Eliminating the Metro Service Gap with Virtualized Optical Networks
[First Full Draft • 05/30/08]
Introduction
Service providers have a problem with their metro Ethernet infrastructures today: a
sizable gap between existing services, such as Internet access, virtual private networks
(VPNs) and disaster recovery applications, and a variety of new, potentially lucrative
ones. Examples of these new services include transaction processing, mobile
aggregation, datacenter interconnect and storage mirroring.
The gap exists because current wave division multiplexing and packet optical transport
technologies are unable to deliver low-latency, fractional 10 Gbps services costeffectively—or at all. The fundamental problem is the discontinuity between packetbased Ethernet and the underlying circuit-based topology employed in all wave division
multiplexing (WDM) and packet optical transport systems. Putting it another way:
point-to-point circuits undermine Ethernet’s hallmark any-to-any connectivity.
This white paper—suitable for both a technical and business decision-maker audience—
explores this service gap, explains its evolutionary causes and introduces a revolutionary
new technology—Packet WDM—that closes the service gap by eliminating the
underlying packet-circuit discontinuity.
The Service Gap in Metro Ethernet Networks
Packet-based networks, particularly those utilizing Ethernet and the Internet protocol
(IP), are far more efficient and, therefore, far more cost-effective than circuit-oriented
networks. This indisputable fact explains why voice over IP (VoIP), for example, is now
displacing the legacy Time Division Multiplexing (TDM) architecture in both enterprise
and carrier networks.
In early metropolitan deployments, the aggregate bandwidth requirements quickly
exceeded the capacity of Ethernet links operating, at the time, at 1 Gbps. As the demand
for IP and Ethernet services grew, carriers began to scale their network infrastructures
with WDM and Packet Optical Transport System (POTS) solutions.
Both WDM and POTS today provide scalability beyond Ethernet’s current maximum rate
of 10 Gbps. But they do so in a way that makes it difficult if not impossible to provision
flexible-bandwidth, low-latency services. And this inability creates a sizable gap in the
continuum of services desirable across the full spectrum of performance. The diagram
below shows this service gap, which is at its worst for applications that require fractional
10 Gbps bandwidth and relatively low latency.
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The gap in the continuum of metropolitan services is caused by limitations inherent in
today’s wave division multiplexing and packet optical transport systems.
The Evolution of the Service Gap
The service gap was inevitable as the LAN and the WAN merged in the MAN. LANs
evolved as packet-based networks with Ethernet ultimately obtaining monopoly status.
WANs traced their roots to the circuit-based communications of the Public Switched
Telephone Network.
When these two worlds merged in the metropolitan area, it was only natural to adopt
conventional long-distance technologies when deploying Ethernet in the “last mile.”
Initially, this meant the use of traditional wave division multiplexing equipment. A next
generation of packet optical transport systems soon evolved, based in part on WDM
technology. Each of these technologies warrants a closer look to explain how such a
sizable service gap could have evolved.
Wave Division Multiplexing (WDM) and Dense WDM (DWDM) equipment offer a way
to scale IP/Ethernet networks beyond today’s maximum Ethernet link rate of 10 Gbps.
With WDM technology, a single pair of fiber optic cabling is able to transport numerous
separate packet streams on different wavelengths.
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DWDM equipment does indeed afford virtually unlimited scalability. But that scalability
comes at a price: enormous complexity that increases capital and operational
expenditures. DWDM configurations can become quite complex in packet-based
networks because the underlying topology remains point-to-point. It s these complexities
and inefficiencies that motivated the industry to seek a better solution.
Packet Optical Transport Systems (POTS) attempt to unite Ethernet, SONET/SDH and
WDM into a more cohesive solution. The idea is to preserve the benefits of
SONET/SDH—a deterministic network with sub-50-millisecond recovery, and robust
operations, administration and maintenance (OA&M) capabilities—while adding
scalability beyond a single SONET/SDH ring’s maximum capacity of 10 Gbps (OC-192).
But because the underlying topology with POTS remains point-to-point, the approach
remains far from optimal for the any-to-any nature of packet traffic. For example, packet
transport based on optical circuits can waste a significant amount of bandwidth because
Gigabit Ethernet signals do not map efficiently into 2.5 Gbps wavelengths. The stackedring architecture is also more expensive owing to the number of non-revenue (noncustomer facing) ports that must be deployed to switch traffic end-to-end.
To alleviate these problems, some POTS solutions incorporate Layer 2 packet switching
modules into their optical circuit platforms. These systems reduce the number of
Ethernet ports required and improve optical circuit utilization by allowing any Ethernet
packet from any Ethernet port to be switched to any of the pre-provisioned wavelengths
serving as optical circuits. While this approach does help by mapping traffic from
multiple Ethernet ports into a single circuit, optical bandwidth must still be deployed
point-to-point.
POTS-based designs also endeavor to enhance flexibility, thereby reducing the
complexity, by using a Reconfigurable Optical Add-Drop Multiplexer (ROADM)
technology that enables remote control of the underlying wave division multiplexer at the
wavelength layer. While in theory this minimizes truck rolls, in practice transponders are
not deployed in an idle mode with the anticipation that they will be needed sometime in
the future. Instead, they are deployed as needed, so truck rolls continue to be required for
optical circuit redesign.
Twin Limitations Open the Service Gap
Both DWDM and POTS solutions suffer from twin limitations that together create the
service gap. The first is their inability to deliver variable increments of bandwidth
efficiently and cost-effectively. Their granularity is limited to 2.5 Gbps or 10 Gbps, with
both wasting bandwidth in the emerging sweet spot of services between Ethernet’s 1
Gbps and 10 Gbps link rates.
