Multi-Core Virtual Concatenation Scheme Considering Inter-Core Crosstalk in Spatial Division Multiplexing Enabled Elastic Optical Networks

Yongli Zhao＊, Liyazhou Hu, Chunhui Wang, Ruijie Zhu, Xiaosong Yu, Jie Zhang

State Key Laboratory of Information Photonic and Optical Communication, Beijing University of Posts and Telecommunications, Beijing,100876, China.

* The corresponding author, email: yonglizhao@bupt.edu.cn

I. INTRODUCTION

As an effective approach to support highspeed optical channels beyond 100 Gbit/s,elastic optical networks (EON) were proposed to improve the spectrum efficiency of optical networks [1]. Sliceable transponder and elastic regenerator are proposed to promote the development of EON [2], and software defined networking control technology can manage the spectrum resource efficiently [3-5]. Nevertheless, the transmission capacity of single-core fiber (SCF) will reach the physical limitation soon [6]. To meet the increasing requirement for network bandwidth, spatial division multiplexing (SDM) has been investigated as an important infrastructural technique, which can improve the transmission capacity of single fiber link by using multi-core fiber (MCF) or multi-mode fiber (MMF) [7-9]. Then, spatial division multiplexing enabled elastic optical networks (SDM-EONs) will become the potential implementation form of future optical transport networks. MCF is considered as one of the most popular and efficient ways for SDM-EONs, and trench-assisted MCFs [10]are mostly preferred in SDM transmission due to the ability of avoiding crosstalk interference[6]. This paper mainly considers SDM-EONs with MCF. Three types of MCFs are considered in the paper, including 7, 12, and 19 core fibers, the structures of which are illustrated in figure 1.

In SDM-EONs, the frequent setup and torn down of lightpaths in dynamic network scenario will break the spectrum resources into non-aligned, isolated and small-sized spectrum fragments. Spectrum fragments result in low spectrum utilization and high blocking probability since these fragments could be hardly used for new incoming requests [11]. Then,how to make use of these spectrum fragments becomes an important issue for the performance improvement in SDM-EONs. How to avoid the spectrum fragmentation problem has been well studied in EONs [12-16]. In [12],the spectrum fragmentation problem is categorized as the fragmentation and misalignment subproblems, and joint routing and spectral fragmentation assignment algorithm is proposed to alleviate the spectral fragmentation in the lightpath provisioning process. Dynamic and adaptive bandwidth defragmentation (DF)in EONs with time varying traffic using connection reconfigurations was investigated in[13]. [14] studied how to take the advantage of the centralized network control and management provided by software-defined EONs for realizing OpenFlow-assisted implementation of online defragmentation (DF). However, this problem in SDM-EONs is more challenging,and there are several new features such as the mitigation of spectrum continuity constraint,which means that the signal can be exchanged from core to core freely while maintaining the same spectrum slice. Furthermore, there is an additional physical constraint introduced by inter-core crosstalk (XT). When the same spectrum slices overlap on the adjacent cores,crosstalk will occur.

Virtual concatenation mechanism [17] has been implemented in terms of multi-path provisioning in EON to improve the spectrum utilization using the non-contiguous spectral fragments based on sliceable transponders[18]. Some requests have huge bandwidth requirement, such as the data migration and data back up. The virtual concatenation can gather the spectrum segments of different cores to prevision the request. In this way, the advantage of SDM can be fully used. However, virtual concatenation is difficult to be implemented in SDM-EONs due to inter-core crosstalk and spectrum contiguity constraint across MCF.

Actually, spatial super-channel has been investigated from hardware perspective to mitigate the traditional contiguity constraint in spectrum dimension. By exploiting the highly correlated properties of different cores in single MCF, B. J. Puttnamet al. show that spatial super-channel can be achieved based on Self-Homodyne Detection (SHD) in MCF[19]. Then, spectrum resources in different cores of SDM-EONs can be fully utilized.This paper focuses on super-channel provisioning based on virtual concatenation solution in SDM-EONs. Crosstalk-aware multicore virtual concatenation scheme (MCVC) is first proposed to improve the performance of SDM-EONs in terms of blocking probability and spectrum utilization.

