Journal of Networks - Academy Publisher

136 JOURNAL OF NETWORKS, VOL. 5, NO. 2, FEBRUARY 2010
communication overhead involved in the process.
Furthermore, it becomes too difficult for a single
centralized NMS to manage the entire network when this
becomes very large. In contrast, decentralized methods
are usually much faster and more efficient than a
centralized NMS, even in small networks with only a few
nodes. This is not only because distributed methods
involve low communication overheads, but also because
certain management functions require rapid and accurate
actions to be taken in response to failed conditions. For
example, if rapid rerouting is required, it is not possible
to rely on a centralized management system to compute a
recovery route for each failed lightpath on demand after a
failure has been detected. Distributed systems would
clearly not have this difficulty.
Network management is performed in a hierarchical
manner involving multiple management systems, which
are usually implemented in a master-slave relationship
between managers and associated agents. The key
concept applied in this manner of implementation is that
of a layering architecture, illustrated in Fig. 2a. Layering
refers to the ability of a network to nest finer-granularity
over coarser-granularity by hiding details and
complexities in different management levels. Thus, these
levels would advertise only a standardized abstraction of
their state information. In such a layered model, the
management functions with associated information can
be decomposed into several layers of abstractions. At the
border between two adjacent layers, for example Layer n-
1 and Layer n, the management view is presented to
Layer n-1. It is presented in the form of management
information that is contained within the agent at Layer n.
For security and other reasons, the management view that
is presented to a higher layer need not unveil all details of
a lower layer. An agent at an arbitrary layer is then
responsible for providing the management information
that is needed at the immediately higher layer. Since the
principle of layering can be applied in a recursive
fashion, the management view of Layer n+1 can be
presented to Layer n, and so on.
In such a hierarchical model, the individual
components to be managed are referred to as Network
Elements (NEs). For AONs, these include Optical Line
Terminals (OLTs), Optical Add/Drop Multiplexers
(OADM), optical amplifiers and Optical-Cross-Connects
(OXCs). Each NE is managed by an Element
Management System (EMS). The NE is represented by an
information model that contains a variety of attributes
associated with it and a set of control and measurement
parameters for monitoring purposes. Typically, a NE
consists of built-in agents that are responsible for
performing control functions such as monitoring and
reporting any performance degradation. As shown in Fig.
2b, an EMS is usually connected to one or more agents
and communicates with neighboring EMSs using
separated control channels. Hence, the EMS has a view
only of the NEs that it can manage and may not have a
comprehensive view of the entire network.
In this model, a lightpath is considered as a sequence
of detection points. An agent therefore, may be able to
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manage one or more detection points, simultaneously. For
example, an OXC node, which is composed of several
active and passive optical components, can be considered
as a transmission segment or a subset of several detection
points that may be managed by one or more agents at the
same time. Accordingly, the corresponding EMS may
communicate with certain agents to examine or alter
some information, without the difficulty of handling each
detection point individually. It will in fact need to collect
a view of performance information, which is only part of
the information illuminated at each detection point. In
short, an agent may act as a proxy for certain EMSs
hosted elsewhere which only need to contact the agent to
get the information they need. In conclusion, additional
management functions need to be implemented at the
agent and EMS components. Special attention needs to be
paid by analyzing and specifying interfaces, protocols
and the control information that should be exchanged
between EMS-EMS and EMS-Agent pairs. Consequently,
the vulnerability analysis of specified interfaces and
protocol stacks from a security perspective is an
important prerequisite. To do so, it is necessary to
generate a detailed picture of the state information of
transmission links in a manner that can be communicated
to EMSs and other control entities involved in the
management process. To satisfy this requirement, control
information should be expanded to include additional
performance measurements, which will be carried and
shared among EMSs. In particular, this needs a
comprehensive synthesis of optical attributes to
distinguish the parameters that change frequently,
requiring continuous exchanges between EMSs, from
those that change infrequently and often depend on the
configuration properties of the individual NEs.
VI. CONTROL PLANE ARCHITECTURES
The design of an optical network is an important and
very practical issue. As stated above, a desirable
architecture should feature, inter alia, flexible
management, automatic lightpath protection and
restoration, and the ability to compile an inventory.
Moreover, network architectures should support the
gradual introduction of new technologies into the network
without time consuming and costly changes to embedded
technologies. However, network architectures currently
used may be categorized in two main models, namely the
overlay model and the peer model. Although both models
consist essentially of an optical core that provides
wavelength services to client interfaces, which reside at
the edges of the network, they are intrinsically different
and offer up two key concepts for managing traffic flows
in the network.
The overlay model hides the internals of the optical
network and thus requires two separate, yet interoperable,
control mechanisms for provisioning and managing
optical services in the network. One mechanism operates
within the core optical network and the other acts as
interface between the core and edge components which
support lightpaths that are either dynamically signaled
across the core optical network or statically provisioned