Minimizing Reconfiguration Times of Optical Cross-

Connects in Distributed Lightpath EstablishmentLihua LUa, Qingji Zenga, Yanna Haob

aRoom 5-501, Building of Electronic and Information, No.800 Dongchuan Road, Shanghai Jiao Tong Univ. 200240, P.R. Chinab825 Zhangheng Road, Shanghai, 201203, P.R.China

Abstract- A new scheme to mitigate the connection setuptime and minimize the reconfiguration times of optical cross-connects (OXCs) for WDM optical networks is proposed inthis study. In this new scheme, we consider thereconfiguration information of switch fabrics in the signalingprotocol, which designated as the signaling with switch fabricstatus (SWFS). Distributed reservation algorithms will reservethe wavelength with minimum of reconfiguration times ofOXCs along the route. Simulation results indicate that ourproposed schemes with switch fabrics status have the shortersetup time, lower switching ratio as well as better blockingperformance than those of the classic schemes. Especially, thereservation schemes all have constant switching ratio underthe environment on different switching delay of OXCs ordifferent requests. Moreover, the proposed schemes withSWFS significantly reduce the number of switch fabrics thatneed to be reconfigured.Key words: optimization, distributed lightpath

establishment, wavelength reservation, signaling protocols,wavelength-routed networks, switch fabrics

I. INTRODUCTION

Nowadays, more and more researches focus on the controland management of lightpath resources dynamically in opticalWDM networks[1] to adapt to the bursty nature andunpredictable traffic patterns of global Internet. The morebursty the traffic is, the higher the dynamic within the networkwill be. One example is the wavelength-routed optical burstswitched (WR-OBS) networks[2], where the average durationof the established lightpaths is only about several tens ofmilliseconds.To cope with the dynamic traffic loads, dynamic

wavelength reservation scheme is required to select a routeand assign a wavelength for a request. In several distributedreservation protocols for dynamic wavelength-routed opticalnetworks, Source Initiated Reservation (SIR)[3] scheme has toover-reserve available wavelengths information underdistributed control networks because of the lack of global stateinformation availability. Generally, SIR scheme has highblocking probability particularly in heavy load networks sinceit tends to temporarily lock many resources which will not beused. Destination Initiated Reservation (DIR)[4] schemes justcollect available wavelengths information forward thelightpath and then initiate a reservation message in the reversedirection from destination to source node. Previous studiesindicate that the blocking performance of DIRs is better thanthat of SIRs in general[5-6]. For the DIRs, the connection

blocking is primarily caused by wavelength contention[6],which is also called outdated information. The IntermediateNode Initiated Reservation (IIR)[7] scheme allows thereservation to be initiated by a set of intermediate nodesbefore the connection request arrives to the destination node.It can reduce the amount of over-reservation and reduce theblocking caused by outdated information due to a shortervulnerable period but it has to increase more control packetsload.

In previous studies, less works evaluate the performance onmetrics of connection setup time. Especially, many worksignore its impact on dynamic optical devices in distributedreservation schemes. In dynamic optical networks, it isdesirable to design a more flexible and efficient optical coreusing the emerging reconfigurable optical components, suchas reconfigurable cross-connects (OXCs), reconfigurableoptical add/drop multiplexers (OADMs), and tunabletransmitters, receivers and filters. Compared with those opticaldevices, the OXC configuration time is a significant factor forthe connection set up time in dynamic optical networks. In M.S. Kumar et al's work[8], a technique to optimize theconnection setup time in centralized dynamic lightpathestablishment (DLE) by considering the delay of switchfabrics configuration is proposed. In this paper, we propose adistributed reservation scheme considering the switch fabricsconfiguration delay. The signaling protocols forward theavailable wavelength together with the switch fabric status ofeach wavelength, which designated as the signaling withswitch fabric status (SWFS). We describe the generalized thereservation schemes with SWFS and propose the DIR schemeswith SWFS and wavelength assignment algorithms for nowavelength conversion networks. Moreover, we simulate andanalysis the performance of our schemes, such as connectionsetup time and blocking probability, especially evaluate theimpact on the switching ratio.

