PROFITIS: Architecture for Location-based Vertical Handovers Supporting Real-Time Applications

Stavros Tsiakkouris and Ian Wassell

Digital Technology Group Computer Laboratory, University of Cambridge,

Cambridge CB3 0FD, United Kingdom {st283, ijw24}@cam.ac.uk

Abstract - Even though numerous proposals have been made for mobility management schemes in heterogeneous wireless IP networks, no single solution has emerged that is capable of supporting seamless and low latency handovers required to satisfy the stringent delay requirements of real-time multimedia applications. In this paper, we present a location-based mobility management architecture for real-time applications in a heterogeneous network environment. Handover latency is minimised by integrating optimised application layer mobility management signalling using the Session Initiation Protocol (SIP) with location-based information of nearby access points (APs). A newly defined Access Point Location Protocol (APLP) is used to disseminate geospatial information and network parameters associated with various APs to location-aware mobile devices that use this information to construct a virtual map of the APs. An intelligent decision engine (DE) implemented in the mobile node (MN) combines the information provided by APLP with knowledge of the users mobility pattern and policy preferences to anticipate handovers. Keywords mobility management; location-based; SIP

I. INTRODUCTION

Wireless IP networking is becoming an increasingly important and popular way of providing global information access to users on the move. New wireless access technologies are spawning to support this increased demand and powerful mobile devices with multiple wireless network interfaces are becoming commonplace.

Wireless access technologies are developed to satisfy specific needs in terms of coverage, bandwidth, latency, cost, and security. By nature, no single technology is capable of delivering optimal performance that can match every users needs and at the same time provide ubiquitous coverage. To overcome this limitation and enable true mobility support for mobile users, an efficient mobility management architecture is required capable of supporting seamless handovers across heterogeneous networks. Mobility support across various access networks is the underlying requirement for supporting the wireless Internet.

There have been several research efforts in the past proposing new mobility management protocols most of which are based on Mobile IP (MIP) [1], [2], each trying to improve certain performance parameters rather than introduce new design philosophies. The most important

contributions of these past research efforts are protocols for address pre-configuration [3] and hierarchical network registrations [4]. Address pre-configuration enables a mobile node (MN) to configure a new Care-of Address (CoA) before it moves to a new sub-network. Therefore, once the connection to the new access router is completed the MN can use its CoA straight away. Hierarchical mobility management reduces the network registration time by minimising the high signalling overheard incurred when MNs perform frequent handovers. This is achieved by introducing a mobility anchor point (MAP) that separates local from global mobility management. Nevertheless, despite these improvements, handover delays are still not sufficient to meet the stringent delay requirements of real-time applications (

Section III describes some of the location sensing technologies that can be used with our architecture and in Section IV we present a newly defined Access Point Location Protocol (APLP). Then in Section V we propose optimisations for application layer mobility management using SIP. Section VI concludes the paper with a discussion and proposals for future work.

II. OVERVIEW

PROFITIS is a location-based mobility management architecture that integrates optimised application layer mobility management signalling based on SIP with a newly defined Access Point Location Protocol (APLP) [9] that disseminates geospatial information associated with various access points to location-aware mobile devices.

The motivation of this research effort is to develop a novel, scalable, mobility management architecture capable of supporting seamless handovers in real-time multimedia applications requiring minimal modifications to network elements.

A. Design Requirements

Before introducing the network architecture, we first outline the general design requirements that guided the development of this architecture. Support anytime, anywhere connectivity.

Flexibility to roam freely, indoors and outdoors, without having to consider the specifics of the underlying wireless technology used.

Minimal handover latency. To support real-time applications, the handover delay should ensure that inter-packet delay is not noticeable by end users, i.e., typically

Fig. 2 Client architecture. associated with registrations to networks that are never actually visited.

To optimise SIP signalling in intra-domain handovers, we have developed regional registration management techniques that restrict signalling within the administrative boundaries of the regional IP network similar to the approach followed by hierarchical mobility protocols in MIP [4]. This reduces the handover delay by minimising the long round trip time associated with signalling between the MN and its home network. When the MN initiates the handover procedure and registers with the new wireless network, Y, the SIP Register message is intercepted by the SIP Mobility Anchor Point (SMAP) that recognises the identity of the MN from the entries in its binding table and locally updates the visited registrar (VR) without having to update the HR.

