The mechanism of efficiently managing power energy is becoming highly significant due to the mobile nature of IEEE802.16e as well as the battery dependency of mobile devices. IEEE802.16e advocates a sleep mode facility so that the power consumption of a Mobile Station (MS) is minimized. As there is no need for MS to send packet always to its base station (BS), both can come to a common agreement that the MS can temporarily go to sleep mode. During the sleep interval, if there is any traffic pending, BS will inform it through MOB_TRF-IND message. When going into sleep mode and getting back to awake mode, MS needs to spend some energy. If an attacker copies a valid MOB_TRF-IND and transmit it to a sleeping MS again and again, he/she will be draining the battery energy of the affected MS at a maximum level. If the energy is not efficiently managed and the battery power of MS is completely drained, it leads to Denial of Service (DoS) attack. Therefore care should be taken to satisfy the security requirement called Availability. In this paper, secured MOB_TRF-IND message is implemented using X.509 certificate so that an attacker cannot do any malpractices in order to exhaust the energy resources of the victim MS.