SDN based Control and Management of WLANs in the 3GPP 5G Network
Pranav Jha Abhay Karandikar
cs.NI

	
	

	

 
  
				
		
		
		
		
	
	
SDN based Control and Management of WLANs in the 3GPP 5G Network
    
 Pranav Jha, Abhay Karandikar
Department of Electrical Engineering, Indian Institute of Technology Bombay 

Email: {pranavjha, karandi}@ee.iitb.ac.in

    December 30, 2023
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The exponential growth in mobile broadband usage <cit.> has catalyzed the need for high data rate communication systems. In this regard, activities for standardizing the next-generation mobile broadband system, known as the Fifth Generation (5G) system are underway. The 5G system also enables the integration of Institute of Electrical and Electronic Engineers (IEEE) Wireless Local Area Networks (WLANs) for providing cost-effective broadband connectivity. It is therefore imperative to find solutions for control and management of WLANs, while providing seamless inter-working capabilities with the cellular network. In this paper, we propose a novel Software Defined Networking (SDN) based architecture for efficient control and management of IEEE WLANs while providing a mechanism for smooth integration of WLANs within the 5G system.









The world is seeing tremendous growth in mobile data <cit.>. This has led to a rapid evolution of cellular wireless communication technology, the development of Fifth Generation Mobile communication system (5G system), which is currently under way, is an important initiative in this direction. The growth in mobile data is also leading to a large-scale deployment of IEEE 802.11 based Wireless Local Area Networks (WLANs) supplementing the 3rd Generation Partnership Project's (3GPP's) Universal Mobile Telecommunication System (UMTS) and Long Term Evolution (LTE) networks. The deployment of WLANs in conjunction with the cellular networks is fuelling the need for an efficient control and management of WLANs and also a seamless integration with cellular networks. This paper proposes an SDN based architecture for effective control and management of IEEE 802.11 based WLANs. The proposed architecture also provides a mechanism for smooth integration of WLANs with the upcoming Fifth generation mobile communication system.

	


§ INTRODUCTION


Multiple initiatives are being undertaken world-wide to handle the rapid growth of mobile data. An important step in this direction is the development of the Fifth Generation (5G) mobile communication system. Various technology and Standard Development Organizations (SDOs) are involved in this exercise and the forthcoming 5G system from the Third Generation Partnership Project (3GPP) is one of the key technologies in this direction.
In addition to the development of new wireless communication technologies, other measures such as the deployment of IEEE 802.11 based Wireless Local Area Networks (WLANs) alongside the cellular network, are also being undertaken 
WLANs are particularly suitable for low mobility access scenarios, e.g., to support stationary or pedestrian users and is a cost-effective mobile broadband 
access technology. Therefore in areas where cellular coverage may be poor or data demand is very high, operators are supplementing their cellular networks with IEEE 802.11 based WLANs <cit.>.


Broadband mobile data usage has experienced a tremendous growth in the past decade, giving rise to multiple standardization initiatives towards the design and development of higher capacity networks known as the Fifth Generation (5G) networks. The 5G network which is being developed by the Third Generation Partnership Project (3GPP), is one such key standard.

In addition to the development of new wireless communication technologies, other measures such as the deployment of IEEE 802.11 based Wireless Local Area Networks (WLANs) alongside the cellular network are also being undertaken. WLANs are particularly suitable for low mobility access scenarios, e.g., to support stationary or pedestrian users and are a cost-effective mobile broadband access technology. Therefore, in areas where cellular coverage may be poor or data demand is very high, operators are supplementing their cellular networks with IEEE 802.11 based WLANs. At present, WLAN equipment vendors provide proprietary mechanisms for Access Point (AP) management. This leads to interoperability issues across APs from different vendors. An attempt to address these issues was  made by the Internet Engineering Task Force (IETF) by standardizing a protocol known as Control And Provisioning of Wireless Access Points (CAPWAP) <cit.>. CAPWAP provides a centralized control and management mechanism for WLAN APs and helps in achieving interoperability across APs from different vendors. However, this protocol does not provide a well-defined programmable interface over the APs and therefore not in agreement with the SDN principles.