When the MN enters the coverage zone of wireless network Y, the multimedia session is redirected to the new AP. Unless the bandwidth or resource allocation parameters in the new wireless network require adaptation of session parameters, the ongoing multimedia session does not need to be re-established after the handover. If some form of rate adaptation is required, i.e., moving to a wireless link with limited bandwidth, the MN must send a SIP re-INVITE message with an updated Session Description Protocol (SDP) [10] entry. An inter-domain handover procedure is required whenever a MN moves from one administrative domain to the next. This is shown by the movement of the MN from position MNX to position MNZ in Figure 1. In this situation, the MN performs a full registration with the HR and the multimedia session is re-established using the newly allocated IP address. To accommodate this additional delay, a more aggressive handover procedure is implemented and a Real-Time Protocol (RTP) [11] translator is used. Unlike traditional handover algorithms that use the radio signal strength (RSS) as the handover trigger, our approach only uses RSS measurements to confirm the existence of a wireless network before the handover is executed.

C. Client Architecture

The handover decision algorithm is implemented by the MN. A tracking agent (TA) monitors the position and velocity of the MN and provides this information in a standardised format to the decision engine (DE). The DE

is responsible for making the mobility management decisions using information provided by the TA, APLP, the SIP user agent, and the users preferences defined by particular policy adopted. The client architecture is shown in Figure 2.

Information received by APLP enables the DE to construct a virtual map of APs for a specified range around the present location of the MN. This information is compared with the MNs most probable path, the roaming preference of the user, and the signalling delay associated with a particular handover to allow the DE to decide if a potential handover needs to be prepared, delayed, or ignored as illustrated in Figure 3.

Fig. 3 Decision engine.

III. LOCATION TRACKING

Location tracking systems for indoor environments come in various flavours differing in accuracy, frequency of location updates, and cost of installation, and maintenance [12]. One of the primary drivers of this research effort is the availability of the Bat location system initially developed in our laboratory as the Active Badge system by Want and Hopper [13]. The original system used infrared ID broadcasts by tags that were picked up by wall-mounted sensors. The current system can determine the 3D position of objects using ultrasonic pulses emitted by the Bats (wireless tag-devices) to reach receivers in known, fixed locations. The accuracy of the system is phenomenal given that 95% of the 3D position readings are accurate to within 3 cm. Even though this level of accuracy is not required to support our location-aided handover algorithm it provides a good perspective on the performance of our system as the accuracy is varied from a few metres all the way down to a few centimetres.

Most of the indoor location systems becoming increasingly available these days are based on radio. Using the WLAN infrastructure, WiFi-enabled devices can achieve accuracies of less than 10 m while more sophisticated technologies that use UWB can achieve accuracies of about 15 cm [14].

For outdoor location, the Global Positioning System (GPS) is the principal location sensing technology. GPS can calculate position with an accuracy of about 20 metres using trilateration even though this precision can be improved considerably using differential GPS and

UDP Transport LayerSIP Transport Module

SIP Transaction Module

SIP User Agent

Optimised REGISTER, INVITE, INFO methods

SDP Parser/Translator

RTP/RTCPStack

RTP Translator

SDPAdaptor

API

CodecsDecision Engine (DE)

Policy Manager (PM)

API

Tracking Agent (TA) APLP Data

Applications

aDecision Engine (DE)

Execute Handover

Delay Handover

IgnoreHandover

Tracking Agent (TA)

Policy Manager

APLP Data

631

other error-correcting techniques. Additional technologies are emerging that use mobile phones and TV broadcasts. Recent experiments by Cambridge Positioning Systems have shown that location accuracies in the order of 20 metres are possible and Rosum has reported accuracies from 3 to 35 metres using digital TV signals.

Undoubtedly, the proliferation of location sensing technologies will enable mobile devices to be location aware, supporting the requirement for implementing an efficient handover algorithm that benefits from critical mobility information conveyed by these systems. We use the Bat system and GPS for indoor and outdoor location tracking respectively, however, our architecture assumes no particular location sensing technology.