These fractional rate services are deemed highly desirable because they enable carriers to
provision inexpensive entry-level services, sizing the service to meet the customers’
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needs, then up-selling additional bandwidth without the need for a truck roll to the
customer premises. But the problem with circuit-based WDM systems is that a full 10
Gbps must be provisioned even when the customer requires only a modest upgrade from
an existing 1 Gbps Ethernet service. Networks with 2.5 Gbps circuits suffer from a
similar problem that effectively wastes available wavelengths with sub-optimal
utilization. In ether case, because the infrastructure to support these fractional services is
expensive, carriers may be forced to charge a premium to preserve profitability.
The second problem is the increased latency caused by the underlying circuit-based
network topology. The lack of any-to-any connectivity in point-to-point topologies
requires traffic to take an indirect route end-to-end across the carrier network
infrastructure. Each node adds some latency, and the cumulative effect can cause
performance problems with some applications. The maximum amount of latency occurs
with solutions that require optical-electrical-optical conversions.
The circuits in the network can be allocated on an exclusive basis, of course, to minimize
latency. The dynamic nature of packet traffic, however, makes any such configuration
short-lived. Every change in customer services requires a corresponding change to
optimize the configuration. And because making these changes is disruptive and
expensive, most carriers choose instead to live with a sub-optimal configuration.
Virtualizing the Metro Network with Packet WDM
A third generation in the evolution of packet transport technology eliminates the service
gap created by the twin limitations in DWDM and POTS designs by integrating
Ethernet’s simplicity and efficiency directly into the optical domain. And by thus
eliminating the packet-circuit discontinuity, the next-generation Packet WDM solution
enables carriers to provision Ethernet services in 1 Mbps increments up to 10 Gbps with
deterministically low latency of less than 1/10TH of a millisecond.
Packet WDM is the next logical step in the evolution of optical networking based on its
ability to combine the any-to-any connectivity of Ethernet with the scalability of wave
division multiplexing. Unlike circuit-based WDM systems that rely on pre-provisioned
optical circuits, Packet WDM technology employs optical burst transponders capable of
switching traffic as packets in bursts at different wavelengths. This underlying
technology, known as optical burst switching (OBS), is widely viewed as the best way to
eliminate the packet-circuit discontinuity that plagues the traditional circuit-based
solutions. The use of such transponders brings statistical multiplexing efficiency to the
optical domain, where each transponder shares bandwidth across all nodes, and available
capacity is reallocated in real-time.
The optical burst transponder’s ability to communicate on any wavelength with every
other transponder in the resilient metro ring topology is what gives Packet WDM its
virtualized any-to-any Ethernet connectivity. The laser in the transponder is able to tune
into and lock onto any wavelength in nanoseconds. Although every node is capable of
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transmitting and receiving any wavelength, individual nodes are configured to receive on
only one wavelength. In effect, this wavelength becomes a destination addresses in the
optical domain. Packets destined for a particular node are, therefore, transmitted on that
node’s wavelength, enabling traffic to flow (in bursts) from any source directly to any
destination with no intermediate processing or latency.
In a Packet WDM network, every node transmits on every wavelength and receives on
just one, enabling the any-to-any connectivity need to eliminate the service gap.
Like SONET/SDH, OBS offers deterministic transport with a sub-50 millisecond
recovery. Like Ethernet, OBS enables bandwidth to be allocated dynamically and
partitioned by user and/or application. Also like Ethernet, OBS employs a special
protocol for synchronizing transmissions network-wide to avoid collisions in the optical
domain. This network-wide coordination of all traffic flows enables the entire Packet
WDM configuration to function as a distributed virtual Ethernet switch.
Significantly, the total cost of ownership is substantially lower with a Packet WDM
solution compared to either DWDM or POTS solutions. By enabling all transponders to
communicate directly throughout the fiber ring (rather than being dedicated in matched
point-to-point pairs), far fewer transponders are needed to deploy a metro-wide Packet
WDM configuration. And because transponders represent the majority of equipment
costs in any WDM network, optical burst switching normally results in an immediate
capital expenditure savings of up to 50%.
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The operational expenditure with a Packet WDM solution is also dramatically lower
owing to its operational simplicity. By consolidating an entire metro optical network into
a single distributed virtual Ethernet switch, the configuration is far easier to deploy,
operate, troubleshoot and manage than complex, circuit-based WDM solutions. In fact,
Packet WDM solutions can entirely eliminate the need for optical engineering, enabling
anyone knowledgeable in Ethernet to install, configure and manage the network.
Conclusion
Until recently, high-bandwidth metropolitan networks could only be built atop a circuitbased transport layer. Packet WDM technology is the breakthrough advance needed for
pure-packet metro networks capable of providing the best of both worlds: the simplicity
and efficiency of Ethernet packet switching combined with the virtually unlimited
scalability of dense wave division multiplexing.
By packetizing and virtualizing the optical domain, Packet WDM technology eliminates
the packet-circuit discontinuity and resulting service gap inherent in circuit-based
solutions. With a Packet WDM solution, carriers can provision a wide range of lowlatency services in 1 Mbps increments up to 10 Gbps—without wasting available
capacity. Carriers also get a lower total cost of ownership that, together with the more
efficient utilization of optical bandwidth, yields far greater profit potential.
To learn more about how your organization can benefit from third-generation Packet
Wave Division Multiplexing technology, please visit Matisse Networks on the Web at
www.matissenetworks.com or call +1-650-938-5100.
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