目录
一、基本介绍（Introduction）
二、进化发展（Evolution）
三、PHY帧（（PHY Frame ）
四、MAC帧（MAC Frame ）
五、协议（Protocol）
六、安全（Security）
七、802.11ac标准
八、802.11ad标准
九、802.11ax ( WiFi 6 )标准
十、WiFi直接连接（WiFi Direct）
十一、测试（Testing）
十二、WiFi射频测量（WiFi RF Meas.）
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*一、基本介绍（Introduction*）
1、总体网络架构（Overall Network Architecture）
使用WLAN的最常见方式如下。在大多数情况下（ In most case），主通信线路将是有线线路，我们将AP（Access Point接入点）连接到有线骨干网（wireline backbone），如下所示（as shown below）。接入点是一种与客户端设备client device（如笔记本电脑Laptop或智能手机）通信并通过有线主干（wireline backbone）传输（convey）数据的设备。接入点有两个不同的接口（interface），一个用于使用有线协议（wireline protocol）连接到有线骨干网，另一个用于通过无线协议（wireless protocol）与各种移动设备通信。
当我们在技术上说WLAN时，它通常是指在无线通信方法中用于连接移动客户端（mobile client）和接入点的技术。我将发布（post）的页面也将是关于移动客户端和接入点之间的无线通信技术。
WLAN（Wireless Local Area Network无线局域网）基本上是一种设置（setup），可以让你在不使用任何电缆（cable）的情况下将笔记本电脑（laptops）、手机或平板电脑（tablet）等设备连接到互联网。无线局域网的主要部分是接入点（APs）、无线设备（wireless devices）和有线骨干网（wired backbone）。
当你将设备连接到Wi-Fi时，它会与接入点通信，然后接入点通过有线骨干网将你的数据发送到需要的地方。这种设置可以让你在没有任何电缆的情况下使用互联网和共享数据，这非常方便（pretty convenient）。
1）接入点（APs）就像是无线设备和有线网络之间的中间人（middleman）。它们会产生一个Wi-Fi信号，这样你就可以将你的设备连接到互联网上。当你有很大的空间需要覆盖时，你可能需要多个AP来确保你在任何地方都能得到良好的覆盖（coverage）。
2）无线设备（wireless devices）是你用来连接Wi-Fi的设备，比如你的手机、笔记本电脑或平板电脑。他们有一个无线适配器（wireless adapter），可以使用Wi-Fi信号与接入点通话。
3）有线主干网（wired backbone）连接所有接入点和其他网络设备，如交换机（switch）或路由器（router）。它有点像整个系统的主干，确保数据到达需要的地方。它使用以太网电缆（Ethernet cable）将所有东西连接在一起，有时甚至使用光纤电缆（fiber-optic cable）来实现更快的速度和更长的距离。
2、协议栈（Protocol Stack）
顾名思义（As the terminology implies），WLAN也是LAN（局域网Local Area Network）的一种，因此它使用了标准LAN协议的大部分，尤其是在更高层。唯一的区别是非常低层的部分（PHY和低MAC）。我将要处理（deal with）的大部分内容将是关于协议栈（protocol stack）的PHY/MAC部分。
让我们以一种更随意的（casual）方式来分解（break down）它，重点关注（focus on）Wi-Fi协议栈和三个主要层：物理层（PHY/Physical）、媒体访问控制层（MAC/Media Access Control）和互联网协议层（IP/Internet Protocol）。
1）物理（PHY）层：
物理层就像无线网络的无线电台（radio station）。它负责（take care of）将您的数据转换为可以通过空中发送的无线电信号（radio signal）。不同的Wi-Fi版本（如802.11a、b、g、n、ac和ax）使用不同的方法和射频（RF：radio frequency）以不同的速度和距离发送数据。它也有一些技巧（trick）来检测（detect）和修复（fix）传输过程中可能发生的错误。
2）媒体访问控制（MAC）层：
MAC层就像一个交通警察（traffic cop），确保所有使用Wi-Fi的设备都能很好地配合使用。由于每个人共享相同的无线电波（airwave），MAC层使用一个名为CSMA/CA的系统来帮助设备确定（figure out）何时轮到他们说话。它还负责（in charge of）寻址（addressing）等事务，以便数据到达正确的位置，并在需要时将大块数据（big chunks）分解（break into）成小块（smaller pieces）。
3）互联网协议（IP）层：
IP层本身并不是Wi-Fi的一部分，但它对于让一切协同工作非常重要。这就像是你的数据的GPS，为每个设备提供一个地址，并帮助你的信息从一个地方找到另一个地方，即使它必须通过（go through）不同的网络。有几个（a couple of ）不同版本的IP，如IPv4和IPv6，可以帮助实现这一点。
因此，简言之（in a nutshell），Wi-Fi协议栈有负责处理无线电信号的PHY层，有帮助设备共享无线电波的MAC层，还有将数据引导到需要的地方的IP层。它们一起确保你可以毫无障碍地（without a hitch）连接Wi-Fi和上网（surf the web）。
3、安全（Security）
安全就是保护你的Wi-Fi网络免受黑客和爱管闲事的邻居（hackers and nosy neighbors）的攻击。有不同的方法来保护你的网络，比如使用密码和加密（encryption）。WPA2和WPA3是确保Wi-Fi安全和数据安全的最佳选择（option）。
注意：WiFi安全的更多细节（Further details）在此处的单独注释（separate note）中进行了解释。
4、网络管理（Network Management）
管理Wi-Fi网络意味着确保一切顺利运行。你需要规划网络，设置它，关注（keep an eye on）它的运行情况，解决问题（fix problems），并随着时间的推移使其变得更好。有一些工具和软件可以帮助网络管理员（network admin）完成所有这些工作。
5、服务质量（QoS/Quality of Service）
QoS是指确保网络上的重要内容(（important stuff）得到优先（priority）考虑。例如，如果你正在流媒体播放电影（stream a movie）或进行视频通话（video call），你不希望因为别人正在下载（download）一个巨大的文件而速度缓慢或断断续续（choppy）。QoS通过优先考虑重要的东西来帮助保持一切顺利运行。
6、漫游（Roaming）