Along with these developments in wireless communication technology, significant progress is also being made towards the evolution of newer architectural and design principles for the next-generation communication systems. Software Defined Networking (SDN) is one such emerging architectural paradigm, which is transforming the approach for control and management of a communication network. It promises to provide significant advantages over conventional networks in terms of flexibility, programmability, vendor interoperability, optimized resource utilization, and granular control over the deployment of policies/services in the network.

SDN enables the separation of control and forwarding (data) plane functions in a network through a standardized programmable interface <cit.>. This is made possible by transposing the control plane functionality from the network devices such as routers, switches etc., to SDN Controllers in a logically centralized fashion. These modified network devices form a part of the forwarding plane in the network and are responsible for forwarding data. The data forwarding decisions in the devices are taken on the basis of the rules provided by the SDN Controller. In this paper, we propose an SDN based architecture for control and management of IEEE 802.11 WLANs. The proposed architecture brings the advantages of SDN to IEEE 802.11 based Radio Access Network (RAN).

Due to increasing WLAN deployments alongside the 3GPP cellular networks, there is an increasing need for unified control and management of 3GPP cellular networks and non-3GPP access networks (IEEE 802.11 based WLANs). 3GPP has made continued efforts for efficient inter-working of WLANs with 3GPP cellular networks. Multiple enhancements to the 3GPP Long Term Evolution (LTE) standard, across different 3GPP releases, have been made in order to make it inter-working with WLANs <cit.>. The proposed 3GPP standard for the 5G System <cit.>, i.e., the Release 15 of the 3GPP standard, aims to enable inter-working between WLAN based access network and the 5G Core (5GC) network through a logically independent inter-working function known as the Non 3GPP Inter-Working Function (N3IWF). This function is still under standardization in 3GPP. In this paper, we also present an SDN based technique for implementing the inter-working functions for seamless integration of WLAN with the 5GC.


The core idea behind SDN is the separation of control and forwarding (data) plane functions in a network through a standardized programmable interface <cit.>. Control plane functionality of networks is typically removed from the network devices, e.g., switches, and routers and is handled by one or more SDN controllers in a logically centralized fashion. The  network devices form part of the forwarding  plane in the network and perform data forwarding functionality. The data forwarding decisions in the devices are taken on the basis of the rules provided by the SDN controller.  Though the paradigm originated for control and management of devices in the wired networks, it is gradually being adopted in wireless networks too <cit.>, <cit.>, <cit.>. A good example in this regard is 3GPP's 5G system, especially the 5G Core Network, which follows the SDN paradigm and has been organized as a set of Network Functions (NF) embodying either the control or the data plane functionality separated through standardized interfaces <cit.>. 
In large-scale deployments of IEEE 802.11 standard based WLANs, usage of an SDN based network architecture, i.e., with logically centralized control and management functions separated through a standardized interface from the distributed Wireless Access Points (AP) devices is likely to bring in substantial advantages. Unfortunately no such standard is in use in today's WLAN deployments and the individual vendors use proprietary mechanisms to manage their WLAN AP devices, leading to interoperability issues across Access Point devices from different vendors. "Control And Provisioning of Wireless Access Points" (CAPWAP) <cit.>, a protocol standardized by Internet Engineering task Force (IETF) supports a centralized control and management mechanism for WLAN APs and also provides a way to achieve interoperability across APs from different vendors. However the protocol does not follow the SDN paradigm, i.e., a well-defined programmable interface over the data plane is missing in CAPWAP.
In this paper, we propose an SDN based architecture for control and management of IEEE 802.11 WLANs. The proposed architecture brings advantages of SDN to IEEE 802.11 based radio access network (RAN).

