Power Management Features

Wireless clients operate in one of two power management modes specified by the IEEE 802.11 standard. These power management modes are active mode, which is commonly called continuous aware mode (CAM) and power save, which is commonly called power save polling (PSP) mode. Conserving power using a power-saving mode is especially important to mobile users whose laptops or PDAs run on batteries. Extending the life of these batteries allows the user to stay up and running longer without a recharge. Wireless LAN cards can draw a significant amount of power from the battery while in CAM, which is why power saving features are included in the 802.11 standard.


Continuous Aware Mode

Continuous aware mode is the setting during which the wireless client uses full power, does not “sleep,” and is constantly in regular communication with the access point. Any computers that stay plugged into an AC power outlet continuously such as a desktop or server should be set for CAM. Under these circumstances, there is no reason to have the PC card conserve power.


Power Save Polling

Using power save polling (PSP) mode allows a wireless client to “sleep.” By sleep, we mean that the client actually powers down for a very short amount of time, perhaps a small fraction of a second. This sleep is enough time to save a significant amount of power on the wireless client. In turn, the power saved by the wireless client enables a laptop computer user, for example, to work for a longer period of time on batteries, making that user more productive.

When using PSP, the wireless client behaves differently within basic service sets and independent basic service sets. The one similarity in behavior from a BSS to an IBSS is the sending and receiving of beacons.

The processes that operate during PSP mode, in both BSS and IBSS, are described below. Keep in mind that these processes occur many times per second. That fact allows your wireless LAN to maintain its connectivity, but also causes a certain amount of additional overhead. An administrator should consider this overhead when planning for the needs of the users on the wireless LAN.


PSP Mode in a Basic Service Set

When using PSP mode in a BSS, stations first send a frame to the access point to inform the access point that they are going to sleep, (temporarily powering down). The access point then records the sleeping stations as asleep. The access point buffers any frames that are intended for the sleeping stations. Traffic for those clients who are asleep continues arriving at the access point, but the access point cannot send traffic to a sleeping client.

Therefore, packets get queued in a buffer marked for the sleeping client. The access point is constantly sending beacons at a regular interval. Clients, since they are time-synchronized with the access point, know exactly when to receive the beacon. Clients that are sleeping power up their receivers to listen for beacons, which contain the traffic indication map (TIM) If a station sees itself listed in the TIM, it powers up, and sends a frame to the access point notifying the access point that it is now awake and ready to receive the buffered data packets. Once the client has received its packets from the access point, the client sends a message to the access point informing it that the client is going back to ‘sleep’. Then the process repeats itself over and over again. This process creates some overhead that would not be present if PSP mode were not being utilized. The steps of this process are shown in Figure 7.18.



PSP in an Independent Basic Service Set

The power saving communication process in an IBSS is very different than when power saving mode is used in a BSS. An IBSS does not contain an access point, so there is no device to buffer packets. Therefore, every station must buffer packets destined from itself to every other station in the Ad Hoc network. Stations alternate the sending of beacons on an IBSS network using varied methods, each dependent on the manufacturer. When stations are using power saving mode, there is a period of time called an ATIM window, during which each station is fully awake and ready to receive data frames. Ad hoc traffic indication messages (ATIM) are unicast frames used by stations to notify other stations that there is data destined to them and that they should stay awake long enough to receive it. ATIMs and beacons are both sent during the ATIM window. The process followed by stations in order to pass traffic between peers is:

• Stations are synchronized through the beacons so they wake up before the ATIM window begins.

• The ATIM window begins, the stations send beacons, and then stations send ATIM frames notifying other stations of buffered traffic destined to them.

• Stations receiving ATIM frames during the ATIM window stay awake to receive data frames. If no ATIM frames are received, stations go back to sleep.

• The ATIM window closes, and stations begin transmitting data frames. After receiving data frames, stations go back to sleep awaiting the next ATIM window.

This PSP process for an IBSS is illustrated in Figure 7.19.


As a wireless LAN administrator, you need to know what affect power management features will have on performance, battery life, broadcast traffic on your LAN, etc. In the example described above, the effects could be significant.