漫游是指当你带着设备四处移动（move around）时，它会自动切换（switch）到最佳的Wi-Fi信号。它在办公室或校园（campuse）等有很多不同Wi-Fi接入点的大地方非常有用。漫游可以确保您保持联系，不会出现任何问题（without any hiccups）。
7、范围和覆盖范围（Range and Coverage）
范围和覆盖范围是关于你的Wi-Fi信号能走多远，以及它覆盖该地区的程度。很多事情都会影响（affect）这一点，比如接入点的功率有多大，它有什么样的天线（antenna），甚至墙上的建筑材料。有时，为了获得更好的覆盖，你可以使用网状网络（mesh network）之类的东西，这有助于扩展（extend）Wi-Fi信号，而不需要更多的电线。
8、基本功能（Basic Features）
WiFi基于半双工（TDD/Half-Duplex）。
使用CSMA/CA（Carrier Sense Multiple Access with Collision Avoidance，带冲突避免的载波侦听多路访问），而有线以太网（wired ethernet）使用CSMA/CD
使用WEP（Wired Equivalent Privacy，有线等效隐私）、WPA（WiFi Protected Access，WiFi保护访问）、WPA2（802.11i）
YouTube
IEEE 802.11 Wireless LAN (WLAN) Part 1 - Fundamental Concepts (Sep 2013)
IEEE 802.11无线局域网（WLAN）第2部分（2013年9月）
IEEE 802.11无线局域网（WLAN）第3部分（2013年9月）
The story of Wi Fi (Mar 2015)
802.11ac Frames - What’s Changed? (Sep 2015)
The Evolution of IEEE 802 11 standards - BAG NAC (Sep 2018)
802.11ax - What’s New Webinar (May 2018)
2018 Wi-Fi Trek - Mark Williams (Wireless Power Saving Mechanisms) (Feb 2019)
Explained: WiFi 1, 2, 3, 4, 5 and 6 (May 2019)
二、信标（Beacon）
信标是从AP（接入点）定期（periodically）广播的一种特殊类型的信号（消息），AP将其ID和能力（capability）携带（carry）到AP周围的每个设备。Beacon的整体框架结构（overall frame structure）如下。
以下是显示信标帧头部分（Beacon Frame Header part）的示例
IEEE 802.11 Beacon frame, Flags: …m…C
Type/Subtype: Beacon frame (0x08)
Frame Control Field: 0x8020
.... ..00 = Version: 0
.... 00.. = Type: Management frame (0)
1000 .... = Subtype: 8
Flags: 0x20
.... ..00 = DS status: Not leaving DS or network is operating in AD-HOC mode
(To DS: 0 From DS: 0) (0x00)
// (To DS: 0 From DS: 0) mean the address 1 is Destination address
// the address 2 is Source address
.... .0.. = More Fragments: This is the last fragment
.... 0... = Retry: Frame is not being retransmitted
...0 .... = PWR MGT: STA will stay up
..1. .... = More Data: Data is buffered for STA at AP
.0.. .... = Protected flag: Data is not protected
0... .... = Order flag: Not strictly ordered
.000 0000 0000 0000 = Duration: 0 microseconds
Receiver address: Broadcast (ff:ff:ff:ff:ff:ff)
Destination address: Broadcast (ff:ff:ff:ff:ff:ff)
Transmitter address: Anritsu_07:91:0e (00:00:91:07:91:0e)
Source address: Anritsu_07:91:0e (00:00:91:07:91:0e)
BSS Id: Anritsu_07:91:0e (00:00:91:07:91:0e)
Fragment number: 0
Sequence number: 78
Frame check sequence: 0xe0fbec2b [correct]
[Good: True]
[Bad: False]
下是Beacon信号的[Frame Body]中携带的信息广播（information broadcast）的列表。并非所有这些信息都是由AP广播的。其中许多字段（fields）仅在特定条件下广播（您可以在802.11 - 8.3.3.2 Beacon frame format中找到更多详细信息）。
以下是信标帧内容的示例。
< Example 1 >
以下是旧802.11规范（specification）中的一个示例，您将看到信标中包含的信息集相对（relatively）较小。将此示例与来自最新（the latest）规范（802.11ac）的下一个示例进行比较，以及功能集（feature set）是如何增长的。
< Example 2 >
这是一个名为Anritsu MT8862的实验性（experimental）AP（接入点）的例子，它支持802.11ac。日志（log）是在这里显示的测试设置中捕获（capture）的。除非您完全理解802.11 PHY/MAC规范，否则很难理解此处显示的所有参数的含义。但是，如果你把这里的所有项目都浏览（go through）了几次（a couple of times），即使你不明白这意味着什么。阅读与802.11相关的其他技术材料或规范将对您有所帮助。（学习任何新事物的第一步是习惯（get used to）它，即使你不了解全部细节）
IEEE 802.11 Beacon frame, Flags: …m…C
Type/Subtype: Beacon frame (0x08)
Frame Control Field: 0x8020
.... ..00 = Version: 0
.... 00.. = Type: Management frame (0)
1000 .... = Subtype: 8
Flags: 0x20
.... ..00 = DS status: Not leaving DS or network is operating in AD-HOC mode
(To DS: 0 From DS: 0) (0x00)
