Routing in all-optical DWDM Networks with Sparse Wavelength Conversion Capabilities

Transcription

1 Routng n all-optcal DWDM Networks wth Sparse Wavelength Converson Capabltes Ala I. Al-Fuqaha LAMBDA Optcal Systems 200 Sunset Hlls Rd. Reston, Vrgna U.S.A ala@eee.org Ghulam M. Chaudhry SCE-CSEE Unversty of Mssour-Kansas Cty Kansas Cty, Mssour U.S.A. ChaudhyG@umkc.edu Mohsen Guzan, Ghassen Ben Brahm Department of Computer Scence Western Mchgan Unversty Kalamazoo, Mchgan U.S.A. mguzan@cs.wmch.edu Abstract Ths work focuses on the Routng and Wavelength Assgnment (RWA) problem n all-optcal DWDM networks wth Sparse Wavelength Converson (SWC) capabltes. By sparse wavelength converson, we mean that nodes wthn the optcal network doman mght or mght not support optcal wavelength converson. For these nodes that support optcal wavelength converson, the number of wavelength converters mght be lmted. As such, optcal lghtpaths mght or mght not be able to fnd the wavelength converson resources that mght be needed for t to be establshed. In ths work, we present the RWA problem n all-optcal WDWM networks wth sparse wavelength converson capabltes (RWA-SWC). We also provde Integer Lnear Programmng (ILP) formulaton for Statc Lghtpath Establshment (SLE) n all-optcal networks wth sparse wavelength converson capabltes. Fnally, we propose a new opaque extenson to the OSPF routng protocol to advertse wavelength usage and converter avalablty throughout the optcal network doman. I. INTRODUCTION The telecommuncatons ndustry s currently facng an unprecedented demand for more bandwdth that s substantally hgher than that offered by electro-optc networks. Electro-optc networks use electrcal form of the sgnals to swtch network traffc from the source through some ntermedate nodes towards the fnal destnaton. These networks also use electro-optcal regenerators to strengthen the transmtted sgnals and ther sgnal to nose ratos. These network are not fully utlzng the bandwdth [WENB] of the optcal fber (approxmately 0 THZ) because they use a sngle carrer frequency (wavelength) that s modulated at a maxmum speed of 40 Gbps. Dense Wavelength Dvson Multplexng (DWDM) s consdered a promsng transmsson technology that mproves that utlzaton of optcal fber bandwdth. Thus, future transmsson networks should employ technologes that overcome electro-optcal bottlenecks and offer better utlzaton of the optcal fber bandwdth. It s beleved that all-optcal DWDM networks wll provde the answer to these challenges. These networks wll elmnate the electro-optcal bottlenecks by transmttng optcal sgnals from source to destnaton wthout the need for electro-optcal converson. They wll also offer better utlzaton of the avalable fber bandwdth by modulatng multple carrer frequences (wavelengths) allowng a sngle strand of fber to carry multple optcal sgnals. Whle all-optcal DWDM transport networks offer capactes above those offered by tradtonal electro-optcal networks, several challenges are ntroduced beyond those known n tradtonal electro-optcal networks. In ths work we focus on the routng and wavelength assgnment problem n all-optcal DWDM transport networks wth sparse wavelength converson capabltes. The remander of ths paper s organzed as follows. Secton II provdes an overvew of the RWA problem n networks wth the LWC constrant. Secton III provdes ILP formulaton for the RWA-LWC problem. Secton IV presents new opaque extenson to the OSPF routng protocol to handle all-optcal DWDM networks wth LWC constrant. Secton V presents our floodng polcy Secton VI concludes ths study and dscusses future extensons. II. THE RWA-LWC PROBLEM A lghtpath s an end-to-end connecton that mght span multple lnks n the optcal DWDM network. In order to be able to establsh lghtpaths n all-optcal DWDM networks, a sgnalng protocol s needed to request and set up the lghtpath through the optcal network. When a connecton request arrves to the network through the User to Network sgnalng protocol (for example, OIF), the routng protocol (for example, OSPF) fnds a route and assgns wavelength(s) to the ncomng lghtpath. Then the network-to-network sgnalng protocol s used to set up the lghtpath through the optcal network. The problem of fndng a route and assgnng wavelength(s) to