Bell Labs OTN
Transcript of Bell Labs OTN
◆ Realizing the Optical Transport NetworkingVision in the 100 Gb/s EraSilvano Frigerio, Alberto Lometti, Juergen Rahn, Stephen Trowbridge, and Eve L. Varma
After a half-decade hiatus, stimulated by dramatic service-driven increases inbackbone network bandwidth requirements, industry focus has once againturned to realizing a vision of optical transport networking (OTN). In thetimeframe since the first OTN standards were stabilized, technology hascontinued to evolve, and additional new service requirements havematerialized. The ability to provide optimized support for gigabit Ethernetservices, ranging from 1Gb/s to 100 Gb/s, has become a high priority. Thispaper examines how evolving OTN standards provide a multi-service capablebackbone infrastructure supporting lambda and sub-lambda services withguaranteed quality, the role of optical control plane technology in realizingdynamically configurable OTN and Internet Protocol (IP) over opticaltransport networking solutions, and emerging technology enablers. Thepaper concludes by providing a vision of optical transport networkinfrastructure evolution in the 100 Gb/s era. © 2010 Alcatel-Lucent.
emerging ultra-high bit rate services (e.g., IEEE
100GBASE-R, 40GBASE-R).
Leveraging optical control plane advances, dynami-
cally configurable OTN and IP-over-OTN solutions
deliver on the promise of rapid provisioning, increased
automation, and richer sets of service functionality.
The control plane enabled OTN opens the door to new
services, similar to how signaling system 7 (SS7)
opened up the possibilities for advanced intelligent
networking (AIN) for the public switched telephony
network (PSTN). IP-over-OTN solutions not only pro-
vide a dynamically configurable optical layer respon-
sive to IP networking demands, but enable multi-layer
optimization with superior service resiliency.
This paper examines OTN standardization advances
and how OTN-based networking helps service providers
IntroductionBandwidth demand continues to grow world-
wide, fueled by new Internet Protocol (IP)-based ser-
vices and multimedia applications. The availability of
higher bandwidth service offerings, coupled with
applications needing higher speeds, has resulted in
dramatic increases in access rates in order to enable
faster consumer access to these services. This dramatic
increase in access rates has created a domino effect,
rippling through metro networks and ultimately driv-
ing dramatic increases in backbone network band-
width requirements. With higher volume, lower
revenue service mixes driving the need for increased
profitability, there has been increased service provider
attention towards converging multiple services onto a
future-proof next-generation optical transport net-
work (NG-OTN) infrastructure positioned to support
Bell Labs Technical Journal 14(4), 163–192 (2010) © 2010 Alcatel-Lucent. Published by Wiley Periodicals, Inc.Published online in Wiley InterScience (www.interscience.wiley.com) • DOI: 10.1002/bltj.20410
Panel 1. Abbreviations, Acronyms, and Terms
3R—Reshape, retime, retransmitAIN—Advanced intelligent networkingAIS—Alarm indication signalAMP—Asynchronous mapping procedureAPS—Automatic protection switchingASIC—Application-specific integrated circuitASON—Automatically switched optical
networkASSP—Application specific standard productATM—Asynchronous transfer modeBER—Bit error rateBIP—Bit interleaved parityBoD—Bandwidth on demandCAPEX—Capital expenditureCBR—Constant bit rateCDR—Clock and data recoveryCM—Connection monitoringCMOS—Complementary metal-oxide
semiconductorDC—Direct currentDPSK—Differential phase shift keyingDQPSK—Differential quadrature phase shift
keyingDWDM—Dense wavelength division
multiplexingE-NNI—External network-network interfaceFDI—Forward defect indicationFEC—Forward error correctionGbE—Gigabit EthernetGFP—Generic framing procedureGFP-F—GFP-framedGFP-T—GFP-transparentGMP—Generic mapping procedureGMPLS—Generalized multiprotocol label
switchingHO—Higher orderIEEE—Institute of Electrical and Electronics
EngineersIaDI—Intra-domain interfaceIP—Internet ProtocolIrDI—Inter-domain interfaceITU—International Telecommunication UnionITU-T—ITU Telecommunication Standardization
SectorLAN—Local area networkLCAS—Link capacity adjustment schemeLH—Long haulLO—Lower orderLOS—Loss of signalMII—Media independent interfaceMPLS—Multiprotocol label switchingNE—Network elementNG-OTN—Next-generation OTNNMS—Network management systemNOC—Network operations center
OADM—Optical add/drop multiplexerOAM—Operations, administration, and
maintenanceOCh—Optical channelODU—Optical channel data unitO/E—Optical/electricalOEO—Optical-electronic-opticalOMS—Optical multiplex sectionONE—Optical network elementOOK—On-off keyingOPEX—Operating expensesOPSMnk—Optical physical section multilane
(n � number of lanes)OPU—Optical channel payload unitOSC—Optical supervisory channelOSNR—Optical signal-to-noise ratioOSS—Operations support systemOTLC—Optical transport lane carrierOTLCG—Optical transport lane carrier groupOTM—Optical transport moduleOTN—Optical transport networkOTS—Optical transport sectionOTU—Optical channel transport unitP2P—Point-to-pointPCS—Physical coding sublayerPDH—Plesiochronous digital hierarchyPHY—Physical layerPMD—Polarization mode dispersionPSTN—Public switched telephony networkPXC—Photonic cross connectQoS—Quality of serviceQPSK—Quadrature phase shift keyingROADM—Reconfigurable optical add/drop
multiplexerSDH—Synchronous digital hierarchySE—Spectral efficiencySLA—Service level agreementSONET—Synchronous optical networkSRLG—Shared risk link groupSS7—Signaling system 7STM—Synchronous transfer modeTCM—Tandem connection monitoringTDM—Time division multiplexingTOADM—Tunable optical add/drop multiplexerULH—Ultra long haulUNI—User network interfaceVCAT—Virtual concatenationVCG—VCAT groupVLAN—Virtual local area networkVP—Virtual pathVT—Virtual tributaryWAN—Wide area networkWDM—Wavelength division multiplexingWSS—Wavelength selective switchWXC—Wavelength cross connect
DOI: 10.1002/bltj Bell Labs Technical Journal 165
evolve to a unified, optimized layer of high-capacity,
high-reliability bandwidth management, providing
solutions for delivering existing and emerging packet-
based services with guaranteed quality.
HistoryDuring the late 1990s, the telecommunications
industry became swept up in a desire to capitalize on
an unprecedented demand for network capacity, mostly
driven by rapidly growing packet-based services—in
particular, by Internet/Intranet-based applications.
The industry view was that transport networks would
need to become optimized for much larger capacity
channels carrying broadband data, voice, and video.
At the same time, driven by the vision of virtually
infinite information bandwidth and transport in the
optical domain, research thrusts accelerated in explor-
ing sophisticated photonic devices and techniques to
allow the transport and routing of signals in the opti-
cal domain [1].
While these early visions focused upon “optical
transparency,” it became clear that practical visions
for optical networking involved the use of opto-
electronics to support carrier grade transport capabili-
ties. The optical transport network was born out of this
recognition, leveraging industry synchronous digi-
tal hierarchy (SDH)/synchronous optical network
(SONET) experience and considering optical technol-
ogy factors, and was considered the next step beyond
SDH/SONET in supporting data-driven needs for
bandwidth and the emergence of new broadband
services [26]. The industry was swept up in a wave of
standardization initiatives to create a suite of OTN rec-
ommendations.
It was expected that optical transport networks
would quickly evolve from dense wavelength divi-
sion multiplexing (DWDM) remedies for capacity
exhaust, to DWDM optical networking solutions opti-
mized for support of fully transparent Gb/s services.
However, with the “bursting” of the Internet bubble,
bandwidth requirements were lower than predicted
with little demand surfacing for “wavelength leased
lines,” and client signals remained predominantly at
the sub-SDH and synchronous transfer mode (STM)-
16 rates. Exacerbating the situation, there was a sig-
nificant amount of installed excess capacity in
long-haul networks that had been laid in the expec-
tation of its imminent need. The market downturn in
succeeding years resulted in deferred deployment of
new photonic networking technologies. Thus, in the
timeframe during which the OTN standardization
effort came to fruition, the market stalled.
In the past few years, the anticipated bandwidth
demands have finally materialized, as exemplified by
intensive Institute of Electrical and Electronics
Engineers (IEEE) standards initiatives for specifica-
tion of 100 GbE/40 GbE. Concurrently, packet-based
services optimization demands have driven interest
in OTN capabilities well down into the sub-lambda
ranges. These forces have triggered OTN evolution
initiatives, expanding its scope from 1 Gb/s through
100 Gb/s, with further upward growth towards
400 Gb in the next decade.
OTN DriversDrivers for evolution from SDH/SONET to OTN
have evolved over the timeframe from its conception
to its rebirth. A case in point is the meaning and role
of OTN “transparency,” referring to the set of charac-
teristics of a client signal that are preserved when that
client is carried over the OTN. Examples of types and
levels of transparency include dark fiber, wavelength,
bit, symbol or codeword, Ethernet (media independ-
ent interface [MII], frame plus preamble, frame), and
timing. As noted previously, the early “optical trans-
parency” visions of transport of arbitrary client sig-
nals over wavelengths of a fiber-optic network were
found problematic given the various impairments
(e.g., chromatic and polarization mode dispersion,
attenuation) that occur when traversing various fiber
types and optical components. There can also be chal-
lenges in preserving the same set of client character-
istics when a client is transported on a dedicated
wavelength versus when that same client is digitally
multiplexed with other client signals onto a higher
bit rate wavelength.
Even the concept of “bit transparency” for digital
client signals is elusive considering that client signals
need to be recovered using clock and data recovery
(CDR) and framing circuitry, which requires a certain
frequency of transitions (clock content) and direct
current (DC) balance that is generally guaranteed by
a client-specific line coding or scrambler. Proper opera-
tion of the network, ability to isolate faults, and abil-
ity of a server network to generate a meaningful
replacement signal towards a client device in the case
of failures require a certain amount of client-specific
processing. Additionally, in a digital multiplexing hier-
archy, more efficient encoding techniques may need
to be employed to enable efficient transport of cer-
tain client signals over channels of the selected hier-
archical bit rates.
Thus, what was originally thought to be a simple
idea of client transparency has become a complex set
of trade-offs that involve identifying the set of char-
acteristics of a client signal that need to be preserved
for proper operation of a service and developing map-
pings that preserve those characteristics when the
client is transported over the OTN. It has also become
essential for the OTN to be able to offer efficient trans-
port of not only the new ultra-high bandwidth packet
services, but also the lower granularity services of
importance to network operators. Finally, the OTN
must continue to satisfy the challenge of reducing
operations complexity for next-generation networks
composed of existing and emergent opto-electronic
(OEO) network elements and wholly photonic optical
network elements (ONEs) [16].
Foundation OTN Problem DomainHybrid solutions involving SDH/SONET [15] inte-
gration with DWDM technology were increasingly
being deployed for tapping into the full capacity of fiber
plant to maximize the return on existing facilities.
