Global Standardization Activities

Standardization Activities on OpticalNetworking
Naohide Nagatsu† and Kazuhiro Oda
Abstract
This article overviews global standardization activities on next-generation optical networking, which
has been promoted in ITU-T SG15/WP3. Work on OTN (Optical Transport Network) and ASON
(Automatic Switched Optical Network) and their intended objectives are introduced, highlighting therelationship between ASON and GMPLS (Generalized MultiProtocol Label Switching), which is undergoing standardization in IETF (Internet Engineering Task Force).

1. Introduction
The amount of Internet traffic has been explosivelyincreasing. In Japan, for instance, the amount of traffic handled at an Internet Exchange is growing at therate of 150% or higher every six months [1]. Such asignificant growth in IP-based traffic in Japan iscaused by the recent rapid penetration of inexpensive
but bandwidth-abundant Internet access using ADSL(Asynchronous Digital Subscriber Line). Furthermore, taking into account the possible growth ofFTTH (Fiber-To-The-Home) services, which are
already available, the amount of IP-based traffic is
expected to continue to grow, maintaining a pace ofapproximately trebling every year. It must also berecognized that IP-based traffic exhibits rapidchanges in its distribution pattern, which may occasionally be associated with social events and otherincidents, and that may severely impact the quality ofservice (QoS). There is therefore a need for network
operators to deploy a robust and future-proof IP backbone into their networks. The aim of the optical network is to meet such a requirement. One of the key
technologies of the optical network is WDM (Wavelength Division Multiplexing), and ITU-T plays themost important role in standardizing the specifications of optical network systems that employ WDM
technologies. The coverage of ITU-T’s concern has† NTT Network Innovation Laboratories
Yokosuka-shi, 239-0847 Japan
E-mail: nagatsu@exa.onlab.ntt.co.jp

Vol. 1 No. 4 July 2003

been extensively expanded to deal with the networking aspects of WDM-based optical networks, which
SG15/WP3 has the responsibility to standardize. The
particular task of SG15/WP3 is to specify requirements, network/functional architecture, signal format
and multiplexing structure, protection/restoration
mechanisms, and network element management andcontrol architecture/functionality/protocols, all ofwhich are necessitated by optical networking, wherein the term “OTN”, rather than “optical network”, andASON are used. The OTN is assumed to use management systems such as EMS (Element Management System) and/or NMS (Network ManagementSystem) for its operation, while ASON also employs
control plane functions, which can be similar to thosethat IP routers provide. The ASON work in
SG15/WP3 is closely related to GMPLS work in
IETF. This article therefore overviews the recentstandardization activities on OTN and ASON in ITUT SG15/WP3, and then makes some remarks regarding the relationship between ASON and GMPLS.
2. OTN standardization
2.1 OTNIn response to the rapid increase in the amount ofIP-based traffic, so-called DWDM (Dense WDM)
systems have been deployed in existing networks. Ina DWDM system, several tens of wavelengths are
multiplexed into an optical fiber (each wavelength
can be a bearer at 2.5 Gbit/s, 10 Gbit/s, or higher).
Since DWDM systems, can only enhance the capaci65

Global Standardization Activities

ty of point-to-point links, electrical processing using
a lower speed switching regime such as IP packet
routing or lower-order SDH (Synchronous DigitalHierarchy) path cross-connection is required at every
node, where a bunch of traffic, comprising streams
that have different source and destination node pairs,
is aggregated to form a 2.5- or 10-Gbit/s signal. This
point-to-point nature of the DWDM system makes it
difficult to achieve cost-effective transport of IPbased traffic between two remote nodes across multiple intermediate nodes, if gigabit-class direct traffic isprovisioned between them. An OTN is intended tooffer an end-to-end connection to convey such highbit-rate traffic directly. This direct connection isdenoted as an “OCh” (Optical Channel)*1. An OChcorresponds to a wavelength path and is routed byOXCs (Optical Cross-Connects) or OADMs (OpticalAdd/Drop Multiplexers) along its route to offer adirect connection between two remote nodes (Fig. 1).
There is no low-speed regime switching at each intermediate node, wherein only the necessary portion ofelectrical switching or termination equipment needs
to be installed to process only the added or dropped
traffic. This significantly reduces the necessary scaleor amount of electrical equipment at each node.
The architecture of OTN has been standardized in
Recommendation G.872 (1st version Feb. 02). This
recommendation specifies the functional architecture*1 OCh is defined such that it is characterized by physical parameters, and as such it is intended to be all-optical between its two termination points. Current optical technologies, however, may notallow for all-optical end-to-end OCh networking; therefore, a specific implementation based on a digital frame was also standardized, using different terminologies from OCh. In this article, theterm “OCh” is used in a general sense that it may hold such a particular implementation.