an ncomng lghtpath s known as the Routng and Wavelength Assgnment (RWA) problem. Two types of swtchng systems can be supported wthn the optcal network doman [ZANG]: Wavelength Selectve Cross-Connects (WSXC) and Wavelength Interchangng Cross-Connects (WIXC). In wavelength selectve cross-connects, wavelength converson s not supported and a lghtpath must occupy the same wavelength throughout ts route from source to destnaton. Ths constrant s known as the wavelength contnuty constrant. Ths means that a lghtpath can be blocked even f there are avalable wavelengths on all lnks. Allowng the lghtpath to change from one wavelength to another at an ntermedate node can reduce the blockng GLOBECOM /03/$ IEEE

2 probablty. Swtchng systems that allow wavelength converson are called Wavelength Interchangng Cross- Connects (WIXC). Wavelength converters may lower the blockng probablty n the optcal network by resolvng the conflcts between the lghtpaths. A swtchng system that s capable of convertng every sngle ncomng wavelength to any other wavelength s called a full wavelength converson capable system. An optcal network that s composed of such swtchng systems s equvalent to a crcut-swtched telephone network [ZANG]. In ths case, the routng problem only needs to be solved and wavelength assgnment s not an ssue. Unfortunately, wavelength converters are expensve and adaptng full wavelength converson soluton wthn an optcal network s an expensve strategy that does not always offer hgher performance mprovements n terms of lower blockng probabltes. A more cost effectve strategy can be adapted by dstrbutng the wavelength converson resources on all optcal swtches throughout the optcal network doman. In ths case, each swtch has a lmted number of wavelength converters that t can use to convert an ncomng wavelength to another outgong one. In optcal networks that posses LWC capabltes, the RWA problem s more challengng snce each lghpath has to be assgned a route and a set of wavelengths dependng on the avalablty of the wavelength converson resources through the optcal doman. It should be noted here that the RWA- LWC problem s a generalzaton of the RWA problem that holds for networks wth the wavelength contnuty constrant as well as networks that posses more wavelength converson capabltes. III. ILP FORMULATION FOR THE RWA-LWC PROBLEM The RWA-LWC problem can be formulated as an Integer Lnear Programmng (ILP) problem n whch the obectve functon s to mnmze the total cost of all lghtpaths that need to be establshed n the optcal network. Let us defne the followng: N: Number of swtches. E: Number of lnks. W: Number of wavelengths per lnk. T: Total number of lghtpaths that need to be establshed. Π: Number of source-destnaton pars. Q = { q }, =,2,..., Π : Vector of sze Π, where element q represents the number of requested lghtpaths between the th source-destnaton. R = { r }, =,2,..., Π : Vector of sze Π, where element r represents the number of all possble paths between the th source-destnaton par. V = { v }, =,2,..., N : Vector of sze N, where element v represents the number of wavelength converters nstalled on the th node. P = { p }, =,2,..., r : A lst of Π vectors that represent the paths on whch each of the sourcedestnaton pars can be routed, P s the th vector of the lst. Element p represents the th path on whch the th source-destnaton par can be routed. These paths can be enumerated usng the k-shortest paths algorthm. Notce that two paths are consdered to be dstnct f they go through dfferent fbers or dfferent wavelengths n ther route from source to destnaton. U = ( u, k ), =,2,..., r, k =,2,..., E : A lst of Π vectors that represent the usage of the lnk resources by the dfferent paths, vector U s the th vector of the lst. Element u, k = f the th path between the th sourcedestnaton par uses lnk k, otherwse u, k = 0. X = ( x, k ), =,2,..., r, k =,2,..., N : A lst of Π vectors that represent the usage of the wavelength converson resources by the dfferent paths, vector X s the th vector of the lst. Element x, k = f the th path between the th source-destnaton par uses a wavelength converter that s nstalled on node k, otherwse x, = 0. Y = ( y ), =,2,..., r : A lst of Π vectors that represent the cost of the dfferent