalent of an electronic
crossbar. Optical switches enable optical net-
works to be constructed, so a given optical sig-
nal can be routed toward its appropriate
destination. Another important optical compo-
nent is the wavelength converter. A wavelength
converter is a device that converts an optical
signal coming at a given wavelength into anoth-
er signal on a different wavelength, maintaining
the same digital content. This capability is
important for WDM networks, because it pro-
vides more flexibility in routing optical signals
across the network.
An OTN is composed of WXC nodes, plus a
management system which controls the setup
and teardown of lightpaths through supervisory
functions, such as monitoring of optical devices
(amplifier, receivers), fault recovery, and so on.
The setup and teardown of lightpaths are to be
executed over a large timescale, such as hours or
even days, given that each of them provides
backbone bandwidth capacity.
There is a lot of flexibility in how OTNs are
deployed, depending on the transport services
to be provided. O ne of the reasons for this
flexibility is that most optical components are
transparent to signal encoding. O nly at the
boundary of the optical layer, where the opti-
cal signal needs to be converted back to the
electronic domain, does the encoding matter.
Thus, transparent optical services to support
various legacy electronic network technologies,
such as SO NE T, ATM, IP, and frame relay,
running on top of the optical layer, is a likely
scenario in the future [3].
The optical layer is further divided into three
sublayers: the optical channel layer network,
which interfaces with OTN clients, providing opti-
cal channels (OChs); the optical multiplex layer
network, which multiplexes various channels into
a single optical signal; and the optical transmis-
OPTICAL TRANSPORT NETWORKS
WDM networks are constructed by connecting
wavelength crossconnect (WXC) nodes in a cer-
tain topology of choice. WXCs are realized by
wavelength multiplexers and demultiplexers,
switches, and wavelength converters. Figure 3
depicts a generic WXC node architecture. Opti-
cal signals, multiplexed in the same fiber, arrive
at an optical demultiplexer. The signal is decom-
posed into its several wavelength carriers, and
sent to a bank of optical switches. The optical
switches route the several wavelength signals
into a bank of output multiplexers, where the
signals are multiplexed and injected into the out-
going fibers for transmission. Wavelength con-
verters may be used between the optical switch
and the output multiplexers in order to provide
more routing flexibility. WXCs have been
researched for a number of years, although these
devices have not yet matured to the point of
becoming commercially available at the time of
this writing. Among the many difficulties are
crosstalk and extinction ratio.
λ1
OS
λ1 λ2…λW
λ1 λ2…λW
λ2
WC
WC
λ1
λW
λ1 λ2…λW
λ1 λ2…λW
λ2
λW
λ1 λ2…λW
λ1 λ2…λW
WC
Optical transport networks (OTNs) are WDM
networks providing transport services via light-
paths. A lightpath is a high-bandwidth pipe car-
rying data at up to several gigabits per second.
Optical
demux
Optical
mux
Tx
Rx
ꢀ Figure 3. A wavelength cross-connect node.
IEEE Communications Magazine • June 2000
167

length, and reduces the number of repeaters. A
Reed-Solomon code is expected to be used.
Several OChs are to be multiplexed together
in the optical domain, to form the optical multi-
plex signal (OMS). This parallels to the multi-
plexing of several STS-1 frames into an STS-Nc
SONET frame format. ITU-T Study Group 15 is
working on a draft [4] which addresses, among
other issues, the format of the optical frame for
both network–network (NNI) and user–network
interfaces (UNI). For NNI, OCh rates will be
defined so that multiple OChs can easily be mul-
tiplexed to form the OMS.
The optical client signal is placed within the
OCh payload signal. Notice that the client signal
is not constrained by the O ch frame format.
Instead, the client signal is required to be only a
constant bit rate digital signal. Its format is also
irrelevant to the optical layer.
OCh
OH
Payload signal
FEC
ꢀ
Figure 4. An optical channel frame.
sion section layer network, which provides the
transmission of the optical signal across the fiber.
