Architecture on Mobility Management in OpenFlowbased Radio Access Networks Wei Tan Guolin Sun, Guisong Liu, Hangming Zhang School of Computer Science and Engineering University of Electronic Science and Technology of China Chengdu, China
[email protected],
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Abstract—With the aim to simplify network management and control, Software defined network is proposed as a new paradigm and architecture in networking areas. The concept of Cloud and cognitive cellular network will be important features in the next generation radio access networks. In this paper, we propose a distributed hierarchical architecture for heterogeneous radio access networks based on OpenFlow. SDN architecture enables resource and infrastructure sharing among heterogeneous radio access networks. Mobility management in this new heterogeneous radio access network architecture and a OpenFlow-enabled node architecture for AP infrastructures are defined. We compared it with the one defined in 3GPP LTE standard to show the required changes. An architecture of cognitive information processing is defined to support new features of mobility management, which is taken as an service of network operation system. In the end, typical network applications of mobility management in this SDN architecture are introduced. New topics with technical challenges are analyzed in this SDN based heterogeneous RANs towards the ongoing research and prototypes. Keywords—OpenFlow;Software defined network; Heterogeneous radio access network; Mobility management
I.
INTRODUCTION
With paradigm changes from operator-oriented to serviceoriented in networking, the current architecture of radio access network has major limitations in the future. First, a complex heterogeneous radio access network environment leads us to access information on some isolated islands. Service-oriented network should provide us a way to use information just like the water, electricity and gas. It will change our means on information transmission, data storage and resource sharing greatly. With various types of network interfaces available on hardware terminals, such as LTE, Wi-Fi and UMTS, seamless mobile services and Quality of user Experience (QoE) can be improved in heterogeneous radio access network environment. Therefore, service-oriented SDN architecture in heterogeneous radio access networks is required by heterogeneous network fusion. The concept of SDN targets to merge these networks with a Cloud network of controllers.
Communication Technology Lab. Huawei Technologies LTD Co. Shenzhen, China
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Second, Big Data brought a lot of new challenges for our current wireless access networks. This leads to an increase in the amount of traffic and network load will increase in form of orders of magnitude in the coming years. While the available radio spectrum and spectral efficiency are both difficult to be promoted further. In fact, spectrum efficiency of 4G reaches within 20%, which is quite close to Shannon capacity limits[1]. To solve challenges brought by Big Data, LTE femtocell and Wi-Fi network are taken as an important way to offload traffic on 4G macrocell networks in the next generation network. The concept of small cells in femtocell and Wi-Fi network without cell planning may appear to provide more spectrum occupancy per user by reducing the number of users per cell. However, this will lead to a more complex network management with current network architecture. The concept of SDN can provide a centralized way to manage it with a network of view. With the motivations above, SDN architecture defined for broadband radio access networks is our interest in this paper to simplify design and management of heterogeneous wireless access network and create a variety of new services. As far as we know, this is the first one to discuss mobility management problem of heterogeneous wireless access networks with SDN architecture. This architecture enables seamless services and improve QoE through monitoring rich network state statistics to make context-aware decisions for network control. Actually, the proposed cognitive information processing is also another new feature in the next generation of wireless access networks. We organize this paper as following: Section II provides the concept of SDN and mobility management, combined with LTE femtocell and Wi-Fi networks. The literature of SDN application in wireless network is reviewed and summarized. We propose a SDN architecture in heterogeneous radio access networks to enable heterogeneous network handover in section III. We analyze the required changes to current architecture defined in the 3GPP LTE standards. An universal architecture for wireless Access Point(AP) is provided to support SDN architecture. Section IV provides technical challenges in the defined SDN enabled network handover environment. We make conclusion for this paper in Section V.
II.
