A Seamless Handoff Scheme for UMTS-WLAN Interworking

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1 A Seamless Handoff Scheme for UMTS-WLAN Interworking Hyun-Ho Choi, Osok Song and Dong-Ho Cho Department of Electrical Engineering and Computer Science Korea Advanced Institute of Science and Technology (KAIST) Telecommunication R&D Center, Samsung Electronics Co. Ltd, Korea Abstract In this paper, we present a practical UMTS-WLAN interworking architecture based on 3GPP standards, and propose a seamless handoff scheme that guarantees low delay and low packet loss during UMTS-WLAN handoff. For low handoff delay, the proposed handoff scheme performs pre-registration and preauthentication processes before the layer 2 handoff. Moreover, it uses packet buffering and forwarding functions in order to reduce packet loss during the handoff period. Numerical and simulation results show that the proposed scheme performs well with respect to signaling cost, handoff delay, and packet loss compared with conventional schemes. I. INTRODUCTION The Universal Mobile Telecommunication System (UMTS) provides high mobility with wide area coverage and supports low to medium data rates. However, these rates are not sufficient to satisfy various data-intensive applications and the service charge is high. The Wireless Local Area Network (WLAN) provides a higher speed data rate and low price, but covers only small areas and allows limited mobility. Therefore, a UMTS-WLAN interworking approach can make the best use of the advantages of each type of network and can eliminate the stand-alone defects of the two services [1]. Several interworking issues, such as dual mode support of mobile, seamless handoff or roaming, mobility management, security and charging problems are currently under discussion by a number of standards organizations. Among these interworking items, the issue of seamless handoff is of great importance for guaranteeing service continuity and Qualityof-Service (QoS), which means low latency and low packet loss during handoff. To provide seamless handoff between two networks, The 3rd Generation Partnership Project (3GPP) discusses basic interworking architectures that use a Mobile Internet Protocol (M) or gateway approach [2]. Moreover, the Internet Engineering Task Force (IETF) suggests preregistration and post-registration handoff schemes based on the M protocol for seamless mobility management in both v4 and v6 networks [3], [4]. However, 3GPP has not, at the time of writing, presented a specific seamless handoff procedure [5]. The IETF s pre/postregistration schemes need to be modified to be suitable for the integrated UMTS/WLAN network architecture, because they operate only in networks using the M protocol and do not consider the authentication process, despite the fact that authentication is an essential procedure and induces large latency during handoff [6]. Therefore, in this paper, we survey current standard techniques for seamless handoff, and propose an effective UMTS-WLAN interworking architecture and an efficient seamless handoff procedure that supports low handoff delay and low packet loss in the proposed architecture. II. PREVIOUS WORKS M is the most widely known mobility management proposal and is the most common solution for offering seamless roaming to mobile devices on the Internet [7]. Its basic operation is consisted of advertisement, movement detection and registration, and packet delivery processes. M was originally designed to operate at the Layer 3 (L3) only, without regard to the underlying link layer (L2). This approach implies a clear separation between L2 and L3 handoff functionality but may lead to unacceptable handoff latencies. Indeed, the messages generated by the registration process need some time to propagate through the network, and the mobile node (MN) is unable to send or receive packets during that time [6]. For these reasons, the IETF s mobile workgroup has proposed so-called low latency handoff schemes, which are distinguished as Pre-Registration and Post- Registration handoff. In pre-registration handoff, the network assists the MN in performing an L3 handoff before the L2 handoff is completed [3]. Fig. 1 presents the operation procedure of the preregistration handoff scheme. First of all, a Router Solicitation is sent from an ofa to an nfa and a Router Advertisement is returned from the nfa to the ofa. The ofa should solicit and cache advertisements from the nfa in advance of the preregistration handoff in order not to delay the handoff. Both the MN and the FAs can initiate a handoff. As a consequence of L2 handoff MN ofa nfa HA (GFA) Proxy Router Sol. Proxy Router Adv. (Reg) Reg Request (Reg) Reg Reply Router Solicitation Router Advertisement (Reg) Reg Request (Reg) Reg Reply This work was supported by the Samsung Electronics Co., Ltd. Fig. 1. Operation procedure of pre-registration handoff. Globecom /04/$ IEEE