Due to increasing WLAN deployments along side the 3GPP cellular networks, a need for unified 3GPP cellular networks and non-3GPP access networks (IEEE 802.11 based WLANs) has also been felt in recent years. 3GPP has made continued efforts for efficient inter-working of WLANs with 3GPP cellular networks. Multiple enhancements to 3GPP Long Term Evolution (LTE) standard, across different 3GPP releases, have been made in order to make it inter-working with WLANs <cit.>. The proposed 3GPP standard for the 5G System<cit.>, i.e., the Release 15 of the 3GPP standard,  aims to enable inter-working between WLAN based access network and 5G core network through a logically independent inter-working function.
The SDN based architecture for control and management of IEEE 802.11 WLAN, proposed in this paper, also enables a seamless integration of WLANs with 3GPP's 5G core network and facilitates the support for some key features of the 5G system over WLANs.










The rest of the paper is organized as follows. Section II describes related work and also provides details on the 3GPP's standardization efforts towards the 5G system. Section III provides details on the proposed SDN based architecture to control and manage WLANs. Section III also dwells on the proposed mechanism for integration between WLANs and 3GPP defined 5G system. Advantages of the proposed architecture are highlighted in the section IV of the paper. The proposed architecture has also been compared with the 3GPP's 5G access network architecture in the same section. Section V summarizes and concludes the paper.


The rest of the paper is organized as follows. Section II discusses some of the available literature in this area. Section III provides details regarding the 3GPP defined 5G architecture. The proposed SDN based architecture is discussed in Section IV. The subsequent section highlights the advantages of the proposed architecture. Section VI concludes the paper and indicates the directions for future work.



§ RELATED WORK

In this section, we present a brief summary of the existing literature in this area. Some of them <cit.> define mechanisms for SDN based control and management of WLANs. Authors in <cit.> propose a solution based on Lightweight Virtual APs (LVAPs) which can be created and controlled by a centralized SDN Controller. The solution combines two south bound protocols viz., OpenFlow for traffic steering and Odin for managing the wireless radio. Another work <cit.> provides a scalable SDN-based monitoring and management framework for WLAN. 


The second set of articles explore the integration of WLANs with the Fourth Generation (4G) 3GPP LTE systems.  Authors in <cit.> propose an SDN based converged architecture for LTE and WLAN. This architecture provides  seamless mobility at the service level between LTE and WLAN using virtualization. This solution involves the use of a virtual middlebox in the UE for choosing the network interface to/from which data is to be steered and route the control messages.



§ 3GPP DEFINED 5G ARCHITECTURE


Standardization activities for the Fifth Generation Mobile Communication System is currently underway. Efforts being made by 3GPP is a notable example in this regard. 3GPP's 5G system, illustrated in Fig. 1, can broadly be divided into two parts, the Core network and the Access network. While the 5G access network can support varied radio access technologies, including 3GPP as well as non-3GPP technologies, there is a single unified 5G core network to support all these access technologies. The system has also been organized as a set of Network Functions (NFs) <cit.>. The 3GPP Access Network for 5G system is also called the Next Generation Radio Access Network(NG-RAN). The Network Functions in the Core Network follow the SDN paradigm and are identifiable either as control or data plane functions. These Network Functions interact with each other over standardized interfaces. However, the 3GPP Access Network nodes belonging to the NG-RAN, e.g., gNB and ng-eNB incorporate both the control and the data plane functionality without a standardized programmable interface between the two. The Figure 2 provides the details on the Architecture of the New Radio based 3GPP NG-RAN. The NG-RAN may comprise of New Radio based radio access nodes called the gNBs or LTE based next generation radio access nodes called the ng-eNBs. A gNB provides New Radio data and control plane protocol interface towards the UE and an ng-eNB, provides E-UTRA data and control plane protocol interface. For the sake of brevity, this paper describes the architecture of a gNB only.


