Seamless WiFi-roaming in IMS network

Marek Golha, Ondrej Lábaj, Sebastian Schumann, Pavol Podhradský Slovak University of Technology (STU), Ilkovičova 3, 812 19 Bratislava, Slovak Republic

E-mail: {golha;labaj,schumann,podhrad}@ktl.elf.stuba.sk

Abstract - Over recent years, the explosion of radio access technologies and wireless networking devices has triggered the intensive use of nomadic computing. Mobile devices receive intermittent network access, and alternate between connected and disconnected states. However, today’s personal gadgets have more networking capabilities, wireless network coverage is becoming ubiquitous, and always-on IP-based services are now closer to reality. This paper shows a possible implementation of mobility in the IMS platform with using two wireless access points. A practical test was also done to investigate the quality of VoIP call as well as defects of this implementation. Keywords - IMS, mobility, user equipment, NGNlab, ELMAR, Croatia 1. INTRODUCTION The main focus of this document is to show possible implementation of seamless roaming in the IP Multimedia Subsystem (IMS) environment through the tests we carried out and the results obtained. Two wireless access points were used in the implementation. These two wireless access points creates two areas of WiFi coverage that overlaps with each other. A roaming client with active VoIP call was used to test the quality of the VoIP call when passing between these two areas with WiFi coverage. The test was done on an OpenIMS platform and the results were reported. 2. DESCRIPTION OF ELEMENTS AND DEFINITIONS 2.1. Open IMS

The Open IMS Core is an Open Source implementation of IMS Call Session Control Functions (CSCFs) and a lightweight Home Subscriber Server (HSS), which together form the core elements of all IMS/NGN architectures as specified today within 3GPP, 3GPP2, ETSI TISPAN and the PacketCable intiative. All the four components are all based upon Open Source software (e.g. the SIP Express Router (SER) or MySQL). [1] 2.2. SIP protocol

The Session Initiation Protocol was chosen to be the protocol for session control in IMS. SIP was originally developed within the SIP working group in the IETF. Even though SIP was initially designed to invite users to existing multimedia conferences, today it is mainly used to create, modify and terminate multimedia sessions. In addition, there

exist SIP extensions to deliver instant messages and to handle subscriptions to events. The main goal of SIP is to deliver a session description to a user at his current location. Once the user has been located and the initial session description delivered, SIP can deliver new session descriptions to modify the characteristics of the ongoing sessions and terminate the session whenever the user wants to. [2] 2.3. User Equipment - UE

IMS services require an IMS/SIP client (including GUI, service logic, routing and discovery functionality) in the user equipment to communicate with the network servers – in terms of, mirroring the service logic in the network. The IMS/SIP [3] client is structured in such a way that the core functions are reused for many applications, and that many applications can be co-located on the same user equipment. The extra work of deploying a new service with IMS is significantly smaller, as the core functions are already in place. Implementing of IMS logic in the terminals means that the architecture truly will span end-to-end. Two UEs was used in the tests. MONSTRER (Multimedia Open InterNtet Services and Telecommunication EnviRonment) Framework has been developed at Fraunhofer Institute FOKUS. The Framework includes a JSR281 – oriented IMS client, services for accessing XDMS and for consuming data within distributed applications build with JSON or SOAP web services. The IMS engine combines implementations of SIP, RTP, MSRP and XCAP according to specifications of 3GPP, OMA and IETF. [4] The Mercuro IMS Client is fully compliant with the main 3GPP IMS specifications and partially compliant with RCS (Rich Communication Suite) phase 1 and OMA specifications. The features of Mercuro IMS Client are: Unified Contact List

Management (XCAP/XDMS), incoming/outgoing Voice calls and Video calls (QCIF, CIF), instant messaging (SIMPLE, MSRP), file transfer (MSRP), OMA Presence Management, multi-codec support (G711, H.263, H.264…) and RSC compliance. [5] 2.4. Seamless roaming or service continuity

Seamless roaming, sometimes referred to as service continuity, means that network end users can move easily between WiFi/WLAN technology and 3G technology without really noticing any effects on their data connections. Service requirements mandate the session transfer to be as seamless as possible in order to offer service continuity with minimum impacts on user experience. [6] 3. DESCRIPTION OF TEST EXECUTION Two ZyXEL G-1000 v2 [7] access points were installed, with about twenty percent overlap between cells. Both access points were configured with the option “enable roaming”. Two laptops were used as user agents, on each laptop an IMS client was installed and registered on Open IMS. Wireshark network sniffer [8] was also running on each laptop to analyse the communication between the two IMS clients. During the experiment the subjective quality of the VoIP communication was investigated when one or both user agents have moved between wireless cells. Also the subjective quality was analyzed when a Video call was created between these two mobile nodes. The communication process was also analyzed using the captured SIP messages from Wireshark. 3.1. Testbed of experiment Fig.1 shows the scheme of our tests.

Fig. 1. Testbed schema

3.2. Environment Specifications

IMS Platform: Open IMS Core Access Points: ZyXEL G-1000 v2 OS: Microsoft Windows XP and Microsoft Windows Vista User Equipment: MONSTER IMS Client, Mercuro IMS Client Notebooks: ASUS, HP Wireshark network sniffer

3.3. Passing between the access points In the Fig.2 there is a chart where the strength of WiFi signal is displayed by passing from one access point to another and back and also the moment when the connection is switched from access point 1 to access point 2 or vice-versa. Timeframe on the x axis specifies the time period during which the Atheros client utility measured the strength of the signal. The values on the y axis specify the measured strength of the signal. These values were measured by passing from access point 1 to access point 2 and back. The Atheros Client Utility has been used as a measuring tool.