OTN FRAME FORMAT
Similar to the use of a SONET frame, access to
the OCh is expected to be through an OC frame,
which is currently being defined [4]. The basic
frame size corresponds to OC-48 speed, or 2488.32
Mb/s, which constitutes the basic OCh signal. Fig-
ure 4 depicts a possible OCh frame format.
The leftmost region of the frame is reserved
for overhead bytes. These bytes are to be used
for OAM&P functions, similar to the overhead
bytes of the SONET frame, discussed earlier.
However, additional functions are likely to be
supported, such as the provision of dark fibers
WDM RINGS
Conceptually, a WDM ring is not much different
from a SONET ring. WXCs are interconnected
in a ring topology, similar to SONET ADMs in a
SONET ring. The major architectural difference
between a SONET ring and a WDM ring is root-
ed in the WXC capabilities of wavelength switch-
ing and conversion. These features can be used,
for instance, to provide levels of protection with
no parallel in SO NE T technology. In other
words, wavelength or lightpath protection can be
provided, in addition to path and line protection.
Various laboratories around the world have
been experimenting with WDM rings for a num-
ber of years, as documented in recent research lit-
erature. Reference [5] identifies at least eight
types of WDM rings that are realizable. It is diffi-
cult to predict, however, which types of WDM
rings will be widely deployed in the future.
Regardless of the types chosen, optical APS pro-
tocols at least as complex as SONET APSs are
necessary. Protection can be provided either at
the O Ch level or the optical multiplex
section/optical transmission section levels. Some
extra protection capabilities can be implemented,
with no parallel in SONET rings. For instance, a
failed lightpath (e.g., a laser failure) can be fixed
by converting an optical signal from a given wave-
length into a different one, avoiding the rerouting
of the signal. This is equivalent to span switching
in SONET, with the difference that even two-
fiber WDM rings can provide such capability for
OCh protection. In the OMS layer, however, span
protection will require four fiber rings, as in
SONET. These extra features will undoubtedly
introduce extra complexity in the optical-layer
APS protocols. R eference [2] studied several
types of failure-resilient WDM rings. One of the
key research opportunities in optical transport
systems is the design of APSs for WDM rings.
Once the WDM ring is up, lightpaths need to
be established in accordance with the traffic pat-
tern to be supported. This amounts to solving the
problem of routing and wavelength assignment.
Essentially, the lightpath routes need to be deter-
mined, in conjunction with the appropriate assign-
ment of wavelengths in each span of the ring.
Signaling protocols are yet to be defined for the
management of lightpaths, and should be part of
an optical network management platform.
(
reservation of a wavelength between two end-
points for a single user) and wavelength-based
APS (to be discussed shortly). The rightmost
region of the frame is reserved for a forward
error correction (FEC) scheme to be exercised
on all payload data. An FEC over an optical
transmission layer increases the maximum span
WXC
WXC
WXC
WXC
Backbone
network
WXC
WXC
WDM backbone
WXC
WXC
WDM
Regional
network
WXC
WXC
ADM
Ring
WXC
WXC
D/O
Tx
Rx
CS
Rx
Tx
ADM
SONET
Ring
PON
Access
network
ADM
Tx
Rx
ADM
Rx
Tx
Tx Rx
ꢀ
Figure 5. WDM network infrastructure.
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IEEE Communications Magazine • June 2000

MESH WDM NETWORKS
ATM
IP
Mesh WDM networks are constructed with the
same optical components as WDM rings. However,
the protocols used in mesh networks are different
from those used in rings. For instance, protection in
mesh networks is a more complex proposition [6],
as is the problem of routing and wavelength assign-
ment in WDM mesh networks [7].
IP/ATM layer
ATM
IP
IP
DXC
DXC
DXC
Mesh networks are likely to be used as back-
bone infrastructures connecting WDM rings.
Some of these connections are expected to be
optical, avoiding optical/electronic bottlenecks
and providing transparency. Others will require
the conversion of the optical signal into the elec-
tronic domain for monitoring, management, and
perhaps billing purposes. Figure 5 depicts a
WDM network infrastructure.