This section provides details on the 3GPP-defined 5G system architecture. This architecture, illustrated in Fig. <ref> can be broadly divided into two parts viz., the 5GC and the access network known as the Next Generation Radio Access Network (NG-RAN). The access network supports a multitude of access technologies, both, 3GPP and non-3GPP. The 5GC is common for all of these diverse access networks. The 5G system is constituted as a set of Network Functions (NFs) <cit.>. NFs are entities with well-defined functionality and interfaces. The NFs in the 5GC follow the SDN paradigm and are identifiable either as control or data plane functions. These NFs interact with each other over standardized interfaces. However, the 3GPP access network nodes belonging to the NG-RAN, e.g., gNB and the next generation eNodeB (ng-eNB) incorporate both the control and the data plane functionality without a standardized programmable interface between the two planes. The NG-RAN may comprise of NR based radio access nodes called the gNBs or LTE based next-generation radio access nodes known as the ng-eNBs. A gNB provides NR data and control plane protocol interface towards the UE and an ng-eNB, provides the Evolved Universal Terrestrial Radio Access (E-UTRA) data and control plane protocol interface. For the sake of brevity, this paper uses only the gNB as the reference.






The gNB as illustrated in Fig. <ref> consists of two types of units, a gNB Central Unit (gNB-CU) and one or more gNB Distributed Units (gNB-DUs). A gNB-CU is a logical node, which controls the operation of one or more gNB-DUs. It hosts the control plane protocol as well as a few data plane protocols on the radio interface. It also interfaces with the 5GC for both, data plane (over N3 interface) and control plane over the N2 interface. The gNB-CU also comprises of the Radio Resource Management (RRM) functionality, e.g., admission control, connected mode mobility control, and radio bearer control functionality. The gNB-DU is a logical node hosting the Radio Link Control (RLC), Medium Access Control (MAC) and Physical Layers of the gNB. It is partly controlled by a gNB-CU. Every gNB-DU supports one or more cells. Although the UE access and session management functionality is handled by the 5GC network in a unified manner, RRM functions are likely to be handled at individual access network entities, i.e., at a gNB-CU, possibly leading to sub-optimal decisions. The N3IWF connects the 5GC to the non-3GPP access networks. 


TR-069 <cit.> is one of the most widely used protocol suites for the management of Consumer Premise Equipment (CPE). These protocols are intended to support discovery of Auto-Configuration Server (ACS), CPE association with the ACS as well as the management of CPEs. 3GPP recommends the usage of TR-069 protocol suite for the management of the Femto Access Points, i.e., the  LTE Home eNodeBs (HeNBs) and 3G-UMTS Home NodeBs. Even though the management of WLAN APs is not  specifically addressed in TR-069, the protocols suite can be used to mange the WLAN APs. However the protocols support only the management plane functionality and does not address the control plane aspects.
Other standardization efforts towards centralized control of WLAN APs include Secure Light Access Point Protocol (SLAPP) <cit.>, Lightweight Access Point Protocol (LWAPP)<cit.>, and Wireless LAN Control Protocol (WiCoP)<cit.>. These protocols were merged by IETF to form CAPWAP. CAPWAP defines a centralized Access Controller (AC) to be used for control and management of WLAN APs. However the protocol tunnels both data as well as control frames from the Access Points to the AC and therefore may not provide the benefits of an SDN based architecture.
Other proposals to utilize the principle of SDN in WLANs include CloudMAC<cit.>, SDWLAN<cit.>, OpenRF<cit.>, Odin<cit.>,  DenseAP<cit.>, and ISD-WiFi<cit.>.




§ PROPOSED ARCHITECTURE



In this section, we propose an SDN based architecture for control and management of IEEE 802.11 based WLANs. The proposed architecture has been depicted in Fig. <ref>. This architecture separates the control plane functions in WLANs from the forwarding (data) plane functions. It also enables the seamless integration of WLANs with the 5GC with the help of a WLAN Aggregation Entity (WAE).
The proposed WLAN architecture consists of the following logical entities: WLAN APs, WAE, the SDN based WLAN Controller (also called the RAN Controller), Application Function (AF) and the Network Virtualizer (NV). The RAN Controller together with the AF implements the control plane functionality in the network. NV is also a control plane entity. However, it is primarily responsible for virtualizing the physical network and facilitating the creation of multiple logical networks over a physical network. The WAE and the WLAN APs constitute the data plane in the network.
The functionality of the various component functions in the control and data plane are described in detail below:


  * Control Plane Functions


  * NV: The NV function creates multiple logical networks, also called known as `network slices' over a single physical RAN. Each of these logical networks or network slices may comprise of individual data and/or control plane entities/functions as shown in Fig. <ref>.

