Since its inception in 2006, software defined network (SDN) has brought tremendous changes to networks. In the areas from data centers to enterprise networks and WANs, SDN has been widely deployed and applied. With the features of centralized control and openness, a network becomes simpler and its O&M is more efficient. The network is no longer "dead", but can have service awareness. According to service features, the network is automatically adjusted to improve its quality and ensure service level agreement (SLA). As the rise of 5G needs simple operations of many technologies and efficient service configuration, SDN is also the choice that brings highly-efficient automatic O&M and end-to-end service configuration.
SDN has found typical application scenarios in the transmission field such as SD-WAN and IP+OPTIC multi-layer synergy. As SDN in microwave transmission is still in its infancy, operators and equipment vendors have different views on it. However, standards organizations and European and American operators, such as ONF and AT&T, have begun to study SDN standards and application scenarios in microwave transmission.
SDN Architecture for Microwave Transmission
In compliance with the transmission architecture, microwave has its own characteristics that are reflected in northbound and southbound interfaces and cascaded controllers. The SDN architecture for microwave transmission contains three layers: application, controller, and microwave transmission equipment or network. The controller can also be divided into H-Controller and D-Controller. When the network under control involves devices from different vendors or covers microwave, IP, and optic fields, different D-Controllers are needed to control the devices of different vendors or their corresponding technical domains, and H-Controllers are used for inter-domain or end-to-end functional coordination.
Interfaces between the controllers and devices include Openflow, Netconf, and PCEP/BGP-LS. In terms of smooth network evolution and scalability, Netconf is more suitable for existing microwave scenarios and services. As microwave services evolve from Native ETH to IP/MPLS, PCEP/BGP-LS will be an option in the future. Some operators are also considering using Openflow. As an alternative choice for future SDN, Openflow might come in handy because the operators hope all future devices will be white-box devices. Control devices deployed at a higher layer will bring more flexible programmable networks and more innovative business models. The industry and certain standards organizations are also studying a better southbound protocol such as P4 to replace Openflow.
Features of Microwave Transmission
Microwave transmission has the following features:
● It has dynamic features at the air interface such as frequency interference, fading, and multipath. The corresponding function includes adaptive modulation and coding (ACM). A change in the weather brings dynamic adjustment to air interface modulation and accordingly results in dynamic bandwidth adjustment.
● It is sensitive to bandwidth capacity, so an important function—physical link aggregation (PLA) is introduced. The insufficient capacity at the air interface can be addressed by binding links.
● It has a tree or chain network topology in most cases and a ring topology in a few cases. This topology results in few redundant paths. For 5G backhaul, such as small cell backhaul or dense site deployment, there will be more mesh networks.
● It is in the access part of transmission and has a large number of nodes.
● Microwave devices are geographically distributed. The bandwidth of the control channel is limited and unreliable, and the real-time requirement is also difficult to meet.
Analysis of SDN Application in Microwave Transmission
Compared to the almost static configuration and management in a traditional O&M, SDN offers an automatic end-to-end O&M to microwave transmission.
Network Topology Discovery
A centralized controller brings not only centralized management but also an overall network view including the global topology view and global resource view. With standard southbound interfaces, the wireless link topology of microwave transmission involving different vendors can be easily shown on the same view, which makes significant progress over the traditional EMS.
Wireless Link Configuration
The standards organization ONF committed to standardizing microwave air interface data models has introduced the TR532 technical standard that provides a new way to automatic configuration of cross-vendor wireless air interfaces. Wireless links of NEs including the frequency, modulation, and XPIC, can be configured by the controller or through flexible applications. With programmed applications, changes to wireless link configuration can be automatically triggered by an event such as weather.
End-to-end service management is an important feature of microwave transmission. The SDN architecture can be used to effectively enhance end-to-end service capabilities of microwave transmission. Through open northbound and southbound interfaces and cascaded multilayer controllers, end-to-end cross-domain and cross-vendor management that is unavailable in the traditional O&M can be supported. In addition to multi-vendor management, operators also hope to deliver end-to-end cross-domain services that cover microwave transmission, IP, and optic domains.
Energy-Efficient Wireless Links Based on Service Awareness
Microwave transmission has a unique PLA/LAG function that aggregates wireless links. Traditionally, modem boards and outdoor units (ODUs) at the air interfaces are all open, regardless of air interface traffic. However, the traffic flow at night or before dawn reduces dramatically compared to that at busy hours, and one or more aggregated air interface channels may be free. In this case, a centralized controller can be used based on certain policies to close some free channels or mute ODUs to save energy (Fig. 1). When the traffic flow is predicted to rise, the free channels can be open again by the controller.
Rerouting Triggered by ACM
Rain fade triggers adaptive coding and modulation (ACM), which results in bandwidth decrease at the air interfaces and damage caused to lower-priority services (Fig. 2). For a ring network, the traditional solution is to upgrade existing protocols to trigger service switching such as ERPS. However, SDN can be an easier solution and brings more adaptability such as MESH. When a service fault is detected through the corresponding O&M, a controller can receive the fault notice, obtain a redundant path through the path algorithm, and reroute the damaged service to the redundant path.
5G Application Prospect
There will be more SDN scenarios for 5G networks. For example, frequencies can be automatically allocated for unlicensed bands. This reduces interference and maximizes frequency utilization. Advanced features such as ad hoc networks and network slicing may also be introduced. These application scenarios need to be further studied.
Although SDN is relatively new to microwave transmission, it will be practically applied as the SDN standards and applications have gradually matured. Microwave transmission is a part of the transmission network, and SDN will be more widely used as the entire transmission network is restructured and upgraded.