2 MN nfa ofa Handoff Request Handoff Reply Bi-directional Edge Tunnel M tunnel HA (GFA) network requirements in an integrated UMTS/WLAN network [10]. In this section, we present a network architecture suitable for UMTS-WLAN interworking and explain in detail the operation of the proposed seamless handoff scheme and handoff requirements. L2 handoff optional Fig. 2. Agent Solicitation Agent Advertisement (Reg) Reg Request (Reg) Reg Reply optional M tunnel Operation procedure of post-registration handoff. the, a Proxy Router Solicitation is generated by the MN. A Proxy Router Advertisement is sent by the ofa as a result of the MN s solicitation message. Then, the (Regional) Registration Request to the nfa is sent via the ofa and is relayed to the HA (or GFA) by the nfa, which can verify the HA information within the message from the MN. Finally, the (Regional) Registration Reply is sent by the HA (or GFA) to the nfa, and is delivered to the MN, via the ofa if the L2 handoff is not completed, or to the MN directly if the L2 handoff is finished. In post-registration handoff, the L3 registration occurs after the L2 handoff has been completed. This technique uses a bidirectional edge tunnel (BET) to support low latency and low packet loss during the change of the L2 attachment point of the MN [8]. The sequence of messages for post-registration handoff is depicted in Fig. 2. The FA receiving the trigger sends a Handoff Request to the other FA. The FA receiving the handoff request sends a Handoff Reply to the other FA. This establishes a BET. When the L2 handoff occurs and the MN is no longer connected to the ofa, the ofa begins forwarding the MN packets to the nfa through the BET, and then the nfa delivers these packets to the MN. If the MN receives the L2 trigger, it decides to initiate the M registration process with the nfa by using Agent Solicitation and Agent Advertisement. Once the registration process is ended by the exchange of a (Regional) Registration Request and a (Regional) Registration Reply with the HA (or GFA), the nfa takes over the role of the ofa. III. PROPOSED SEAMLESS HANDOFF SCHEME Pre-registration and post-registration handoff schemes are difficult to apply to the UMTS-WLAN interworking architecture currently under consideration because their operation was originally based on the M and they deal with only the registration procedure [3]. However, in the event of handoff between heterogeneous networks, both the registration process and the authentication process must be included in the entire handoff procedure [9]. Therefore, we propose a new handoff procedure taking into account actual handoff situations and A. Interworking Architecture The considered network architecture for UMTS-WLAN interworking is based on interworking network models of 3GPP standards documents [5]. After due consideration of WLAN interworking reference models presented in [5], we were able to design the network architecture, as in Fig. 3. The User Equipment (UE) is a mobile node that can communicate with both a WLAN access network and a UMTS network. While this UE can access both WLAN and UMTS systems, it connects with only one access network at a time, since it cannot generally execute dual processes for two communications [2]. This makes soft handoff impossible, and so hard handoff must be performed during WLAN-UMTS handoff. The WLAN Access Network (WLAN AN) provides WLAN access services for the UE. It is not limited to any specific WLAN technology and may consist of several WLAN entities, such as Access Point (AP) and Access Point Controller (APC) [5]. The WLAN AN is connected to the UMTS network via the WLAN Access Gateway (WAG) and to the 3GPP Authentication, Authorization, Accounting (AAA) server for the WLAN authentication process. The WAG is a gateway through which the data to/from the WLAN AN is routed. The UMTS is divided into the UMTS Terrestrial Radio Access Network (UTRAN) and the UMTS core network. The UTRAN consists of the Node B and Radio Network Controller (RNC) which perform functions related to access control of the UE. The UMTS core network contains the Serving GPRS Support Node (SGSN), Gateway GPRS Support Node (GGSN), Packet Data Gateway (PDG), AAA server, Subscriber Server (HSS), and Location Register/Authentication Center (HLR/AuC) [5], [10]. The GGSN serves as a gateway between the SGSN and the public data network (PDN), and UE WLAN Access Network Node B Fig. 3. Intranet/ Internet UTRAN RNC Wr/Wb Wn Wu HSS Wg WLAN Access Gateway SGSN UMTS Core Network Wx D /Gr 3GPP AAA Server Wp Gn HLR/ AuC Wf Wm Wo Packet Data Gateway/FA GGSN/FA CGw/ CCF OCS Network architecture for UMTS-WLAN interworking. Wi Gi PDN/ Internet HA Globecom /04/$ IEEE

