non-interfering wavelengths. To improve the scalability, Chen et al. in [12] proposed a
WSS-based optical network called WaveCube, of which the topology is an n-dimensional cube.
WaveCube can support tens of thousands of ToR switches, and has a cabling complexity 2-3
orders smaller than that of a Fat-tree. However, it could only provide 70% - 85% bisection
bandwidth of a non-blocking network with a complex route and wavelength assignment
algorithm. Furthermore, WaveCube needs hop-by-hop electrical relays, which increase not
only queuing delay of the traffic but also the switching overhead of electrical ToR switches.
Another kind of optical network called OvS network was constructed in [2,13], based on
flattened butterfly topology [14]. This design possesses the properties similar to the WaveCube,
except that end-to-end communications require at most one electrical relay. Kitayama et al. in
[15] proposed to improve the scalability using optical packet switching technology, which
however will not be commercially available in the near future. Thus, the design of scalable
optical networks to provide network-level nonblocking interconnections for all the ToR
switches in the mega data centers remains an open issue.
To deal with the slow-varying bulk traffic in mega data centers, this paper studies multiple
ROADM rings based OCS networks, which are essentially the combination of MEMS
crossbars and ROADM rings proposed in [11,16]. This type of optical network is designed
according to the idea of classical Clos networks [17], such that the size of each switching
element can be small though the number of server racks is large. Also, this kind of network
possesses another important advantage that the number of long optical cables can be further
reduced since the ring-based crossbars can be arbitrarily stretched to pass through the areas that
ToR clusters are installed such that each rack could connect to the OCS network through short
fiber cables.
Two network structures are considered: ring-MEMS-ring (RMR) network and
MEMS-ring-MEMS (MRM) network. In the RMR network, the ring-based crossbars
constitutes the input stage and output stage and the MEMS crossbars associated with different
wavelengths form the central stage. In the MRM network, the ring-based crossbars in the
central stage are sandwiched by the MEMS crossbars in the input stage and output stage. We
show that the MRM network is better than the RMR network, since the MRM network does not
need tunable lasers to achieve nonblocking routing.
2. Preliminary
In this section, we introduce structure and features of a ring-based switch proposed in [11],
since it is the building block of our designs. We show that the switching network directly
constructed from the ring-based switches is not a good idea to build a large scale switching
network, which actually motivates our work in Section 3 and 4.
2.1 Ring-based switches
Fig. 1. The ring-based switches: (a) legacy ROADM ring, (b) input-output ring.
Figure 1 displays two ring-based switches including a legacy ROADM ring in Fig. 1(a), an
input-output ring in Fig. 1(b). The legacy ring in total contains n ROADM nodes comprising a 1
× 2 OC and a 1 × 2 WSS to add, drop or pass the signals simultaneously. Herein, the WSS has
much longer reconfiguration time (on the order of ms) than that in Ref [16], because the OCS
Received 7 Sep 2015; revised 18 Oct 2015; accepted 19 Oct 2015; published 22 Oct 2015
2 Nov 2015 | Vol. 23, No. 22 | DOI:10.1364/OE.23.028546 | OPTICS EXPRESS 28548