In the figure we show three topology layers:
the access network, the regional network, and
the backbone network. We have included both
SO NE T rings and passive optical networks
SDH electronic layer
WDM optical layer
DXC
WXC
WXC
WXC
WXC
ꢀ
Figure 6. Overlaying a WDM transport network carrying ATM/IP traffic.
(
PONs) as access networks. PONs are networks
constructed with passive optical components.
They are generally based on a bus or star topol-
ogy, and a medium access control (MAC) proto-
col is used to coordinate transmissions among
users. No routing functionality is provided in
such networks. These architectures are practical
for networks supporting at most a few hundred
users over short distances. Although PONs are
less expensive networks than WDM rings, due to
the lack of active components and features such
as wavelength routing, the lasers necessary at the
PON sources make the first generation of such
equipment still more expensive than SONET
rings. This favors the SONET solution at the
access network level, at least in the near future.
If anything, PONs may replace SONET technol-
ogy at some time in the future as an affordable
access network technology. Backbone networks,
on the other hand, contain active optical compo-
nents, hence providing functions such as wave-
length conversion and routing.
they are unlikely to embrace any new technology
that does not interoperate with SONET/SDH.
Many fora are actively working on interoperabil-
ity issues regarding these two technologies.
Among them are the Optical Internetworking
Forum (http://www.oiforum.com), the ITU-T
Study Group 15 (transport networks, systems,
and equipment: http://www.itu.int/ITU -
T/com15/index.html), and the Sonet Interoper-
ability Forum (www.atis.org/atis/sif/sifhom.htm).
In this section we discuss the requirements and
issues for a gradual migration from SONET to
WDM networks.
SONET FRAME ENCAPSULATION
The O Ch frame must be defined so that
SONET/SDH frame encapsulation can be easily
done. The entire STS-48c signal, for instance,
has to be carried as an OCh payload. If a basic
OC-48 optical channel is used, it might not be
possible to encapsulate SONET STS-48c into
OC-48, due to the OCh overhead bytes. The
OCh frame format is currently being defined.
Figure 7 exemplifies SONET frame encapsula-
tion into an OCh frame.
OPTICAL NETWORKS TESTBEDS
Several initiatives to build WDM backbones are
under development. In the United States, we
have the MONET project [8], which consists of
OADMs or rings interconnected to a long-dis-
tance backbone network of optical cross-con-
nects (O XCs). In E urope, a pan-E uropean
optical network is being deployed under the
Advanced Communications Technologies and
Services (ACTS) program [9]. These and other
backbone networks will have to somehow inter-
face with legacy transport technologies, such as
IP, ATM, public switched telephone network
SONET INTERFACES TO WDM
WDM equipment with physical SONET inter-
faces will deliver optical signals to SO NE T
devices. These interfaces must be in accord with
[10] for backward compatibility with SONET
technology. Therefore, the SONET device need
not be aware of the WDM technology used to
transport its signal (e.g., the device can belong to
a BLSR/4 ring). In this case, the WXC will drop
and add into the optical medium the wavelength
originally used in the SONET ring. This way,
WDM and SONET layers are completely decou-
pled, which is necessary for WDM interoperabili-
ty with SONET legacy equipment. Notice also
that this puts extra constraints on the selection of
wavelengths in the optical layer, since the last-hop
wavelength, the one interfacing with the SONET
device, must be the same one used by the SONET
device to terminate the optical path, if wavelength
conversion is not provided within the SONET
device.
(
PSTN), and SONET. The overall scenario is
depicted in Fig. 6. Notice the several types of
interface involved in the figure.
SONET/SDH AND
WDM INTEGRATION
Current network operators are just now finishing
the migration from pleisiochronous digital hier-
archy (PDH) to SONET/SDH. It took roughly
1
0 years for this migration to take place. Thus,
IEEE Communications Magazine • June 2000
169

can be used at higher layers to avoid early reac-
tion to faults at lower layers.
OH SONET payload
A failure recovery at the OMS sublayer can
replace recovery procedures of several instances
of the SONET signals being served by the optical
layer. Thus, a potentially large number of
SONET clients are spared from starting failure
recovery procedures at their layers. Therefore, a
single failure recovery at the optical OMS sublay-
er can spare hundreds if not thousands of routing
table updates at the IP layer, for instance.
