Resource Allocation in Electrical/Optical
Hybrid Switching Data Center Networks
Zhangxiao Feng, Weiqiang Sun, Jie Zhu, Junyi Shao, and Weisheng Hu
Abstract—Hybrid switching combines the strengths of
electronic packet switching and high-speed optical circuit
switching and is thus widely believed to be a cost-effective
solution for current and future data center networks
(DCNs). In designing a hybrid switching DCN, it is essential
to know how many resources should be devoted to optical
circuit switching and electronic packet switching so the
performance requirements in both planes can be satisfied,
and, at the same time, objectives such as minimizing the
overall system building cost or the power consumption
can be realized. In this paper, we introduce BLOC, the
Blocking LOss Curve, as a tool to characterize hybrid
switching DCNs. With BLOC, the three interacting compo-
nents in hybrid switching systems—resource allocation,
traffic partitioning, and system performance—can be natu-
rally integrated and studied together. We show how BLOC
can be used to obtain feasible resource allocation/traffic
partition combinations. We also show how system parame-
ters, such as flow arrival characteristics, may affect re-
source allocation and system costs. Our study reveals
that the total amount of resource required to satisfy 95th
percentile traffic can be as low as 60% of that required
for 99.9th percentile traffic.
Index Terms—Data center networking; Hybrid switching
networks; Network performance analysis; Network re-
source allocation; Optical circuit switching; Traffic
scheduling.
I. INTRODUCTION
T
o meet the increasing demands for cloud computing,
streaming video, and other emerging cloud applica-
tions, data center networks (DCNs) have been constructed
that can accommodate hundreds of thousands of servers.
The data-intensive applications hosted in the servers
require significant link capacity for data transmission.
Annual global data center traffic is expected to grow at a
rate of 27% over the next few years and will reach 15.3
zettabytes by the end of 2020 [1]. Cisco reports that 77%
of data center traffic remains within data centers [1].
This huge rack-to-rack traffic presents major challenges
for high-speed and low-cost network interconnection
between top-of-rack (ToR) switches.
To meet the bandwidth demands in data centers, hierar-
chical multi-layer tree topologies based on conventional
electronic packet switching (EPS) have been proposed,
such as Fat-Tree [2] and VL2 [3]. However, the number
of required switches increases as the size of the data center
itself increases, and EPS-based DCN architectures con-
sume a large amount of power and have extremely complex
cabling. Hybrid switching architectures that combine opti-
cal circuit switching (OCS) and EPS have recently been
introduced into DCNs [4–10]. By offloading large flows
or relatively stable portions of traffic to slow-changing
and coarser-granular OCS, the cost-effectiveness of deliver-
ing the same amount of traffic in DCNs can be improved. In
[11], the author claimed that using hybrid switching may
bring about 2.8×, 6×, and 4.7× reductions in cost, power,
and cabling complexity, respectively.
In designing a hybrid DCN with both OCS and EPS, it is
essential to know how many resources should be allocated
to each switching plane so the performance requirements
in both planes can be satisfied and objectives such as min-
imizing the overall system power consumption can be real-
ized. Here, resource can refer to the overall budget for
realizing the network or the available cabling/interfaces
used to interconnect the ToRs. This problem is complicated
by the fact that, in hybrid switching systems, the portions
of traffic to be delivered by each switching plane may not be
static or known in advance. It is further complicated by the
fact that the performance requirements in the two switch-
ing planes are different, and, given a cost constraint, the
resource allocation is essentially a tradeoff between the
performances in the two switching planes. In summary,
the interactions between the three components, i.e.,
resource allocation, traffic partitioning, and system perfor-
mance, are essential in the design of hybrid switching
DCNs and must therefore be studied together.
In this paper, we study the design of DCNs with a par-
ticular emphasis on resource allocation. We assume the de-
sign problem is subject to a budget constraint, denoted by
the number of ToR uplink interfaces. We also assume the
performances in the two switching planes are bounded by
maximal allowable values. The overall traffic demand,
comprising both mice and elephant flows, is assumed to
be served by EPS in part and the remainder by OCS,
depending on a system-wide flow size threshold.
Our research makes the following contributions:
• We introduce a framework called BLOC (Blocking LOss
Curve), into which the three components in hybrid
switching DCN design can be naturally integrated and
studied.
https://doi.org/10.1364/JOCN.9.000648
Manuscript received January 18, 2017; revised May 19, 2017; accepted
June 6, 2017; published July 25, 2017 (Doc. ID 284910).
The authors are with the State Key Laboratory of Advanced Optical
Communication Systems and Networks, Shanghai Jiao Tong University,
Shanghai 200240, China (e-mail: sunwq@sjtu.edu. cn).
648 J. OPT. COMMUN. NETW./VOL. 9, NO. 8/AUGUST 2017 Feng et al.
1943-0620/17/080648-10 Journal © 2017 Optical Society of America