3 contains the FA function-managing UEs in the UMTS. The SGSN performs the UMTS authentication and interacts with the HSS/HLR. The PDG routes the packet data received from/sent to the PDN and allows allocation of the WLAN UE s address. Further, the FA function that handles the WLAN AN is located in the PDG and manages the mobility of WLAN UEs. Lastly, the HA is located in the PDN/Internet and manages FAs of both the WLAN and UMTS networks [2]. B. Handoff Procedure To reduce the handoff delay, we use the basic idea of the pre-registration handoff scheme. That is to say, the proposed seamless handoff scheme performs both registration and authentication processes prior to the L2 handoff. Although our proposal is based on the pre-registration handoff method, we present different handoff procedures considering the authentication process and characteristics of both UMTS and WLAN systems, and use minimum signaling messages to decrease latency and signaling costs. Moreover, in order to prevent packet loss during handoff, the proposed scheme allows the HA to manage the UE s s assigned by two networks participating in the handoff. By using this ability, the HA can buffer data packets destined for the mobile node during handoff. The conventional HA manages only the current according to the UE s home address, but the HA in the proposed scheme registers the next assigned in the newly handed-off network as it allocates extra space to store the UE s next in the address table, as in Table I. The next column is not written in general cases, but is filled by the proposed handoff procedure when the UE is in a handoff situation. Table I shows that the UE1 and the UE2 are processing the handoff procedure. If both current and next s are written, the HA simultaneously begins to buffer packets destined for the UE s home address. Thereafter, if the UE s handoff is completed and the next entity becomes a blank, the HA forwards the buffered packets to the UE. For the operation of the proposed scheme, seven new signaling messages are defined. Before explaining the handoff operations in detail, we briefly introduce the additional signaling messages as follows. Pre-authentication Request is issued by the UE after the handoff decision and is sent to the PDG/FA in the WLAN network or to the SGSN in the UMTS network. It demands the UE s pre-authentication while delivering information required for the UE s authentication. Pre-authentication Reply is generated by the PDG/FA or the SGSN and is sent to the UE. It informs the UE of TABLE I ADDRESS TABLE MANAGED IN THE HA address UE1 C1 N1 UE2 C2 N2 UE3 C3 the result of pre-authentication completion, whether the UE s authentication is successful or not. Pre-registration Request is initiated by the UE and is transmitted to the FA of the new network in order to request pre-registration. This message contains the UE s home address. Once the FA receives this message, it starts the pre-registration process. Pre-registration Reply is delivered from the FA to the UE and informs the UE of the completion of pre-registration together with the assigned next. Pre-reg. Update Request is issued by the FA and is transmitted to the HA. This message contains the UE s home address and the new assigned by the FA. It instructs the HA to register the new assigned as the next. Pre-reg. Update Reply is sent by the HA as a result of a pre-reg. update request message. This message informs the FA that the UE s next is registered in the HA. Handoff Completion is transmitted from the UE to the HA. It reports to the HA that the UE has been connected with the new network perfectly. Therefore, it requests the HA to update the current and next s of the address table. The detailed operation depicted in Fig. 4, the handoff procedure from UMTS to WLAN, may be explained by dividing the process into five parts as follows. 1) Handoff Initiation Phase: At the beginning, the UE communicates with the UMTS network. So, a packet transmission route is formed between the HA and the UE through the UMTS network entities. Here, the HA manages the current ( UMTS ) assigned by the UMTS FA according to the UE s home address ( UE ). If a UE in the UMTS system enters the handoff region, the UTRAN judges its location and notifies it of the Service Set Identifier (SSID) of its neighbor WLAN AP. For this operation, the UTRAN is assumed to have previously stored the SSIDs of neighbor WLAN APs within each UTRAN cell. After the UE receives this SSID and if the occurs, it decides the acceptance or rejection of the handoff attempt into the WLAN AN. 2) Pre-authentication Phase: After the handoff decision, the UE transmits the pre-authentication request message to the GGSN/FA and the GGSN/FA relays it to the PDG/FA of the WLAN in order to request WLAN authentication. Here, the GGSN/FA is the