166 Bell Labs Technical Journal DOI: 10.1002/bltj
In these applications, most of the transport networking
functionality was provided by the underlying SDH/
SONET systems that used the DWDM spans. As net-
work traffic grew and DWDM deployment continued,
utilization of this approach for networked DWDM
applications resulted in limitations in supporting multi-
carrier and multi-service networking requirements.
Specifically, supporting networked DWDM applications
using SDH/SONET layer functionality ran into several
barriers, described further in the following subsections.
Transport of SDH/SONET connection services. At the
time SDH/SONET was developed, it was assumed
that this technology would always serve as the lowest
networking layer. SDH/SONET offers path layer trans-
parency (for the payload), multiplex section trans-
parency, and finally regenerator section transparency
(as provided by the physical media layer). Only path
layer transparency was considered as supporting a
“user service.” Thus, there was no capability offered to
support a “carrier’s carrier” application in which the
“user service” was an SDH/SONET connection service.
That is, a “carrier’s carrier” could not transparently
carry both SDH/SONET payload and overhead, as
multiplex/regenerator section overhead would always
be terminated upon multiplexing or cross-connection.
An example of the problem this presents is illus-
trated in Figure 1, which depicts carriage of a service by
network operator A, that in turn makes use of network
facilities provided by an intervening network provided
by network operator B (serving as a carrier’s carrier).
In this example, network operator A desires to sup-
port end-to-end protection via usage of a SDH/SONET
Network operator B domain
SDH/SONET technology
Userdomain
Userdomain
Network operator A domainNetwork operator A
domain
SDH/SONET ring
SDH—Synchronous digital hierarchySONET—Synchronous optical network
Figure 1.Multi-operator network example.
DOI: 10.1002/bltj Bell Labs Technical Journal 167
ring. However, this is not possible as it would require
that SDH/SONET equipment within network operator
B’s network not terminate the multiplex section over-
head. The only solution for the carrier’s carrier in such
a case would be to deploy a passive all-optical solution,
but then they would not have the necessary opera-
tions, administration, and maintenance (OAM) capa-
bilities to maintain their own network.
Operations complexity challenges: multi-carrier scenarios. With movement towards networked
DWDM solutions involving deployment of flexible
ONEs, it became essential to overcome any associated
limitations/barriers that added to operations com-
plexity and increased network cost. At the same time,
there was an increasing need to provide enhanced
service level agreement (SLA) verification and fault
sectionalization capabilities in multi-carrier/multi-
domain networking scenarios.
Supporting SLA verification and fault localization
in multi-carrier scenarios brings additional challenges.
The transport OAM that enables fault isolation and
SLA validation in multi-domain environments is
known as tandem connection monitoring (TCM).
Existing SDH/SONET network OAM standards sup-
port one level of TCM. Specifically, again considering
the multi-operator scenario of Figure 1, SDH/SONET
TCM either allows network operator A to monitor the
end-to-end connection or allows each network opera-
tor to separately monitor its own network. As it is not
possible to concurrently perform both types of moni-
toring, SLA verification requires tight inter-
carrier cooperation. For example, if the decision is
made to provide end-to-end monitoring, manual
processes are required for fault localization. If the deci-
sion is made to provide per-operator monitoring, an
end-to-end carrier serving as the prime contractor of
the service cannot determine overall signal quality. This
carrier would therefore need to rely upon customer
complaints or work to assure tight coupling of man-
agement systems across carrier boundaries. Such lack of
direct end-to-end monitoring for service assurance
ended up being a barrier to lowering operations cost.
Operational challenges: photonic networking faultsectionalization. Further challenges arise with appli-
cation scenarios involving networked DWDM systems
and flexible ONEs. Consider a client time division
multiplexing (TDM) service transported over an opti-
cal network composed of flexible ONEs such as recon-
figurable optical add-drop multiplexers (ROADMs)
and photonic cross connects (PXCs), which are inter-
connected by SDH/SONET-based DWDM line sys-
tems. In order to support client service-transparent
transport, the overhead of the client signal could not
be terminated, requiring usage of non-intrusive moni-
toring to check its health. If impairments occurred on
one of the DWDM line systems, causing client signal
impairments (bit errors), a threshold crossing alert
would be detected not only at the first downstream
SDH/SONET section BIP (bit interleaved parity) moni-
tor point, but also at all downstream monitor points.
Similarly, if a misconnection occurred within a pho-
tonic cross connect, a trace mismatch defect would
be detected not only at the first downstream SDH/
SONET section trace monitor point, but also at all
downstream monitor points. In both cases, manage-
ment system intervention would again be required
for fault localization.
Inability to do autonomous fault sectionalization
also adversely impacts shared protection/restoration
capabilities, even if fault isolation to a specific span is
not required to initiate survivability actions. In particu-
lar, it is necessary to know whether the fault occurred
within the protected domain or outside the protected
domain, since faults occurring outside the domain can-
not be protected against. If switching/ restoration
activity is initiated by faults occurring outside of their
protection domain, resultant unnecessary switches will
increase, versus decrease, downtime (since upon clear-
ing of the fault, an additional switch back to the nor-
mal path must be made). Unnecessary switching also
wastes spare capacity that could otherwise have been
used to restore traffic disrupted by a fault within the
protected domain (with the potential consequence
being the inability to restore this traffic).
Operational challenges: photonic networking alarmstorms. SDH/SONET networks control faults by pro-
viding a specific alarm indication signal (AIS) indicat-
ing that the fault has been detected, and that
downstream elements need not raise an alarm. As dis-
cussed earlier, since it was originally assumed that
SDH/SONET would always serve as the lowest net-
work layer, no provision was originally made for an
alarm indication signal between SDH/SONET regen-
erators. To address this omission, a generic AIS was
defined in standards for use by SDH/SONET-based
DWDM systems to prevent downstream SDH/SONET
network elements from alarming because of a DWDM
line system failure. However, generic AIS can only be
inserted at points supporting opto-electronic regen-
eration (i.e., OEO points).
Consider the implications of a major failure such
as a cable cut in a network composed of conventional
SDH/SONET-based DWDM systems and ONEs, which
do not support OEO capabilities. If there is a DWDM
line system failure, there are no OEO points available
for insertion of the generic AIS to prevent down-
stream SDH/SONET network elements from alarm-
ing. In a transport network with several cables per
duct, dozens of fibers per cable, and hundreds of
wavelengths per fiber, a cable cut occurring within
such a photonic subnetwork could result in hundreds
of thousands of loss of signal (LOS) indications, which
would flood the management communications net-
work. Further, localizing the fault to the specific
DWDM line system would require the network man-
agement system to handle and correlate huge numbers
of LOS alarms. Aside from operations considerations,
lack of alarm suppression capabilities also inhibits cost
reduction of photonic subnetworks by preventing
removal of opto-electronic transponders at points
where they are not already needed for other reasons
(e.g., a signal which does not require regeneration,
or for demarcation).
OTN Evolution Problem DomainOTN was designed to support both TDM
(SDH/SONET, PDH) and packet services. However,
since the foundation OTN bit rates were established,
several new packet transport-related market forces
have emerged.
Foundation OTN was developed in the context of
considering STM-N [15] and emerging Ethernet inter-
faces, as well as maximum line rates of 10G transoceanic
line systems. It could efficiently transport the SDH STM-
64 compatible IEEE 10GBASE-W (10GbE WAN PHY)
[9] as a constant bit rate (CBR) service or use the generic
framing procedure (GFP) [18] to map packet streams
including Ethernet, asynchronous transfer mode (ATM),
168 Bell Labs Technical Journal DOI: 10.1002/bltj
and IP directly into OTN containers. However, some
IEEE 802.3 standards non-compliant applications
emerged carrying layer 2 client application data in
Ethernet 10GBASE-R (10 GbE LAN PHY) [9] frame
structure entities, such as the preamble and inter-packet
gap. As the standard OTN container sizes could not effi-
ciently carry the slightly higher rate 10GbE LAN PHY as
a CBR service, a proliferation of various “semi-standard”
mechanisms resulted (e.g., “over-clocking”) and were
ultimately documented in the informative International
Telecommunication Union Telecommunication Stan-
dardization Sector (ITU-T) G.Sup43 [19]. While these
mechanisms were used for point-to-point applications,
concern arose that this proliferation would migrate to
40G, via 4�10G local area network (LAN) PHY imple-
mentations. Lack of coherent integration, and a solu-
tion for “capping” the 10G LAN/WAN PHY perturbation,
has been a “thorn in the industry’s side” [8].
Demand for more optimized solutions for IEEE
1000BASE-X (1GbE) signals has emerged from opera-
tors moving to cap SDH/SONET deployments, as well
as those simply increasing investment in OTN infra-
structure deployments. For operators planning to
transition to an OTN infrastructure, it was considered
important to provide a finer granularity container
with the same OAM capabilities as those present in
the foundation OTN hierarchy. For service scenarios
where GbE is adapted and payload mapped to incum-
bent SDH/SONET transport systems, it was thought
logical to maintain a robust SDH/SONET payload mul-
tiplexing scheme overlaid on OTN through the core.
There are also scenarios in which there is no incum-
bent SDH/SONET deployed at the edge of the net-
work, and here the option of adopting an optimized
Ethernet client payload mapping directly onto OTN
is quite attractive.
At the other end of the bandwidth spectrum,
demand emerged that the OTN be capable of effi-
ciently supporting IEEE 802.3ba 100GBASE-R
(100 GbE) signals. By adding a new higher tier to the
OTN hierarchy, transport networks could continue to
support the highest bit rate enterprise services. In addi-
tion to transport of ultra-high rate Ethernet mappings,
this new tier allows mapping and multiplexing of
foundation OTN signals into higher bit rate lambdas
for improved spectral efficiency. The IEEE 802.3ba
DOI: 10.1002/bltj Bell Labs Technical Journal 169
definition of 40GBASE-R (40 GbE) signals provides a
strong driver for assuring compatibility with founda-
tion OTN to leverage the deployed infrastructure. The
mapping into OTN is independent of Ethernet polari-
zation mode dispersion (PMD) technology choices and
evolution, enabling end-to-end 40 GbE and 100 GbE
services [25]. As high-speed Ethernet development is
expected to produce cost effective single-client paral-
lel interfaces for up to 40 km reach, architectures
enabling usage of 40 GbE/100 GbE optical modules for
corresponding SDH/SONET and OTN client side inter-
faces facilitate common cost curves, becoming a major
driver for reducing capital expenditure (CAPEX).
Recent developments in synchronous Ethernet
[7], which require transparency of the timing of the
signal as well as the data content, have driven design
of new mappings for Ethernet as CBR services over
OTN, similar in concept to the timing transparent
mappings of SDH over OTN.
Finally, as packet technologies continue to
advance, it becomes increasingly valuable to have the
ability to create packet trunks of variable sizes for car-
rying packet flows (e.g., virtual local area networks
[VLANs]) through the OTN, enabling usage of lower
order (LO) optical channel data unit (ODU) layer 1
switching versus needing to route packets at every
node at higher cost per bit.
