Generally, wireless local area networking (WLAN) devices must support two channel access methods, namely, contention-based access and polling driven access. Contention based access allows any wireless terminal or access point to capture the channel and transmit a data frame after monitoring the channel in accordance with carrier sensing procedures. When using the polling driven access procedure, on the other hand, all frame exchanges are initiated by an access point, either through a polling signal or by simply transmitting data from the access point to one of a plurality of stations. Either contention based or polling driven access techniques may be used to transport voice traffic between an access point and a plurality of wireless terminals, and extensions to both techniques are available to provide service differentiation between voice and data traffic classes sharing a single WLAN channel. However, the frame exchange sequences defined by both procedures are inefficient when used to deliver voice traffic between an access point and a plurality of stations. Additionally, neither technique offers adequate support for power savings operations in a wireless terminal that is transmitting and receiving voice traffic.
ANSI/IEEE Standard 802.11, 1999 Edition (hereinafter “the 802.11 standard”), defines two different methods of accessing the channel. Section 9.3 of the 802.11 standard defines a polling-based contention-free access method. Section 9.2 of the 802.11 standard defines a carrier sensing contention-based access method.
The WLAN frame exchange sequence using the contention-free access method is illustrated in FIG. 1. Under this method, a communication device such as a mobile terminal (MT), wakes up prior to each beacon transmission and waits to be polled by the access point (AP). The duration between transmission of the beacon and receipt of a polling message at a particular MT can depend on the loading level of the network as well as the scheduling algorithm at the AP. Along with the polling message, the AP also transmits a downlink voice packet. Then, the MT responds with an acknowledgement (ACK) to the downlink voice packet and an uplink voice packet destined to the AP. Upon successful reception of the ACK and uplink voice packet, the AP acknowledges the successful reception by sending an acknowledgement message to the MT. Following a successful frame exchange sequence, the MT can stop monitoring the channel and turn off its radio transceiver. It should wake up again to receive the next beacon transmission. Because the frame exchange sequence depicted in FIG. 1 has to be initiated by the AP, the MT has to be alert at all times waiting for the initiation. This causes a power drain on the MT's battery, which on average, is proportional to the number of MTs on the polling list for a particular AP.
FIG. 2 shows a prior art method 200 of polling in a wireless local area network. The method illustrated in FIG. 2 is similar to the method illustrated in FIG. 1, except that the initial poll frame 202 does not contain any data, and is only a polling frame. In this instances, the polling station seeks to receive data from the polled station, here the MT. In response to the polling frame, after a short interframe space (SIFS) time period 204, the polled station sends a data packet 206 to the polling station. After another SIFS 208, the polling station acknowledges receipt of the data by sending an acknowledgment frame 210.
The problem with the prior art method of polling is that it is driven by the AP, and requires the MT to remain on for long periods, which consumes significant battery life. One solution to this is to let the mobile terminals control polling, as is described in pending U.S patent application No. 60/421490, filed Oct. 25, 2002, titled “Method Of Communication Device Initiated Frame Exchange,”and assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference. While allowing the mobile terminals to control polling could allow for better management of power consumption, it has been found that many access point controllers cannot respond fast enough because the AP must locate data for the MT, place it in a transmit buffer, and then transmit it. Since a SIFS time period is on the order of 9 to 20 microseconds, AP equipment is not always able to respond in time. Therefore, there is a need whereby a mobile terminal can power down for longer periods, but allow an AP time to respond to a polling frame.