Computer Networks 52 (2008) 1975–1987

Contents lists available at ScienceDirect

Computer Networks

journal homepage: www.elsevier .com/ locate/comnet

Introducing OMS protection in GMPLS-based optical ring networks

Luis Velasco *, Salvatore Spadaro, Jaume Comellas, Gabriel JunyentOptical Communications Group, Universitat Politècnica de Catalunya (UPC), C/Jordi Girona, 1-3 D6-107, 08034 Barcelona, Spain

a r t i c l e i n f o

Article history:Received 1 June 2007Received in revised form 12 February 2008Accepted 13 February 2008Available online 21 March 2008

Keywords:ASON/GMPLSROADMLMPOMS dedicated protectionOMS shared protection

1389-1286/$ - see front matter � 2008 Elsevier B.Vdoi:10.1016/j.comnet.2008.02.022

* Corresponding author. Tel.: +34 93 401 69 99.E-mail addresses: luis.velasco@tsc.upc.edu (L. V

upc.edu (S. Spadaro), comellas@tsc.upc.edu (J. Comupc.edu (G. Junyent).

a b s t r a c t

Legacy ring-based networks have been deployed in conjunction with SONET/SDH technol-ogy to provide survivability to transport networks, and they achieve service recoverywithin 50 ms after fault detection. Current generalized multiprotocol label switching(GMPLS)-controlled optical transport networks need efficient resilience mechanisms toallow recovery times equivalent to those granted by SONET/SDH networks. In this paper,dedicated and shared optical multiplex section (OMS) protection systems are proposed.Both solutions consist of a mechanism based on extensions of the GMPLS link managementprotocol (LMP) to properly manage the protection actions, and both utilize a new reconfig-urable optical add/drop multiplexer (ROADM) design to support the protection schemes.The performance of both solutions has been experimentally evaluated.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Legacy SONET/SDH ring-based networks are well-knownfor their inherently fast protection switching capability,which allows service recovery within 50 ms after faultdetection [1]. An interruption of 50 ms or less in a transmis-sion signal is perceived by higher layers as a transmission er-ror. It may cause a packet retransmission handled by TCP/IPat the IP layer, but no TCP sessions will be affected at all. InVoIP applications, users do not perceive 100 ms outages[2]. A complete discussion about the 50 ms figure can befound in [3].

The introduction of OADMs into transport networks al-lows them to be configured in ring-based topologies simi-lar to traditional SONET/SDH networks. An OADM allowsthe dropping of a specific wavelength out of the bundleof dense wavelength division multiplexing (DWDM)-mul-tiplexed signals and the addition of another channel onthe same wavelength. OADMs are currently evolving intoreconfigurable (and remotely controlled) devices, paving

. All rights reserved.

elasco), spadaro@tsc.ellas), junyent@tsc.

the way for future flexible optical networks. In fact, dy-namic optical rings using ROADMs play a crucial role inthe migration to the ASON/GMPLS paradigm [4,5]. An auto-matically switched optical network (ASON) [4] is an opticaltransport network that has dynamic connection set up/teardown capability. This functionality is accomplished bymeans of a control plane that carries out, among otherthings, routing and signaling functions. GMPLS is a tech-nology that provides enhancements to MPLS to supportswitching capabilities not only at the packet level but alsoat the time slot, wavelength, or even fiber levels [5]. GMPLSprovides a suitable control plane for dynamic optical net-works. It includes the traffic engineering (TE) extensionsof RSVP-TE [6] for signaling and the intra-domain link-state OSPF-TE [7] for routing.

The use of DWDM technology implies a very large num-ber of parallel links (i.e., hundreds or even thousands ofwavelengths if multiple fibers are used) between two adja-cent nodes. The manual configuration and control of such ahuge amount of resources becomes impractical. The linkmanagement protocol (LMP) [8] has been specified to re-solve this issue. The LMP specification defines two core pro-cedures, namely the control channel management and thelink property correlation [8]. The former procedure covers,among other functionalities, the maintenance of an IP

1976 L. Velasco et al. / Computer Networks 52 (2008) 1975–1987

control channel between each pair of neighboring LMPnodes. It monitors the periodic exchange of Hello messagesbetween neighbor optical node connection controllers(OCC) to confirm that the control channel is operational.The latter procedure can be used for fault management incases of failure.

Currently, the GMPLS recovery framework covers onlyoptical channel (OCh) resilience [9,10]. However, opticalmultiplex section (OMS) protection schemes allow recoveryof the complete bundle of DWDM channels in a fiber withjust one protection action. This paper focuses on OMS pro-tection for GMPLS-controlled optical ring networks.

Previous work in the literature regarding protection atthe OMS level must be taken into consideration. In [11],different architectures for resilient ring and mesh basedoptical networks are described and compared. In [12], dif-ferent optical protection ring architectures are described.In particular, OCh shared protection rings (OCh-SPRing)including node architecture designs are discussed in detail.In [13], the authors propose a dedicated protection mech-anism based on extensions to the RSVP-TE protocol forfault location and notification. The proposed mechanismcan be applied to small metropolitan networks. This solu-tion is lacking in terms of scalability, however, duringextrapolation from the obtained protection time (45 msfor a three-node optical ring with links of 35 km) to largeroptical rings.

In this paper, we focus on dynamic optical rings sup-porting either dedicated or shared link protection (hereaf-ter OMS DPRing and OMS SPRing, respectively). The OMSDPRing scheme is deployed over two-fiber unidirectionalrings. One fiber is dedicated to the working traffic, whereasthe other is reserved for protection. The OMS SPRingscheme is deployed over two-fiber bidirectional rings.The total capacity of each fiber is thus divided in two wave-bands: one waveband is reserved for transporting workingchannels, and the other is used for transporting protectionchannels. Working and protection channels share each fi-ber in this case.

We propose and evaluate complete solutions for build-ing ring-based dynamic optical networks with OMS DPRingand OMS SPRing protection capabilities. Both proposalsconsist of: (1) a novel GMPLS automatic protection switch-ing (GAPS) mechanism that coordinates the protection ac-tions after failures and (2) a new reconfigurable opticalROADM design to support OMS protection. The perfor-mance of both solutions has been experimentally evaluatedover the ASON/GMPLS CARISMA network test-bed [14].