Foundation OTN DefinedFoundation OTN represents a transport network-
ing layer that has been considered the next step
beyond SDH/SONET in supporting data-driven needs
for bandwidth and the emergence of new broadband
services. It provides a multi-service capable core infra-
structure that leverages lessons learned from the
SDH/SONET experience and adds optical technology
to meet the challenges of the evolving telecommuni-
cations networking environment. It provides gigabit-
level bandwidth granularity required to scale and
manage multi-terabit networks, that:
• Maximizes nodal switching capacity, which is the
gating factor for reconfigurable network capacity.
• Avoids very large numbers of fine granularity
pipes that stress network planning, administration,
survivability, management systems, and control
protocols.
• Allows networks to support end-to-end monitor-
ing of client services while decoupling the switch-
ing granularity from the DWDM line system
capacity.
The OTN value proposition has primarily been
based on building upon the industry’s positive experi-
ence with SDH/SONET, providing 1) support for new
revenue generating services and 2) solutions for offer-
ing enhanced OAM capabilities, while addressing
inherent optical transmission challenges that did not
exist for SDH (e.g., DWDM system engineering rules
with/without flexible ONEs). Key features include:
• Ability to offer enhanced SLA verification capabilities
in support of multi-carrier, multi-service environment.
This was expected to offer additional revenue
generating opportunities by allowing operators to
lease capacity to other operators while still being
able to provide high-quality SLA verification.
• Provision of scalable maintenance solutions encompass-
ing introduction of flexible ONEs. This required sup-
port for client-independent fault and signal
degradation isolation, client independent monitor-
ing, and prevention of alarm storms in all-optical
sub networks that would reduce OPEX.
Foundation OTN Structure and FormatThe optical transport network architecture [10],
similar to SDH/SONET, encompasses three hierarchi-
cal transport layers:
• Optical Transport Section (OTS). An optical regenera-
tion section layer that is devoted to the manage-
ment of line optical amplifiers and related links.
The OTS represents a multi-wavelength signal
over a single optical span (e.g., between line
amplifiers).
• Optical Multiplex Section (OMS). An optical multi-
plex section layer devoted to the multiplexing of
“lambdas,” and thus to the management of mul-
tiplexers/demultiplexers. The OMS represents a
multi-wavelength signal over multiple optical
spans (e.g., between DWDM equipment).
• Optical Channel (OCh). An optical path layer
devoted to end-to-end management of “lambdas”
within the OTN. The OCh represents a single opti-
cal channel over multiple optical spans having
flexible connectivity.
ITU-T Recommendation G.872 also defines two
types of OTN interfaces, which are specified in Rec.
G.709 [20]: inter-domain (IrDI) and intra-domain
(IaDI), as illustrated in Figure 2. The IrDI interfaces, by
definition, employ reshape, retime, retransmit (3R)
processing at each end of the interface (which could
be between different operator domains, or between
different vendors in a given operator domain). This
assures digital processing capabilities may be leveraged
to validate the quality of “signal handoff” between
these domains. It should also be noted that G.709 inter-
faces are logical interfaces; i.e., there is no specification
of the corresponding electrical or optical interfaces that
would also be required for their implementation.
The logical structure of the OTN networking inter-
face, the optical transport module (OTM), is described
further below and illustrated in Figure 3 [26].
The OCh is composed of an optical channel pay-
load unit (OPU), ODU, and optical channel transport
unit (OTU). The OPU provides the functionality for
the mapping of client signals into the ODU. The ODU
is a network-wide transport entity that can transpar-
ently transport a wide range of client signals.
Foundation OTN defines three rates of approximately
2.5 Gb/s, 10 Gb/s, and 40 Gb/s that are referred to as
the ODUk (k � 1, 2, or 3).
Client signals mapped into the OPU include bit syn-
chronous constant bit rate, asynchronous CBR, ATM
streams based on virtual path (VP), and mapping of
170 Bell Labs Technical Journal DOI: 10.1002/bltj
data clients via GFP. The CBR streams are limited to the
average data rates corresponding to the related
SDH/SONET rates of 2.488Gb/s for OPU1, 9.995Gb/s
for OPU2, and 40.150Gb/s for OPU3, each with a long
term frequency tolerance of �20ppm. Values for these
CBR streams, and the related OPUk, ODUk, and OTUk
(k � 1, 2, 3) are provided in Table I. OPUk overhead
includes information on payload type, supporting rate
adaptation for CBR signals using fixed and flexible
stuffing (justification) and providing justification
control.
The ODU adds overhead to support managed ser-
vices in multi-operator DWDM-based optical networks
in the client-independent manner that is essential for
operating such networks. The overhead enables moni-
toring to support end-customer, service provider, and
network operator needs, providing for multiple levels
of nested and overlapping connection monitoring.
Foundation G.709 provides virtual concatenation
(VCAT) of OPUk signals in order to decouple the path
establishment from the actual physical network
resources, such as:
• Ability to transport ultra-high rate services on
foundation infrastructure, including CBR 10 G
and CBR 40 G signals across fibers supporting less
than 10 G and/or 40 G wavelengths.
• Finer granularity bandwidth allocation to map
packet streams into the most efficiently sized
pipes that, in conjunction with the link capacity
OTN technology
Network operator Adomain
IrDI Network operator Bdomain
Vendordomain 1
Vendordomain 2
Domain 3IrDI
IaDI
Non-OTNtechnology
Non-OTNtechnology
IaDI
IaDI—Intra-domain interfaceOTN—Optical transport network
Figure 2.OTN interface classification.
DOI: 10.1002/bltj Bell Labs Technical Journal 171
adjustment scheme (LCAS) [12], provide hitless
bandwidth modification and built-in resilience
when the signal components are routed via two
or more diverse routes.
Since the supervision of a number of ODUk
belonging to a VCAT group (VCG) is more complex
than the management of a single, per service, trans-
port entity, and the allowed great flexibility (individual
VCG members on different wavelengths or provi-
sion of bandwidth higher than the bit rate of a
wavelength) requires large buffers for differential
delay compensation, an additional mechanism (see
the section describing ODUflex) was subsequently
introduced to provide flexible allocation of band-
width within a single wavelength without this com-
plexity.
Optical transport module
OTUk
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OTSn
OPS0
FEC—Forward error correctionOCC—Optical channel carrierODU—Optical channel data unitOH—OverheadOMS—Optical multiplex section OOS—OTM overhead signalOPS—Optical physical section
OPU—Optical channel payload unitOSC—Optical supervisory channelOTM—Optical transport moduleOTN—Optical transport networkOTS—Optical transmission section OTU—Optical channel transport unit
Figure 3.Foundation OTN: structure of optical transport module.
Table I. Values for CBR streams and related OPU, ODU, and OTU.
OTU type and rate ODU type and rate CBR client OPU type and OPU payload nominal bit rate
OTU1 ODU1 STM16 OPU12 666 057 kb/s 2 498 775 kb/s 2 488 320 kb/s
OTU2 ODU2 STM64 OPU210 709 225 kb/s 10 037 273 kb/s 238/237 � 9 953 280 kb/s �
9 995 276.962 kb/s
OTU3 ODU3 STM256 OPU343 018 413 kb/s 40 319 218 kb/s 238/236 � 39 813 120 kb/s�
40 150 519.322 kb/s
OTU, ODU, and OPU payload bit rate tolerance is �20 ppm each
CBR—Constant bit rate OTU—Optical channel transport unitODU—Optical channel data unit STM—Synchronous transfer modeOPU—Optical channel payload unit
To condition the ODU for transport over a wave-
length in an optical transport network, it is trans-
ported within an OTU frame that includes a forward
error correction (FEC) code. The OTUk adapts the
ODUk for transport over 3R sections. Some OTUk sig-
nals offer standards interoperability to support the
interconnection of two networks of different opera-
tors and/or subnetworks of different vendors. Other
OTUk signals are vendor proprietary (OTUkV) and
will be deployed in vendor-specific subnetworks only.
Overhead is also provided for single and multi-
channel optical signals to support management of the
“all-optical” parts of the OTN network. Unlike sub-
lambda overhead, this optical signal overhead is typically
transported via a separate optical supervisory channel
(OSC) wavelength and is called “non-associated.” While
sub-lambda overhead (syntax and semantics) has been
fully standardized in ITU-T G.709, only the function-
ality of lambda overhead for single and aggregated
channels has been standardized, with the supporting
OAM mechanisms yet to be provided.
The OTN will therefore consist of vendor- and/or
operator-specific OTN subnetworks (with IaDI inter-
faces), interconnected via standard optical-transport
module (OTM0, OTM-n) inter-domain interfaces.
ITU-T G.709 interface and G.798 equipment [14]
specifications, together with G.959.1 [17] optical physi-
cal layer specifications, describe both the single chan-
nel OTM0 and 16 and 32 channel DWDM IrDI with
simplified OTS, OMS, and OPS layer (the OPS0 denot-
ing the single channel section layer) specifications for
short-haul single and multi-channel interfaces.
Foundation OTN Solution DomainThis section illustrates how foundation OTN capa-
bilities can be leveraged to address the challenges
described in the section titled “Foundation OTN
Problem Domain.”
Referring back to the example illustrated in Figure
1, transport of SDH/SONET connection services is no
longer an issue if network operator B deploys an OTN
network. In this case, the entire SDH/SONET frame is
mapped into an OCh, which provides networking
capabilities (cross-connection, protection) at the OCh
level, and is transparently carried through network
operator B’s network.
172 Bell Labs Technical Journal DOI: 10.1002/bltj
Multi-carrier scenarios can easily be supported
via ITU-T G.709 connection monitoring capabilities,
as illustrated in Figure 4, enabling a wide range of
SLA verification capabilities. The ODUk signal pro-
vides nested and overlapping connection monitoring
(CM) capabilities for every stakeholder in the trans-
port domain: customer, service provider, and network
operators. The customers can own the OCh endpoints
(and their monitoring capabilities), and service
providers can own the OCh leased circuits for which
the network operators provide the OCh connections.
Two fixed levels of CM capabilities (path and section
CM) and six variable levels of nested and overlap-
ping connection monitoring are defined for this pur-
pose. These also can be applied for protected domain
monitoring, testing, and optical-link connection
monitoring.
Photonic network fault sectionalization is easily
supported via leveraging OTUk section overhead:
• For the case of the DWDM line system impair-
ments resulting in bit errors, OTUk section moni-
toring at the adjacent downstream network
element detects a bit error rate (BER) threshold
crossing. However, OTUk overhead inserted at
downstream nodes allows independent monitor-
ing of OTUk sections. Thus, downstream nodes
will not detect bit errors caused by the upstream
degradations in other OTUk sections, and the
degradation can be easily isolated to the correct
DWDM line system.
• For the case of the photonic cross connect mis-
connection, trace mismatch will only be detected
within the OTUk OH of the impacted section.
Again, since new OTUk overhead is inserted to
monitor downstream sections, those sections will
not detect the misconnection, and the isolation
of the fault is relatively simple.
Finally, within a photonic subnetwork, OTN non-
associated overhead carried within the OSC prevents
alarm storms. In the event of a fiber cut, OCh for-
ward defect indication (FDI) signals are simply sent
via the OSC to downstream nodes to prevent LOS
alarms from being reported. Thus, only the OTN net-
work element (NE) adjacent to the fault will report an
LOS alarm.
