VGSN : A Gateway Approach to Interconnect UMTS/WLAN Networks
VGSN : A GATEWAY APPROACH TO INTERCONNECT UMTS/WLAN NETWORKS Shiao-Li Tsao and Chia-Ching Lin Computer & Communications Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, R.O.C., {[email protected], [email protected]}
Abstract - High tier wireless systems such as GPRS and UMTS provide users high mobility but less bandwidth. On the other hand, low tier wireless systems such as Wireless LAN offer high bandwidth with less mobility. To support seamless roaming between heterogeneous wireless networks is regarded as one of the key issues in the future mobile communication system. The paper presents method and system for seamless roaming between UMTS and WLAN networks. The two networks are assumed to construct and operate independently, but wish to offer roaming services to their customers. A new logical node, called Virtual GPRS Support Node (VGSN), is designed for this purpose to connect UMTS and WLAN core networks. VGSN is a normal GPRS Support Node (GSN) in a UMTS system and also acts as an access route in a WLAN network. The node converts control signals and routes data packets between two networks for roaming users. With the presented approach, mobile IP is not required and users can have a seamless roaming between networks based on one IP address.
present a number of approaches to interconnect heterogeneous networks [1]. However, their work is mainly to evaluate handoff efforts of different approaches but lacks of the system design of interworking strategies. ETSI BRAN project [8] further classifies the strategies into no coupling, tight coupling and loose coupling. [2][3][4][5] present several approaches to interconnect different IP-based radio access networks. They are basically tight coupling approaches. Stemm et al. [9] and 3GPP [6][7] suggest integrating the networks by using mobile IP. However, mobile IP introduces serious latency and packet lost for frequent roaming users in between two networks frequently.
Keywords – UMTS, WLAN, Packet Routing, Gateway, Interworking, Handover I. INTRODUCTION
Figure 1. Connecting UMTS and WLAN networks.
A number of wireless network systems have been proposed and deployed during the past few years. It is believed that multiple standards will coexist in the same environment for the future wireless communication system. To provide seamless roaming between heterogeneous networks becomes more and more important in the future mobile communication system. Different radio access networks have their own properties. High-tier system such as General Packet Radio Service (GPRS) and Universal Mobile Telecommunication System (UMTS) provides high mobility but with less data transmission bandwidth. On the other hand, low-tier system such as wireless local area network (Wireless LAN, WLAN) provides high data bandwidth but with less mobility. For the hot spot area such as a building, a station or an airport, it is normally covered by number of networks. Therefore, it is important to consider dual mode users roaming in between high tier and low tier wireless networks. UMTS and WLAN interworking is regarded as an important working item for 3G beyond systems. Figure 1 shows that two different wireless networks are connected based on current standards/specifications. Pahlavan et al.
A new logical node, called Virtual GPRS Support Node, is introduced in this paper to interconnect UMTS and WLAN backbones. The logical node acts a GPRS Support Node in UMTS and is an access router in WLAN. The node exchanges necessary information between networks, converts signals, and forward the packets for roaming users. Our approach aims to separate the operations of the two networks. In other words, two networks are peer networks and handle their single mode subscribers independently. For two network operators have a roaming agreement, VGSN helps them to enable intersystem roaming in two networks. The merits of this approach are that the two networks can be operated independently and mobile IP is not necessary. The packets for roaming users go through VGSNs to Internet without processing by mobile IP. The design reduces packet lost and delay. The rest of the paper is organized as follows. Section 2 proposes the VGSN. Section 3 shows and discusses simulation results. Finally, Section 4 concludes this study.
0-7803-7589-0/02/$17.00 ©2002 IEEE
PIMRC 2002
II. VIRTUAL GPRS SUPPORT NODE Several approaches can be applied to interconnect UMTS and WLAN networks. The paper aims to enable dual mode devices roaming in two wireless networks that are separately operated. Assumptions in this paper are listed below. (1). A user has a dual mode device. In other words, a device has two radio access interfaces. (2). Users with the dual mode devices subscript a data service of one network can use the radio access network of the other wireless network if the two networks have a roaming agreement. (3). Applications use only one IP address to access Internet. (4). Services, transactions and IP connections will not break during the handovers between networks. (5). Mobile IP is not required. (6). Two networks are peer networks. Two networks are managed and operated separately. For single mode users, traffics to/from a network will handle by the network itself without involving the other network. Figure 2 and Figure 3 depict the packet flows for UMTS users roaming to a WLAN and WLAN users roaming to a UMTS network respectively. A new logical node, called virtual GPRS support Node (VGSN), is presented in this paper. The logical entity can be either implemented as a separated node or integrated with Gateway in WLAN or SGSN or GGSN nodes. Since two networks are managed separately, we can classify the scenarios into two basic categories. The first case is a UMTS subscriber (we also use UMTS users or UMTS mobile station (MS) in this paper) attaches to a UMTS network and moves to a WLAN. It might handover to WLAN to have a higher data transmission rate. Figure 2 shows the scenario. We assume the IP address allocated to the users (or mobile station (MS)) will not change during the whole period of data service in the different network. In order to achieve this, traffic from MS to Internet will go through the UMTS network. For the MS moves to a WLAN, the incoming packets go to the UMTS network through the VGSN to the WLAN and ended at the MS. The out-going packets can go through WLAN directly. Another scenario is a WLAN user registering in the WLAN but might moves to a UMTS network. Figure 3 shows the case. In this case, incoming/outgoing packets must go through the VGSN if a WLAN MS camps in a UMTS network. The two cases are presented below.
