IP Over Optics: Where Are We Going With It?
BY DR. BILLY WU
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Today's networks are struggling to meet the demand created by new users,
new technologies, and new high-bandwidth applications. To meet this
expanding market demand, carriers are re-evaluating how they architect their
Traditional networks -- developed using a multilayer model where layers
are built and managed independently -- make it difficult for carriers to
operate, optimize network resources, and provide networks with
cost-effective reliability. An IP network, for example, may run over a frame
relay network, which runs over an ATM network, which runs over a SONET/SDH
network, which runs over an optical (wavelength) network, and eventually
over fibers. This multilayer model allows each layer to evolve
independently, while continuing to support legacy services.
However, the large number of different devices in the network complicates
the development, deployment, and management. In addition, each layer of the
network typically has an independent management structure and associated
processes that only have visibility of the topology and state information of
that one layer. The number of management systems increases network cost
while adding complexity to network-wide operation tasks such as
provisioning, performance monitoring, and fault isolation. These challenges
are the driving forces behind the need for a much simpler IP over smart
optics architecture, where the vast majority of services -- voice, data,
video, and private line -- are all carried through IP.
REDEFINING NETWORK ARCHITECTURE
To make the transition from today's complicated multilayer network
architecture to the much simpler IP Over Optics architecture, three key
enablers are needed:
A standards-based intelligent optical layer -- Today's
optical networks are mostly static. Services take a long time to provision,
networks are susceptible to manual errors, and service providers must
overprovision their networks to avoid congestion and provide carrier-grade
availability. When bandwidth grows fast and unpredictably, there is an
urgent need to switch optical bandwidth so that a lightpath should be set up
and torn down when and where needed. The International Telecommunication
Union (ITU) standard for Automatically Switched Transport Networks (G.ASTN)
provides dynamic optical connectivity though automatic routing and switching
of transport bandwidth. With G.ASTN, it is possible to sustain a simpler and
less resource-intensive network with automation. Dynamic connectivity leads
to significant reduction in capital, operational, and maintenance costs for
both service providers and end-users.
The packet layer needs to be unified, from today's multilayer or
separate networks to one network that runs the same protocol -- The
Multi-Protocol Label Switching (MPLS) standard developed by Internet
Engineering Task Force (IETF) allows service providers to carry potentially
all traffic types over one universal packet core while continuing to provide
ATM-like Quality of Service. The MPLS standard is based on the blending of
connectionless protocols, like IP, with the virtual-circuit networking
concept. The term "IP Over Optics" is indeed for new generation
switching routers running IP or MPLS over intelligent optical switches.
Interworking of the packet and the optical layers -- With
interworking, it is possible to monitor both packet and optical layers to
recommend or automatically make changes to either the packet layer (create
or tear down MPLS paths) or the optical layer (create or tear down new
lightpaths). The optical layer can then dynamically adapt to the change of
traffic pattern in the IP layer. The key issue is how to exchange routing
information and control signaling between IP networks and optical networks.
IP OPTICAL NETWORKING MODELS
New IP Over Optics architecture proposals can be mainly classified into
overlay model and peer model. The overlay model has separate routing and
signaling protocol sets for each layer, while the peer model has a single
monolithic routing and signaling protocol set spanning both the IP and the
optical layers. In the case of the overlay model with separate protocol sets
for each layer, models can be further distinguished by the level and mode of
interaction between the two layers.
Typically, accessing the optical layer is done through
user-network-interface (UNI) while the interconnection between the optical
components is done through network-network-interface (NNI). In the peer
model, IP routers and optical switches act as peers. There is no
distinguishing between the UNI and NNI, or between UNI and other
router-to-router protocols. A single routing protocol runs across both
layers to discover the topology of IP routers and optical switches. The IP
and optical layer can be truly integrated. Over the short term, however,
this seems unrealistic to implement, because it requires a single overall
control plane, and consequently requires the optical switches to become as
intelligent as the IP routers. The optical switches would have to be able to
discover topology, and signal lightpath setup as easy as routers.
The overlay model is similar to traditional IP over ATM or ATM over SONET
models. The main benefit of this model is that the optical layer can
independently define its own control plane to support a variety of services
such as wavelength private lines, optical virtual private networks (OVPNs),
and transport bandwidth brokering. The optical layer control plane can also
adapt to the optical layer infrastructure, be it electro-optical like today,
or migrating to all-optical in the future.
