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Hybrid coupling scheme for UMTS and wireless LAN interworking
Int. J. Electron. Commun. (AEÜ) 61 (2007) 329 – 336 www.elsevier.de/aeue
LETTER
Hybrid coupling scheme for UMTS and wireless LAN interworking Jee-Young Songa,∗ , Hye Jeong Leea , Sun-Ho Leeb , Sung-Won Leeb , Dong-Ho Choa a Department of EECS, Korea Advanced Institute of Science and Technology, 373-1 Guseong-Dong, Yuseong-Gu, Daejeon 305-701,
Republic of Korea b Telecommunication Network Business, Samsung Electronics Co., Ltd., Suwon, Republic of Korea
Received 11 November 2005
Abstract We propose a hybrid coupling scheme to support interworking between UMTS and WLAN networks. Under the Tightcoupled system, it is expected that WLAN users can also use UMTS services with guaranteed QoS and seamless mobility. However, the interworking is problematic. The capacity of UMTS core network nodes cannot accommodate the bulky data traffic from WLAN, since the core network nodes are designed to handle the small-sized data of circuit voice calls or short packets. The proposed coupling scheme differentiates the data paths according to the type of the traffic and can accommodate traffic from WLAN efficiently, with guaranteed QoS and seamless mobility. We compare the handover procedures of the proposed coupling strategy with those of the loose and tight coupled schemes. In addition, we analyze the delay based on signaling costs during vertical handover. It is shown that the handover latency decreases when the UMTS and WLAN are coupled in the proposed way. 䉷 2006 Elsevier GmbH. All rights reserved. Keywords: UMTS; WLAN; Interworking; Handover latency
1. Introduction Recently, wireless LANs (WLAN) have operated as complementary solutions to cellular networks and have enabled high-speed wireless internet access. It is expected that the connection between high data-rate WLAN with narrow coverage and low data-rate cellular networks with broad coverage could support better services for users [1]. 3GPP introduced six scenarios for interworking between UMTS and WLAN in [2] and there have been discussions about functionality and architecture to couple WLAN with UMTS, especially with respect to scenario 2 and 3. One of the ∗ Corresponding author. Tel.: +82 42 869 8067; fax: +82 42 867 0550.
E-mail addresses: [email protected] (J.-Y. Song), [email protected] (H.J. Lee), [email protected] (S.-H. Lee), [email protected] (S.-W. Lee), [email protected] (D.-H. Cho). 1434-8411/$ - see front matter 䉷 2006 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2006.06.003
outcomes of those discussions in Release 6 is described in [3]. Discussions to standardize scenario 4 and beyond are expected to be continued in Release 7. Refs. [4–6] summarize recent results about UMTS-WLAN interworking well. Also, standardization activity and research about interworking between CDMA2000 and WLAN are summarized in [7,8]. European Telecommunications Standard Institute (ETSI) defined two approaches for the interworking of WLAN and cellular networks: tight-coupled and loose-coupled. The main difference between tight coupling and loose coupling is whether or not the user’s traffic is delivered through the core network of UMTS. That is, when UMTS and WLAN are tightly coupled, traffic from the WLAN flows into the core network of the UMTS and out to the external packet data network (PDN) via a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN). In contrast, in the loosely coupled case, WLAN does not share any of the core network nodes of UMTS except for
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External PDN
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Fig. 2. Interworking architecture of tight coupling scheme. Fig. 1. Interworking architecture of loose coupling scheme.