DOI: 10.1002/bltj Bell Labs Technical Journal 173
OTN Evolution DefinedThe foundation OTN structures and formats pre-
viously described were designed to provide an easily
evolvable modular approach. The goal of OTN evolu-
tion is to extend and enrich the foundation hierarchy
as a seamless transition towards enabling optimized
support for an increasingly abundant service mix.
ITU-T G.709 Amendment 3 [20], approved April
2009, extended the hierarchy “at both ends” and added
the capability to support new services, as illustrated in
Figure 5. At the lower end, a new ODU0 hierarchical
layer was added that was optimized to support 1 GbE
client signals. At the upper end, a new ODU4 hierar-
chical layer was added, optimized to support transport
of emerging new 100GbE services, and also designed to
be a server capable of carrying all current and future
OTN services. Clarification of client/server relationships
was provided by the definition of higher order (HO)
and lower order OPU and ODU transport entities. The
LO ODUk represents the container transporting a client
of the OTN that is either directly mapped into an OTUk
or multiplexed into a server HO ODUk container.
Consequently, the HO ODUk represents the entity
transporting a multiplex of LO ODUj tributary signals in
its OPUk area. Note that the LO OPU and HO OPU,
and related LO ODU and HO ODU, have the same
information structures though they represent different
entities. Great care was taken to assure preservation of
the integrity of the foundation OTN hierarchy.
• There is a single standardized server line rate at
each tier of the hierarchy; HO ODUk/OTUk (k �
1–4) at 2.5 Gb/s, 10 Gb/s, 40 Gb/s, and 100 Gb/s.
• There is a single standardized client container
rate at 1.25 Gb/s, 2.5 Gb/s, 40 Gb/s, and 100 Gb/s;
LO ODUj (j � 0, 1, 3, 4) for both CBR and GbE
clients.
Operator A Operator B Operator A
User
Working
Protection
End-to-end path supervision (PM)
User QoS supervision (TCM1)
Service provider QoS supervision (TCM2)
Protection supervision (TCM4)
Operator domain and domain interconnect supervision (TCM3)
User
OTN ingress/egressclient mapping
OTN ingress/egressclient mapping
OTN—Optical transport networkPM—Path monitoringQoS—Quality of serviceTCM—Tandem connection monitoring
Figure 4.OTN tandem connection monitoring levels.
• There are two standardized client rates at 10 Gb/s:
— LO and HO ODU2 for SDH and most other
c l i e n t s , a n d
— LO ODU2e for transparent 10GBASE-R and
transcoded FC1200. (Originally described in
G.Sup43, the new LO ODU2e represents one
of the most widely deployed over-clocked
ODU2 options for 10 GbE LAN PHY signal
transport.)
• A standard mapping provides codeword-
transparent 10 GbE, which is networkable over
standard ODU2 bit-rate networks. (Originally
described in ITU-T G.Sup43, and elevated to stan-
dards status, this method maps the Ethernet pac-
kets, preamble, and ordered set information into
GFP-F frames, with only the inter-frame gap
information not preserved.)
174 Bell Labs Technical Journal DOI: 10.1002/bltj
• Two flavors of non-normative ODU3 rates (HO
ODU3e1, ODU3e2), for transport of four over-
clocked ODU2s (ODU2e) over a single wave-
length, were included in G.Sup43.
The ODU0 frame structure is consistent with that
of foundation ODUj, with a rate of 1.244 Gb/s. As this
rate is too low for bit-transparent transport of the
1GbE line code, a 10B codeword transparent mapping
has been defined using the same 64B/65B transcoding
method used for the mapping of 1 GbE into virtually
concatenated SDH containers (VC4-7v). The ODU0
carrying 1 GbE can be cleanly multiplexed into the
foundation hierarchy, e.g., two per ODU1 and eight
per ODU2. This signal is then mapped into the ODU0
frame using GFP-T [18], using sigma-delta justifica-
tion to handle clock tolerance differences. This timing
transparent method supports synchronous Ethernet.
ODU2e
ODU4 OTU4
40GbE
100GbE
ODU01GbE
10GbE LAN
ODU1
ODU4
x2
x8
x3
ODU1
ODU2
ODU3
OTU1
OTU2
OTU3
CBR2G5
CBR10G
CBR40G
ODU2
x4
x16
x4
ODU3
Foundation G.709 Hierarchy
G.709 Amendment 3
10GbE LAN
x32
OTU2e
OTU3e2ODU3e2
Non-normative (G.sup43)
ODUflexCBRx2G5+
GFP data
xn
xn
HO ODU/OTULO ODUODU clients2009 standards agreements
AMP—Asynchronous mapping procedureBMP—Bit-synchronous mapping procedureCBR—Constant bit rateGbE—Gigabit Ethernet
GFP—Generic framing procedureGMP—Generic mapping procedureHO—Higher orderLAN—Local area network
AMP/BMP
GMP
OTU3e1ODU3e1x4
x4
LO—Lower orderODU—Optical channel data unitOTN—Optical transport networkOTU—Optical channel transport unit
x40x80
x10x10x2xn
Figure 5.OTN hierarchy evolution.
DOI: 10.1002/bltj Bell Labs Technical Journal 175
between IEEE and ITU-T which assures there will be
no future issues regarding OTN compatibility and no
need for proprietary over-clocked solutions [25]. This
mapping supports timing transparency for synchro-
nous Ethernet.
Use of low-cost 40 GbE/100 GbE multilane optical
modules for STM-256/OTU3/OTU4 client side inter-
faces is supported by inversely multiplexing the OTU
bit stream synchronously in 16 byte blocks, which are
round robin distributed to the multiple lanes with
lane rotation at each frame boundary. OTU3 is trans-
ported via four lanes of 10.755 Gb/s, while OTU4 is
transported via four lanes of 27.952 Gb/s.
This is reflected in the logical structure of the OTN
networking interface and the enhanced optical trans-
port module, as illustrated in Figure 6.
FEC—Forward error correctionOCC—Optical channel carrierODU—Optical channel data unitOH—OverheadOMS—Optical multiplex sectionOOS—OTM overhead signalOPS—Optical physical sectionOPU—Optical channel payload unitOSC—Optical supervisory channelOPSMnk—Optical physical section multilane (n = number of lanes)
OTL—Optical channel transport laneOTLC—Optical transport lane carrierOTLCG—Optical transport lane carrier groupOTM—Optical transport module OTN—Optical transport networkOTS—Optical transmission section OTU—Optical channel transport unit
Optical transport module
OTUk
OCC OCC OCC
Client
ODUk FECOH
OPUkOH
ClientOH
Dig
ital
do
mai
n
Ass
oci
ated
ove
rhea
d
OSCOOS
OHOH
OH
No
n-a
sso
ciat
edo
verh
ead
OMSn
OTSn
OPS0
OTUkMultilane OPSMnk option
. . .
OPSMnk
OTLCG
OTM0-kvn
OTL 0
OTLC OTLC
OTL n-1
OSC
Figure 6. OTN evolution: optical transport module multilane option.
The ODU0, which has no corresponding OTU physical
layer interface, can be mapped into two newly defined
1.25 Gb tributary slots of the ODU1.
The ODU4 container size was selected as
104.794 Gb/s to assure efficient transport of 100 GbE
(and corresponding OTU4 rate of 111.81Gb/s) selected
by balancing optical physical layer constraints with
client needs. The ODU4 a d d i t i o n a l l y s u p p o r t s
80 t r ibutary s lo t s o f 1 .25Gb/s for mapping LO
ODUs in a flexible non-blocking manner.
To support 40 GbE services, a physical coding sub-
layer (PCS) codeword-transparent mapping has been
specified that allows mapping into a standard ODU3
container by transcoding the 64B/66B line code of
the Ethernet interface into a 512B/513B code. A
strictly controlled 66B line code has been agreed
The G.709 revision (October 2009) incorporates a
flexible ODU container (ODUflex) friendly to packet
transport for port and sub-port level grooming, as
illustrated in Figure 7. In addition to transport of spe-
cific physical layer clients that are synchronously
wrapped, this provides a scalable vehicle for transport
of packet streams mapped into a flexibly sized con-
tainer using GFP-F [18]. Distinct from the fixed size
containers of foundation G.709, the ODUflex enables
service providers to allocate bandwidth as needed by
each logical connection within a physical interface.
While any bit rate is possible in principle, maximum
efficiency for ODUflex carrying packets is achieved by
choosing the size of the ODUflex to fill an incremen-
tal number of tributary slots of the HO ODUk (k�2, 3,
4), which carries the ODUflex. In the event that an
ODUflex is expected to traverse multiple different HO
ODUk, then increments of the smallest tributary slot
size of any HO ODUk in the path should be used. The
ODUflex is being developed in such a way that will
not preclude the possible introduction of resizing func-
tionality in case of GFP-F mapped packet streams.
176 Bell Labs Technical Journal DOI: 10.1002/bltj
A generic mapping procedure (GMP) has been
introduced to provide a more flexible way of map-
ping new clients into fixed-size ODUs (e.g., ODU4)
as well as mapping of LO ODUflex and ODU2e into
HO ODUk. GMP supports a wider range of client rate
variations and bit rates than the asynchronous map-
ping procedure (AMP) of foundation OTN. GMP is
capable of encapsulating any new LO ODUj into
the 1.25 Gb/s tributary slot structure of the OTN.
For example, ODU2e has a clock tolerance of
�100 ppm and does not fit into a standard OTU2
but is mapped via GMP into 9 � 1.25 Gb/s tributary
slots of an ODU3, or 8 � 1.25 Gb/s tributary slots of
an ODU4.
OTN Network Architecture EnablersNetwork operator architecture evolution is
dependent upon a range of characteristics including
service mix offered and relative dominance, scalabil-
ity, reliability, technology breakthrough, manageabil-
ity, and economic considerations. While no single
future network architecture will meet every service
HO ODUk (�)
ODUj (not flex)
ODUflex nn FC PHY
HO ODUk (�)
ODUflex 1
ODUflex m
ODUj (not flex)
Logical Flow(VLAN #1)
Eth PHY
ODUflex mN Eth PHY
ODUflex nLogical flow(VLAN #n)
N Eth PHY
TDM CBR
TDM CBR
ODU k
ODUflex
ODUk
Circuit ODUflex
ODUflex Packet ODUflex
Circuit ODUflex
Supports any possible client bit rate as a service in circuit transport networks.
Packet ODUflex
Creates packet trunks of variable sizes for transporting packet flows using layer 1 switching of a LO ODU.
CBR—Constant bit rateEth—EthernetFC—Fiber channelHO—Higher orderLO—Lower order
OCh—Optical channelODU—OCh data unitPHY—Physical layerTDM—Time division multiplexingVLAN—Virtual local area network
Figure 7.Flexible ODU (ODUflex).
DOI: 10.1002/bltj Bell Labs Technical Journal 177
provider need, there are some unifying service
provider objectives:
• Flexibility to govern the selection of technology,
architecture, and products that facilitate cost
effective and scalable solutions:
— Maximizing their network resource efficiency
considering the range of external users/clients
for whom they are providing services.
— Allowing network optimization to be per-
formed within their own administrative
domain.