The rest of the paper is organized as follows. Section 2 introduces some basic knowledge of SDM-EONs including switch fabric with MCF and inter-core crosstalk. Crosstalk-aware MCVC scheme is presented in section 3. Section 4 describes the simulation results in terms of blocking probability and spectrum utilization, and section 5 concludes the paper.

A crosstalk-aware multi-core virtualization concatenation(MCVC) scheme is proposed in this paper for SDN-EONs.

II. SDM-EON

2.1 Switch fabric in SDM-EON with multi-core fiber

Fig. 1 Multi-core fiber with different number of cores

Spectrum slots in different cores are the most important resources in SDM-EONs. A spatially and spectrally resolved optical switching node is designed in figure 2. As the most important resource in frequency domain, spectrum slot is the basic bandwidth unit in optical layer. Spectrum continuity must be followed,which means that an end-to-end lightpath must use the same spectrum slots from the source node to the destination node unless wavelength converter or OEO units are deployed at the middle nodes. Meanwhile, the service can be carried by several spectrum slots within the same core. These spectrum slots must be continuous in frequency dimension, which can be considered as spectrum contiguity constraint[4]. Of course, all the spectrum slots in one core channel should guarantee the orthogonal property.

Fig. 2 Spatially and spectrally resolved optical switching fabric

Fig. 3 Core switching with spectrum continuity

Fig. 4 (a) Schematic of trench-assisted seven-core fiber; (b) Schematic of a core with index trench

Spatially and spectrally resolved optical switching fabric is shown in figure 2. Different spatial cores on a fiber link are de-multiplexed,and bandwidth-variable wavelength selective switches (WSS) consist of a reconfigurable optical add-drop multiplexer (ROADM),allowing adding, dropping and switching of different flexible channels with granularity down to the wavelength level. The transceiver resources consist of a transceiver pool, supplying the appropriate sub-transceivers according to the traffic requirement. In the switch fabric,different spectrum slots can be switched between different cores, but must follow spectrum continuity as shown in figure 3.

2.2 Inter-core crosstalk

Inter-core crosstalk [20-22] is a key physical constraint in SDM-EONs, which severely impacts the signal quality during transmission. To decrease the inter-core crosstalk and achieve dense core arrangement, a trench-assisted MCF (TA-MCF) was developed [23].Figure 4(a) shows a schematic diagram of the seven-core model used in this paper. The schematic of a core with index trench is shown in figure 4(b). To evaluate the statistical mean inter-core crosstalk of a MCF, we adopt formulation (1). Furthermore, the coupled-power theory is considered to form formulation (2),where is the mean inter-core crosstalk [5].

In formulation (1), denotes the mean increase in inter-core crosstalk per unit length. k,r, β and wthare the relevant fiber parameters,representing the coupling coefficient, bend radius, propagation constant, and core-pitch.In formulation (2), is the number of adjacent cores and represents the fiber length. We note that the inter-core crosstalk is mainly affected by the number of adjacent cores and the length of the fiber. So, when routing, spectrum and core assignment (RSCA) algorithms are conducted in SDM-EONs, crosstalk should be considered and a certain threshold of inter-core crosstalk should be set [24].

III. MCVC SCHEME

3.1 Network model

In this paper, we consider the problem of virtual concatenation in SDM-EONs, where the spectrum resource can be simplified as Frequency Slot (FS) in each MCF link. We formulate the physical network as a graph G(V, E), where V is a set of network nodes, and E is a set of MCF links. Each link L (L∈E) is composed of a core set C, and each core has a set of FSs. Matrix Alis defined to denote the occupation status of different FS on each link l as formulation (3).

