QoS based vertical handoff method between UMTS systems and wireless LAN networks

Sungkwan Jung and Dong-ho Cho Div. of EE, Dept. of EECS

Korea Advanced Institute of Science and Technology Daejeon, Rep. of Korea

skjung@comis.kaist.ac.kr

Osok Song Telecommunication R&D center Samsung Electronics Co., LTD

Suwon, Rep. of Korea osok.song@samsung.com

Abstract—Future mobile telecom systems will most probably consist of various radio access networks to utilize various merits. A typical scenario of these network integrations is an interworking between UMTS and WLAN network. UMTS network is one of most prevalent network that can cover wide area and wireless LAN (WLAN) network is a good and cheep solution for local high speed data network. Thus, the integration of these two networks enables users to be satisfied with an enhanced service coverage and quality. But, the different characteristics of these integrated networks may degrade the service quality during the changes of access networks, so called, handoffs. To minimize the service quality degradations, for examples, longer handoff delay, more packet losses, decreased throughput and network disconnection, we proposed a QoS based vertical handoff scheme for UMTS and WLAN integrated networks. The main points of our proposed scheme are target cell searching through the connected access network and QoS based handoff. Simulation results show that our proposed scheme gives an increased QoS support during vertical handoffs.

Keywords- UMTS, WLAN, interworking, QoS support handoff

I. INTRODUCTION Most of people agree that future wireless

telecommunication networks will consist of overlaid various mobile networks.[1] Due to their different coverage and costs, various mobile networks have pros. and cons. To enhance a future mobile telecommunication system, people try to gather the pros of each mobile network to make heterogeneous networks interwork as a single wireless mobile network.

UMTS network [2] can cover a wide area and support user mobility and high speed mobile users. But complex signal processing technologies to overcome the problem of varying wireless channel conditions and to support multiple user access limit the maximum data transmission rate and cause high cost to use. Wireless LAN networks using IEEE 802.11(a/b/g) [3] can not cover a wide area but it can support a high speed transmission rate with very cheep cost. This high speed data rate can be achieved by simple signal processing and radio resource control.

The interworking between UMTS networks and Wireless LAN networks can provide better services to the subscribers. But these two networks have different characteristics. One is

telecommunication network and the other is data communication network. So, the integration of these two networks to provide services as a single interworking network has some difficult problems to solve. Because wireless LAN network doesn’t have mobility management and fair radio resource management, wireless LAN networks need some mobility management functions and radio resource control functions.

In UMTS networks, GTP based mobility management is provided. [2] But this mobility management scheme is a local mobility management. It can support the user mobility within its own network and can not support the user mobility between other communication networks, such as public Internet. Thus, IETF Mobile IP protocol is used to support macro mobility management. [4] Using this Mobile IP protocol based mobility management scheme can make UMTS and wireless LAN networks easily interwork each other. In addition, the interworking network architecture will be based on IP protocol which enhances the interoperability and flexibility. But, IP protocol and Mobile IP protocol were not basically designed to support the real-time applications and user mobility. So, during the handoff between wireless LAN networks and UMTS systems, users will experience the service discontinuity, such as long service time gap or network disconnection. Besides this service discontinuity, the different service characteristics of these interworked networks may degrade the Quality-of-Services (QoSs).

In this paper, we research the problems that might occur during handoffs between the interworked networks, especially QoS supporting method. In section II, we briefly describe the interworking standardization works of UMTS and wireless LAN and the QoS supporting problem when users handoff between two networks. And, we mention our proposed solution that can enhance service quality during handoffs in section III. Then, we show that our proposed scheme can provide the enhanced handoff performance by simulations in section IV, and conclude this paper in section V.

II. STANDARDIZATION OF UMTS-WLAN INTERWORKING 3G Partnership Project (3GPP), which is a world wide

specification group that considers the GSM and UMTS standards, has been studying the interworking architecture of

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UMTS systems and wireless LAN. After some year works, the basic interworking architecture shown in Fig. 1. was established.[5] When the UE tries to connect UMTS system through wireless LAN, it is connected to GPRS networks via WLAN Access Gateway (WAG) which manages access network security, accounting and routing functions. Through the WAG, UE’s access is connected to the Packet Data Gateway (PDG) that controls the major functions of the access through wireless LAN, such as IP address allocations, tunnel establishments, routing managements and etc. But, upto now, the interworking scenario concerning how users can access UMTS networks through wireless LAN has been studied. This access standard just includes interworking architecture, authentication and authorization methods, billing and account methods, and basic requirements on the UE and interworking networks. After the establishment of this basic access method, the user mobility support method that concerns the handoff between these two networks for service continuity has been researched. The fundamental working assumption about the inter-network handoff standards is to use IETF Mobile IP protocol for supporting user mobility between UMTS systems and wireless LAN.

