4/8/2001:Rapid service delivery with optical switching
Optical switches will manage traffic throughout
the network and act as a gateway between
network layers.
ERIC PROSSER,
Broadwing Communications
Strong demand for network capacity coupled with
aggressive delivery intervals has been the driving
force behind the evolution of network architecture
to incorporate optical-switching technology.
Service providers compete with buying power,
available capital, internal processes, and highly
skilled engineers at a frenetic pace to meet the
challenge of matching demand with supply.
Recent technology advances offer service
providers additional building blocks with which to
construct their networks. Enabling technologies
include Raman amplification for ultra-long-haul
core networks, along with development of
vertical-cavity surface-emitting lasers (VCSELs),
micro-electromechanical systems (MEMS), and
tunable-laser technologies for optical switches.
Network architectures deployed today exploit the
potential of optical switching to enable rapid
delivery of services with customer-defined
protection.
Promise of intelligence
Optical switching is the cornerstone of the new
network architecture. The promise of optical
switches has fueled innovation in product design,
manufacturing, and network deployment. The
goal is to use the intelligence of the switch to
rapidly turn up circuits with point-and-click
provisioning and manage traffic around network
bottlenecks and failures.
Carriers traditionally respond to demand by
deploying significant levels of raw transport
capacity (both wavelength count and channel
size), but then terminate this capacity in fiber
crossconnect panels at major network junctions.
Optical circuits are then provisioned across the
network through dedicated paths, which are
connected and accessed across the crossconnect
panels. Dedicated line terminating equipment is
deployed to fill these paths in discrete SONET
increments, while stranding the remaining
bandwidth.
These networks are predominantly SONET rings
or linear transport systems. The network is
designed segment by segment, span by span, and
high-speed circuits are equipped and provisioned
across the network segment by segment and span
by span. The resulting network complexity
requires manual intervention at multiple network
points, creating delivery intervals measured in
weeks and months. As demand continues to
increase, this "network on demand" approach will
consume planning, engineering, provisioning, and
operations resources at greater and greater rates.
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Ultimately, carriers compete on their ability to
rapidly deliver services. The initial race among
service providers was to obtain a national
footprint with coast-to-coast fiber networks; the
next race was to lay fiber to connect end users to
these networks. The carriers that win will provide
networks that come closest to real-time
end-to-end provisioning.
Advances in optical switching continue the trend
toward merging ATM, Interneet Protocol (IP), and
transport-layer technologies. The backbone
network becomes a "cloud" with simple ingress
and egress points. Ultimately, customers will be
able to buy high-speed capacity (up to 40
Gbits/sec), define quality of service, and
dynamically reconfigure their circuits across a
network with no intervention by the service
provider.
With intelligent network-aware switches, new
traffic can be routed efficiently around
bottlenecks. Moreover, as network constraints are
resolved, existing network resources can be
optimized with automatic traffic grooming. As
technology progresses, network engineers will be
able to add a new DWDM system and allow the
optical switch to optimize the network by
grooming traffic to the new system. Additions to
one layer or region of the network will
automatically resolve constraints in other parts of
the network.
As Multiprotocol Lambda Switching standards
are developed, optical switches will integrate
point-and-click provisioning, traffic-engineering
functions, and restoration capabilities with
label-switching routers. This integration will
allow high-speed networks to be more responsive
to traffic flow, ultimately resulting in instant
provisioning of service by customers across a
carrier's network, for example, providing an
end-to-end 40-Gbit/sec connection for a customer
in the same time that a telephony circuit is
established today.
Optical switching throughout
Equipment vendors typically segregate the optical
and electrical switch fabrics into separate
products, with different roles in the network.
Beyond service-level granularity, the switching
philosophy of these products progresses from
STS-1 through wavelength, band, and fiber
switching.
STS-1 switching will be required to aggregate
and protect service domain traffic, classified as
OC-12 (622 Gbits/sec) and below services.
Wavelength and band switching will route the
optical transport-rate traffic across the core. Fiber
switching is generally thought of as a replacement
for lightwave crossconnect panels, but it can be
deployed to support switching of entire core
DWDM systems.
Switches in each of these spaces are being
deployed in networks today. Available products
have different combinations of strengths and
weaknesses. Each layer of network architecture
has unique characteristics, so a product well
suited for deployment on the edge of the network
will most often be inefficient in the core. A
best-of-breed optical strategy by service providers
will require that switches deployed in each layer
interact with standard protocols to efficiently
route traffic across the network.
Optical architecture
First-generation optical switches will replace the
broadband crossconnect switch and become the
gate node between regional and core networks.
Carriers will build their networks around this
architecture to take advantage of the ease of
provisioning, restoration, traffic management, and
the service-ready characteristics of these
switches. The architectures will include dedicated
bandwidth conduits between switches and STS-1
granular switching to efficiently aggregate traffic.
