◆ 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

OCC OCC OCC

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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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