The rest of this paper is organized as follows: Section IIproposes our generalized distributed reservation scheme withSWFS, DIR schemes with SWFS with no wavelengthconversion and wavelength assignment algorithms. Section IIIpresents and discusses the simulation results. Section IVprovides a summary and concludes the paper.

II. DISTRIBUTED WAVELENGTH PROVISIONING WITH SWITCHFABRIC STATUS

A. Distributed Reservation Scheme with SWFSIn dynamic optical mesh networks, one OXC has an electric

control element to manage and control lightpath in the bursty

traffic environment. In a reconfigurable OXC, especially in atoday's popular MEMS switches[9], the optical data signalscan be passed through the interconnection port pairs of theswitch fabrics for each wavelength although the connectionthrough the port pairs of the interconnection is not establishedby software. On the other hand, when the cut-throughinterconnection port pairs of a node are not selected bydistributed control plane, the switch fabrics of the node will bereconfigured. Fig. 1 shows a sample optical switch fabrics with4*4 port pairs for one wavelength. If distributed reservationschemes just select and reserve the wavelength between theport pairs <inport2, outport3>, the switch fabrics need not tobe reconfigured. If reservation schemes reserve the lightpathbetween<inport2, outputl>, the switch fabrics have to bereconfigured(Fig.1 right figure). The nodes can collect, storeand management the information of switch fabricsconfiguration with each wavelength. For distributed opticalnetworks, nodes do not know the global state information. Wepropose the signaling protocol schemes with switch fabricsstatus, which designated as SWFS. The signaling with SWFScollects the available wavelengths information as well as theextra status of the switch fabrics. Reservation schemes willselect the wavelength with cut-through port pairs for reducingthe connection setup time in priority.

IOW01 2 XtMo 'w 2 fit =t .1.2.H

I'g1 nRI1 1fFigure 1 A sample optical switch fabrics with 4*4 port pairs

To provide a formal definition of the schemes with SWFS,we define the following parameters:

NNR: The set of a sequence of nodes on route R, fromsource to destination.

NR <j<nR R R

Here, m denotes the total number of nodes in route R.* SR~:At any node ni E N , S. denotes the status of the

switch fabrics status with the wavelength k at node niRalong the route R.

Sk ={ ¢ no reconfigured <i<R

1¢ reconfigured

S1k R At any node ni EN,SNI iR denotes the times of

switch fabrics reconfigured with the wavelength k fromnRto .

iSk -v>Sk

1,2, iR SJR

j=l

1<i<m

aWi At any node n7 NR WR denotes the sets of

available wavelengths to be collected at node n7 in the

classic distributed scheme.* WSR: At any node n7R E N> WS[' denotes the sets of

wavelengths k as well as S R of each wavelength to be

collected at node ni7 in the distributed reservation schemewith SWFS.For the classic distributed reservation schemes, control

packets only collect the wavelength availabilityinformationW17 along the route of a request. Being differentfrom classic reservation schemes, the reservation schemeswith SWFS collect the available wavelengths information aswell as the extra status of the switch fabrics WS,. The extraswitch fabrics status indicates if the switch fabrics should bereconfigured for the specified wavelength at the node.

The generalized protocols may include algorithms for theoptimal reservation of the wavelengths with least times ofswitch fabrics need to be reconfigured and the algorithms fordetermining the optimal selection of the nodes for initiation(SIR/DIR/IIR). The choice of the parameters in theoptimization process could be based on various requirementsand properties of the specific connection request, such asquality of service requirements, priorities, hop distance, andother constraints. Meanwhile, the schemes with SWFS can beeasily realized within the GMPLS[10] framework such as CR-LDP[11] and RSVP-TE[12] by attaching an optional

parameter Sk2 for each available wavelength. In this paper,

we investigate the DIR schemes with SWFS for networks withno wavelength conversion.

