A message encoding specification and implementation where a receiver of an encoded message determines if the message can be decoded. A pre-authentication request is received by a first access point associated with a first subnet from a mobile node requesting that is requesting pre-authentication with a second access point associated with a second subnet. Each main coordinates of a direct image obtained as an object point on the object is imaged on the imaging device is assigned to corresponding sub-coordinates of an indirect image of the object point formed from the reflection surface on the imaging device. The removable component has a connecting end which is connected into the placing space, as the hard disk is placed in the placing space, the removable component fits the hard disk, and thus the hard disk can be easily removed by applying force on the removable end of the removable component.