paths, vector Y s the th vector of the lst. Element y s the cost of the th path between the th source-destnaton par. Z = ( z, k ), =,2,..., r, k =,2,..., E * W : A lst of Π vectors that represent the usage of the wavelength resources (lambdas) by the dfferent paths, vector Z s the th vector of the lst. Element z, k = f the th path between the th source-destnaton par uses wavelength k k E + on lnk W, otherwse z, = 0. S = ( s ), =,2,..., r : A lst of Π vectors that represent the cost of the dfferent paths, vector S s the th vector of the lst. Element s = f the th path between k k the th source-destnaton par s selected, otherwse 0. s = The obectve functon of the RWA-LWC problem s to mnmze the total cost of all requested lghtpaths. The RWA- LWC problem s then formulated as follows: GLOBECOM /03/$ IEEE

3 Π Mnmze = ( Y Subect to the followng constrants: Q r S = ) T S S Π r Π T ( S ) U W Π T ( S ) X V Π T ( S ) Z Π () (2) (3) (4) (5) In ths formulaton, the symbol T ndcates the transpose operaton. Equaton () ndcates that a path can be selected or not selected (bnary varable). Equaton (2) ndcates that all the requested lghtpaths need to be establshed for a soluton to be feasble. Equaton (3) verfes that no more than W wavelengths are used on a sngle lnk. Equaton (4) verfes that the wavelength converson capablty constrants are respected. Fnally, Equaton (5), guarantees that no more than one connecton s carred on any gven wavelength of all lnks n the network. Table shows two scenaros to whch we appled the ILP formulaton presented above. Table also ndcates the optmal resources that need to be allocated to each lghtpath. Fg. shows the topology of the network on whch the lghtpaths ndcated n Table need to be establshed. In ths example, each wavelength converter was assumed to have a cost of 00. We mplemented the k-shortest paths algorthm to enumerate the dfferent paths for the ILP formulaton then we used CPLEX to solve the formulaton. A c = 2 c = B C c = 2 c = 0 D E c = 3 C : Number of wavelength converters nstalled on the node Number of wavelengths = 3 (per each b-drectonal lnk) Fgure : Sample network wth LWC capabltes F c = Scenaro Source Destnaton Route Wavelengths Converters A B A F AB BD DF A F AC CE EF E A EC CA 0 E A EC CA E B ED DB C D CB BD 0 E A EC CA E B ED DB 0 E B ED DB E B ED DB E B EC CB 0 E B EC CB A B AB 0 A B AB 2 0 A B AB 3 0 A B AC CB 3 Table : Examples of route, wavelength, and converter assgnment n LWC networks IV. GENERIC OPTICAL ROUTING EXTENSION (GORE) The Open Shortest Path Frst (OSPF) s an effcent and commonly used lnk-state protocol [MOY2] that can be employed to dstrbute QoS parameters through the optcal network utlzng ts floodng protocol and ts opaque LSA opton. In ths secton, we ntroduce a Generc Optcal Routng Extenson (GORE) to the OSPF Verson 2 protocol descrbed n RFC The purpose for ths extenson s to advertse the wavelength and converters avalablty throughout the optcal network doman. Earler Internet drafts by Chaudhur et al. [CHAU] and Basak et al. [BASA], take the stand that an optcal adaptaton of the OSPF protocol should not advertse any nformaton pertanng to wavelength avalablty or wavelength converters' avalablty. They contend that the set of avalable wavelengths as well as the number of converters used to convert an ngress wavelength to a dfferent wavelength at the egress of the swtch change so frequently that advertsng these changes would not yeld a performance ncrease proportonal to the communcaton cost of ncreased control traffc. Our GORE extenson breaks from ther proposal and adopts a new strategy that conssts of adaptng the OSPF protocol to advertse the number of avalable wavelengths per fber and the number of wavelength converters avalable wthn the swtch. Our ratonale for advertsng wavelength avalablty nformaton as well as the wavelength converters avalablty nformaton s as follows. In networks wth sparse wavelength converson capabltes, a sgnfcant number of swtches do not posses a large number of wavelength converters; the absence of wavelength nformaton from the descrpton of network lnks causes the route computaton algorthm to operate wth nsuffcent nformaton about the network state. As wavelength utlzaton of network lnks ncreases, the probablty of selectng a source route that can be provsoned dramatcally decreases because the lmted number of wavelength converson resources wll render most of the source routes nfeasble. It s unacceptable that the nfeasblty of these routes would not be GLOBECOM /03/$ IEEE

