01-02-2013, 10:10 AM
Trends in Optical Switching Techniques: A Short Survey
1Trends in Optical Switching.pdf (Size: 121 KB / Downloads: 54)
Abstract
We are currently witnessing a strong worldwide push toward bringing fiber closer
to individual homes and businesses. The emerging FTTX access networks will move
the bandwidth bottleneck from the first/last mile toward metropolitan and wide
area networks, creating a need for efficient optical-switching mechanisms. In this
article, we review the current trends in optical switching that help to improve the
bandwidth efficiency, as well as to decrease the cost and power consumption of
next-generation optical networks. Our review provides an overview of the optical
switching domain and facilitates the understanding of newly emerging switching
techniques and their interpretation as derivatives of the presented main optical
switching trends.
INTRODUCTION
Wide area networks (WANs) were one of
the first network segments that experienced
the widespread deployment of optical
technologies to provide sufficient
capacity in support of heavy long-haul traffic. Currently, by
means of wavelength division multiplexing (WDM), optical
WANs offer large bandwidth pipes where a single fiber can
carry tens or even hundreds of wavelength channels, each
operating at a bit rate of 10 Gb/s or higher. Given these vast
amounts of available bandwidth, one of the major design criteria
of current backbone networks has been not to maximize
the utilization of bandwidth resources but to simplify network
operation and reduce capital and operational expenditures.
Toward these goals, a few optical network technologies were
commercially adopted, for example, Erbium-doped fiber
amplifier (EDFA), reconfigurable optical add-drop multiplexer
(ROADM), wavelength cross-connect (WXC), and tunable
laser, whereas others still must demonstrate their practical
importance [1]. Most current operational optical WDM backbone
networks deploy circuit switching at the wavelength
granularity. The resultant point-to-point wavelength channels
are often referred to as lightpaths (or light-trees in the case of
point-to-multipoint wavelength channels). At present, there is
a strong worldwide push toward bringing fiber closer to individual
homes and businesses. Fiber-to-the-home/business
(FTTH/B) or close to it (FTTX) networks are poised to
become the next major success story for optical fiber communications
[1]. For a survey of recent developments and deployments
of FTTX networks, see [2]
Waveband Switching
Compared to ordinary optical cross connects (OXCs), socalled
multi-granularity OXCs (MG-OXCs) hold great
promise to reduce significantly the complexity, size, and cost
of OXCs by switching fibers and wavebands together without
demultiplexing the arriving WDM comb signal into its individual
wavelengths, giving rise to WBS. As a result, the size of
ordinary cross connects that traditionally switch at the wavelength
granularity can be reduced, including the associated
control complexity and cost, by using a single input/output
port instead of multiple input/output ports, one for each individual
wavelength.
Waveband Grouping
To determine which wavelengths to group together into a single
waveband, several waveband grouping strategies exist,
which can be categorized into end-to-end or intermediate
approaches. In [4], an end-to-end waveband grouping strategy
that groups wavelengths with the same source-destination pair
into a waveband was compared with an intermediate waveband
grouping strategy that groups wavelengths with the same
destination at an intermediate node. The obtained results
indicate that intermediate waveband grouping strategies outperform
end-to-end grouping strategies in terms of required
ports at MG-OXCs.
Routing and Wavelength Assignment
The routing and wavelength assignment (RWA) problem in
WBS networks that use MG-OXCs is, in general, more
involved than that in conventional wavelength-switching networks
due to additional constraints, apart from wavelength
continuity. Several new RWA-related problems in WBS networks
were identified and solved. The so-called routing and
wavelength/tunnel assignment (RWTA) problem deals with
the bundling and switching of wavebands and fibers and routing
lightpaths through them [5].
TDM Switching and Grooming
The additional DXC in Fig. 1 is used to perform TDM switching
and grooming in the electrical domain by means of OEO
conversion of wavelengths and wavebands. Grooming allows
for grouping multiple low-volume traffic flows into a wavelength
or waveband and thereby improve their bandwidth utilization.
In [9], a design study of networks based on a hybrid
WBS-OEO grooming switch architecture was performed taking
physical transmission impairments into account to study
the maximum distance and number of nodes the optical signal
can traverse without undergoing OEO conversion.
Photonic Slot Routing
To improve the utilization of lightpaths under bursty traffic
without requiring electronic traffic grooming at the source
node, a cost-efficient design approach for WDM networks
called PSR was proposed in [10]. In PSR networks, time is
divided into fixed-size slots, whereby each slot spans all wavelengths,
and slot boundaries are aligned across all wavelengths.
The resultant multi-wavelength slot is called a
photonic slot. Each wavelength in the photonic slot may contain
a single fixed-size packet. Furthermore, all packets in a
given photonic slot must be destined for the same node, but
each photonic slot may be destined for a different node.
Optical Flow Switching
One of the main bottlenecks in the current optical Internet is
electronic routing at the IP layer. To alleviate this electrooptical
bottleneck, routers can be offloaded by switching large
transactions and/or long-duration flows at the optical layer,
leading to so-called OFS [14]. In OFS, a dedicated lightpath is
set up for the transfer of large data files or upon detection of
long-duration flows, whereby flows with similar characteristics
(e.g., same destination IP router) may be aggregated and
switched together by means of grooming to improve the utilization
of the established lightpath. Because in OFS the setup
of a lightpath takes at least one round-trip time between
source and destination IP routers, clearly, the size of a transaction/
flow should be in the order of the product of roundtrip
propagation delay and line rate of the lightpath.
The set-up lightpath enables the optical bypassing of intermediate
IP routers and thereby eliminates the need for (electronic)
packet processing, for example, buffering, routing, and
so on. It is important to note that OFS can be end-user initiated
or IP-router initiated [15]. OFS offers the highest-grade
quality of service (QoS) because the established lightpath provides
a dedicated connection. However, OFS must determine
carefully when to set up a lightpath because wavelengths are
typically a scarce network resource.
Conclusions
We have presented the current trends in switching techniques
for next-generation optical networks and explained their
underlying concepts and mechanisms. Each of these trends
has its own specific strengths and limitations and may be
deployed depending on various criteria, such as costs, capacity
requirements, and traffic characteristics. Our comprehensive
overview of the current trends of optical switching
provides a framework for anticipating new switching techniques,
such as hybrid techniques that exploit the different
switching granularities offered by the presented switching
techniques. The discussed optical switching techniques significantly
improve the flexibility of the data plane by providing a
wide range of different temporal and spatial switching granularities.
For commercial viability, however, further research is
required to reduce the complexity introduced to the control
and management planes by each of these optical switching
techniques.