BACKGROUND OF MOBILITY MANAGEMENT AND SDN
OpenFlow is one kind protocol, initiated at Stanford, to enable all switches on the wired network programmable and intelligent via a standard interface. The Open-Flow protocol is standardized by ONF to lower operation cost while enhance network functionality through simplified hardware, software and management[2]. Open-Flow moves forwarding intelligence into a controller, while keeps the switches simple. With the method of SDN, we could customize networks according to local needs, eliminate the unuseful features and create our virtual network. The thought of OpenFlow extends from wire switches to wireless infrastructures now. The OpenRoad is dedicated to explore and test new solutions for mobility with new routing protocols and controllers based on OpenFlow[3][4]. OpenRoads was tested on a topology with five switches, thirty Wi-Fi APs and a WiMax AP. The seamless handover between Wi-Fi and WiMax systems is successfully managed with the mobility management controllers. The OpenFlow for wireless mesh networks are also investigated[5]. The CellSDN is the first one architecture that brought SDN concept into cellular networks, but it is an initial step, not in deep[6]. The Follow-Me Cloud(FMC) is a technology developed at NEC Laboratories Europe, which allows transparent migration of services in TCP/IP networks with dynamic configuration of a set of coordinated OpenFlow switches located at the edge of the network[7]. However, in this paper, we are working toward a method based on Cloud MAC instead of IP layer, which can improve handover performance on real-time demand-response. With introduction of SDN and OpenFlow, OpenFlow based mobility management can enable heterogeneous radio access network fusion, because SDN makes network service-oriented with a centralized network control. Mobility management can be defined as a new service on the network controller and implemented as a component of network OS, e.g. NOX.
control-plane protocols. They perform hop-by-hop signaling to handle session setup, tear-down and reconfiguration, as well as mobility in coordination with Mobility Management Entity (MME), e.g. location update, paging and handoff.
Figure 1 Data/Control Plane Architecture for LTE Femtocells Based on the thought of SDN, we need decouple data plane and control plane in the 3GPP architecture, shown in Fig.1. The network control applications are all centralized programs on Controllers, as opposed to the distributed algorithms over low-level address we are forced to work today. A SDN controller is configured with a network operating system, e.g. NOX, to manage applications in this local wireless access network[9]. The location of controller just like replaces MME, as show in Fig.2.
In this paper, we design an Open-Flow based architecture for the coexistence scenario in a heterogeneous network of the LTE femtocell and Wi-Fi networks. Based on my knowledge, this paper is the first one to consider Cloud-MAC based mobility management in a SDN based heterogeneous wireless access network till now. So, what will happen in the future with the introduction of SDN and OpenFlow? III. THE ARCHITECTURE ISSUES IN SDN BASED HETEROGENEOUS RADIO ACCESS NETWORKS A. SDN Architecture for Heterogeneous WAN In 3GPP architecture, LTE Femtocell network connects HeNB to Internet using IP networking equipment. The UEs connect to HeNB, who directs traffic via Serving Gate Way (SGW) over a GPRS Tunneling Protocol[8]. The SGW serves as a local mobility coordination entity to guarantee the seamless communication when UEs move from one AP to another. The SGW must handle frequent changes of a UE’s location and store a large amount of state information since UEs retain their IP addresses when they move. The HeNB, HeNB-GW and SGW, as shown in Fig. 1, are all involved in data-plane and
Figure 2 SDN-based Mobility Management Architecture Most of the control plane functions in 3GPP standards are all moved to controllers as components. To improve real-time response to events and control the traffic volume to controllers, we propose an architecture with local controller (LC) and global controller(GC) in the Fig.2. The LC process the events inside single a standard network situated on the entry of the wireless local area network access to Internet. The GC will deal with the events among different standard networks as an entrance of access network to the backbone Internet. The LC1
manages the LTE femtocell A. The LC1 controls Wi-Fi access network B. GC will control this heterogeneous network with LTE and Wi-Fi. We take the scenario of LTE Femtocells and Wi-Fi network as an example, a local controller manages the intra-network handover with a local monitoring database. A global view of network state will be stored in a database of a monitoring server, which gets data through query local sensing database, as show in Fig. 2. Mobile terminals can access LTE HeNBs or Wi-Fi APs under decisions of network controller. LC is able to handover wireless devices from one AP to the other inside an access network, while GC can switch mobile terminals from one wireless access network to another with network state statistics. In Section C, we will discuss how to collect network state statistics with SNMP. Therefore, network resource could be