Survey of Seamless Vertical Handoff Schemes for WiFi/WiMAX Heterogeneous Network

With the development of wireless communication technology the next generation MN has to move around different access technology. Handoff is the mechanism that enables this feature. Handoff mechanism can be broadly classified into two (a) Horizontal handoff: It is the process of handoff that occurs during the movement of a mobile station among homogeneous base stations, i.e. BSs of same wireless standard. Based on the homogeneous network protocols, generally soft-hand-off process is followed, i.e. there is no disconnection during the process of handoff. This is shown in Fig.1 where horizontal handoff process occurs between BS_A and BS_B. (b) Vertical handoff: It is the process of handoff that occurs during the movement of a mobile station among the heterogeneous base stations, i.e. BSs of different wireless standards. Due to the packet formats of Layer2 and Layer3 and the differences between the communication protocols, there will generally be a disconnection during the process of handoff. This is shown in Fig. 1, where a vertical handoff occurs between BS_C and AP_D. IEEE 802.16 WiMAX (World wide Interoperability of micro wave access) is a evolving wireless technology to provide coverage for metropolitan area with a radius of 25 kms. The limited coverage range of typically 500 metres for WiFi (Commonly known as WLAN) makes it difficult to provide always on connectivity services at any time, any where. It is economically infeasible to use a WiMAX base station for small cells or office. Hence we consider the heterogeneous combination of WiFi and WiMAX to provide both indoor /local and Medium distance/outdoor wireless connectivity.