A Review of Mobility Supporting Tunneling Protocols in Wireless Cellular Networks

With recent technology advancements mobility support is one of the major needed parameters by any wireless or mobile networks. Continuous mobile movement from one cell to another or from one network to another requires continuous mobility support. Previously, tunneling protocols employment was the technique to support UE’s inter or intra network mobility. More specifically, GRE, GTP, MIPv6 or PMIPv6 were employed for mobility support. In tunneling encapsulation of one protocol over another protocol is performed to deliver IP packet during inter network or intra network handover. In terms of usage scenario of each tunneling protocol, tunnel establishment, data transfer and tunnel release, an overview and comparison of tunneling protocols is presented in this paper. 3GPP and WLAN interworking, and GAN based usage scenarios and supported tunneling mechanisms has been discussed. Some insights regarding security, multiplexing, multiprotocol and packet sequencing support are also provided for each tunneling protocol. Keywords—Tunneling; mobility; 3GPP; WLAN; interworking


I. INTRODUCTION
With the recent advancements in wireless and mobile networks there is need of mechanisms that can support coexistence of multiple radio access technologies. These technologies should not also coexist but also provide seamless mobility between different radio access technologies. Looking on this thing various developments has been performed from different researchers. However, basically mechanism that provides support seamless handover are depends on tunnel that is created between different radio access technologies for establishing connectivity to the core network. Generic Routing Encapsulation (GRE) is one of the pioneering tunneling protocols that provide support for switching from one radio access technology to another. This protocol is based on encapsulation of one protocol over another protocol. Proxy Mobile IPv6 (PMIPv6) is mostly used for encapsulation. It is also specified in standard as IP session continuity signaling being used by Evolved Packet Core (EPC). This protocol supports mobility of terminals or UEs for various radio access technologies, e.g., Long Term Evolution (LTE) radio access, WLAN, 3G radio access, WiMAX and radio standards from 3GPP2. Basically, protocols just like PMIPv6 and MIPv6 manage the path for IP packets destined for different network or radio access technology. This kind of mobility approaches not also support seamless handover but also ensures efficient utilization of network resources, user privacy and network security.
Another type of tunneling protocol being used for supporting terminals mobility is GPRS Tunneling protocol (GTP). This protocol is also being used between different 3GPP core network entities. In which once a tunnel is established between different network entities then IP packets can be encapsulated and tunneled between these network entities. IP Security (IPSec) is another tunneling protocol used for protecting data integrity of wireless devices being delivered to the core network entities. This is the security association mechanism in which mutual authentication between terminal and access gateway is performed. Negotiation of security keys for a connection is also performed. Packets in IPSec are encrypted and encapsulated within a new packet with new control information and are delivered by IPSec tunnel from terminal or user equipment to access gateway.

II. BACKGROUND
In Fig. 1, we have tried to cover architectural elements needed for supporting different type of technologies. As we can see in upper part of the figure 2G network components that will be needed for Voice Call Continuity (VCC) which provides support for anchoring circuit switched voice call in IP infrastructure by transferring speech path between these two domains transparently to end user [1,2]. Tunnel is needed when VCC supported UE moves from 3G/4G network to 2G network then signaling messages between MME and Interworking solution Function for 3GPP2 1xCS (1xCS IWS) are transferred using S102 Tunnel. Another tunnel named IPSec is being used in Generic Access Network (GAN) and Wireless Local Area Network (WLAN) between UE to GAN Controller and ePDG network components respectively. GTP, PMIPv6 and GRE tunnels are used for secure data transfer between different network components like SGW, PGW, SGSN MME from 3G or 4G networking technologies, respectively. However, authors in [3,4] surveyed various tunneling protocols for supporting mobility. These tunneling protocols are IP based tunneling protocols used to support mobility in IPv6 based networks. Implementation of these protocols depends on usage scenario, as scenario can be host based, network based, soft handoff or hard handoff based Micro and Macro Mobility usage scenarios. Depending on these each different protocol is discussed for supporting seamless handover and mobility support in IPv6 networks [5]. However, some authors discussed these protocols from centralized or distributed point of view. Implementation of these protocols can be dependent on centralized or distributed support of these protocols. Authors suggested that use of protocols like GTP and PMIPv6 in centralized fashion may be not an effective solution because of single point of failure and scalability issues. So, they think that some distributed usage scenarios should be discussed. However, they mentioned the few by employing some already defined techniques just like de coupling control and user plane.

