The future fourth-generation (4G) systems will coexist with different radio access technologies, such as GSM/GPRS, UMTS, WiFi, and WiMAX. Multimode or multi-RAT MS (Mobile Station) shall have the ability to seamlessly roam among these heterogeneous networks without disrupting ongoing sessions or connections. Intensive research efforts have been made to define efficient seamless mobility procedures, commonly referred to as vertical handover or inter-RAT (Radio Access Technologies) handover. Hence, many proposals to solve the mobility management problems in heterogeneous wireless networks can be found in the literature. For example, [1] defines the Mobile IPv6 within the network layer to provide transparent support for host mobility. It introduces movement detection, IP address configuration, and location update procedures. However, many experiments and simulations have shown difficulties in efficiently maintaining sessions' continuity during handover, due to long latency and high packet loss rate issues [2]. Another typical and successful handover solution is the 3GPP inter-RAT handover solution in [3]. It can achieve lossless handover procedure by applying sequence number synchronization mechanism at LLC layer (Logical Link Control). Unfortunately, the 3GPP inter-RAT solutions only support inter-RAT handover between cellular networks, for example, between GSM (Global System for Mobile communications) and UMTS (Universal Mobile Telecommunications System). Hence, inter-RAT handover between 802-based networks and cellular networks, for example, between WiMAX (Worldwide Interoperability for Microwave Access) and UMTS, is not supported.
Thus, a seamless and smooth inter-RAT handover procedure does not only depend on the handover mechanism design, for example, the location update, address re-configuration, and data flow rerouting, but it is also tightly related to several important network issues, as follows.
1.1. Coupling Approaches
When applying an inter-RAT handover scheme to the 802-based networks and non-802-based networks, a given interworking architecture, allowing some forms of coupling between these networks, shall be deployed. However, different interworking architectures provide different service qualities for data access and different levels for the seamless roaming. Therefore, one important issue about future wireless system is the design of the coupling architecture allowing the implementation of efficient inter-RAT handover schemes. Depending on the existence of the coupling point and where it is implemented, there are several interworking architectures, namely: no coupling, loose coupling, tight coupling, and very tight coupling (integrated coupling). Taking the interworking scenario between WiMAX and UMTS, for example, as illustrated in Figure 1, loose coupling indicates that the interworking point is behind the GGSN (Gateway GPRS Support Node). Tight and integrated coupling assume interworking at the UMTS SGSN (Serving GPRS Support Node) level or GGSN (Gateway GPRS Support Node) level, and at the UMTS RNC (Radio Network Controller) level, respectively.
Reference [2] summarizes existing handover solutions' performance in different coupling architectures. For example, in UMTS-WiMAX networks, in general, the loose coupling architectures often use Mobile IPv6 or part of Mobile IPv6 as the handover management protocol and require few complicated modifications of the existing UMTS architecture. However, loose coupling approaches often suffer from handover latencies, varying from hundreds of milliseconds to several seconds. Mobility management schemes for integrated and tight coupling approaches are often based on the existing mobility solutions of the UMTS network. Therefore, these approaches require significant modifications to the existing access network architectures and redesign of network protocols, for example, deployment of a NodeB emulator or an RNC emulator. However, integrated and tight coupling approaches can achieve a better handover performance compared to loose coupling approaches. For the design of future high reliable heterogeneous wireless networks, integrated and tight coupling architectures appear as the preferred option.
1.2. Context Transfer
Inter-RAT handover performance is usually evaluated with some recognized metrics, such as drop rate, blocking rate, handover delay, and packet loss. Since most of the future radio access networks are based on data technologies, eliminating packet losses becomes the primary objective. To achieve a lossless handover procedure, the most common solution is applying the context transfer mechanisms to forward context-related data and parameters from the source network to the target network. Thus, what are the most suitable context transfer schemes for a future inter-RAT handover procedure is another important issue.
In 3GPP UMTS network, the PDCP (Packet Data Convergence Protocol) sequence number synchronization procedure [4] is applied to guarantee reliable data transmission service during SRNS (Service Radio Network Subsystem) relocation. PDCP entity maintains PDCP sequence numbers to avoid any data loss. After a successful relocation, the data transmission starts from the (first) unconfirmed SDU (Service Data Unit) having a sequence number equal to the next expected sequence number by the PDCP entity. Unfortunately, in the case of inter-RAT handover, the PDCP sequence number synchronization mechanism cannot be used any more because non-3GPP systems usually do not have a similar synchronization mechanism, especially IEEE 802-based RATs, such as WiMAX or WiFi.