.... .0.. = More Fragments: This is the last fragment
.... 0... = Retry: Frame is not being retransmitted
...0 .... = PWR MGT: STA will stay up
..1. .... = More Data: Data is buffered for STA at AP
.0.. .... = Protected flag: Data is not protected
0... .... = Order flag: Not strictly ordered
.000 0000 0000 0000 = Duration: 0 microseconds
Receiver address: Broadcast (ff:ff:ff:ff:ff:ff)
Destination address: Broadcast (ff:ff:ff:ff:ff:ff)
Transmitter address: Anritsu_07:91:0e (00:00:91:07:91:0e)
Source address: Anritsu_07:91:0e (00:00:91:07:91:0e)
BSS Id: Anritsu_07:91:0e (00:00:91:07:91:0e)
Fragment number: 0
Sequence number: 78
Frame check sequence: 0xe0fbec2b [correct]
[Good: True]
[Bad: False]
IEEE 802.11 wireless LAN management frame
Fixed parameters (12 bytes)
Timestamp: 0x0000000000f3c041
Beacon Interval: 0.204800 [Seconds]
Capabilities Information: 0x0001
.... .... .... ...1 = ESS capabilities: Transmitter is an AP
.... .... .... ..0. = IBSS status: Transmitter belongs to a BSS
.... ..0. .... 00.. = CFP participation capabilities:
No point coordinator at AP (0x0000)
.... .... ...0 .... = Privacy: AP/STA cannot support WEP
.... .... ..0. .... = Short Preamble: Not Allowed
.... .... .0.. .... = PBCC: Not Allowed
.... .... 0... .... = Channel Agility: Not in use
.... ...0 .... .... = Spectrum Management: Not Implemented
.... .0.. .... .... = Short Slot Time: Not in use
.... 0... .... .... = Automatic Power Save Delivery: Not Implemented
...0 .... .... .... = Radio Measurement: Not Implemented
..0. .... .... .... = DSSS-OFDM: Not Allowed
.0.. .... .... .... = Delayed Block Ack: Not Implemented
0... .... .... .... = Immediate Block Ack: Not Implemented
Tagged parameters (134 bytes)
Tag: SSID parameter set: MT8862A6000000008
Tag Number: SSID parameter set (0)
Tag length: 17
SSID: MT8862A6000000008
Tag: Supported Rates 6(B), 9, 12(B), 18, 24(B), 36, 48, 54, [Mbit/sec]
Tag Number: Supported Rates (1)
Tag length: 8
Supported Rates: 6(B) (0x8c)
Supported Rates: 9 (0x12)
Supported Rates: 12(B) (0x98)
Supported Rates: 18 (0x24)
Supported Rates: 24(B) (0xb0)
Supported Rates: 36 (0x48)
Supported Rates: 48 (0x60)
Supported Rates: 54 (0x6c)
Tag: Traffic Indication Map (TIM): DTIM 0 of 0 bitmap
Tag Number: Traffic Indication Map (TIM) (5)
Tag length: 4
DTIM count: 0
DTIM period: 1
Bitmap control: 0x00
.... ...0 = Multicast: False
0000 000. = Bitmap Offset: 0x00
Partial Virtual Bitmap: 00
Tag: HT Capabilities (802.11n D1.10)
Tag Number: HT Capabilities (802.11n D1.10) (45)
Tag length: 26
HT Capabilities Info: 0x007e
.... .... .... ...0 = HT LDPC coding capability:
Transmitter does not support receiving LDPC coded packets
.... .... .... ..1. = HT Support channel width:
Transmitter supports 20MHz and 40MHz operation
.... .... .... 11.. = HT SM Power Save: SM Power Save disabled (0x0003)
.... .... ...1 .... = HT Green Field: Transmitter is able to receive PPDUs
with Green Field (GF) preamble
.... .... ..1. .... = HT Short GI for 20MHz: Supported
.... .... .1.. .... = HT Short GI for 40MHz: Supported
.... .... 0... .... = HT Tx STBC: Not supported
.... ..00 .... .... = HT Rx STBC: No Rx STBC support (0x0000)
.... .0.. .... .... = HT Delayed Block ACK:
Transmitter does not support HT-Delayed BlockAck
