In a mobile communications network, the subscriber can move around freely, & to maintain the constant connection with subscriber, so that he can use all his services without any disturbance is done with the help of Hand-Over.The basic concept is when the subscriber moves from the coverage area of one cell to another, a new connection with the target cell has to be set up and the connection with the old cell has to be released.
Types of handover
Intra cell - Intra BSC handover - The smallest of the handovers is the intra cell handover where the Subscriber is handed over to another traffic(generally in another frequency) within the same cell. In this case the BSC controlling the cell makes the decision to perform handover.
Inter cell - Intra BSC handover - The subscriber moves from cell 1 to cell 2. In this case the handover process is controlled by BSC. The traffic connection with cell 1 is released when the connection with cell 2 is set up successfully.
Inter cell - Inter BSC handover - The subscriber moves from cell 2 to cell 3, which is served by another BSC. In this case the handover process is carried out by the MSC, but, the decision to make the handover is still done by the first BSC. The connection with the first BSC (and BTS) is released when the connection with the new BSC (and BTS) is set up successfully.
Inter MSC handover - The subscriber moves from a cell controlled by one MSC/VLR to a cell in the domain of another MSC/VLR. This case is a bit more complicated. Considering that the first MSC/VLR is connected to the GMSC via a link that passes through PSTN lines, it is evident that the second MSC/VLR can not take over the first one just like that. The MSC/VLR currently serving the subscriber (also known as the anchor MSC), contacts the target MSC/VLR and the traffic connection is transferred to the target MSC/VLR. As both MSCs are part of the same network, the connection is established smoothly. It is important to notice, however, that the target MSC and the source MSC are two telephone exchanges. The call can be transferred between two exchanges only if there is a telephone identifying the target MSC. Such a situation makes it necessary to generate a new the Handover Number (HON).
When a mobile user travels from one area of coverage or cell to another cell within a callâ€™s duration the call should be transferred to the new cellâ€™s base station. Otherwise, the call will be dropped because the link with the current base station becomes too weak as the mobile recedes. Indeed, this ability for transference is a design matter in mobile cellular system design and is call handoff.
Types of handoff
Hard handoff -In a mobile cellular communication network, a Hard handoff (or Hard handover) is a typical Handoff mechanism in a communication network which is designed to work by first breaking off from the initial connection with a base station before switching to another base station. This is done in order to retain communications in a session for mobile users after incurring a non perceptible and insignificant brief interruption. A Hard handoff is also referred to as â€œBreak-before-Makeâ€ handover
Soft handoff -In cellular telephone communication, soft handoff refers to the overlapping of repeater coverage zones, so that every cell phone set is always well within range of at least one repeater also called a base station). In some cases, mobile sets transmit signals to, and receive signals from, more than one repeater at a time.Soft handoff technology is used by code-division multiple access (CDMA) systems. Older networks use frequency division multiplex (FDM) or time division multiplex (TDM). In CDMA, all repeaters use the same frequency channel for each mobile phone set, no matter where the set is located. Each set has an identity based on a code, rather than on a frequency (as in FDM) or sequence of time slots (as in TDM). Because no change in frequency or timing occurs as a mobile set passes from one base station to another, there are practically no dead zones. As a result, connections are almost never interrupted or dropped.