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Thursday, June 23, 2011

Heterogeneous Wireless Networks: integration architectures that allow seamless handover

Various wireless network technologies have been developed to offer Internet access to end users. The technologies have been developed for different purposes and thus provide different services, coverage areas, network bandwidths, and so on. For example, the second-generation (2G) and third-generation (3G) cellular networks have been developed to cover large areas, whereas wireless LANs (WLANs) offer smaller areas. Furthermore, 2G/3G cellular networks provide relatively low data rates up to 14.4 Mbps while WLANs offer higher data rates up to 54 Mbps. According to these heterogeneous and complementary characteristics, various wireless networks will coexist and interwork together to support the different requirements of end users such as high usability, seamless connectivity, delay-sensitive applications, etc., in next-generation (4G) wireless networks.

4G wireless networks are required to support network heterogeneity and complementariness for satisfying end users’ requirements. The features of 4G networks can be summarized as follows: (1) providing high-data-rate services to accommodate numerous multimedia applications; (2) supporting the mobility of outdoor pedestrians and vehicles; and (3) providing end users with high-speed, large volume, good quality, global coverage, and flexibility. According to such features, 4G networks will be adapted to various forms in several fields such as medical centers, natural environments, information industries, etc. Moreover, 4G networks will coexist with successfully deployed wireless networks because it enables a reduction in developing cost and risk. Therefore, 4G networks will be expected to have heterogeneous wireless networks and to be deployed with variable physical, networking, and architectural characteristics. 3GPP/3GPP2 has introduced two integration architectures for 3G and WLANs as one of the first steps toward achieving seamless handovers. Since both networks have complementary characteristics and have already been well deployed in the world, the integrated architecture can provide integrated service capabilities across 3G and WLANs. One of the two integration architectures is the tightly coupled architecture, where WLANs appear as one of 3G access networks and provide 3G services to WLAN users. This architecture uses WLAN gateways in order to hide 3G protocol details from WLAN users. The illustration below shows the tightly coupled architecture between 3G and WLANs.
A WLAN gateway is connected to SGSN/GGSN of 3G networks and provides translation services between the 3G and the WLAN protocols in order to offer 3G services to WLAN users. However, this architecture requires a new interface between the SGSN/GGSN and the WLAN gateway, which requires modifications or extensions to already deployed 3G networks. Therefore, it introduces a network deployment cost as well as a network integration cost during the initial network deployment of integrated 3G and WLANs.

The other integration architecture is the loosely coupled architecture. Contrary to the tightly coupled architecture, it integrates 3G and WLANs based on the Internet architecture. The illustration below shows the loosely coupled architecture. As shown in the figure, this architecture completely separates 3G and WLANs since networks are integrated into the Internet. According to this characteristic, the loosely coupled architecture does not require any modifications or extensions to deployed 3G networks, contrary to the tightly coupled architecture. However, the loosely coupled architecture needs a mobility management protocol to support handovers between 3G and WLANs.
4G wireless networks will consist of various wireless networks and will be integrated into IP-based networks, which further require seamless vertical handovers. Once the decision of a vertical handover has been made, the key issue for the seamless handover is a mobility management system.

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