the authentication, authorization and accounting (AAA) functionality. Through tight coupling scheme, users from the WLAN can access the UMTS services with guaranteed QoS and seamless mobility. However, it is a problem that the capacity of the UMTS core network nodes cannot accommodate the bulky data traffic from the WLAN, since the core network nodes are designed to handle the small-sized data of circuit voice calls or short packets. When WLAN and UMTS are loosely coupled, each network operates independently and is connected to the other via the Gi reference point, where the GGSN is connected to an external PDN. The architecture of a loose-coupled network is shown in Fig. 1. Since each network operates independently, under the loose coupling scheme, networks do not need to change their network architectures or protocol stacks. However, a loose-coupled network cannot support service continuity during handover to other access networks, and this behavior causes significant handover latency and packet loss. Fig. 2 shows the network architecture when the WLAN and the UMTS are tightly coupled. The WLAN is connected to the UMTS through an Iw -interface which is similar to the Iu_ps -interface connecting a radio network controller (RNC) to an SGSN. To support the Iw -interface and connect the WLAN to an SGSN, a new node named access point gateway (APGW) should be added. In the core network of the GPRS, the WLAN seems to be a kind of radio access network. In this paper, we propose a new interworking scheme with lower handover latency and lower packet loss probability,
and evaluate its performance. The rest of this paper is organized as follows. In Section 2, the proposed interworking scheme between UMTS and WLAN is described. Handover procedures of three coupling schemes are analyzed in Section 3. Some numerical results about handover latency are provided in Section 4. Section 5 presents conclusions.
2. Proposed network architecture We propose a hybrid coupling scheme that differentiates traffic paths according to the type of the traffic. For real-time traffic, the tightly coupled network architecture is chosen, and for non-real time and bulky traffic, the loosely coupled network architecture is chosen. For example, traffic generated from the session initiation protocol (SIP) would be delivered along the path APGW-SGSN-GGSN to an application server in an external PDN, but FTP traffic would be forwarded to access routers (AR). The network architecture for the proposed coupling scheme is shown in Fig. 3. The proposed coupling scheme could accommodate traffic from WLAN efficiently with guaranteed QoS. In addition, it guarantees mobility and provides seamless service, just as does the tightly coupled scheme, while users are moving or need to change their access technologies. Similar to the tight-coupled network, APGW should be added to connect the WLAN to the UMTS network. The functions of APGW are as follows: 1. Forwarding packets to/from an access point (AP) from/to the SGSN or AR.
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External PDN GGSN
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Gi Gn
(WLAN) Real time traffic
SGSN
AR Iu_ps * RNC Node B
(WLAN) Non-real time traffic
APGW Node B
AP
MS
AP
AP *Iw- interface signaling path data path
Fig. 3. Interworking architecture of hybrid coupling scheme.
2. Support of Iu_ps -like interface: radio access network application part (RANAP)-like signaling protocol and Iu -interface data bearer transport are needed. 3. Management of radio resources in the WLAN and mapping them onto radio resources in the cellular network. 4. Setting data path to SGSN or AR according to the type of traffic. 5. Differentiation of service types: the user equipment (UE) sets its service type by using the type of service (TOS) or DiffServ Code Point (DSCP) field of the IP header. APGW checks this field to decide the traffic path.
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When a WLAN and UMTS are coupled by the proposed scheme, it is possible to support seamless handover as in the tight coupling case, using an inter-RNC handover-like procedure. In this case, the Iu -bearer between the SGSN and the radio access network can be reused and handover does not need any procedure to set up new bearers, so latency, dropping probability and packet loss probability during handover would decrease. In addition, the Hybrid coupling scheme supports UMTS services such as the location based service (LCS), and the broadcast/multicast service (MBMS), to users in the WLAN. To support the IP-level mobility of the user for real-time and non-real time traffic, it is assumed that the Mobile IP and its simultaneous binding option are used. That is, when a user accesses the network through a WLAN, it belongs to two Foreign Agents (FA). Then, for real-time traffic of the WLAN users, an FA in the UMTS acts as the FA of those users, and in the other case, the FA in the WLAN acts as the FA. To use the simultaneous binding option, the home agent (HA) should have the ability to support this option and a user sets the s-bit in the Mobile IP header to 1. The packet transmission procedure is the same as in the case with no simultaneous binding option. To receive packets, two kinds of operation are possible: (1) the user receives the same packets twice from two FAs. This is the default operation of simultaneous binding, but this option wastes network resources. (2) the operation needs to use TOS or DSCP in IP header. Users set the TOS or DSCP field according to their traffic type, then the HA selects one of two FAs based on the value of the field and forwards the packets to the corresponding FA.