• Capability to offer “managed services,” which
involves being able to:
— Validate SLA compliance with their cus-
tomers, taking into account possible network
and/or equipment fault conditions.
— Support interoperability with other operators,
as needed, to realize an end-user’s request for
services.
— Rely upon multi-vendor interoperability
across one or more dimensions.
Within the following sections, a description is pro-
vided of OTN capabilities that may be leveraged to
satisfy the aforementioned objectives.
Scalable SolutionsScalability reflects a network’s ability to grow in
number of users, number of network nodes, geo-
graphic reach, and total bandwidth. The challenge
is to achieve scalability within the confines of other
network requirements, especially those pertaining to
cost, performance, and reliability.
From a technology perspective, OTN is character-
ized by a graceful mix of photonic and opto-electronic
switching, which play complementary roles.
• In photonic switching, an optical signal transit-
ing a network node is switched as a wavelength.
Optimally suited for cases where the granularity
of the service is close to the wavelength capacity,
photonic switching is primarily used to provision
and restore such “lambda” services.
• In electronic switching, an optical signal is termi-
nated and the entire signal, or individual tributary
slots contained in the signal, can be switched.
Opto-electronic switching is primarily used to
provision and restore “sub-lambda” services that
consume less than a wavelength of bandwidth.
While photonic switches can be extremely low
cost when a full wavelength can be switched, they do
not allow access to any of the channel content. At the
same time, state-of-the-art optical switching architec-
tures are typically characterized by non-zero block-
ing probability. OEO points may then be leveraged to
provide additional flexibility for wavelength conver-
sion, which becomes essential as network load and
complexity increase. Additionally, when transmission
impairments such as optical signal-to-noise ratio
(OSNR), dispersion, and non-linear effects accumulate
after a substantial transmission distance, regeneration
is required even if the channel does not need to be
switched. Further, OEO points are used at operator
domain borders in order to establish a quantifiable
assessment of the client signal quality.
The contribution of the OTN to fostering scalabil-
ity is elaborated in the sections that follow.
Scalability offered by sub-lambda multiplexing.Technological changes coming in transmission, coupled
with the continued growth in traffic, are motivating a
migration to 100G optical channel rates in core DWDM
transport systems. In parallel, the line rate of interfaces
interconnecting client devices to the optical transport
network is growing from 10Gb/s to 40Gb/s and now to
100Gb/s. Such high transmission speeds, while reduc-
ing the number of interfaces installed at the edge of the
transport network, in many instances provide a far
higher capacity than the overall amount of bandwidth
actually required for communication between peer
client network elements: i.e., there will be many net-
work connections that do not require a full optical
channel (lambda). In reality, it can be expected that
high-speed client interfaces will carry various traffic
flows, each representing a logical channel between two
different peering client network elements.
Thus, while capable of supporting terabit net-
working, OTN serves as the convergence layer for
transporting a wide range of services whose bit rates
do not allow efficient usage of the entire bandwidth
associated with a single lambda. Efficient transport of
such line rates involves supporting sub-lambda mul-
tiplexing technologies on network elements located
at the edge of the optical transport network.
For example, instead of having a single interface
that uses the entire available bandwidth, each port
could be partitioned into smaller data channels (inter-
face channelization), each one building a logical
point-to-point link between a virtually adjacent pair
of peer routers as shown in Figure 8. Traffic belong-
ing to the different logical channels may be distin-
guished within these transport network elements by
looking at information located at layer 2 or below.
VLAN tags are among the ideal candidates for layer
2 because of their scope being limited to a single physi-
cal interface, the presence of quality of service informa-
tion, and the lack of control plane dependency between
the router network and transport networks. Moreover,
router manufacturers currently provide VLAN tags on
Ethernet interfaces of any line rate. At the transport net-
work boundary, all packets that transit on the same
physical interface and carry the same VLAN tag iden-
tify a logical data channel whose maximum bandwidth
can be, in the case of core and metro core networks,
pre-calculated by means of traffic planning tools.
A packet flow belonging to a logical data chan-
nel can then be transformed in a CBR flow and car-
ried across the optical transport network by means of
an ODU pipe whose bit rate is similar to the CBR flow
rate, allowing the flow to be carried through the
transport network using lower cost-per-bit layer 1
switching technologies.
178 Bell Labs Technical Journal DOI: 10.1002/bltj
Like SDH, foundation OTN supports flexible
bandwidth allocation using virtual concatenation of a
set of basic container sizes. Additionally, as discussed
in “OTN Evolution Defined,” the ODUflex container
provides bandwidth flexibility by leveraging tributary
slot concatenation.
The introduction of CBR channels supported by
partitioning of client interfaces by means of VLAN
tags enables the transport network to efficiently allo-
cate and route smaller router channels over larger
bandwidth optical transport connections.
With interface channelization, the OTN infra-
structure is not artificially constrained to transport/
switching of 10 Gb/s, 40 Gb/s, and 100 Gb/s pipes of
large granularity. Thus, sub-lambda multiplexing leads
to resource efficiencies and cost savings.
Scalability benefits for IP over optical architectures.IP core networks are increasing both in node count
and size, and it is generally accepted that “IP over
point-to-point DWDM” does not scale because router
throughput and port count increase in proportion to
the overall traffic transmitted. Further, the through
traffic is sometimes as high as 70 to 80 percent of the
overall traffic [6]. The primary challenges faced in
scaling new generations of routers to larger sizes relate
to power and heat dissipation. This can even affect
Logical channel 1,VLAN “X”
Logical channel 1,VLAN “X”
Logical channel 2,VLAN “Z”
Logical channel 2,VLAN “Z”
Logical channel 3,VLAN “Y”
Logical channel 3,VLAN “Y”
OTN—Optical transport networkVLAN—Virtual local area network
Figure 8.OTN interface channelization example.
DOI: 10.1002/bltj Bell Labs Technical Journal 179
the central office layout, as it may require increased
spacing between racks to avoid violating the station
cooling requirements. Performing routing only when
actually needed (transit traffic off-load) reduces over-
all energy costs and environmental concerns, as the
increased complexity of packet processing will always
force a power and cost increment over equivalent
bandwidth through the transport layer. Thus, it is
more expensive to put 1 Gb/s through the service
layer than the transport layer at any point in time.
Hence, the service layer pass-through tax of using IP
over point-to-point DWDM is becoming insupport-
able in terms of cost, power, and footprint.
Consider the network example in Figure 9, illus-
trating an IP over point-to-point DWDM architecture,
where certain nodes experience sudden spikes in
demand, for example, when a major new customer
comes online [22]. To handle the increased capacity,
the client layer network must be upgraded, poten-
tially including additional intermediate nodes to pro-
vide bandwidth management and survivability
functions for the engineered routes. The underlying
issue is that the client-layer logical topology is tied to
the network’s physical-link topology. This coupling
leads to a de-optimization of the more complex
IP/multiprotocol label switching (MPLS) transport
layer at a time when there are growth and churn in
new packet-based services.
Utilization of OTN offers the potential to reduce
power levels, carbon footprint, and cost where trans-
port functions suffice, especially at intermediate nodes
along an end-to-end route, which minimizes the num-
ber of excursions up to the more complex service layer.
A hierarchical approach, detailed in Figure 10,
reduces overall network cost by enabling the service
layer network to grow efficiently, without requiring
costly capacity upgrades at intermediate core routers,
and only performing routing when really needed [22].
P2P DWDM
IP
DWDM
Demand spikes
=P2P DWDM
with larger transport bandwidth
IP
DWDM
All routers mustupgrade to handle
more through traffic
DWDM—Dense wavelength division multiplexingIP—Internet ProtocolP2P—Point-to-point
Figure 9.Scalability arising from client layer demand spikes.
Optical transport networking
Servicelayer
Opticaltransportnetwork
Controlled upgrades
Figure 10.Networking scalability via hierarchy.
Moreover, further cost reduction can be achieved
by proactively adapting IP topologies as traffic war-
rants, making use of reconfigurable optical transport
connections, which will be discussed in a later section.
Finally, it is important to note that increasingly
demanding real-time services (i.e., audio, video,
images) present more challenges to the design of
next-generation networks than do traditional data
applications such as e-mail and Web. Although still
bandwidth adaptive, these real-time services have
stringent latency, packet delay variation, and packet
loss requirements. Enabling carriage of transit traffic
in the OTN layer offers a way to prevent long multi-
hop cascades of routers, thus avoiding unnecessary
delay, jitter, and network instability in case of cata-
strophic events. Meeting stringent multimedia service
requirements is becoming a critical factor in deter-
mining the success of operators engaged in deliver-
ing high volumes of these services over complex
network architectures.
Thus, as a general engineering principle, it makes
sense to decouple the services layer from the transport
and keep transit traffic in the transport domain at the
lowest possible layer.
In reality, Figure 9 and Figure 10 are oversimpli-
fied, as they do not address the roles of the photonic
and opto-electronic switching layer technologies, as
discussed in the previous section.
180 Bell Labs Technical Journal DOI: 10.1002/bltj
Optimization for Multi-Domain/Multi-CarrierApplications
Let us consider a realistic scenario in which several
network operators are involved in the connection of a
client data service between two endpoints, as shown in
Figure 11. The multiple-carrier model is one where the
service provider (represented as operator B) owns part of
the transport path but does not have access to the
edge(s) of the optical transport network. The customer-
supplier relationship is between the client data customer
and service provider who holds the end-to-end contract
with the customer; however, the service provider does
not have a physical presence at the service edges.
Since there are performance guarantees pursuant to
this contract, there is a resulting SLA between the ser-
vice demarcation points, denoted the user network
interface (UNI). This service level agreement guarantees
provider edge-to-provider edge performance. In order
to complete the service offering, the service provider
buys mapping/de-mapping functionality and optical
transport connectivity from a carrier with a physical
presence near the client data customer. The presence
carriers wholesale their service to the service provider.
This relationship requires another level of agreements at
the demarcation between carriers, denoted the external
network-network interface (E-NNI).
OTN TCM, as described in “Foundation OTN Struc-
ture and Format,” explicitly provides the necessary
UNI
E-NNI
E-NNIService provider
OTN networkUNI
G.709 Network (lambda)
G.709 Network (sub-lambda)
Operator A
G.709 Network (lambda)
G.709 Network (sub-lambda)
Operator B
G.709 Network (sub-lambda)
G.709 Network (lambda)
Operator C
Customerequipment
Customerequipment
E-NNI—External network-network interfaceOTN—Optical transport networkUNI—User network interface
Figure 11.Service provider without direct access to service edge.
DOI: 10.1002/bltj Bell Labs Technical Journal 181
service demarcation capabilities allowing for SLA veri-
fication and fault localization among multiple
domains, while retaining monitoring capabilities
needed for fault sectionalization and restoration/
protection activities.
SurvivabilityOne of the key challenges for next-generation net-
works is to bring the reliability of the circuit-based voice
network to packet-based networks. Appropriately,
many of today’s problems with data-network reliabil-
ity are being solved in the service layers themselves.