Some other work related to protection has also beenconsidered. In [15], the concept of differentiated reliability(DiR) was introduced. A reliability degree was assigned toeach individual connection irrespective of the underlyingprotection mechanism. In our proposal, all of the networklightpaths have the same priority. In [16], a routing algo-rithm with shared-risk link groups (SRLG) disjoint protec-tion for mesh networks was presented. In general, linkfailure dependency is an important factor to be consideredwhen calculating disjoint routes. However, we assume thatthe optical topology has been designed during the planningphase in such a way that no common infrastructure (e.g.,optical cables or conduits) is used by any two links in the

network. The authors in [17] provide a framework forwaveband switching (WBS). In WBS, wavelengths aregrouped into bands and switched as single entities. Thus,a waveband is an intermediate entity between fibers andwavelengths. Our solution for OMS SPRing is based on sep-arating wavelengths into two bands, one for working andone for protection. When a failure occurs, working and pro-tection bands are switched. Our solution for OMS DPRing,on the other hand, is based on fiber switching.

The remainder of the paper is organized as follows. Sec-tion 2 provides an overview of protection mechanisms forring-based networks. In Section 3, an availability model forlightpaths that is useful for comparing the performance ofthe proposed schemes is presented. Section 4 is devoted tothe basic description of the GAPS mechanism. In Section 5,the design for the utilized optical nodes is presented. Someexperimental results are presented in Section 6. Finally,Section 7 draws the main conclusions of this work.

2. Protection mechanisms for ring-based networks

Failures at the optical layer have a high impact on over-all optical network performance due to the high bandwidthavailable per wavelength and the number of wavelengthsper fiber. For example, fiber cuts resulting from diggingworks or the failure of individual transmitters or receiversare quite common [18].

In ring-based dynamic optical networks, protection atthe optical layer can be implemented at either the OMSor OCh layer. At the OCh layer, the protection action is per-formed when the ROADMs inject/extract the selectedwavelength to/from the ring network. At the OMS layer,the protection action is performed by the ROADMs adja-cent to the failure. OMS protection is more appropriatefor fiber failures, whereas OCh protection is adequatewhen a single channel fails. A general comparison in termsof cost, availability, and recovery time of different networkarchitectures can be found in [11]. With the OCh schemes,it is possible to mix protected and unprotected traffic; incontrast, OMS schemes are more rigid because all channelsare simultaneously recovered. On the other hand, if most ofthe channels need to be protected in some subareas of thenetwork (typically, on the core network), OMS schemes area good option. For example, in a network where all light-paths are protected, OMS protection allows recovery ofall connections with only one action. On the contrary,OCh protection requires one action for each impactedlightpath. This hardly increases the quantity of signalingmessages required, as well as the optical node complexity.

The protection scheme can be dedicated (dedicated pro-tection ring, DPRing) or shared (shared protection ring,SPRing). Although the shared protection scheme consumesfewer resources than the dedicated approach, its imple-mentation and management are typically more complex[11,19].

A detailed classification of resilience schemes for ringnetworks including both OCh and OMS layers can be foundin [3] and [19]. In the next two subsections, we introducetwo OMS protection schemes for ring-based dynamic opti-cal networks.

L. Velasco et al. / Computer Networks 52 (2008) 1975–1987 1977

2.1. OMS dedicated protection ring (OMS DPRing)

OMS DPRing consists of two counter-rotating unidirec-tional rings, each of which transmits in an opposite direc-tion relative to the other (Fig. 1a). Only one fiber isdedicated for working traffic, and the other is reservedfor protection. Both flows of a bidirectional lightpath arerouted on different sides of the ring using the same wave-length. There is thus no possibility of reusing wavelengthson the ring for different lightpaths. Therefore, the maxi-mum capacity that can be allocated on the ring is limitedto the capacity of a single link. When a link failure occurs,it is detected by the two optical nodes adjacent to the fail-ure. Both nodes loop back the bundle of optical channels onthe protection ring in the opposite direction (dashed linesin Fig. 1b). To perform and manage efficient switching tothe protection fiber, an automatic protection switching(APS)-like protocol [1] is required.

2.2. OMS shared protection ring (OMS SPRing)

In OMS SPRing, the total capacity of each fiber is dividedin two wavebands (B1, B2) as shown in Fig. 2a. One wave-band on each fiber (B1 clockwise and B2 counter-clockwise)is reserved to transport working channels, whereas theother is used to transport protection channels. In theOMS SPRing scheme, therefore, working and protectionchannels share each fiber. Working connections in one fi-

F E D

A B C

WorkinProtec

a b

Fig. 1. An OMS DPRing transporting one lightpath (a

F E D

B CA

WorkingProtection

BB

a b

Fig. 2. An OMS SPRing transporting two lightpaths (a

ber are protected by the available capacity in the other fi-ber in the opposite direction of the ring. This way, nowavelength converters are needed when channels aremoved from working to protection bands.

Both directions of a bidirectional lightpath are routedalong the same side of the ring in different fibers. The samewavelength can therefore be reused to accommodate aconnection between other nodes whose routes do notoverlap the existing connection (connections A–D and E–F in Fig. 2a).

When a link (or node) failure is detected at the OMS le-vel, the nodes adjacent to the failure will loop back alllightpaths at once on the protection channels of the ring(Fig. 2b). Similar to the OMS DPRing scheme, an APS-likeprotocol is required to manage the switching actions andensure the correct use of the shared protection capacity.

Although the implementation of OMS SPRing is morecomplex than that of OMS DPRing, it provides better band-width efficiency. For example, the maximum number ofprotected lightpaths that can be transported in an n nodering with OMS DPRing is limited to the number of wave-lengths available in each link (e.g., W). On the contrary,the maximum number of protected lightpaths that canbe transported using OMS SPRing depends on the trafficpattern. In our example, it ranges from W for hub-like traf-fic (one node sources all traffic) to a maximum of Wn/2 forthe case where the nodes only send traffic to their adjacentnodes.

F E D

A B C

gtion

) before and (b) after a failure in the link B–C.

F E D

B CA

12

) before and (b) after a failure in the link B–C.

1978 L. Velasco et al. / Computer Networks 52 (2008) 1975–1987

3. Availability model for OMS protection schemes

When comparing different protection schemes in opti-cal networks, a crucial aspect involves the lightpaths’ avail-ability. In this section, we define an availability model forthe OMS DPRing and OMS SPRing schemes. The aim of thismodel is to justify that a protection scheme is strictlyrequired.

