802.11 Power Management Mode

In wireless network terminal devices, handheld devices and laptops should be large. This type of equipment tends to rely on battery power when used. Therefore, the IEEE 802.11 provides several different power management modes, especially power-saving modes that require improved battery endurance.

This article describes several power management modes for engineers who want to learn basic WLAN knowledge.

The first is the Active Mode)

It is called when the power source of the terminal device is the AC power that connects to the wall power outlet. Such as desktop computers with wireless NICs or laptops with power cords. In this case, the power supply comes from an endless alternating current system, and there is no need for power saving. Therefore, IEEE 802.11 allows the maximum data transmission capability in this case, improve Network Transmission Performance as much as possible.

The second Mode is called Power Save Mode)

Compared with the first model, if the power source of a terminal device is a battery, IEEE 802.11 has designed a mechanism to keep the device running for a long time, enable the wireless network adapter to switch regularly to dozing to save power consumption. This is simple, but there is a series of complex mechanisms behind it. When the workstation is in a nap state, it is bound to be unable to process incoming message frames. At this time, it will rely on the AP in charge of traffic to temporarily keep the incoming message boxes. Of course, the IEEE 802.11 Protocol does not require the AP to provide unlimited assistance to the data boxes of the client in the escrow nap. For the sake of efficiency, the AP will require the workstation to regularly stay awake and retrieve its own message boxes from the AP, at the same time, the buffer space of the AP is also released to meet the subsequent data processing requirements.

Here, you should understand-in power-saving mode, the wireless Nic of the workstation will remain in "dozing"-"awake"-"dozing"-"awake "....... the two-State loop, switching several times per second. In dozing, you have made great efforts and worked hard during awake.

However, there are several questions that need further consideration. When the workstation is in the dozing stage, will there be data transfer? How does the AP know that the workstation has been awakened by the dozing status and can receive the message box back? Can an AP accept all large broadcast or group broadcast frames? Will there be a buffer burst?

If you can raise these questions, it means that you are really an engineer who understands how to think. It is worthy of Hubert's Pat on your shoulders and encouragement. To explain this series of questions, we must start with Beacon, which is regularly issued by the AP.

The Beacon message box contains a set of information named Traffic Indication Map, which can be translated as a Traffic indicator chart 」. After the User device connects to the AP, the AP assigns each user a group of Association IDS (AID. When the AP receives traffic from a certain AID, assuming that the endpoint is just "doze", it will issue Beacon again, transmit the list of AID files that are waiting to be received through TIM. When the client device wakes up, he will naturally find that he is in the TIM's receiving list, so he quickly sends a Poll message to the AP, indicating that he has ready, you can retrieve your own documents. In addition, in order to digest a large number of instant broadcast boxes, the AP makes a rule that every few TIM has a DTIM (Delivery Traffic Indication Message). When DTIM arrives, all clients must be awake at any time, because the AP will use this time to pour all broadcast or group broadcast boxes to all clients. In order to get timeliness, the user end does not even need to use the Poll message box to tell the AP that he is awake, because this is a game rule and cannot be violated by the user end.

If you have a wireless base station at hand, go to the management interface to find it. You will find that in the preset situation, the Beacon Interval Value is 100, that is, 100 ms (0.1 seconds), indicating that a Beacon is sent every 0.1 seconds, since every Beacon has TIM information, it means that there will be 10 TIM messages per second, and the wireless manager can modify this intermittent value on his own. In addition, you can find a DTIM column in the Set screen. If the default value is 2, it indicates that there is a DTIM in every two TIM, there are up to five DTIM instances every second.

The above two power management modes are introduced. In principle, the mode of terminal equipment is based on the power source to allow automatic switching of the system. However, Nic drivers often provide advanced settings so that experienced users can fix the power management mode in "Active Mode" based on the transmission efficiency considerations "; in addition, from the perspective of energy saving, regardless of the power supply type, the "power-saving mode" can be adopted at a rate, and then the inevitable efficiency discount is acceptable.

Finally, there is a third mode calledPower-saving mode automatic transmission (APSD) Mode. This mode is a power-saving mode adopted by AP for QoS operation. It can enable user-side devices with QoS functions to enjoy a longer battery life. The Wi-Fi Alliance has developed a WMM certification standard, which is specially issued to devices supporting IEEE 802.11e. If used with APSD, it is called the WMM energy saving function. In addition, APSD is divided into two types, one called "non-scheduled APSD (U-APSD)", the other is "scheduled APSD (S-APSD )", I will not introduce it here. If you are interested, you can refer to the IEEE 802.11e specification file.