3. Handover procedures 3.1. Loose coupling
Functions (1)–(3) are also required for the tight coupling scheme, but the functions related to differentiating service types and paths, such as (4) and (5), are additionally required for implementation of the hybrid coupling scheme. In a Loose-coupled network, real-time traffic cannot be assured to have appropriate delay and jitter when forwarded through a WLAN and the internet. Because of long handover latency, traffic experiences large packet loss, and even whole packet calls may be blocked. However, when connecting a WLAN to a UMTS network using the proposed Hybrid coupling scheme, it is possible to support QoS for real-time traffic and service continuity during vertical handover. When the tight-coupling scheme is applied, handover latency and packet loss may decrease, but if bulky data traffic, for example, FTP traffic, flows into the cellular core network from the WLAN, the packet loss rate in the core network would increase abruptly. However, the proposed coupling scheme can prevent core network nodes from experiencing traffic overflow by rerouting non-real time traffic of the WLAN and not sharing the capacity of core network nodes.
To support IP-level mobility in a loose-coupled network, it is assumed that the Mobile IP (MIP) is applied and that the WLAN and the UMTS networks have their own FAs. (A GGSN acts as an FA in UMTS, and the HA and the Correspondent Node (CN) are located in external IP networks.) The handover procedures to/from WLAN from/to UMTS are shown in Figs. 4 and 5. 3.1.1. Handover to the WLAN (Fig. 4) A UE turns its power on, and is associated with an AP. Then, the UE and the AP start the 802.1x authentication procedure. After completing authentication successfully, an IP address is assigned to the UE for MIP registration. Using this IP address, the UE registers to the FA in the WLAN and restarts transmitting and receiving packets. 3.1.2. Handover to the UMTS (Fig. 5) First, a UE sets up an radio resource control (RRC) connection with an RNC and starts authentication and ciphering.
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Packet transmission/reception Deactivate PDP Context (if required) RAB Assignment (Deletion) (if required) Release RB, RL, RRC Connection (if required)
Fig. 4. Handover to WLAN in loose coupling scheme.
After successful setup of the RRC connection, the UE starts the GPRS attach procedure and requests the activation of a PDP context. Then, an SGSN starts to set up an Iu bearer to the RNC, and requests that the RNC assigns a radio access bearer (RAB) and that the GGSN creates a PDP context with the UE. The RNC initiates the setup of a radio bearer (RB) and additional radio links (RL) for the PDP context. With the IP address assigned by the GGSN, the SGSN sends back an Activate PDP Context message to the UE. Now, the UE can transmit IP packets using the new PDP context with the assigned IP address. To receive packets from the home network, the UE sends a Registration Request message using MIP, then the FA (GGSN) performs MIP registration to the HA of the UE. Thereafter, the UE can receive IP packets forwarded by the HA. When the UE leaves the UMTS network and moves to the WLAN, it is possible for the PDP
context or the signaling connections to remain active, to enable a return to the UMTS upon completion of the highrate data transmission in the WLAN. This operation could save signaling costs, since the UE can skip long and complicated setup procedures. So, it is also expected that handover latency and packet loss would decrease.
3.2. Tight coupling 3.2.1. Handover to the WLAN (Fig. 6) The RNC decides whether or not to start handover based on the measurement results from the UE, and if required, it sends a Relocation Required message to the SGSN. Then, the SGSN sends Handover Required to the appropriate APGW. The UE is associated with an AP, and the SGSN requests the APGW to begin the radio access bearer setup
J.-Y. Song et al. / Int. J. Electron. Commun. (AEÜ) 61 (2007) 329 – 336
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Agent Solicitation & Advertisement MIP registration Registration Request & Reply
Packet transmission / reception
Fig. 5. Handover to UMTS in loose coupling scheme.
procedure. After completing the bearer setup procedure, the SGSN requests the GGSN to update the PDP context to the UE. 3.2.2. Handover to the UMTS (Fig. 7) Handover to the UMTS is similar to handover to the WLAN. After deciding upon handover to the UMTS, the APGW sends a Relocation Required message to the SGSN to start relocation procedures. After successful setup of a PDP context, which means RRC connections and RAB-including a RB, RLs, an Iu bearer are set up successfully, the UE and the GGSN restart transmitting and receiving packets.
