Technology Business Unit White Paper SDN in Wireless Cellular Networks: Gearing Up to Meet the Growing Bandwidth Demand
About the Author Thomas Lee Sebastian Dr. Thomas Lee Sebastian works as a Solution Architect at Tata Consultancy Services (TCS), where his R&D activities span several related technologies such as SDN, OpenStack and NFV. Prior to joining TCS, Dr. Sebastian was associated with Universidad de Concepcion, Chile, Fiber Optika Technologies and Tech Mahindra. He holds a graduate degree in Physics and a doctorate in Fiber Optics from Cochin University of Science and Technology, India. Dr. Sebastian also has a postdoctoral fellowship from Universite de Nice, France, in the field of Fiber Amplifiers.
Abstract The explosive growth in devices and the corresponding increase in the need for higher bandwidth are compelling cellular service providers to look beyond traditional technologies. In addition, wireless networks have been riddled with several issues such as inter-cell interference and mobility management. This volatile situation calls for disruptive technologies such as Software Define Networking (SDN) to address and solve existing issues as well as future challenges. SDN brings in programmability and dynamism to communication networks. These two attributes have been the major driving forces for its immediate acceptance in Data Center Networks (DCN), where the network topology and connected host configuration remain fluidic. SDN works as an effective facilitator for wireless networks due to its dynamic nature. Compared to wired networks, wireless networks are diverse in terms of technology (LTE, Wi-Fi, CDM, and GSM), cell ranges (macro, micro, pico, and femto), and user density levels. This paper proposes a federated SDN controller framework with a dedicated SDN controller for each network domain such as EPC, IMS, and E-UTRAN. Such a solution can help service providers effectively address the increasing demand for higher bandwidth, while controlling costs. It also presents a high level architecture for potential SDN deployment in wireless cellular networks.
Contents The Rising Demand for Increased Bandwidth 7 The Successful Evolution of SDN 7 The State of Cellular Networks Today 8 The Challenges with Cellular Networks 9 Centralization of Data Plane Functions 9 Integration between Data and Control Planes 9 Inter-cell Interference 9 Offloading technologies An SDN Architecture for Cellular Networks 10 10 SDN EPC Controller 11 SDN E-UTRAN Controller 12 SDN Wi-Fi Controller 13 SDN IMS Controller 13 Improving Wireless Cellular Networks with SDN 14
List of Abbreviations AAA Authorization, Authentication and Accounting ANDSF Access Network Discovery and Selection Function CAGR Compounded Annual Growth Rate CDMA Code Division Multiple Access CSCF Call/Session Control Function DCN Data Center Networks DOCSIS Data over Cable Service Interface Specifications DSL Digital Subscriber Link EPS Evolved Packet System E-UTRAN Evolved UTRAN FSF Federated SDN Framework FTTX Fiber to the X GERAN GSM EDGE Radio Access Network GPRS General Packet Radio Service GSM Global System for Mobile Communication GTP GPRS Tunneling Protocol HSS Home Subscriber Server IMS IP Multimedia Subsystem LTE Long Term Evolution MIP Mobile IP MME Mobility Management Entity NE Network Element NFV Network Functions Virtualization OF OpenFlow ONF Open Networking Foundation PCRF Policy Control and Rules Function P-CSCF Proxy CSCF PDN Packet Data Network
List of Abbreviations P-GW PDN Gateway PMIP Proxy Mobile IP PSTN Public Switched Telephone Network QoS Quality of Service S-CSCF Serving CSCF SDN Software Defined Networking S-GW Serving Gateway SIP Session Initiation Protocol SNR Signal to Noise Ratio USF Unified SDN Framework UTRAN Universal Terrestrial Radio Access Network VNI Virtual Networking Index VoLTE Voice over LTE WiMAX Worldwide Interoperability for Microwave Access WMWG Wireless and Mobile Work Group
The Rising Demand for Increased Bandwidth Wireless devices such as smart phones, tablets, and laptops have contributed to the explosive growth in internet traffic over the past few years. An average 4G connection generates 15 times more traffic than a 3G connection, and requires proximally located mobile towers to meet the Signal to Noise Ratio (SNR) requirements of advanced modulation techniques. The phenomenal growth in traffic from mobile devices poses new challenges for service providers (SPs) and network operators in meeting customer needs, without compromising the bottom line. The obvious but costly solution to increased bandwidth demand would be to increase the link capacity. However, this can impact profit margins. In addition, an increase in link capacity from the wireless side may not completely resolve the issues because the available