Figure 2. UMTS users use WLAN as a radio carrier.
Figure 3. WLAN users use UMTS as a radio carrier. UMTS to WLAN roaming is first presented. Here, we assume a MS is registered in a UMTS network and the UMTS network has a roaming agreement with WLAN operators. Figure 4 shows the procedures of UMTS to WLAN roaming. From the step 1 to the step 3, these are normal UMTS attach/PDP context activation procedures. At the step 4, a MS finds a WLAN is accessible and wants to handover to a WLAN. The step 4 performs L1/L2 handover procedures. In the step 5, the MS tries to obtain a VGSN address in order to perform following procedures. An example to resolve a VGSN address in a WLAN environment is to use DHCP (dynamic host configuration protocol). The step 5.1 sends a DHCPDISCOVER to ask VGSN address in the WLAN network. The VGSN responses the MS with its IP address. After the MS obtains the VGSN address, it sends routing area update (RA update) to the VGSN using its original IP address of the UMTS network. Then, the VGSN sends standard Update PDP contexts Request to a GGSN to ask a GGSN changing its SGSN address-in-use. At this period, a VGSN simulates a SGSN in a UMTS network. Once a GGSN receives PDP context request from a VGSN, it knows the MS moves to a WLAN environment. The VGSN becomes the SGSN temporally. Packets to the MS should go to the VGSN instead of the old SGSN. Since the MS might moves back to the UMTS network, data stored in the SGSN cannot be deleted even it moves to a WLAN temporally. The GGSN sends a new packet data protocol/mobility management context standby command to the old SGSN. The message is to ask the SGSN to hold the PDP/MM context till the MS comes back to the UMTS or detaches. After the whole roaming procedures, the MS sends the packets out using its original IP address of the UMTS network. For the packets from the MS to Internet, they can be sent through the WLAN network if the WLAN does not perform ingress filtering on these UMTS IP address. If the WLAN applies ingress filtering on non-WLAN IP addresses, packets should go to the VGSN and then to Internet. For these packets to the MS, it should go to the UMTS network based on IP routing. Then, the GGSN recognizes the MS moves to the VGSN and it tunnels packets to the VGSN. There are several implementation choices of tunnelling data, i.e. GPRS tunnelling protocol (GTP) tunnelling and IP in IP
tunnelling. In order to reduce tunnelling overhead, IP in IP tunnel between VGSN and GGSN is suggested. The procedures are summarized as : Step 1 to Step 3 : Normal UMTS attach and PDP context activation procedures.
the GGSN will be forwarded to the new SGSN instead of the VGSN.
MS
Step 4 : L1/L2 handover procedures.
WLAN AP1
Step 8 : GGSN sends PDP/MM context standby to the old SGSN. The message to SGSN is to present the deletion of PDP/MM context in SGSN. Step 9.1 : Packets send from MS to Internet hosts. Step 9.2 : Packets from internet hosts to MS through VGSN.
GGSN
Virtual GSN
Host
1.1 Attach Req. 1.2 Attach Acc. 2.1 Activate PDP Context Req. (IP: NULL, APN:GGSN) 2.2 Active PDP Context Activation Acc. (IP_GPRS) 3. Packet transmission(IP: IP_GPRS)
3. RA update procedures 4. Packet transmission(IP: IP_GPRS)
Figure 5. WLAN to UMTS handovers for UMTS MSs (back to the same RA). Another case is that the MS moves to another RAs. In this case, the MS sends a standard RA update to the new SGSN and then involves a standard inter-SGSN update procedure. To support this type of handover, the MS is assumed to store the information of the original SGSN information. The original SGSN is the SGSN that the MS first moves to the WLAN.
MS
4. L1/L2 handover procedures
(IP in IP tunnel)
2. L1/L2 handover procedures
Step 7 : VGSN simulates SGSN to send Update PDP Context Request and responses Update PDP Context Request to GGSN.
SGSN
Host
1.2 Packet transmission(IP: IP_GPRS) : downlink
Step 6 : MS sends a RA Update to VGSN to handover to VGSN.
WLAN AP1
Virtual GSN
GGSN
1.1 Packet transmission(IP: IP_GPRS) : uplink
Step 5 : Procedures to obtain VGSN IP address. DHCP is used here as an example.