Optical cross-connect

SDH equipment
IP router etc.

Optical fiber(WDM link)of OTN layers, management capability of the OTN
layers, connection supervision techniques, protection
architecture, interworking principles, and so forth.
Note that, from a functional viewpoint, an OTN isdivided into three layers including the OCh that provides end-to-end networking capability.
There were extensive discussions as to the conceptof OTN and in particular on the capability of OCh in
the early stage of G.872 work. NTT significantly contributed to the development of the current OTN concepts. The management capabilities at the OCh level,
as defined in G.872, are assured by introducing a digital frame structure for it. Recommendation G.709(1st version Feb. 01) specifies the signal structure of
the OTN layers including the digital frame structurefor OCh. Three classes of bit rates for this digitalframe have been specified, i.e., approx. 2.5, 10, and40 Gbit/s, to allow for various mapping of digitalclient signals such as STM-N (Synchronous Transport Module level N, where N=16, 64, and 256),
ATM-VP (Asynchronous Transfer Mode - VirtualPath), GFP (Generic Framing Procedure) packets,and other CBR (Constant Bit Rate) signals with thesame bit rates as specified above.
2.2 Extensions of OTN standards
From the viewpoint of IP-based networking, GFP
has the potential to be a major client signal of OTN.
It is a brand-new data link protocol that encapsulates
IP-based signals including Ethernet for efficient mapping to SDH/OTN. To date there seems to be no OTN
equipment that enables direct mapping of GFP packets, though GFP itself has already gained considerable popularity in the SDH world. This should be
noted as one of the successful cases in which ITU-T
has led the Telecom industry to a brand-new standard,

No SDH/IP level processing
at this node

OCh #1
OCh #2

SDH/IP levelprocessing
takes placeat this nodeFig. 1. End-to-end OCh networking in OTN.

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Global Standardization Activities

i.e., GFP. In addition to GFP, ITU-T has put forth
great effort to extend the virtual concatenation function, which is truly of great importance in the dataoriented era. LCAS (Link Capacity Adjustment
Scheme) was specified as a mechanism for achieving
dynamic link capacity adjustment according to thechange in the amount of traffic. It varies the link
capacity by changing the number of virtually concatenated payloads according to the requests from themanagement systems. LCAS will soon be widely
available for SDH cross-connect systems. Both GFPand LCAS are expected to play important roles in theOTN.
3. ASON/GMPLS standardization
3.1 ASONAn OTN is operated using centralized management
systems such as NMS or EMS. In general, these are
specifically used to supervise and control a particulartechnology-based network, i.e., OTN, SDH, and soforth. On the other hand, a transport network has alayered structure to accommodate these different
technology-based networks. Therefore, multiple
management systems, each specific to a particulartechnology-based network or type of equipment, areoperated at the same time in existing networks. To
provide a connection across different technologybased networks, it is essential to maintain the database of each network or set of equipment so that theircorrelation can be fully ensured. To date, however,most of such correlation work entails complicatedmanual processes, which can only be achieved byskilled operators (Fig. 2 (a)). It is therefore difficult,e.g., in private line services, to instantaneously provi-Higher layer
equipment

Controlsion a connection upon request from a customer,
resulting in the loss of business opportunities. Stimulated by such problems, ITU-T initiated ASON standardization around 1999. Since then, the capabilitiesof IP-like distributed control have been examined toautomate OTN/SDH control by introducing the control plane functions, as can be seen in Fig. 2(b).
ASON is characterized by (1) its instantaneous path
provisioning capability, (2) more flexible failure
recovery scheme attained by IP-like distributed control rather than the conventional protection scheme,and (3) synergy of centralized and distributed control
schemes. The first application of ASON is thought tobe the SPC (Soft Permanent Connection), which isinstantaneously setup/released using the control
plane upon request initiated by the management system.
It is of great importance to specify how to create thecontrol plane that performs the call and connection
control functions (Note that, through signaling, thecontrol plane establishes and releases connections,and may restore a connection in the case of a failure).
The work on ASON had been promoted on the aspectof the architecture of the control plane and its requirements, and as a consequence Recommendations
G.807 and G.8080 were approved in May and October 2001, respectively. Recommendation G.807 specifies high-level requirements that are applicable toother automatically switched transport networks aswell as ASON, whereas G.8080 specifies the set ofcontrol plane components that are used to manipulatetransport network resources to provide the functionality of establishing, maintaining, and releasing connections. The focus of ASON work has been expanded to cover the details of specific control plane func-

Higher layer
equipment

Higher layer MS

Control (less-functionalized)
Higher layer MS(*)

Manual correlation

SDH
equipment

SDHMS

SDHequipment

OTN
equipment

OTN MSOTN
equipment

MS: Management System(a) OTN/SDH with centralized control

Control

Control plane

Multiplexing, mapping

Automated correlation
SDH MSOTN MS(*) GMPLS does support this, while ASON does not.
(b) GMPLS/ASON with distributed control

Fig. 2. OTN/SDH and GMPLS/ASON.

