Optical Network Evolution to SDN Helps build future broadband optical networks

With the revolutionary progress of ICT technology, software-defined network (SDN) technology using IT and software-based ideas and architecture has become a research hotspot in the industry, and has rapidly extended from the field of data network to the field of optical network, becoming an important direction for the development and evolution of optical networks in the future, which will bring new value and vitality
to broadband optical networks.

First, the driving force of SDN introduced into optical networks

The introduction of software-defined network (SDN) technology in the field of optical networks mainly aims to decouple control and transmission, adopt centralized control strategies, simplify the complex and private control management protocols of existing optical networks, and provide network programmability through open network and application layer interfaces to meet the development needs
of future data center optical interconnection, network virtualization, flexible and fast service provision, network and service innovation.

At present, the trend of SDN in optical networks is obvious, and the main driving forces include the following aspects:

1.
Cloud data center interconnection

With the development of cloud computing, data center interconnection services (data center backup, virtual machine migration) have proposed large-bandwidth, dynamic and flexible service requirements for backbone and metro optical networks, and the total bandwidth provided by domestic operators' optical networks for data center interconnection has reached several terab/s levels
.
Optical networks generally provide data center interconnection according to the maximum peak configuration static bandwidth mode, because the bandwidth demand of the data center is not always in the peak state, this mode causes high user rental costs and waste
of operator network resources.
The dynamism of data center interconnection also includes changes in access points, requiring optical networks to achieve flexible scheduling and dynamic adjustment
of connections between data centers.

2.
Centralized maintenance, management and scheduling across manufacturer networks

There are many devices and manufacturers in optical networks, especially metro transmission networks and access networks with massive sites, which brings great challenges
to the centralized management and maintenance of networks.
At the same time, due to the complexity of optical network physical layer technology, the network control and management protocols of each manufacturer have different degrees of private information, and it is difficult to achieve centralized management and scheduling of cross-vendor equipment, resulting in long service provisioning time and low efficiency, and often a service provisioning takes days or even weeks
.
SDN shields the forwarding plane details of different manufacturers through forwarding and control separation and network virtualization technology, and provides standard interfaces to achieve controller interconnection, providing new opportunities
for solving the centralized management and interconnection of multi-vendor optical network devices and simplifying network operation and maintenance.

3.
Intelligent interconnection and collaborative scheduling of heterogeneous networks

With the rapid growth of broadband network flow, the scissor gap between the growth of expansion costs and the slowdown of service profit growth has become the primary problem
faced by operators.
SDN technology can realize resource optimization and collaborative control between optical networks and IP networks, effectively reduce the capacity requirements of routers, solve network capacity bottlenecks, and reduce network comprehensive bearer costs
.
At the same time, through SDN controller collaboration, intelligent interconnection and joint resource scheduling of heterogeneous networks such as access networks, metro networks, and backbone networks can be realized, improving traffic operation capabilities and providing more intelligent pipeline services, which is of great significance
to operators and users.

4.
Network virtualization and open business application provision

The virtualization of network resources can better play the advantages of optical network infrastructure resources, enable customers to quickly and effectively access and control network resources according to the application requirements of different services, under the premise of ensuring service quality, and optimize the utilization of
network resources 。 Based on network virtualization technology, optical networks can provide service openness in two aspects: on the one hand, it is open to operators' own services, and the same physical network can have a logical independent control system for government and enterprise customers, home customers, and backhaul networks, that is, to realize logical sharding of the network; On the other hand, it is open to customers and provides "soft network" capabilities
that adjust on demand, open innovation, and efficient collaboration.

Second, there are different technical routes for the development of optical network SDN

Unlike data network SDN, optical network SDN has its own unique characteristics
.
First of all, due to the characteristics of the physical layer, the optical network itself has a control architecture
with control and forwarding separation.
Secondly, in terms of centralized control, optical networks already have mature centralized management and control systems
such as network management and path computing unit (PCE).
Third, the optical network has the characteristics of connection-oriented, and all services are preconfigured, and there is no need for the controller to forward packets according to the service packet flow in real time, which reduces the performance requirements of the controller and has better network expansion capabilities
.
It can be said that optical networks already have some of the characteristics of SDN, laying a good foundation
for the evolution to SDN.
On the other hand, the physical layer programmability of optical network equipment is weak, and resource virtualization and slicing implementation are difficult, which makes it difficult
to introduce SDN technology into optical networks.