utilized in an efficient manner. Data packet forwarding function of network infrastructures is supported with the Open-Flow protocols. OpenFlow enables operators to distribute data-plane rules over cheaper switches and provides a flexible way to manage network . The SGW in the Fig. 2 provide a data tunnel only for LTE femtocell to internet without control plane. The GC and LCs can support mobility management applications with component, shown in Fig. 2. Except the original features embedded in network OS NOX mobility management is a new feature including gather AP statistics and STA statistics. SDN provides operators network view through GUI and reconfigure virtual network on controllers via web access.The virtual network management will left to virtual operators with Web access interface instead of physical network operators. B. Open-Flow Wireless Infrastructure Architecture
based AP provides us measurements from PHY to High-Level. For example, wireless channel utilization rate will be collected from MAC layer as a metric of traffic load on infrastructures. The control agent is used to carry out the decision of controller. Control signaling in PHY and MAC layer will be decoupled from data transmission to enable Cloud MAC protocols via security link(SSL) to a controller[9]. As shown in Fig. 3, data block and path are all drawn in grey, but control plane in white. The software defined radio architecture enable PHY-MAC for LTE and Wi-Fi to be reconfigurable. C. Cognitive Information Processing Architecture SDN controller cooperate with a monitoring server to collect network statistics, which is taken as information support to make decision on network control. The SDN architecture can extend handover function in mobility management to enhance its intelligence. To coordinate heterogeneous resources in an efficient way, channel utilization, the number of associated clients, traffic load of each AP, SINR and RSSI of each client are all useful statistics in monitoring server[10]. How to define statistics for heterogeneous networks mobility management? How to collect statistics for SDN controllers? How to define an software architecture to handle a lot of events in LC and GC? We will consider the questions above from information processing aspects in this section. In this SDN architecture, REM is one kind of databases on the monitoring server[11]. Local SNMP manager at local controller needs such information to decide single network control on mobility management. Global SNMP manager at GC collects information that LC can’t deal with it to control heterogeneous network on mobility management. The SNMP agent at each AP collects the statistics from the measured radio environment and radio signal characteristics from client stations in each network based on air interface specification, 3GPP LTE or IEEE 802.11. LC gathers the measured statistics from all of the APs via SNMP. SDN controllers can query information in database via Jason, XML etc. Global SNMP Manager
Local SNMP Manager
SNMP Agent OF_AP_Get_state
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OF_AP_Response_state
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OF_LTE_Response_state
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Figure 3 OpenFlow based Architecture of SDR AP Infrastructures in heterogeneous wireless access network include OpenFlow switches and wireless APs. Each AP will be configured with physical wireless transmission functions of LTE femtocell or Wi-Fi. Each physical interface will provide two logical protocol interfaces. One is used to transfer control signaling with SNMP for network monitoring and statistics collection. The other is built on SSL to report events to NOX. The sensing agent is used to get statistics from each protocol layer, defined in SNMP. For handover operation, OpenFlow
OF_AP_Get_state
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Figure 4 SNMP based Statistics Collection Information processing in the defined SDN architecture is a cognitive procedure. First, LCs can gather statistics from the SNMP agents at APs in a database. Second, LC will parse and
handle the incoming events from the monitoring server and network. Third, LC decides to control network with the aid of statistics in database. LC will send control signals to AP to instruct it what need to deal with. Based on event type, LC will deliver it to the On-Line Transaction Processing (OLTP), On-Line Analytical Processing (OLAP) block, or redirect it to GC. The components in OLTP as well as OLAP will handle events with the predefined algorithms. On function, OLTP handles the time-constraint, low-level events with current measurements, e.g. mobility based handover. OLAP usually handles high-level events due to historical changes of network state, e.g. Traffic load balance based network handover. In this paper, we figure out four typical handover scenarios of mobility management in this software defined heterogeneous wireless access network architecture, shown in Fig. 5. They are intra-network mobility based handover, load-balance based network handover, price-based network handover and QoSbased network handover. The specific research topics and technique challenges will be explained in the section IV.
Figure 5 Architecture for Cognitive Information Processing IV.
