A hand-off (later referred to as “horizontal handoff” in this article) occurs when a
mobile system changes its BS. The mobile station communicates via the BS using one
of the wireless frequency sharing technologies such as FDMA, TDMA, CDMA etc. Each
BS is connected to a Mobile Switching Center (MSC) through fixed links, and each MSC
is connected to others via Public Switched Telephone Network (PSTN). The MSC is a
local switching exchange that handles switching of mobile user from one BS to another.
It also locates the current cell location of a mobile user via a Home Location Register
(HLR) that stores current location of each mobile that belongs to the MSC. In addition,
the MSC contains a Visitor Locations Register (VLR) with information of visiting mobiles
from other cells. The MSC is responsible for determining the current location of a target
mobile using HLR, VLR and by communicating with other MSCs. The source MSC
initiates a call setup message to MSC covering target area for this purpose.