in a radio telecommunication system , the performance of a mobile unit can be significantly improved during a hand - over procedure by reusing existing security associations that correspond to the mobile unit . by reusing existing security associations , a mobile unit can begin secure communications immediately following the hand - over . otherwise , and in accordance with conventional practice , the mobile unit will have to undertake the time consuming task of renegotiating the required security associations , before it can begin transmitting and receiving secure communications .