Interoperability procedures between access technologies using IEEE 802.21

Alexandru Vulpe, Şerban-Georgică Obreja, Octavian Fratu Facultatea de Electronică, Telecomunicaţii şi Tehnologia Informaţiei

Universitatea Politehnica din Bucureşti Bucureşti, România

alex.vulpe@radio.pub.ro, serban@radio.pub.ro, ofratu@elcom.pub.ro

Abstract— Next generation wireless communication will integrate

the heterogeneous wireless technologies in order to provide better

services, with improved quality, higher coverage, and seamless

access to the system resources. Providing support for the

connection transfer between different access technologies –

vertical handover – together with radio resources management

for the integrated networks are the major integration challenges.

IEEE has introduced the 802.21 standard which provides a

technology-independent abstraction layer. It provides a

framework to be used as a starting point for the integrated

wireless networks development. In this paper, a solution, built on

the IEEE 802.21 - Media Independent Handover - framework is

presented, along with some aspects related to the implementation

of the system modules in QualNet simulator. The proposed

system tries to offer a solution for both the vertical handover

process and radio resource managemnet. The paper focuses on

system functionalities evaluation for the vertical handover

process implemented on a heterogeneous wireless network,

implemented with WiMAX and WiFi technologies. The

experiments were performed on a simulated platform

implemented in QualNet.

Keywords: IEEE 802.21, MIH, vertical handover, simulation;

I. INTRODUCTION

Wireless 4G communication systems, although not deployed yet, are characterized by larger bandwidths, higher data rates, and fast and seamless handovers. In order to achieve this, it is essential that 4G capabilities be integrated into existing mobile technologies. Operators must manage rationally their networks in terms of both intra-system (horizontal) handovers and inter-system (vertical) handovers and mobility, quality of service and security.

In all these aspects, the architecture proposed by the IEEE 802.21-2008 standard is a key element in easing the implementation of these 4G systems. IEEE 802.21 provides a framework that allows higher levels to interact with lower layers to provide session continuity without dealing with the specifics of each technology. It provides the missing, technology-independent abstraction layer, thus hiding technology-specific primitives. This abstraction can be exploited by the IP stack (or any other upper layer) to better interact with the underlying technologies, ultimately leading to improved handover performance [1][2][3].

This paper presents a QualNet implementation of a mobility management system for heterogeneous wireless networks, which is based on the Media Independent Handover (MIH) framework. This system was proposed by the RIWCoS (Reconfigurable Interoperability of Wireless Communications Systems) NATO Project, and aims to develop a mobility management system which integrates different wireless communications technologies into a common hybrid communication infrastructure. Besides the system architecture proposed by the project, the MIH implementation represents a novelty of our work since the QualNet does not have support for MIH framework.

The paper is structured as follows. Section II summarizes the main features and functionalities described by the IEEE 802.21 standard, and the next one is a short overview on the RIWCoS system architecture. In sections IV and V a sample simulation scenario and its results are presented. Section VI contains some conclusions and future work.

II. OVERVIEW OF IEEE 802.21

A. Key features of the standard

The standard provides MIH functionality that facilitates both mobile-initiated and network-initiated handovers. It consists of the following three main elements:

• A framework that enables service continuity while a mobile node (MN) transitions between heterogeneous networks. It relies on the presence of a mobility management protocol stack within the network elements that support the handover.

• The MIH function (MIHF), which is an entity consisting of a set of handover-enabling functions within the protocol stacks of the network elements.

• Service access points (SAPs), which define both media-independent and media-specific interfaces. They offer both access for MIH Users to the MIH function and help for the MIHF to collect link information and control link behavior during handovers

B. MIH function services

IEEE 802.21 defines three services that facilitate handovers across heterogeneous networks. These are:

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• Media Independent Event Service (MIES). Events relevant to handover originate from MAC, PHY, or MIHF at the MN or at the network PoA.

• Media Independent Command Service

(MICS). A number of commands are defined in this standard to allow the MIH users to configure, control, and retrieve information from the lower layers including MAC, Radio Resource Management, and PHY.

• Media Independent Information Service

(MIIS). Provides a framework by which an MIHF, residing in the MN or in the network, discovers and obtain network information within a geographical area to facilitate network selection and handovers.

These services are managed and configured by a fourth service which is called the management service. Through the service management primitives, the MIHF is capable of discovering other MIHF entities. Fig. 1 summarizes the relationships between the MIHF and the OSI layer stack, using these services.

C. The MIH Protocol

The MIH Protocol is used for remote communication between peer MIHFs. It mainly defines the format of the messages exchanged between them that are based on the primitives defined by the MIH Services. In addition, the following are defined:

• MIH Transaction defined by a sequence of messages with the same ID between two peer MIHFs

• MIH protocol acknowledgement service used when the MIH transport used for remote communication does not provide reliable services

• General frame format, fragmentation and reassembly mechanisms

III. RIWCOS ARCHITECTURE

RIWCoS is a project that aims to integrate different wireless communications technologies into a common hybrid communication infrastructure.

