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Architecture on Mobility Management in OpenFlow-

based Radio Access Networks

Guolin Sun, Guisong Liu, Hangming Zhang

School of Computer Science and Engineering

University of Electronic Science and Technology of China

Chengdu, China

[email protected], [email protected]

Wei Tan

Communication Technology Lab.

Huawei Technologies LTD Co.

Shenzhen, China

[email protected]

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 radioaccess 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; Heterogeneousradio access network; Mobility management

I.

I NTRODUCTION

With paradigm changes from operator-oriented to service-oriented in networking, the current architecture of radio accessnetwork has major limitations in the future. First, a complexheterogeneous radio access network environment leads us toaccess information on some isolated islands. Service-orientednetwork should provide us a way to use information just likethe water, electricity and gas. It will change our means oninformation transmission, data storage and resource sharing

greatly. With various types of network interfaces available onhardware terminals, such as LTE, Wi-Fi and UMTS, seamlessmobile services and Quality of user Experience (QoE) can beimproved in heterogeneous radio access network environment.Therefore, service-oriented SDN architecture in heterogeneousradio access networks is required by heterogeneous networkfusion. The concept of SDN targets to merge these networkswith a Cloud network of controllers.

Second, Big Data brought a lot of new challenges for ourcurrent wireless access networks. This leads to an increase inthe amount of traffic and network load will increase in form oforders of magnitude in the coming years. While the availableradio spectrum and spectral efficiency are both difficult to be

promoted further. In fact, spectrum efficiency of 4G reacheswithin 20%, which is quite close to Shannon capacity limits [1].To solve challenges brought by Big Data, LTE femtocell andWi-Fi network are taken as an important way to offload trafficon 4G macrocell networks in the next generation network. Theconcept of small cells in femtocell and Wi-Fi network withoutcell 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 withcurrent network architecture. The concept of SDN can providea 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 tosimplify design and management of heterogeneous wirelessaccess network and create a variety of new services. As far aswe know, this is the first one to discuss mobility management problem of heterogeneous wireless access networks with SDNarchitecture. This architecture enables seamless services andimprove QoE through monitoring rich network state statisticsto make context-aware decisions for network control. Actually,the proposed cognitive information processing is also anothernew feature in the next generation of wireless access networks.

We organize this paper as following: Section II providesthe concept of SDN and mobility management, combined withLTE femtocell and Wi-Fi networks. The literature of SDNapplication in wireless network is reviewed and summarized.We propose a SDN architecture in heterogeneous radio access

networks to enable heterogeneous network handover in sectionIII. We analyze the required changes to current architecturedefined in the 3GPP LTE standards. An universal architecturefor wireless Access Point(AP) is provided to support SDNarchitecture. Section IV provides technical challenges in thedefined SDN enabled network handover environment. Wemake conclusion for this paper in Section V.

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II.

BACKGROUND OF MOBILITY MANAGEMENT AND SDN

OpenFlow is one kind protocol, initiated at Stanford, toenable all switches on the wired network programmable andintelligent via a standard interface. The Open-Flow protocol isstandardized by ONF to lower operation cost while enhancenetwork functionality through simplified hardware, softwareand management[2]. Open-Flow moves forwarding intelligenceinto a controller, while keeps the switches simple. With the

method of SDN, we could customize networks according tolocal needs, eliminate the unuseful features and create ourvirtual network. The thought of OpenFlow extends from wireswitches to wireless infrastructures now. The OpenRoad isdedicated to explore and test new solutions for mobility withnew routing protocols and controllers based on OpenFlow[3][4].OpenRoads was tested on a topology with five switches, thirtyWi-Fi APs and a WiMax AP. The seamless handover betweenWi-Fi and WiMax systems is successfully managed with themobility management controllers. The OpenFlow for wirelessmesh networks are also investigated[5]. The CellSDN is thefirst one architecture that brought SDN concept into cellularnetworks, 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 migrationof services in TCP/IP networks with dynamic configuration ofa set of coordinated OpenFlow switches located at the edge ofthe network [7]. However, in this paper, we are working towarda method based on Cloud MAC instead of IP layer, which canimprove handover performance on real-time demand-response.With introduction of SDN and OpenFlow, OpenFlow basedmobility management can enable heterogeneous radio accessnetwork fusion, because SDN makes network service-orientedwith a centralized network control. Mobility management can be defined as a new service on the network controller andimplemented as a component of network OS, e.g. NOX.

In this paper, we design an Open-Flow based architecture

for the coexistence scenario in a heterogeneous network of theLTE femtocell and Wi-Fi networks. Based on my knowledge,this paper is the first one to consider Cloud-MAC basedmobility management in a SDN based heterogeneous wirelessaccess network till now. So, what will happen in the futurewith the introduction of SDN and OpenFlow?

III. THE ARCHITECTURE ISSUES IN SDN BASED

HETEROGENEOUS R ADIO ACCESS NETWORKS

A. SDN Architecture for Heterogeneous WAN

In 3GPP architecture, LTE Femtocell network connects HeNBto Internet using IP networking equipment. The UEs connectto HeNB, who directs traffic via Serving Gate Way (SGW)over a GPRS Tunneling Protocol[8]. The SGW serves as alocal mobility coordination entity to guarantee the seamlesscommunication when UEs move from one AP to another. TheSGW must handle frequent changes of a UE’s location andstore a large amount of state information since UEs retain theirIP addresses when they move. The HeNB, HeNB-GW andSGW, as shown in Fig. 1, are all involved in data-plane and

control-plane protocols. They perform hop-by-hop signaling tohandle session setup, tear-down and reconfiguration, as wellas 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 planeand control plane in the 3GPP architecture, shown in Fig.1.The network control applications are all centralized programson Controllers, as opposed to the distributed algorithms overlow-level address we are forced to work today. A SDNcontroller is configured with a network operating system, e.g. NOX, to manage applications in this local wireless accessnetwork [9]. The location of controller just like replaces MME,as show in Fig.2.