Figure 1. Interworking network architecture between UMTS and WLAN

Considering a vertical handoff based on this interworking architecture, we have a serious problem that it is difficult to support a seamless service during inter-AN handoff. The service interworking architecture and procedures, the way to provide the network and user securities, the control scheme for minimizing performance decrease caused by different service data rate, and the interworking network detection and selection methods are typical problems and to be solved to provide the stable and continuous services to users.

Among these many problems, we focus on the control scheme for providing continuous services to users. When there is a user movement, it is necessary to control the data service rate. Especially, when the user is handoffed from wireless LAN to UMTS network, affordable QoS control scheme is needed for continuous and seamless service support because there is a high probability of network congestion or service quality decrease due to data rate difference between high data rate of wireless LAN and medium to low data rate of UMTS. Though it can not always possible to support high data rate services in UMTS unlike wireless LAN, using a suitable QoS control mechanism may give smooth service continuity to subscribers.

But UE can not know whether there is any cell that can support UE’s required service rate until UE connects to the target network because this handoff is an inter access network (AN) handoff. If UEs try to handoff between different ANs without these information, they experience service discontinuities, performance decreases, and service delays. To solve these QoS support problems, UEs need to know QoS support information in target networks before they actually initiate handoff.

III. PROPOSED SEAMLESS QOS HANDOFF SCHEME In this section, we propose a new handoff method that can

support UE’s required QoS during inter-network handoff for the UMTS and wireless LAN interworking. The fundamental idea of our proposed scheme is to search QoS supportable cells or APs through already connected wireless AN. Before executing a vertical handoff, a UE gathers IDs of target cells or APs using target wireless interface, and then sends this information through the network that the UE already has a connection to negotiate the QoS availability. Thus, we can increase the success rate of vertical handoff and minimize the handoff time delay until the UE gets the required QoS services due to the advanced QoS negotiation and resource preparation.

A. Functional requirements for entities Functional requirements for entities to support seamless

QoS handoff are described below.

1) GGSN and PDG • Data forwarding function of inter GGSN (and PDG)

There is no data forwarding function between GGSNs, which causes long IP service delays after the changes of ANs due to the time required for reconfiguration of IP parameters. This inter-GGSN data forwarding function can eliminate the necessity of IP mobility support for vertical handoff.

2) WAG • Inter SGSN handoff-like data forwarding function

• Management of mapping table between AP/Node-B ID and SGSN/WAG

To adopt our proposed vertical handoff procedure, it is required for WAG to provide more functions. Inter SGSN data forwarding function is required to minimize the packet loss during a vertical handoff. And to adopt our proposed scheme, WAG has to maintain mapping information between AP/cell ID and SGSN/WAG. Based on this mapping table, WAG can find the target SGSN or WAG to forward the request when it receives a vertical handoff information request from a UE.

3) UE • WLAN/UTRAN layer 2 trigger function

It is needed to initiate layer 3 handoff procedure as soon as the UE detects/decides layer 2 handoff. Previous layer 3 handoff procedure is initiated when IP mobility support layer detects the change of IP network, but this IP network detection consumes too much time after an IP network change. Thus,

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layer 2 triggering function is required to minimize the time delay and to enhance the seamless property of vertical handoff.

B. Seamless QoS handoff procedure 1. When a UE decides to perform a vertical handoff, it

gathers a neighboring cell information by receiving cell broadcast information. Thereafter, it sends a Vertical Handoff Info Request message that contains the received cell information such as target network ID, received cell ID and signal strength, and UE’s QoS requirement, to WAG.

2. When the WAG receives this message from UE, the WAG forwards it to the corresponding SGSNs that control the reported cell ID according to the mapping table.

3. After receiving the message, SGSN examines the message to identify the RNC corresponding to the reported cell ID, and sends the message to the RNC.

