Adaptive transport layer protocol for highly dynamic environment
© Jubara et al.; licensee Springer. 2012
Received: 30 March 2012
Accepted: 21 June 2012
Published: 23 July 2012
Computer and wireless communication require Internet accessibility at anytime and anywhere; this includes in a high-speed mobile station such as in speedy trains, fast moving cars as vehicle-to-infrastructure communication. However, wireless Quality of Service (QoS) provisioning in such an environment is more challenging. This increased the development of numerous schemes concerning the need of smooth handover of the mobile nodes. Conversely, transport layer (L4 in ISO layers) protocols such as stream control transmission protocol can support such a seamless handover in high-speed mobility users. This article highlights on the issues of moving users in mobile WiMAX networks. An adaptation of transport layer protocol of the high mobility vehicle that supports seamless handover can guarantee and maintain QoS for rapid handover rates. The results show an improvement of L4 protocol in terms of delay time and throughput in order to enable efficient and robust mobility aware protocols.
With more users moving around in need of Internet connection from their home to their office, vehicular ad-hoc network (VANETs) has increasingly become popular. However, to have infrastructure of 3G and 4G around VANET expands its usage by attaching the users to the backbone infrastructure for additional support and usage applications. Thus, in VANET there are two types of communication, which are vehicles-to-vehicle (V2V) and vehicle-to-infrastructure (V2I). V2V deals with communication between vehicles themselves, while V2Itransmits information between vehicles and the fixed infrastructure which are installed on the sides of the road. This infrastructure includes gateways or base stations that provide services such as Internet access. VANET is very similar to mobile ad-hoc network (MANETs). However, the network topology in vehicular networks is highly dynamic and the topology is often constrained by the road structure [1, 2].
Furthermore, V2I is likely to encounter a lot of obstacles such as poor channel quality and connectivity due to high moving speeds. Thus, there is a crucial need for effective protocols that take the specific characteristics of vehicular networks into account [3, 4].
Most of the existing transport layer techniques proposed for mobility cannot deal with mobility on their own, since they depend on the network layer mobility management required by handovers. The main purpose is simply to minimize the degradation of transport layer performance caused by handovers. Some of the newly emerging protocols, such as stream control transmission protocol (SCTP), suggest the possibility of independent management of mobility by the transport layer. The multi-homing features of SCTP provide a basis for mobility support since it allows a mobile user to add new IP address, while holding the old IP address already assigned to itself [5–9].
The rest on the article is organized as follows. The following section presents the highly dynamic environments literature review, and the related works. An overview of vehicular network mobility management in terms of five requirements is detailed in “Vehicular network mobility management”. The cross-layer design of the high speed to overcome the problem statement is discussed in Section “Proposed transport layer adaptation for high-speed vehicle”. Section “Simulation topology” describes simulation topology and parameters. Section “Results and discussion” presents results of the protocol design mentioned, and the final section concludes the article.
Highly dynamic environments
Mobility management is one of the most challenging research issues for vehicular networks to support a variety of intelligent transportation system (ITS) applications. Some traditional mobility management schemes for Internet as MANET have to meet the requirements of vehicular networks, and characteristics of vehicular networks (e.g., high mobility). Therefore, mobility management solutions developed specifically for vehicular networks would be required.
Vehicular network mobility management
Mobility management flows
Fast handover is needed for delay sensitive ITS applications (e.g., safety, Internet access, etc.). Fast handover is also a crucial requirement for wireless networks with small coverage area (e.g., WiFi network), since the vehicle with high speed spends short period of time at each point of attachment (e.g., Base station). Consequently high handover rate
The global reachability requires a comprehensive reliable routable IP address for each MN. IPv6 with large address space can support a unique address for all mobile devices in the vehicles. In addition, IPv6 also has better support of security and quality of service (QoS) which are the necessary requirements of ITS applications
High mobility speed
The Internet access is expected to be constantly connected regardless of the movement speed. It is highly desirable to make these contents available and reliable regardless of time, place, fixed, or mobile. As the speed of vehicle increases, the successful probability of handover decreases as the handover execution time is increased
Vehicle needs to detect the availability of different types of access networks (e.g., WiMAX base station) known as data link layer handover (L2), and obtain addresses in these networks for communication
Location management scheme, which deals with the storage, maintenance, and retrieval of MN location information, is needed in VANETs 
Mobility support for users and vehicular networks requires network connection as interactive and real-time applications become increasingly important. Therefore, many seamless-mobility approaches have been developed to avoid service disruption and minimize the awareness of service degradation while the mobile device is moving fast. The study of  proposed a cross-layer scheme called CEAL to support mobility of transport layer protocol mSCTP using data link layer primitives. The performance evaluation shows less handover delay in WLAN environments. In [11, 13, 14], various approaches that support seamless and lossless handover in the high-speed transportation system were described. The study of  exploits prediction technique to improve and optimize the performance in high-speed environments. Thus, there would be no problem regarding insufficient time in connection establishment as the speed increase. A study in  also suggested that 802.21 centric approaches used to exploit a prior knowledge method where network information is gathered from both mobile terminal and network infrastructure to establish an earlier connection with the new subnet. In order to reduce the effect of service interruption in the high movement speed environment, the study of  propose a packet forwarding control scheme to select a common ahead point as the tunnel source to forward packets. Using this method, packets can be sent through a shorter delivery path during handover. The authors of  proposed network mobility protocol for VANETs NEMO protocol for VANET in highway. Since every car is moving in a fixed direction with high moving speed, the car adopting this protocol can acquire IP address from the VANET through the V2V communications. In , they presented a cross-layer handover scheme, called vehicular fast handover scheme, where the physical layer information is shared with the MAC layer, to reduce the handover delay. Using lower layer’s handovers, the transportation layer will not be aware of the handover which may cause packet loss and degradation of the network QoS.