4 detected untl sgnalng was attempted and faled, because ths would cause lghtpath setup to experence a large number of crankbacks. The cumulatve effect of not advertsng wavelength avalablty nformaton as well as the swtch wavelength converson capablty beng that as the load on the network ncreases, the lghtpath setup latency ncreases prohbtvely. Ths problem gets worse when a large number of swtches n the network have lmted number of wavelength converson resources. It should be emphaszed here that [KOMP0] s desgned to handle a network comprsed of Packet Swtchng Capable (PSC), Tme Dvson Multplexng Capable (TDMC), Lambda Swtchng Capable (LSC), and Fber Swtchng Capable (FSC) equpment. We thnk ths approach; even though t s generc; complcates the routng protocol and makes t neffcent snce the routng protocol should handle the advertsements of equpment employng all prevously mentoned swtchng technologes even though such equpment mght belong to dfferent overlays. In ths work, we take the stand that telecom networks employ overlay archtecture and t s more effcent and feasble to desgn a routng protocol that s specfc to each of the employed overlays. In ths case, despte that each overlay would employ ts own routng protocol, each overlay would be able to advertse more nformaton that s specfc to that overlay resultng n more effcent routng and better provsonng of network resources. In ths secton, we present an extenson to the OSPF protocol that addresses the routng problem faced n all-optcal DWDM networks wth lmted number of wavelength converson resources. Even though that the routng extenson presented n ths paper s an overlay specfc one that pertans to all-optcal DWDM networks regardless of ther wavelength converson capabltes a smlar approach can be adapted to desgn routng protocols for other overlays. A. Overvew OSPF-GORE Extenson OSPF opaque Lnk State Advertsement (LSA) opton provdes a generalzed mechansm for OSPF to carry addtonal nformaton especally for traffc engneerng. Opaque LSAs [COLT] are of types 9, 0, and. Opaque LSAs consst of a standard LSA header followed by a 32-bt algned applcaton-specfc nformaton feld. The traffc engneerng LSA opaque data s dvded nto a number of tuples, each consstng of a Type (T), a Length (L), and a Value (V). The general defnton of TLV s used, except that the length feld sze depends on the type feld. The nformaton carred by an opaque LSA s structured nto one TLV and zero or more traffc engneerng sub-tlvs as needed. Fg. 2 represents the detaled structure of an opaque LSA header whle Fg. 3 represents the structure of the nformaton carred by the opaque LSA n terms of TLV and sub-tlvs. In the next two subsectons, a detaled descrpton of the message formats used by our OSPF-GORE extenson to advertse the wavelength usage and the number of avalable wavelength converters per swtch s llustrated. Lnk State ID Sub-TLV Type LS Age Opton LS Type = 0 LS checksum TE LSA ID Advertsng Router LS Sequence Number Length LSA # (0..255) Traffc Eng. Type Identfes OSPF Instance For old and duplcate Opaque LSA detecton (Sequence of the same LSA #) Fgure 2: Opaque LSA header nformaton Type Type Opaque LSA Header.. Value Fgure 3: Opaque LSA structure Length Length ) Wavelength-Avalablty Opaque LSA Message Format The Wavelength-Avalablty Opaque LSA descrbes a DWDM lnk that can carry multple wavelengths. Ths LSA descrbes the avalablty of the dfferent wavelengths carred over the DWDM lnk, the local and remote nterface IP addresses and ther dentfers. Fg. 3 shows the Wavelength-Avalablty Opaque LSA format. Ths LSA contans the followng felds: LS Age: Tme n seconds snce the LSA was orgnated. Optons: The optonal capabltes supported by the descrbed porton of the routng doman. LSA Type: Ths feld s set to 0 descrbng an opaque LSA to be advertsed nsde a sngle area. Lnk State ID: In our case (pont-to-pont), ths feld s set to the source nterface IP address. Advertsng Router: The Router ID of the router that orgnated the LSA LS Sequence Number: Successve nstances of an LSA are gven successve LS sequence numbers. GLOBECOM /03/$ IEEE

