Wide-Sense Nonblocking Conditions for Flex-Grid OXC-Clos Networks

The emerging high-capacity optical networks make it urgent to design large-scale flexible mesh optical cross-connect (OXC). Though Clos networks are the theory for building scalable and cost-effective switching fabrics, the nonblocking conditions for flex-grid optical Clos networks without wavelength conversion remain unknown. This paper studies the nonblocking conditions for the flex-grid OXC-Clos network, which is constructed from a number of small-size standard OXCs. We first show that a strictly nonblocking (SNB) OXC-Clos network will incur a high cost, as small-granularity lightpaths may abuse central modules (CMs), rendering them unavailable for large-granularity requests due to frequency conflicts. Hence, we propose a granularity differential routing (GDR) strategy, the idea of which is to restrict the set of CMs that can be utilized by the lightpaths of each granularity. Under the GDR strategy, we investigate two system models, called granularity-port binding and unbinding models, and prove the wide-sense nonblocking (WSNB) conditions. We show that the cost of WSNB network is remarkably smaller than that of SNB network, and find that the granularity-port unbinding model can offer more flexible network-bandwidth utilization compared to the binding model, with a slight cost associated with switching fabrics.

I. Introduction

In recent years, the surge in Internet traffic caused by high-performance computing services and multimedia services is driving the continuous growth of optical network capacity. On one hand, the bandwidth of single-fiber links will be exhausted and the use of multiple fibers on optical links has been put on the agenda. As Ref. [1] points out, optical fiber deployment is growing at an annual rate of 15%. On the other hand, the data rates of signals launched into the optical links climb up from 10 Gb/s to 100 Gb/s, and will soon reach 400 Gb/s and above. It is necessary to maximize spectral efficiency according to the data rate and the transmission distance of each demand [2]. In this context, the traditional fixed-grid optical networks, which divide the optical spectrum into fixed wavelength grids, are becoming inefficient. In contrast, the flex-grid optical network, also called elastic optical network (EON) [3], [4], [5], [6], [7], is emerging to support lightpaths of different bandwidths. Accordingly, the design of optical network components should adapt to the new changes.