4. The RNC decides whether it can support the QoS requirement of UE if the UE is connected to it. And, according to the decision, it generates the response message that contains the possible QoS profile and transfers it to the SGSN.

5. The SGSN gathers the response messages and transfers them to WAG.

6. The WAG integrates the received cell information messages into Vertical Handoff Info Response message and transfers it to the UE.

7. After receiving the response message, the UE decides the handoff target cell ID that can support its required QoS profile. And then, it really tries to handoff to the specific target cell by sending Vertical Handoff Request message.

8. The WAG sends a Relocation Request message to the corresponding SGSN to inform a vertical handoff.

9. The SGSN forwards this message to the RNC that controls the target cell.

10. When the RNC receives a Relocation Request message, it recognizes the initiation of vertical handoff procedure and reserves the wireless resources to support the required QoS profile. After the reservation, the RNC sends a Relocation Request Acknowledge message to the SGSN.

11. The SGSN forwards this message to the WAG to inform a reservation result.

12. After the WAG receives a Relocation Response message, it starts the forwarding of buffered data to the corresponding RNC for seamless data transfer.

13. The WAG informs the UE of the fact that the target network is prepared to accept the UE’s handoff.

14. The WAG also informs the PDG that the UE performs a vertical handoff.

15. The WAG sends the packet transfer context to the SGSN for context management.

16. The SGSN forwards the received context to the RNC.

Figure 2. Signaling chart for our proposed QoS handoff scheme

17. When the RNC receives the packet transfer context, it prepares the continuous packet transmission according to the received context. And it sends the acknowledge message to the SGSN.

18. The initiation of the vertical handoff is notified to the PDG by the message from WAG. Then, the PDG sends context information to the GGSN for supporting a continuous service after the vertical handoff in UMTS networks.

19. Based on the received context information, the GGSN regenerates a PDP context to support the UE after vertical handoff, and sends the acknowledge message to the PDG.

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20. After the receiving of RNC relocation message from the RNC, the SGSN sends a PDP context update message to the GGSN.

21. The GGSN activates the PDP context based on the received PDP context information from SGSN, and notifies the SGSN that the PDP context update is completed.

22. The UE tries to connect to the UTRAN by sending RRC messages.

23. The RNC confirms the vertical handoff of the UE by receiving RRC messages and notifies the vertical handoff execution to the SGSN.

24. The SGSN sends the Relocation Complete Message to the WAG to notify that the vertical handoff procedure is completed.

25. By receiving the Relocation Complete Message, the WAG confirms that the UE is disconnected and sends the Relocation Complete Message to the SGSN in reply.

26. After the vertical handoff completion, all the data packets destined to the PDG are forwarded to the GGSN.

IV. PERFORMANCE EVALUATION For the performance evaluation of the proposed QoS

support vertical handoff scheme, we simulated our proposed scheme and the basic vertical handoff scheme. This performance comparison by simulation depends on cell structure, cell load distribution, QoS requirement of UEs, and the mobility of UEs, but it can provide the relative performance and the operability of the proposed scheme.

Figure 3. Configuration of simulation network

At first, we assume the following simulation network shown in Fig. 3. 5 UMTS cells are located in 3 Km by 3 Km rectangular area and 25 wireless LAN APs are installed in the center of the area as hot spots. Each UMTS cell has a circular coverage of radius 1.1 Km and each wireless LAN AP has a circular coverage of radius 128 m.

Each UMTS cell has a limited traffic bandwidth of maximum 128 kbps and background traffic as a cell load

parameter. For wireless LAN APs, we assume a maximum traffic bandwidth is 512 kbps and they also have the background traffic to change their traffic loads. In this environment, 20 UEs move in a random direction with the variable velocity of 1~8 m/s and have the required service bandwidth of 25 kbps, 50 kbps and 125 kbps according to the simulation parameter.

Figure 4. State transition diagram of UE

To describe the UE’s operation, we define 6 states and the state transition diagram as shown in Fig. 4. The defined states are described as following.

• IDLE UE is connected to the networks, and is not located in handoff region.

• PROBE_SENT For the vertical handoff, UE sent a vertical handoff candidate probing message. Using this message, UE tried to find cells that can support its QoS requirement in the target network.