.... 0... .... .... = HT Max A-MSDU length: 3839 bytes
...0 .... .... .... = HT DSSS/CCK mode in 40MHz:
Won't/Can't use of DSSS/CCK in 40 MHz
..0. .... .... .... = HT PSMP Support: Won't/Can't support PSMP operation
.0.. .... .... .... = HT Forty MHz Intolerant: Use of 40 MHz transmissions
unrestricted/allowed
0... .... .... .... = HT L-SIG TXOP Protection support: Not supported
A-MPDU Parameters: 0x1f
.... ..11 = Maximum Rx A-MPDU Length: 0x03 (65535[Bytes])
...1 11.. = MPDU Density: 16 [usec] (0x07)
000. .... = Reserved: 0x00
Rx Supported Modulation and Coding Scheme Set: MCS Set
Rx Modulation and Coding Scheme (One bit per modulation): 1 spatial stream
.... .... .... .... .... .... 1111 1111 = Rx Bitmask Bits 0-7: 0x000000ff
.... .... .... .... 0000 0000 .... .... = Rx Bitmask Bits 8-15: 0x00000000
.... .... 0000 0000 .... .... .... .... = Rx Bitmask Bits 16-23: 0x00000000
0000 0000 .... .... .... .... .... .... = Rx Bitmask Bits 24-31: 0x00000000
.... .... .... .... .... .... .... ...0 = Rx Bitmask Bit 32: 0x00000000
.... .... .... .... .... .... .000 000. = Rx Bitmask Bits 33-38: 0x00000000
.... .... ...0 0000 0000 0000 0... .... = Rx Bitmask Bits 39-52: 0x00000000
...0 0000 0000 0000 0000 0000 000. .... = Rx Bitmask Bits 53-76: 0x00000000
.... ..00 0000 0000 = Highest Supported Data Rate: 0x0000
.... .... .... ...0 = Tx Supported MCS Set: Not Defined
.... .... .... ..0. = Tx and Rx MCS Set: Equal
.... .... .... 00.. = Maximum Number of Tx Spatial Streams Supported: 0x0000,
TX MCS Set Not Defined
.... .... ...0 .... = Unequal Modulation: Not supported
HT Extended Capabilities: 0x0006
.... .... .... ...0 = Transmitter supports PCO: Not supported
.... .... .... .11. = Time needed to transition between 20MHz and 40MHz:
5 msec (0x0003)
.... ..00 .... .... = MCS Feedback capability:
STA does not provide MCS feedback (0x0000)
.... .0.. .... .... = High Throughput: Not supported
.... 0... .... .... = Reverse Direction Responder: Not supported
Transmit Beam Forming (TxBF) Capabilities: 0x0000
.... .... .... .... .... .... .... ...0 = Transmit Beamforming: Not supported
.... .... .... .... .... .... .... ..0. = Receive Staggered Sounding:
Not supported
.... .... .... .... .... .... .... .0.. = Transmit Staggered Sounding:
Not supported
.... .... .... .... .... .... .... 0... = Receive Null Data packet (NDP):
Not supported
.... .... .... .... .... .... ...0 .... = Transmit Null Data packet (NDP):
Not supported
.... .... .... .... .... .... ..0. .... = Implicit TxBF capable: Not supported
.... .... .... .... .... .... 00.. .... = Calibration: incapable (0x00000000)
.... .... .... .... .... ...0 .... .... = STA can apply TxBF using CSI explicit
feedback: Not supported
.... .... .... .... .... ..0. .... .... = STA can apply TxBF using uncompressed
beamforming feedback matrix: Not supported
.... .... .... .... .... .0.. .... .... = STA can apply TxBF using compressed
beamforming feedback matrix: Not supported
.... .... .... .... ...0 0... .... .... = Receiver can return explicit CSI
feedback: not supported (0x00000000)
.... .... .... .... .00. .... .... .... = Receiver can return explicit
uncompressed Beamforming Feedback Matrix:
