802.11协议节电学习memo

根据802.11-2012标准中，章节 10.2 Power management 中的描述，目前 802.11协议中存在的节电技术有以下几种：


节电方式	介绍	引入时间
PS-Poll	通过PS-Poll报文通告AP发送缓存的报文	802.11
S-APSD	配合QoS工作，AP按照事先协商的时间片发送缓存报文	802.11e
U-APSD	配合QoS工作，STA发送Trigger报文通知AP发送缓存报文	802.11e
PSMP	AP通过发送单个PSMP帧i组织所有STA的缓存报文收发	802.11n
TDSL Peer PSM	未了解，SDK中未找到	802.11n（？）
WNM Sleep	协议中可选内容，SDK中未找到	未知
SM Power Save	通过关闭多条receive chains中一部分来省电	802.11n

基础框架＆ PS-Poll
节电方式有时显得很多，很复杂，但是都遵循最基本的框架运作。
- 电源管理状态
  
  Active Mode
  对应 STA 的 Awake 状态，STA 全功率运行，可以随时接收报文。
  
  PS Mode
  对应 STA 的 Doze 状态，STA is not able to transmit or receive and consumes very low power，定期苏醒接收报文。
- 更改电源管理状态的方法
  以下内容节选自 802.11-2012 章节10.2.1, 这些内容规定了状态切换的基本原则。
  
  STAs changing Power Management mode shall inform the AP of this fact using the Power Management bits within the Frame Control field of transmitted frames。
  
  Power Management mode shall not change during any single frame exchange sequence.
  
  The Power Management bit shall be ignored in frame exchanges initiated by the AP.
  
  A non-AP STA shall not change power management mode using a frame exchange that does not receive an ACK or BlockAck from the AP.
  
  NOTE—A PS-Poll frame exchange does not necessarily result in an ACK from the AP, so a non-AP STA cannot change power management mode using a PS-Poll frame.
  
  the Power Management bit is the same for all MPDUs in an A-MPDU.
- PS Mode 下的动作
  - 缓存报文
    - 缓存到 STA 的可缓存的单播报文（某些medium access control (MAC) management protocol data unit不可缓存）
    - 缓存所有的广播报文
  - 发送 TIM 和 DTIM
    
    TIM
    traffic indication map, 每个Beacon都携带
    
    DTIM
    delivery traffic indication map，特殊的TIM，关注广播包围。在携带DTIM的Beacon发出后，AP会随即发送缓存的广播报文。
    
    TIM 具体格式参看 802.11-2012，8.4.2.7 TIM element。
    - Bitmap 中每一个bit与一个STA的AID对应。具体计算方法参加协议。
    - 需要指出的是 Partial Virtual Bitmap 字段指示的是所有 STA 的节点状态，不包括广播报文。Bit 0 对应与 AID 为 1 的 STA。
      
      The Bitmap Control field is a single octet. Bit 0 of the field contains the Traffic Indicator bit associated with AID 0. This bit is set to 1 in TIM elements with a value of 0 in the DTIM Count field when one or more group addressed MSDUs/MMPDUs are buffered at the AP or the mesh STA.
    - TIM 与 DTIM 的区分
      
      The DTIM is indicated by the DTIM count field of the TIM element having a value of 0.
  - STA 接收报文
    - PS-Poll 与 TIM 的关系
    - PS-Poll 获取报文
    - 切换到 Active Mode
      
      When an AP is informed that a STA has changed to the Active mode, then the AP shall send buffered BUs (if any exist) to that STA without waiting for a PS-Poll. When
youtube上有个 Video 介绍了节电的基本概念，对初学者有参考价值。（不过英文着实难懂）
- 节电介绍视频1/2
- 节电介绍视频2/2
U-APSD
- 在基础构架的之上，不同之处在于STA唤醒后于AP交互的方式。Trigger Frame不再是PS-Poll，而是特定优先级的QoS报文作为Trigger报文。
  - Trigger 报文的决定可以在管理过程中协商，也可以关联后用ADDTS报文协商。
- BU 按照 Qos 优先级分别进行缓存。有如下两个概念：
  - trigger-enabled AC
  - delivery-enabled AC
- 注意点：
  - STA 只需要发送一个Trigger报文来通告AP，AP在随后的处理中会发送buffer的多个报文。和 PS-Poll方式不同。
    
    If the STA has set up to use unscheduled SPs, the AP shall buffer BUs using delivery-enabled ACs until it has received a trigger frame using a trigger-enabled AC from the non-AP STA, which indicates the start of an unscheduled SP. A trigger frame received by the AP from a STA that already has an unscheduled SP underway shall not trigger the start of a new unscheduled SP
  - 以下内容指明何时返回休眠。
    
    The STA shall remain awake until it receives a QoS data frame or QoS Null frame addressed to it, with the EOSP subfield in the QoS Control field equal to 1
- 更多内容参考 802.11-2012 的 10.2.1.6，10.2.1.8，10.2.1.9

PSMP
- 原理
  
  Power Save Multi-Poll (PSMP) is a feature of 802.11n that extends the Automatic Power Save Delivery (typically called APSD) mechanism defined in 802.11e by allowing the client stations to operate on a group schedule rather than individually.
  - AP 通过发送 PSMP 报文通知所有STA 之后的上下行缓存报文发送的安排。

SM Power Save
- 原理
  - Spatial Multiplexing Power Save 的缩写
  - 通过关闭硬件的部分流，从而节电。
    
    A STA consumes power on all active receive chains, even though they are not necessarily required for the actual frame exchange. The SM Power Save feature allows a STA to operate with only one active receive chain for a significant portion of time.
  - 有静态和动态两种模式。
    - 静态模式下，STA 将一直关闭部分硬件，持续以单流的形式与AP通行。
    - 动态模式下，STA 关闭部分硬件，处于单流模式。当接受到一个单流的单播报文后，开启省电关闭的硬件，切换到多条流的工作状态。通常使用RTS/CTS报文完成这个动作。
      
      In dynamic SM power save mode, a STA enables its multiple receive chains when it receives the start of a frame sequence addressed to it. Such a frame sequence shall start with a single-spatial stream individually addressed frame that requires an immediate response and that is addressed to the STA in dynamic SM power save mode. An RTS/CTS sequence may be used for this purpose.
- SDK 现状
  - AP 自身处于 HT_CAP_MIMO_PS_OFF 状态，即始终不使用 SM Power Save。
  - STA 可根据自身情况决定 SM Power Save 状况，并通告 AP。关联时或通过Action报文。

其他之前北京XX出现过 Intel 6205 网卡的笔记本无故变成单流，之后发现是笔记本使能了静态的SM Power Save导致。
- 为了避免这种情况对笔记本进行了多种设置和改动，均不能解决。
- AP 方面做实验强制将 STA 设为非 SM Power Save 模式也不能生效。由于这个过程不是协商的，STA 仅是通过自己的 SM Power Save状态。
最终只能认定为笔记本端硬件，或者驱动的动作导致进入该状态。

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2012-11-10

802.11协议节电学习memo

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