Generally speaking, availability is the probability that asystem will be found in the operating state at a randomtime in the future. Steady state availability can be ex-pressed as [3]:

A ¼ UpTimeUpTimeþ DownTime

� MTTFMTTFþMTTR

ð1Þ

where

� MTTF is the mean time to failure, the expected time tothe next failure of the network component followingcompletion of the repair. MTTF is usually expressed inhours or in FITs (number of failures in 109 h).

� MTTR is the mean time to repair, the expected timeneeded to repair the network component.

The probabilistic complement of the availability A isunavailability (U), defined as

U ¼ 1� A ð2Þ

For the purpose of availability analysis purpose, let us con-sider the figures shown in Table 1 for the MTTF and MTTR[18,20]. In long-haul networks, the system componentswith the highest failure rate are the optical cables (Table1). Therefore, the availability model can be accurately esti-mated when taking into account only link failures.

Let us denote Ex and Ex as an event and a negate event,respectively, associated with a functional element or sys-tem x. In our study, Ex ðExÞ implies that x is (not) operatingat the time t independent of the past history of events.Thus, PfExg represents the x availability ðAxÞ and PfExg itscorresponding unavailability ðUxÞ.

In an OMS DPRing, the lightpaths’ availability is givenby the union of two disjoint groups of events, namely:(1) all links i in the ring are available and (2) one link inthe ring is unavailable but the rest of the links are availableand can be used for ring protection. In OMS DPRing, light-paths use resources in every link of the ring as shown inFig. 1. Therefore, ring and lightpath availability are coinci-dent, and this value is given by the following mathematicalexpression:

Table 1MTTF and MTTR values

Optical node failure rate 10,867 FITsFiber-optic cable failure rate 311 FITs/kmPlug-replacement equipment MTTR 2 hFiber-optic cable MTTR 12 h

PfEDPRingg ¼ PfElightpathg

¼ P\8i2ring

Ei [[8i2ring

Ei \\8j6¼i

i;j2ring

Ej

0BB@

1CCA

8>><>>:

9>>=>>;

ð3Þ

Generally speaking, optical links may be supported bycommon cables or conduits, and thus links in the ringmay fail dependently [16]. When long-haul core networksconnecting main cities are deployed, however, the plan-ning phase must address this issue by choosing fibers sup-ported by a disjoint infrastructure. In the present study,therefore, we consider the links in the ring to be mutuallyfailure-independent and can express availability in OMSDPRing as

ADPRing ¼ Alightpath ¼Y8i2ring

Ai þX8i2ring

Ui �Y8j6¼i

i;j2ring

Aj

0BB@

1CCA ð4Þ

As an example, we calculate the availability for the light-path A–D in the OMS DPRing network shown in Fig. 1aassuming that all links have the same length (300 km).Using the values given in Table 1, the availabilitywill be:

ADPRing ¼ A6link þ 6UlinkA5

link ¼ 99:9981% ð5Þ

Availability figures close to 100% are difficult to compare.For this reason, we will use the unavailability figure.Applying (2), the lightpath A–D unavailability is

UDPRing ¼ 1:88E� 5 ð6Þ

This is equivalent to saying that the A–D lightpath will beunavailable, on average, for 9.86 min per year over theOMS DPRing.

We can also use (4) to calculate the lightpaths’ availabil-ity in OMS SPRing, taking into account that the network isbidirectional and lightpaths will be routed strictly throughthe shortest route in this case. In OMS SPRing, therefore,each lightpath has a different availability depending on itsroute. According to this, we can express the lightpath avail-ability over an OMS SPRing as

AlightpathSPRing ¼

Y8i2lightpath

Ai þX

8i2lightpath

Ui �Y8j 6¼i

i2lightpathj2ring

Aj

0BBB@

1CCCA ð7Þ

In this case, the unavailability for the lightpath A–D in theOMS SPRing shown in Fig. 2a is

UA—DSPRing ¼ 1� ðA3

link þ 3UlinkA5linkÞ ¼ 1:50E� 5 ð8Þ

The A–D lightpath will be unavailable, on average, 7.89 minper year over this OMS SPRing.

L. Velasco et al. / Computer Networks 52 (2008) 1975–1987 1979

On the basis of the previous results, we can say thatOMS shared protection provides better lightpath availabil-ity than OMS dedicated protection. OMS shared protectionmakes it possible to find shortest routes for the lightpaths.This is opposite to the path protection case, where dedi-cated path protection provides better lightpath availabilitythan shared path protection due to the fact that the pro-tecting route is shared by several lightpaths. In fact,‘shared’ in OMS protection refers to the fact that workingand protection resources share one fiber; each opticalworking channel has been assigned a backup optical chan-nel in the protection capacity.

Fig. 3 shows the unavailability of the longest possiblelightpaths in an OMS SPRing (solid lines) and OMS DPRing(dashed lines) as a function of the number of nodes (n) inthe ring for several average link lengths (L).

The target lightpath availability in a network has to bechosen according to the distances in that network. Inmetropolitan networks, an availability objective of0.99999 (five nines) or 5.26 min/year of total outage issometimes referred to as the availability objective. Inlong-haul core networks, however, a four nines availabilityobjective (less than 53 min/year of total outage) is moreappropriate. Note that the values of Ulink range from 10�4

to 10�3 for lengths ranging from 30 to 300 km, respectively,in Table 1. Thus, the graph in Fig. 3 also draws the unavail-ability objective of 10�4, which corresponds to a targetavailability of 0.9999.

From Fig. 3, we can conclude that the maximum overalllength that allows us to meet the strict unavailabilityobjective is roughly 3800 km for the OMS DPRing schemeand roughly 4500 km for the OMS SPRing scheme. TheOMS SPRing scheme provides an improvement of about25% in the expected lightpaths’ unavailability over theOMS DPRing scheme.

If no protection scheme is implemented or applied (1)to a ring network whose length is 3800 km, an unavailabil-ity of 1.4 � 10�2 would be found. This implies more than 5days/year (or 20 min/day) of total outage, which clearlydoes not meet the required network availability. Therefore,a protection scheme is strictly required.