The general scientific objective of the project is to study, define and evaluate a flexible, integrated architecture, considering heterogeneous access supporting technologies, able to offer secure content transport and distribution (unicast, multicast, broadcast) of multimedia-oriented services to mobile and residential users with given Quality of Service (QoS) restrictions. It is based on MIH framework to offer service continuity to mobile users while they are crossing access networks based on different mobile technologies, like Cellular Mobile Telecommunication Networks (HSDPA, UMTS), and IP-based Wireless Networks (WLAN, WMAN) [4].

The overall RIWCoS architecture consists of an Interoperability Manager module for each wireless communication system with the main function of mobility

management. In a MIH capable network, this module should interoperate with the MIH function (Fig. 1) to ensure the cooperation between the different component technologies. It has the following functionalities: provision of information to mobile terminal about the neighbor networks, provision of link state information, MIH messages handler.

Link Interoperability Modules (one for each of the link-layer technologies) ensure cooperation between technology-specific link layers and the Interoperability Manager They are used to add MIH functionalities to each type of MAC layer.

There is also a Resource Manager (RM) entity whose main function is to collect information and perform handover decisions based on a certain algorithm. The information is based on parameter monitoring performed by the Link Interoperability Modules, and the selection process takes into account, the user profile, link quality parameters, network load, and quality of service.

More details about implementing the RIWCoS architecture can be found in [5][6]. Details about an implementation of an IEEE802.21 Media Independent Information Server developed within the framework of the same project can be found here[7].

IV. DEPLOYMENT SCENARIOS AND IMPLEMENTATION

In order to prove the functionality of the proposed system a handover scenario between WiMAX and Wi-Fi access networks managed by the same operator was considered.

In order to implement key features of the IEEE 802.21 standard we have chosen the QualNet simulator which uses a layered architecture similar to that of the TCP/IP network protocol stack, within which data moves between adjacent layers. The QualNet protocol stack consists of the following layers (from top to bottom): Application, Transport, Network, Data Link (MAC) and Physical Layers [8].

The RIWCoS modules are integrated in QualNet like this: the Interoperability Manager (IM) at the Network Layer and the Link Interoperability Modules at the MAC Layer. This

Figure 1 RIWCoS system architecture

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choice ensured an easier integration with the QualNet stack, because the Network and MAC Layer framework could be used instead of developing a new layer. The Resource

Manager was implemented as an external module and will be connected with QualNet via socket interface. In order to debug the simulator implementation a minimal RM test module was also developed in QualNet. The results presented in this paper use this RM version.

The following logical components were implemented in QualNet:

• The Event Dispatcher of the IM. It will process received MIH messages from its peer and forward them, if necessary to the lower or higher levels either to process a request or to receive a response message or an indication.

• The MIH Protocol including its ACK service. The ACK service is fully based on the implementation of the finite state machine and procedures described by the standard.

• The Link Interoperability Modules. Each module controls the MAC state by answering received Link Commands and sending Link Events to the MIH function.

The MIH signaling involved in triggering the handover between two heterogeneous networks is described in more detail in [6]. A mobile node attached to an 802.16 base station detects that the link quality is degrading and sends a Link_Going_Down (LGD) indication primitive to its Interoperability Manager (IM). The IM forwards this indication with the help of the MIH Protocol via MIH_Link_Going_Down (MIH_LGD) indication to the

Resource Manager (RM) located within a node inside the network. In order to find out the link parameters of the potential candidate points of attachment, the RM sends a MIH_Link_Get_Parameters (MIH_LGP) request primitive to all the available points of attachment located in the vicinity of the mobile node. The IM located within each of the nodes receives it and queries its MAC layer through the Link_Get_Parameters (LGP) request primitive. The MAC acquires the necessary information and returns it to the IM. Now, the IM can send the response to the RM and after it receives all the information from all the networks it can make a decision as to which point the mobile node will be attached to.

The scenario simulated in QualNet (Fig. 2) consists of 7 nodes, form 1 to 7, of which node 4 is the Mobile Node with dual WiFi-WiMAX radio interfaces, nodes 2, 3, 5 and 6 represent Points of Attachment (nodes 2 and 3 are 802.16 base stations and nodes 5 and 6 are of 802.11 technology) and nodes 1 and 7 are switching centers that connect all the points of attachment through a wired backbone. Node 4 maintains a Constant Bit Rate (CBR) application with node 7, while the Resource Manager is designated to be node 1. The routing protocol used is Ad-hoc On-demand Distance Vector (AODV) which is a reactive routing protocol, meaning that it establishes a route to a destination only on demand. Also, the connection setup delay is lower. This makes it one of the commonly used routing protocols in Mobile Ad-Hoc Networks [9].