spectrum shrinks with more customers connecting to the mobile tower. This challenge can be mitigated by using the unlicensed spectrum of radio frequency, wherever possible. In addition, wireless networks have been plagued by other issues such as inter-cell interference and mobility management. However, there is an upside to these challenges. They are fueling the advent of new technologies such as Network Functions Virtualization (NFV) and Software Defined Networking (SDN) into the wireless network landscape. The Successful Evolution of SDN The concept of SDN has its roots in OpenFlow protocol. The goal of SDN was to decouple the control plane from the data plane of all network equipment (NE) and keep it in a centralized location that could control the NE through OpenFlow messages. This centralized control plane architecture generated several benefits in terms of cost, manageability, and programmability. The Open Networking Foundation (ONF), established by leading SPs and equipment vendors (EVs), created the Wireless and Mobile Work Group (WMWG) to identify enhancements to OpenFlow that will improve the operation of mobile and wireless networks. The current use cases being considered at WMWG include separation of control and data planes and traffic steering in Evolved Packet Core (EPC), and wireless backhaul optimization wherein the SDN controller optimizes radio parameters in the data plane using enhanced OpenFlow. Several network equipment vendors have already made heavy investments in OpenFlow and SDN with their OpenFlow switches and SDN controllers. Today, SDN is a broad term and is not limited to OpenFlow protocol but encompasses all technologies that can abstract lower level functionality and provide network programmability. 7
The State of Cellular Networks Today In today's 4G Long Term Evolution (LTE) network, the user equipment (UE) connects to the outside world (internet or Public Switched Telephone Network (PSTN)) through enodeb, EPC, and IP Multimedia Subsystem (IMS) as shown in Figure 1. The EPC, along with E-UTRAN is classified as an Evolved Packet System (EPS). EPC comprises of multiple components such as Serving Gateway (S-GW), Mobile Management Entity (MME), Policy Control and Rules Function (PCRF) and PDN Gateway (P-GW). 3GPP AAA Server HSS Internet MME S-GW UE P-GW IMS Core Data Traffic Control/Management Traffic E-UTRAN PCRF EPC IP Services Figure 1: 4G LTE Architecture with IMS The UE connects to enodeb over Uu interface, which then directs the traffic to S-GW in EPC over S1 interface (S1-U for user plane traffic and S1-MME for management plane traffic) using a GPRS Tunneling Protocol (GTP) tunnel. Besides data plane functionality, the enodeb, S-GW, and P-GW contribute to several control plane protocols. The S-GW serves as a local mobility anchor that enables seamless communication when the user moves from one base station to another while retaining its IP address. The S-GW channels traffic to the P-GW over another GTP tunnel. In coordination with the MME, P-GW performs hop-by-hop signaling to handle session setup, teardown, and re-configuration, as well as location update, paging, and handover. Moreover, the P-GW enforces quality-ofservice (QoS) policies and monitors traffic to enable billing. The P-GW also connects to the IMS and other cellular data networks, and acts as a firewall to block unwanted traffic. The PCRF manages flow-based charging in the P-GW. The PCRF also provides the QoS authorization (QoS class identifier and bit rates) that decides how to treat the traffic flow, based on the user s subscription profile. The Home Subscriber Server (HSS) contains subscription information for each user, such as the QoS profile, any access restrictions for roaming, and the associated MME. The policies at the P-GW can be very fine-grained, based on whether the user is on roaming services, properties of the user equipment, usage caps in the service contract, parental controls, and so on. 8
IMS hosts a set of SIP servers called Call Session Control Function (CSCF) servers, of which Serving CSCF (SCSCF) is the hub of all signaling functions of IMS. The UE first registers with the S-CSCF through Proxy CSCF (P-CSCF) and then sends the S-CSCF a Session Initiation Protocol (SIP) invite message, which contains a Session Description Protocol (SDP) with the required QoS values. The S-CSCF examines the SIP message and informs the PCRF in EPC, which in turn sets the appropriate policies in the P-GW for the given QoS values. The Challenges with Cellular Networks While it is clear that cellular networks today have evolved in terms of functionality and speed, there are still some residual challenges such as latency, spectrum availability, and rising costs. These challenges can be partially ascribed to the current