  * AF: AF is responsible for the implementation of the RAN resource management algorithms, such as load balancing, admission control and mobility management, spectrum management etc.. It is integrated with the  RAN Controller through a standardized Interface.

  * RAN Controller: The RAN Controller is responsible for the following functionalities in the control plane. It supplies configuration to the data plane entities and also performs the mapping from the UE to the AP. The RAN Controller also manages the RAN specific control plane communication for the UEs. It provides the Quality of Service (QoS) attributes to the RAN data plane entities for an individual  traffic flow. This can be achieved with the help of the following sub functions viz., the RAN Management Function (RMF), Flow Control Function (FCF), RAT Control Function (RCF) and the Data Plane Interface Function (DPIF). The RMF provides the relevant entity specific configuration parameters to the data plane entities. The FCF  deals with an abstract view of the underlying network in terms of UE specific traffic flows and is responsible for setting up these flows with requisite Quality of Service (QoS) attributes through the RAN data plane entities. The flow specific QoS attributes are received by the RAN Controller from the 5GC. The RCF exchanges RAN specific control messages with the underlying data plane entities for every UE and manages the UE to AP mappings. This mapping is established under the control of algorithms running as part of the AF. The DPIF comprises of the NETCONF and OpenFlow protocols and acts as the RAN Controller interface towards the data plane i.e., the WAE.


  * Data Plane Functions


  * WAE: The WAE is a data plane aggregation function built over the WLAN APs. It presents a unified management and control interface of the underlying RAN to the control plane. It receives relevant control and management information from the RAN Controller and provides them to the APs and UEs. A NETCONF and OpenFlow protocol based standardized interface is used for the exchange of information between the RAN Controller and the WAE. The WAE also facilitates integration of WLANs with the 3GPP 5GC. It relays data and UE specific control (Non-Access Stratum (NAS) signaling) messages exchanged between UEs and the 5GC. The relayed control messages include information for the authentication of UEs. It also establishes an Internet Protocol Security (IPSec) tunnel with the UEs for secure communication between the two. This entity also segregates the data path from the control path. Control signals (NAS signaling messages) are exchanged with the 5GC network over the N2 interface, whereas the user data is exchanged over the N3 interface with the 5GC. It also applies the required QoS attributes to the individual data flows in the RAN. This entity also acts as the mobility anchor for the UEs, i.e., the WAE for a particular UE does not change when the UE moves across APs.

  * WLAN AP: The WLAN APs are connected to the UEs on one side and the WAE on the other side and is responsible for relaying data/UE specific control signaling exchanged between UEs and the 5GC via the WAE. The APs communicate with the UEs over the IEEE 802.11 based radio interface. The APs also exchange RAN specific control information with RAN controller via the WAE. The RAN Controller shares RAN specific control information with APs to control the UE-to-AP mappings and also broadcast information by APs in the basic service area.



In our proposed architecture <ref> the SDN controller controls and manages the WLAN access network while the 5G core is used to control the User Equipment which includes authentication and data service. We introduce a new logical entity called EIF in this architecture. This is used to achieve separation of control and data plane and integration of WLAN with 3GPP 5G network. 
 
 SDN controller uses Control and Management Interface which is a standardized interface to exchange control and management messages with WLAN base stations through EIF. Applications running on top of SDN controller can use CMI to achieve functionalities like UE admission control, UE mobility management, flow configuration, spectrum management, load balancing, interference management etc. The EIF abstracts the messages received from CMI into South Bound Interface(SBI) messages and vice versa. This is necessary due to the proprietary nature of SBI. 
 
 The proposed 3GPP 5G standard uses N3IWF to support non-3GPP networks in its architecture. In our architecture EIF exactly mimics the functionality of N3IWF. The EIF provides two interfaces towards 5G core. The N2 interface towards AMF is used for UE authentication and NAS signaling while N3 interface towards UPF is used to route user data to 5G core.
 