MS
SGSN
WLAN AP1
New SGSN
5.1 DHCPDISCOVER(VGSN) 5.2 DHCPOFFER(VGSN_IP) 6. RA Update
Old SGSN
Virtual GSN
GGSN
Host
1.1 Packet transmission(IP: IP_GPRS) : uplink 1.2 Packet transmission(IP: IP_GPRS) : downlink
7. Update PDP Context Req./Res. 8. PDP/MM context Standby 9.1 Packet transmission(IP: IP_GPRS) : uplink
(IP in IP tunnel)
2. L1/L2 handover procedures 3. Inter SGSN RA update procedures
9.2 Packet transmission(IP: IP_GPRS) : downlink 4. Packet transmission(IP: IP_GPRS)
(IP in IP tunnel)
Figure 4. UMTS to WLAN roaming. After UMTS users handover to a WLAN environment, it might move to another WLAN cells or go back to UMTS networks. For the MS handovers from one WLAN base station to another WLAN base station, it only introduces Layer 1(L1) and Layer 2(L2) procedures. In that way, both incoming and outgoing packets go through the same path. The only difference is that the final WLAN access node is different. The handover procedures are done by the WLAN L1/L2. Another situation is that the MS moves from the WLAN back to a UMTS network. Since PDP/MM context are buffered in the SGSN that the MS first moves out to the WLAN, the handover procedures involve that the old SGSN. There are two possible cases. First one is the MS goes back to the same routing area (RA). The procedure is shown in Figure 5. The step 3 is a normal RA update procedure. It only updates SGSN-in-use in the GGSN. Then, packets to
Figure 6. WLAN to UMTS handovers for UMTS MSs (back to the different RA).
MS
WLAN AP1
SGSN
GGSN
Virtual GSN
1.Packet transmission(IP: IP_WLAN) 2. L1/L2 handover procedures 3.1 Attach Req. 3.2 Attach Acc. 4.1 Activate PDP Context Req. (IP: IP_WLAN, APN:VGSN) 4.2 Active PDP Context Activation Acc. (NULL) 5.Packet transmission(IP: IP_WLAN)
Figure 7. WLAN to UMTS roaming.
Host
Another handover category is a WLAN user handover from WLAN to UMTS networks. Here, we assume a MS is registered in a WLAN network and the WLAN has a roaming agreement with a UMTS network. Here, we also assume MS is equipped with a dual mode device. Moreover, the MS must be also a subscriber of the UMTS network. The reason is that the MS can access UMTS radio access network only if it is the subscriber of the UMTS network. Figure 7 shows the step-by-step procedures of WLAN to UMTS roaming. A WLAN user first connects to a WLAN network and starts to access Internet. The IP address belongs to the WLAN. Once the MS detects the signal received from the UMTS network stronger than the WLAN, it might decide to handover to the UMTS network. If it has already attached to the UMTS network and in the same RA, it can just start the service. If it is the first time to attach to the network, it should perform attach and PDP context activation and then can start the service. In the attach procedures, MS uses VGSN as its access point name (APN) to inform the SGSN that it wants to use WLAN IP. During the PDP context activation procedure, the MS use the WLAN IP to request the PDP context. Once the VGSN detects the IP is WLAN IP and the security process is passed, it responses the MS with the same WLAN IP. The MS can use the same IP address that was used in the WLAN. Here, VGSN simulates the GGSN in a UMTS network. The SGSN will send packets to the VGSN and the VGSN sends packet to the SGSN. Both incoming and out-going packets go through the same path. Internet hosts send packets to the WLAN MS will go to the WLAN network, received by the VGSN, send to the SGSN and ended to the MS. The detail procedures are listed as : Step 1 : Packets transmission through WLAN. Step 2 : L1/L2 handover procedures. Step 3 : Standard UMTS attach procedures (not necessary if MS has attached to UMTS network). Step 4.1 : Standard UMTS PDP context activation procedures (not necessary if MS has activated a PDP context for WLAN/UMTS roaming). MS uses VGSN as its APN and asks to use the same IP address in WLAN network. SGSN receives the message and passes it to its local VGSN. Step 4.2 : VGSN receives the commands and checks that if the MS is allow to use the same IP address in the WLAN. Then, it replies the MS. If the admission of the MS is passed by the VGSN, the VGSN will then handle the packet forwarding of the MS packets. Step 5 : Packets from MS to SGSN will go to its GGSN (i.e. VGSN) to the WLAN and Internet. Packets from Internet hosts to the MS will go to WLAN first, and then be routed to VGSN and SGSN to the MS. To update the routing table between WLAN internal routers, one approach is to use proxy ARP to enforce update the packet routing to the VGSN. Another approach is using tunnelling approach. In
other words, packets to the MS should be tunnelled from the original router to the VGSN and then send to the SGSN.
MS
WLAN AP1
Virtual GSN
SGSN #2
SGSN #1
Host
1.Packet transmission(IP: IP_WLAN) 2. GPRS/WLAN handover procedures 3.Packet transmission(IP: IP_WLAN) 4. L1/L2 handover procedures 5.1 Routing Area Update Req. 5.3 Update PDP Context Req./Res. 5.4 Routing Area Update Acc./Comp. 6.Packet transmission(IP: IP_WLAN)
Figure 8. UMTS to UMTS handovers for WLAN MSs. As for the WLAN user moves from the UMTS back to the WLAN, the case only involves WLAN L1/L2 procedures. For the WLAN user moves from one RA to another RA, it has two situations. The first situation is the two RAs are controlled by the same SGSN. The MS initiates a standard RA update procedure. It introduces no change to the intersystem roaming. The second situation is the two RAs are controlled by two SGSNs. It requires updating the current SGSN-in-use in the VGSN. Figure 8 shows the scenario. The procedures are normal inter SGSN RA update procedure. The VGSN here has to simulate a GGSN and updates the new SGSN address.
