Abstract: In this paper, a novel Infrastructure as a Service architecturefor future Internet enabled by optical network virtualization is proposed.Central to this architecture is a novel virtual optical network (VON) compo-sition mechanism capable of taking physical layer impairments (PLIs) intoaccount. The impact of PLIs on VON composition is investigated based onboth analytical model of PLIs and industrial parameters. Furthermore, theimpact of network topology on VON composition is evaluated.
References and links1. Cisco white paper, “Cisco Visual Networking Index: Forecast and Methodology, 2009-2014”.2. S. Figuerola and M. Lemay, “Infrastructure Services for Optical Networks (Invited),” J. Opt. Commun. Netw.
1(2), A247–A257 (2009).3. M. Chowdhury and R. Boutaba, “A Survey of Network Virtualization,” Comput. Netw. 54(5), 862–876 (2010).4. T. Takeda, “Framework and Requirements for Layer 1 Virtual Private Networks,” RFC 4847 (2007).5. C. V. Saradhi and S. Subramaniam, “Physical Layer Impairment Aware Routing (PLIAR) in WDM Optcial
Networks: Issues and Challenges,” IEEE Commun. Surv. Tutor. 11(4), 109–130 (2009).6. S. Peng, R. Nejabati, S. Azodolmolky, E. Escalona, and D. Simeonidou, “An Impairment-aware Virtual Optical
Network Composition Mechanism for Future Internet,” in Proceedings of ECOC, Tu.6.K.3 (2011).7. G. P. Agrawal, Fiber-Optic Communication Systems, 3rd. ed. (Wiley-Interscience, 2002).8. W. Lin, ”Physically Aware Agile Optical Networks,” Ph.D. dissertation (Montana State University-Bozeman,
2008).9. W. Lin, T. Hahn, R. S. Wolff, and B. Mumey, “A distributed impairment aware QoS framework for all-optical
networks,” Opt. Switching Networking 8(1), 56–67 (2011).10. G. Ellinas, N. Antoniades, T. Panayiotou, A. Hadjiantonis, and A. M. Levine, “Multicast Routing Algorithms
Based on Q-Factor Physical-Layer Constraints in Metro Networks,” IEEE Photon. Technol. Lett. 21(6), 365–367(2009).
11. X. Cheng, S. Su, Z. Zhang, H. Wang, F. Yang, Y. Luo, and J. Wang, “Virtual network embedding throughtopology-aware node ranking,” ACM SIGCOMM Comput. Commun. Rev. 41(2), 38–47 (2011).
1. Introduction
Future Internet is characterized by global delivery of high-performance network-based appli-cations, such as Cloud Computing and (Ultra) High Definition Video-on-Demand Streaming[1], over a high-capacity dynamic optical network. Due to the requirements for high capacity,low latency, and deterministic quality of service (QoS), dedicated optical network services are
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B251
desired. However, as these types of applications evolve, the current technical and operationalcomplexities as well as CAPEX and OPEX considerations will limit the ability of network op-erators to setup and configure dedicated network for each application in a scalable manner [2].Therefore, a key challenge for network operators is the deployment of dynamic optical infras-tructures capable of supporting all application types, each with their own access and networkresource usage patterns.
Infrastructure as a Service (IaaS) framework [2] is a key enabler to address this challenge.By adopting IaaS framework and its key technology enabler, Network Virtualization [3], net-work operators are able to partition their physical infrastructures into virtual networks, basedon user/application requirements, and offer them as infrastructure services to users. In that case,multiple virtual networks will coexist over a shared physical infrastructure. These different co-existing virtual networks should be isolated, without interferences between each other. Layer 1virtual private network (L1VPN) [4] has been proposed as a solution for optical network virtu-alization. However, the existing L1VPN solutions involve mainly point-to-point dedicated andpre-defined connectivity with deterministic QoS. They are not able to create multiple coexist-ing virtual networks each with its own network topology where the owners of these virtual net-works have full control in terms of routing and management over their portion of the network.Furthermore, optical networks are analogue in nature, which differentiates optical network vir-tualization from other network virtualization technologies, i.e., layer 2 (L2) and layer 3 (L3)virtualization [3]. Due to the existence of physical layer impairments (PLIs), the adjacent ac-tive channels interfere each other, which will impact the isolation of multiple coexisting virtualoptical networks (VONs) and the way that VONs are composed. The “PLI-aware” networkingis not a new concept, and a lot of work has been done in PLI-aware routing algorithms [5].However, in optical network virtualization, the impact of PLIs not only on routing paths butalso on all the active coexisting virtual networks needs to be investigated.