mobile network architecture and the way it operates. The major constraints in the mobile network architecture include: Centralization of Data Plane Functions Today s cellular networks are beset by complex and inflexible architecture wherein most of the data plane related functionality is centralized (traffic monitoring for billing, policy enforcement and so on) and the control plane is highly distributed (radio resource allocation). Centralizing data-plane functions such as monitoring, access control, and QoS functionality at P-GW creates scalability challenges. This forces all traffic to travel all the way to the P-GW, including traffic between users on the same cellular network, making it difficult to host popular content inside the cellular network. Integration between Data and Control Planes Typically, network equipment comes with proprietary configuration interfaces that communicate through complex control-plane protocols, as well as a large and growing number of proprietary configuration variables. Today, service providers can at best configure the network, with little opportunity to create new services such as Application Aware Routing or Bandwidth on Demand. Moreover, there is no clear separation between control and data planes, with the control plane embedded in the network element. However, there is a clear separation between the data plane and management plane. Inter-cell Interference The industry has been deploying more and more base stations to handle the ever increasing bandwidth needs. By bringing the infrastructure closer to the client, networks can theoretically improve the link quality to each user and reduce the number of users that each base station needs to support. However, this cell splitting technique has created new issues in terms of inter-cell interference due to the scarcity of radio spectrum. Due to the lack of adequate spectrum, neighboring base stations in a dense deployment have to operate on the same channel, with a frequency reuse factor of one. Managing such networks is very complex because the radio resource management (RRM) decisions taken at one base station (power and spectral values) significantly impact the performance of devices connected to neighboring base stations. These dense base station deployments are detrimental to network performance in two ways - inter-cell interference at the UE and load fluctuation due to user mobility in relatively smaller cell ranges. 9
Consequently, handovers, cell association, and resource (spectrum) allocation have to be managed in a coordinated manner across UEs and base stations with minimal impact on the overall network performance. Traditionally, radio access network (RAN) is a collection of independent base stations each with its own control plane and therefore not suitable for working in a coordinated manner. Moreover, distributed coordination algorithms do not scale well. Nor do they meet latency requirements as they work with a larger number of base stations. Offloading technologies The scarcity of access radio spectrum has prompted the networking industry to adopt other access technologies such as small cells and unlicensed Wi-Fi networks that are connected to the internet through fixed broadband technologies. This heterogeneous network (HetNet) deployment has achieved higher data rates by bringing networks closer to end users through increased spatial use. 3GPP has laid out the specifications for the HetNet architecture with different offloading models having different Wi-Fi and cellular network integration levels. The introduction of alternative technologies to address the spectrum issue has given rise to a set of new challenges. Interference management in a HetNet deployment is a major challenge due to unplanned deployment of user deployed cells, leading to power and coverage differences between cells that share the same spectrum. Interference scenarios include inter-smallcell interference and macrocell smallcell interference. Another challenge is the way a UE selects an access technology, when multiple technologies such as Wi-Fi and WiMAX are available; this is usually done based on the signal strength. However, the access mechanism with the highest signal strength may not provide the best QoS and this issue could be fixed by creating appropriate policies. An SDN Architecture for Cellular Networks SDN can effectively mitigate the challenges of present day cellular networks. It offers a logically centralized control plane for the networks wherein the network equipment has only data plane or data plane with basic control plane functionality. The centralized nature of SDN enables better coordination among network elements. Similarly, SDN can enable common control protocol across diverse wireless technologies for seamless mobility