Mobility is one of the most important features of a wireless communication system. Usually, continuous service is achieved by supporting handoff (or handover) from one cell to another. Handover is defined as a process, in which a MN shifts from the air interface provided by another BS. It is often initiated either by crossing a cell boundary or by deterioration in quality of the signal in the current channel. Handoff is divided into two broad categories-hard and soft handoffs. They are also characterized by "break before make" and "make before break." In hard handoffs, current resources are released before new resources are used; in soft handoffs, both existing and new resources are used during the handoff process. Poorly designed handoff schemes tend to generate very heavy signaling traffic and, thereby, a dramatic decrease in quality of service (QoS). The reason why handoffs are critical in cellular communication systems is that neighboring cells are always using a disjoint subset of frequency bands, so negotiations must take place between the mobile station (MS), the current serving base station (BS), and the next potential BS. Other related issues, such as decision making and priority strategies during overloading, might influence the overall performance.
The reason for HO can be various:
RSS is not enough for maintaining proper connection at the edge of the cell.
BS capacity is full and more traffic is pending.
Faster or cheaper network is available (in the case of vertical handover).
HO process occurs is two stages:
Fig. 3.1 handover scenario 
3.2 Qos in WI-FI and WiMAX handover:
The mobility with quality of service in wireless networks is a reality ever closer. The need for mobility while communicating becomes more and more necessary. Technologies for wireless networks with interoperability develops the concept of "communications anytime anywhere". Interoperability uses the handover to ensure the mobility of a mobile terminal through various communication infrastructures, supported by different technologies of wireless networks. Maintain the connection between base stations and mobile terminal is the key of the customer's satisfaction, with the guarantee of continuity of services and applications running, moving between different cells, using the technology of wireless communications to provide better coverage in the position where is. The aim of this study is to examine how the mobility between different access technologies in wireless networks can be optimized and what their behavior in scenarios of interoperability. To examine the quality of the service of mobility, we analyze the time required for the handover and the existing loss of packages, the latter being a very important factor to satisfy the user.
3.3 Different types of handovers:
Handovers can be divided into hard and soft handover:
A hard handoff is one in which the channel in the source cell is released and only then the channel in the target cell is engaged. Thus the connection to the source is broken before the connection to the target is made-for this reason such handoffs are also known as break-before-make. Hard handoffs are intended to be instantaneous in order to minimize the disruption to the call. A hard handoff is perceived by network engineers as an event during the call.
A soft handoff is one in which the channel in the source cell is retained and used for a while in parallel with the channel in the target cell. In this case the connection to the target is established before the connection to the source is broken, hence this handoff is called make-before-break. The interval, during which the two connections are used in parallel, may be brief or substantial. For this reason the soft handoff is perceived by network engineers as a state of the call, rather than a brief. When a call is in a state of soft handoff the signal of the best of all used channels can be utilised for the call at a given moment or all the signals can be combined to produce a clearer copy of the signal. The latter is more advantageous, and when such combining is performed both in the downlink and the uplink the handoff is termed as softer. Softer handoffs are possible when the cells involved in the handoff have a single cell site, termed as softer. Softer handoffs are possible when the cells involved in the handoff have a single cell site.
3.4 horizontal handover and Vertical Handover:
3.4.1 Horizontal handover
The main concern of horizontal handover is to maintain on-going service although the change of IP address due to the movement of a mobile node. Maintaining on-going service is done by hiding the change of IP address (e.g., Mobile IP) or dynamically updating the changed IP address (e.g., mSCTP).
To hide the change of IP address during the movement of a mobile node, Mobile IP keeps two types of IP address; one permanent IP address (Home address) might be used above transport layer and one changeable IP address (Care-of address) might be used under transport layer. mSCTP could dynamically update the IP address during the on-going service.
The majority of proposed handover mechanism mights be included in horizontal handover because they focus on maintain on-going service even though only the IP address is changed.
3.4.2 Vertical handover
Vertical handover is happened when a mobile node moves across heterogeneous access networks. Differently from horizontal handover, the used access technology is also changed as well as IP address, because the mobile nodes moves different access network which uses different access technology. In this case, the main concern of vertical handover is to maintain on-going service although not only the change of IP addresses but also the change of network interfaces, QoS characteristics, and etc.
For example, a suitably equipped laptop might be able to use both a high speed wireless LAN and a cellular technology for Internet access. Wireless LAN connections generally provide higher speeds, while cellular technologies generally provide more ubiquitous coverage. Thus the laptop user might want to use a wireless LAN connection whenever one is available, and to 'fail over' to a cellular connection when the wireless LAN is unavailable. Vertical handoffs refer to the automatic failover from one technology to another in order to maintain communication. This is different from a 'horizontal handoff' between different wireless access points that use the same technology in that a vertical handoff involves changing the data link layer technology used to access the network. Vertical handoffs among a range of wired and wireless access technologies including WiMAX can be achieved using Media independent handover which is standardized as IEEE 802.21.
