Software Defined Networking
(SDN) is a term that has quickly risen to the forefront of the networking
industry, with the objective of solving many of the challenges facing
network providers today. While there are many varying opinions of exactly
what SDN means, there is one common trait amongst them – “programmability”.
SDN is a means to open up the network and support programmability of the
network, often comprised of multiple vendors, multiple domains, and multiple
networking layers. For some providers’ networks, programmability and SDN’s
functions are viewed as a means to enabling automated, on-demand networking
with optimal resource utilization.
Initial SDN efforts are largely focused
on decoupling the control plane from the data plane and enabling a higher
level of programmability into packet forwarding tables of switches. But what
does this mean for the emerging converged optical transport layer with
integrated switching, which is now playing an increasingly important role in
optimizing networks and underlies most of the world’s Internet backbone?
What does the optical layer need to adequately and economically support a
programmable network with on-demand capabilities?
The Evolution of Optical Transport:
Opportunities and Challenges
The architecture of the
optical network is undergoing a significant transformation – and with this
transformation lays some new challenges around automation, elasticity, and
capital/operational efficiency.
The Network Efficiency
Challenge
Over the past many years, network providers were deploying 10Gbps
wavelengths on a large scale, and the predominant service that was
transported over the fiber backbone was 10Gb. Initially, it was SONET/SDH
OC192/STM64 service, running at 10Gb rate, and more recently, 10Gb Ethernet
(10GbE) services have rapidly risen in popularity, driven by Ethernet
convergence. In this environment, the transport service speed matches the
wavelength bitrate, and the term “wavelength” was used synonymously to mean
the optical, analog transmission wavelength as well as the transparent
wavelength-like digital service.
Today, however, there is a
growing divergence between the wavelength bitrate and the transport services
the network needs to support. Layer 0 transmission is rapidly evolving
towards 100Gb optical wavelengths equipped with coherent detection, as
carriers and network providers strive to increase fiber capacity to multiple
Terabits. But the supported transport services are still largely 10Gb,
sometimes less. Moreover, the services mix will continue to include a
variety of service rates, as dictated by the economics of these services.
While the quest for achieving
optimal cost/bit transmission economics is driving the need for 100Gb
wavelength technology, the business of transport services and market demand
for a broad set of service rates necessitates a different approach than in
the 10Gb wavelength era. With the current economics of 10GbE vs. 100GbE
services, it is generally expected that 10GbE services will continue to
dominate in volume for some time while networks are upgraded with 100Gb
optical technology. Further, this divergence of service from optical
wavelength speeds will likely continue, as optical technology takes its next
step forward to super-channels – wavelengths with bitrates beyond 100Gb.
State of the art technology today offers 500Gb super-channels, with 1Tb
super-channels soon to follow, as the drive to increase fiber capacity
continues.
The Bandwidth Elasticity Challenge
A second challenge facing providers today is the need for on-demand
“elastic” bandwidth to efficiently and cost-effectively deliver bits
whenever and wherever needed. Evolving traffic patterns driven by cloud
network and datacenter communications are driving providers to relook at
their network architecture and the relationship between IP and optical
transport layers. The conventional practice of over-provisioning the IP
layer and running links at low utilization rates, while constraining the
optical layer to provide static, “always-on” 100Gb point-to-point capacity,
is being scrutinized, as new optical transport solutions with integrated
digital switching emerge that can readily flex and adapt to varying and
unpredicted bandwidth needs.
While this level of
flexibility and adaptability begets greater elasticity, it highlights the
more general challenge of multi-layer resource optimization. With resources
that can be allocated and repurposed at multiple network layers, network
providers require ways to optimally allocate resources to provide the
appropriate bandwidth connectivity services that meet the service
requirements of applications.
The Network Automation
Challenge
In order for the network to provide on-demand bandwidth at Internet speeds,
operational processes need to devoid themselves of human intervention. This
encompasses not just automation of processes across multiple network layers,
from transmission and transport up through IP/MPLS, but also the
orchestration of resources between separate domains and amongst multiple
vendors. In the optical transport layer, this means enabling rapid delivery
of transport bandwidth in a manner that is cost and resource efficient,
without burdensome wavelength engineering processes.