MS
WLAN AP1
SGSN
GGSN
Virtual GSN
Host
Host #2
1.Packet transmission(IP: IP_WLAN) 2. L1/L2 handover procedures 3.1 Attach Req. 3.2 Attach Acc. 4.1 Activate PDP Context Req. (IP: IP_WLAN, APN:VGSN) 4.2 Active PDP Context Activation Acc. (NULL) 5.Packet transmission(IP: IP_WLAN) 6.2 Request PDP Context Act.
6.1 Incoming packet
7.1 Activate PDP Context Req. (IP: NULL, APN:GGSN) 7.2 Active PDP Context Activation Acc. (IP_GPRS)
Figure 9. UMTS paging for WLAN MSs. Regarding of a WLAN user locating in a UMTS network, some hosts in the Internet want to access the WLAN users by their UMTS addresses. The packets from hosts to the MS will first handled by the GGSN. Since the original PDP context maintained in the SGSN and the VGSN are for the WLAN roaming not for UMTS service. A new PDP must be allocated to handle the new UMTS connection. Step 6 and 7 in Figure 9 shows the case. In other words, the present approach uses two separated PDP contexts to handle (1) WLAN address in WLAN to UMTS roaming service (2) UMTS address for UMTS service. Figure 9 shows the detail message flow.
[6] ETSI EN GSM 03.60, “General Packet Radio Service (GPRS), Service Description, Stage 2”, April 2000. [7] 3GPP TS 23.060, “General Packet Radio Service (GPRS), Service Description, Stage 2”, Dec. 2001. [8] ETSI BRAN Project, http://portal.etsi.org/bran/home.asp. [9] Mark Stemm and Randy H. Katz, “Vertical Handoffs in Wireless Overlay Networks”, Mobile Networks and Applications, Vol. 3, No. 4, pp. 335-350, 1998.
UMTS/WLAN Gateway (15Mbps) 70
40 30 20 10
Figure 10. Average bandwidth per user for different UMTS/WLAN interworking strategies. UMTS/WLAN Gateway (15Mbps) 450
350 300
Tight coupling VGSN Mobile IP
250 200 150 100 50
10
Handover Delay (ms)
400
[1] Kaveh Pahlavan et al., “Handover in Hybrid Mobile Data Networks”, IEEE Personal Communications, April 2000.
[4] US patent 6,292,891 B1, “Method of connecting base station to cellular system”
91 0 12 10 15 10 18 10 21 10 24 10
Number of User
0
[3] US patent 6,314,108 B1, “Method and apparatus for providing network access over different wireless networks”
31 0 61 0
10
0
REFERENCES
[2] US patent 6,243,581, “Method and system for seamless roaming between wireless communication networks with a mobile terminal”.
Mobile IP/VGSN
50
IV. CONCLUSIONS A gateway approach to connect UMTS and WLAN networks was proposed in this paper. A logical node, call Virtual GPRS Support Node (VGSN), is designed to convert control signals and to route data packets between two networks for roaming users. With the presented approach, mobile IP is not required and users can have a seamless roaming between networks based on single IP address. Simulation results show that our approach obtains a similar performance but reduces handover latency significantly comparing with mobile IP approach.
Tight coupling
60
51 0 10 10 15 10 20 10 25 10 30 10 35 10 40 10 45 10
Simulations are conducted to examine the performance of the proposed approach. The simulation environment is covered by a UMTS network and WLAN. The UMTS network has 100Mbps backbone with 5 radio network subsystems and offers 32Kbps data services. WLAN has 25 access points and provides 100Kbps data service. We assume 50% registers as WLAN users and 50% registers as UMTS users. Among all users, 50% are dual mode users and might moves in between two networks, and the other 50% are single mode users using either WLAN or UMTS. We assume dual mode users have 50% to enter the WLAN and 50% to enter the UMTS. We compare three different approaches, i.e. mobile IP, VGSN and tight coupling (WLAN acts as the radio access network of UMTS). Figure 10 shows the average bandwidth per user. We learn that mobile IP and VGSN approaches obtain the same the average bandwidth that is much better than the tight coupling approach. The reason is that packets of tight coupling approach always need to go to the UMTS network that degrades the system performance. Mobile IP and VGSN approaches separate single mode users and roaming users. It much improves the scalability and performance. Figure 11 shows the handover latency. Mobile IP has the poorest performance since the signaling packets need to go to Internet (home agent and foreign agents). VGSN and tight coupling approaches only involve the message exchange within intra-network. The latency of VGSN approach is a little bit higher than the tight coupling but is much better than mobile IP.