Furthermore, the networks will be deployed to
optimize the role of the core, regional, and
metropolitan layers.
Providers will pre-position capacity from the edge
points-of-presence to the optical switch and
pre-position capacity between optical switches.
The resulting bandwidth conduits will look and
feel like trunks in a switch network. Customers
will buy capacity into the switch as a conduit for
on-demand services. Initially, the switches will
route traffic over dedicated wavelength conduits.
As technology advances, the switches will
translate and route wavelengths over dedicated
fiber conduits.
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Networks are designed as a hierarchical
architecture to efficiently aggregate traffic and
limit the effects of regeneration. Networks with
core, regional, and metro layers will use the
optical switches to manage traffic within the layer
and act as the gateway between each layer (see
Figure 1). The network between the core
switches will be built as a mesh terminating in the
switches. The network between the core junction
and underlying regional network can be built with
a mesh topology, with express systems connecting
the regional switches to the core switch (see
Figure 2).
These networks are distinctly different designs.
Capacity in the metro layer is aggregated to feed
capacity into the regional layer, which is
aggregated to feed capacity into the core. The
primary function of the core network is to carry
capacity between core optical switches with a
minimal level of regeneration, meaning that the
core network is optimized for express links and
built over ultra-long-haul transport equipment.
The regional network must be flexible enough to
service both intra-regional traffic (short paths) and
inter-regional traffic (long paths). In contrast, the
average distance of a circuit in the metro layer
will be relatively short, and the role of
regeneration is insignificant.
Core optical switches route wavelengths at
backbone fiber junctions, keeping the traffic in the
optical domain and eliminating unnecessary
optical-electrical-optical conversions. The cost of
these conversions can be measured by the
additional latency, failure points, floor space, and
hardware expense introduced. The closer the
switch is to the core, the larger the role of the
optical matrix.
While the demand for optical rate circuits for
application platforms and private-line customers
continues to increase, the primary demand across
networks is still in the service domain at speeds of
OC-12 and below. The closer the switch is to the
edge of the network, the greater the role of the
STS-1 granular switch fabric. The STS-1 granular
switch fabric's primary role is to efficiently bundle
service domain traffic across the optical domain in
an effort to minimize the cost of stranded
bandwidth.
A gating item to the widespread deployment of
STS-1 granular switches is the available
unprotected OC-48 matrix size (256x256 to
512x512). That represents a significant increase
beyond the matrix afforded by a traditional
digital-crossconnect switch. But to the extent that
the core switch replaces other time-division
multiplexing elements and switches optical
domain traffic-OC-48s and above-this matrix will
be quickly exhausted.
Protection architecture
Optical switches will automatically reroute
wavelengths around fiber cuts or equipment
failures and enable customers to choose from a
menu of protection classes. Carriers will build one
network and offer different classes of service:
unprotected, automatic protection switching, ring,
or mesh. The services will be defined in the
switch rather than by the transport system.
Protection of traffic across transport networks is
being pushed from fully redundant SONET rings
toward distributed mesh networks.
Restoration strategies with first-generation optical
switches are performed by switching line-rate
circuits in the electrical matrix to a different
wavelength route or switching wavelengths
between fibers. As work on tunable lasers
continues, carriers will have the flexibility of
installing line-side transmitter ports in the switch
capable of a spectrum of frequencies, rather than
deploying a wavelength-dependent inventory of
core capacity. Switches will be able to
automatically reroute wavelengths between spans
without contention, which will further mitigate
the capital risk of over-investing in the protection
network.
With optical switches bridging the network
layers, incremental channels will be equipped
only at the far edge of the network to establish a
complete circuit. In the not-too-distant future,
establishing a customer's 10-Gbit/sec circuit will
involve simply equipping the interfaces at the
customer's premises and allowing the optical
switches to dynamically route the wavelengths
across the network.
From the core to the edge
The optical network is evolving from a static,
segmented architecture to a dynamic,
bandwidth-on-demand, integrated infrastructure.
Carrier-network management and provisioning
systems must be redesigned to incorporate the
new intelligence of the optical network. Circuit
provisioners will need tools to design and
document circuits across this new architecture, as
the network itself makes increasingly
sophisticated circuit-design decisions.
Today, core networks require all-optical switches
that route traffic between core sites without
electrical regeneration. The evolution of the
network is to push the optical core toward the
edge and ultimately directly to the customer; the
goal is to push the all-optical architecture closer
to the customer. The question is how long this
migration from the core to edge will take. The
answer dictates how long carriers will continue to
deploy electrical switching in the core.
Eric Prosser is a senior manager of network
transport planning at Broadwing
Communications (Austin, TX).
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