A. Generic Routing Encapsulation (GRE)
Currently, various protocols support encapsulation of one protocol over another. Generic Routing encapsulation (GRE) is among one of the encapsulation protocols which provide support for encapsulation of one protocol over another protocol. Simple IP packets are encapsulated in GRE header and transmitted to different intervening routers [6,7]. 1) Usage scenarios: GRE tunnel implementation is based on GRE encapsulation of data from network entity to another network entity over some other mobility supported protocol i.e., PMIPv6. Basically, GRE is used with PMIPv6 that can support mobility for UE if it is moved from one network to another. Then data can be transferred by using GRE encryption and PMIPv6 mobility support option [8].
2) Tunnel establishment and data transfer: Fig. 2 illustrates GRE tunnel establishment procedure. By adopting PMIPv6 as mobility support protocol and employing GRE as data encryption technique for security purposes a tunnel can be established between as Mobility Access Gateway (MAG) and Local Mobility Anchor (LMA). GRE key option is needed for establishing a tunnel between MAG and LMA. GRE keys are exchanged between these two entities by using Proxy Binding Update (PBU) defined in PMIPv6. MAG and LMA establish a GRE tunnel by the agreed GRE keys to transmit uplink and downlink traffic [9].
3) Tunnel release: Once a tunnel is established for data transfer it can also be removed from network. By following some steps tunnel between MAG and LMA can removed. MAG transmits a PBU message to LMA for release of the LMA binding. LMA completes release of binding by transmitting an acknowledgment message to MAG. During tunnel release process all resources just like IPv6 Home Network Prefix, IPv4 Home Address, Downlink and Uplink GRE Key, GRE Tunnel Tear-down deletion and deassignment is performed [6]. 25 | P a g e www.ijacsa.thesai.org

B. GPRS Tunneling Protocol (GTP)
For transporting IP packets with in network or outside network GPRS tunnels concept is used. In which a tunnel is established between different network entities for successful transmission of IP packets within the network. For this purpose, a tunnel is established between different end points for data transmission and a unique identity named Tunnel End Point Identifier (TEID) is assigned to IP packets. Packets being received at different end points are forwarded based on their TEID's. According to 3GPP technical documents there are two types of protocols used in GTP one is GTP-U [6] and other is GTP-C [10]. GTP-U is used for delivering user data to different network entities. However, GTP-C is used to exchange control plane messages among different network entities.
1) Usage scenarios: GPRS Tunneling protocol implementation can be observed in various scenarios depending on type of interfaces being used. As, previously in [11] enhanced GTP was employed for GSM network which adopted Packet Data Protocol (PDP) to reduce tunneling overhead being faced at that time. Basically, one usage scenario that can be observed in different standard documents is 3GPP wireless LAN interworking. GTP can be implemented on different interfaces for providing connectivity between different network interfaces. GTP implementation using 3GPP plus trusted Non-3GPP access over S2a interface or 3GPP plus untrusted Non-3GPP access over S2b are provided to support usage scenarios. Seamless IP session continuity is supported between cellular networks and wireless local networks without the change of the address [12].
2) Tunnel establishment and data transfer: Fig. 3 illustrates GTP tunnel establishment procedure. First of all, Non-3GPP IP Access specific procedures takes place after that UE Authentication and authorization is performed by looking into HSS where subscription information of the user/subscriber is stored. 3GPP AAA transmits a reply to trusted non-3GPP network with information on all the authorized APNs and additional PDN GW selection. Upon completion of authentication and authorization, L3 attach procedure specific to non-3GPP access is initiated. UE transmits a request for session start from the obtained list of available APNs. Otherwise, PDN gateway selection procedure takes place. If UE connection to the particular APN becomes successful, trusted non-3GPP network transmits a message for making a connection (IMSI, APN, RAT type, Trusted non-3GPP IP Access TEID, etc.) message to PGW. After that PDN GW initiates the IP-CAN Session Establishment Procedure with the PCRF. PCRF creates IP-CAN session related information and responds to the PDN GW with PCC rules and event triggers. Then, the selected PDN GW informs 3GPP AAA server about PDN GW identity and the APN corresponding to the UE's PDN connection and also information about selected S2a protocol (GTP). PDN GW replies with a create session response (PDN GW Address for the user plane, PDN GW TEID of the user plane, PDN GW TEID of the control plane, PDN Type, PDN Address, EPS Bearer Identity, EPS Bearer QoS, APN-AMBR, Protocol Configuration Options and Cause message to the Trusted non-3GPP IP Access, with IP address assigned to the UE. Then, GTP tunnel is set up between trusted Non-3GPP network and the packet gateway. Once a tunnel is established, data transfer can take place [12]. 3) Tunnel release: To release the tunnel or detach procedure is accomplished by following a procedure. First of all, a mobile or Trusted Non-3GPP network starts an access specific detach procedure from the network. Access technology specific detach trigger procedure is performed for tunnel release. Then, active bearers for UE and PDN connection are deactivated by the trusted non-3GPP IP Access sending a Delete Session Request to the gateway. Then the gateway informs AAA of PDN detach. PDN Gateway on receiving message deletes IP session associated with that particular UE. PDN Gateway and PCRF perform PCEF-Initiated IP CAN Session Termination Procedure. Then PDN Gateway sends a message to acknowledge with delete session response message. Finally, resources for trusted Non-3GPP network will be freed [9]. 26 | P a g e www.ijacsa.thesai.org