1 E-06

1 E-05

1 E-04

1 E-03

4 6 8 10

Exp

ecte

d U

nav

aila

bili

ty (

U)

Number o

U(SPRing) (L=100 Km) U

U(SPRing) (L=200 Km) U

U(SPRing) (L=300 Km) U

U lim

Fig. 3. Lightpaths unavailability in OM

Finally, let us analyze the behavior of OMS schemes in amultiple failure scenario. In OMS DPRing, all lightpaths willbecome unavailable under a double-link failure since light-paths have the same route through all links in the ring. Thisis different in OMS SPRing, since each lightpath may have adifferent route. One lightpath will remain working under adouble-link failure if the failure affects links that do notsupport a given lightpath; in all other cases, the lightpathwill become unavailable. As an example, let us considerthe OMS SPRing in Fig. 2a, where links A–F and F–E failsimultaneously. In this case, lightpath F–E will becomeunavailable whereas lightpath A–D will remain working.

Eqs. (4) and (7) can be used to calculate the expectedlightpath availability under any arbitrary number offailures.

4. GMPLS-controlLed OMS protection

In this Section, we present the GAPS mechanism. Thismechanism is based on extensions of the LMP protocol thatcan be used for fault management purposes in OMS protec-tion. We first introduce the mechanism to control the OMSDPRing protection scheme, and then we extend the GAPSmechanism to control the OMS SPRing scheme. Protectiontime models for GAPS-controlled OMS protected rings aredefined.

4.1. The GAPS mechanism

As stated above, the OMS DPRing configuration consistsof two counter-rotating rings. In the normal state (Fig. 4a),the working links in the transport plane carry regular traf-fic. When a network component fails, a switch event occursand the working link is protected using backup links. Let usassume that OMS DPRings are remotely controlled by aGMPLS control plane that can be transported out-of-bandin-fiber or out-of-fiber [5]. For the sake of simplicity, weassume that the topology of both the control and transportplanes is the same (Fig. 4a), although the GAPS mechanismwould work for any control plane topology independent ofthe transport plane.

12 14 16 18 20

f nodes in the ring (n)

(DPRing) (L=100 Km)

(DPRing) (L=200 Km)

(DPRing) (L=300 Km)

S DPRing and in OMS SPRing.

Fig. 4. OMS DPRing controlled by the GAPS mechanism.

1980 L. Velasco et al. / Computer Networks 52 (2008) 1975–1987

The link failure implies a Loss of Light (LoL) detection.After the detection, the failure must be corrected by itsadjacent transport nodes. These nodes, called switchingnodes, use the bridge and switch actions for the protectionof the working link (Fig. 4b). Specifically, when an opticalnode detects a LoL, it notifies the failure to its correspond-ing OCC in the GMPLS-based control plane; this OCC be-comes the head end. It conveys the failure detection tothe OCC (tail end) corresponding to the other adjacentoptical node, which executes a bridge. GAPS messages in-clude the information depicted in Table 2.

To illustrate how the GAPS mechanism works, Fig. 5shows the recovery from a link failure. The initial state ofthe ring is the normal state. In this state (T0), all OCCs inthe ring have exchanged normal state messages 1–4 withtheir neighbors.

At time T1, node A detects a LoL on its working link andnotifies its OCC. When an OCC receives notification of fail-ure detection, it sends a switching request (i.e. GAPS mes-sages) to the OCC of the adjacent node over the controlnetwork on both the short and long paths. The short pathconnects the head and tail OCCs directly, whereas the longpath connects them through intermediate OCCs using theopposite side of the ring. Node A then becomes a switchingnode, and its OCC becomes the head end. The head end OCCsends a bridge request. All intermediate OCCs on the longpath enter the full pass-through state. OCC D, upon receiv-ing the bridge request from OCC A on the short path, trans-

Table 2Information transported by GAPS messages

Source/destinationNode ID

Identifies the origin/destination nodes for this GAPS messaDepending on the semantics for the message type, origin adestination nodes represent head and tail nodes or vice ve

Path Indicates whether the path the message is being sent to thshort or long path

mits a LoL ring bridge. OCC D, upon receiving the bridgerequest from OCC A on the long path, executes a bridgeand updates its status. OCC A, upon receiving the ACK fromOCC D on the long path, executes a ring switch and updatesits status. Signaling then reaches the steady-state.

At time T2, the LoL clears. Node A notifies its OCC ofthis, and OCC A enters the wait-to-restore (WTR) stateand advertises its new state to OCC D. Upon receiving theWTR bridge request on the short path, OCC D sends out amessage with the WTR code.

At time T3, the WTR interval expires. OCC A sends out ano request message. OCC D, upon receiving the no requestfrom OCC A on the long path, drops its bridge and gener-ates the Idle code. OCC A, upon receiving the Idle code onthe long path, drops its switch and also generates the Idlecode. All OCCs return then to the normal state.

Since the protection channels are shared among alllinks, contention among the nodes may arise when multi-ple simultaneous failures occur. In these cases, the requestwith the lowest head node identifier (ID) has priority. Thismechanism is useful in OMS SPRing, where some light-paths can continue working in a double-failure scenario.

If in-fiber signaling is used, the GAPS mechanism is ableto efficiently manage node failures. When a LoL is detectedin this case, the detecting OCC includes the identifier of thedestination node in the GAPS message. If the GAPS messagereaches the other end of the failure (i.e., the destinationOCC), it will find itself as the destination OCC. If this occurs,

ge.ndrsa

Requesttype

Indicates the type of request. A request can be acondition (LoL), a state (normal) or an external request(not covered in this paper)

e Status Indicates the status of the protection switch

OCCD

OCCC

OCCB

OCCA

OCCC

T0 1a 1b 2a 2b4a 4b 3a3b

NodePassthrough

NodePassthrough

NodeBridge

NodeSwitch

5a 5b

5b5b

6b

6a6b

6b7b 7a

8a 8b

7b

7b

8b

8b

T1LOL/A

LOL/A6b

6a

LOL/A

LOL/A7b

7a

LOL/D

LOL/D8b

8a

WTR/D

WTR/D9b

9a

NR/D

NR/D

D/S/IDLE

D/L/IDLE

D/S/Br

D/L/Br

A/S/RDI

A/L/Sw

A/S/Sw

A/L/Sw

A/S/Sw

A/L/Sw10b

10a

NR/D

NR/B1b

1a

NR/A

NR/C2b

2a

NR/B

NR/D3b

3a

NR/C

NR/A4b

4a

LOL/D

LOL/D

A/S/IDLE

A/S/IDLE

B/S/IDLE

B/S/IDLE

C/S/IDLE

C/S/IDLE

D/S/IDLE

D/S/IDLE

A/S/RDI

A/L/IDLE5b

5a

LOL receivedEnter switching state

Enter switchingstate

10a 10b

10b10b

NodeNormal

4b 4a

4b

4b

NodeNormal

NodeNormal

NodeNormal

WTR expires

1a 1b

2a 2b

3a3b

T3

LOL clearedWTR starts

WTR Dest.