Table 3.1: Changed items in horizontal/vertical handover 
Could be changed
Could be changed
It is noted that the any handover mechanisms for horizontal handover could not directly be used for vertical handover. Because handover mechanisms for horizontal handover have only the ability to solve the problem of the change of IP address, they could not maintain on-going service when network interfaces or QoS characteristics are changed. To support vertical handover, the modifying of legacy Mobile IP is needed . The main capabilities of vertical handover as compared to horizontal handover are as follows;
Usage of different access technologies
Usage of multiple network interfaces
Usage of multiple IP addresses
Usage of multiple (changeable) QoS parameters
Usage of multiple network connections (multi-homing features)
Horizontal handover assume that the mobile node has only single network interface, single IP address, and single network connection at a time. Break-before-make in horizontal handover is that a mobile node can make a new network connection only after disconnecting the old connection.
If we use multiple network interfaces, multiple IP addresses, and multiple network connections for vertical handover, we can develop MM mechanism based on make-before-break. So, the latency time of handover could be decreased or eliminated perfectly.
Table 3.2: Capabilities in horizontal/vertical handover 
IP address at a time
Single IP address
Multiple IP address
Because MM mechanisms for horizontal handover assume that the mobile node has only single network interface, single IP address, and single network connection at a time, there are many restrictions to develop MM mechanisms such as break-before-make. Break-before-make in horizontal handover is that a mobile node can make a new network connection only after disconnecting the old connection.
If we use multiple network interfaces, multiple IP addresses, and multiple network connections for vertical handover, we can develop MM mechanism based on make-before-break. Make-before-break is that a mobile node can make a new network connection before disconnecting the old connection. So, the latency time of handover could be decreased or eliminated perfectly. Also, vertical handover mechanism based on soft handover could be available
3.5 How to decide which type of handover suitable:
For the mobile device the steps to perform a horizontal (within a same network type) or vertical (across heterogeneous network) handover can be classified into three logical tasks shown in Figure 1 which in chronological order are, i) "when and why" should the device transition to a new network, ii) "where" should the device transition to, and iii) "how" should the device transition between networks to maintain connectivity and session persistence. The "when and why" is triggered when the mobile device receives an indication that a transition should take place. These can include external conditions like signal degradation or network congestion, discovery of a more suitable network providing lower cost, higher bandwidth or better energy efficiency, or be user or network initiated. The "where" step is when the mobile device selects the next network to connect to, and the "how" defines how the device performs the transition, e.g. either doing a horizontal or vertical handover.
The network can facilitate the mobile device network selection by providing a network map describing the presence and characteristic of networks available in the vicinity of the mobile device. Additionally the network itself can initiate a handover request when for example it detects that the load on one of the access networks is nearing capacity. Watch the video below to hear researcher Vijay Kesavan discussing the technology behind a WiFi/WiMAX handover.
"When and why?" to handover. The goal is to provide accurate and predictive link layer information about an impending degradation or loss of signal to start a handover - for example a make-before-break operation. A three step process predicts with a high level of accuracy the signal quality direction. We
Smooth out the signal characteristics,
Predict the signal level in the near future and
Perform a trend analysis, focusing on methods that provide accuracy with a low computation overhead.
We use exponential average for smoothing, "Straight Line" for prediction and Fast Fourier Transform for trend analysis on both the short and long term window. We predict link layer degradation on WiFi and WiMAX with about 90% accuracy and provide this indication about 800 msec to 1 second before the loss happens thereby providing a significant lead time to take action.
Figure 3.2: System Architecture
"Where?" to handover to? Once indication of an impending disconnection is received, the next decision is to select another suitable network to connect to. In the wireless environment the ability to discover networks depends on the detection and proximity of the signal source and the environment, e.g. interferers, obstacles, etc. Mobile devices periodically scan to "search" wireless networks on each of their interfaces thus consuming power. This is where the information learned from the network on the existing connected channel, e.g. a network information map describing the presence and characteristics of other networks, helps the mobile device improve its energy efficiency by scanning only on appropriate interfaces at specific geographic locations. To evaluate the best network to go to, a cost function taking into account network level (e.g. bandwidth, delay, security, etc.), platform level (e.g. battery load, thermal, etc.), user's preferences (e.g. monetary cost, operator, etc.) and network preferences defined by network operator/virtual network operator/enterprise is used.
"How?" to handover to? This is the execution part of the handover process. There are many methods available for handling session handover, e.g. Mobile IP (MIP), Proxy Mobile IP, Session Initiation Protocol (SIP), Voice Call Continuity (3GPP VCC). We used a SIP based A/V application.
Figure 3.3: Heterogeneous Handover with predictive triggers timing diagram
Using the optimizations mentioned earlier we can see in Figure-2 that we can start a make-before-break where the two radios are concurrently active with enough lead time to support seamless handover for multimedia sessions. (It should be noted that authentication has not been taken into account here). Our implementation is based on the IEEE 802.21 Draft specification for both client and network components (Information Server and Command Server), but this is only one option among many to achieve this.