Moreover, buffering-and-forwarding is the most commonly used context transfer scheme for eliminating packet losses. During a handover period, the source network forwards unsent packets/frames to the target network as well as other connection-related parameters. Generally, this scheme cannot eliminate packet losses effectively. For example, if buffering-and-forwarding scheme is deployed at IP layer, during inter-RAT handover period, packets stored at lower layers of one RAT, for example, at link layer, usually cannot be retrieved by the IP layer. So, IP layer buffering-and-forwarding scheme cannot recover lost packets after handover, and the communication may be broken down.
Sachs et al. [5] proposes the SDU reconstruction scheme. In order to support lossless handover, segments stored in the PDU (Packet Data Unit) buffer of source link are first reconstructed back to an SDU and then forwarded to the target link as well as the SDUs from the SDU buffer. However, during the handover period, an SDU, whose corresponding PDUs have not been totally and successfully transmitted, cannot be reconstructed and will be discarded locally. This is because the successfully transmitted PDUs have been already removed before from the local PDU buffer. Therefore, the packet loss during handover period is generally unavoidable.
In [6], a retransmission mechanism is utilized to eliminate packet losses at a sublayer called R-LLC (Remote Link Layer Control) on the BTS for handover between GPRS and WiFi. In the downlink, when a handover is made, a retransmission timer is set for a transmitted packet at R-LLC sublayer. If the acknowledgement corresponding to a transmitted packet cannot be received before its retransmission timer expiration, R-LLC retransmits this unacknowledged packet that is lost during inter-RAT handover. The packet loss is only indicated by retransmission timer timeout, which is set to 5 sec. Unfortunately, the local retransmission window at R-LLC sublayer is not defined. So, buffer overflow at lower layers is unavoidable. Likewise, inter-RAT handover execution mechanism at R-LLC is not specified in [6].
1.3. TCP-Specific Handover Problems
In addition, for the TCP traffic, inter-RAT handover procedure faces some specific problems, such as BDP (Bandwidth Delay Product) mismatch, premature timeout, false fast retransmit, and spurious RTO (Retransmission TimeOut) [2]. These traffic-specific problems make many conventional handover schemes ineffective, because these schemes are generally based on the assumption of simplified traffic, for example, UDP traffics. Therefore, an effective and traffic-oriented inter-RAT handover scheme is desired for future heterogeneous networks.
For TCP-specific handover problems, the existing solutions are generally classified into network-centric and receiver-centric approaches. Network-centric approaches, such as M-TCP [7], TCP proxy [8–10], and snoop protocol [11, 12] split a TCP connection between a CN (Correspondent Node) and an MS. In M-TCP, when a handover is detected, base station sends back the last byte's ACK with zero window size. This ACK forces TCP sender to enter into persist mode. In the persist mode, the values of congestion window (cwnd) and slow-start threshold (ssthresh) are frozen. When a new base station detects handover completion, it immediately notifies TCP sender to resume transmission with the frozen cwnd and ssthresh. Unfortunately, M-TCP is originally designed for horizontal handover and does not consider the wireless link BDP and RTT (Round Trip Time) variations. In [8], a TCP proxy is used to overcome problems of stemming from a large BDP of WCDMA wireless link. An improved TCP proxy is proposed for inter-RAT handover in [10], and yet it suffers from high signaling cost and cannot work in transparent mode after handover. In [11, 12], the snoop protocol introduces a module, called snoop agent at base station. This agent monitors every TCP packet for a connection in both directions and maintains a cache of TCP packets sent across wireless link that have not been acknowledged by MS. A packet loss is detected thanks to duplicate ACKs for MS or local timeout. On a packet loss, the snoop agent retransmits the lost packet if it has cached it on behalf of TCP sender. This snoop protocol is considered as a link-layer protocol that takes advantage of knowledge of TCP protocol. However, snoop protocols are not designed for handover among heterogeneous networks.