[5] US patent 6,320,873 B1, “CDMA transmission of packetswitch data”
Average bandwidth per user (Kbps)
III. SIMULATION RESULTS
Number of User
Figure 11. Handover latency for different UMTS/WLAN interworking strategies.
Abstract - High tier wireless systems such as GPRS and UMTS provide users high mobility but less bandwidth. On the other hand, low tier wireless systems such as Wireless LAN offer high bandwidth with less mobility. To support seamless roaming between heterogeneous wireless networks is regarded as one of the key issues in the future mobile communication system. The paper presents method and system for seamless roaming between UMTS and WLAN networks. The two networks are assumed to construct and operate independently, but wish to offer roaming services to their customers. A new logical node, called Virtual GPRS Support Node (VGSN), is designed for this purpose to connect UMTS and WLAN core networks. VGSN is a normal GPRS Support Node (GSN) in a UMTS system and also acts as an access route in a WLAN network. The node converts control signals and routes data packets between two networks for roaming users. With the presented approach, mobile IP is not required and users can have a seamless roaming between networks based on one IP address.
present a number of approaches to interconnect heterogeneous networks [1]. However, their work is mainly to evaluate handoff efforts of different approaches but lacks of the system design of interworking strategies. ETSI BRAN project [8] further classifies the strategies into no coupling, tight coupling and loose coupling. [2][3][4][5] present several approaches to interconnect different IP-based radio access networks. They are basically tight coupling approaches. Stemm et al. [9] and 3GPP [6][7] suggest integrating the networks by using mobile IP. However, mobile IP introduces serious latency and packet lost for frequent roaming users in between two networks frequently.
Keywords – UMTS, WLAN, Packet Routing, Gateway, Interworking, Handover I. INTRODUCTION
Figure 1. Connecting UMTS and WLAN networks.
A number of wireless network systems have been proposed and deployed during the past few years. It is believed that multiple standards will coexist in the same environment for the future wireless communication system. To provide seamless roaming between heterogeneous networks becomes more and more important in the future mobile communication system. Different radio access networks have their own properties. High-tier system such as General Packet Radio Service (GPRS) and Universal Mobile Telecommunication System (UMTS) provides high mobility but with less data transmission bandwidth. On the other hand, low-tier system such as wireless local area network (Wireless LAN, WLAN) provides high data bandwidth but with less mobility. For the hot spot area such as a building, a station or an airport, it is normally covered by number of networks. Therefore, it is important to consider dual mode users roaming in between high tier and low tier wireless networks. UMTS and WLAN interworking is regarded as an important working item for 3G beyond systems. Figure 1 shows that two different wireless networks are connected based on current standards/specifications. Pahlavan et al.
A new logical node, called Virtual GPRS Support Node, is introduced in this paper to interconnect UMTS and WLAN backbones. The logical node acts a GPRS Support Node in UMTS and is an access router in WLAN. The node exchanges necessary information between networks, converts signals, and forward the packets for roaming users. Our approach aims to separate the operations of the two networks. In other words, two networks are peer networks and handle their single mode subscribers independently. For two network operators have a roaming agreement, VGSN helps them to enable intersystem roaming in two networks. The merits of this approach are that the two networks can be operated independently and mobile IP is not necessary. The packets for roaming users go through VGSNs to Internet without processing by mobile IP. The design reduces packet lost and delay. The rest of the paper is organized as follows. Section 2 proposes the VGSN. Section 3 shows and discusses simulation results. Finally, Section 4 concludes this study.
0-7803-7589-0/02/$17.00 ©2002 IEEE
PIMRC 2002
II. VIRTUAL GPRS SUPPORT NODE Several approaches can be applied to interconnect UMTS and WLAN networks. The paper aims to enable dual mode devices roaming in two wireless networks that are separately operated. Assumptions in this paper are listed below. (1). A user has a dual mode device. In other words, a device has two radio access interfaces. (2). Users with the dual mode devices subscript a data service of one network can use the radio access network of the other wireless network if the two networks have a roaming agreement. (3). Applications use only one IP address to access Internet. (4). Services, transactions and IP connections will not break during the handovers between networks. (5). Mobile IP is not required. (6). Two networks are peer networks. Two networks are managed and operated separately. For single mode users, traffics to/from a network will handle by the network itself without involving the other network. Figure 2 and Figure 3 depict the packet flows for UMTS users roaming to a WLAN and WLAN users roaming to a UMTS network respectively. A new logical node, called virtual GPRS support Node (VGSN), is presented in this paper. The logical entity can be either implemented as a separated node or integrated with Gateway in WLAN or SGSN or GGSN nodes. Since two networks are managed separately, we can classify the scenarios into two basic categories. The first case is a UMTS subscriber (we also use UMTS users or UMTS mobile station (MS) in this paper) attaches to a UMTS network and moves to a WLAN. It might handover to WLAN to have a higher data transmission rate. Figure 2 shows the scenario. We assume the IP address allocated to the users (or mobile station (MS)) will not change during the whole period of data service in the different network. In order to achieve this, traffic from MS to Internet will go through the UMTS network. For the MS moves to a WLAN, the incoming packets go to the UMTS network through the VGSN to the WLAN and ended at the MS. The out-going packets can go through WLAN directly. Another scenario is a WLAN user registering in the WLAN but might moves to a UMTS network. Figure 3 shows the case. In this case, incoming/outgoing packets must go through the VGSN if a WLAN MS camps in a UMTS network. The two cases are presented below.