OCh
OH
Payload signal
FEC
WAVELENGTH PACKING IN
SONET AND WDM INTERNETWORKS
ꢀ
Figure 7. SONET frame encapsulation.
The performance and cost of a SONET/WDM
integrated network depend on several issues, such
as the number of O ADMs and topology
THE MULTIPLE-LAYER
AUTOMATIC PROTECTION SWITCH
(
ring/mesh). Here we briefly discuss the wave-
length packing problem, which has a direct effect
on the number of SONET devices to be used.
Consider an optical network offering lightpaths
between optical/electronic termination points,
where OADMs and SONET devices are placed
for traffic grooming and delivery. Let’s assume
basic OC-48(2488.32 Mb/s). Given a traffic matrix,
which defines the amount of traffic to be carried
between ingress/egress optical termination points,
the question is: at the ingress points, how should
we populate the various wavelengths with the
originating traffic in order to minimize network
cost while attending to the transport needs repre-
sented by the traffic matrix? In this case network
cost can be assumed to be dominated by the num-
ber of OADMs and SONET devices. Figure 8
exemplifies the optimization problem.
In the figure, two STS-12 (622 Mb/s) light-
paths are required between optical nodes A and
C. If they are packed into the same wavelength,
two ADMs are saved. Wavelength packing and
routing are current research topics. For instance,
[12] studies the wavelength packing problem in
WDM rings. Wavelength packing in WDM mesh
networks, along with the lightpath routing prob-
lem under failures (restoration), is studied in [13].
SO NE T rings have their own APS protocol,
described earlier. OChs will also have protection
capabilities, since some OCh clients may not
possess this capability. Therefore, protection in
the O Ch layer must be such that it does not
adversely interfere with the SONET APS. The
optical APS then must react in a timescale faster
than SONET can, or quicker than 50 ms, so that
recovery is attempted first at the lower OTN
layer. In fact, in a multilayer environment, multi-
ple timescale recovery mechanisms must operate
in an ascending response timescale. Table 1 lists
various recovery mechanisms, with their respec-
tive recovery time [11], in a multilayer network.
Notice that although restoration is faster in
WDM than in SDH technology, failure detection
in WDM is slower. Safer overlay of WDM/SDH
protection mechanisms calls for a faster WDM
protection scheme. Alternatively, SONET APSs
could be artificially slowed down if SO NE T
clients can afford the performance degradation
incurred by such procedures. Unnecessary fail-
ure recovery at higher layers may cause route
instability and traffic congestion; hence, it should
be avoided at all costs. Fault persistence checks
NETWORK MANAGEMENT
Technology
WDM-OMS/Och
SDH
Detection Restoration Details
OAM&P procedures are key to
the success of any high-capacity
transport network. In SONET net-
works these procedures are pro-
prietary; it is left for each vendor
to decide how to implement its
network management system
1–10 ms
0.1 ms
0.1 ms
0.1 ms
0.1 ms
0.1 ms
40 s
10–30 ms
50 ms
Ring/P-P
Ring
WDM
SONET
APS 1+ 1
50 ms
P-P
FDDI
10 ms
Ring
(
NMS). As a result, SO NE T
devices of distinct vendors do not
interoperate. Moreover, these
proprietary solutions normally
work in isolation from one ring to
another, even within the same
vendor. This has proven to be a
serious drawback in the operation
and maintenance of SONET rings.
An OAM&P platform is to be
constructed by defining manage-
ment information blocks (MIBs)
for the various optical sublayers.
The MIBs must include several
types of information, such as phys-
ical characteristics (fiber type,
STM
100 ms
10 ms x N
1–10 s
ATM
ATM PV-C/P 1+ 1
ATM PNNI SPV-C/P, SV-C/P
Border Gateway Protocol
Standby N = # hops
180 s
10–100 s
1–10 s
Interior Gateway Routing Protocol 40 s
and E-OSPF
IP
Intermediate System-Intermediate
System
40 s
1–10 s
Routing Internet Protocol
180 s
100 s
ꢀ
Table 1. Time responses of various APS mechanisms.