3.3. Hybrid coupling The vertical handover procedures in a Hybrid-coupled network are similar to the procedures in a tight-coupled network. The only difference is whether or not the Mobile IP and the simultaneous binding option are used. In a
tight-coupled network, it is possible to support a user’s IP mobility using the tunneling protocols without using the MIP. Of course, the MIP can also be applied in a tightcoupled network, but even in that case, handover between the different the access networks (i.e. the UMTS and the WLAN) does not need to change the FA since those access networks belong to the same FA (GGSN). So, the vertical handover between the UMTS and the WLAN is an intraFA handover, as are other inter-RNC handover or inter-AP (APGW) handovers, which does not need to change the user’s IP address and to register to an FA or the HA. However, in a Hybrid-coupled network, we use the MIP and the simultaneous binding option to support IP mobility for both of the two traffic paths. In case of the UMTS network, a UE belongs to the FA of the UMTS network (i.e. the GGSN) only, but in the case of the WLAN, it belongs to two FAs: the FA of the UMTS and the FA of the WLAN. So, when a user moves from the WLAN to the UMTS, it does not need to register to the GGSN because it has already registered, as shown in Fig. 7. But when moving to the WLAN, it is
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HO Required HO Required. ACK Relocation Command HO command RAB assignment Req. Probe Req. Associate Req. Notification RAB assignment Resp. Notification.ACK Relocation Complete
PDP context update
Update PDP Context Req. & Resp. Agent Solicitation & Advertisement
MIP reg. Registration Request & Reply
Packet transmission / reception Iu Release Cmd. & Comp. (if required)
Fig. 6. Handover to WLAN in tight coupling scheme.
necessary to perform additional registeration to the FA in the WLAN. This additional registration procedure is shown in Fig. 6 as arrows with dotted line.
4. Numerical results We define the transmission delay in one wireless hop as Twireless , and the transmission delay in between wired nodes as Twired . Figs. 8 and 9 show the total handover latency based on signaling cost when Twired and Twireless vary. When the WLAN and UMTS networks are loose-coupled, the 802.1x authentication procedure during handover to the WLAN and the UMTS call setup procedure during handover to the UMTS have the largest portion of each handover procedure. In tight-coupled and hybrid-coupled networks, 802.1x authentication is processed during initial registration after power-on, and does not need to be processed again during handover. In addition, in a loose-coupled network, when a user moves to the UMTS, the GPRS attach and PDP context
activation including RRC connection setup, as well as RAB assignment, should be done during handover. However, in tight-coupled and hybrid-coupled networks, there is no need to process those procedures because it is possible to reuse the signaling and data bearers assigned and used in previous access networks. This explains the gap in signaling costs between the loose-coupled case and the other cases. In the both tight-coupled and hybrid-coupled cases, a handover user experiences the same procedures. However, in case of the handover to the WLAN, a user in a hybridcoupled network has to register to the FA in the WLAN additionally, which results in increased signaling costs in the case of a hybrid-coupled network.
5. Conclusions We proposed a hybrid coupling scheme for interworking between the UMTS and WLAN, and compared its handover procedures and the signaling costs during handover to previous interworking schemes, such as the loose coupling and
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PDP context update
Packet transmission / reception Iw Release Cmd. & Comp. (if required)
Fig. 7. Handover to UMTS in tight coupling scheme.
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Fig. 8. Handover latency in case of Twireless = 10 ms.
Fig. 9. Handover latency in case of Twired = 10 ms.
the tight coupling scheme. When the UMTS and WLAN are connected by the hybrid coupling scheme, it is expected that the packet loss probability will be lower than that in the tight-coupling, and it is shown that the total signaling
cost during handover decreases compared to the case of the loose-coupled network. That is, with the hybrid coupling scheme, the interworking network accommodates real-time traffic from the WLAN efficiently.
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References [1] Salkintzis AK. et al. WLAN-GPRS integration for next-generation mobile data networks. IEEE Wireless Communications 2002;9(5):112–24. [2] 3GPP TR 22.934. Feasibility study on 3GPP system to Wireless Local Area Network (WLAN) interworking. December 2002. [3] 3GPP TR 23.234. 3GPP system to Wireless Local Area Network (WLAN) interworking; Functional and architectural definition. June 2003. [4] Salkintzis AK. Interworking between WLANs and third-generation cellular data networks. IEEE Wireless Communications 2002;9(5):112–24.