9a 9b

9b9b

6b

7a6b

6b

T2NR: No RequestLOL: Loss of LightWTR: Wait To Restore

RDI: Remote Defect IndicationBr: BridgedSw: Switched

S: Short pathL: Long path

Retransmitted Message

Generated Message

Node ANode B

Node C

Node D

Fig. 5. Failures management: GAPS messages.

L. Velasco et al. / Computer Networks 52 (2008) 1975–1987 1981

the failure was indeed a link failure as assumed. On thecontrary, a node failure will be assumed if the messagereaches an OCC adjacent to the destination and the desti-nation OCC is unreachable. In the latter, the OCC adjacentto the failure node will act as the destination and assumeits protecting role.

A simplified finite state machine for the GAPS mecha-nism is illustrated in Fig. 6. When in-fiber signaling is used,messages that head and tail end OCCs would exchangethrough the short path are never received. The transitionfrom the normal state to the ring bridge destination statecan be completed either with an intermediate transitionupon reception of the request message through the shortpath or directly upon reception of the request throughthe long path. Receiving the request message through the

short path permits acceleration of the switching processby preparing the optical node. The same can be done inthe intermediate nodes upon reception of a bridge requestdirected to the tail end.

When considering bidirectional rings (OMS SPRing pro-tection), two GAPS entities are needed so that one entity isavailable for each direction. Under normal conditions, theworking wavebands in the transport plane are used to carryregular traffic. When a network component fails, a switchevent occurs and the working wavebands are protectedusing the protection wavebands. A bidirectional link failureimplies LoL detection in the adjacent optical nodes, whichnotify their OCCs in the GMPLS-based control plane ofthe failure (Fig. 7). The adjacent OCCs exchange bridgerequests.

normalring

switching(head)

ring switched(head)

wtr(head)

ring switchdropped(head)

passthrough

ringswitching

(dest.)

ring bridged(dest.)

wtr(dest.)

no request

workingchannel SF

destinationbridged

workingchannel clear

wtr intervalexpires

no request

no request

no request

ring bridge req. (not dest.)

ring bridge req. long path (dest.) ring bridge req.

(short path)

headswitched

wtr bridgerequest

Initialstate

Fig. 6. GAPS mechanism: finite state machine.

Fig. 7. A failure in a bidirectional link is detected by its adjacent nodes.

1982 L. Velasco et al. / Computer Networks 52 (2008) 1975–1987

4.2. GAPS LMP extensions definition

We define GAPS as an LMP extension running in theGMPLS-based control plane of OMS protected networks.In this way, we avoid the implementation of a new controlprotocol that would increase the signaling overhead. Spe-cifically, GAPS relies on control channel management func-tionalities provided by the LMP protocol. Once a controlchannel is activated between two adjacent nodes, theLMP Hello messages exchanged can be used to maintaincontrol channel connectivity between the nodes. In orderto run the GAPS mechanism, however, the definition of anovel LMP message is required:

GAPS Message

hGAPS Messagei::= hCommon Headeri hGAPSiThis message is used to transmit GAPS information

when the LMP adjacency is part of an OMS protected ring.

Table 3GAPS Object Format

The GAPS message (Table 3) contains all the informationneeded by the GAPS protocol.

From a functional point of view, the GAPS agent is lo-cated on the top of two LMP agents (LMP east and west ).This way, GAPS messages can be sent through either theeast or west control channel.

4.3. Protection time Models for GAPS-controlled OMSprotected rings

In this Subsection, we present models for calculatingthe protection time for OMS DPRing and OMS SPRing run-ning with the GAPS mechanism. Our aim is to determinethe switching time requirements to be imposed on theoptical nodes necessary to meet the protection time target.Let us define the protection time ðTDPRingÞ in an OMSDPRing as the interval from the decision to switch to thecompletion of the switching operation at the node initiat-ing the bridge request. It includes, therefore, the notifica-tion from the initiating optical node to its OCC ðTconfigÞ,the propagation delay in each control network link ðT linkÞ,the processing time in each OCC ðTcontrolÞ, the time to con-figure each optical node in the ring ðTconfigÞ to perform theswitching action, and the time to switch itself ðTswitchÞ.Thus, TDPRing can be expressed as

0

25

50

75

4 8 10 12 14 16 18 20

Th

eore

tica

l pro

tect

ion

tim

e (m

s)

Number of nodes in the ring (n)

T(DPRing)(L=100Km)

T(DPRing)(L=200Km)

T(DPRing)(L=300Km)

Objective

6

Fig. 8. Protection time for OMS DPRings.

0

25

50

75

4 10 12 14 16 18 20Number of nodes in the ring (n)

Th

eore

tica

l pro

tect

ion

tim

e (m

s) T(SPRing)(L=100Km)

T(SPRing)(L=200Km)

T(SPRing)(L=300Km)

Objective

6 8

Fig. 9. Protection time for OMS SPRings.

L. Velasco et al. / Computer Networks 52 (2008) 1975–1987 1983

TDPRing ¼ 2Tconfig þ Tswitch þ ð2n� 1ÞTcontrol þ 2ðn� 1ÞT link

ð9Þ

Tswitch is predefined by the switching device. We use aswitch with a response time below 1 ms, which is in linewith the devices currently available on the market. T link de-pends on the ring link lengths (L) and the signal speedthrough the fiber. Note that T link is negligible for metropol-itan ring networks.

The protection time model for GAPS controlling a bidi-rectional ring is different than that defined in (9) for OMSDPRing. In fact, GAPS has been extended in the case ofOMS SPRing to coordinate both protection actions (onefor each direction) to be performed by the nodes adjacentto the failure. We assume that configuration actions areexecuted in the optical node in a serial manner. Let us de-fine the protection time ðTSPRingÞ in an OMS SPRing as theinterval from the decision to switch to the completion ofthe switch operation at the node initiating the bridge re-quest. In this case, TSPRing can be expressed as

TSPRing ¼ 2Tconfig þ Tswitch þ nTcontrol þ ðn� 1ÞT link

þMaxðTconfig; ðn� 1ÞTcontrol þ ðn� 1ÞT linkÞ ð10Þ

The term Maxða; bÞ expresses the idea of configuration ac-tions that are performed in a serial manner in the opticalnode. Note that (10) will give the same values as (9) whenthe time to configure the optical node is higher than thetime to transport the GAPS message around the ring. Thiswill happen if the number of nodes in the ring is low orthe distances between ring nodes are short. In such cases,the protection time in OMS SPRing will be lower than theobjective even though it is higher than that of the OMSDPRing.