Based on the above characteristics of optical network SDN, the development and evolution ideas adopted by optical network SDN are different
from those of data network SDN.
At the same time, due to the different development requirements and technologies used in the optical network backbone, metro area and access layer, SDN introduction also has different technical routes
.

(1) Route 1: Based on the enhancement of the optical network control plane, emphasize the opening
of the northbound interface.

Based on the existing ASON/GMPLS control plane and centralized path computing unit PCE technology, the control plane of optical network equipment such as backbone network OTN and DWDM can be used to realize SDN centralized controller and provide open programmable application services
for upper-layer networks and applications by expanding and enhancing PCE management and control capabilities and open northbound interfaces.
This route can better protect the existing technical input and is conducive to the smooth evolution of
the network.
Due to the large amount of private information in the physical layer of optical networks, it is difficult to standardize southbound interface protocols, and southbound interfaces may allow multiple interface technologies
.

(2) Route 2: Introduce the OpenFlow protocol to emphasize the opening
of the southbound interface.

For packet transmission PTN and access network PON devices, there is no intelligent control plane, and the openflow protocol can be directly introduced on the device to realize the openness and standardization
of the southbound interface.
Due to the standardization of southbound interfaces, this route is conducive to decoupling devices from services, simplifying network node functions, reducing equipment costs, and facilitating operators' centralized management and control
of multi-vendor massive metro areas and access sites.
For existing network devices, the centralized controller capability
will be realized through network management upgrades.

Third, the development status of optical network SDN and its impact on the industry

At present, optical network SDN is showing a rapid development trend
.
Since 2013, the International Organization for Standardization has competed for standardization work
.
ONF has established the Optical Transport Working Group OTWG, which has completed the architecture and use cases, and will complete the openflow-based optical network protocol extension in 2014; ITU-T SG15 has initiated a transport network SDN study, IETF is carrying out standardization work on PCE-based extended support for SDN, and BBF has proposed a future architecture
for SDN-based access networks.
From the perspective of industrial development, from 2012 to 2013, mainstream optical network equipment manufacturers have launched prototypes of transmission network SDN, domestic and foreign operators actively pay attention to the evolution of PCE and carry out relevant tests and verifications, China Mobile demonstrated its SDN-based Super PTN application, China Telecom set up a new standard project to study access networks (FTTH/B/ C AND IPRAN, ETC.
) TO INTRODUCE THE REQUIREMENTS AND ARCHITECTURE
OF SDN.

From the perspective of the impact of the introduction of SDN into optical networks on the industry: First, since the core competitiveness of optical network equipment is mainly in physical layer technology, SDN will not cause serious value loss for optical network equipment manufacturers in the short term
.
However, in the long run, because SDN provides an open network interface, optical network equipment has stronger multi-vendor interoperability capabilities, which is conducive to the entry of emerging manufacturers and will change the competitive landscape
of equipment manufacturers to a certain extent.

Secondly, the introduction of SDN puts forward new requirements
for the physical layer programmability of optical network equipment.
In order to better support software control and service dynamic scheduling requirements, some flexible and programmable physical layer technologies (such as ODUflex, adaptive optical transceiver, flexible grid optical switching, etc.
) will be gradually introduced and applied
.
L2 layer devices such as PTN and PONT are essentially packet-switched devices, and chips that support openflow will be gradually introduced with the development of data device evolution
.

Third, the introduction of SDN into optical networks provides a variety of heterogeneous network interconnection and open service capabilities, which will greatly promote the convergence and development
of bearer networks.
The introduction of SDN will establish a larger ecosystem, and with the help of SDN's openness, guide the entire optical network to an open and cooperative model, facilitate the rapid introduction of new service features, and improve the profitability
of the network.

Conclusion

The implementation of the national broadband strategy and the development of emerging services will promote the development of optical networks into a new era
.
SDN technology is a key enabling technology
for the broadband, intelligent, and open development of optical networks.
At present, the development of optical network SDN is still in the early stage, and there are still many problems and challenges in terms of technical route, application drive, and industrial promotion, which requires increasing investment in scientific research, promoting collaborative innovation among all parties in the industry, and jointly promoting the construction of future broadband optical networks
.