[5] Ahmavaara K, Haverinen H, Pichna R. Interworking architecture between 3GPP and WLAN systems. IEEE Communications Magazine 2003; 41(11):74–80. [6] Koien GM, Haslestad T. Security aspects of 3G-WLAN interworking. IEEE Communications Magazine 2003; 41(11):82–8. [7] 3GPP2 S.P0087-0. 3GPP2-WLAN Interworking Stage 1 Requirements. July 2003. [8] Buddhikot MM. et al. Design and implementation of a WLAN/CDMA2000 interworking architecture. IEEE Communications Magazine 2003;41(11):90–100.
LETTER
Hybrid coupling scheme for UMTS and wireless LAN interworking Jee-Young Songa,∗ , Hye Jeong Leea , Sun-Ho Leeb , Sung-Won Leeb , Dong-Ho Choa a Department of EECS, Korea Advanced Institute of Science and Technology, 373-1 Guseong-Dong, Yuseong-Gu, Daejeon 305-701,
Republic of Korea b Telecommunication Network Business, Samsung Electronics Co., Ltd., Suwon, Republic of Korea
Received 11 November 2005
Abstract We propose a hybrid coupling scheme to support interworking between UMTS and WLAN networks. Under the Tightcoupled system, it is expected that WLAN users can also use UMTS services with guaranteed QoS and seamless mobility. However, the interworking is problematic. The capacity of UMTS core network nodes cannot accommodate the bulky data traffic from WLAN, since the core network nodes are designed to handle the small-sized data of circuit voice calls or short packets. The proposed coupling scheme differentiates the data paths according to the type of the traffic and can accommodate traffic from WLAN efficiently, with guaranteed QoS and seamless mobility. We compare the handover procedures of the proposed coupling strategy with those of the loose and tight coupled schemes. In addition, we analyze the delay based on signaling costs during vertical handover. It is shown that the handover latency decreases when the UMTS and WLAN are coupled in the proposed way. 䉷 2006 Elsevier GmbH. All rights reserved. Keywords: UMTS; WLAN; Interworking; Handover latency
1. Introduction Recently, wireless LANs (WLAN) have operated as complementary solutions to cellular networks and have enabled high-speed wireless internet access. It is expected that the connection between high data-rate WLAN with narrow coverage and low data-rate cellular networks with broad coverage could support better services for users [1]. 3GPP introduced six scenarios for interworking between UMTS and WLAN in [2] and there have been discussions about functionality and architecture to couple WLAN with UMTS, especially with respect to scenario 2 and 3. One of the ∗ Corresponding author. Tel.: +82 42 869 8067; fax: +82 42 867 0550.
E-mail addresses: [email protected] (J.-Y. Song), [email protected] (H.J. Lee), [email protected] (S.-H. Lee), [email protected] (S.-W. Lee), [email protected] (D.-H. Cho). 1434-8411/$ - see front matter 䉷 2006 Elsevier GmbH. All rights reserved. doi:10.1016/j.aeue.2006.06.003
outcomes of those discussions in Release 6 is described in [3]. Discussions to standardize scenario 4 and beyond are expected to be continued in Release 7. Refs. [4–6] summarize recent results about UMTS-WLAN interworking well. Also, standardization activity and research about interworking between CDMA2000 and WLAN are summarized in [7,8]. European Telecommunications Standard Institute (ETSI) defined two approaches for the interworking of WLAN and cellular networks: tight-coupled and loose-coupled. The main difference between tight coupling and loose coupling is whether or not the user’s traffic is delivered through the core network of UMTS. That is, when UMTS and WLAN are tightly coupled, traffic from the WLAN flows into the core network of the UMTS and out to the external packet data network (PDN) via a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN). In contrast, in the loosely coupled case, WLAN does not share any of the core network nodes of UMTS except for
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External PDN
External PDN GGSN
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BR
BR Gi
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Node B Node B
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APGW Node B
AP
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AP
Node B
AP
AP
AP
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signaling path data path
MS
*Iw- interface signaling path data path
Fig. 2. Interworking architecture of tight coupling scheme. Fig. 1. Interworking architecture of loose coupling scheme.
