IEEE defined several different standards for configuring wireless networks and devices. For example, 802.11, 802.11(a), 802.11(b), 802.11(g), 802.11(h), 802.11(n), 802.16, and 802.20. According to these standards, wireless network devices may be operated in either an infrastructure mode or an ad-hoc mode.
In the infrastructure mode, the wireless network devices or client stations communicate with each other through an access point. In the ad-hoc mode, the wireless network devices communicate directly with each other and do not employ an access point. The term client station or mobile station may not necessarily mean that a wireless network device is actually mobile. For example, a desktop computer that is not mobile may incorporate a wireless network device and operate as a mobile station or client station.
A wireless network that operates in the infrastructure mode includes an access point (AP) and at least one client station that communicates with the AP. Since the client stations are often battery powered, it is important to minimize power consumption to preserve battery life. Therefore, some client stations implement a low power mode and an active, or “awake,” mode. During the active mode, the client station transmits and/or receives data. During the low power mode, the client station shuts down components and/or alters operation to conserve power. Usually, the client station is not able to transmit or receive data during the lower power mode.
A power management device controls and selects different operating modes of the client stations. During operation, the power management device instructs some of the modules to transition to a low power mode to conserve power. Additional information may be found in U.S. patent application Ser. No. 10/650,887, filed on Aug. 28, 2003, Ser. No. 10/665,252, filed on Sep. 19, 2003, and Ser. No. 11/070,481 filed on Mar. 2, 2005, which are hereby incorporated by reference in their entirety.
Referring now to FIG. 1, a first wireless network 10 is illustrated in an infrastructure mode. The first wireless network 10 includes one or more client stations 12 and one or more access points (AP) 14. The client station 12 and the AP 14 transmit and receive wireless signals 16. The AP 14 is a node in a network 18. The network 18 may be a local area network (LAN), a wide area network (WAN), or another network configuration. The network 18 may include other nodes such as a server 20 and may be connected to a distributed communications system 22 such as the Internet.
The client station 12 does not continuously transmit data to or receive data from the AP 14. Therefore, the client station 12 implements a power savings mode when the client station 12 and the AP 14 do not have data to exchange. Data commonly remains intact in a network for a predetermined amount of time before it is dropped. The client station 12 informs the AP 14 when the client station 12 is entering a low power mode (and will not be capable of receiving data for a predetermined period). After notifying the AP 14, the client station 12 transitions to the low power mode. During the low power period, the AP 14 buffers data in memory 15 that is intended to be transmitted to the client station 12. Following the low power period, the client station 12 powers up and receives beacon transmissions from the AP 14. If the beacon transmissions indicate that the AP 14 has data for the client station 12, or the host processor or the client station 12 indicates it has data to transmit, the client station 12 remains active. Otherwise, the client station 12 enters the low power mode again.
Before the AP 14 sends out a beacon transmission, the AP 14 determines whether other devices are currently transmitting data so that other devices are able to use the network. The AP 14 transmits a beacon at a target beacon transmission time (TBTT). The client station 12 transitions to the active mode prior to a beacon transmission to queue frames to transmit to the AP 14 in a buffer. Following a beacon transmission, the AP 14 may transmit multicast or broadcast frames to one or more client stations 12 in a deterministic order. To minimize collisions that may occur when one or more client stations 12 simultaneously attempt to transmit data to the AP 14, each station 12 may wait for a random delay prior to a transmission. The random delay, commonly referred to as backoff, reduces the chance of multiple station transmissions occurring simultaneously.
Referring now to FIG. 2, a second wireless network 24 operates in an ad-hoc mode. The second wireless network 24 includes multiple client stations 26-1, 26-2, and 26-3 that transmit and receive wireless signals 28. The client stations 26-1, 26-2, and 26-3 collectively form a LAN and communicate directly with each other. The client stations 26-1, 26-2, and 26-3 are not necessarily connected to another network. The client stations 26-1, 26-2, and 26-3 do not continuously transmit data to and receive data from each other. The client stations 26 implement a power savings mode when one of the client stations 26-1 does not have data to exchange with the other client stations 26-2 and 26-3.
The client stations 26-1, 26-2, and 26-3 periodically transition to the active mode prior to the TBTT and generate a random number. The client station with the smallest random number transmits the beacon to the other client stations. Following the beacon, the client stations 26-1, 26-2, 26-3 may transmit an announcement traffic indication messages (ATIM) frame during a predefined ATIM window to notify other client stations of any buffered data waiting at the client station. For example, if client station 26-1 has buffered data intended for station 26-2, station 26-1 will transmit an ATIM frame subsequent to the beacon to inform station 26-2 of the buffered data. Once station 26-2 acknowledges the ATIM frame, station 26-1 transmits the buffered data to station 26-2.
FIG. 3 depicts a typical system on chip (SOC) circuit 40 that can be used to implement a wireless Ethernet network device, that is, a client station and/or an AP. The SOC 40 generally includes one or more processors 42; a medium access controller (MAC) device 44; a base band processor (BBP) 46; and a host interface. Additionally, the SOC 40 may include a radio frequency (RF) transceiver 48 or the transceiver may be located externally.
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