1. Field of the Invention
The present invention relates to a power saving method in a wireless LAN system, and more particularly, to a power saving method in a wireless LAN system conforming to IEEE802.11.
2. Description of the Related Art
Power saving in a wireless LAN system conforming to IEEE802.11 is performed in sequences illustrated in FIGS. 1A, 1B, for example, as disclosed in IEEE Std 802.11, 9.7 Frame exchange sequences Table 21 Frame sequence.
FIG. 1A is a sequence chart illustrating a sequence of operations when no data destined to a terminal station is stored in a base station. The terminal station transmits PS-Poll, which is a control packet for prompting the base station to transmit data, to the base station for requesting the base station to transmit downlink data destined thereto after the terminal station has transitioned to an active mode, in which a transmission/reception function turns on (step 101). Upon receipt of PS-Poll without error, the base station transmits a successful reception notification signal ACK (step 102). The terminal station transitions to a doze state in which the transmission/reception function turns off after it has completed the transmission of ACK.
FIG. 1B is a sequence chart illustrating a sequence of operations when data destined to the terminal station has been stored in the base station. The terminal station transmits PS-Poll to the base station, after it has transitioned to the active mode, for requesting the base station to transmit downlink data destined thereto (step 111). Upon receipt of PS-Poll without errors, the base station transmits a data frame (step 112). Upon receipt of the data frame without errors, the terminal station transmits ACK (step 113). The terminal station transitions to the doze state after it has completed the transmission of ACK.
FIG. 2 is a flow chart illustrating the operation of the terminal station in the foregoing situation. As the terminal station starts a receiving operation, it transitions to an active state (step 201). Next, the terminal station transmits PS-Poll to the base station for requesting the base station to transmit data destined to the terminal station (step 202). Next, the terminal station determines whether a response to PS-Pall from the base station is ACK or data (step 203). If the terminal station fails to receive either ACK or data, the terminal station again transmits PS-Pall (step 202). Upon receipt of ACK, the terminal station determines that there is no data destined thereto stored in the base station, and transitions to the doze state (step 205). Upon receipt of data, the terminal station transmits ACK to the base station (step 204), and transitions to the doze state (step 205), followed by termination of the receiving operation.
In the foregoing sequence of operations, when data destined to the terminal station is stored in the base station, the base station returns a data frame when it receives the PS-Poll signal from the terminal station without errors. However, conventional general wireless LAN base stations are often configured to return ACK to a terminal station in response to PS-Poll from the terminal station irrespective of the presence or absence of data to the terminal station. This is because it takes long time to complete the transmission of the data frame so that the base station informs, as a temporary measure, the terminal station that the PS-Poll signal has been received without errors. IEEE802.11 also approves that ACK is returned.
FIGS. 3A, 3B illustrate sequence charts when a base station returns ACK to a terminal station in response to PS-Poll from the terminal station irrespective of the presence or absence of data destined to the terminal station.
FIG. 3A is a sequence chart when no data destined to a terminal station is stored in the base station. The terminal station transmits PS-Poll to the base station after it has transitioned to the active mode for requesting the base station to transmit downlink data destined thereto (step 301). Upon receipt of PS-Poll without errors, the base station transmits ACK (step 302).
FIG. 3B is a sequence chart when data destined to a terminal station is stored in a base station. The terminal station transmits PS-Poll to the base station after it has transitioned to the active mode for requesting the base station to transmit downlink data destined thereto (step 311). Upon receipt of PS-Poll without errors, the base station transmits ACK to the terminal station (step 312). When data destined to the terminal station is stored in the base station, the base station further transmits a data frame to the terminal station (step 313). Upon receipt of the data frame without errors, the terminal station transmits ACK (step 314).
In this way, data communications in a power save mode in a general wireless LAN differs in a frame sequence chart between the terminal station and base station depending on the presence or absence of data destined to the terminal station stored in the base station. Then, as illustrated in FIGS. 3A, 3B, when the base station returns ACK to the terminal in response to PS-Poll from the terminal station irrespective of the presence or absence of data destined to the terminal station, the terminal station experiences difficulties in determining whether to transition to the doze state when no data is stored in the base station, resulting in difficulties in a reduction in power consumption. This is because the terminal station cannot determine the presence or absence of data destined thereto at the time it receives ACK, and therefore cannot promptly transition to the doze state. This is because the terminal station does not know whether or not data destined thereto is present in the base station at the time it receives ACK. Therefore, even if there is no data destined to the terminal station, the terminal station cannot transition to the doze state immediately after the receipt of ACK.