the authentication, authorization and accounting (AAA) functionality. Through tight coupling scheme, users from the WLAN can access the UMTS services with guaranteed QoS and seamless mobility. However, it is a problem that the capacity of the UMTS core network nodes cannot accommodate the bulky data traffic from the WLAN, since the core network nodes are designed to handle the small-sized data of circuit voice calls or short packets. When WLAN and UMTS are loosely coupled, each network operates independently and is connected to the other via the Gi reference point, where the GGSN is connected to an external PDN. The architecture of a loose-coupled network is shown in Fig. 1. Since each network operates independently, under the loose coupling scheme, networks do not need to change their network architectures or protocol stacks. However, a loose-coupled network cannot support service continuity during handover to other access networks, and this behavior causes significant handover latency and packet loss. Fig. 2 shows the network architecture when the WLAN and the UMTS are tightly coupled. The WLAN is connected to the UMTS through an Iw -interface which is similar to the Iu_ps -interface connecting a radio network controller (RNC) to an SGSN. To support the Iw -interface and connect the WLAN to an SGSN, a new node named access point gateway (APGW) should be added. In the core network of the GPRS, the WLAN seems to be a kind of radio access network. In this paper, we propose a new interworking scheme with lower handover latency and lower packet loss probability,
and evaluate its performance. The rest of this paper is organized as follows. In Section 2, the proposed interworking scheme between UMTS and WLAN is described. Handover procedures of three coupling schemes are analyzed in Section 3. Some numerical results about handover latency are provided in Section 4. Section 5 presents conclusions.
2. Proposed network architecture We propose a hybrid coupling scheme that differentiates traffic paths according to the type of the traffic. For real-time traffic, the tightly coupled network architecture is chosen, and for non-real time and bulky traffic, the loosely coupled network architecture is chosen. For example, traffic generated from the session initiation protocol (SIP) would be delivered along the path APGW-SGSN-GGSN to an application server in an external PDN, but FTP traffic would be forwarded to access routers (AR). The network architecture for the proposed coupling scheme is shown in Fig. 3. The proposed coupling scheme could accommodate traffic from WLAN efficiently with guaranteed QoS. In addition, it guarantees mobility and provides seamless service, just as does the tightly coupled scheme, while users are moving or need to change their access technologies. Similar to the tight-coupled network, APGW should be added to connect the WLAN to the UMTS network. The functions of APGW are as follows: 1. Forwarding packets to/from an access point (AP) from/to the SGSN or AR.
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External PDN GGSN
BR
Gi Gn
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SGSN
AR Iu_ps * RNC Node B
(WLAN) Non-real time traffic
APGW Node B
AP
MS
AP
AP *Iw- interface signaling path data path
Fig. 3. Interworking architecture of hybrid coupling scheme.
2. Support of Iu_ps -like interface: radio access network application part (RANAP)-like signaling protocol and Iu -interface data bearer transport are needed. 3. Management of radio resources in the WLAN and mapping them onto radio resources in the cellular network. 4. Setting data path to SGSN or AR according to the type of traffic. 5. Differentiation of service types: the user equipment (UE) sets its service type by using the type of service (TOS) or DiffServ Code Point (DSCP) field of the IP header. APGW checks this field to decide the traffic path.
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When a WLAN and UMTS are coupled by the proposed scheme, it is possible to support seamless handover as in the tight coupling case, using an inter-RNC handover-like procedure. In this case, the Iu -bearer between the SGSN and the radio access network can be reused and handover does not need any procedure to set up new bearers, so latency, dropping probability and packet loss probability during handover would decrease. In addition, the Hybrid coupling scheme supports UMTS services such as the location based service (LCS), and the broadcast/multicast service (MBMS), to users in the WLAN. To support the IP-level mobility of the user for real-time and non-real time traffic, it is assumed that the Mobile IP and its simultaneous binding option are used. That is, when a user accesses the network through a WLAN, it belongs to two Foreign Agents (FA). Then, for real-time traffic of the WLAN users, an FA in the UMTS acts as the FA of those users, and in the other case, the FA in the WLAN acts as the FA. To use the simultaneous binding option, the home agent (HA) should have the ability to support this option and a user sets the s-bit in the Mobile IP header to 1. The packet transmission procedure is the same as in the case with no simultaneous binding option. To receive packets, two kinds of operation are possible: (1) the user receives the same packets twice from two FAs. This is the default operation of simultaneous binding, but this option wastes network resources. (2) the operation needs to use TOS or DSCP in IP header. Users set the TOS or DSCP field according to their traffic type, then the HA selects one of two FAs based on the value of the field and forwards the packets to the corresponding FA.