But to maintain its performance and cost effective-
ness, the service layer also needs to rely on the trans-
port layer for the first line of defense against
big-network faults, such as fiber cuts. The fastest pos-
sible recovery from these optical-layer outages is espe-
cially important given the growing bandwidth and
number of users per fiber. OTN protection is, as for
SDH/SONET, very fast and always provides accepta-
ble transport layer recovery. In general, providing
transport layer recovery as close as possible to the
physical media layer tends to be most efficient as spare
capacity over all the affected layers, and the number
of transport entities involved, is minimized.
OTN survivability. OTN currently supports shared
and dedicated ODUk linear protection schemes, with
the automatic protection switching (APS) protocol
and protection switching operation as specified in
ITU-T G.873.1 [11]. These schemes encompass ODUk
subnetwork connection protection with:
• Inherent monitoring (1�1, 1:n),
• Non-intrusive monitoring (1�1), and
• Sublayer monitoring (1�1, 1:n).
While standardization activity had been initiated
on APS-based OTN ring protection (draft G.873.2), this
work became dormant when the market stalled, and
the draft Recommendation was not completed. Control
plane-enabled OTN restoration schemes (ODUk and
OCh) may also be supported, including mesh-based
restoration. With the resurgence of standardization
activities related to OTN evolution and photonic net-
working in general, it is expected that a resurgence of
activities on OTN survivability will also occur.
Survivability for IP over optical architectures. Multi-
layer survivability refers to the possible nesting of
survivability schemes among these layers, and the
way in which these mechanisms may interact with
each other. A coherent multi-layer survivability strat-
egy enables the desired level of quality of service
(QoS) and network bandwidth optimization and
minimizes cost on a per-service basis. Within the con-
text of multi-layer survivability, the most important
parame-ters to focus upon are the fault type and the
effect of this fault on the traffic. Faults such as physi-
cal medium faults, node faults, and some hardware
faults affect all services in all the network layers and
consequently have to be recovered from concurrently
(and quickly). The effects of other types of hardware
faults, provisioning errors, and performance degra-
dations are often less catastrophic, as fewer services
are affected or services are not all affected at the same
time [2].
Single layer recovery can be performed in the
transport layer as well as in the service layer. There are
a number of trade-offs to be considered, which are typi-
cally application dependent. If we consider a scenario
involving traditional IP/MPLS as the service and OTN
as the transport, traffic impacted by a physical medium
fault can be restored by the transport layer in larger
granularity bundles, making the recovery approach
more effective (especially for catastrophic faults like
fiber cuts) and simplifying network maintenance.
Architectures focused upon IP/MPLS service layer
protection provide service layer rerouting for all fail-
ures, including fiber cuts and optical port failures. The
intuitive appeal is that there is theoretically less need to
reserve spare capacity in advance, and the statistical
nature of the service layer means that any available
protection route can be shared among many services.
The disadvantage is that each incremental unit of capac-
ity in the service layer is relatively more expensive, and
it must be available in every intermediate hop.
Support for service layer survivability also
requires allocation of bandwidth in the transport lay-
ers. This bandwidth provides the alternative routes
used by the service layer survivability mechanism,
which may not be used for transport survivability. The
total cost involved in this survivability solution is
related to the total amount of bandwidth required in
all the layers. The total amount of spare capacity
required in the service layer may depend on the faults
against which it has to protect, which may be large if
the intention is to protect against catastrophic faults in
this layer.
Alternatively, providing a nested IP/MPLS and
OTN multi-layer survivability solution that appropri-
ately leverages OTN shared protection architectures
is particularly valuable for the meshed traffic patterns
found in core networks, where such capabilities are
ideal. Figure 12 illustrates efficient survivable trans-
port networking with shared protection [22]. Such
architectures hold protection-capacity overbuilds to a
minimum, on a par with that achieved by any realis-
tic service-layer scheme, and they achieve this at a
lower network cost.
Thus, nesting IP/MPLS and OTN-based surviva-
bility mechanisms can be extremely attractive.
Role of Optical Control PlaneOptical transport is undergoing a critical transi-
tion in which the network is migrating from static to
dynamic intelligent optical transport networking solu-
tions. Improved network efficiency, operational
improvements, and new revenue opportunities are
some of the advantages linked to the migration from
182 Bell Labs Technical Journal DOI: 10.1002/bltj
static to dynamic intelligent optical transport net-
working solutions [21].
Optical control plane—automatically switched opti-
cal network/generalized multiprotocol label switching
(ASON/GMPLS) enabled solutions [3, 13]—simplify
network operations by delegating several key opera-
tions support system (OSS) processes to the control
plane for automation with the goal of a “self-running”
network where “the network is the database.”
Automated processes include network topology/
resources/services discovery, end-to-end optical con-
nection routing for optimal resource utilization, flow-
through service provisioning, and mesh restoration.
Overall, the following benefits are anticipated:
• Automation that results in reducing operating
expenses (OPEX) by minimizing the manual and
time-intensive procedures present in today’s pro-
visioning processes.
• CAPEX improvements due to elimination of
stranded resources through high-quality inven-
tory databases, populated by the optical control
plane auto-discovery process.
• Increased optimized network-wide resource uti-
lization resulting from more dynamic multi-layer
OTN
IP/MPLS
Share
d prote
ction
bandwidth
IP—Internet ProtocolMPLS—Multiprotocol label switchingOTN—Optical transport network
Figure 12.Efficient survivable transport networking with shared protection.
DOI: 10.1002/bltj Bell Labs Technical Journal 183
networking coupled with integrated traffic engi-
neering solutions.
• Higher bandwidth-efficiency transport via sup-
port for mesh topologies together with dynamic
rerouting and restoration mechanisms.
• Network efficiency improvement by ensuring
flow-through interoperability across multi-vendor,
multi-layer, and multi-regional networks by
way of standardized signaling protocols and pro-
cedures.
• Provision of control plane-enabled protection and
restoration schemes, increasing the solutions
toolkit for meeting different customer needs, and
improving network reliability and availability.
• Facilitation of multi-layer network engineering
that enables an automated process of coopera-
tively tailoring the server layer capacity based on
the network topology and resource-usage of the
client layer.
The ASON/GMPLS control plane complements
management plane based solutions in providing the
operator with enhanced capabilities. For example,
the control plane relies upon management system
configuration of some static traffic engineering data,
which is useful for calculation of disjoint cost equiva-
lent paths pertaining to shared risk link groups
(SRLGs). In turn, the management plane leverages
the control plane for periodic retrieval of actual traf-
fic flows to be compared to nominal traffic flows when
performing network re-optimization.
Dynamically Configurable OTN ApplicationsWhile technological innovations in optical net-
working have led to huge leaps in network capacity
while driving down the cost per bit, these innovations
matter little if the network capacity is unavailable for
use when needed. In order to use network resources
cost-efficiently, carriers will need to ensure that the
right amount of network capacity is allocated where
the traffic demand resides in the network. In today’s
highly competitive environment, there is no tolerance
for service provisioning delays. In fact, failure to
deliver services faster than competitors can limit a
carrier’s ability to compete for new services and ulti-
mately can drive a carrier out of business.
Compounding the dilemma, bandwidth con-
sumers only want to pay for what they use and are
expressing more and more reluctance to sign long-
term service contracts. In order to deal with these
market pressures, carriers, in turn, are demanding
network solutions that facilitate faster and more flexi-
ble service delivery.
The struggle lies in their ability to deliver, quickly
and efficiently, the managed-bandwidth services that
best address their customers’ needs. Customers of
managed bandwidth services—enterprises, Internet
service providers, applications service providers, and
other carriers—are looking for bandwidth services
that more closely resemble their business needs. They
require bandwidth without long lead provisioning
times, available on an as-needed (bandwidth-on-
demand) basis. They also require more flexible band-
width increments that allow them to purchase the
quantity needed instead of being locked into fixed
bandwidth chunks. Lastly, they require flexibility in
terms of their service contracts in the form of QoS-
based pricing since they have varying service needs.
Migrating to an intelligent and flexible optical core
network architecture will also support mesh topolo-
gies. As traffic continues to grow, mesh topologies are
becoming more interesting to service operators. For
high traffic density, mesh topologies provide for lower
capital expenditures due to more efficient filling of
direct shortest links. In this type of environment, ring-
based networks require expensive and complex ring
stacking. Also, in high traffic density growth environ-
ments, growth is easier in a mesh network, since only
direct links are affected, versus entire rings. Finally,
meshed networks enable simpler provisioning of cir-
cuits in comparison to the complex routing required
for stacked interconnected rings [21]. (It should be
noted that in areas of lower traffic density and lower
connectivity, however, ring networks continue to pro-
vide an advantage, providing highly reliable transport
that is well adapted to a feeder topology.)
Optimized IP Over Optical SolutionsDeployment of ASON/GMPLS-powered optical
transport networking capabilities results in further
reduction of cost per bit by enabling proactive adap-
tation of IP topologies as traffic warrants via recon-
figuration of the underlying optical transport
connections. As traffic between major core nodes con-
sistently starts to consume substantive bandwidth,
direct links may be put in place. Evolving cost-
optimized topologies will result in a decreasing num-
ber of intermediate core router hops for high band-
width traffic traversing long distances. The
combination of adaptive topologies and more closely
engineered router links results in cost per bit improve-
ment as traffic grows.
Multi-layer network engineering provides for the
most optimized topologies. In this case, proactive pre-
diction of IP traffic demands, coupled with ranking
the most effective optical transport network configu-
ration changes (consistent with the timescale of mid-
term packet traffic pattern variations to maintain
packet network routing stability), can be used to drive
“where” and “when” to trigger the appropriate addi-
tion, modification, or deletion of particular optical
transport network connections [5] via optical control
plane signaling and routing protocols.
Bandwidth on demand (BoD) services may also
be supported that assure optical transport network
responsiveness to the needs of IP client customers.
This involves subscription to a BoD service for a suite
of connection services among a set of sites, which can
be triggered via user network interface signaling [23,
24]. Examples of UNI service attributes include ser-
vice level (class of service), directionality, diversity
(node, link, SRLG, shared path), traffic parameters,
and bandwidth modification support. This enables the
IP clients to use UNI signaling (including attributes
that describe the service requirements for the con-
nection) to dial up the service between any two sites
based upon their business needs, send information
over the optical transport connection for an unspeci-
fied period of time, and then “hang up.”
Role of Emerging TechnologiesOTN evolution is also assisted by underlying tech-
nology enablers, including advances in modulation
formats, optical switching, high-speed electronics, and
innovative approaches to photonic OAM.
Modulation FormatsFor many years, fiber has been considered an
“infinite bandwidth” medium. Approaching 10 Gb/s,
some limitations have begun to appear, and more
recently, moving to multi-lambda 40 Gb/s and
100 Gb/s transmission, fibers are exhibiting impair-
184 Bell Labs Technical Journal DOI: 10.1002/bltj
ments that call for much more sophisticated modula-
tion schemes than the traditional simplistic on-off key-
ing (OOK). Beyond that, multi-level modulation
schemes are also needed in order to keep signal pro-
cessing at “acceptable” rates and improve the spectral
efficiency, expressed in bit/s/Hz, as data rates increase.