In this paper, a new IaaS architecture for the future Internet utilizing optical network virtu-alization is proposed. For this architecture, a novel PLI-aware VON composition mechanism isproposed capable of creating multiple co-existing VONs, each with its own network topologyand QoS requirements. The analytical model for assessing the impact of PLIs is also introduced.The impact of PLIs on VON composition is investigated based on both analytical assessmentmodel of PLIs and industrial parameters. Furthermore, the impact of network topology on theVON composition is evaluated. To the authors’ best knowledge, no previous work in the litera-ture has addressed solutions for the network virtualization and optical infrastructure as a servicein the context of optical network considering the effect of PLIs.
Virtualization is of paramount importance in next generation networks (NGN) to enable dy-namic and efficient infrastructure services [3]. However, virtualization in optical networks en-tails certain challenges inherent to their analogue nature that do not arise in L2 and L3 vir-tualization. The main concept of optical network virtualization is to allow the composition ofisolated VONs, which are coexisting simultaneously over the same physical infrastructure. AVON, as such, is composed of a set of virtual optical switches (or virtual optical nodes) in-terconnected by virtual optical links. The virtualization of an optical switch is achieved bypartitioning or aggregation of physical optical switches. The associated virtual optical linksare defined as the connections between virtual nodes. Therefore, a virtual link may consist ofseveral physical links traversing several physical switches as shown in Fig. 1. The link virtu-alization granularity (e.g., wavelength or sub-wavelength) depends on the available technologythat can guarantee the isolation between virtual links that share the same physical medium. Theoptical transmission technologies such as DWDM allow the separation of wavelength channels
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B252
in a single fibre. However, PLIs may heavily affect the isolation between adjacent wavelengths,thus interfering signals that belong to different channels, or in our case, different VONs.
Virtual Networks
Physical Infrastructures
Management and Control of OXC
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Fig. 1. Example of the virtualization of an optical network.
In this paper, a new IaaS architecture enabled by a PLI-aware optical network virtualizationmechanism is proposed. The architecture is composed of three layers, as shown in Fig. 2: (i)the optical physical infrastructure layer, which is composed of optical network resources; (ii)the optical virtualization layer, in which the physical resources are virtualized and composedforming VONs; and (iii) the virtual optical network control and management layer that providescontrol and management functionalities for each VON. The optical virtualization layer is thekey innovation and main focus of this architecture. Its functionalities can be simplified in:abstraction to hide technological details and unify the representation of physical resources forvirtualization, partitioning/aggregation of nodes and links, and VON composition. VONs arecomposed by selecting and interconnecting the virtual resources. The functionalities of thecontrol and management layer could be provided by conventional provisioning mechanismssuch as GMPLS, so it will stay out of the scope of this paper. Since multiple VONs can coexistsharing the same physical substrate, the interferences caused by the PLIs between VONs willimpact the isolation and the performance of each of them. Therefore, the proposed architecturedeploys a PLI monitoring and evaluation system within the optical virtualization layer alongwith a PLI-aware VON composition mechanism. This enables composition and operation ofVONs considering the effect of PLIs and guaranteeing a perfect isolation while satisfying therequested QoS.
3. PLI-aware VON composition
The proposed IaaS architecture enabled by optical network virtualization allows serviceproviders to lease resources on demand from infrastructure providers. Moreover, the parti-tioning and/or aggregation of these resources empowers the creation of multiple simultaneousVONs, each with its own topology and QoS requirements, running over the same optical net-
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B253
Optical Virtual Network Control and Management Layer
...
Network Control Plane
PLI monitor & calculator
Fig. 2. The reference model of the PLI-aware IaaS architecture.
work infrastructure. Requests for composing VONs are usually generated by service providersor operators. Each request has associated requirements that need to be fulfilled when composingthe VON. Given the information of physical network resources and the requirements of VONrequests, an intelligent and dynamic composition mechanism is needed to create VONs on de-mand, utilizing the available physical resources. In this paper, a PLI-aware VON compositionmechanism is proposed.