Figs. 8 and 9 show the theoretical protection time forOMS DPRing and OMS SPRing, respectively, as functionsof the number of nodes (n) in the ring for several linklengths. They show the scalability of GAPS when the num-ber of nodes in the ring is increased. In this analysis, we as-sume Tconfig to be less than 5 ms and Tcontrol to be about0.2 ms. From the GAPS mechanism, we can conclude thatthe typical target protection time (i.e. 50 ms) is reachedeven when rings are composed of a large number of nodes.However, it implies strict requirements for the hardware ofthe optical nodes (e.g., Tcontrol and Tconfig).

5. ROADM design to support OMS protection schemes

Besides the design of the GAPS mechanism, we have de-signed two new optical nodes (one for OMS DPRings andanother for OMS SPRings) capable of satisfying the require-ments derived in the last section. Optical nodes are basedon two wavelength selective switches (WSS). One of theseis used for adding and the other for dropping the local traf-fic [21].

In the OMS DPRing node (Fig. 10a), two optical powermeters (labeled with M in Fig. 10) measure the incomingoptical power at the east and west ports. Two 2 � 2 opticalswitches have been added to the WSS components to allowOMS protection. The two pairs of optical Mux/demux areresponsible for coupling the WDM-multiplexed bundlewith the in-fiber optical supervisory channel (OSC), whichtransports the control channel. Two 1300 nm optical tran-sponders ðkOSCÞ are used to convert electrical fast Ethernetsignals to the optical domain. Additionally, a node control-ler (not shown in Fig. 10) is needed to manage the extraresources.

The optical power meters monitor the incoming opticalpower levels at the west and east inputs and notify thenode controller upon receiving out-of-bounds levels. Uponreceiving this information, the node controller will send aLoL notification to the OCC. Note that if the link is not af-fected by a failure, optical power must always be receivedat each end of the link. When considering out-of-fibersignaling, the OSC does not transport the control chan-nel. Nevertheless, the associated optical hardware can-not be eliminated because an optical pilot signal is stillnecessary to detect the repair of any failure affecting thelink.

The OMS SPRing optical node is also based on WSS asshown in Fig. 10b. We use WSS components with a capac-ity of 40 channels in the C-band. Although the optical nodeis defined as bidirectional, it is important to highlight that,as in the OMS DPRing optical node, only two WSS are used;this keeps the cost and complexity of the node low. Wehave defined waveband B1 as channels 1–20 and B2 aschannels 21–40.

Under normal conditions, B1 received from east and B2received from west are used to transport the traffic and B2east and B1 west are used for protection. Band splitter (BS)

λOSC

East

West

λOSC

Mx

Mx

Dem

x

Dem

x

M

M

WSS WSS

West

Mx

Dem

x

M

λOSC

East

Mx

Dem

x

M

λOSC

B1

B2

BS

BS

BS

B1

B2

B1

B2

s

s

s

s

s

s

sWSS WSS

B1+B2

a b

Fig. 10. Optical nodes design to support OMS protection. (a) OMS DPRing scheme (b) OMS SPRing scheme.

1984 L. Velasco et al. / Computer Networks 52 (2008) 1975–1987

components are used to divide the WDM bundle into twobands, and splitters (S) are used to separate optical signalsor join both bands. To implement OMS SPRing protection,four 2 � 2 optical switches decide which (B1, B2) bandsare used to transport the traffic and which bands are usedfor protection. The resulting cost and complexity of theOMS SPRing optical node are not much higher than thoseof the OMS DPRing optical node; in fact, only two 2 � 2optical switches and a set of passive components (splittersand band splitters) are added.

The functionality depicted in Fig. 10 has been separatedinto several building blocks. Each block has been imple-mented as a separated card. Therefore, two additional cardsequipped with active components other passive compo-nents (mux/demux, splitters, and band splitters), the tran-sponder and optical switching and monitoring (OSNL)cards, have been added to the WSS components. The OSNLcard includes the monitoring and switching devices. Theswitching device has a response time below 1ms and inser-tion losses lower than 0.9 dB. The monitoring device ex-tracts a small part of the incoming optical power,transforms the sample into a digital value by means of anA/D converter, and stores the converted value in a register.The monitoring sweep time is 10 ls.

Each active card in the optical node is equipped with anARM7 32-bit RISC processor [22] running at 100 MHz. Theprocessor card controls the different cards componentsand manages the communication with the node controllerimplemented in a separate card (the so-called ‘Master card’).

The Master card communicates through an internal se-rial bus with the rest of the optical node cards and througha fast Ethernet interface with the control and managementplanes. The Master card is based on the UNC90 microcon-troller module [23], which is equipped with an ARM9 32-bit RISC processor [22] running at 180 MHz. In additionto other elements, the UNC90 module includes a RISC pro-cessor, 32 MByte SDRAM, and 32 MByte Flash Memory. Theinternal architecture of the optical node is shown in Fig. 11.

The OSNL card processor includes an interrupt-drivensystem to allow the CPU to continue processing instruc-

tions when a request from the Master Card arrives. In themean time, the card processor executes a polling loop tocontinuously read samples from the monitoring register.If the values for the samples read within 1 ms are consid-ered to be out-of-bounds, the card processor declares aLoL condition. This condition has to be communicated tothe Master card by sending a proprietary message throughthe serial bus.

The Master processor runs using a Linux Operating Sys-tem. An application (the node agent) has been developed tomanage the entire optical node by providing an interfacebetween the cards and the control and management planes.The agent listens for incoming data from serial and TCP/UDPports. When a message indicating a LoL condition is re-ceived through the serial port, the agent sends a simple net-work management protocol (SNMP) trap that brings therelated information to the OCC in the control plane.

The agent on the Master card accepts request-responsecommands using an XML-based proprietary protocol.When a message through a TCP/UDP port is received, theagent decodes it and initiates the appropriate communica-tion to another card in the optical node through the serialbus.

