What happens when SDN meets 5G?

SDN is a profound change to traditional IP networks. It has a far-reaching impact on IP networks in three aspects: first, it makes the network programmable. By providing a northbound interface, applications can adjust the network according to needs, promoting network innovation; second, the control plane is centralized. The centralized control plane has a global view of the network, which solves the segmented autonomy and disorder of the IP network; finally, because the control plane is separated from the forwarding plane, the behavior of the forwarding plane is abstracted and a standard southbound interface is used, so that the forwarding plane can be as universal and simplified as possible.

After the traditional IP network is deployed and launched according to business needs, if the business demand for the network changes, the configuration of the corresponding network equipment (routers, switches, firewalls) needs to be modified again, which is a very cumbersome task. In the ever-changing business environment of the Internet/mobile Internet, the high stability and high performance of the network are not enough to meet business needs. Flexibility and agility are more critical. After the introduction of SDN, the control rights on the network equipment can be separated and managed by a centralized controller without relying on the underlying network equipment, shielding the differences from the underlying network equipment. The control rights are completely open, and users can customize any network routing and transmission rule policies they want to implement, making it more flexible and intelligent.

In SDN networks, the configuration of network devices is greatly simplified. The ports and links of devices are automatically discovered through default protocols. Applications only need to define simple network rules according to their needs. SDN provides an open programming interface based on business logic, making the development space of the network as a "pipeline" infinitely possible. If the business application model of cloud computing in the future can be simplified to "cloud-pipe-end", then SDN is an important technical support for the "pipe" link. SDN has been widely used in data center networks. In view of the traffic characteristics of data centers, the use of SDN technology can significantly improve the bandwidth utilization of the network; SDN can greatly simplify network configuration and save operation and maintenance costs, so it has also achieved remarkable results in the fields of metropolitan area networks and access networks.

With the evolution of 4G networks, the forms of CloudRAN and future 5G networks have gradually become clearer. The xHaul network has evolved from a single backhaul to a fronthaul, backhaul, and CO (Central Office) switching network. In mobile networks, end-to-end service guarantees are emphasized. The xHaul network carries data transmission between multiple network elements/computing nodes, and the QOS of data packets must be guaranteed throughout the process. The use of SDN technology can coordinate the scheduling of network resources throughout the entire network, thereby providing end-to-end service guarantees.

The mobile network itself has the characteristics of separation of control plane and data plane, and separation of service and bearer, and is inherently equipped with the concept of SDN. The 3GPP specification also clearly defines the network elements of the network controller, such as BSC in the GSM network and RNC in the UMTS network. However, since the bearer network has historically belonged to the fixed network transmission part, there is no open unified interface. In addition, these transmission devices come from different manufacturers, resulting in the RAN network controller BSC/RNC having very limited control over the bearer network and failing to meet the service guarantee requirements of the entire network.

Therefore, as the bearer network in the future wireless network, 5G xHaul not only needs to introduce SDN technology to solve the basic needs of network open programming, control plane centralization, and control and forwarding separation, but also needs to deeply integrate the SDN technology of the transmission network with the RAN network to meet the future 5G-oriented business diversification, hundreds of billions of IoT business connections, and ultra-low latency for industrial control/ARVR.

The 5G xHaul network has unique characteristics of wireless networks. The current SDN architecture designed for data centers and fixed-line bearer networks as the main application scenarios cannot be completely wireless, especially in the 5G era. The xHaul Controller and the transmission equipment of the bearer network must meet the following requirements:

1. Fusion scheduling of multiple networks

The 5G xHaul network will inherit the existing Backhaul network and build or enhance some new networks. Therefore, there will be a situation where multiple transmission technologies coexist, including optical wavelength division or PON equipment, microwave-based mmW equipment, Gbit-level Ethernet, and TDM-based point-to-point equipment. The 5G xHaul network needs to be able to perform global resource scheduling between heterogeneous transmission resources, allocate the optimal transmission path for business matching, and achieve coordination between multiple transmission technologies to achieve dynamic adjustment, energy saving, and improved reliability.

2. End-to-end slicing of 5G xHaul networks

The requirements for QOS of 5G's diversified services vary greatly, and RAN networks and core networks already have slicing technology to solve this problem. Traditional bearer networks can only make simple resource reservations or provide differentiated services for QOS, and the use of network resources is still preemptive, and the QOS services provided are limited. As a bearer network connecting network elements or computing nodes, xHaul must be able to provide separate QOS capabilities for different slices, without affecting the slices, and this capability is full-process and full-network. To achieve this challenging goal, the xHaul network also needs to provide slicing capabilities, slice the transmission equipment resources and network topology physically and logically, implement QOS management for each slice, and match each type of service.

3. Control of ultra-low latency networks

CPRI is a technology based on fixed rate, with exclusive bandwidth and latency guarantee. After CPRI is packetized, the quality of QOS cannot be reduced. In particular, some 5G services for industrial control and AR/VR require extremely low latency. 5G xHaul needs to provide a capability to provide a certain forwarding path and the lowest forwarding latency in a given slice through unified scheduling of the entire network by the controller to meet business needs.

4. Routing algorithm based on optimal bandwidth and latency

The existing SDN technology has a single strategy in routing algorithms, and can only select the shortest path or the optimal path with cost. In the 5G xHaul network, it is necessary to provide a routing algorithm with bandwidth guarantee and optimal latency. This algorithm calculates the end-to-end bandwidth reservation and latency reference based on the input bandwidth parameters and latency parameters, and selects the optimal path for business use.

5. Rapid linkage of wireless and transmission resources

Due to the fast-changing characteristics of mobility and air interface quality, wireless networks require rapid adjustments to 5G xHaul networks. This adjustment is different from the dynamic configuration of general transmission networks. It has a very high frequency. For example, when users switch cells at high speeds and cell load changes in crowded areas, 5G xHaul is required to respond quickly and adjust to the new optimal path. This places high demands on the linkage of wireless resources and transmission resources. The current SDN controller is difficult to meet this scenario through the northbound interface. The 5G xHaul network provides a unified controller to deploy the wireless control unit and the transmission network control unit in one, thereby providing rapid linkage.

6. Real-time monitoring of QOS performance

When the service experience changes, the network needs to be aware of it in advance. For 5G xHaul, it is necessary to be able to promptly detect changes in packet loss, latency, and throughput on the service link, and provide real-time feedback to the network control unit so that timely adjustments can be made to optimize the transmission network. It uses similar IPPM technology to the existing one, and further enhances latency, throughput, and packet loss rate to match the new SDN architecture.

7. Refined network management

To cope with 5G service carrying, the xHaul network will be more intelligent. In order to support slicing and low latency requirements, in addition to the traditional SDN flow table resources, the switching node will open more interfaces for the controller to schedule uniformly, such as the thread, queue depth, buffer, logical port, etc. of the switch processing the message. The SDN controller can achieve end-to-end precise QOS control of the entire network through global unified and refined management of these resources.

8. Openflow protocol extension

The Openflow protocol is a standard southbound interface under the existing SDN architecture, used for communication between the SDN controller and the SDN switch to complete configuration and query. The Openflow protocol abstracts the data plane message forwarding, which consists of a series of match-actions. The message matches the flow table and executes the corresponding action to complete the data plane forwarding task. In the 5G xHaul network, the behavior abstraction of the data plane needs to be further extended, such as supporting GTPU tunnels and supporting new encapsulation and actions after Fronthaul-Split. Therefore, the Openflow protocol needs to be extended to match wireless services.