In our work, the physical optical network is modelled as a weighted undirected graph anddenoted as Gp = (Np,E p), where Np is the set of physical optical nodes and E p is the set ofoptical links. Each np
i ∈ Np is associated with the geolocation and the switching capability ofthe optical node. Each link ep(i, j) ∈ E p between nodes i and j, with the weight value l(ep)denoting its length, is associated with the number of wavelengths W and the data rate perwavelength R. A VON request is specified as a virtual network topology Gv = (Nv,Ev). Thegeolocation and attributes of each virtual node nv
i ∈ Nv are indicated in the request, as well asthe capacity of each virtual link ev(i, j) ∈ Ev, which also shows how virtual nodes are inter-connected by virtual links.
Upon the arrival of a VON request, a proper mapping of the VON to the available physicalresources is executed by the VON composition mechanism. The proposed PLI-aware VONcomposition mechanism can be decomposed into three major phases:
1. Node mapping: Each virtual node nvi ∈ Nv is mapped to a physical node np
i ∈ Np accord-ing to the requirements of geolocation and switching capability of nv
i .
2. Link mapping: Each virtual link ev(i, j) ∈ Ev, requesting for certain number of wave-lengths, is mapped to a path or a set of physical links ep(i, j) ∈ E p by adopting routingalgorithms (e.g., shortest path or load-balancing routing algorithms). It must be notedthat a virtual link connecting two virtual nodes may need to traverse several physicalnodes (see Fig. 1). This requires the virtual link mapping process to be able to performreservation and configuration of such nodes.
3. Quality verification: The main novelty of the proposed VON composition mechanism isthat the PLIs inherent to the optical layer are taken into account, that is, even though thereare enough available physical resources to satisfy the requirements of the VON request,the quality of the VON still needs to be verified. Due to the nature of the interferencesintroduced by PLIs, different active VONs will intervene with each other. Therefore, the
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B254
quality of all the existing and the newly to be established VONs needs to be verified.For each wavelength wk ∈ ep(i, j) to be used in the VON, the quality qk(i, j) of all theactive wavelengths in the link ep(i, j) will be verified. If the quality of all the involvedVONs is acceptable, the wavelength will be allocated to the VON, and the quality of allthe involved VONs will be updated. Otherwise, the phase 2 will be executed again tillthere is no available wavelength resources. The quality verification leads to much morerealistic outputs or solutions for service providers to satisfy their users’ requirements.
When a successful VON mapping is achieved, the associated physical network resources areallocated to that VON, ensuring the required level of signal quality. Once the VON is no longerrequired, all the allocated resources are released.
In our previous work [6], a simple PLI model is used to verify the quality of VONs. Inthis work, an analytical model for assessing PLIs, also with industrial parameters as inputs, isadopted, which is elaborated in the following section.
4. Analytical model for assessing PLIs
In this study, the nonlinear impairments XPM (Cross-Phase Modulation) and FWM (Four-WaveMixing) are considered, as well as the linear impairment ASE (Amplified Spontaneous Emis-sion), since they are the major impairments that can impact the signal quality significantly [7].
In order to measure the signal quality, the Q factor is obtained using the following definition,
Q = 10lg(Psig peak√
σ2ASE +σ2
XPM +σ2FWM
), (1)
where Q is the Q factor, Psig peak is the peak power of the wavelength channel, σ2ASE repre-
sents the ASE noise, σ2XPM and σ2
FWM represent the non-linear impairments generated by XPMand FWM respectively.
ASE is the noise generated from optical amplifier, which is given as follows,
σ2ASE = Fh fc(G−1)B0, (2)
where F is the EDFA noise figure, h is Planck’s constant, fc is the carrier frequency of achannel under consideration, G is the EDFA optical gain and B0 is the optical bandwidth of thechannel.
XPM, as modelled in Eq. (3), is the non-linear phase modulation of an optical channel causedby intensity fluctuations of other co-propagating optical channels, which will be eventuallyconverted to the intensity modulation of the channel [8].
σ2XPM =
12π ∑
j∈pumps
∫ ∞
−∞
∣∣Hj(w)∣∣2PSDj(w)dw, (3)
where Hj(w) is the XPM transfer function of the pump channel j, and PSDj(w) is the powerspectrum of the channel j.
FWM originates from the third order non-linear susceptibility in optical links. When threeoptical signals in different wavelengths are co-propagating within a fibre simultaneously, a newoptical signal is generated in a frequency close to one of the three existing ones, which mayinterfere with them. The total FWM noise generated on the probe channel is given in Eq. (4).