C. Proxy Mobile IPv6
For supporting mobility of end users in different networks a protocol was introduced named Mobile IPv6. This protocol allows the users to keep online with mobility within IPv6 network. The basic concept behind this protocol is each node has a home address when connected to some network. However, by moving to another network it is associated to other network based on Care-of-Address (CoA). The address provides information on UE's latest point-of-contact. The protocol supports IPv6 nodes to store information on UE's home address and CoA. It also allows to send packets destined for UE by utilizing UE CoA. Tunneling mechanisms like GTP or GRE can also be used to support MIPV6 data transfer [13].
In network-initiated mobility control, network side maintains of the location of UE and triggers the necessary mobility message exchange. A proxy mobility agent is responsible for performing the mobility signaling with home agent. UE upon reaching to another network will try to connect to an access link. MAG over the link will perform authorization for network-based mobility. After authorization UE can perform address configuration and can move anywhere in PMIPv6 domain [14]. Authors in [15] presented some analytical studies regarding PMIPv6 where LMA is placed far from current MAG. In that case PMIPv6 will suffer from significant handover delay. So, based on mechanisms for supporting mobility whether predictive or reactive the authors in [15][16][17][18] suggested some enhancements to reduce handover latency, signaling cost and network utilization. Similarly, authors in [19][20][21][22] investigated messaging, data transmission, and tunneling overhead of different protocols with respect to PMIPv6.

1) Usage scenarios:
Proxy Mobile IPv6 implementation is basically for supporting for transfer of control information between different network entities whether it is 3GPP or WLAN for handling network-based mobility. Basically, GRE is used with PMIPv6 that can support mobility for UE if it is moved from one network to another. Then data can be transferred by using GRE encryption and PMIPv6 mobility support option.
2) Tunnel establishment and data transfer: Fig. 4  Address Mobility Option if the MAG supports the capability to receive from the LMA an alternate LMA address for user plan. On PBU reception, LMA selects the PDN based on APN information delivered in the PBU. LMA allocates an IPv4 or IPv6 address on receipt of PBU. Also, LMA generates uplink GRE key distinct from existing PDN connection's uplink traffic for that UE. After tunnel is established between different network entities data can be transferred by using any type of encapsulation, i.e. GRE [6].
3) Tunnel release: MAG initiates the release of PDN connection to tear down an existing PDN connection with LMA. MAG first transmits a PBU to LMA to delete the LMA binding for the UE's PDN connection. LMA completes deletion of the binding by transmitting a PBA to MAG. During tunnel release procedure, all resources such as IPv6 prefix, IPv4 address, Downlink and Uplink GRE Key, GRE Tunnel Tear-down deletion and de-assignment is performed [6].

D. Dual Stack Mobile IPv6
Dual Stack Mobile IPv6 (DSMIPv6) is an extension to MIPv6 functions. It allows UEs to request their home agent to forward IP packets addressed to their home address, and to their IP care-of address. A dual stack mobile can simultaneously enable both IPv4 and IPv6. Hence, two different mobility protocols are not needed at the same time [23,24].

