How will the optical layer change today's networks?
By Nicholas DeVito, Tellium Inc.
Optical switches--transparent or opaque--will play an
important role in controlling the new wavelength layer
in communications networks.
Growing but unpredictable bandwidth demand coupled with
increasing tributary and line-rate speeds have created
problems for existing network operators and opportunities
for emerging providers. Many network operators and
equipment suppliers have responded to customers' evolving
capacity requirements by adding a wavelength or optical
layer to the existing network hierarchy. This wavelength
layer is also being sold as services--leased wavelengths,
OC-48c (2.5-Gbit/sec) and OC-192c (10-Gbit/sec)
bandwidth pipes, and protected OC-N--underscoring the
need for low cost and high reliability.
Despite the current debate about where intelligence ought
to reside in the network, the wavelength layer still requires
all the operations, administration, maintenance, and
provisioning functions of traditional electrical layers. To
meet these demands and reduce the risk of actual growth
significantly deviating from forecasts, most carriers are
considering the use of optical switches to manage the
wavelength layer just as digital crossconnects have been
used to control the electrical layers.
What's required
To be effective in this role, optical switches must combine
the best features of both Synchronous Optical
Network/Synchronous Digital Hierarchy (SONET/SDH) and
the digital-crossconnect worlds. They must provide
maximum flexibility and scalability with high reliability. Since
the wavelength layer is also sold as a service layer, it will
require that switches support SONET/SDH
network-management features to guarantee service-level
agreements and provide efficient maintenance and
troubleshooting. In this vein, optical switches must
incorporate the broadcast, multicast, and bridging features
necessary for restoration and to transmit optical test
signals to downstream offices, obviating the need for
multiple test sets. To serve as network gateways or
regenerators in long dense wavelength-division multiplexing
(DWDM) spans, optical switches must also perform
full-regeneration functions.
These switches must scale to hundreds of terabits in
capacity, with interfaces that range from OC-48 to OC-768
(40 Gbits/sec) and beyond. They must also incorporate the
capability to convert electrical switch cores to all-optical
switch cores during the next five years if that technology
proves reliable and cost-effective. Finally, optical switches
will assume the responsibility for optical-layer routing and
network-restoration functions at OC-48 and above.
Optical-layer architecture
Optical switches that combine these characteristics will
deliver tremendous cost savings through flatter
architectures, fewer interfaces, and faster restoration and
provisioning times. Flat network architectures, flexible
protocols, and powerful management systems that
automate the optical layer will support on-demand
provisioning of high-speed data services and reduce
maintenance to a scheduled activity. Fast provisioning
coupled with automated restoration are the only ways to
address increasing demand and keep network costs falling
faster than bandwidth prices.
Applications
Given the wide variety of features in optical switches, their
applications can vary, ranging from traditional crossconnect
functions to new optical-domain needs. However, the
primary application migrates most network-restoration
functions to the wavelength or optical layer, using optical
switches as the major restoration vehicle. This migration
pushes traditional SONET/SDH or time-division multiplexing
devices to the edges of the network as termination
equipment. Optical switches will restore services according
to the priority of the service-level agreements in the ring
and mesh architectures.
New switching protocols
This level of automation would not be possible without
next-generation switching protocols and
network-management systems. There are several new
protocols being proposed in a variety of standards bodies
for routing and restoring services on the optical layer. All
these proposals rely on either assigning a piece of the
unused, unassigned SONET/SDH overhead bytes to the
optical layer for signaling or adding additional overhead
bytes to the overall optical signal (increasing its bit rate by
7% to 8%). However these standards emerge,
optical-switch providers will respond with flexible software
to adapt to the changes.
To make the network transparent to the operator,
network-management systems will integrate functions
between the Layer 3 applications and the Layer 1 optics.
With SONET/SDH multiplexing and remultiplexing essentially
removed from the "ideal" network architecture, this task is
significantly less daunting than it may first appear. Several
new equipment providers are already building systems to
perform such integrated tasks.
Cost benefits
This level of automation and service is compelling, but does
it really save customers money? Tellium has performed
several network-architecture cost studies and comparisons
using actual data from regional and national networks. The
most striking result? In regional networks, using the optical
switch with restoration routes is about half the cost of
using traditional sonet/sdh network elements. Since there is
additional operational savings with the optical-switch model
because of the reduced number of managed elements,
these projected cost savings are conservative.
In the case of no restoration, the addition of the optical
switches still results in small savings due to the improved
grooming of DWDM channels. Therefore, adding optical
switching to a network does not necessarily increase costs,
even as it adds flexibility and manageability to the network.
National-scale mesh networks are more cost-effective
(60% savings) using optical switches, but they make it
imperative that the optical layer be self-protected.
High-speed mesh restoration becomes a necessity and is
made possible by doing the restoration at the optical layer
using optical switches. Such restoration can be performed
in 50 to 100 msec, compared with the minutes to
tens-of-minutes required in today's traditional mesh
restoration architectures. The use of optical-layer
switching and mesh restoration will be a key factor in
providing manageable, high-capacity backbone networks.
Transparent or opaque?
While the case for flexibility in the face of uncertainty, cost
reduction through simplification of network architectures,
and bandwidth management that delivers optical-layer
services is clear, how will optical switches meet the
daunting challenges proposed? Two primary options for
constructing an optical switch are available to equipment
manufacturers.
First, an all-optical crossconnect, or "transparent" optical
crossconnect, can take advantage of advances in
optical-component technology. Transparent all-optical
crossconnects have several drawbacks, however.
All-optical crossconnects introduce additional cascaded
optical impairments. They cannot deliver 3R regeneration.
They are costly to use with today's devices. They cannot
capture SONET/SDH performance monitoring. They do not
deliver sub-50-msec protection switching. And they cannot
achieve broadcast capabilities without introducing
significant cost, complexity, and performance limitations.
A second option for constructing optical crossconnects is
the use of an opaque optical-electrical-optical (O-E-O)
conversion. These devices overcome all of the previously
mentioned difficulties. Opaque optical switches also offer
network operators the low cost, flexibility, scalability, and
reliability that they need to manage the optical or
wavelength layer.
Down the road
O-E-O crossconnects will remain state-of-the-art through
2002. However, large-capacity switches (more than 400
Tbits/sec) may have to use an all-optical (transparent)
approach. These products might include a
micro-electromechanical system (MEMS), or other
optomechanical and optical waveguide-based technologies.
The industry will watch these technology developments
closely to ensure the realization of very large optical-switch
products.
As the wavelength layer becomes more dominant, optical
switches will be at the heart of every carrier's
telecommunications network. These devices will provide
both ring and mesh restoration, automated provisioning
functions, and gateway and optical-layer on-ramp access,
thus truly enabling telecommunications for the new
millenium.
Nicholas DeVito is director of marketing and product
management at Tellium Inc. (Oceanport, NJ).
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