σ2FWM = ∑
fi+ f j− fk= fpi, j �=k
σ2i, j,k, (4)
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B255
σ2i, j,k = PiPjPk
∣∣∣∣γd3
1+ e−2αL −2e−αL cos(K ∗L)α2 +K2
∣∣∣∣2
, (5)
K = 2πλ 2
p
c( fi − fp)( f j − fp)[D− λ 2
p
c(
fi + f j
2− fp)S], (6)
where σ2i, j,k is the FWM noise generated by one FWM mixing term with the frequency group
( fi, f j and fk), and fp is the frequency of the probe channel. If fi, f j or fk equals to fp, Pi,Pj or Pk equals to Psig peak, otherwise, 1
2 Psig peak. γ is the nonlinear coefficient of Non-zerodispersion-shifted fibre (NZDSF), d is 3 (degenerate FWM) or 6 (non- degenerate FWM), α isattenuation of NZDSF, L is the length of the optical link, D is the dispersion of NZDSF and Sis the dispersion slope of NZDSF.
5. Simulation studies
The simulation studies investigate the impact of PLIs on the VON composition. The PLIs, in-cluding ASE, XPM, and FWM, are assessed according to the analytical model elaborated inSection 4. The physical parameters used in the assessment model are given as follows: the datatransmission rate per wavelength is 10Gbps; σ2
ASE of a single EDFA is 10−9 [9]; the chan-nel power Psig peak is 1mW ; the attenuation α , the dispersion D and the dispersion slope Sof NZDSF are 0.25dB/Km, 4ps/(Km ∗ nm) and 0.08ps/(Km ∗ nm2) respectively; the non-linear coefficient of NZDSF is 2/(Km ∗W ). The ITU-T G.694.1 Grid, anchored to 193.1THz(1552.52nm) is adopted and different channel spacing (e.g., 50GHz, 100GHz, and 200GHz) aretaken into account in the simulations. The mutual impact of existing VONs and newly requestedVON in terms of PLIs is considered by checking the quality of all the active wavelength chan-nels in half of the wavelength window (the wavelength window is the same as the total numberof wavelengths). The Q factor threshold is set to a reference value of 8.5dB according to [10].
To investigate the possible issues when facing the challenge of deploying dynamic VONrequests, request arrivals are assumed to follow a Poisson process with the average inter-arrivalperiod of 20 time units, and each request has an exponentially distributed holding time withthe mean value varying from 100 to 1000 time units [11]. In one instance of simulation, 500VON composition requests are generated randomly. In each VON request, the number of virtualnodes is randomly determined by a uniform distribution between 2 and half the number ofphysical network nodes, and each pair of virtual nodes are connected with the probability 0.5.Virtual links are mapped to the physical paths calculated by using the shortest path routingalgorithm. For simplicity, only one wavelength is requested for each virtual link. In this study,both scenarios with and without wavelength convertors deployed in core nodes are explored.Three network topologies have been investigated as depicted in Fig. 3.
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Fig. 3. Physical topology with average node degree shown in paranthesis.
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B256
5.1. The impact of wavelength channel spacing on VON composition
We first evaluate the impact of different wavelength channel spacing values on the VON compo-sition. The failure rate of VON composition is collected as the performance comparison criteria,which is defined as the ratio of the number of declined VON requests relative to the total numberof VON requests. In this simulation, the NSF topology with 14 nodes-21 links is adopted, andthe number of wavelengths per link is set to 16. The channel spacing is ranged from 50GHz to200GHz. As the channel spacing increases, the interference between adjacent channels causedby PLIs (XPM and FWM) is reduced. Therefore, two VONs are more likely to use two adja-cent wavelength channels and wavelength resources are used more efficiently. This is reflectedin Fig. 4, where results verify the analytical model for assessing PLIs and the established simu-lation platform. As expected, results show that when virtualizing optical networks, more VONscan be composed over the same optical physical substrate by increasing the channel spacing. Onthe other hand, the figure also depicts that for longer VON holding time, requests are blockedwith a higher probability.
Fig. 4. The impact of different channel spacing on the VON composition.