As shown in formulation (3), Alis composed of c columns and f rows, which represent c cores in link l and f FSs in each core.The matrix element Oi,jis a binary value,which is used to denote the occupation status of frequency slot (FS) i in core j. For example,Oi,j=1 means FS i in core j is available, while Oi,j=0 means the corresponding FS is occupied. For a pending end-to-end connection request, we formulate it as R(s, d, b), where s and d are the source and destination nodes, and b is the required bitrate of this request. Once an end-to-end request arrives, the network operator needs to select modulation format such as Binary Phase Shift Keying (BPSK) and Quadrature Amplitude Modulation (QAM)[25] according to Signal-to-Noise Ratio (SNR)in the selected path.

Fig. 5 Super-channel provisioning with virtual concatenation

3.2 Super-channel provisioning based on virtual concatenation

In principle, high bitrate data streams are transported as groups of sub-channels in spatial super-channel occupying the same FSs in separate cores. To simplify the problem,we assume that the transceivers located at each optical node can be sliceable, so that the spatial super-channel can be constructed with different spectrums in separated cores. This paper discusses the problem of super-channel provisioning based on virtual concatenation in SDM-EONs. Traditionally, contiguity is one of the most important constraints for building super-channel in SDM-EONs. Based on spectrum contiguity constraint, several RSCA algorithms are proposed [26-28]. However,many discontinuous FSs cannot be used to carry high bitrate requests. When a new service request arrives, maybe no core has enough vacant continuous frequency slots, and this request will be blocked. To handle this kind of un-optimal condition, we propose a multi-core virtual concatenation scheme (MCVC) considering inter-core crosstalk to build super-channels on both core and spectrum dimensions based on SHD. The corresponding pilot tones are branched from a separate core to act as the local oscillator (LO) in coherent detection of the dropped data [9], and not considered in the paper. The idea of MCVC can be illustrated as figure 5, where the FS groups of {O1,6, O2,7,O2,8, O1,9, O1,10} and {O3,1, O3,2, O4,3, O3,4} are selected to construct the super-channel for S1 and S2, respectively. MCVC scheme follows spectrum continuity constraint along the route,though the spectrum may be distributed in different cores. Then, the MCVC scheme implements the super-channel by using available continuous spectrums not only in one core, but also in other cores. It is able to make full use of spectrum resources and minimize spectral fragments. It is worth noting that the differential delay between the routing cores can be addressed with additional electronic buffering in the higher layer of the destination node [29].

3.3 Crosstalk-aware MCVC

Fig. 6 SCVC and MCVC comparison

The main ideas of crosstalk-aware multi-core virtual concatenation (MCVC) and single core virtual concatenation (SCVC) schemes are described in figure 6. Figure 6(a) is a lighpath with two hops. SCVC scheme is a typical spectral super-channel, for which each request is carried by consecutive frequency slots only in single core, where contiguity is one of the most important constraints, and many discontinuous FSs cannot be used to carry high bitrates requests [30]. For example, a super-channel with only two FSs can be built in core 0 along the lightpath as shown in figure 6(b). While for MCVC scheme, a super-channel with four FSs can be built crossing two cores along the lightpath as shown in figure 6(c). When a lightpath is setting up, the crosstalk of the lightpath will be calculated using Eq. (2). If the corresponding crosstalk value is larger than the threshold, the crosstalk will affect the signal seriously and the lightpath will not be set up. Details of MCVC scheme can be found as follow.

Scheme: Crosstalk-aware MCVC

20: end if 21: end if 22: end for 23: if there is no satisfactory FS segment 24: block the request;25: end if 26: update FSs status;27: end if

Compared with MCVC scheme, SCVC scheme does not have steps 13 to 20 in the procedure. As described above, line 2 completes the KSP algorithm, the time complexity is K*V2. Line 3 checks whether there is candidate in {P}. Lines 6 to 20 allocates the modulation format and frequency slots for the connection request. Line 7 decides the modulation format and frequency slots number. Lines 8 to 19 allocate the frequency slots. Lines 9 to 12 allocates the frequency slots in the same core,while lines 13 to 18 allocates the frequency slots in different cores. They have same time complexity, i.e.Line 24 updates the frequency slots status in the entire networks, the time complexity of which isThen, the total time complexity of MCVC iswhich is polynomial.