In fact, optical transmission is increasingly inheriting
radio modulation formats and techniques, progres-
sively moving from basic amplitude modulation with,
for example, spectral efficiency (SE) up to 0.4bit/s/Hz,
to a variety of more efficient phase modulation meth-
ods. These include differential phase modulation
(DPSK) with, for example, SE from 0.4 to 0.8 bit/s/Hz,
and quadrature modulations (QPSK, DQPSK) approach-
ing 1bit/s/Hz. These techniques can be further improved
by dual polarization mixing (the spectral efficiency of
any modulation format is in this case doubled) and
finally by coherent detection, as featured by more
recent industrial implementation at 40 Gb/s.
This is presumably not the end of the story, since
coherent detection implies complex digital signal pro-
cessing, which calls for very high speed analog-digital
conversion of photo detector outputs, and in turn,
opens the possibility of soft-decoding of FEC codes,
and possibly also even more complex multi-level
modulation formats with error correcting codes
embedded in the signal space. Another key topic to be
addressed is the effect of interference among signals
characterized by different bit rates, which again may
need further electronic countermeasures. In a nut-
shell, optical transmission techniques are evolving
very rapidly, mainly via adoption of digital signal pro-
cessing techniques that have been commonly
employed in radio transmission for many years. These
techniques are only now being adopted due to the
requirements imposed by support for very high speed
optical transmission (aiming at multi-lambda
100 Gb/s), in conjunction with the availability of
unprecedented processing power in complementary
metal-oxide semiconductor (CMOS) application-specific
integrated circuits (ASICs) and application-
specific standard products (ASSPs).
Optical SwitchingIn wavelength-routed networks, switching is per-
formed through optical add/drop multiplexers
DOI: 10.1002/bltj Bell Labs Technical Journal 185
(OADMs) and PXCs supporting provisioning, protec-
tion, and restoration at the optical layer. Notable
architectures and recent advances for supporting
wavelength-routed networks include:
• ROADM architectures, which are characterized by
two DWDM ports and N single wavelength add/
drop ports, enabling evolution of wavelength divi-
sion multiplexing (WDM) systems from point-to-
point to ring or linear add/drop topologies. The
first to be available, they are usually realized in
the field using wavelength blocker or planar light-
wave circuit technologies. They can be evolved
to “colorless” (any multiple “lambda” from any
port to any port) using tunable filters at the drop
and tunable lasers at the add.
• 1�N wavelength selective switch (1�N WSS) ROADM
architectures, which are characterized by N�1
DWDM ports. They can be used either for multi-
degree (mesh) connectivity or for channel
add/drop (in a “colorless” way). Note that a
degree N�1 node requires N�1 1�N WSS mod-
ules to support mesh connectivity alone.
• Wavelength cross connect (WXC) architectures, which
provide complete N�N connectivity for mesh net-
works. For a degree N node and L wavelengths
per fiber, a WXC needs N demuxes, N muxes, and
L N�N switches).
Despite these advances, barriers exist to estab-
lishing complex wavelength-routed networks in a
purely photonic domain. For example, there is no
commercially available technology to support pho-
tonic wavelength conversion or regeneration.
Additionally, the intrinsically slow switching time of
many solutions precludes satisfying traditional carrier-
class 50 ms protection switching requirements.
It therefore becomes interesting to explore hybrid
photonic and electrical switching architectures that
can provide selective wavelength regeneration/
conversion, while supporting the aggregation of con-
nections at sub-wavelength granularity. As illus-
trated in Figure 13, the optical/electrical (O/E)
converters can be seen as a pool of “floating” shared
resources usable for any client as well as for any
wavelength.
XIN
YIN
ZIN
A/DOUT
XOUT
YOUT
ZOUT
A/DIN
Electrical switching
O/E
O/E
O/E
O/E
O/E
O/E
O/E
O/E
O/E
O/E
O/E O/E
O/E
Photonicswitching
A/D—Add/dropO/E—Optical/electrical
�1
�3
�N
�N-1
�2
Figure 13.Hybrid photonic and electrical switching architecture.
This type of architecture provides the following
benefits:
• An O/E converter could be used as an adaptation
device to convert the client signal into the appro-
priate DWDM line signal. Coupled with another
O/E converter, and cross-connected over the elec-
trical matrix, it offers regeneration and wave-
length conversion. Support of multiple functions
via one pool of shared resources allows for better
resource utilization and reduces the need for
accurate forecasts when designing the network.
• It offers the possibility of combining fast elec-
tronic protection switching, with the flexibility
of photonic restoration in mesh networks.
Additionally, it enables efficient 1:N protection
support against failures of client and line side
optical devices.
High-Speed ElectronicsHigh-speed and high-capacity electronic devices
are key connection-routing technology enablers for
OTN node ingress and egress signals at lambda and
sub-lambda layers.
Protocol-agnostic cross-point switches are devices
with M inputs and N outputs where each channel
operates independently (M�N spatial matrix); they
can be profitably used for regeneration and wave-
length routing/conversion in systems working with
“lambda” granularity and similar transparency attrib-
utes. The larger the matrix in a single device, the more
signals can be routed without suffering from the cost
and power dissipation penalty introduced by the
interconnection technology. Single chip capacities in
excess of 1.5 Tb/s, with bit rates of up to 11 Gb/s, can
already be found on the market.
However, purely spatial (asynchronous) architec-
tures may be non-optimal for systems that also aggre-
gate sub-lambda rate signals. For example, nodes that
do grooming of LO ODUs (from ingress HO ODUs,
cross-connecting them towards the desired outgoing
HO ODUs) are typically based on non-blocking scala-
ble synchronous matrices using time-based or
time/space-based switching. Key technology building
blocks for such architectures include CMOS ASIC and
ASSP devices, which are able to provide fully non-
blocking switching with finer service granularity
186 Bell Labs Technical Journal DOI: 10.1002/bltj
(down to 1 Gb/s) and a capacity exceeding 1 Tb/s per
chip that can scale to multi-Tb/s when combining sev-
eral devices together.
Photonic OAMThe OTN maintenance philosophy is a balanced
combination of “opto-electronic enabled maintenance”
(where opto-electronics are present), coupled with
targeted OAM capabilities for the optically transpar-
ent segments. As discussed previously, only the func-
tionality of lambda overhead (non-associated for single
and aggregated channels) has been standardized, with
the supporting OAM mechanisms yet to be provided.
However, this does not preclude vendor provi-
sion of associated photonic overhead in the context of
the IaDI. For example, it has been demonstrated (e.g.,
via Wavelength TrackerTM) that it is possible to support
the following associated photonic overhead capabili-
ties in networks of metro/regional scale that provide:
• Path trace management (continuity and connec-
tivity supervision for path set-up with instant
diagnostics in case of a failure): every optical
channel is encoded with a unique “tag” to identify
wavelength.
• Measurement of the optical power level of each
individual channel in the WDM spectrum without
embedded optical spectrum analyzers.
In the near future, advances are expected
that should enable supervision of the optical signal
quality necessary for determining its performance
(i.e., measurement of dispersion and OSNR) and
extension of the range of applicability from metro to
long haul/ultra long haul (LH/ULH) distances.
Such associated information allows for compre-
hensive, yet simple and cost-effective, monitoring in
many points of the network (e.g., amplifier input/output,
T/ROADM input/output, and multiplex input/output)
without requiring opto-electronic signal termination.
This enables, in a cost effective way, support for a
wavelength management paradigm very similar to
that for SDH/SONET and LO/HO ODU, where pho-
tonic OAM is closely coupled with the network man-
agement system (NMS) to facilitate ease of service
commissioning, continuous monitoring of the net-
work’s “optical health,” and failure diagnosis from a
remote network operations center (NOC).
DOI: 10.1002/bltj Bell Labs Technical Journal 187
Network Design and Optical Network Planning ToolsOptical network evolution from point-to-point to
mesh topologies is demanding increasing system
automation (intelligence) of DWDM equipment for
guaranteed system performance in “any-to-any” con-
nectivity scenarios (optical power balance for channel
add, removal or re-routing, enhanced resilience
schemes, and other possibilities).
Optical network design and planning tools have
therefore become essential to operators for network
design and optimization, as well as for an automated
end-to-end connection setup, tear down, and restora-
tion in case of failure.
• Automated design and engineering of a DWDM optical
network. The DWDM link/network is automati-
cally optimized and engineered, taking into
account the physical topology and parameters of
the fiber infrastructure plus the features/capabili-
ties of the WDM equipment.
• Traffic routing and wavelength assignment. Starting
from the traffic demand on the given infra-
structure, the planning of the network is car-
ried out through aggregation of low rate traffic
services (sub-lambda multiplexing), wavelength
assignment, and routing of main and protection
paths.
Optical Transport Network InfrastructureEvolution Vision
As described in [1], introduction of DWDM rep-
resented the first step towards optical networking
because it employed wavelength-based transport.
However, these backbone DWDM deployments were
generally point-to-point (P2P) applications, with the
necessary flexibility for service multiplexing, aggre-
gation, and networking provided by the underlying
TDM systems. Operators regarded such optical net-
works simply as “fat pipes” connecting switching
nodes. Primary applications in the field falling into
these categories are SDH/SONET over DWDM and
IP/MPLS over DWDM.
SDH/SONET over DWDM offered a more cost-
effective approach to core/long-haul capacity expan-
sion than other alternatives, such as adding fiber, or
upgrading/replacing lower capacity TDM systems with
new, higher-rate TDM systems. However, as described
in the earlier section “Foundation OTN Problem
Domain,” SDH/SONET faced challenges introduced
by new “carrier’s carrier” services, increasingly multi-
domain market environments, and deployment of
photonic technology.
IP/MPLS over (P2P) DWDM offered direct inter-
connection of IP/MPLS core routers, bypassing
SDH/SONET standalone network elements, whose
interface functionality was being increasingly sub-
sumed within router edge ports. The appeal of IP over
DWDM was in reducing the number of layers in the
network infrastructure (replacing IP over SDH/SONET
over DWDM by IP over DWDM). In reality, the num-
ber of layers remained the same, as IP over WDM was
typically implemented as IP packets mapped into
SDH/SONET, coupled with SDH/SONET-based point-
to-point DWDM systems. So while SDH/SONET
standalone network elements were not required,
SDH/SONET remained an integral element of the data
networking equipment interface [4]. The actual net-
working impact was limiting the number of poten-
tially switchable layers, the implications of which
were described earlier in the “Scalability offered by
sub-lambda multiplexing” and “Scalability benefits for
IP over optical architectures” segments of the “OTN
Network Architecture Enablers” section.
Evolution to fully featured OTN facilitates evolu-
tion from point-to-point capacity expansion to scalable
and robust optical transport networking applications,
catering to the expanding range of layer 1 to layer 3
services (and including carrier’s carrier services). With
service granularity moving from narrowband to
broadband, OTN enables shifting the cross-connection
granularity from VC-4/VC-11 or STS-1/VT 1.5 to
ODU0/ODU1/ODU2/ODU3 and ODU4 to satisfy the
grooming requirements of a new generation of
Terabit machines targeted for optimization around the
dominant Ethernet clients (GbE/10 GbE/40 GbE and
100 GbE).
Encompassing both photonic (OCh) and electronic
or circuit (ODU) transport entities, the emergent opti-
cal transport paradigm employs complementary appli-
cation of photonic and opto-electronic technologies
supporting:
• Photonic switching for an agile photonic layer, trans-
parent to protocol and bit rates, providing
flexibility by optical add/drop (ROADM/TOADM)
capabilities at the lambda level.