B. DIR scheme with SWFS for networks with no wavelengthConversion

Fig.2 shows a successful example of DIR scheme. Eachnode of the route reserves the selected wavelength andbackwards the control packet to the upstream node along theroute after the switch fabrics is configured.A successful example of the DIR scheme with SWFS is

shown in Fig.3. We suppose the route of a requestis NR =< A, B, C, D >. In the scheme with SWFS, the source

node A collects available wavelengths as well as SAk for eachavailable wavelength k and forwards the message with theset of < k, Sk > to the next node B. The intermediate node Bupdates the set of available wavelengths according to thereceived message from upstream node and information from

itself, collects SB of each available wavelength k, calculates

SAB(kSASk+=SB) and forwards the message with < k,SkB>to the next node C. The next node C also collect thewavelength availability and SC for each wavelength k,

calculates SkBC (SAC =SAB+SC ) and forwards the message

with < k,SBc > to the next node D. When the message issent to the destination node, the destination node D calculates

SkBCD ( SABCD =SAkB +Sk ) for each available wavelength k,selects and reserves the available wavelength with

minimal SABCD That means the control message collects thesum of switch fabrics reconfigured in the nodes along the

route that the message traverses. Both SA = 0 and Sc = 0 inthe example of Fig.2 at that time. It means only node B and Dneed to configure the switch fabrics after reserve the selectedwavelength and backward the message to the upstream nodealong the route in the example.

I Switching Fabnrc Reconfigured

Fig. 2 A successful example of classic DIR scheme

available available available

< 2> i<,A SAB ><A2!1A A' AR

3227S 2 7 AB > <2 SAC >

<3A,> <A SB >A >

<A,, S !,AB reserve

I Switching Fabric Reconfigured

Fig. 3 A successful example of the DIR scheme with SWFS

Several distributed wavelength assignment algorithms withSWFS we proposed as shown in Table. 1.

Table 1: Wavelength Assignment at the Destination NodeSelect the first wavelength in the order of the

S-Firstfit wavelength number from the set of availablewavelengths with minimal S k

R1,2 m

Select one available wavelength randomly from theS-Random set of available wavelengths with minimal S12 mR

Select the first available wavelength in the order ofFirstfit the wavelength number from the set of available

wavelengthsRandom Select one available wavelength randomly from theRandom set of available wavelengths

C Connection setup time andswitching ratioWe introduce a parameter, switching ratio p, to express the

ratio of the number of nodes that OXC need to bereconfigured to the total number of nodes on the route. Thusthe average establishment time is:

T = (2H + 1)P + 2HD + p(H +1)CHere, C is the time of the switch fabrics configuration. P is

the message processing time in a node. D and H are thepropagation delay and the average hop distance between anytwo nodes, respectively. The value of p can reflect thereconfigured situation of the switch matrixes with thevariation of the dynamic traffic.

III. NUMERICAL RESULTS

We study the performance of our proposed algorithms usingthe Pacific-Bell network topology (Fig.4), which has 8wavelengths on each directional link. The fiber length of twoadjacent nodes is set to 100km. The propagation delay D isproportional to the fiber length. The message processing timeP is set to 10ps. Each node has a fully configurable opticalswitch matrix and the number of add/drop ports are enough toadmit any request. No wavelength converter is installed. Eachsimulation result is obtained over a simulation of 106 requeststhat follows a Poisson traffic model, and the connectionholding time of requests is negative exponentially distributedwith mean 1/,u. The fixed shortest path routing is employed.

In all of the following simulations, B denotes the averageholding time (1/Ip) of all requests. Our simulations areperformed with different values of B because performances ofour schemes mainly depend on those time parameters. We use1Oms as the delay of the switch fabrics configuration, whichfit well with the parameters of up-to-date popular MEMSswitches, whose switching time is on the order of milliseconds[10]. For the source node and destination node of a request, itis also necessary to consider the tuning time of transmittersand receivers. In this paper, simulations only consider theswitching time is presented because the switching time isregarded to have more significant impact on the performanceof the network.

Fig.4 Pacific-Bell network topologiesA. Performance ofconnection setup time

Fig.5 presents the average setup time versus traffic load ofdifferent wavelength assignment algorithms with C= Oms .

