12. K. Chen, X. Wen, X. Ma, Y. Chen, Y. Xia, C. Hu, and Q. Dong, “WaveCube: a scalable, fault-tolerant,
high-performance optical data center architecture,” in Proc. INFOCOM (INFOCOM, 2015), pp. 1–9.
13. Z. Zhu, S. Zhong, L. Chen, and K. Chen, “Fully programmable and scalable optical switching fabric for petabyte
data center,” Opt. Express 23(3), 3563–3580 (2015).
14. J. Kim, W. J. Dally, and D. Abts, “Flattened butterfly: a cost-efficient topology for high-radix networks,” in
Proceeding of ISCA 126–137 (2007)
15. K.-I. Kitayama, Y.-C. Huang, Y. Yoshida, R. Takahashi, T. Segawa, S. Ibrahim, T. Nakahara, Y. Suzaki, M.
Hayashitani, Y. Hasegawa, Y. Mizukoshi, and A. Hiramatsu, “Torus-topology data center network based on
optical packet/agile circuit switching with intelligent flow management,” J. Lightwave Technol. 33(5), 1063–1071
(2015).
16. N. Farrington, A. Forencich, G. Porter, P.-C. Sun, J. E. Ford, Y. Fainman, G. C. Papen, and A. Vahdat, “A
multiport microsecond optical circuit switch for data center networking,” Photonics Technol. Lett. 25(16),
1589–1592 (2013).
17. C. Clos, “A study of nonblocking switching networks,” Bell Syst. Tech. J. 32(2), 406–424 (1953).
18. K. Christodoulopoulos, K. Katrinis, M. Ruffini, and D. O’Mahony, “Tailoring the network to the problem:
topology configuration in hybrid electronic packet switched/optical circuit switched interconnects,” Concurr.
Comput. Pract. Exper. 25(17), 2412–2432 (2013).
1. Introduction
Emergence of mega data centers in near future imposes a big challenge on traditional electrical
data center networks (DCNs). A mega data center [1] will contain more than 10
5
server racks
[2] distributed over 10
5
square feet [3]. Sometimes, each pair of racks may generate a lot of
traffic needed to be transmitted [4], though the bulk of communication typically changes very
slowly [5]. To support nonblocking interconnection for so many racks, different kinds of
electrical DCNs, such as folded Clos [6] and FatTrees [7], have been deployed. These new
designs can provide full bisection bandwidth for each pair of racks, but would require a large
number of long cables and switches, which remarkably increase the network cost and power
consumption, and thus become the development obstacle of data centers. Hence, to meet the
requirement of mega data centers, it is very important to design a scalable DCN architecture
that can provide nonblocking interconnection with a low complexity.
An interesting effort in recent years is so-called hybrid electrical/optical DCN architecture,
e.g., Helios [8] and c-Through [9], which employs the nonblocking electrical packet switching
network together with a nonblocking optical circuit switching (OCS) network. Compared to the
electrical network, the optical network can provide much higher bandwidth, but requires much
longer reconfiguration time. Thus, the electrical network is used to deal with fine-granularity
but delay sensitive traffic, while the optical network is employed to clean up the hotspot traffic
which is typically delay insensitive and slowly varying. Because of the employment of
high-speed optical network, the network complexity, in terms of number of needed switches,
number of long cables, and power consumption, can be remarkably reduced. The simulation
results in [8] confirm that a factor of 3 reduction in cost, a factor of 6.5 reduction in number of
cables, and a factor of 9 reduction in power consumption can be achieved by the introduction of
optical networks.
However, the scalability of the nonblocking OCS network in the hybrid DCN architecture is
still a big challenge. In [8] and [9], the micro-electro-mechanical-system (MEMS) based
crossbar was used to perform rack-to-rack non-blocking optical circuit switching. Constrained
by the port count of the most updated MEMS that is 320 in practice or 1000 in Labs [10], such
architecture is not applicable in the scenario of mega data centers which contains more than 10
5
server racks. What’s more, if the number of racks is large, a large number of long fibers are still
needed to connect the aggregation top-of-rack (ToR) switches to the MEMS. In [11], Porter et
al. proposed a reconfigurable optical add-drop multiplexers (ROADM) ring based switch, in
which a collection of optical couplers (OCs) and wavelength-selective switches (WSSs) are
combined alternatively to form ROADM stations, connecting together by a fiber ring. In this
architecture, each input is associated with a specific wavelength, and each output can connect to
that input if it is tuned to the wavelength that is associated with that input. Though this design
looks interesting, the port count of the ring-based crossbar is limited by the number of available
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 28547