support within and across technologies (WiMAX, 4G LTE, 3G UMTS, and Wi-Fi). In addition, due to its centralized control plane, SDN architecture can give cellular operators greater control over their equipment, simplify network management, and introduce value-added services. Since SDN centralizes the existing distributed control plane and management plane functionality, it enables a significant reduction in control traffic that increases rapidly with the introduction of new services. With SDN, the centralized nature of special data plane features such as rate limiting, traffic monitoring or firewalling can be decentralized into multiple NEs. This creates hotspots in the network and reduces NE cost, as expensive and dedicated hardware is no longer required for each function. 10
SDN concepts can be applied to several network domains involved in today s cellular networks. The SDN framework can be brought into the mobile network as a Federated SDN Framework (FSF) as shown in Figure 2. Another approach that can be adopted is to merge the capabilities of the individual SDN controllers into a Unified SDN Framework (USF) that manages the entire gamut of mobile network technologies. However, USF may present several challenges associated with scalability and latency. Since SDN controllers host dedicated functionalities on traditional NE in the form of SDN applications, Diameter protocol can be used for communication between these applications residing in various SDN controllers. Moreover, it is the de facto protocol used in traditional EPC and IMS for communication between various elements such as P-GW, S-GW, and MME. Diameter protocol messages involve policy information exchange and authentication messages. Each SDN controller can host a Diameter protocol stack which can be extended to cater to the specific needs of inter-sdn communication. The advantage offered by the Diameter protocol is its ability to extend itself through the creation of new Attribute Value Pairs (AVPs) for new and future requirements. In contrast to OpenFlow that is used as a southbound protocol to control NE, Diameter caters to east-west inter SDN communication. Figure 2: Federated SDN Framework In the FSF, a dedicated SDN controller is used for each type of network such as SDN Wi-Fi Controller, SDN Small Cell Controller, SDN E-UTRAN Controller, SDN EPC Controller, and SDN IP Multimedia Subsystem (IMS) Controller. Although all these SDN controllers use OpenFlow or its extensions such as Control Plane protocol, the logic and applications running on these controllers will vary significantly. The functionality of each of the network components in the Federated SDN framework is discussed below. SDN EPC Controller SDN enables an easy transformation from separated functionalities into a centralized entity. Basically, the SDN EPC controller replaces the existing control plane modules in today's EPC and hosts them as applications (depicted in Figure 4). These applications include Resource Manager, Mobility Manager, Subscriber Information Base (SIB), PCRF, and Infrastructure Routing that decide how user data packets are handled in the network. 11
The Mobility Manager has the location information of the user and creates the tunnel for the packets with the help of Infrastructure Routing that calculates the best route. The PCRF monitors the traffic and packet scheduling. The SIB stores and maintains subscriber information that includes static subscriber attributes as well as dynamic data. In addition, service providers can distribute data-plane rules (firewalling, rate limiting, etc.) through the SDN EPC Controller over multiple, cheaper network switches, reducing the scalability pressure on the P-GW and enabling flexible handling of traffic. Similarly, the SDN EPC Controller can provide distributed traffic monitoring at switches deep inside the core network to ease the burden on the packet gateway. Mobility Manager Subscriber Information Base Policy and Charging Rules Function Infrastructure Routing SDN Controller Core South Bound Plugin Extended OpenFlow East West Plugin Diameter Figure 3: SDN EPC Controller Architecture SDN E-UTRAN Controller Challenges with traditional radio access networks such as distributed radio resource management (RRM) can be mitigated by introducing the SDN E-UTRAN Controller. Distributed base stations can be centralized into a big virtual base station by decoupling the individual control plane of each base station and centralizing it. The major challenge with SDN E-UTRAN Controller may be the inherent delay (5-10 minutes) between the base stations and the controller. These issues can be overcome by splitting the distributed control plane into two: a base station handling local control decisions that do not affect the neighboring radio elements (for example, resource block allocation), and a centralized controller making decisions that affect the global network state (for example, handovers and power levels). An X2 interface can be used between the split control planes in E-UTRAN with some extensions. Applications that run on the SDN controller include Radio Resource Manager, Mobility Manager, RAN Information Base and Interference Map. Interference Map can take the form of a weighted graph where each node represents a base station with edge weight representing the signal strength between the nodes. RAN Information Base holds the subscriber information and operator preferences. Radio Resource Manager with the help of 12