Since the SWFS schemes minimize the delay of switch fabricsreconfigured in the nodes along the route, less average setuptime is acquired. This is verified in Fig.5, in which the averagesetup time of S-Firstfit or S-Random algorithm is significantless than Firstfit or Random at whole traffic load period no

(2.

matter the traffic load is light or heavy. Among them, Randomalgorithm has the longest average setup time. Moreover, theaverage reservation time has little change in the low trafficload period and decrease gradually under the high traffic loadin each scheme.

B=ls, C=1rOOms

0.015 A VU)E

C,0.010

o 0.005 --*-S-Firsffit-- S-Random-A- Firstfit-v- Random

o.ooo0.1 10

Traffic Load(Erlang)

0.7

0.6

0.5 F-0

Cu 0.4

.gc,m-c 0.30

(I)

0.1 _

1E-5 1E-4 1E-3 0.01

Switching Time(s)

0.7

Fig.5 Average setup time versus traffic load of different wavelengthassignment algorithms with C=lOms

B. Performance ofswitching ratio p

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0.0

0.6

0.5

0.4

0.3

0.2

0.1

0.0

Traffic load=1 Erlang

. *

*-S-Firsffit*-S-Random-* Firsffit

-v- Random

10 100 1000 10000 100(

Average Holding Time(s)

...I

Traffic lo,

-*- S-Firsffit-*- S-Random-A- Firstfit-v- Random

....... .......1 10 100 1000

Average Holding Time(s)

Fig.6 Switching ratio p versus switching time C of diffe

assignment algorithms with B=Is

a =10.E ...

bad =10 Erlang-

0.6 -

0.5 ,-

0so 0.4 -

D 03

0)C,)

0.2

0.1 -

0.0 -1E-5

)000

0.1 1

100

1E-4 1E-3 0.01

Switching Time(s)0.1 1

Fig.7 Switching ratio p versus average holding time B of different

wavelength assignment algorithms with C=lOmsWe also investigate switching ratio p versus different time

parameters B and C. Fig.6 describes p versus C and Fig.7describes p versus B of different wavelength assignmentalgorithms under different given traffic load. It is shown thatp has little change at a constant traffic load no matter how Bor C changes. We can find from the figures that our proposed

- schemes have obviously reduced the number of thereconfigured switch fabrics in contrast to the schemes withoutSWFS.C. Blockingperformance ofSWFS

Fig.8 plots blocking probability versus traffic load ofdifferent wavelength assignment algorithms. We can point outfrom this figure that the blocking performance of S-Firstfitand S-Random algorithms are better than that of Firstfit andRandom schemes in the whole traffic load period. This isbecause less average setup time and vulnerable period [7] ofS-Firstfit and S-Random lead to less backing blocking causedby outdated information [6]. Moreover, the blocking

-rent wavelength performance with Random algorithm is better than those withFirstfit algorithms no matter with or without SWFS. We can

Traffic load=1 Erlang

* * *0

--S-Firstfit*-S-Random--Firsffit-v- Random

mI0)

C,C.)n1

Traffic load=10 Erlang.

V V V~,

--S-Firstfit-- S-Random-A- Firsffit-v- Random

0

0).1lcn

.i i i .1 i s i .1 i l

.u-..... ..... ...

Iiiiiiiiiiii iii

k0.2 F

1

U.U

also draw the conclusion that S-Random algorithm has thebetter performance than S-Firstfit algorithm under all of theenvironments. The setup time of Random algorithm is longerthan that of Firstfit though the blocking perfornance ofRandom is better than that of Firstfit. We can find from thefigures that only the algorithms with SWFS have betterimprovement in both of the perfornance of average setup timeand blocking probability.

0.01

Z0

c-

.0

2cm

1E-3

1E-4

1E-510

Traffic Load (Erlang)

Fig.8 Blocking probability versus traffic load of different wavelengthassignment algorithms with C=lOms

In Fig.9, we also investigate the effects of the switchingtime C to the network blocking probability. We can find fromthe figure that the blocking probability is insensitive to thedelay of switch fabrics configuration in light trafficenvironment. Even though the blocking probability tends to bea constant with the decreasing of the switching time, choosingthe wavelength that need not to be reconfigured still can

decrease the blocking probability.