3. Handover procedures 3.1. Loose coupling
Functions (1)–(3) are also required for the tight coupling scheme, but the functions related to differentiating service types and paths, such as (4) and (5), are additionally required for implementation of the hybrid coupling scheme. In a Loose-coupled network, real-time traffic cannot be assured to have appropriate delay and jitter when forwarded through a WLAN and the internet. Because of long handover latency, traffic experiences large packet loss, and even whole packet calls may be blocked. However, when connecting a WLAN to a UMTS network using the proposed Hybrid coupling scheme, it is possible to support QoS for real-time traffic and service continuity during vertical handover. When the tight-coupling scheme is applied, handover latency and packet loss may decrease, but if bulky data traffic, for example, FTP traffic, flows into the cellular core network from the WLAN, the packet loss rate in the core network would increase abruptly. However, the proposed coupling scheme can prevent core network nodes from experiencing traffic overflow by rerouting non-real time traffic of the WLAN and not sharing the capacity of core network nodes.
To support IP-level mobility in a loose-coupled network, it is assumed that the Mobile IP (MIP) is applied and that the WLAN and the UMTS networks have their own FAs. (A GGSN acts as an FA in UMTS, and the HA and the Correspondent Node (CN) are located in external IP networks.) The handover procedures to/from WLAN from/to UMTS are shown in Figs. 4 and 5. 3.1.1. Handover to the WLAN (Fig. 4) A UE turns its power on, and is associated with an AP. Then, the UE and the AP start the 802.1x authentication procedure. After completing authentication successfully, an IP address is assigned to the UE for MIP registration. Using this IP address, the UE registers to the FA in the WLAN and restarts transmitting and receiving packets. 3.1.2. Handover to the UMTS (Fig. 5) First, a UE sets up an radio resource control (RRC) connection with an RNC and starts authentication and ciphering.
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Fig. 4. Handover to WLAN in loose coupling scheme.
After successful setup of the RRC connection, the UE starts the GPRS attach procedure and requests the activation of a PDP context. Then, an SGSN starts to set up an Iu bearer to the RNC, and requests that the RNC assigns a radio access bearer (RAB) and that the GGSN creates a PDP context with the UE. The RNC initiates the setup of a radio bearer (RB) and additional radio links (RL) for the PDP context. With the IP address assigned by the GGSN, the SGSN sends back an Activate PDP Context message to the UE. Now, the UE can transmit IP packets using the new PDP context with the assigned IP address. To receive packets from the home network, the UE sends a Registration Request message using MIP, then the FA (GGSN) performs MIP registration to the HA of the UE. Thereafter, the UE can receive IP packets forwarded by the HA. When the UE leaves the UMTS network and moves to the WLAN, it is possible for the PDP
context or the signaling connections to remain active, to enable a return to the UMTS upon completion of the highrate data transmission in the WLAN. This operation could save signaling costs, since the UE can skip long and complicated setup procedures. So, it is also expected that handover latency and packet loss would decrease.
3.2. Tight coupling 3.2.1. Handover to the WLAN (Fig. 6) The RNC decides whether or not to start handover based on the measurement results from the UE, and if required, it sends a Relocation Required message to the SGSN. Then, the SGSN sends Handover Required to the appropriate APGW. The UE is associated with an AP, and the SGSN requests the APGW to begin the radio access bearer setup
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Fig. 5. Handover to UMTS in loose coupling scheme.
procedure. After completing the bearer setup procedure, the SGSN requests the GGSN to update the PDP context to the UE. 3.2.2. Handover to the UMTS (Fig. 7) Handover to the UMTS is similar to handover to the WLAN. After deciding upon handover to the UMTS, the APGW sends a Relocation Required message to the SGSN to start relocation procedures. After successful setup of a PDP context, which means RRC connections and RAB-including a RB, RLs, an Iu bearer are set up successfully, the UE and the GGSN restart transmitting and receiving packets.