SDN SERVICES OF MOBILITY MANAGEMENT
A. Mobility-based hand-off The SDN based heterogeneous radio access networks must support mobile handoff due to the mobility of client stations. Heterogeneous network resources available vary over time and space, as make it difficult to provide seamless and reliable connection to mobile clients going across multiple domains[13,14]. Mobile client handoff is an inherent operation in heterogeneous radio access networks to keep resilient and continuous communications. SDN based heterogeneous radio access networks can mitigate resource burden by controller through heterogeneous network handoff, which makes client stations access information blind to air interface types. The four types of handoff events and components, shown in Fig. 4, are discussed below. The classical intercell handoff in cellular network due to physical user mobility. In the SDN based heterogeneous radio access networks, all the infrastructures of wireless are shared as a transmission tunnel of data. The hand-off and location
update will be handled in the Cloud of controllers. They don’t need care about network type, but choose the AP around with the best signal quality to client stations. Selection of the radio access network at application launch. This role is ensured by mobility management functions here referred to as service-toradio mapping control. Triggering of the handover during a session.The mobility management function aims at always providing the best access network to the terminal. Terminal-centric selection without network assistance is recommended. Network-controlled handover selection within network entities is based on both terminal and access network measurements, enforcing decisions on the terminal. Networkassisted selection on the terminal side, the network providing operator policies and access/core load information (joint terminal/network decisions). When only one access remains available, network-assisted selection is applied; when access selection is triggered by network load considerations, network control may be used for load balancing. Finally, for access network selection, the mobility management function must retrieve the status of resource usage in each access network. This information is provided by an REM database in controller, which computes a technology-independent abstracted view of access resource availability B. Load Balance-based network hand-off In mobile handover scenario, clients dynamically access different radio access networks around them. To improve the QoE and increase network capacity, the Cloud-MAC scheme with SDN controller could provide a large gain in network capacity[15]. The Cloud-MAC algorithms considering multiuser scenario with heterogeneous network resources should be an important technical challenge in SDN based radio access network. It is difficult to achieve a perfect solution with single objective decision theory due to heterogeneity. Each client is required to access options of multiple objects at the same time to achieve the best solution. How to achieve a trade-off under constraints with limited resources is a technical challenge in this multi-objective optimization problem. With network topology and statistics stored in databases, controller has a view of network state. Load balance is one of basic requirements from network to enable full utilization resource among infrastructures. The metric of Load definition is the most important issue for isolated wireless access networks. Therefore, load balance is an important issue with mobile hand-off operation. In this problem, the definition of load can be varied. How to choose overload APs and the right clients is the main algorithm challenge. C. Price-based Inter-network Handoff Resource slicing allow to isolate and separate traffic on different resource slices with tags defined in semantic space. Flow-visor should provide functions to create and delete resource slice at least[16]. Therefore, how to configure virtual APs to create a resource slice will be a technical challenge. In this SDN architecture, slice configuration of APs can be asked by controller via Jason file. If resource is not available at the
SDR based Open-Flow AP, the Jason configuration file will point to controller. The Flow-visor can also support high-level semantic space definition[16]. A slice of semantic space is the set of packets whose subscriber attributes satisfy the same predicates. For example, client stations in the heterogeneous radio access networks would be configured with different capability, e.g. Mixed transmission rates. This allows network provider to isolate traffic for clients with a certain capability using legacy protocols. How to slice and map radio resources to high-level semantic space depends on specific application scenarios. In home network, semantic spaces include smart grid, security monitoring and smart appliance control[17].
ACKNOWLEDGMENT This study is supported by Grant YB2012120193 from Research Fund for Huawei Technologies Co., Ltd, China and the Fundamental Research Funds for the Central Universities. REFERENCES [1]
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With the fact that load changes spatially and temporally with changing user demand. In this SDN defined architecture, all of the virtual operators provide their services sharing a common physical network. Therefore, there always is an opportunity for Virtual Operators(VO) to maximize their profits by selling current unutilized spectrum, if it exists, directly to secondary temporarily for a fixed price per fixed time window set by VOs[18]. The fixed price within the time window could be the price per minute (price/min) or the price per Megabyte (price/MB) depending on the application class in consideration. As a simple user case, clients can switch and handoff its operation resource based on dynamic pricing to save money.
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D. QoS-based inter-network hand-off Although physical network interface is data transfer tunnels to clients, it still provide different QoS guarantee for specific service. The QoS is a collection of a variety of criteria, such as RSSI, delay, throughput. The CSMA-based network might coexist gracefully in terms of very low packet error rate, but with significantly increased channel access time, whereas the TDMA-based systems depends on both load and scheduling mechanism used. OFDMA based system provide better QoE for the mixed-rate clients than CSMA-based one.
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V.
CONCLUSION
This paper presents a critical study of service-oriented, Open-Flow and SDN enabled architecture for heterogeneous radio access networks on mobility handover scenarios of LTE femtocells and Wi-Fi. The required changes on 3GPP LTE specification are analyzed with the hierarchically distributed SDN architecture. Open-Flow enabled wireless infrastructure architecture is defined with sensing and control agents for network management. With event-component architecture on NOX, cognitive information processing for handover is given with the thoughts of event-classification. A possible stepwise approach to different functional elements of the presented architecture is defined. New components include mobilitybased, price-based, load-balance based, QoS-based handover for intra-network and inter-network operations. This work is an initial step towards SDN and Open-Flow enabled mobility management in heterogeneous radio access networks, which will be further developed on our prototype .
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