5 LS Checksum: The Fletcher checksum of the complete contents of the LSA excludng the LS age feld. Length: Ths feld represents the length n bytes of the whole LSA. Type = 2: The Lnk TLV descrbes a sngle lnk. Lnk ID: Ths feld dentfes the other end of the lnk. In our case (pont-to-pont), ths s the Router ID of the neghbor. The Lnk ID sub-tlv s of Type = 2, and s four bytes n length. Local Interface IP Address: Ths feld specfes the IP address of the nterface correspondng to ths lnk. The local nterface IP address felds are used to dscern multple parallel lnks between systems. The type of the sub-tlv correspondng to ths feld s 3. Remote Interface IP Address: Ths feld specfes the IP address of the neghbor's nterface correspondng to ths lnk. The remote nterface IP address felds are used to dscern multple parallel lnks between systems. The type of the sub-tlv correspondng to ths feld s 4. Outgong Interface Identfer: A lnk from Swtch A to B may be assgned an outgong nterface dentfer. Ths feld represents a non-zero 32-bt number assgned by swtch A. It should be unque wthn the scope of A. The type of the sub-tlv correspondng to ths feld s. Incomng Interface Identfer: A lnk from Swtch A to B may be assgned an ncomng nterface dentfer, whch s the outgong nterface dentfer from B's pont of vew. The type of the sub-tlv correspondng to ths feld s 2. Type = 32777: The type of the new ntroduced sub-tlv should be out of the range , whch s reserved for Csco-specfc extensons. Our OSPF-GORE extenson range should not conflct wth the requred or optonal sub-tlv types or wth range reserved for Cscospecfc extensons. The range wll be used by OSPF-GORE for further extensons. We decde to assgn a type value of to the new Wavelength- Avalablty opaque sub-tlv. Length of Mask: Number of bts used to represent the bandwdth mask. Bandwdth Mask: Ths feld represents the avalable as well as used wavelengths over a specfc lnk. Ths feld s 20 bts long (see Fg. 4). Bt (where 20) represents the status of λ : λ = f wavelength λ s currently used λ = 0 f wavelength λ s currently free 2) Wavelength Converter Avalablty Opaque LSA Message Format The Wavelength Converter Avalablty Opaque LSA descrbes the number of wavelength converters avalable wthn the swtch. We use the same concept used to defne the Wavelength Avalablty opaque LSA except for the sub-tlv type feld. Fg. 5 depcts the Wavelength Converter Avalablty Opaque LSA format. Ths LSA contans the followng felds: Type = 32776: We use the same concept used n defnng the Wavelength Avalablty opaque LSA. Agan, the type of the newly ntroduced sub-tlv should be out of the range We decde to assgn a type value of to the Wavelength Converter Avalablty opaque sub-tlv. Number of converters: The total number of converters that are not used wthn the swtch. V. OSPF-GORE LSA FLOODING POLICY Some of the Traffc Engneerng (TE) and Qualty of Servce (QoS) parameters change very frequently, rasng the ssue of when to advertse the changes of the network characterstcs throughout the whole network. The orgnal OSPF standard mandates a varety of tunable parameters controllng the floodng of LSAs, ncludng the MnLSInterval that specfes the tme between any two consecutve LSA orgnatons, and the MnLSArrval that lmts the frequency of acceptng newer nstances of LSAs. In [APOS], Apostolopoulos et al. present other polces dealng wth the ssue of when a router should flood a new LSA to advertse changes n ts lnk metrc. Some of the proposed polces nclude: Threshold based polces, whch trgger updates when the dfference between the prevously flooded and the current value of avalable lnk bandwdth s larger than a confgurable threshold. Class based polces, whch partton the capacty of a lnk nto a number of classes and re-advertse when a class boundary s crossed. Tmer based polces, whch generate updates at fxed ntervals to enforce a mnmum spacng between two consecutve updates. Our OSPF-GORE extenson adopts the followng update polcy to advertse the wavelength usage and converter avalablty metrcs: The traffc comng to the swtch over a specfc wavelength uses the same wavelength at the output port of the swtch: In ths case, no wavelength converson s needed and only a Wavelength Avalablty LSA has to be orgnated and flooded to all neghborng swtches. The traffc comng to the swtch over a specfc wavelength needs to be swtched to a dfferent wavelength at the output port of the swtch: In ths case, a wavelength converter s needed and two opaque LSAs have to be orgnated and flooded to all neghborng swtches. The frst opaque LSA s the Wavelength Avalablty, whch refers to the change n the wavelength GLOBECOM /03/$ IEEE