not supported (0x00000000)
.... .... .... ...0 0... .... .... .... = STA can compress and use compressed
Beamforming Feedback Matrix:
not supported (0x00000000)
.... .... .... .00. .... .... .... .... = Minimal grouping used for explicit
feedback reports: No grouping supported
(0x00000000)
.... .... ...0 0... .... .... .... .... = Max antennae STA can support when CSI
feedback required: 1 TX antenna sounding
(0x00000000)
.... .... .00. .... .... .... .... .... = Max antennae STA can support when
uncompressed Beamforming feedback
required: 1 TX antenna sounding
(0x00000000)
.... ...0 0... .... .... .... .... .... = Max antennae STA can support when
compressed Beamforming feedback required:
1 TX antenna sounding (0x00000000)
.... .00. .... .... .... .... .... .... = Maximum number of rows of CSI explicit
feedback: 1 row of CSI (0x00000000)
...0 0... .... .... .... .... .... .... = Maximum number of space time streams
for which channel dimensions can be
simultaneously estimated: 1 space time
stream (0x00000000)
000. .... .... .... .... .... .... .... = Reserved: 0x00000000
Antenna Selection (ASEL) Capabilities: 0x00
.... ...0 = Antenna Selection Capable: Not supported
.... ..0. = Explicit CSI Feedback Based Tx ASEL: Not supported
.... .0.. = Antenna Indices Feedback Based Tx ASEL: Not supported
.... 0... = Explicit CSI Feedback: Not supported
...0 .... = Antenna Indices Feedback: Not supported
..0. .... = Rx ASEL: Not supported
.0.. .... = Tx Sounding PPDUs: Not supported
0... .... = Reserved: 0x00
Tag: HT Information (802.11n D1.10)
Tag Number: HT Information (802.11n D1.10) (61)
Tag length: 22
Primary Channel: 36
HT Information Subset (1 of 3): 0x05
.... ..01 = Secondary channel offset: Secondary channel is above the primary
channel (0x01)
.... .1.. = Supported channel width: Channel of any width supported
.... 0... = Reduced Interframe Spacing (RIFS): Prohibited
...0 .... = Power Save Multi-Poll (PSMP) stations only: Association requests are
accepted regardless of PSMP capability
000. .... = Shortest service interval: 5 ms (0x00)
HT Information Subset (2 of 3): 0x0000
.... .... .... ..00 = Operating mode of BSS: All STAs are - 20/40 MHz HT
or in a 20/40 MHz BSS or are 20 MHz HT in a 20 MHz BSS
(0x0000)
.... .... .... .0.. = Non-greenfield STAs present:
All associated STAs are greenfield capable
.... .... .... 0... = Transmit burst limit: No limit
.... .... ...0 .... = OBSS non-HT STAs present: Use of protection for non-HT STAs
by overlapping BSSs is not needed
0000 0000 000. .... = Reserved: 0x0000
HT Information Subset (3 of 3): 0x0000
.... .... ..00 0000 = Reserved: 0x0000
.... .... .0.. .... = Dual beacon: No second beacon is transmitted
.... .... 0... .... = Dual Clear To Send (CTS) protection: Not required
.... ...0 .... .... = Beacon ID: Primary beacon
.... ..0. .... .... = L-SIG TXOP Protection Full Support: One or more HT STAs
in the BSS do not support L-SIG TXOP protection
.... .0.. .... .... = Phased Coexistence Operation (PCO): Inactive
.... 0... .... .... = Phased Coexistence Operation (PCO) Phase: Switch to or
continue 20 MHz phase
0000 .... .... .... = Reserved: 0x0000
Rx Supported Modulation and Coding Scheme Set: Basic MCS Set