Fig. 2. Strength of WiFi signal 4. TEST EXPERIMENTS 4.1. Test groups

Testing of the mobility implementation between two access points was carried out in three scenarios:

1. First IMS client moved from access point 1 to access point 2. Second IMS client was static and he was placed in coverage area of access point 1.

2. First IMS client moved from access point 2 to access point 1. Second IMS client was static and he was placed in coverage area of access point 1.

3. Both IMS clients were moving. First IMS client moved from access point 1 to access point 2 and second IMS client moved from access point 2 to access point 1.

First, these three scenarios were tested with a voice call established between the two IMS clients and after with a video call between these IMS clients.

Table captions are placed above the appropriate table. The example is given bellow. 4.2. Test results On both laptops a Wireshark network sniffer was used for analyzing the flow of messages between the two IMS clients. The main purpose of this was to find out whether or not a re-registration or disconnection of an active call occured while moving from one access point to another. In the Fig.3,4 a call flow of a voice call with passing between two access points captured in Wireshark is shown.

Fig.3. Movement of client 1 from AP1 to AP2

Fig. 3 displays a call flow, where client 1 (IP address 147.175.103.51) initiated a voice call with client 2 ( IP address 147.175.103.242). Client 1 created a call with client 2 and was moving during the call from AP1 to AP2, while client 2 was statically placed. When client 1 came close to client 2, client 2 ended the call. Only the communication between client 1 and the IMS core (IP address 147.175.103.216) was captured. When client 1 was passing from AP1 to AP2, the message 408 Request Timeout appeared because of re-association process in WiFi card on laptop.

Fig.4. Movement of client 2 from AP2 to AP1

Fig. 4 displays a call flow, where client 2 initiates a voice call with client 1. Client 2 is moving from AP2 to AP1, while client 1 was statically

placed. When client 1 came close to client 2, client 2 ended the call. Again only the communication between client 2 and the IMS core was captured. Message 408 Request Timeout appeared again, because of the same reason as was in the previous test. The difference is that other laptop was used with the different network card and re-association process took longer. 4.3. Subjective quality measurement Voice call In all voice scenarios the quality of the call was on a good level. When one client was a mobile client and the other client was static a short interruption of the call occurred by passing from one access point coverage area to coverage area of another access point. Average length of the interruption was from one second up to two seconds. However, the Fig.3,4 where the call flow chart captured in Wireshark is displayed that the call wasn’t dropped and was terminated correctly. The length of the interruption also depended on the speed of the transition from one access point coverage area to another access point coverage area. An important role also played also the strength of the signal, in the Fig.2 it can be seen that the strength of signals from access points at the moment of passing from one access point to another access point are not so high. As a consequence of this issue when we the movement in interrupted at the moment when the handover between the access points has to be realized, the signal from the both access points is very weak and the length of the interruption is therefore more than 2 seconds. One good solution for this problem can be the extension of the overlapping area between these two access points. Video call In the case of a video call between the mobile nodes, the quality of voice and video was also good. The difference was in the length of the interruption at the moment of handover between the two access points that was comparable longer. Video similar to the voice was in the moment of handover stopped and resumed after about five seconds (voice resumed earlier after about three seconds). Similarly to the voice scenario the length of the interruption also depended on the speed of the transition from one access point coverage area to another access point coverage area as well as from the strength of the wifi signal and switching of wireless network interface. Similarly to the voice scenario, the video call wasn’t dropped when moving form coverage area of one access point to the coverage area of another access point.

5. IMS MOBILITY IMPELMENTATION IN NGNLAB NETWORK ARCHITETURE (STUBA) Fig.5 shows overview of whole NGNlab network architecture at Slovak Technical University Bratislava [9] and also the described implementation of mobility (in the left bottom corner).

Fig.5. NGNlab architecture 6. CONCLUSION The experiments with seamless WiFi-roaming in an IMS network was presented in this paper. The tests that was carried out shows that it is possible to roam between two WiFi cells without the lost of a voice or video call. However, at the moment of handover a few second interruption of the voice or video call occurs. Some parameters that affect the length of the interruption are: overlapping of WiFi cells and speed of the re-association process in WiFi cards. This implementation is applicable in cases where a slight interruption can be acceptable.

ACKNOWLEDGEMENT This paper also presents some of the results and acquired experience from various research projects such as NGNlab project [9], European Celtic-EURECA project Netlab [10], Leonardo da Vinci projects InCert [11] and Train2Cert [12], AV project: Converged technologies for next generation networks (NGN) No. AV/4/0019/07, Slovak National basic research projects VEGA No. 1/0720/09 and VEGA 1/4084/07. REFERENCES [1] Open IMS, http://www.openimscore.org/ [2] RFC 3261 recommendetion: SIP: Session

Initiation Protocol [3] 3GPP TS 23.228 V5.13.0 (2004-12) IP

Multimedia Subsystem (IMS) Stage 2 (Release5)

[4] MONSTER– the IMS client, http://www.monster-the-client.org/

[5] Mercuro – the IMS client, http://www.mercuro.net/

[6] Seamless Roaming, Ove Johnsson, November 2007

[7] ZyXEL G-1000 v2, http://www.zyxel.com/ [8] Wireshark, http://www.wireshark.org/ [9] NGNlab - NGN laboratory at Slovak

Technical University in Bratislava, http://www.ngnlab.eu

[10] NetLab - Use Cases for Interconnected Testbeds and Living Labs, http://www.celticinitiative.org/Projects/NETLAB/

[11] InCert Next Generation Network Protocols Professionals certification in InCert, International Certificates of Excellence in Selected Areas of ICT, http://incert.eu

[12] Train2Cert, Vocational Training for Certification in ICT, http://train2cert.eu