Other ways to solve TCP-specific handover problems are receiver-centric approaches. In these approaches, the MS knows the handover occurrence; thus, the MS takes charge of congestion control by modifying ACK message protocol header or feeding back specific ACK messages. For instance, in Freeze-TCP [13], the tasks of base station of M-TCP are shifted to the MS side.
1.4. Mobility Management Framework
IEEE 802.21 [14] is a standard to enable handover and interoperability between heterogeneous network types including both 802-based and non-802-based networks. It defines an extensible media-independent handover (MIH) framework enabling transparent service continuity as an MS roams among different access networks. In this framework, a logical entity MIH function (MIHF) is proposed between layer-2 and layer-3. It can provide cross-layer abstract services to the higher layers through a media-independent interface, and it can obtain cross-layer information from the lower layers through media-specific interfaces. These MIHF abstract services may be either local or remote, that is, local operation occurring within a protocol stack and remote operation occurring between two distant MIHF entities. Thus, MIHF is a very useful facility for link layer inter-RAT handover.
However, how to apply IEEE 802.21 framework to UMTS-WiMAX integrated and tight coupling architectures is still an open problem. In IEEE 802.21 standard, for coupling 802-based networks with cellular networks, generally the MIH PoS (Point of Service) resides deeper inside the access or core networks. Hence, when connected to a 3GPP network such as UMTS, an MS uses layer 3 IP transport to conduct signaling or messages exchanges [14]. This becomes a problem for UMTS-WiMAX integrated coupling architecture, because the layer 3 (IP layer) terminates at SGSN network entity, which is not included in this coupling architecture. In addition, IEEE 802.21 MIHF only defines abstract service model in control plane, and it does not attempt to specify the actual handover execution mechanism. Therefore, how to design an effective handover execution mechanism in user plane remains an implementation issue.
1.4.1. Our Contributions
According to above inter-RAT handover-related issues, in this paper, we propose our traffic-oriented inter-RAT handover as a complete solution. In consideration of the advanced features and limitations of IEEE 802.21 framework, a novel IEEE 802.21 MIHF variant is proposed to realize a seamless inter-RAT handover procedure for integrated and tight coupling architectures. Different from conventional IEEE 802.21 MIHF, this MIHF variant is deployed in both control plane and user plane. In the user plane, it introduces a new intersystem retransmission mechanism and applies cross-layer mechanism to resolve packet loss and long handover latency problems. In the control plane, it simplifies MIHF services model by only defining a few cross-layer triggers and information from lower layers or to upper layers. In order to differentiate it from standardized MIHF framework, this MIHF variant is renamed InterWorking (IW) sublayer scheme. Thus, the primary objective of this paper is to illustrate the flexibility and suitability of this IW sublayer scheme for future heterogeneous wireless networks. In the following, due to the simplified service model in the control plane in IW sublayer scheme, we will not deliberately differentiate control and user planes for the sake of simplicity.
In this paper, we also propose a new TCP snoop agent (TCP Snoop) solution with cross-layer mechanism. Different from enhanced TCP proxy in [10], new inter-RAT handover mechanisms are defined in TCP Snoop and only few pieces of signaling are needed. Furthermore, TCP Snoop can work in transparent or disabled mode after handover. Then, the secondary objective of this paper is to evaluate and highlight the benefits of cross-layer interaction between IW sublayer and the new snoop agent in achieving a smooth inter-RAT handover procedure for TCP traffics.
In our previous work [15, 16], only the IW sublayer itself was discussed. Compared with this previous work [10, 15, 16], the novelties of this paper are the enhanced IW sublayer with cross-layer mechanism and the new TCP Snoop. In the sequel, the terminologies "vertical handover" and "inter-RAT handover'' will be used interchangeably with "handover." Likewise, we only consider the downlink. The remainder of this paper is organized as follows: in Section 2, firstly, we describe the common IW sublayer framework. Then, we specify the intersystem retransmission and cross-layer mechanisms at IW sublayer in the tight and integrated coupling architectures. Afterwards, in Section 3, the new TCP Snoop and its cross-layer interaction with IW sublayer are also specified in detail. The simulation results are given. Finally, our conclusions are drawn in Section 4.