Figure 2. UMTS users use WLAN as a radio carrier.
Figure 3. WLAN users use UMTS as a radio carrier. UMTS to WLAN roaming is first presented. Here, we assume a MS is registered in a UMTS network and the UMTS network has a roaming agreement with WLAN operators. Figure 4 shows the procedures of UMTS to WLAN roaming. From the step 1 to the step 3, these are normal UMTS attach/PDP context activation procedures. At the step 4, a MS finds a WLAN is accessible and wants to handover to a WLAN. The step 4 performs L1/L2 handover procedures. In the step 5, the MS tries to obtain a VGSN address in order to perform following procedures. An example to resolve a VGSN address in a WLAN environment is to use DHCP (dynamic host configuration protocol). The step 5.1 sends a DHCPDISCOVER to ask VGSN address in the WLAN network. The VGSN responses the MS with its IP address. After the MS obtains the VGSN address, it sends routing area update (RA update) to the VGSN using its original IP address of the UMTS network. Then, the VGSN sends standard Update PDP contexts Request to a GGSN to ask a GGSN changing its SGSN address-in-use. At this period, a VGSN simulates a SGSN in a UMTS network. Once a GGSN receives PDP context request from a VGSN, it knows the MS moves to a WLAN environment. The VGSN becomes the SGSN temporally. Packets to the MS should go to the VGSN instead of the old SGSN. Since the MS might moves back to the UMTS network, data stored in the SGSN cannot be deleted even it moves to a WLAN temporally. The GGSN sends a new packet data protocol/mobility management context standby command to the old SGSN. The message is to ask the SGSN to hold the PDP/MM context till the MS comes back to the UMTS or detaches. After the whole roaming procedures, the MS sends the packets out using its original IP address of the UMTS network. For the packets from the MS to Internet, they can be sent through the WLAN network if the WLAN does not perform ingress filtering on these UMTS IP address. If the WLAN applies ingress filtering on non-WLAN IP addresses, packets should go to the VGSN and then to Internet. For these packets to the MS, it should go to the UMTS network based on IP routing. Then, the GGSN recognizes the MS moves to the VGSN and it tunnels packets to the VGSN. There are several implementation choices of tunnelling data, i.e. GPRS tunnelling protocol (GTP) tunnelling and IP in IP
tunnelling. In order to reduce tunnelling overhead, IP in IP tunnel between VGSN and GGSN is suggested. The procedures are summarized as : Step 1 to Step 3 : Normal UMTS attach and PDP context activation procedures.
the GGSN will be forwarded to the new SGSN instead of the VGSN.
MS
Step 4 : L1/L2 handover procedures.
WLAN AP1
Step 8 : GGSN sends PDP/MM context standby to the old SGSN. The message to SGSN is to present the deletion of PDP/MM context in SGSN. Step 9.1 : Packets send from MS to Internet hosts. Step 9.2 : Packets from internet hosts to MS through VGSN.
GGSN
Virtual GSN
Host
1.1 Attach Req. 1.2 Attach Acc. 2.1 Activate PDP Context Req. (IP: NULL, APN:GGSN) 2.2 Active PDP Context Activation Acc. (IP_GPRS) 3. Packet transmission(IP: IP_GPRS)
3. RA update procedures 4. Packet transmission(IP: IP_GPRS)
Figure 5. WLAN to UMTS handovers for UMTS MSs (back to the same RA). Another case is that the MS moves to another RAs. In this case, the MS sends a standard RA update to the new SGSN and then involves a standard inter-SGSN update procedure. To support this type of handover, the MS is assumed to store the information of the original SGSN information. The original SGSN is the SGSN that the MS first moves to the WLAN.
MS
4. L1/L2 handover procedures
(IP in IP tunnel)
2. L1/L2 handover procedures
Step 7 : VGSN simulates SGSN to send Update PDP Context Request and responses Update PDP Context Request to GGSN.
SGSN
Host
1.2 Packet transmission(IP: IP_GPRS) : downlink
Step 6 : MS sends a RA Update to VGSN to handover to VGSN.
WLAN AP1
Virtual GSN
GGSN
1.1 Packet transmission(IP: IP_GPRS) : uplink
Step 5 : Procedures to obtain VGSN IP address. DHCP is used here as an example.