Transport layer-based approach such as mobile SCTP (mSCTP) influences the ability of SCTP to have multiple IP addresses per association. mSCTP utilizes a feature of SCTP, which allows an MN dynamically switch between available access networks thus affecting seamless handovers. The authors of  provide analysis that mSCTP can provide lower handover latency than mobile IP and give much smaller handover latency for vertical handover. Hierarchical transport layer mobility protocol which is a new proposed option that deals with the local and global mobilities to improve throughputs during the handoff period. This protocol exploits the dynamic address reconfiguration feature of SCTP and introduces an anchor mobility uniting order to complete more efficient handoff procedures. A novel error recovery mechanism associated with a handover was discussed in  where the error recovery time of this mechanism is analyzed and compared to that of the plain SCTP for handover cases. The previous work mainly focuses on low or medium speeds. However, the needs to maintain a seamless communication in the high-speed situations is becoming highly attractive and challenging issue that needs to be tackled [3, 4, 11].
Proposed transport layer adaptation for high-speed vehicle
Handover procedure of SCTPcd
where L2 data link layer delay, L4: Transport layer delay.
To evaluate our idea, a simulation used was OMNET++ cooperatively with MATLAB. As shown in Figure 4, the vehicle is multi-homed node moving with speed of 70–120 km/h along highway and connected to the Internet through wireless access point (WiMAX BS). The coverage area of each BS about 2000 m, and the overlapping region between two BSs is 200 m. Moreover, from the network side, each two BSs connected to one AR, and both of two ARs connect one MAP. This MAP directly joins this network to the Internet as Gateway. As shown in Figure 2, other part of the network connect the CN as a single-homed node sending traffic to the vehicle, which corresponds to the services like file downloading or web browsing by mobile users. However, LM uses by SIGMA as a network control entity.
Results and discussion
The simulation scenario taking accounts the MS speeds between 1 and 40 m/s. 40 m/s (equals to 144 km/h), which is above the 100 km/h limit described in IEEE 802.16e for a seamless handover. When the vehicle is moving to the border of one BS in a certain speed, the signal quality of the SBS begins to degrade. Consequently, either the signal strength becomes low to initiate handover by sending (MOB-MSHO-REQ/RSP) messages. Alternatively, when the signal strength is below threshold (WiMAX standard 2 dB) and the actual process of HO would be executed as (MOB–HO–IND) sends.
Comparison of cross-layer and SIGMA throughputs for different speeds (15–40 m/s)
Internet accessibility in high-speed vehicles as V2I is more challenging and raise the need of least delay. In this article, an adaptive algorithm was proposed on L2 to support seamless handover in high-speed vehicles that connecting to a CN through the Internet. Moreover, a proposed cross-layer design at L2 has adapted L4 of SIGMA protocol design for global reachability to network. The cross-layer design dynamically updates L4 of handover at the time when network parameters (RSSI, SNR) degrade to unacceptable level. The results show that our design achieves better performance about 90% when speed is higher than SIGMA protocol design.
The authors would like to thank all those who contributed toward making this research successful. Also, we would like to thank all the reviewers for their insightful comments. The authors wish to express their gratitude to Ministry of Higher Education (MOHE), Malaysia and Research Management Center (RMC), Universiti. Teknologi Malaysia for the financial support of this project under GUP research grant no: Q-J I 30000.71 23–02 J93.
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