1
70
IEEE Communications Magazine • June 2000

maximal rates, wavelength conversion capabili-
ties), protection control, and power levels. They
can belong to separate optical sublayers, or
include information about various sublayers.
Some of the data can be replicated in several
MIBs belonging to different sublayers. The exact
definition of the MIBs to be used depends heav-
ily on the implementation of the network man-
agement platform.
λ1 λ2
λ1 λ2
WXC
A
WXC
B
WXC
C
λ1
λ2
λ1
λ2
There is an ongoing effort to integrate and
manage existing transport networks in a vendor-
independent manner [14]. Major telecommuni-
cation industries have joined forces to produce a
Common Object Request Broker Architecture
SONET
ADM
SONET
ADM
SONET
ADM
SONET
ADM
STS-12
LTE
STS-12
LTE
STS-12
STS-12
LTE
(
CORBA) NMS. The resulting CORBA NMS is
LTE
to provide an integrated management architec-
ture for various network types, including
SONET, WDM, and ATM networks. In the mul-
tilayer multitechnology network environment of
today, such a network management platform is
not only necessary but mandatory.
λ1
λ1
WXC
A
WXC
B
WXC
C
EVOLUTION TOWARD AN
ALL-OPTICAL TRANSPORT NETWORK
λ1
λ1
Evolution toward an all-optical WDM network is
likely to occur gradually. First, WXC devices will
be connected to existing fibers (e.g., transatlantic
ones). Some extra components might be neces-
sary in the optical link, such as EDFAs, in order
to make legacy fiber links suitable to WD M
technology. WXCs will interface with legacy
equipment, such as SONET and fiber distributed
data interface (FDDI). At this stage, the archi-
tecture and logical topology of the WDM and
SONET layers, discussed in the previous section,
are important issues.
Optical subnetworks (possibly rings) will be
deployed, interconnected by SDH or ATM equip-
ment. Thus, the optical signal will be brought
back into the electronic domain at each intercon-
nection point. This approach allows tight moni-
toring of failures, such as fiber cuts and
laser/detector failures, making feasible the imple-
mentation of sophisticated protection mechanisms
in the optical layer. New provisioning mechanisms
and network management tools need to be devel-
oped for this scenario. Several startup companies
are currently pursuing this avenue, trying to tap
into the DWDM market as early as possible. The
downside of this scenario is that a lightpath across
WDM subnetworks will not be transparent, in the
sense that only SONET encoded signals can be
transmitted appropriately.
As SONET devices depreciate, new optical
access solutions will appear, squeezing the
SONET layer off the protocol stack, together
with optical–electronic conversion at intermedi-
ate points of a lightpath. This will clear up the
lightpaths from electronic–optical conversion,
making the optical layer truly transparent. The
down side of this scenario is that failure detec-
tion becomes a challenging task in a transparent
optical layer.
A plus of an all-optical transparent transport
network is that the transferring of SONET func-
tions into either the layer above(IP/ATM) or
below (WD M) SO NE T is likely to happen,
bringing savings in terms of network upgradabili-
ty and maintenance. In fact, discussions about
SONET
ADM
SONET
ADM
STS-12
STS-12
LTE
STS-12
STS-12
LTE
LTE
LTE
ꢀ
Figure 8. The wavelength packing problem.
new optical access layers, such as packet over
OTN, have already started, driven by the rapid
advances in WDM technology. To give the read-
er an idea of the extent to which such layer reor-
ganization could affect transport networks,
assume that real-time traffic, including voice, is
packetized (IP/ATM). This could lead to the
extinction of VTs’ SONET signals. A key issue
then would be how to most efficiently pack pack-
ets into SO NE T, or even directly into O Ch
frames. Whatever new encapsulation method
emerges, backward compatibility with IP/PPP/
HDLC and ATM encapsulation is a must.