Node 4 roams from the area covered by node 2 to the one covered by node 6. During this time it handovers between the 4 PoAs so each time the received signal strength (RSS) from each of them drops below a threshold of -78 dBm with a margin of 1 dBm it sends a MIH_LGD indication to the RM (node 1) which in turn sends MIH_LGP request to all the candidate PoAs minus the current PoA (which means 3 primitives).

So, 3 handover situations occurs, and in every such situation, the RM sends 3 MIH_LGP request primitives to the IMs of the candidate PoAs, which means a total of 12 primitives. Each IM receives these requests 3 times and triggers a local link-layer command to find out the link parameters, and when it receives the confirm primitive, it forwards the conveyed information to the RM.

Each IM of a candidate PoA queries its MAC layer by means of the LGP request primitive, and the confirm primitive will contain the necessary information for the RM. For now, the handover information is based solely on CINR and RSS metrics, as QualNet APIs have convenient ways of extracting them at the mobile node as well as at PoA side. The scenario was simulated in three situations, in order to demonstrate the effectiveness of the implementation:

• The first scenario was run without any QualNet custom code modifications. Handover decisions thereof lack, and the handover is being done by means of the mobile node exiting the coverage area of the second Base Station (node 3) and then entering the

Figure 2 The scenario simulated in QualNet

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a) b) c)

Figure 3. Simulation graphs

coverage area of the first access point. At that moment, the mobile node starts associating itself with the AP, and the handover takes place.

• In the second stage, MIH support was introduced at layer 3. The MIH signaling was used by RM to gather information, and trigger the handover based on that information. This was done by sending commands to the routing protocol, to update the route at that moment, instead of at the time a route expired.

• In the third stage, layer 2 support was introduced. Problems occured because QualNet 5.0.2 WiFi source code lacks an implementation of the disassociation and reassociation procedures for the STA. Therefore, the STA remained associated with an AP until in exited its coverage area, and disassociation could not be commanded by the RM. After implementing disassociation and reassociation functionalities for 802.11, the RM souce code was modified so that it could command association and disassociation with and from a certain access point and an improvement in the results was noticed.

In each stage, 60 simulations were run, and the results were averaged. Table 1 shows the results obtained from the Statistics Analyzer of QualNet 5.0.2. We notice that the three stages described earlier show progressive improvements in the simulation results. As the RM can command disassociation from an access point and association with the AP of choice, it can be seen that the number of packets received jumps from 5823 to 6761 out of 6940 sent. This means an improvement from an initial packet loss of 16% to 2.5%. Also, average end-to-end delay becomes significantly lower (from 36.07 to 30.7 s). An increase in throughput is achieved too.

Fig. 3a) shows the RM successfully receiving the MIH_LGD indications sent by the MN. There are 4 messages received and not 3 because of the ACK service which retransmits a MIH message if the ACK response is not received. However, the duplicate message is not processed. Fig. 3b) indicates this and the fact that each of the candidate PoAs receives a MIH_LGP request sent by the RM each time it receives the MIH_LGD indication. Fig. 3c) shows the ACK messages received by the MIHF entities involved in remote communication, which shows the successful transport of the

primitives via the MIH Protocol. The graphs shown in Fig. 3 are selections from a multitude of graphical statistics and are shown here for the purpose of validating the created MIH module.

Fig. 4b) shows the server side packet delay using MIH signaling and control. The 3 spikes represent an increase in delay due to the 3 handovers being executed. The central one corresponds to the inter-technology handover. We see that the handover delays have values well above 200 ms, compared to the average delay which is of around 75 ms. For comparison Fig. 4a) shows the packet delay without this signaling. We notice that the delay for the vertical handover is higher then that obtained when using MIH signaling.

V. CONCLUSION AND FUTURE WORK

Implementing a mobility management system for heterogeneous wireless access networks must take numerous aspects into consideration. The present paper presents a mobility management framework, which includes an Interoperability Manager module, the MIH Protocol, and a Resource Manager. Handover control at the MAC layer is open to improvements, but the tests made with the present model have confirmed the functionality of this framework and the reliability of the MIH Protocol.

Table 1 Simulation results

CBR Client CBR Server

Total Packets Sent

Through-put [kbps]

Total Packets Received

Average End-to-End Delay [ms]

Through-put [kbps]

Stage 1

6940 409,659

5496 38.5 304.235

Stage 2 5823 36.07 345.72

Stage 3 6761 30.7 401.413

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a) without MIH control b) with MIH control

Figure 4. Delay at the server side

Our intention is to further develop the Link Interoperability Modules and to provide support for other technologies like UMTS and the emerging LTE.

VI. ACKNOWLEDGMENTS

This work is sponsored by NATO's Public Diplomacy Division in the framework of “Science for Peace” through the SfP-982469 "Reconfigurable Interoperability of Wireless Communications Systems (RIWCoS)" project and by Romanian Authority of Scientific Research in the framework of PNCDI 2 “Partnership” through the 12-126/2008 “Hybrid wireless access system with unique addressing (SAWHAU)” project and Romanian UEFISCSU PN-2 RU-TE Project no. 18/12.08.2010

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