5.2. The performance evaluation of PLI-aware VON composition
In order to evaluate the PLI-aware VON composition mechanism, two different scenarios havebeen analyzed with and without Q verification (i.e., “Q Verification” and “no Q Verification”in Fig. 5 and 6). In the scenario with “no Q Verification”, VON requests are composed withoutconsidering the impact of PLIs. On the other hand, in the scenario with “Q Verification”, a VONrequest is composed only using the available physical resources that can meet the acceptableQ factor threshold (i.e., 8.5dB). From the results, we can observe the severe impact of PLIs onthe VON composition. The quality verification rejects almost half of the VON requests (about54.72% when the number of wavelengths per link is 16, the VON holding time is 400 units, andthe channel spacing is 50GHz) due to the unacceptable quality despite the availability of spareresources.
Results in Fig. 5(a) also show that, as the number of wavelengths per link increases, thefailure rate of VON composition with Q verification remains almost the same. This is dueto the fact that wavelength selection uses the First-Fit (FF) algorithm. However, the chosenwavelengths are always adjacent to the channels that are currently being used, and most ofthem have unacceptable quality due to PLIs. Moreover, in this scenario, the quality verificationof VON is executed after the wavelength selection, and the quality verification does not give
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B257
any feedback to the resource checking process. Therefore, after the quality verification failure,the VON request is directly rejected. In this case, even if the number of wavelengths per linkincreases, the failure rate of VON composition will not change significantly. To compensatethis effect, in the proposed PLI-aware VON composition algorithm, wavelength selection isjust performed over an already verified wavelength set that can guarantee the required quality.The results of this mechanism, named PLI-aware FFWA, are also shown in Fig. 5 and 6, andcompared with the two previous scenarios. The results indicate that the proposed algorithm canimprove the performance of VON composition (about 65.95% when the number of wavelengthsper link is 16, the VON holding time is 400 units, and the channel spacing is 50GHz). Theimpact of the wavelength conversion (WC) capability is also evaluated, and the results areshown in Fig. 5(b). Without wavelength converters being installed, all the virtual links have tobe mapped to the same wavelength to maintain the connectivity of an entire VON, which willsuffer the limit of physical resources severely, as reflected in the results.
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Fig. 5. The impact of PLIs on the VON composition with different number of wavelengthsper link. VON holding time is 400 units. Channel spacing is 50 GHz.
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Fig. 6. The impact of PLIs on the VON composition with different VON holding times.
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B258
5.3. The impact of node degree on PLI-aware VON composition
The impact of network topology, i.e., node degree, on the PLI-aware VON composition has alsobeen evaluated in the six-node simple and full mesh topologies shown in Fig. 3. It is assumedthat all physical links in the two topologies have the same length and each wavelength channelhas the same data rate. Moreover, to make a fair comparison over the two topologies the totalnumber of resources is the same (120 wavelengths), that is, 8 wavelengths in the full meshnetwork topology (15 links in total) and 15 wavelengths in the simple network topology (8links in total). In Fig. 7, the results show that the optical network with a higher node degree canachieve better VON composition performance (about 32.43% higher when the total numberof wavelengths in each network is 120, the VON holding time is 400 units, and the channelspacing is 50GHz). The improvement is due to the fact that in a full mesh network, virtual linkscan be directly mapped into physical links one to one, whilst with lower node degree a virtuallink may need to traverse multiple physical links experiencing more severely the effect of PLIs.
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Fig. 7. The impact of node degree on the PLI-aware VON composition with different VONholding time. Channel spacing is 50 GHz.
6. Conclusions
This paper proposed a novel IaaS architecture for the future Internet exploiting PLI-aware op-tical network virtualization. For this architecture, a new impairment-aware VON compositionmechanism is proposed. By adopting analytical model of PLIs as well as industrial parameters,network simulation studies show that the proposed mechanism can mitigate the impact of PLIson the VON composition. The impact of the wavelength channel spacing and the wavelengthconversion capability on the VON composition is also evaluated. Furthermore, the results alsoshow that the PLI-aware VON composition mechanism performs better in the network withhigher node degree. The presented results can provide infrastructure providers with guidelinesfor properly planning their networks to serve VON requests with acceptable quality.
Acknowledgments
The work described in this paper has been carried out with the support of the EU funded FP7GEYSERS project and UK funded EPSRC PATRON project.
#155784 - $15.00 USD Received 30 Sep 2011; accepted 1 Nov 2011; published 18 Nov 2011(C) 2011 OSA 12 December 2011 / Vol. 19, No. 26 / OPTICS EXPRESS B259