current FA of the UE and the PDG/FA is the new FA to which the UE will move. Because the GGSN/FA is assumed to record PDG/FA addresses corresponding to the WLAN SSIDs that the UE receives, it can deliver the request message to the PDG/FA without error. This message contains the UE authentication information that is required for standard WLAN authentication [5]. If the PDG/FA receives a pre-authentication request message, it begins the standard Extensible Authentication Protocol/Authentication and Key Agreement (EAP/AKA) or Extensible Authentication Protocol/Subscriber Identity Module (EAP/SIM) procedure for WLAN authentication among the UE, PDG/FA and AAA server [5]. At this point, instead of the WLAN AN, the PDG/FA relays WLAN authentication Globecom /04/$ IEEE

4 UE WLAN AN PDG/FA AAA server HA GGSN/FA SGSN UTRAN UE EAP Success [keying material] WLAN Attachment Fig. 4. UMTS UMTS Start Packet WLAN Stop Packet Pre-reg. Update Request [, WLAN] Handoff Completion [] Buffered Packet Forwarding Pre-reg. Update Reply WLAN Pre-registration Reply [WLAN] UE movement WLAN info. [SSID] Pre-authentication Request (EAP Response/Identity [NAI based on a temp. identifier or IMSI, SSID]) WLAN Authentication (EAP/AKA or EAP/SIM Procedure) Pre-authentication Reply (EAP Success) Pre-registration Request [] UMTS Detachment Handoff Decision Handoff procedure when a UE moves from UMTS to WLAN. messages between the UE and AAA server although the original WLAN authentication is performed through the WLAN AN [9]. If pre-authentication is completed successfully according to the standard authentication process, the EAP success is transmitted to the WLAN AN with authentication keying material. This WLAN AN memorizes the received EAP success information that corresponds to the pre-authenticated UE, in order to use it after the UE is attached to itself. The EAP success is also delivered to the UE via the pre-authentication reply message. 3) Pre-registration Phase: If the pre-authentication phase is completed successfully, the UE sends the PDG/FA a pre-registration request message for M registration. If the PDG/FA receives it, a new ( WLAN ), which will be used in the WLAN, is assigned. The assigned can be either a foreign agent or co-located. Thereafter, the FA sends the HA the pre-reg. update request message, which includes the new assigned ( WLAN ) together with the UE s home address ( UE ). If the HA receives the pre-reg. update request, it inserts the received into the next entity that corresponds to the UE s home address. At this time, the HA starts to buffer Handoff Initiation Phase Pre-authentication Phase Pre-registration Phase L2 Handoff Phase Handoff Completion Phase the packet destined for the UE s home address and sends the PDG/FA the pre-reg. update reply message. If the PDG/FA receives it, the pre-registration reply message is transmitted to the UE through the GGSN/FA. This message informs the UE of the completion of pre-registration with the new assigned, WLAN. 4) L2 Handoff Phase: If both pre-registration and preauthentication are finished, the UE disconnects from the UMTS network by using the L2 detachment process based on the 3GPP standard [11]. Thereafter, it performs the WLAN attachment process by using standard WLAN association procedures. Namely, the L2 handoff is carried out from the UMTS to the WLAN. During this attachment process, the WLAN AN can confirm that the UE is the pre-authenticated user from the previously received EAP success message. Therefore, the UE is able to communicate via WLAN AN immediately, without a further WLAN authentication process [9]. 5) Handoff Completion Phase: After the UE becomes connected to the WLAN AN, it sends the handoff completion message to the HA, which then updates the address table by replacing the current with the next. Then, the HA ceases the packet buffering and forwards the data packets buffered up to this time. Now, the entire handoff procedures are completed and the packet transmission route is formed through the WLAN AN. The overall handoff procedure that the UE uses to move from WLAN to UMTS is depicted in Fig. 5. This case is also divided into five phases and observes the same signaling procedure as in the case of handoff from UMTS to WLAN, except for the pre-authentication phase. When the UE hands off from WLAN to UMTS, pre-authentication is performed among the UE, SGSN and HLR/AuC based on the 3GPP authentication procedure [11]. Because the UMTS authentication is issued by the SGSN, the pre-authentication request message must be sent to the SGSN, and so the SGSN must process the UMTS authentication and transmit the pre-authentication reply message to the UE following successful authentication. C. Requirements for the Proposed Handoff Scheme There are some requirements for the natural operation of the