§ ADVANTAGES OF THE PROPOSED ARCHITECTURE

The proposed architecture provides multiple advantages, some of which are described below:

\begin


 §.§ Interoperability

Network operators may require to deploy WLAN APs with different capabilities from different vendors to meet their requirements. However, in practice each vendor uses proprietary interface between base stations and their controllers. Thus, instead of using a generic controller to control their complete network, they have to use multiple controllers from multiple vendors to manage their network. In this architecture the WAE can abstract the vendor specific south bound message into a standardized message which can be used with a generic controller. This enables the operator to do multi-vendor deployment while using a single controller.



 §.§ Enables Slicing of the Network

Due to the increasing diversity of services, with each service having its own performance requirements, it is difficult for operators to deliver these services over same physical infrastructure. Network slicing resolves this problem by dividing the physical infrastructure into smaller logical networks known as slices. The proposed architecture enables network slicing support for WLAN networks at the RAN. The RAN Controller facilitates the complete life cycle viz., the Create, Read, Update, Delete (CRUD) operations for network slice templates to support diverse applications. Network slice templates are data filters that help in mapping a particular service to a given slice instance. The RAN Controller configures and manages the resources in WLAN APs and the DPIF, based on the slice template. It provides efficient end-to-end service orchestration of RAN deployments, services for network scalability etc.. Slicing also enables multi-tenancy support and flexibility in the network. 




 §.§ Introduction of SDN into the RAN




In the proposed 3GPP 5G NG-RAN specification <cit.> the gNB is connected to the 5G core via the NG interface. The gNB is split into two units gNB-CU and gNB-DU. Multiple gNB-DUs can be connected to, and be controlled by a single gNB-CU node over the F1 interface. F1 interface carries both data as well as user plane frames from gNB-DU to gNB-CU. Data and control frames are aggregated by gNB-CU and sent over the NG interface. Thus, there is an absence of a clear separation between the control and data plane in this architecture. Whereas in our architecture, the WAE aggregates both control and data traffic from WLAN APs. It then acts as the point of separation between control and data plane as control frames are sent over the DPIF interface and data frames are sent over the N3 interface. 




 §.§ Reduction in Control Plane Latency

WLAN Controller architectures like CAPWAP support two  major modes of operation, split-MAC and local-MAC. In the split-MAC mode, data frames need to be tunneled to the Controller after which they are routed to the packet gateway. This approach does not scale well for largescale WLAN networks especially under high load. Due to high data traffic, the link from WLAN AP to the Controller becomes saturated. This increases  the latency of the control messages. In the proposed architecture data is not tunneled to the RAN . As a consequence, the control packets do not exhibit any latency even under high load conditions. 


 §.§ Support for Virtualization

As the proposed architecture uses an SDN based approach, it is possible to implement middlebox functionalities like firewall, load balancing as virtual network functions as an applicartion on the SDN controller. The architecture can also support virtualization of entities like WLAN APs as proposed by various research papers such as Odin<cit.>, CloudMAC<cit.>


 §.§ Thin and Energy Efficient Controller

As the RAN Controller does not have to handle data plane processing, it can be simpler in terms of functionality in comparison to existing SDN Controllers. This translates to lower energy consumption values even under high load conditions as compared to other Controllers. Also, thinner energy efficient controllers enable OPEX reduction for operators due to lower electricity costs. 
















§ CONCLUSION

In this paper we have proposed an SDN based architecture for control and management of WLAN. This architecture also provides a novel mechanism for enabling integration with the proposed 3GPP-defined 5G network. We have also compared other existing solutions and present possible use cases for the proposed architecture.



§ ACKNOWLEDGMENTS

This research work has been funded through a grant on SDN and 5G from Department of Electronics and Information Technology (DeITY), Government of India. We thank Abhishek Dandekar, Ashish Sharma, Rohan Kharade and Akshatha Nayak M. for their contributions to this paper.

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