"This Recommendation defines IP-layer and sub-IP-layer performance parameters for communication scenarios where one or more network technologies provide the access path between a user's communication device and an IP-based network (the Internet is the most common example)@ where the number of access paths to the IP-based network is changing (possibly due to mobility or to other factors such as failure or overload). The performance parameters defined here build on the foundation of point-to-point parameters defined in [ITU-T Y.1540] and point-to-multipoint parameters defined in [ITU-T Y.1544]. One way to view the coverage of this Recommendation in comparison with [ITU-T Y.1540] is to view them both in the context of the 3-by-3 matrix of communication phases and performance categories of speed@ accuracy and dependability. Whereas [ITU-T Y.1540] covers the important phase of communication when the user device has activated a network interface and performed sub-IP layer and IP layer configurations@ this Recommendation recognizes the importance of metrics for access network activation and deactivation corresponding to a user device's mobility into and beyond coverage areas@ and the policy of a user device to use access networks it deems suitable. Furthermore@ this Recommendation recognizes that when more than one access network is accessible to the user device@ the device policy may be to: hand-over communications to one network and deactivate others; prefer one access network but keep the other networks active or in back-up/standby; use the information transfer capabilities of all networks simultaneously. New performance parameters are needed to accommodate all the policies above. At the core of hand-over performance is the simple activation speed and accuracy of the newly accessible network@ followed by the implementation of policy@ or policy action followed by deactivation. Policy implemented before activation@ based on a priori knowledge or field strength measurements is beyond the scope of this Recommendation. This phase of operation is often referred to as access network ""discovery"". Thus@ this Recommendation recognizes policy as an element of activation@ prior to hand-over or possibly precluding hand-over when all information transfer capabilities will be used simultaneously."