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Most of the WLAN clients will be portable systems.The fact that portable systems are typically battery powered led to another area of specialization in the 802.11 MAC specification. Specifically, features were added to the MAC that could maximize battery life in portable clients via power-management schemes.
To support clients that periodically enter sleep mode, the 802.11 specifies that APs include buffers to queue messages. Sleeping clients are required to awaken periodically and retrieve any messages. The APs are permitted to dump unread messages after a specified time, and the messages go unretrieved.
The power management scheme is relatively easy to implement in a BSS or ESS configuration because the AP is always present. Moreover, the AP isnít battery powered and never enters sleep mode. In an IBSS configuration, however, no full-time AP exists and all systems may desire to enter sleep mode.
The 802.11 does provide for power savings in an IBSS configuration. Essentially, all clients in an IBSS configuration must awaken each time a beacon is sent. The clients randomly alternate the task of transmitting a beacon (Figure 4). Immediately after the transmission of each beacon, a short time period, called an announcement traffic information message (ATIM) window, commences. During the ATIM window, any station can indicate the need to transfer data to another station during the ensuing data-transmission window. Clients with no incoming or outgoing frames pending can reenter sleep mode during the data-transmission window.
The power-management capabilities built into 802.11 apply primarily to NICs. System-level power management in a client remains the domain of the system designer. Obviously, however, the NIC must be able to awaken independently of the client system, and the NIC must be able to awaken the client when it learns of pending incoming frames.
Although this mode is not an optional component of the 802.11 standard, relatively few vendors have implemented power savings.