IV. PERFORMANCE EVALUATION

We evaluate the performance of the proposed MCVC scheme through simulations on NSFNET with14 nodes and 21 links as shown in figure7, where each node is configured with sliceable transceivers, and each link is configured as MCF. The number of FSs per core is set as 300. The fiber parameters , in formulation (1) are set as, and the threshold for the crosstalk is -32dB. Under each traffic load, the simulator generates 10000 requests following Poisson model with the fixed departing rate 0.04. The source node and destination node of each request are generated randomly. In this simulation, the bitrate requests are simplified as the number of FSs. How to choose modulation format is not illustrated in this paper,which does not affect the result to a large extent. To simplify the simulation, we choose BPSK for all the requests. Taking the SCVC scheme as the benchmark scheme, we evaluate the performance of MCVC in terms of blocking probability and spectrum utilization. In the simulation with traffic load increasing, we set the number of cores of each MCF link to be 7. The number of required FSs are evenly distributed in two conditions: from 4 to 9 and from 9 to 14. To avoid the linear and nonlinear intra-core impairments, one spectrum slot is assumed as the guardband between each lightpath. Then the total required FSs are actually 5 to 10 and 10 to 15.

Fig. 7 NSFNET topology with 14 nodes

Fig. 8 Blocking probability of MCVC and SCVC

Fig. 9 Spectrum resource utilization of MCVC and SCVC

Fig. 10 Blocking probability of different cores number

Fig. 11 Spectrum resource utilization of different cores number

Figure 8 and 9 show that both blocking probability and spectrum resource utilization grow as the traffic load increasing from 100 to 1000 erlang. It is notable that the advantages of the proposed MCVC scheme are more significant in the condition with more FS requirement, especially when traffic load is beyond 500 erlang. For SCVC scheme, the more FS a request requires, the more likely it is blocked for lacking enough FSs in one single core. However, super-channels crossing different cores can be built with MCVC scheme,which can improve the spectrum utilization and reduce the blocking probability. We also compare the performance of blocking probability and spectrum utilization of these two schemes under different core numbers. Obvious differentiations between SCVC scheme and MCVC scheme in MCFs can be observed in figure 10 and figure 11, which indicate that the proposed MCVC scheme can achieve better performance among 7-core, 12-core and 19-core MCF. The traffic load is 2000 erlang. As shown in figure 10, we can note that blocking probability of both MCVC and SCVC schemes decrease with the increasing cores number in SDM-EON, because there are more spectrum resources with core number increasing. Meanwhile, MCVC scheme can get the best performance when the core number is 12, compared with SCVC scheme. Due to the same reason, the spectrum resource utilization decrease with cores number as shown in figure 11. MCVC scheme can get higher spectrum resource utilization compared with SCVC scheme, because there will be fewer requests blocked for MCVC scheme.

The figures above indicate that the blocking probability of MCVC scheme is lower than that of SCVC scheme, while the resource utilization of MCVC scheme is higher than that of SCVC scheme both in experimental MCFs and traffic load. Both advantages can be achieved due to utilizing distributed FSs to build super-channels in spatial dimension by MCVC scheme. In the traditional methods, the small spectrum fragments are not easy to be utilized to serve the requests. However, in the algorithm we proposed, these small spectrum fragments will be gathered together to serve the requests. Therefore, less fragments will result in higher resource utilization ratio and lower blocking probability eventually.

VI. CONCLUSION

In summary, crosstalk-aware multi-core virtualization concatenation (MCVC) scheme is proposed in this paper for SDN-EONs.Simulation results show that the proposed crosstalk-aware MCVC scheme can get better performance compared with SCVC scheme in terms of blocking probability and spectrum utilization all in 7-core, 12-core, 19-core MCFs.

ACKNOWLEDGEMENT

This work has been supported in part by NSFC project (61571058, 61601052).

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