CONCLUSION
After ten years of SONET/SDH deployment, a
vast investment has been made in SONET equip-
ment all over the world. Only in North America
did the figure total $4.5 billion in 1998. A slow-
down has been detected due to migration of
SONET functionalities into ATM and IP equip-
ment, as well as the market growth of DWDM
equipment. Figures seem to project a double in
DWDM investment in the 1998–2002 period,
from $1.86 to $3 billion by 2002 [15]. The emerg-
ing WDM technology provides an alternative for
fiber access and transport services for the first
time. The WDM promises of unlimited band-
width and fast protection capabilities are not
enough to entice network operators to retire their
SONET/SDH investment in favor of this new and
still unproven technology. The operators are
apparently satisfied with SONET technology on
IEEE Communications Magazine • June 2000
171

[
[
7] A. Mokhtar and M. Aziziglu, “Adaptive Wavelength
Routing in All-Optical Networks,” IEEE/ACM Trans. Net.,
vol. 6, no. 2,Apr. 1998, pp. 197–206.
8] R. E. Wagner et al., “MONET: Multiwavelength Optical
Ne t w o rkin g ,” IEEE JSAC, vo l. 1 4 , Ju n e 1 9 9 6 , p p .
both fronts: provision of 50 ms protection capa-
bilities, and speeds up to OC-192 (9.8 Gb/s). The
deployment of WDM devices thus must be eco-
nomically well justified, as well as well planned.
In this article we have discussed various issues
involved in deploying WDM technology in a
SONET-dominated network transport environ-
ment.
It is clear that gradual evolution of today’s
SONET technology into WDM systems is manda-
tory. The question is how long this evolution can
last before it gets hit by the next revolutionizing
transport technology, whatever that may be.
It is clear that
a gradual
1
349–55.
evolution of
[9] M. Berger et al., “Pan-European Optical Networking
using Wavelength Division Multiplexing,” IEEE Com-
mun. Mag., Apr. 1997, pp. 82–88.
[10] ITU-T Rec. G.957, “Optical Interfaces for Equipments
and Systems Relating to the Synchronous Digital Hierar-
chy,” July 1995.
[11] R. Batchellor, “Coordinating Protection in Multiple Lay-
ers,” Op tical Internetworking Forum , cont. OIF-99-
038.0, Apr. 1999.
today’s SONET
technology into
WDM systems
is mandatory.
The question is
how long can
this evolution last
before it gets hit
by the next
[12] O. Gerstel, P. Lin, and G. Sasaki, “Combined WDM and
SONET Network Design,” Proc. INFOCOM ’99, vol. 2,
1
999, pp. 734–43.
ACKNOWLEDGMENTS
The author would like to thank the several
reviewers for their helpful suggestions.
[13] M. Alanyali and E. Ayanoglu, “Provision Algorithms for
WDM Optical Networks,” Proc. INFOCOM ’98, vol. 2,
1998, pp. 910–18.
[14] R. Pease, “Companies Demonstrate Multivendor, Multi-
technology Network Management,” Lightwave, Nov. 1999.
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[
[
1] W. Sim pson, “PPP over SONET/SDH,” IETF RFC 1619,
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BIOGRAPHY
[
[
DIRCEU CAVENDISH (dirceu@ccrl.nj.nec.com) received his B.E.
in electronic engineering in 1986 from Federal University of
Pernambuco, Brazil, an M.S. in computer science in 1994
from Kyushu Institute of Technology, Japan, and a Ph.D. in
computer science in 1998 from University of California, Los
Angeles. He currently holds a research staff member posi-
tion at NEC USA, where he conducts research on QoS sup-
port in IP networks, as well as photonic switches and WDM
networks. His research interests include network manage-
ment and provisioning, fault tolerance, routing protocols,
as well as packet schedulers for high-speed switches.
technology,
whatever that
may be.
[5] S. Johansson et al., “A Cost-Effective Approach to Introduce
an Optical WDM Network in the Metropolitan Environ-
ment,” IEEE JSAC, vol. 16, no. 7, Sept. 1998, pp. 1109–22.
6] S. Ramamurthy and B. Mukherjee, “Survivable WDM
Mesh Networks, Part I - Protection,” Proc. INFOCOM
[
’99, vol. 2, 1999, pp. 744–51.
1
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IEEE Communications Magazine • June 2000