All-Optical Networking Dilemma
Conventional all-optical
networks based on ROADMs deliver wavelengths on an end-to-end (A-to-Z) path,
and constrain the delivery of transport services statically between those
two sites. Operating on the principle of photonic switching, ROADMs can only
route entire wavelengths and cannot access transport services carried inside
the wavelength. Capacity that is not utilized within the wavelength cannot
be leveraged by other traffic demands that do not originate at the same
locations but which might share the same physical sub-path. As networks evolve to 100Gb
wavelengths and beyond, this not only presents a resource utilization
challenge, but also a bandwidth elasticity challenge.
All-optical networks
empowered with ROADMs are capable of “flexing” to dynamic bandwidth demands
with wavelength granularity only. They facilitate turn-up of new end-to-end
wavelengths, but the inability for all-optical networks to 1) manipulate the
services inside the wavelength or to 2) pool capacity together and
dynamically allocate bandwidth leads to static, underutilized wavelengths
and excessive deployed capital. As the pressure to increase fiber capacity
grows, leading to larger but fewer super-channels, the wavelength
fragmentation challenge is exacerbated.
With the evolution of optical
wavelengths from 100Gb today towards 1Tb super-channels in the future, the
role of ROADMs will inevitably evolve towards steering large chunks of
capacity between major hubs, and less for turning up and delivering digital
services to end users. Tightly coupling the allocation of dedicated
wavelengths between A-to-Z network locations for on-demand delivery of
services does not scale.
Nevertheless, programmability of the ROADM, as well
as key optical transmission parameters such as modulation scheme for trading
off reach versus capacity, are important elements of SDN for the overall
optical transport layer.
In order for a network to offer truly elastic bandwidth, and enable
transport bandwidth service to be efficiently delivered over any optical
wavelength, virtualization of the wavelengths is a necessity. This entails
creation of an abstraction layer that represents the creation of a pool of
optical resources that can be leveraged for any bandwidth demands.
The Solution: Bandwidth
Virtualization
Facilitating SDN’s
programmable networking concept in a manner that simultaneously optimizes
utilization of optical capacity and enabling real-time delivery of
optimally-sized bandwidth requires a means to decouple transport service
delivery from the transmission layer. It requires an abstraction layer that
virtualizes wavelengths and pools the capacity together on each link, and
promotes sharing of that bandwidth for any transport circuit traversing that
link. Instead of a dedicated resource between 2 fixed locations, wavelengths
can be transformed into a shared resource supporting services between any
network locations.
Not surprisingly, this concept is very similar to IT resource
virtualization, where the collective power of multiple physical resources
are pooled together and shared amongst multiple Virtual Machines (VMs). VMs
supporting applications can be dynamically instantiated or decommissioned
from this shared pool of resources, maximizing utilization and efficiency.
Bandwidth Virtualization achieves a similar objective by forming an
abstraction layer representing a bandwidth pool and hiding details of the
underlying optical wavelength resources. Any bandwidth service can be
flexibly mapped to any physical wavelength resource on each digital network
link, whether the wavelength bitrate is 10Gb, 100Gb, or 1Tb.
This is essential as optical transport evolves towards super-channels.
Additionally, through finer granularity switching of transport services
rather than coarse wavelengths, Bandwidth Virtualization provides the
foundation for SDN programmability. With Bandwidth Virtualization, the
transport service provisioning process is decoupled from wavelength
engineering, leading to significant benefits including:
- Reduced time to delivery
of new bandwidth services to meet unexpected demands - Responsiveness and
adaptability of the optical transport layer to dynamic needs of the
application and IP layers with appropriately sized optical transport
capacity - Efficient on-demand
allocation of bandwidth from available resources to maximize wavelength
utilization
The capability of digitally switching individual transport services rather
than just optically redirecting coarse wavelengths provides a level of
bandwidth service flexibility that is decoupled from the evolution of
optical transmission technology.
