which corresponds to the map id of the currently possessed available channel list with the set period if the mode i sta does not receive the cvs corresponding to the map id of the currently possessed available channel list for the cvstimeinterval the mode i sta judges that the channel list corresponding to the map id is not valid anymore in particular the cvstimeinterval can be represented as an expiration date of the available channel list if the mode i sta does not possess a valid available channel list the mode i sta should obtain an available channel list in a manner of performing the mode i caq process again a case of not capable of receiving the cvs which corresponds to the map id of the currently possessed available channel list for the cvstimeinterval by the mode i sta may include a case of not capable of receiving the cvs itself e g a case of getting out from the coverage of the mode ii sta by the mode i sta and a case that the map id of the cvs is not matched with the map id of the currently possessed available channel list although the cvs is received in this case the mode i sta judges that the currently possessed available channel list is not valid anymore the mode i sta should obtain new available channel information corresponding to the map id included in the cvs in a manner of transmitting the mode i caq again and receiving a mode i caq response in case that the mode ii sta moves the cvs and the mode.