As defined in the previous Section, Tconfig is the configu-ration time of the optical node (i.e., the time to process a re-quest from the OCC or inform the OCC of any event). Inorder to achieve a recovery time shorter than 50 ms afterfault detection, we specified 5 ms as the maximum valuefor Tconfig in the previous section. During optimization ofthe system, some bottlenecks were detected and corrected.One of the more important ones relates to the TTY devicedriver architecture in the Linux kernel [24]. Linux considersserial ports to be high latency devices. When data is re-ceived, therefore, the TTY device driver schedules itself topush the data to the user application at some later pointin the near future. This behavior introduces an unaccept-able delay in the system. To avoid this high latency in theserial transmission, the Linux kernel was modified to definethe serial driver as a low latency driver that immediatelypushes the data to the user application, (the node agent).

Fig. 11. Optical node internal architecture.

L. Velasco et al. / Computer Networks 52 (2008) 1975–1987 1985

Fig. 12 shows the physical layout of the resulting OSNLand Master and Transponder Cards. It represents the fron-tal view of the optical node and the test-bed where thecomplete architecture has been tested.

6. Experimental results

The performance of the GAPS mechanism with theROADM nodes discussed in the previous sections hasbeen experimentally evaluated using the ASON/GMPLSCARISMA network test-bed [14]. The CARISMA GMPLScontrol plane uses the RSVP-TE protocol for signaling,the OSPF-TE protocol for routing, and the LMP protocolfor control channel management and link property corre-lation. The OCCs have been implemented using Linux-based routers. Each pair of OCCs communicates througha single IP control channel implemented with full duplexFast Ethernet links. Finally, each OCC has also a connec-tion controller interface (CCI) for communicating withthe optical nodes.

Fig. 13a shows the switching time measured when theprotection decision is made by the OCC at the control planeupon reception of a LoL notification from the optical nodeð2Tconfig þ TswitchÞ. Thus, it does not include any interchangeof GAPS messages.

Fig. 12. Physical layout and testbed.

Fig. 13b shows the experimental results for the protec-tion time as a function of the number of nodes for OMSDPRing and OMS SPRing. Because of the testing environ-ment, T link is negligible. In order to provide enough accu-racy, the figures reflect the average values from 10experiments. From these results and applications (9) forOMS DPRing and (10) for OMS SPRing, we found thatTcontrol is less than 0.1ms and Tconfig is less than 4.5 ms. Theseresults are better than those specified in previous sections.Thus the obtained behavior will be also better than thatshown in Figs. 8 and 9 for OMS DPRing and OMS SPRing,respectively.

Note that the experimental protection times in Fig. 13bdo not include any propagation time (the T link terms in (9)and (10)). Therefore, these experimental times must beincremented with the specific propagation time. For exam-ple, in a 12 node OMS DPRing with links of 300 km, theterm 2ðn� 1ÞT link represents an additional delay of33 ms. The protection time in this case would be11.90 + 33=44.90 ms. This is better than the 48.6 ms valuespecified in previous sections.

As an example of Figs. 13 and 14a and b show theexperimental TDPRing and TSPRing, respectively, for the worstscenario considered with 18 nodes.

When the link failure is repaired, optical power is de-tected again by the adjacent optical nodes. At this moment,the WTR period starts. After the WTR time, the protection isreverted and the signal is switched from the protection tothe working links.

Therefore, it is possible to deploy rings with 20 nodesand a total length of 2000 km or 16 nodes and 3200 kmwith a recovery time below 50 ms. These results show thatour GAPS-based solutions scale linearly with both thenumber of nodes in the ring and link length. Thus, wecan conclude that GAPS in conjunction with the designedROADMs provide OMS protection under 50 ms in ringswith high numbers of nodes.

Finally, a comparison of both solutions is shown in Table4. Although the cost and complexity of the OMS DPRingsolution is lower than that of the OMS SPRing, the incre-ment in the ROADM cost due to the OMS support is verylow in both cases. OMS SPRing is more bandwidth efficientand provides better availability than OMS DPRing.

-45

-40

-35

-30

-25

-20

-15

-10

0 10 15 ms

Op

tica

l p

ow

er (

dB

m)

9.89 ms

9

11

13

15

17

19

3 12 15 18

Exp

erim

enta

l pro

tect

ion

tim

e (m

s)

Number of nodes in the ring (n)

DPRing

SPRing

5

6 9

a b

Fig. 13. Experimental results (a) experimental 2Tconfig þ Tswitch value (b) evolution of protection time with the number of nodes.

-45

-40

-35

-30

-25

-20

-15

-10

0 10 15 20 25 30 35 40 45

Time (ms)

Op

tica

l po

wer

(dB

m)

12.85 ms

-45

-40

-35

-30

-25

-20

-15

-10

0 10 15 20 25 30 35 40 45Time (ms)

Op

tica

l po

wer

(d

Bm

)

15.71 ms

5 5

a b

Fig. 14. Experimental results for rings with 18 nodes (a) OMS DPRing protection time (b) OMS SPRing Protection time.

Table 4Comparison of OMS solutions

OMS DPRing OMS SPRing

Transported traffic(lightpaths)

W (num. wavelengths/link).Independent of the numberof nodes (n) in the ring

From W to Wn/2,depending on thetraffic pattern

Availability High HighestProtection time Fastest (<50 m) Fast (<50 m)Cost Lowest Low

1986 L. Velasco et al. / Computer Networks 52 (2008) 1975–1987

7. Conclusions

In this paper, we have presented two OMS protectionsolutions (DPRing and SPRing) for GMPLS-controlled opticalring networks based on a novel GAPS mechanism. OMS pro-tection makes possible the recovery of all optical channels ina fiber with just one protection action. Since the overall pro-tection time increases linearly with the number of nodes, thescalability of the GAPS mechanism has been demonstrated.From the obtained results, we conclude that a ring-based net-work using the designed ROADM nodes and controlled by the

GAPS protocol will provide survivability with a SDH-like ser-vice recovery time (<50 ms) even in large optical rings.

A pay-as-you-grow strategy can be implemented usingboth schemes. OMS DPRing can be used in networks wherethe expected traffic demand is lower than the number ofwavelengths available in each link. If the traffic grows,the migration from OMS DPRing to OMS SPRing consistsof adding one OSNL card and one card for the passive com-ponents (splitters and band splitters) to every optical nodein the ring.