3.3. Hybrid coupling The vertical handover procedures in a Hybrid-coupled network are similar to the procedures in a tight-coupled network. The only difference is whether or not the Mobile IP and the simultaneous binding option are used. In a
tight-coupled network, it is possible to support a user’s IP mobility using the tunneling protocols without using the MIP. Of course, the MIP can also be applied in a tightcoupled network, but even in that case, handover between the different the access networks (i.e. the UMTS and the WLAN) does not need to change the FA since those access networks belong to the same FA (GGSN). So, the vertical handover between the UMTS and the WLAN is an intraFA handover, as are other inter-RNC handover or inter-AP (APGW) handovers, which does not need to change the user’s IP address and to register to an FA or the HA. However, in a Hybrid-coupled network, we use the MIP and the simultaneous binding option to support IP mobility for both of the two traffic paths. In case of the UMTS network, a UE belongs to the FA of the UMTS network (i.e. the GGSN) only, but in the case of the WLAN, it belongs to two FAs: the FA of the UMTS and the FA of the WLAN. So, when a user moves from the WLAN to the UMTS, it does not need to register to the GGSN because it has already registered, as shown in Fig. 7. But when moving to the WLAN, it is
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Fig. 6. Handover to WLAN in tight coupling scheme.
necessary to perform additional registeration to the FA in the WLAN. This additional registration procedure is shown in Fig. 6 as arrows with dotted line.
4. Numerical results We define the transmission delay in one wireless hop as Twireless , and the transmission delay in between wired nodes as Twired . Figs. 8 and 9 show the total handover latency based on signaling cost when Twired and Twireless vary. When the WLAN and UMTS networks are loose-coupled, the 802.1x authentication procedure during handover to the WLAN and the UMTS call setup procedure during handover to the UMTS have the largest portion of each handover procedure. In tight-coupled and hybrid-coupled networks, 802.1x authentication is processed during initial registration after power-on, and does not need to be processed again during handover. In addition, in a loose-coupled network, when a user moves to the UMTS, the GPRS attach and PDP context
activation including RRC connection setup, as well as RAB assignment, should be done during handover. However, in tight-coupled and hybrid-coupled networks, there is no need to process those procedures because it is possible to reuse the signaling and data bearers assigned and used in previous access networks. This explains the gap in signaling costs between the loose-coupled case and the other cases. In the both tight-coupled and hybrid-coupled cases, a handover user experiences the same procedures. However, in case of the handover to the WLAN, a user in a hybridcoupled network has to register to the FA in the WLAN additionally, which results in increased signaling costs in the case of a hybrid-coupled network.
5. Conclusions We proposed a hybrid coupling scheme for interworking between the UMTS and WLAN, and compared its handover procedures and the signaling costs during handover to previous interworking schemes, such as the loose coupling and
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Fig. 7. Handover to UMTS in tight coupling scheme.
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Fig. 8. Handover latency in case of Twireless = 10 ms.
Fig. 9. Handover latency in case of Twired = 10 ms.
the tight coupling scheme. When the UMTS and WLAN are connected by the hybrid coupling scheme, it is expected that the packet loss probability will be lower than that in the tight-coupling, and it is shown that the total signaling
cost during handover decreases compared to the case of the loose-coupled network. That is, with the hybrid coupling scheme, the interworking network accommodates real-time traffic from the WLAN efficiently.
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[5] Ahmavaara K, Haverinen H, Pichna R. Interworking architecture between 3GPP and WLAN systems. IEEE Communications Magazine 2003; 41(11):74–80. [6] Koien GM, Haslestad T. Security aspects of 3G-WLAN interworking. IEEE Communications Magazine 2003; 41(11):82–8. [7] 3GPP2 S.P0087-0. 3GPP2-WLAN Interworking Stage 1 Requirements. July 2003. [8] Buddhikot MM. et al. Design and implementation of a WLAN/CDMA2000 interworking architecture. IEEE Communications Magazine 2003;41(11):90–100.