• PROBE_FAIL Though a UE sent a candidate probing message, it can’t find any QoS supporting target cell. After some time out, it changes its state into IDLE.

• PROBED This state indicates that the UE found a QoS supporting target cell information through the probing procedure and can initiate a vertical handoff to that cell.

• HANDOFF_INIT This state shows that a UE actually started the vertical handoff procedure.

• DISCONNECTED If there is no cell that has coverage to connect a UE, the UE are disconnected from the network.

The basic handoff policy of UE is based on the wireless LAN preferred operation. If a UE finds an available wireless LAN AP during a UE movement, it tries to connect to the wireless LAN AP though it is connected to a UMTS cell. If there is no available wireless LAN AP, it tries to connect to a

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UMTS cell and if there is no UMTS cell, it is disconnected from a network. In addition, if a UE could not acquire a required bandwidth from the connected cell or AP, it also tries to find another AP or cell that can support better QoS, and if any, it performs a horizontal handoff to that AP or cell.

Figure 5. Numbers of handoff vs. WLAN AP available bandwidth difference

The numbers of handoff events vs. the difference of available bandwidth between wireless LAN APs is shown in Fig. 5, in which HOCount, QoSSuc, QoSFail, VrtHO and HoriHO mean the numbers of handoffs, QoS supported handoffs, QoS unsupported handoffs, vertical handoffs and horizontal handoffs, respectively. The results of our proposed scheme are drawn using a dashed line, and the results of previous one are drawn using a solid line. If there is small available bandwidth, the difference between the results of proposed scheme and previous one is small. In this case, because the network is somewhat saturated, our proposed scheme can’t find the QoS supported cell or AP around the UE. But, if there is more available bandwidth, our proposed scheme increases the handoff performance, because our proposed scheme decides the handoff target cell or AP based on not only the received signal strength, but also the available bandwidth that could be used after the handoff.

Figure 6. Ratio of QoS successful handoff vs. WLAN AP available bandwidth difference

The ratio of the number of QoS supported handoffs to the number of total handoffs is shown in Fig. 6, in which the result of our proposed scheme is shown using rectangles and the result of previous one is shown using triangles. If there is no QoS supportable cell or AP around the UE, our proposed scheme doesn’t try a vertical handoff. Thus, the performance of handoff supporting QoS should be considered with respect to the total number of handoffs. Fig. 6 shows that our proposed scheme always provides more QoS supported handoffs regardless of network load.

V. CONCLUSIONS In this paper, we study the QoS supported handoff scheme

in the interworking of UMTS systems and wireless LAN to provide a seamless packet data service. Among many problems in the interworking networks, we focus on how to minimize the different service characteristics of two interworking networks, and provide a new handoff scheme that enhances the target cell or AP selection scheme during a vertical handoff.

Our proposed scheme uses two additional procedures before executing a basic vertical handoff procedure. One is to probe a cell or AP that can support UE’s required bandwidth through the already connected network interface. And the other is to select a best one based on not only the signal strength but also the reported available bandwidth. Using these procedures, our proposed scheme increases the number of QoS supported handoffs and decreases the numbers of useless handoffs. In addition, using our proposed scheme makes the network entities prepare for the QoS supporting handoff before a UE actually is handoffed to a new network, thus it can minimize the total handoff delay time.

Besides this QoS supporting handoff problem, there are too many problems to be solved to provide a seamless service to a UE when UMTS and wireless LAN are interworked. By solving these problems, we can enhance the upcoming network environments to provide better services for the subscribers.

REFERENCES [1] K. Ahmavaara, H. Haverinen and R. Pichna, “Interworking Architecture

Between 3GPP and WLAN Systems”, IEEE Comm. Mag., vol. 41, no. 11, pp 74-81, Nov. 2003.

[2] 3GPP, “General Packet Radio Service (GPRS); Service description; Stage 2 (Release 5),” 3GPP TS 23.060. v5.4.0, Dec. 2002

[3] “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specification,” ANSI/IEEE Std 802.11:1999 (E) Part 11, 1999

[4] C. Perkins, Ed., “IP Mobility Support for IPv4,” IETF RFC 3344, Aug. 2002

[5] 3GPP, “3GPP Systems to Wireless Local Area Network (WLAN) Interworking; System Description (Release 6),” 3GPP TS 23.234. v.2.3.0, Nov. 2003

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