6 avalablty. The second opaque LSA s the Wavelength Converter Avalablty, whch refers to the change n the avalable number of converters wthn the swtch. VI. CONCLUSIONS AND FUTURE WORK In ths work, we ntroduced a new extenson to the OSPF routng protocol for all-optcal DWDM networks wth sparse wavelength converson capabltes. We also presented an ILP formulaton for the RWA-LWC problem faced n these networks. Ths ILP formulaton can be used n small networks wth statc traffc load. In the future, a heurstc approach can be ntroduced to deal wth large networks or networks wth dynamc traffc loads. The obectve of any proposed heurstc should be to mnmze the blockng probablty whle offerng paths wth reasonable QoS. A fuzzy-nference rule base can be used to assgn a fuzzy cost to each path based on the crsp metrcs of the network lnks and the QoS requrements of the lghtpaths that need to be establshed. LS Age Opton LS Type=0 TE Type TE LSA ID LSA# Advertsng Router LS Sequence Number LS checksum Length=08 Type=2 Length=84 Type=2 Lnk ID Type=3 Local Interface IP Address Type=4 Remote Interface IP Address Type= Outgong Interface Identfer Type=2 Incomng Interface Identfer Type=32773 Lnk Protoceton Type Not Used Type=32774 Length=8 Shared Rsk Lnk Group (SRLG) Shared Rsk Lnk Group (SRLG2) Type=32775 Length=20 Length of Mask Bandwdth Mask Reserved for Future Use Fgure 4: Wavelength avalablty opaque LSA LS Age Opton LS Type=0 TE Type TE LSA ID LSA# Advertsng Router LS Sequence Number LS Checksum Length=32 Type=2 Length=8 Type=32776 Number of Converters Number of Used converters Fgure 5: Converter avalablty opaque LSA REFERENCES [APOS] George Apostolopoulos, RochGuérn, Sanay Kamat, SatshTrpath, Qualty of Servce Based Routng: A Performance Perspectve, n Proc. Of ACM SIGCOMM, pp. 7-28,, September 998. [BANE] D. Baneree and B. mukheree, A Practcal Approach for Routng and Wavelength Assgnment n Large Wavelength-Routed Optcal Networks, IEEE Journal on Selected Areas n Communcatons, Vol. 4, No. 5, pp , June 996. [BASA] D. Basak, D. Awduche, J. Drake, Y. Rekhter, "Mult-protocol Lambda Swtchng: Issues n Combnng MPLS Traffc Engneerng Control Wth Optcal Crossconnects," Internet Draft, Work n Progress, July [CHAU] Sd Chaudhur, Gsl Halmtysson, Jennfer Yates, Control of Lghtpaths n an Optcal Network, Internet Draft, Work n Progress, August [COLT] R. Coltun, The OSPF Opaque LSA Opton, RFC 2370, July 998. [KATZ] D. Katz, D. Yeung, K. Kompella, "Traffc Engneerng Extensons to OSPF Verson 2," Internet Draft, Work n Progress, October [KOMP] K. Kompella, Y. Rekhter, "Routng Extensons n Support of Generalzed MPLS," Internet Draft, Work n Progress, August [KOMP2] K. Kompella, Y. Rekhter, "OSPF Extensons n Support of Generalzed MPLS," Internet Draft, Work n Progress, August [LEEK] Kyungsk Lee, Kug Chang Kang, Taehan Lee, Sungsoo Park, An Optmzaton Approach to Routng and Wavelength Assgnment n WDM All-Optcal Mesh Networks wthout Wavelength Converson", ETRI Journal, Vol. 24, No. 2, pp.3-4, [MOKH] Ahmed Mokhtar, Murat Azzoğlu, Adaptve wavelength routng n all-optcal networks, IEEE/ACM Transactons on Networkng, v.6 n.2, pp , Aprl 998. [WANG] G. Wang, D. Fedyk, V. Sharma, K. Owens, G. Ash, M. Krshnaswamy, Y. Cao, M. Grsh, H. Ruck, S. Bernsten, P. Nquyen, S. Ahluwala, L. Wang, A. Dora, H. Hummel, Extensons to OSPF/IS-IS for Optcal Routng, Internet Draft, [WENB] March Bo Wen and K. M. Svalngam, "Routng, Wavelength and Tme-Slot Assgnment n Tme Dvson Multplexed Wavelength-Routed Optcal WDM Networks", IEEE INFOCOM, June [ZANG] Hu Zang, Jason P. Jue, and Bswanath Mukheree, "A revew of routng and wavelength assgnment approaches for wavelength-routed optcal WDM networks," SPIE Optcal Networks Magazne, Vol., No., Jan GLOBECOM /03/$ IEEE