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

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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

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 ofLTE 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

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 forLTE and Wi-Fi to be reconfigurable.

C. Cognitive Information Processing Architecture

SDN controller cooperate with a monitoring server to collectnetwork statistics, which is taken as information support tomake decision on network control. The SDN architecture canextend handover function in mobility management to enhanceits intelligence. To coordinate heterogeneous resources in anefficient way, channel utilization, the number of associatedclients, traffic load of each AP, SINR and RSSI of each clientare all useful statistics in monitoring server [10]. How to definestatistics 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 andGC? We will consider the questions above from information processing aspects in this section.

In this SDN architecture, REM is one kind of databases on themonitoring server [11]. Local SNMP manager at local controllerneeds such information to decide single network control onmobility management. Global SNMP manager at GC collectsinformation that LC can’t deal with it to control heterogeneousnetwork on mobility management. The SNMP agent at eachAP collects the statistics from the measured radio environmentand radio signal characteristics from client stations in eachnetwork based on air interface specification, 3GPP LTE orIEEE 802.11. LC gathers the measured statistics from all ofthe APs via SNMP. SDN controllers can query information in

database via Jason, XML etc.

Local

SNMP

Manager

SNMP

Agent

OF_AP_Get_state

OF_AP_Response_stateOF_InformRequest_State

Global

SNMP

Manager

OF_LTE_Get_state

OF_LTE_Response_stateOF_InformRequest_State

OF_AP_Get_state

OF_AP_Response_stateOF_InformRequest_State

OF_LTE_Get_state

OF_LTE_Response_stateOF_InformRequest_State

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

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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 currentmeasurements, 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 QoS-

based 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 networksmust support mobile handoff due to the mobility of clientstations. Heterogeneous network resources available vary overtime and space, as make it difficult to provide seamless andreliable connection to mobile clients going across multipledomains[13,14]. Mobile client handoff is an inherent operation inheterogeneous radio access networks to keep resilient andcontinuous communications. SDN based heterogeneous radioaccess networks can mitigate resource burden by controllerthrough heterogeneous network handoff, which makes clientstations access information blind to air interface types. Thefour 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 radioaccess networks, all the infrastructures of wireless are sharedas a transmission tunnel of data. The hand-off and location

update will be handled in the Cloud of controllers. They don’tneed care about network type, but choose the AP around withthe best signal quality to client stations. Selection of the radioaccess network at application launch. This role is ensured bymobility management functions here referred to as service-to-radio mapping control. Triggering of the handover during asession.The mobility management function aims at always providing the best access network to the terminal.

Terminal-centric selection without network assistance isrecommended. Network-controlled handover selection withinnetwork entities is based on both terminal and access networkmeasurements, enforcing decisions on the terminal. Network-assisted selection on the terminal side, the network providingoperator policies and access/core load information (jointterminal/network decisions). When only one access remainsavailable, network-assisted selection is applied; when accessselection is triggered by network load considerations, networkcontrol may be used for load balancing. Finally, for accessnetwork selection, the mobility management function mustretrieve 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 accessdifferent radio access networks around them. To improve theQoE and increase network capacity, the Cloud-MAC schemewith SDN controller could provide a large gain in networkcapacity[15]. The Cloud-MAC algorithms considering multi-user scenario with heterogeneous network resources should bean important technical challenge in SDN based radio accessnetwork. It is difficult to achieve a perfect solution with singleobjective decision theory due to heterogeneity. Each client isrequired to access options of multiple objects at the same timeto achieve the best solution. How to achieve a trade-off under

constraints with limited resources is a technical challenge inthis 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 utilizationresource among infrastructures. The metric of Load definitionis the most important issue for isolated wireless accessnetworks. Therefore, load balance is an important issue withmobile hand-off operation. In this problem, the definition ofload can be varied. How to choose overload APs and the rightclients is the main algorithm challenge.

C. Price-based Inter-network Handoff

Resource slicing allow to isolate and separate traffic ondifferent resource slices with tags defined in semantic space.Flow-visor should provide functions to create and deleteresource slice at least[16]. Therefore, how to configure virtualAPs to create a resource slice will be a technical challenge. Inthis SDN architecture, slice configuration of APs can be asked by controller via Jason file. If resource is not available at the

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SDR based Open-Flow AP, the Jason configuration file will point to controller.

The Flow-visor can also support high-level semantic spacedefinition[16]. A slice of semantic space is the set of packetswhose subscriber attributes satisfy the same predicates. Forexample, client stations in the heterogeneous radio accessnetworks 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-levelsemantic space depends on specific application scenarios. Inhome network, semantic spaces include smart grid, securitymonitoring and smart appliance control[17].

With the fact that load changes spatially and temporally withchanging user demand. In this SDN defined architecture, all ofthe virtual operators provide their services sharing a common physical network. Therefore, there always is an opportunityfor Virtual Operators(VO) to maximize their profits by sellingcurrent unutilized spectrum, if it exists, directly to secondarytemporarily for a fixed price per fixed time window set byVOs[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 itsoperation resource based on dynamic pricing to save money.

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.

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 presentedarchitecture is defined. New components include mobility-

based, 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 .

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.

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