Rx Modulation and Coding Scheme (One bit per modulation): Reserved:8
.... .... .... .... .... .... 0000 0000 = Rx Bitmask Bits 0-7: 0x00000000
.... .... .... .... 0000 0000 .... .... = Rx Bitmask Bits 8-15: 0x00000000
.... .... 0000 0000 .... .... .... .... = Rx Bitmask Bits 16-23: 0x00000000
0000 0000 .... .... .... .... .... .... = Rx Bitmask Bits 24-31: 0x00000000
.... .... .... .... .... .... .... ...0 = Rx Bitmask Bit 32: 0x00000000
.... .... .... .... .... .... .000 000. = Rx Bitmask Bits 33-38: 0x00000000
.... .... ...0 0000 0000 0000 0... .... = Rx Bitmask Bits 39-52: 0x00000000
...0 0000 0000 0000 0000 0000 000. .... = Rx Bitmask Bits 53-76: 0x00000000
.... ..00 0000 0000 = Highest Supported Data Rate: 0x0000
.... .... .... ...0 = Tx Supported MCS Set: Not Defined
.... .... .... ..0. = Tx and Rx MCS Set: Equal
.... .... .... 00.. = Maximum Number of Tx Spatial Streams Supported: 0x0000,
TX MCS Set Not Defined
.... .... ...0 .... = Unequal Modulation: Not supported
Tag: VHT Capabilities (IEEE Stc 802.11ac/D3.1)
Tag Number: VHT Capabilities (IEEE Stc 802.11ac/D3.1) (191)
Tag length: 12
VHT Capabilities Info: 0x03800022
.... .... .... .... .... .... .... ..10 = Maximum MPDU Length: 11 454 (0x00000002)
.... .... .... .... .... .... .... 00.. = Supported Channel Width Set:
Neither 160MHz nor 80+80 supported
(0x00000000)
.... .... .... .... .... .... ...0 .... = Rx LDPC: Not supported
.... .... .... .... .... .... ..1. .... = Short GI for 80MHz: Supported
.... .... .... .... .... .... .0.. .... = Short GI for 160MHz and 80+80MHz:
Not supported
.... .... .... .... .... .... 0... .... = Tx STBC: Not supported
.... .... .... .... .... .000 .... .... = Rx STBC: None (0x00000000)
.... .... .... .... .... 0... .... .... = SU Beam-former Capable: Not supported
.... .... .... .... ...0 .... .... .... = SU Beam-formee Capable: Not supported
.... .... .... .... 000. .... .... .... = Compressed Steering Number of Beamformer
Antennas Supported: 1 (0x00000000)
.... .... .... .000 .... .... .... .... = Number of Sounding Dimensions: 1
(0x00000000)
.... .... .... 0... .... .... .... .... = MU Beam-former Capable: Not supported
.... .... ...0 .... .... .... .... .... = MU Beam-formee Capable: Not supported
.... .... ..0. .... .... .... .... .... = VHT TXOP PS: Not supported
.... .... .0.. .... .... .... .... .... = +HTC-VHT Capable (VHT variant HT Control
field): Not supported
.... ..11 1... .... .... .... .... .... = Max A-MPDU Length: 1 048 575 (0x00000007)
.... 00.. .... .... .... .... .... .... = VHT Link Adaptation: No Feedback
(0x00000000)
...0 .... .... .... .... .... .... .... = Rx Antenna Pattern Consistency:
Not supported
..0. .... .... .... .... .... .... .... = Tx Antenna Pattern Consistency:
Not supported
00.. .... .... .... .... .... .... .... = Reserved: False
VHT Supported MCS Set
Rx MCS Map: 0xfffe
.... .... .... ..10 = Rx 1 SS: MCS 0-9 (0x0002)
.... .... .... 11.. = Rx 2 SS: Not Supported (0x0003)
.... .... ..11 .... = Rx 3 SS: Not Supported (0x0003)
.... .... 11.. .... = Rx 4 SS: Not Supported (0x0003)
.... ..11 .... .... = Rx 5 SS: Not Supported (0x0003)
.... 11.. .... .... = Rx 6 SS: Not Supported (0x0003)
..11 .... .... .... = Rx 7 SS: Not Supported (0x0003)