1) Usage scenarios:
Usage scenarios can be based on for either 3GPP-WLAN interworking or mobility support within 3GPP. Solutions that enable seamless mobility between 3GPP-WLAN interworking and within 3GGP are needed such that current 3GPP based packet sessions can be served without interruption to UE's received service perception during the change of access network [25].
2) Tunnel establishment and data transfer: If UE is already on 3GPP Access and discovers the 3GPP I-WLAN domain, it may decide to change point-of-contact to 27 | P a g e www.ijacsa.thesai.org discovered 3GPP I-WLAN. For that purpose, UE will establish an IPSec Tunnel with ePDG. After establishing IPSec tunnel UE sends Binding Update message to HA and informs HA of its IP address. As a result of BU, DSMIPv6 tunnel is set up between UE and HA. After tunnel establishment, data plane messages can be transmitted using I-WLAN [25]. Fig. 5 illustrates DSMIPv6 tunnel establishment procedure.

E. IP Secure Tunnel (IPSec)
For securing data integrity and for transmission another form encryption protocol is IPSec. This protocol provides support to encapsulate original IP packet and assign a new IP header to deliver the encapsulated data to other side of the network. Authentication header is also included in together with ESP when IPSec is being used in tunnel mode [26][27][28] for providing security and mobility support. 1) Usage scenarios: UE with connectivity to trusted or un-trusted non-3GPP access needs some security architecture to connect to 3GPP EPS. User identity confidentiality and devices identity confidentiality is needed in 3GPP EPS and Non-3GPP access for providing connectivity for non-3GPP access devices to 3GPP EPS [29].
2) Tunnel establishment and data transfer: If connection of UE is already established with non-3GPP access network then that UE can also access 3GPP access network by using this security mechanism, during which a secure tunnel is established. An example of such tunnel establishment is one between UE and ePDG. They first exchange some messages known as IKE_SA_INIT. Then UE sends user identity and APN information in IKE_AUTH step. It initiates negotiation of child security associations. ePDG sends Authentication and Authorization Request message to the 3GPP AAA Server containing user identity and APN information. Then 3GPP AAA server checks authentication vectors from HSS/HLR, and stores the information like IMSI and EAP-AKA requested authentication method in HSS. Then, 3GPP AAA server initiates authentication challenge by transmitting a reply to ePDG. ePDG then responds with its identity and a certificate. It sends the AUTH parameter to protect the previous message it sent to the UE. Message sent by 3GPP AAA server is also attached in that response message. UE checks the authentication parameters and responds to the authentication challenge. Then, the ePDG transmits the EAP-Response/AKA-Challenge message to the 3GPP AAA. AAA checks whether the authentication response is correct. If everything is correct, AAA shall initiate the Subscriber Profile Retrieval. It registers to the HSS and checks user's subscription whether it is authorized for non-3GPP access. If all checks are done, AAA transmits a final answer to ePDG. Information consists of MSK (EAP-Master-Session-Key-AVP). ePDG uses the MSK to authenticate IKE_SA_INIT step signalings. The EAP Success/Failure message is delivered to the UE over IKEv2. UE shall take its own copy of the MSK as input to generate the AUTH parameter to authenticate the first IKE_SA_INIT message. ePDG checks the correctness of the AUTH sent by the UE. ePDG calculates the AUTH parameter which authenticates the second IKE_SA_INIT message. Then, AUTH parameter is sent to the UE together with the configuration payload, security associations and the rest of the IKEv2 parameters. Now IKEv2 negotiation ends [29].
In Table I given below, we have some insights of each tunneling technique. First of all is security mechanism that should be supported by each tunneling protocol. Of the abovementioned protocols IPSec supports complete security mechanism. In IPSec, Authentication Header (AH) protocol provides data origin authentication. Encapsulating Security Payload (ESP) supports data confidentiality [26]. Similarly, GRE provides some security by providing a four-byte key field for the purpose of origin authentication [3]. While regarding GTP security can be provided by assigning a unique tunnel end point identifier. Another key feature for each tunneling protocol is support for multiplexing which means supporting multiple simultaneous end devices. Separate tunnels may be set up; however separate tunnels impose processing overhead and increased delay for tunnel establishment. So, a better option is to share one tunnel among all end devices. A unique field is needed in tunneling IPSec provides this by Security parameter index. However, GRE and GTP provide multiplexing support by using GRE key field and GTP TEID field, respectively [6,12]. Multiprotocol support is also needed by each tunneling protocol. GRE provides multiprotocol support as it was defined as general encapsulation protocol. However, IPSec fails to provide support for multiple protocols. Another important parameter for each protocol is packet sequencing. IPSec has sequence number such that in-order delivery of packets can be feasible.
28 | P a g e www.ijacsa.thesai.org  Fig. 6 represents a generic session mobility scenario in which different tunneling protocols can be employed for handling mobility and data transfer. Initially Non-3GPP intrinsic L2 signaling is completed. Then EAP authentication procedure is started between UE Trusted Non-3GPP and AAA. As authentication reply by AAA, a group of all the authorized APNs plus additional PDN gateway selection is returned to the access gateway. Upon completion of authentication and authorization, non-3GPP radio specific L3 attach procedure is 29 | P a g e www.ijacsa.thesai.org initiated. Then, ePDG transmits a message to establish a data session to the gateway. PDN gateway initiates the IP CAN Session Establishment Procedure with the PCRF and the PCRF provides the APN-AMBR and Default Bearer QoS to the PDN GW in the response message. In response to the create session request message PGW sends a create session response message caching the all required information. After that session is established successfully and ePDG is also authenticated by the UE. IP session between UE and P-GW is now established. Packets from UE to ePDG are tunneled using IPSec Tunnel and then onward using some other tunneling protocol i.e., GRE or GTP [34].