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tions such as signaling, auto-discovery, routing, link
resource management, and connection admission
control functions.
3.2 GMPLS and ASON
The original intention of ASON standardization
work was to develop protocol independent specifications. This approach allows for various protocol specific implementations, and specifically implies that
the PNNI (Private Network to Network Interface), aprotocol suite for ATM signaling/routing standardized in the ATM Forum, can also be reused for theASON control plane. Taking into account the need toprovide concrete specifications to industry, however,
ITU-T subsequently developed protocol specific recommendations by translating its generic signaling
specifications to the specific protocols such as RSVPTE (Resource reSerVation Protocol with Traffic
Engineering extension), CR-LDP (Constraint-basedRouted Label Distribution Protocol), and PNNI. Notethat the first two protocols, i.e., RSVP-TE and CRLDP, are derived from the MPLS paradigm describedbelow.
In IETF, considerable effort has been devoted to thedevelopment of label switching standards to realize
effective packet forwarding mechanisms, and MPLS
is the resultant standard reached by IETF consensus.
MPLS brings connection-oriented features to thepacket-based IP network, and this consequently
enables sophisticated networking in IP-based networks such as QoS control, routing control, and VPN(Virtual Private Network). Its connection-oriented
feature as above may be regarded as the driving forceof the current MPLS paradigm rather than its original
concept, i.e., effective packet forwarding. GMPLS isan extension of MPLS, and its standardization wasinitiated in response to the proposals from NTT andothers that presented the concept of Photonic MPLS,
also known as MPLambdaS where “Lambda” stands
for wavelength or OCh, in early 2000 at IETF andOIF (Optical Internetworking Forum). These original
concepts have been more generically extended to thecurrent GMPLS, where MPLS-based control can beapplied to transport entities other than the packet, i.e.,
TDM (Time Division Multiplexing) time slot, wavelength, wavelength band, and optical fiber. The implications of the TDM time slot and wavelength are theapplication of MPLS to SDH virtual containers andOTN OCh, respectively.
Currently, IETF’s GMPLS has attracted muchattention from both the IP and Telecom industries,and efforts on bilateral collaboration between IETF
68

and ITU-T have been examined. This may lead tosuccessful updates of each standard, and in fact, todate, some of the functional requirements raised byITU-T ASON have drawn much attention from IETF,
and vice versa (Note that ITU-T has also examined
RSVP-TE and CR-LDP in their standards as previously stated).
4. Conclusion
This article overviewed recent standardization
activities on optical networking. OTN andASON/GMPLS are keys to optical networking, andas such continuous efforts should be made toward
their standardization.
Reference[1]

http://www.jpix.ad.jp/en/techncal/traffic.html

Naohide Nagatsu
Senior Research Engineer, Photonic Transport
Network Laboratory, NTT Network Innovation
Laboratories.
He received the B.E. and M.E. degrees in electrical and information engineering from Yokohama National University, Kanagawa, in 1990and 1992, respectively. Since joining NTT
Transmission Systems Laboratories in 1992, hehas been engaged in the research of photonic networking, in particular, optical path network planning and dimensioning. In 1997, he received theYoung Investigators Award from the Institute ofElectronics, Information and Communication
Engineering (IEICE) of Japan. He began to contribute to ITU-T SG 13 on OTN architecture in1997, and is now active in SG15 on OTN andASON. He is a member of IEICE and IEEE, andis currently a visiting researcher at University
College London, UK.

Kazuhiro Oda
Senior Research Engineer, Second Promotion
Project, NTT Network Service Systems Laboratories.
He received the B.E. degree in electronics
engineering from Fukui University, Fukui, in1985. He also received the Ph.D. degree from
Osaka University, Osaka, in 1996. In 1985, hejoined Yokosuka Electrical Communication
Laboratories of NTT. He has been engaged in the
research of dense wavelength-division-multiplexing (DWDM) transmission, and has been
involved in the standardization activities in theITU-T SG15 on SDH, OTN, ASON, and Ethernet Transport Network. In 1997, he received theOECC ‘97 Best Paper Award by his paper, “First
demonstration of 1580nm wavelength band
WDM transmission”. He is a member of IEICEof Japan and IEEE LEOS.

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