0.1 Traffic load=100 Erlang

a,,,,,,,, .. ---

0.01 0 S-Firsffita 0-*-S-Random

FirsffitjU v Random

1IE-4

- Traffic load=10 Erlangm

1 E-5

1E-61E-4 1 E-3 0.01

switching Time(s)0.1

Fig.9 Blocking probability versus switching time C of different wavelengthassignment algorithms with B=Is

IV. CONCLUSION

In this paper, we presented and investigated a noveldistributed reservation scheme considering the switch fabricstatus in distributed signaling schemes as well as wavelengthassignment in wavelength-routed optical networks. Simulationresults indicated the proposed algorithms with SWFS have theshorter setup time, lower switch ratio and better blockingperformance than the classic schemes. Moreover, the S-Firstfitand S-Random algorithms significantly reduced the number ofswitch fabrics that need to be reconfigured. The less switchingratio not only leads to a shorter average reservation time ofrequests but also extends the lifetime of OXC. The schemeswith SWFS are compatible with the current signaling schemeand easily realized within the GMPLS framework.

ACKNOWLEDGMENT

The work was jointly supported by the National NatureScience Fund of China (No. 60632010 and No. 60572029) andthe National "863" Hi-tech Project of China (No.2006AA0 lZ25 1).

100

REFERENCES

[1]H. Zang, J. P. Jue, L. Sahasrabuddhe, R. Ramamurthy, B.Mukherjee,"Dynamic lightpath establishment in wavelength routedWDM networks", IEEE Communications Magazine, vol. 39, no. 9, pp.

100-108, 2001[2] M.Duser, P. Bayvel,"Analysis of a dynamically wavelength routed optical

burst switched network architecture", IEEE/OSA Journal of LightwaveTechnology, vol. 20, no.4, pp.574-585, 2002

[3] H. Zang, L. Sahasrabuddhe, Jason P. Jue, S. Ramamurthy, B. Mukherjee,"Connection Management for Wavelength-Routed WDM Networks,"Proc. IEEE GROBECOM'99, pp. 1428-32, 1999.

[4] X. Yuan, R. Melhem, R. Gupta, Y. Mei, C. Qiao, "Distributed controlprotocols for wavelength reservation and their performance evaluation,"Photonic Network Commun., vol.1, no.3, pp.207-218, 1999.

[5] K. Lu, G. Xiao, I. Chlamtac, "Analysis of blocking probability fordistributed lightpath establishment in WDM optical networks," IEEETrans. on Networking, vol. 13, no.1, ppl 87-197, 2005.

[6] D. Saha, "A comparative study of distributed protocols for wavelengthreservation in WDM optical networks," Optical Networks Magazine,vol.3, no.1, pp. 45-52, 2002.

[7] K. Lu, J. P. Jue, G. Xiao.,I. Chlamtac, T.Ozugur., "Intermediate-nodeinitiated reservation (IIR): A new signaling scheme for wavelength-routed networks," IEEE Journal on Selected Areas in Commun., vol.21,no.8, pp.1285-1294, 2003.

[8] M. S. Kumar, P. S. Kumar, "Light setup time optimization in wavelengthrouted all-optical networks," Computer Communication, 24, pp.984-995,2001

[9] A. Neukermans, R. Ramaswami, "MEMS technology for opticalnetworking applications," IEEE Commun. Mag., vol.39, pp.62, 2001

[10] L. Berger, Generalized multi-protocol label switching (GMPLS)signaling functional description, IEEE RFC 3471, Jan 2003.

[11] P. Ashwood-Smith, L. Berger., Generalized multi-protocol labelswitching (GMPLS) signaling constraint-based routed label distributionprotocol (CR-LDP) extensions, IETF RFC 3472, Jan.2003

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

Average holding time B =0.1s

S/-Firstfit_S-RandomA Firsffit

- /-v- Random

1