IV. APLP

APLP is a newly defined protocol that disseminates geospatial information and network parameters associated with various APs to location-aware mobile devices [9]. This information is updated each time a mobile node performs a SIP registration (or requires additional information for a new coverage area it is about to enter) using the SIP INFO method and is used to facilitate low latency handovers. Mandatory information delivered include the AP geographical position (longitude/latitude), ID (unique identifier, typically the MAC address), IP address of serving access router (AR), supported wireless network technology and coverage zone. The MN requests this information for a specified region around its location (at the time of the request) depending on its mobility pattern, available bandwidth, and policy preferences. This information is used to construct a virtual map of APs and their coverage zones. Optional information can also be requested including bandwidth utilisation, QoS constraints and billing indicators. All APLP-related information is stored in databases at the location service and the requested information is sent as a text payload in the SIP INFO message body. The message sequencing is illustrated in Figure 4.

Fig. 4 Message sequence to retrieve APLP information.

V. APPLICATION LAYER MOBILITY MANAGEMENT

To improve signalling delays related to mobility management, we propose some optimisations to SIP mobility that minimise the loss of transient data packets during handover. This is achieved by reducing the network registration time through the use of a SIP Mobility Anchor Point (SMAP) that introduces a hierarchical network management structure and by reducing address resolution time by implementing address pre-configuration.

A. Intra-Domain Handover

Figure 5 shows a typical SIP signalling flow during an intra-domain handover as the user moves from position MNX to position MNY. During power-on, the MN associates with APX and obtains a local IP (LoIP) address and the address of the serving SMAP (using SIP DHCP options [15]) from the serving AR. The LoIP address can be either private or public depending on how the network is administered. Once the MN is informed of its regional SMAP, it sends a SIP REGISTER message using LoIP as its contact address. This creates a binding in the VR location service associating the address-of-record-URI* with the LoIP address. At the same time the SMAP requests a globally routable proxy IP (PIP) address that uniquely identifies the MN and updates the address binding to . The SMAP completes the mobile node registration by sending a SIP REGISTER message to the HR using PIP as its contact address.

Fig. 5 Intra-domain handover message sequence. This hierarchical registration approach eliminates the need to send additional registration messages to the home network that usually introduces high round trip delays during frequent intra-domain handovers. Assigning both

* An address-of-record is a SIP Uniform Resource Identifier (URI) that points to a domain with a location service that can map the URI to another URI where the user might be available.

MNXSIP UA

MNYSIP UA

CNSIP UA

Obtain LoIP and SMAP address

SIP REGISTER (Contact: LoIP1)

SIP 200 OK

SIP INVITE

Request RIP address and bind with LoIP

Media session established using wireless network X

AR

APX

DHCP

APY

DHCP

AR SMAPSIP

Proxy Server

LocationService

VRHR

Handover Trigger

SIP REGISTER (Contact: PIP)SIP 200 OK

SIP INFO (APLP Network Info)

SIP 200 OK

Obtain new LoIP address

SIP REGISTER (Contact: LoIP2)Update binding

New media session redirected to MN via wireless network Y

Location Service

URI/Contact address binding

database

APLPdatabase

MNSIP UA

VR(2) Request/Update (non-SIP)

(3) Response (non-SIP)

Required information1. Position [lat/lon]2. AP ID3. Access Technology4 .Coverage zone5. Serving AR IP address

Optional information1. Bandwidth utilisation2. QoS constraints3. Billing indicators4. Security options

(4) S

IP 200 OK

(5) S

IP INFO

(1) S

IP (r

e-)REGISTER

632

LoIP and PIP addresses to the MN differentiates inter-domain mobility from intra-domain mobility by effectively hiding the local mobility of the MN within a regional network.

If the registration with the HR is successful, a SIP INFO message is used to pass APLP network information to the MN that will enable it to anticipate and prepare for future handovers.

When a handover trigger is received, the MN obtains a new LoIP address from the AR and registers this address with the SMAP which in turn, updates its binding. Since the PIP does not change, the media session can be redirected by the SMAP without the need for a SIP re-INVITE message.

B. Inter-Domain Handover

In the case of inter-domain handovers, the PIP and LoIP address of the MN changes requiring a SIP registration with the HR. Moreover, after a handover, a SIP re-INVITE message is required to re-establish the media session.

Fig. 6 Inter-domain handover message sequence.