Acknowledgements

This work has been partially funded by the i2Cat Foun-dation through the TRILOGY project and by the SpanishScience Ministry through the TEC-2005-08051-C03-02RINGING project.

References

[1] ITU-T Rec. G.841, Types and characteristics of SDH networkprotection architectures, 1998.

L. Velasco et al. / Computer Netwo

[2] G. Iannaccone, C. Chuah, R. Mortier, S. Bhattacharyya, C. Diot,Analysis of link failures in an IP backbone, in: Proceedings of ACMSIGCOMM IMW’02, Marseille, France, November 2002.

[3] W.D. Grover, Mesh-Based Survivable Networks, Prentice Hall PTR,New Jersey, 2004.

[4] ITU-T Rec. G.8080/Y.1304, Architecture for the AutomaticallySwitched Optical Networks, 2001 and Am. 1, 2003.

[5] E. Mannie, Generalized multi-protocol label switching (GMPLS)architecture, RFC-3945, 2004.

[6] L. Berger, Generalized multi-protocol label switching (GMPLS)signaling resource reservation protocol-traffic engineering (RSVP-TE) extensions, RFC 3473, 2003.

[7] D. Katz, K. Kompella, D. Yeung, Traffic engineering (TE) extensions toOSPF Version 2, RFC 3630, 2003.

[8] J. Lang, Link management protocol (LMP), RFC 4204, 2005.[9] J.P. Lang et al., RSVP-TE extensions in support of end-to-end

generalized multi-protocol label switching (GMPLS) recovery, RFC4872, 2007.

[10] L. Berger et al., GMPLS segment recovery, RFC 4873, 2007.[11] P. Arijs et al, Design of ring and mesh based WDM transport

networks, Optical Networks Magazine 3 (2000) 25–40.[12] M.-J. Li et al, Transparent optical protection ring architectures and

applications, IEEE Journal of Lightwave Technology 23 (10) (2005)3388–3403.

[13] R. Muñoz et al., Experimental GMPLS fault management for OULSRtransport networks, in: Optical Fiber Communications, OFC/NFOEC3, 2005, paper JWA50.

[14] J. Perelló, E. Escalona, S. Spadaro, J. Comellas, G. Junyent, Resourcediscovery in ASON/GMPLS transport networks, IEEECommunications Magazine 45 (10) (2007) 86–92.

[15] A. Fumagalli, M. Tacca, Differentiated reliability (DiR) in wavelengthdivision multiplexing rings, IEEE/ACM Transactions on Networking14 (1) (2006) 159–168.

[16] L. Guo, L. Lemin, A novel survivable routing algorithm with partialshared-risk link groups (SRLG)-disjoint protection based ondifferentiated reliability constraints in WDM optical meshnetworks, IEEE Journal of Lightwave Technology 25 (6) (2007)1410–1415.

[17] X. Cao, V. Anand, C. Qiao, Framework for waveband switching inmultigranular optical networks part I-multigranular cross-connectarchitectures, Journal of Optical Networking 5 (12) (2006) 1043–1055.

[18] M. To, P. Neusy, Unavailability analysis of long-haul networks, IEEEJournal on Selected Areas in Communication 12 (1994) 100–109.

[19] J.-P. Vasseur, M. Pickavet, P. Demeester, Network Recovery –Protection and Restoration of Optical, SONET-SDH, IP and MPLS,Elsevier, San Francisco, 2004.

[20] S. Verbrugge et al., General availability model for multilayertransport networks, in: Proceedings of DRCN, 2005, pp. 85–92.

[21] S. Sygletos, A. Tzanakaki, I. Tomkos, Numerical study of cascadabilityperformance of continuous spectrum wavelength blocker/selectiveswitch at 10/40/160 Gb/s”, IEEE Photonics Technology Letters 18(24) (2006) 2608–2610.

[22] ARM: <http://www.arm.com>.[23] DIGI UNC90 - Datasheet: <http://www.digi.com/pdf/hwref_ cc9u.

pdf>.[24] J. Corbet, A. Rubini, G. Kroah-Hartman, Linux Device Drivers, third

ed., O’Reilly Media, Sebastopol, 2005.

Luis Velasco (luis.velasco@tsc.upc.edu) recei-ved the M.Sc. degree in TelecommunicationsEngineering from Universidat Politécnica deMadrid (UPM), in 1989. In the same year, hejoined Telefónica de España and was involvedon the specifications and first office applica-tion of Telefónica’s SDH transport network. In2003 he joined Universitat Politècnica deCatalunya (UPC), where currently he is assis-tant professor. He is currently workingtowards the Ph.D. degree at the Optical Com-munications group of UPC. His research

interests include signaling, routing and resilience mechanisms architec-tures in ASON/GMPLS-based networks.

Salvatore Spadaro (spadaro@tsc.upc.edu)received the M.Sc. and the Ph.D. degrees inTelecommunications Engineering from UPC(Barcelona, Spain) in 2000 and 2005, respec-tively. He also received the M.Sc. degree inElectrical Engineering from Politecnico diTorino, Italy, in 2000. He is currently associateprofessor in the Optical Communicationsgroup of the Signal Theory and Communica-tions Department of UPC. He has beeninvolved in international and nationalresearch projects. He has co-authored about

60 papers in international journals and conferences. His research interestsare in the fields of all-optical networks with emphasis on traffic engi-neering and resilience.

rks 52 (2008) 1975–1987 1987

Jaume Comellas (comellas@tsc.upc.edu) received M.S (1993) and Ph.D. (1999) degrees inTelecommunications Engineering from UPC.Since 1992 he has been a staff member of theOptical Communications Research Group ofUPC. His current research interests mainlyconcern optical transmission and IP overWDM networking topics. He has participatedin different research projects funded by theSpanish government and the European Com-mission. He has co-authored more than 100research articles in national and international

journals and conferences. He is associate professor at the Signal Theoryand Communications Department of UPC.

Gabriel Junyent (junyent@tsc.upc.edu) is atelecommunications engineer (UniversidadPolitécnica de Madrid, UPM, 1973), and holdsa Ph.D. degree in communications (UPC,1979). He has been a teaching assistant (UPC,1973–1977), adjunct professor (UPC,1977–1983), associate professor (UPC, 1983–1985),and professor (UPC, 1985–1989), and has beena full professor since 1989. In the last 15 yearshe has participated in more than 30 nationaland international R&D projects, and has pub-lished more than 30 journal papers and bookchapters and 100 conference papers.