A method of indicating downlink traffic to a mobile subscriber station, hereinafter called MSS, the method comprising the steps performed by the MSS: transmitting (S51, S61) a sleep request message to a base station, hereinafter called BS; entering a sleep mode by receiving a sleep response message from the BS, wherein the sleep response message includes a first sleep identifier, hereinafter called SLPID; and receiving (S53, S54, S63, S64) a traffic indication message from the BS during a listening window of the sleep mode, wherein the traffic indication message includes: a management message type field, and an one-bit format field, hereinafter called FMT field, indicating that an SLPID bit-map based format or an SLPID based format is used for configuring the traffic indication message, wherein if the FMT field indicates that the SLPID based format is used, the traffic indication message further includes a Num_SLPID field and a second SLPID instead of a SLPID bitmap; determining whether the second SLPID included in the traffic indication message is same as the first SLPID included in the sleep response message when the SLPID based format is used; and terminating the sleep mode to receive the downlink traffic when the second SLPID in the traffic indication message is same as the first SLPID included in the sleep response message. transmitting (S51, S61) a sleep request message to a base station, hereinafter called BS; entering a sleep mode by receiving a sleep response message from the BS, wherein the sleep response message includes a first sleep identifier, hereinafter called SLPID; and receiving (S53, S54, S63, S64) a traffic indication message from the BS during a listening window of the sleep mode, wherein the traffic indication message includes: a management message type field, and an one-bit format field, hereinafter called FMT field, indicating that an SLPID bit-map based format or an SLPID based format is used for configuring the traffic indication message, wherein if the FMT field indicates that the SLPID based format is used, the traffic indication message further includes a Num_SLPID field and a second SLPID instead of a SLPID bitmap; determining whether the second SLPID included in the traffic indication message is same as the first SLPID included in the sleep response message when the SLPID based format is used; and terminating the sleep mode to receive the downlink traffic when the second SLPID in the traffic indication message is same as the first SLPID included in the sleep response message. a management message type field, and an one-bit format field, hereinafter called FMT field, indicating that an SLPID bit-map based format or an SLPID based format is used for configuring the traffic indication message, wherein if the FMT field indicates that the SLPID based format is used, the traffic indication message further includes a Num_SLPID field and a second SLPID instead of a SLPID bitmap; determining whether the second SLPID included in the traffic indication message is same as the first SLPID included in the sleep response message when the SLPID based format is used; and terminating the sleep mode to receive the downlink traffic when the second SLPID in the traffic indication message is same as the first SLPID included in the sleep response message.