11.. .... .... .... = Rx 8 SS: Not Supported (0x0003)
...0 0000 0000 0000 = Rx Highest Long GI Data Rate
(in Mb/s, 0 = subfield not in use): 0x0000
Tx MCS Map: 0xfffe
.... .... .... ..10 = Tx 1 SS: MCS 0-9 (0x0002)
.... .... .... 11.. = Tx 2 SS: Not Supported (0x0003)
.... .... ..11 .... = Tx 3 SS: Not Supported (0x0003)
.... .... 11.. .... = Tx 4 SS: Not Supported (0x0003)
.... ..11 .... .... = Tx 5 SS: Not Supported (0x0003)
.... 11.. .... .... = Tx 6 SS: Not Supported (0x0003)
..11 .... .... .... = Tx 7 SS: Not Supported (0x0003)
11.. .... .... .... = Tx 8 SS: Not Supported (0x0003)
...0 0000 0000 0000 = Tx Highest Long GI Data Rate
(in Mb/s, 0 = subfield not in use): 0x0000
Tag: VHT Operation (IEEE Stc 802.11ac/D3.1)
Tag Number: VHT Operation (IEEE Stc 802.11ac/D3.1) (192)
Tag length: 5
VHT Operation Info
Channel Width: 80 MHz (0x01)
Channel Center Segment 0: 42
Channel Center Segment 1: 0
Basic MCS Map: 0xfffc
.... .... .... ..00 = Basic 1 SS: MCS 0-7 (0x0000)
.... .... .... 11.. = Basic 2 SS: Not Supported (0x0003)
.... .... ..11 .... = Basic 3 SS: Not Supported (0x0003)
.... .... 11.. .... = Basic 4 SS: Not Supported (0x0003)
.... ..11 .... .... = Basic 5 SS: Not Supported (0x0003)
.... 11.. .... .... = Basic 6 SS: Not Supported (0x0003)
..11 .... .... .... = Basic 7 SS: Not Supported (0x0003)
11.. .... .... .... = Basic 8 SS: Not Supported (0x0003)
Tag: Vendor Specific: Microsof: WMM/WME: Parameter Element
Tag Number: Vendor Specific (221)
Tag length: 24
OUI: 00-50-f2 (Microsof)
Vendor Specific OUI Type: 2
Type: WMM/WME (0x02)
WME Subtype: Parameter Element (1)
WME Version: 1
WME QoS Info: 0x00
0... .... = U-APSD: Disabled
.... 0000 = Parameter Set Count: 0x00
.000 .... = Reserved: 0x00
Reserved: 00
Ac Parameters ACI 0 (Best Effort), ACM no , AIFSN 3, ECWmin 4 ,ECWmax 10, TXOP 0
ACI / AIFSN Field: 0x03
.00. .... = ACI: Best Effort (0)
...0 .... = Admission Control Mandatory: No
.... 0011 = AIFSN: 3
0... .... = Reserved: 0
ECW: 0xa4
1010 .... = ECW Max: 10
.... 0100 = ECW Min: 4
TXOP Limit: 0
Ac Parameters ACI 1 (Background), ACM no , AIFSN 7, ECWmin 4 ,ECWmax 10, TXOP 0
ACI / AIFSN Field: 0x27
.01. .... = ACI: Background (1)
...0 .... = Admission Control Mandatory: No
.... 0111 = AIFSN: 7
0... .... = Reserved: 0
ECW: 0xa4
1010 .... = ECW Max: 10
.... 0100 = ECW Min: 4
TXOP Limit: 0
Ac Parameters ACI 2 (Video), ACM no , AIFSN 2, ECWmin 3 ,ECWmax 4, TXOP 94
ACI / AIFSN Field: 0x42
.10. .... = ACI: Video (2)
...0 .... = Admission Control Mandatory: No
.... 0010 = AIFSN: 2
0... .... = Reserved: 0
ECW: 0x43
0100 .... = ECW Max: 4
.... 0011 = ECW Min: 3
TXOP Limit: 94
Ac Parameters ACI 3 (Voice), ACM no , AIFSN 2, ECWmin 2 ,ECWmax 3, TXOP 47
ACI / AIFSN Field: 0x62
.11. .... = ACI: Voice (3)
...0 .... = Admission Control Mandatory: No
.... 0010 = AIFSN: 2
0... .... = Reserved: 0
ECW: 0x32
0011 .... = ECW Max: 3
.... 0010 = ECW Min: 2
TXOP Limit: 47
三、服务质量（QoS Field）
< Example for (5) >
IEEE 802.11 QoS Data, Flags: …TC
Type/Subtype: QoS Data (0x28)
Frame Control Field: 0x8801
.... ..00 = Version: 0
.... 10.. = Type: Data frame (2)
1000 .... = Subtype: 8
Flags: 0x01
.... ..01 = DS status: Frame from STA to DS via an AP (To DS: 1 From DS: 0) (0x01)