Enabling Bandwidth
Virtualization
The virtualization of the optical wavelengths requires abstraction into a
shared pool of digital bits that can then be rapidly allocated to support
any transport service. Key enablers of the Bandwidth Virtualization paradigm
include (a) cost-effective OEO conversion to gain accessibility to the
individual services being transported via optical carriers and (b)
integrated digital switching. Conversion of wavelengths into the electrical
domain normalizes the traffic into a form where it can be managed,
independent of wavelength bitrate and origin, and enables individual
bit-based services to be sorted (demultiplexed), switched, and groomed,
before being remapped on to an outbound optical carrier. These functions
provide important network capabilities including:
- Redirection of
individual transport services for optimal latency routes - Optional redirection for
protection against network failures - Maximum level of
wavelength utilization through mixing and matching of any services on to
any wavelengths - Mitigation of wavelength
reach limitations or wavelength blocking situations - Decoupling of the
service provisioning process from complex analog wavelength engineering
and turn-up
Critical design requirements
for Bandwidth Virtualization solutions include minimal latency, minimal
space/power, and scalability commensurate with the optical domain.
Additionally, Bandwidth Virtualization must be economically viable –
implementations that necessitate excessive “boxes” linked together with
optics may technically deliver the same capability, but are not as economic
as converged solutions with internal integrated switching.
Along with integrated
switching, considerations for supporting the broad set of transport services
must be made. Generalized optical transport infrastructures typically need
to support multiple rates and protocols, and full transparency of the
service with stringent performance requirements is mandatory. Additionally,
dynamic scaling of the transport service upwards and downwards is also
important for optimizing consumption of optical capacity. With this set of
capabilities, networks can capitalize on a new level of transport elasticity
that provides appropriately sized bandwidth services whenever and wherever
needed in the network.
A New Approach: Elastic Optical
Transport
The emergence of optical
transport with integrated switching is changing the way architects design
cloud networks. Instead of the traditional model of “dumb pipes”
interconnecting large routers and relegating all bandwidth management
functions within routers, network providers now have the option of deploying
cost-efficient, flexible optical transport networks with integrated
switching and offloading transport bandwidth from routers.
Evolving traffic
patterns in clouds coupled with large amounts of data traffic between data
centers warrants traffic more optimally being transported and switched
within the optical layer, not solely at the more expensive router layer.
This evolution towards a more flexible architecture with multiple dynamic
switching layers calls for more intelligence in managing not just
multi-layer networks, but also networks involving multiple network domains
and multiple vendors. The convergence of WDM, OTN and packet bandwidth
management functions into the next generation optical transport layer is
creating new opportunities for network providers to further reduce the total
cost of ownership of their network infrastructure, inclusive of the IP/MPLS
layer, while also providing a more scalable, adaptable, and cost-efficient
solution that meets the dynamic demands of emerging cloud architectures.
SDN: Ready for Elastic Optical
Transport?
The SDN philosophy of
decoupling the control plane from the data plane is an important paradigm
many in the industry are investigating as a means for automating processes
across a multi-layer, multi-vendor, multi-domain network, and orchestrating
the many moving parts through network Application Programming Interfaces
(APIs) to provide an optimal bandwidth solution for applications that takes
maximum advantage of what each network layer has to offer. Centralization of
information creates the opportunity to make better over-arching decisions,
as it provides globalized visibility across layers, domains, and vendors
that is necessary to understand and make appropriate tradeoffs between cost,
performance, survivability, and other key SLA metrics.
In order for SDN to be truly
useful in multi-domain & multi-layer networks, SDN needs to incorporate not
just broader network management functionality, such as network discovery and
monitoring and correlation, but also deepen its control to include the
emerging next-generation optical transport layer, where integrated switching
adds substantial network value and has significant impact on overall network
architecture, including what happens at higher layers. This broader vision
ensures the entire network stack becomes open and programmable. With
expansion of SDN to include elastic optical transport and abstractions like
Bandwidth Virtualization, network providers will be able to unlock the real
potential of the multi-layer network and fully leverage the resources
available at all layers.