IV. TUNNELING PROTOCOLS FOR 3GPP-WLAN INTERWORKING
Radius or diameter based 3GPP WLAN interworking is discussed in [35] which represents some network some network components WLAN UE, WLAN Access Network (WLAN AN), 3GPP AAA Server and Home Subscriber Server (HSS) database. To connect to WLAN, WLAN UE uses a SIM or USIM (UMTS Subscriber Identity Module) containing the authentication keys for the mobile subscriber. To connect UE to 3GPP network, WLAN Access Network (WLAN AN) plays a role of anchor between them. 3GPP AAA performs authentication and authorization for UE. When the WLAN AN receives a WLAN UE connection request, it may perform an initial access negotiation with the WLAN UE to obtain identity information and then pass this information to the 3GPP AAA server as part of an authentication/authorization request. WLAN AN may be RADIUS or Diameter-based. The 3GPP AAA Server matches data from the authentication/ authorization request with information in a trusted database, called a Home Subscriber Server (HSS). If a match is found, and the subscriber's credentials are correct, the 3GPP AAA server responds with a reply to WLAN AN. This indicates the acceptance of the request. Otherwise or if a problem is found with the subscriber's credentials, 3GPP AAA returns a reject message. This results in the termination of WLAN UE connection.
Upon establishment of WLAN UE connection, WLAN AN may forward accounting information to 3GPP AAA to record the transaction for future price charging. 3GPP AAA must be able to translate between RADIUS and Diameter (and vice versa), to support connection requests from legacy RADIUS WLAN ANs to the 3GPP network which uses the Diameter protocol. HSS stores subscriber data like keys to complete authentication to allow access for user device. It also performs authorization to enable access to limited service and functions. Overall for interworking support tunneling is the mechanism adopted as discussed in [36]. Also, another mechanism that supports offloading end users from Radio Access Network to WLAN using IPSec transport mode has been discussed in [37] to reduce networking security overhead caused by IPSec Tunnel mode. Fig. 6 illustrates 3GPP to WLAN mobility procedure and required tunnel establishment.

A. Directly Accessing to the Internet
In this case, the Internet is connected through WLAN AN. Users access WLAN AN. IMS AAA is responsible for authentication of users. It uses either the EAP-SIM or EAP-AKA protocol that originates from RADIUS and Diameter WLAN ANs. The SIM-authentication mechanism is used against the subscriber information stored in the HSS. Authentication is performed directly from the WLAN AN. Upon completion of authentication, authorization will return policy information for session establishment [35,38].

B. Accessing through 3GPP
In this case, users can access connection service to the Internet via a secure tunnel to 3GPP IMS. IP packets are forwarded through tunnel to 3GPP IMS network via WLAN Access Gateway (WAG) and ePDG. WAG acts as a dynamically configured firewall. ePDG is a tunnel end-point. Multiple tunnels are possible to support any number of simultaneous services. ePDG requests authorization separately from the authentication request. For example, WLAN UE may initiate a tunnel towards the ePDG. This is followed by authentication and tunnel establishment [35].