To overcome these signalling delays, a more aggressive handover decision algorithm is used that prepares the MN for handover earlier at the expense of additional signalling overheads. An RTP translator originally suggested in [6] is also implemented to provide application-layer forwarding of RTP packets for a given address and UDP port. When the SMAP sends a SIP REGISITER message to the HR with a new PIP address for the MN, it also sends a message to the RTP translator to inform it that the MN has performed an inter-domain handover. The RTP translator binds the old IP address used by the MN and forwards all incoming packets to the new IP address of the MN for a set interval or until no more new packets are received. Once the SIP re-INVITE message is successful, a new media session is established via the new SMAP server.

IV. CONCLUSION

In this paper we have presented PROFITIS, an architecture that can support low latency handovers for real-time multimedia applications in a heterogeneous network infrastructure. To accommodate the stringent delay requirements of real-time applications we incorporate AP location information together with the mobility patterns of mobile users to anticipate handovers. The MN uses a decision engine (DE) to make mobility managements decisions based on inputs provided by a tracking agent (TA), APLP, and the users policy manager. Using this information, the DE can decide if a potential handover should be executed, delayed, or ignored. To reduce the signalling overheads and improve handover latency for frequent intra-domain handovers, a SIP Mobility Anchor Point (SMAP) is implemented that introduces a hierarchical network management structure.

In future work, we plan to present our simulation results based on different mobility scenarios and evaluate the performance of our proposed architecture compared to other mobility management schemes. We are also working on developing efficient ways of defining information about AP coverage zones without compromising the performance of APLP.

REFERENCES [1] C. Perkins, IP Mobility Support, RFC 2002, IETF, Oct. 1996. [2] D. Johnson, C. Perkins, and J. Arkko, Mobility Support in IPv6,

RFC 3775, IETF, June 2004. [3] R. Koodli, Fast Handovers for Mobile IPv6, RFC 4068, IETF, July

2005. [4] H. Soliman, et al, Hierarchical Mobile IPv6 Mobility Management

(HMIPv6), RFC 4140, IETF, Aug. 2005. [5] R. Hsieh, et al, Performance analysis on Hierarchical Mobile IPv6

with Fast-handoff over End-to-End TCP, in Proc. of GLOBECOM, Tapei, Taiwan, 2002.

[6] E. Wedlund and H. Schulzrinne, Mobility support using SIP, in Second ACM/IEEE International Conference on Wireless and Mobile Multimedia (WoWMoM), Seattle, Washington, 1999.

[7] S. Goebbels, et al, Simulative evaluation of location aided handover in wireless heterogeneous systems, in Proc. of 15th IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), Barcelona, Spain, 2004.

[8] R. Hsieh, Z. G. Zhou, and A. Seneviratne, S-MIP: A Seamless Handoff Architecture for Mobile IP, in Proc. of INFOCOM, San Francisco, California, 2003.

[9] S. Tsiakkouris and I. Wassell, Mobility Management for Real-Time Multimedia Applications across a Heterogeneous Network Infrastrcuture, in Proc. of Het-Nets 04, Ilkley, UK, 2004.

[10] M. Handley and V. Jacobson, SDP: Session Description Protocol, RFC 2327, IETF, Apr. 1998.

[11] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, RTP: a transport protocol for real-time applications, RFC 1889, IETF, Jan. 1996.

[12] M. Hazas, J. Scott, and J. Krumm, Location-Aware Computing Comes of Age, in IEEE Computer, Feb. 2004, pp. 95-97.

[13] R. Want, A. Hopper, V. Falcao, and J. Gibbons, The Active Badge Location System, ACM Transactions on Information Systems, Jan. 1992, pp. 91-102.

[14] P. Stegglesand S. Gschwind, The Ubisense Smart Space Platform, http://www.ubisense.net/Product/whitepapers&downloads.html.

[15] H. Schulzrinne, DHCP Option for SIP Servers, RFC 3361, IETF, Aug. 2002.

MNXSIP UA

MNZSIP UA

CNSIP UA

ARAPZ

SMAPOLDRTP

SMAPNEW HR

Obtain LoIP address

SIP REGISTER (Contact: LoIP) SIP REGISTER (Contact: PIP)SIP 200 OKSIP 200 OK

} Transient media pathSIP re-INVITESIP 200 OK

Handover Trigger

Media Session

New media session

633

634