MS
SGSN
WLAN AP1
New SGSN
5.1 DHCPDISCOVER(VGSN) 5.2 DHCPOFFER(VGSN_IP) 6. RA Update
Old SGSN
Virtual GSN
GGSN
Host
1.1 Packet transmission(IP: IP_GPRS) : uplink 1.2 Packet transmission(IP: IP_GPRS) : downlink
7. Update PDP Context Req./Res. 8. PDP/MM context Standby 9.1 Packet transmission(IP: IP_GPRS) : uplink
(IP in IP tunnel)
2. L1/L2 handover procedures 3. Inter SGSN RA update procedures
9.2 Packet transmission(IP: IP_GPRS) : downlink 4. Packet transmission(IP: IP_GPRS)
(IP in IP tunnel)
Figure 4. UMTS to WLAN roaming. After UMTS users handover to a WLAN environment, it might move to another WLAN cells or go back to UMTS networks. For the MS handovers from one WLAN base station to another WLAN base station, it only introduces Layer 1(L1) and Layer 2(L2) procedures. In that way, both incoming and outgoing packets go through the same path. The only difference is that the final WLAN access node is different. The handover procedures are done by the WLAN L1/L2. Another situation is that the MS moves from the WLAN back to a UMTS network. Since PDP/MM context are buffered in the SGSN that the MS first moves out to the WLAN, the handover procedures involve that the old SGSN. There are two possible cases. First one is the MS goes back to the same routing area (RA). The procedure is shown in Figure 5. The step 3 is a normal RA update procedure. It only updates SGSN-in-use in the GGSN. Then, packets to
Figure 6. WLAN to UMTS handovers for UMTS MSs (back to the different RA).
MS
WLAN AP1
SGSN
GGSN
Virtual GSN
1.Packet transmission(IP: IP_WLAN) 2. L1/L2 handover procedures 3.1 Attach Req. 3.2 Attach Acc. 4.1 Activate PDP Context Req. (IP: IP_WLAN, APN:VGSN) 4.2 Active PDP Context Activation Acc. (NULL) 5.Packet transmission(IP: IP_WLAN)
Figure 7. WLAN to UMTS roaming.
Host
Another handover category is a WLAN user handover from WLAN to UMTS networks. Here, we assume a MS is registered in a WLAN network and the WLAN has a roaming agreement with a UMTS network. Here, we also assume MS is equipped with a dual mode device. Moreover, the MS must be also a subscriber of the UMTS network. The reason is that the MS can access UMTS radio access network only if it is the subscriber of the UMTS network. Figure 7 shows the step-by-step procedures of WLAN to UMTS roaming. A WLAN user first connects to a WLAN network and starts to access Internet. The IP address belongs to the WLAN. Once the MS detects the signal received from the UMTS network stronger than the WLAN, it might decide to handover to the UMTS network. If it has already attached to the UMTS network and in the same RA, it can just start the service. If it is the first time to attach to the network, it should perform attach and PDP context activation and then can start the service. In the attach procedures, MS uses VGSN as its access point name (APN) to inform the SGSN that it wants to use WLAN IP. During the PDP context activation procedure, the MS use the WLAN IP to request the PDP context. Once the VGSN detects the IP is WLAN IP and the security process is passed, it responses the MS with the same WLAN IP. The MS can use the same IP address that was used in the WLAN. Here, VGSN simulates the GGSN in a UMTS network. The SGSN will send packets to the VGSN and the VGSN sends packet to the SGSN. Both incoming and out-going packets go through the same path. Internet hosts send packets to the WLAN MS will go to the WLAN network, received by the VGSN, send to the SGSN and ended to the MS. The detail procedures are listed as : Step 1 : Packets transmission through WLAN. Step 2 : L1/L2 handover procedures. Step 3 : Standard UMTS attach procedures (not necessary if MS has attached to UMTS network). Step 4.1 : Standard UMTS PDP context activation procedures (not necessary if MS has activated a PDP context for WLAN/UMTS roaming). MS uses VGSN as its APN and asks to use the same IP address in WLAN network. SGSN receives the message and passes it to its local VGSN. Step 4.2 : VGSN receives the commands and checks that if the MS is allow to use the same IP address in the WLAN. Then, it replies the MS. If the admission of the MS is passed by the VGSN, the VGSN will then handle the packet forwarding of the MS packets. Step 5 : Packets from MS to SGSN will go to its GGSN (i.e. VGSN) to the WLAN and Internet. Packets from Internet hosts to the MS will go to WLAN first, and then be routed to VGSN and SGSN to the MS. To update the routing table between WLAN internal routers, one approach is to use proxy ARP to enforce update the packet routing to the VGSN. Another approach is using tunnelling approach. In
other words, packets to the MS should be tunnelled from the original router to the VGSN and then send to the SGSN.
MS
WLAN AP1
Virtual GSN
SGSN #2
SGSN #1
Host
1.Packet transmission(IP: IP_WLAN) 2. GPRS/WLAN handover procedures 3.Packet transmission(IP: IP_WLAN) 4. L1/L2 handover procedures 5.1 Routing Area Update Req. 5.3 Update PDP Context Req./Res. 5.4 Routing Area Update Acc./Comp. 6.Packet transmission(IP: IP_WLAN)
Figure 8. UMTS to UMTS handovers for WLAN MSs. As for the WLAN user moves from the UMTS back to the WLAN, the case only involves WLAN L1/L2 procedures. For the WLAN user moves from one RA to another RA, it has two situations. The first situation is the two RAs are controlled by the same SGSN. The MS initiates a standard RA update procedure. It introduces no change to the intersystem roaming. The second situation is the two RAs are controlled by two SGSNs. It requires updating the current SGSN-in-use in the VGSN. Figure 8 shows the scenario. The procedures are normal inter SGSN RA update procedure. The VGSN here has to simulate a GGSN and updates the new SGSN address.
