ng system (GPS) [29] receiver that enables the MN to determine its location with high accuracy. Therefore the network coverage area can be easily determined by having MNs report their positions to the network whenever the RSS at the MN falls below a pre-defined © 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security threshold. As the radio propagation environment varies over time with such factors as weather conditions and human activities, determination of network coverage area should be an ongoing process. The movement of an MN can be predicted based on past location informa- tion. Assume that the MN tracks its location every AT time and the past h samples of the recorded positions are (Lx,k, Ly,k), where k E [0, h - 1] and Lx,k and L,k is the position of the MN in the X and Y directions at the k-th time unit. As the handoff only lasts for several seconds, we can consider the movement trajectory of the MN during this period as a straight line. Denot- ing the movement direction and velocity of the MN as O and v, we can get Lx = Lx,0 + vt COS 0 and Ly = Ly,0 + vt sin O. We can estimate the value of V and O by linear regression as: h1)() (11.5) 0= argtan((o'Wo)-1(/WY)) (11.6) Here Wk is the weight on the past k-th time unit and Wk-1 > Wk > 0. = (Lx,0 - Lx,1, Lx,1,..., Lx,h-2 - Lx,h-1)', Y = (Ly,o - Ly,1,..., Ly,h-2 - Ly,h-1)' and W = diag (w0, . . , Wh-2). As the positions are sensed at regular intervals, the MN should initiate the handoff when it is leaving the access network within AT + Dhandoff time units. Since the prediction may not always be correct, if the MN leaves the access network earlier than the estimated time, packets will be lost before the handoff is complete. If it leaves later than the estimated time, the overall service will be unnecessarily degraded. The balance depends on the traffic type and user's preference. Consequently, we suggest adding an adjustable factor in the expression of the remaining time threshold (i.e., it becomes + Dhandoff), where the value of