proposed scheme. In order that the UE confirms the WLAN cell to which it moves later, the UTRAN should store the SSIDs of WLAN APs located in each UMTS cell beforehand and send them to the UE in the handoff region. Further, when the interworking network is initially constructed, the GGSN must record the PDG address according to each WLAN SSID for message delivery to the PDG that corresponds to the SSID. However, in the case that the UE moves to the UMTS network, there is no requirement to deliver signaling messages since the CN can easily identify the UMTS cell to which the UE will hand off. This is because a UMTS cell is much wider than a WLAN cell and so covers the WLAN cell area [10]. Finally, instead of the WLAN AN, the PDG should relay WLAN authentication messages between the UE and the AAA server. This requirement makes it possible to perform WLAN authentication via PDG, even though the UE has not yet connected to the WLAN AN [5]. Globecom /04/$ IEEE

5 UE WLAN AN PDG/FA HLR/AuC HA GGSN/FA SGSN UTRAN UE Handoff Decision Pre-registration Reply [UMTS] WLAN WLAN Start Packet UMTS Stop Packet Pre-authentication Request [, IMSI, Auth. info.] UMTS Authentication (Distribution of Auth. Vector & Auth. and Key Establishment) WLAN Detachment Fig. 5. Pre-authentication Reply Pre-registration Request [] Pre-reg. Update Request [, UMTS] UE movement UMTS Pre-reg. Update Reply UMTS Attachment Handoff Completion [] Buffered Packet Forwarding Handoff procedure when a UE moves from WLAN to UMTS. IV. PERFORMANCE EVALUATION It is difficult to measure handoff performance accurately because actual system performance depends on a variety of factors, such as network topology and location of entities. Therefore, we investigate the overall performance with respect to signaling cost and handoff delay by using a simple analytic calculation with certain assumptions [12]. Signaling cost is defined as the message traffic exchanged between network nodes during handoff. We define signaling cost parameters as follows. C A B : signaling cost for message transmission between node A and node B C L2HO : signaling cost needed for L2 handoff C auth : signaling cost needed for UMTS or WLAN authentication Thus, the signaling costs of the basic M, pre/postregistration handoff schemes, and the proposed scheme may be computed as follows. C M = C L2HO + C auth +4C MN FA +2C FA HA (1) C pre = C L2HO + C auth +4C MN of A +4C of A nf A +2C nf A HA (2) Handoff Initiation Phase Pre-authentication Phase Pre-registration Phase L2 Handoff Phase Handoff Completion Phase C post = C L2HO + C auth +4C MN nf A +2C of A nf A +2C nf A HA (3) C prop = C L2HO + C auth +2C MN of A +2C of A nf A +2C nf A HA + C MN HA (4) The M consumes six signaling costs for registration, and two signaling costs for L2 handoff and authentication are added. In the pre/post-registration handoff schemes, each of four and two signaling costs between the ofa and nfa are inserted into the signaling costs of the M. These added signals are used to achieve pre-registration before L2 handoff or post-registration after L2 handoff. The proposed handoff uses a total of seven messages for registration and needs the signaling cost for L2 handoff and authentication. Similarly, we use a simple analytic calculation in order to evaluate the handoff delay under the following assumptions [12]. T link(a B) : average link delay between node A and node B T proc(a) : average processing delay of node A T L2HO : average delay needed for L2 handoff T auth : average delay needed for UMTS or WLAN authentication By using these delay parameters, the total handoff delay of each scheme may be calculated as follows. D M = T L2HO + T auth +4T link(mn FA) +2T link(fa HA) +T proc(fa) +T proc(ha) (5) D pre = T L2HO + T auth +4T link(mn of A) +4T link(of A nf A) +2T link(nf A HA) +T proc(fa) + T proc(ha) (6) D post = T L2HO + T auth +4T link(mn nf A) +2T link(of A nf A) +2T link(nf A HA) +T proc(fa) + T proc(ha) (7) D prop = T L2HO + T auth +2T link(mn of A) +2T link(of A nf A) +2T link(nf A HA) +T link(mn HA) +T proc(fa) +2T proc(ha) (8) In the case of M, delays for L2 handoff and authentication, link delays among the MN, FA and HA, and processing delays in the FA and the HA are consumed. In the pre/postregistration handoffs, more link delays are required for the preor post-registration process. By contrast, the proposed scheme needs seven link delays for message transmissions and three processing delays by operations at the FA and the HA, together with L2 handoff and authentication delays. In every handoff operation, a period during which the MN is not connected with any access network occurs. During this interval, data packets destined for the MN may be lost. This disconnected interval is an important factor to evaluate the handoff performance in view of packet loss and could be obtained as some portion of the total handoff delay as follows. M = D M (9) pre = T L2HO + T auth + T link(nf A HA) +T link(of A nf A) + T link(mn of A) (10) Globecom /04/$ IEEE