MS
WLAN AP1
SGSN
GGSN
Virtual GSN
Host
Host #2
1.Packet transmission(IP: IP_WLAN) 2. L1/L2 handover procedures 3.1 Attach Req. 3.2 Attach Acc. 4.1 Activate PDP Context Req. (IP: IP_WLAN, APN:VGSN) 4.2 Active PDP Context Activation Acc. (NULL) 5.Packet transmission(IP: IP_WLAN) 6.2 Request PDP Context Act.
6.1 Incoming packet
7.1 Activate PDP Context Req. (IP: NULL, APN:GGSN) 7.2 Active PDP Context Activation Acc. (IP_GPRS)
Figure 9. UMTS paging for WLAN MSs. Regarding of a WLAN user locating in a UMTS network, some hosts in the Internet want to access the WLAN users by their UMTS addresses. The packets from hosts to the MS will first handled by the GGSN. Since the original PDP context maintained in the SGSN and the VGSN are for the WLAN roaming not for UMTS service. A new PDP must be allocated to handle the new UMTS connection. Step 6 and 7 in Figure 9 shows the case. In other words, the present approach uses two separated PDP contexts to handle (1) WLAN address in WLAN to UMTS roaming service (2) UMTS address for UMTS service. Figure 9 shows the detail message flow.
[6] ETSI EN GSM 03.60, “General Packet Radio Service (GPRS), Service Description, Stage 2”, April 2000. [7] 3GPP TS 23.060, “General Packet Radio Service (GPRS), Service Description, Stage 2”, Dec. 2001. [8] ETSI BRAN Project, http://portal.etsi.org/bran/home.asp. [9] Mark Stemm and Randy H. Katz, “Vertical Handoffs in Wireless Overlay Networks”, Mobile Networks and Applications, Vol. 3, No. 4, pp. 335-350, 1998.
UMTS/WLAN Gateway (15Mbps) 70
40 30 20 10
Figure 10. Average bandwidth per user for different UMTS/WLAN interworking strategies. UMTS/WLAN Gateway (15Mbps) 450
350 300
Tight coupling VGSN Mobile IP
250 200 150 100 50
10
Handover Delay (ms)
400
[1] Kaveh Pahlavan et al., “Handover in Hybrid Mobile Data Networks”, IEEE Personal Communications, April 2000.
[4] US patent 6,292,891 B1, “Method of connecting base station to cellular system”
91 0 12 10 15 10 18 10 21 10 24 10
Number of User
0
[3] US patent 6,314,108 B1, “Method and apparatus for providing network access over different wireless networks”
31 0 61 0
10
0
REFERENCES
[2] US patent 6,243,581, “Method and system for seamless roaming between wireless communication networks with a mobile terminal”.
Mobile IP/VGSN
50
IV. CONCLUSIONS A gateway approach to connect UMTS and WLAN networks was proposed in this paper. A logical node, call Virtual GPRS Support Node (VGSN), is designed to convert control signals and to route data packets between two networks for roaming users. With the presented approach, mobile IP is not required and users can have a seamless roaming between networks based on single IP address. Simulation results show that our approach obtains a similar performance but reduces handover latency significantly comparing with mobile IP approach.
Tight coupling
60
51 0 10 10 15 10 20 10 25 10 30 10 35 10 40 10 45 10
Simulations are conducted to examine the performance of the proposed approach. The simulation environment is covered by a UMTS network and WLAN. The UMTS network has 100Mbps backbone with 5 radio network subsystems and offers 32Kbps data services. WLAN has 25 access points and provides 100Kbps data service. We assume 50% registers as WLAN users and 50% registers as UMTS users. Among all users, 50% are dual mode users and might moves in between two networks, and the other 50% are single mode users using either WLAN or UMTS. We assume dual mode users have 50% to enter the WLAN and 50% to enter the UMTS. We compare three different approaches, i.e. mobile IP, VGSN and tight coupling (WLAN acts as the radio access network of UMTS). Figure 10 shows the average bandwidth per user. We learn that mobile IP and VGSN approaches obtain the same the average bandwidth that is much better than the tight coupling approach. The reason is that packets of tight coupling approach always need to go to the UMTS network that degrades the system performance. Mobile IP and VGSN approaches separate single mode users and roaming users. It much improves the scalability and performance. Figure 11 shows the handover latency. Mobile IP has the poorest performance since the signaling packets need to go to Internet (home agent and foreign agents). VGSN and tight coupling approaches only involve the message exchange within intra-network. The latency of VGSN approach is a little bit higher than the tight coupling but is much better than mobile IP.
[5] US patent 6,320,873 B1, “CDMA transmission of packetswitch data”
Average bandwidth per user (Kbps)
III. SIMULATION RESULTS
Number of User
Figure 11. Handover latency for different UMTS/WLAN interworking strategies.