Interference Map and RAN Information Base can decide on the power levels of base stations, while the Mobility Manager handles UE handovers. Overall, it functions as a single SDN controller with global control plane functionalities to manage a set of enodeb, each with limited control plane functionalities, local scope and impact. Radio Resource Manager Mobility Manager RAN Information Base Interference Map SDN Controller Core South Bound Plugin Extended OpenFlow East West Plugin Diameter Figure 4: SDN E-UTRAN Controller Architecture SDN Wi-Fi Controller The SDN Wi-Fi Controller has to host a set of applications that may vary depending on the type of underlying Wi-Fi access points (Aps). For instance, this could involve managing a set of public hotspot APs where most of the applications are used for cellular data offloading. Similarly, the federation of SDN controllers in this case depends on the way the hotspot network is connected to the internet. The hotspot network may or may not be connected to EPC, and may be a trusted or untrusted one. Accordingly, the SDN Wi-Fi controller has to interact with SDN EPC Controller or the SDN IMS Controller along with SDN E-UTRAN controller. Thus, to provide users with the best QoS or meet the SLAs, the SDN Wi-Fi controller and SDN E-UTRAN need to federate with SDN EPC Controller or SDN IMS Controller. Another interesting use case for the SDN Wi-Fi controller is where some of the applications on UE use Wi-Fi access (high bandwidth applications, videos), whereas other applications such as voice (low latency) can use E-UTRAN. In an office environment, the SDN Wi-Fi Controller will need to host other applications such as IP Mobility, Bring Your Own Device, Unified Wired-Wireless network management, and so on, along with Wi-Fioffloading functionality. SDN IMS Controller The SDN architecture can also be introduced into the core network to make it smarter. A centralized SDN controller at the core can provide the services envisioned by IMS in a simpler and more efficient manner. Some of the IMS components such as CSCF can be retained as applications on the SDN controller that controls the OpenFlow switch-based core network. 13
Other components of IMS such as Home Subscriber Server (HSS) that contain user data related to subscription of services can also be retained as applications in the centralized controller. The CSCF application handles the SIP data that comes to the IMS SDN Controller, containing the Session Description Protocol (SDP). This SDP information is sent to PCRF, which implements the policy control and flow based charging. The PCRF authorizes the Policy and Enforcement Rules Function to setup an end-to-end path between the source and destination with the required QoS values at the P-GW. For IMS and EPS implementation, the SDN controller may have to be enhanced by means of southbound plugins to support traditional signaling protocols such as SIP and Diameter. Call/Session Control Function Policy and Charging Rules Function Home Subscriber Server Policy and Charging Enforcement Function SDN Controller Core Extended OpenFlow SIP Diameter Figure 5: SDNIMS Controller Architecture Improving Wireless Cellular Networks with SDN Today s cellular wireless networks need to handle the rapidly escalating customer demand for higher bandwidth. As a result, service providers are scrambling to meet customer expectations, without impacting their profit margins. SDN offers tremendous potential to solve the current network issues because of its architectural and cost benefits, enabling service providers to scale operations efficiently while mitigating costs. Although SDN adoption benefits are perceptible, the return on investment of SDN networks and the capex made on existing networks may be the deterrents for its deployment. However, new technological advancements such as 5G, with very low latency requirements, and programmable cities (e.g., Bristol Is Open) will act like a tipping point at which the benefits of SDN networks may be too overwhelming to ignore. 14
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