C. Generic Access Network (GAN)
GAN was developed as an advancement of Unlicensed Mobile Access. GAN is another type of network that can coexist with 3GPP core network. User equipment with multiple radios (WLAN and 3GPP) can access 3GPP through GAN. GAN is applied usually through IEEE 802.11 WLAN. Gateway in 3GPP core named as Generic Access Network Controller (GANC) is responsible for handling traffic coming from WLAN. Initially, UE starts working on by default 3G settings when powered on using WLAN. UE connects to appropriate AP. IP address is configured to perform GAN discovery. It establishes an IPSec tunnel with Security Gateway (SeGW). It registers with GANC. If GANC accepts connection UE GAN mode is enabled [39,40].

1) GAN Discovery and registration:
First of all, for GAN mode selection is done during Discovery phase. MS transfers its GAN Mode Support information to GANC. GANC can assign appropriate port on default GANC based on the GAN mode support information provided by MS. During discovery phase, MS obtains the address of default GANC. It also discovers that of associated SEGW. Then it establishes a secure IPSec Tunnel. After sending IP address query for both SEGW and GANC, MS opens a TCP session with GANC. GANC responses with Discovery accept message. After discovery phase, MS initiates registration with default GANC, which can act as the serving GANC after establishment of connection and registration procedure. GAN registration procedure confirms adequate registration of a mobile to the controller. The procedure helps the MS for appropriate GAN mode selection. After establishing a secure tunnel with SEGW and IP address has also been obtained. MS can then send a registration request message to GANC. Information contained in registration request message is current camped cell of MS i.e., GERAN/UTRAN/EUTRAN, Last LAI or TAI, IMSI and information about required GAN services. If GANC accepts registration request it responds by sending register accept message to MS. 30 | P a g e www.ijacsa.thesai.org Fig. 7 illustrates GAN tunnel establishment procedure using EAP/SIM(AKA) over IKE. In GAN different tunneling protocols are being supported on different levels of network. During establishment of connection to GANC UE establishes an IPSec tunnel with SEGW. The security association of IPSec tunnel is established. Another tunnel protocol used between GANC and GGSN is GPRS Tunneling protocol. This tunnel is established during data transfer from between UE and the network i.e., 2G or 3G. GA-RRC Packet Transport Channel is made for packet switching domain on both sides of network entities. Connection status of GA-RRC is active or inactive. Some triggers are used for GA-RRC PTC state activation. First one is when GANC receives RAB assignment message from GGSN and second one is when GANC receives relocation request from SGSN. When these two triggers happen SGSN includes the information like RAB ID, IP Address and GTP-U TEID in RAB Assignment Request or Relocation Request message sent to GANC. During that time GA-RRC channel on UE is activated by GANC by forwarding the received message from GGSN to UE. After channel activation at UE now that particular UE will be in GA-RRC-Connected PTC-ACTIVE sub state. RAB Assignment is transmitted to GGSN using the same information received already. Upon establishment of PDP context, a mobile may start transmission upward data in GA-RRC PDU. GANC relays the payload part of PDU to SGSN in Iu-PS G-PDU message. SGSN transmits downward user data in Iu-PS G-PDU toward GANC. The message includes MS TEID already received during RAB Assignment Request or Relocation Request messages [39]. On the other hand, previously some work also focused on mobility management. Mechanism for supporting GAN handoff is presented in [41], in which authors focused on adaptive keep alive interval messages for allocation of resources and mobility management and reducing the handoff failure probability. Similarly, authors in [42] presented a VoLGA based solution. They suggested to with some software and interface addition connectivity for VoLGA can be provided.

V. CONCLUSION
In this paper, an in-depth review of various tunneling protocols employed by various wireless and mobile networks for supporting mobility is presented. Tunnel establishment, data transfer, tunnel release and respective usage scenarios for each tunneling protocol has been considered for systematic and thorough comparison of existing tunneling protocols. An insight of each tunneling protocol is provided and discussed. Specifically, mechanisms like security, multiplexing support, multiprotocol support and packets sequencing support has been discussed. Focusing usage scenarios, tunneling mechanism being used in 3GPP wireless LAN interworking, and Generic Access, Network (GAN) has been discussed.
ACKNOWLEDGMENT This work was supported by the Dong-A University research fund. W. Yoon is the corresponding author.