Another advantage of the overlay model is isolation of IP and optical
layer information. This allows for different life cycles and evolution of
the two layers, and makes the implementation and deployment of UNI and NNI
more straightforward. Finally, this model can be applied to incumbent or new
multiservice carriers who either own or lease their transport facilities.
The overlay model does have overlapping functions in the IP layer and the
optical layer, however. For example, both layers have the function of
topology and reach-ability discovery, which leads to low efficiency. In
addition, the intelligence in both layers might cause a coordination problem
in areas such as protection and restoration.
To improve the coordination, information from one layer must be passed
into the other. An enhanced overlay model (Overlay Plus) could be based on a
trusted intelligent layer in between the IP and optical layers that uses
topology and status information from both networks to prepare policy-based
(e.g., SLA, revenue...) end-to-end connection and release requests. These
requests can be rapidly invoked to avoid network abnormalities such as
congestion and failures, increase infrastructure utilization, and automate
engineering by rebalancing the network and forecasting needed resource
upgrades (such as node and link capacity) for both the IP and optical
IMPLICATIONS OF IP OVER OPTICS
Many vendors are working on products and solutions that usually classify
themselves as "IP Over Optics." The meaning of the term "IP
Over Optics" has moved away from the simple physical connection of
router and optical equipment toward the full interworking and integration of
the control and signaling as well as the management of routers and optical
switches. The intelligence introduced to the picture with technology
innovation is changing the way many carriers operate. Therefore:
Carriers should consider building or migrating to a
simple IP Over Optics network architecture, with the three key enabling
technologies: MPLS, G.ASTN, and Packet Optical Interworking.
The new network architecture with agile and auto
provisioning will reduce operating costs as fewer personnel are needed to
operate the network. The simple architecture should also help in reducing
capital cost. New technology like MPLS overcomes the scalability problem of
ATM or TDM networks. It is also easier to grow the network, thanks to the
automation of topology discovery and end-to-end path establishment/tear
down. Coordinated and unified cross-layer protection can lead to
differentiated SLA options. Such scalability and reliability eventually
means more revenue to the carriers.
With the variation in today's service provider environment,
which model applies will likely depend on a number of factors such as: Metro
versus regional versus long haul networks; incumbent versus new providers;
multiservice versus IP centric providers; and facility ownership.
More importantly, the maturity of technology and
practicality of deployment in service providers' network should be carefully
examined. The overlay model and its variations offer the smooth evolution
from today's network, to a fully integrated IP Over Optical network. While
the peer model represents the future of network architecture, it is not
realistic to implement it today for the reason mentioned earlier.
Some other approaches call for a smart IP layer to attempt to use
rudimentary impairment and fiber-type primitives of a relatively dumb
optical layer, to assess what lightpath connections can be established.
These approaches ignore the fact that optical network is quickly developing
its intelligence and is emerging to provide rich optical services directly,
and may consequently bypass the IP layer.
- Carriers should fully leverage new network applications enabled by
the new architecture and network to differentiate their service
offerings from competitors.
The new IP Over Optics architecture opens a whole range of new network
applications such as protection/restoration. A number of protection
mechanisms exist across packet and optical layers including guaranteed 1+1,
protected 1:N, non-pre-emptible unprotected, and pre-emptible unprotected.
Carriers can choose to protect their networks at the packet layer only, the
optical layer only, or a combination coordinated through cross-layer
interworking intelligence. Such flexibility allows carriers to provide rich
SLA options to their customers.
The other application is resource allocation that provides optical
resources for the packet layer in an automatic manner. Capacity is added
when needed, then relinquished as an available resource later when the need
RETHINKING THE STANDARDS OF SERVICE
All of these can be performed dynamically and automatically to enable
service providers to provide faster, better services to their customers.
With a new network architecture and maturing of the technology, more network
applications will be discovered and consequently benefit both the carriers
and their customers.
Dr. Billy Wu is senior manager of technology marketing for Nortel
Networks' Optical Internet business. He can be reached at [email protected].
Nortel Networks is a global Internet and communications leader with
capabilities spanning optical, wireless, local Internet, and
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