6 post = T L2HO + T auth +2T link(of A nf A) (11) prop = T L2HO + T link(mn HA) + T proc(ha) (12) In M, the packet loss duration is the same as its total handoff delay, because it becomes disconnected from the network immediately after the handoff starts and remains so until the entire handoff procedure has been completed. In the case of the pre/post-registration handoff schemes, the duration of packet loss is determined by the link delays in making the connection path, plus L2 handoff and authentication delays [6]. Because the proposed scheme performs authentication before L2 handoff, the interval for which it is disconnected does not contain the authentication processing time and only includes the L2 handoff delay with link and processing delays after L2 handoff. If we assume that signaling costs and delay components for both L2 handoff and authentication are 2, and the other components mentioned above are 1 in all [12], the performance comparisons of each scheme can be briefly summarized as shown in Table II. The pre/post-registration handoff schemes have large signaling costs and handoff delays compared with the other schemes because they observe the basic operations of M and use additional messages between ofa and nfa for pre- or post-registration. However, the proposed scheme shows low signaling cost and handoff delay than the other schemes, because it uses a smaller number of signaling messages for handoff. In addition, its packet loss duration is shorter than that of the other schemes. Using the analyzed packet loss duration, we could obtain the packet loss performance during handoff by computer simulation. For reasons of computational tractability, all network nodes are modeled as simple M/M/1 queues [6]. The service time of a packet is assumed to be exponentially distributed, including the processing time and the transmission time. We assume that the CN transmits 500 byte packets every 20 ms to the MN, and the service rate is 4 Mbps in all nodes. In addition, all link delays between two nodes and processing delays are assumed to be 5 ms, and both T L2HO and T auth are assumed to be 100 ms [8]. Fig. 6 shows the expected number of dropped packets versus playout time. The playout time is the maximum allowed endto-end delay between the CN and MN. Hence, the number of dropped packets falls as the playout time is increased. Packet loss occurs more in M than in the other schemes because it has the longest duration of packet loss during handoff. The pre-registration and post-registration handoff schemes perform similarly to each other because the durations of packet loss in the two schemes are almost the same. The proposed handoff scheme shows the best performance because its duration of packet loss is shorter than that of the others. Therefore, the proposed scheme can guarantee a low packet loss rate, because its pre-registration and pre-authentication processes enable fast handoff. V. CONCLUSIONS In this paper, we introduced a practical UMTS-WLAN interworking architecture based on 3GPP standards and proposed a seamless handoff method which guarantees low delay and TABLE II PERFORMANCE COMPARISONS M Pre-reg. HO Post-reg. HO Proposed Signaling Cost (C) Handoff Delay (D) Loss Duration ( ) Expected number of dropped packets Mobile Pre reg. HO Post reg. HO Proposed Scheme Playout Time (sec) Fig. 6. Expected number of dropped packets vs. playout time. low packet loss during UMTS-WLAN handoff. As results of analysis and simulation, the proposed handoff operation performs well with respect to signaling cost, handoff delay, and packet loss compared with the conventional schemes. Therefore, the proposed handoff scheme is able to guarantee seamless handoff supporting QoS and service continuity for the UMTS-WLAN interworking. REFERENCES [1] Vijay K. Varma, et al., Mobility Management in Integrated UMTS/WLAN Networks, IEEE ICC 2003, vol. 2, pp , May [2] S. Taso and C. Lin, Design and Evaluation of UMTS-WLAN Interworking Strategies, IEEE VTC 2002 Fall, vol. 2, pp , Sep [3] K. El Malki, et al., IETF draft, Low latency Handoff in Mobile v4, May [4] Rajeev Koodli, IETF draft, Fast Handovers for Mobile v6, Sep [5] 3GPP system to Wireless Local Area Network (WLAN) Interworking; System Description, Rel.6, 3GPP TS , v1.15.0, Aug [6] C. Blondia, et al., Low Latency Handoff Mechanisms and Their Implementation in an IEEE Network, Proceedings of ITC18, Teletraffic and Engineering Vol. 5b, pp , Berlin, Germany, Sep [7] C. Perkins, Mobility Support, RFC 2002, Oct [8] O. Casals, Ll. Cerda, G. Willems, C. Blondia, N. 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