— Provides the lowest cost for high bandwidth
optical multiplexing on a fiber and transpar-
ent pass-through, eliminating unnecessary
OEO conversions and signal delay accumu-
lation.
— Avoids need for intensive network “lambda”
planning required to efficiently deploy the first
generation of WDM network elements that
were based on fixed-OADM (i.e., to avoid
blocking even in cases where capacity was
188 Bell Labs Technical Journal DOI: 10.1002/bltj
available, but the lambda was the wrong
color).
• Opto-electronic switching for an agile sub-lambda layer,
enabling aggregation and protection of traffic and
avoiding stranded bandwidth, when the service
granularity is less than the wavelength capacity.
— Enables optimization of overall network
bandwidth allocation, by decoupling the ser-
vice rate from the OTN line system capacity.
— Supports fast shared and dedicated protection
solutions, as described in “Scalability benefits
for IP over optical architectures,” avoiding the
(a) Photonic switching node
(b) Photonic and electronic switching node
NE “A”
HO-ODU
OTS
OMS
OCh
OTU
HO ODU
NE “B”
HO-ODU
LO-ODU
OTS
OMS
OTU
LO ODU
OCh
LO/HO ODU
NE “A”NE “B”
XC
XCXC XC XC
XC XC
XC
XC XC
HO-ODU HO-ODU HO-ODU
HO-ODU
HO-ODU
� level networking
Sub–� level networking
LO-ODU LO-ODULO-ODU
HO-ODU
LO-ODU
HO-ODU
Clear channel clients
Switching/routing clients
NE “B” NE “A”
HO—Higher orderLO—Lower orderNE—Network elementOCh—Optical channelODU—OCh data unit
OMS—Optical multiplex sectionOTN—Optical transport networkOTS—Optical transmission section OTU—OCh transport unitXC—Cross connect
Figure 14.OTN enabled multi-service core.
DOI: 10.1002/bltj Bell Labs Technical Journal 189
high cost of 1�1 replication in the photonic
domain.
Building upon this paradigm, it is possible to
realize the OTN vision of a multi-service core for
“any service” (comprising IP and CBR clients) that
can maximally leverage photonic technology evolu-
tion, while providing OAM capabilities meeting the
high benchmark for reliability and operational sim-
plicity that carriers have come to expect from
SDH/SONET.
This OTN-enabled multi-service core is illustrated in
Figure 14, which shows a number of photonic domains
interconnected by opto-electronic (gateway) nodes that
subdivide the overall photonic infrastructure into
smaller regions. The two basic supporting node types
are:
• Photonic switching nodes that are well suited to loca-
tions with large amounts of transit traffic having
coarse granularity (OCh switching).
• Opto-electronic capable switching nodes, integrating
photonic and electronic (LO ODU or service layer)
switching, that are well suited to the aggregation
and protection of “sub-lambda” granular services
and/or in locations that process large amounts of
add/drop traffic.
Figure 15 illustrates the high-level architecture of
these two OTN node types; their primary characteris-
tics are summarized in Table II.
HO-ODU
OTS
OMS
OTU
LOODU
OCh
LO/HOODU
NE “B”
Tunable m-degree OADMOOO
Tunable TRPUNI-NNI
OEO
Tunable TRPNNI-NNI
OEO
OTH XC (sub�)Tunable i/fs
OEO
OTS
OMS
OCh
OTU
HOODU
NE “A”
HO-ODU
OTS
OMS
OCh
OTU
HOODU
Tunable m-degree ROADMOOO
Tunable TRPUNI-NNI
OEO
Tunable TRPNNI-NNI
OEO
HO—Higher orderLO—Lower orderNE—Network elementNNI—Network-network interfaceOADM—Optical add/drop multiplexerOCh—Optical channel
ODU—Optical data unitOEO—Optical-electronic-opticalOMS—Optical multiplex sectionOOO—Optical-optical-opticalOTN—Optical transport networkOTS—Optical transmission section
OTU—OCh transport unitROADM—Reconfigurable optical add/drop multiplexingTRP—Total radiated powerUNI—User network interfaceXC—Cross connect
LO-ODU
Figure 15.OTN node architectures.
ConclusionsWith resurgence of industry interest in optical
transport network evolution, the OTN is poised to
truly emerge as the converged optical transport infra-
structure solution “offering carriers unprecedented
architectural flexibility—the client protocol (and bit
rate) independence, and service differentiation” envi-
sioned a decade ago [1].
AcknowledgementsThe authors would like to thank the many friends
and colleagues in the Alcatel-Lucent community who
have (in one way or another) contributed to the
material presented in this paper. Special thanks are
extended to Alberto Bellato, Pietro Grandi, Thomas
Mueller, and Kevin Sparks.
References[1] R. C. Alferness, P. A. Bonenfant, C. J. Newton,
K. A. Sparks, and E. L. Varma, “A PracticalVision for Optical Transport Networking,” BellLabs Tech. J., 4:1 (1999), 3–18.
190 Bell Labs Technical Journal DOI: 10.1002/bltj
[2] Alliance for Telecommunications IndustrySolutions, T1A1.2 Working Group on NetworkSurvivability Performance, “Technical Reporton Enhanced Network SurvivabilityPerformance,” ATIS T1.TR.68-2001, Feb. 2001.
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Table II. High-level architecture and primary characteristics of OTN node types.
Type “A” node (photonic switch) Type “B” node (photonic and electronic switch)
Switching Switching
“Lambda” (photonic) switching matrix WSS based “Lambda” (photonic) switching matrix WSS based
OEO conversion for drop and back-to-back OEO conversion for drop and back-to-back “lambda” regeneration and color conversion “lambda” regeneration and color conversion
Electronic OTH switching/multiplexing
Characteristics Characteristics
End-to-end OAM G.709 (IrDI) End-to-end OAM G.709 (IrDI)
TCM OAM Associated overhead TCM OAM Associated overhead (IaDI) (IaDI)1 � 1 protection @ 1�1 protection @ client client or line (colored) or line (colored) interfacesinterfaces
Resilience l mesh restoration Resilience l mesh restorationFast protection/restorationvia ODUk switching
IaDI—Intra-domain interface OTH—Optical transport hierarchyIrDI—Inter-domain interface OTN—Optical transport networkOAM—Operations, administration, and maintenance monitoring TCM—Tandem connection ODU—Optical channel data unit WSS—Wavelength selective switchOEO—Optical-electronic-optical
DOI: 10.1002/bltj Bell Labs Technical Journal 191
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(Manuscript approved August 2009)
SILVANO FRIGERIO is a member of technical staff within the Alcatel-Lucent Optics Product UnitChief Technology Office (CTO) in Vimercate,Italy. He received a degree in electronicengineering at the Politecnico of Milan, Italy.He has extensive experience as a system
architect for optical multi-service nodes (OMSN) and asan SDH/SONET ASIC designer. As a member of the CTONetwork Architecture & Engineering team, he iscurrently focusing upon aspects regarding multi-technology (hybrid) solutions and optical transportnetwork (OTN) transformation at both the equipmentand network levels. His professional interests encompasstransport equipment system design, networkdevelopments, and convergence trends. Mr. Frigerio isan Alcatel-Lucent Italia TCT Principal Engineer and holdsseveral patents concerning transmission networks.
ALBERTO LOMETTI is network architecture director within the Chief Technology Office (CTO)organization of Alcatel-Lucent’s OpticsProduct Division in Vimercate, Italy. Hereceived a diploma degree in electricalengineering from the University of Pavia,
Italy. He has been with Alcatel-Lucent for over 20 years,spanning different technical experiences from board andASIC design to system and network design. His currentresponsibilities include defining an overall optics networkvision while designing coherent end-to-end, inter-workable solutions across the division product portfolio.He is author or co-author of about 10 technical journaland conference papers and holds over 10 patents invarious transmission fields. He was appointed a Bell LabsFellow in 2007 and Alcatel-Lucent Italia Fellow in 2008.
JUERGEN RAHN is a member of technical staff within the Alcatel-Lucent Optics/Cross Connects/R& D/Architecture organization in Nürnberg,Germany. He received a Diplom-Ingenieur(FH) degree in electrical engineering at theHochschule für Technik Bremen, Germany,
and subsequently a degree of Diplom-Ingenieur at theUniversity of Bremen in electrical communications andhigh frequency techniques. He joined the opticaldevelopment (at that time TeKaDe) in 1982. His currentarea of interest is networking of high capacity opticaltransport systems, and in this role he representsAlcatel-Lucent in OTN standardization and also aseditor of OTN standards including G.798, OTNequipment, G.873.1, OTN linear protection, andG.8251, OTN synchronization.
192 Bell Labs Technical Journal DOI: 10.1002/bltj
STEPHEN TROWBRIDGE is a consulting member of technical staff within the Alcatel-LucentOptics Product Organization ChiefTechnology Office (CTO) in Boulder,Colorado. He received his Ph.D. in computerscience from the University of Colorado at
Boulder and has been with Alcatel-Lucent, originallyhaving joined AT&T Bell Laboratories, for over 30 years.He has been contributing to global standards since1995 and has been a key transport networkingstandardization leader across ITU-T, IEEE 802, ATISOPTXS, and OIF. He is the chairman of ITU-T workingparty 3/15, responsible for transport network structures(including SDH, OTN, ASON, and packet transport). Heis vice chairman of the ITU-T telecommunicationstandardization advisory group, chairman of the ATISOPTXS-OHI (optical hierarchal interfaces) workinggroup, and a member of the editorial team for the IEEEP802.3ba (40 Gb/s and 100 Gb/s Ethernet) project. Hehas authored numerous papers and conferencepresentations including High Speed Ethernet Transport(IEEE Communications Magazine, December 2007) and served as co-author for a chapter within AComprehensive Guide to Optical Networking forProfessionals (Springer, 2006). He has helped fostercooperation across standards organizations bydeveloping the procedure for handling liaisonstatements to and from the IETF (RFC 4053/BCP 103).
EVE L. VARMA is director of standardization within the Alcatel-Lucent Optics Product OrganizationChief Technology Office (CTO) in Murray Hill,New Jersey. She received an M.A. degree inphysics from the City University of New Yorkand has been with Alcatel-Lucent, originally
having joined AT&T Bell Laboratories, for 30 years. Shehas been contributing to global standards since 1984and continues to be actively engaged in supporting thedevelopment of specifications relevant to transportnetworking solutions within global standards andindustry fora spanning ITU-T, IETF, and OIF. Previousresearch experience includes specification of transmis-sion jitter requirements, optical transport and its controland management, and associated enabling technologyand methodology evolution. She has co-authored twobooks, Achieving Global Information Networking,Artech House (1999), and Jitter in Digital TransmissionSystems, Artech House (1989), and co-authored twochapters in A Comprehensive Guide to OpticalNetworking for Professionals, Springer (2006). She is aBell Labs Fellow and a member of the Alcatel-LucentTechnical Academy. ◆
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