.... .0.. = More Fragments: This is the last fragment
.... 0... = Retry: Frame is not being retransmitted
...0 .... = PWR MGT: STA will stay up
..0. .... = More Data: No data buffered
.0.. .... = Protected flag: Data is not protected
0... .... = Order flag: Not strictly ordered
.000 0000 0011 1100 = Duration: 60 microseconds
Receiver address: Anritsu_07:91:0e (00:00:91:07:91:0e)
BSS Id: Anritsu_07:91:0e (00:00:91:07:91:0e)
Transmitter address: e0:cb:ee:f9:4a:de (e0:cb:ee:f9:4a:de)
Source address: e0:cb:ee:f9:4a:de (e0:cb:ee:f9:4a:de)
Destination address: Anritsu_07:91:0e (00:00:91:07:91:0e)
Fragment number: 0
Sequence number: 13
Frame check sequence: 0x1adb5852 [correct]
[Good: True]
[Bad: False]
Qos Control: 0x0000
.... .... .... 0000 = TID: 0
[.... .... .... .000 = Priority: Best Effort (Best Effort) (0)]
.... .... ...0 .... = QoS bit 4: Bits 8-15 of QoS Control field are TXOP Duration Requested
.... .... .00. .... = Ack Policy: Normal Ack (0x0000)
.... .... 0... .... = Payload Type: MSDU
0000 0000 .... .... = TXOP Duration Requested: 0 (no TXOP requested)
四、无线局域网（WLAN：wireless local area networks）：CSMA/CA碰撞检测的载波侦听
CSMA/CA（Carrier Sense Multiple Access with Collision Detection）由以下三个组成部分（过程）组成（be made up of）。因此，如果您对每个组件流程都有清晰的理解，那么您就会理解什么是CSMA/CA。
● CS：载波感应（Carrier-Sensing）
● MA：多址接入（Multiple Access）
● CA：防撞（Collision Avoidance）
现在，让我们考虑（think of）一下这些组件过程中的每一个。其中的质心分量是“MA”，所以我将从MA开始。
1、什么是MA？
MA（Multiple Access）代表（stand for）多址接入。这是什么意思？
简单地说（Simply put），多路访问是在“多个”实体entity（例如，多个设备、多个人等）之间共享（访问/accessing）单个物理资源。
例如，在我们的日常生活中（in our daily life），我可以说“任何有很多汽车行驶的道路（或街道）都是一种多路访问。你家门前的道路是一种物理资源，许多汽车共享（访问）这种物理资源。
在类似的逻辑中（In similar logic），以太网Ethernet（例如LAN系统）可以是多址系统。在局域网（Wired LAN）系统中，许多网卡（network card）都连接到一根局域网电缆主干（cable backbone）上。甚至无线LAN也可以是多址系统，因为多个无线设备共享相同的物理资源，该物理资源是相同的RF频率。
2、什么是CS？
为了使这种多址系统（Multiple Access system）正常（properly）工作，所有共享资源的设备都应该具有一定的技术，以避免多个设备之间争夺（ compete for）同一资源的冲突（conflict）。这种用于以太网的“冲突避免技术（conflict avoidance technology）”（collision avoidance system）被称为“CS（Carrier-Sensing）”。
CS（Carrier-Sensing）听起来可能很吓人（intimidating），但概念（concept）很简单。这就是你每次开车从家里驶入家门前的一条大道时所做的事情。当你开车上路时，你不只是开车上路。如果你这样做，你可能很容易被路上行驶的其他汽车撞到（get hit）。通常你会先看一看（ take a look at）路上的交通情况。如果路上没有汽车经过，那么你就开车上路。如果前面有一辆车经过（pass by），你就等它经过。换句话说（In other words），你“感觉（Sense）到”了交通，然后开车进入（或等待）。
这与以太网系统（ Ethernet System）中的逻辑相同。在一个设备发送任何数据之前，它首先检查是否有其他设备发送的任何其他流量。换句话说，如果有其他设备发送的流量，它就会“感知（Sense）”。如果没有流量，它会发送自己的数据。如果有交通，它会一直等到交通经过。
3、什么是CA？
在有线以太网的情况下，它们使用CD（Collision Detection/碰撞检测），但在无线情况下，设备无法像在有线情况下那样直接检测（detect）碰撞，因为无线LAN设备在传输时无法检测到任何传入信号（incoming signal），因为它是TDD（half Duplex/半双工）。这意味着设备在传输（使用发射器链transmitter chain）时无法检测到（使用接收器链reciever chain）任何东西，因此设备在传输时无法执行CD（collision dection）。
在CD的情况下，设备直接检查电线上的电压水平（voltage level），但在无线情况下，该设备无法以这种方式检查碰撞（Collision ）。
那么无线设备如何检测碰撞（detect collision）呢？
在CA（Collision Avoidance）方法中，设备正在等待来自接收器（reciever）的ACK帧。如果在某个时间帧（time frame）内没有检测（detect）到来自接收器（reciever）的ACK帧，则发送器（transmitter）假设（assume）发生了一些冲突（collision），并在某个退避时间（backoff time）内停止发送。
这种机制（mechanism）的问题是，由于（due to）冲突检测（collision detection）和ACK等待时间，它产生了大量开销（overhead），开销可能需要大约50%。
4、通道感应（载波感应）工作****Channel Sensing(Carrier Sensing) in Action
载波感应（Carrier Sensing）和数据传输（Data Transmission）的总体程序如下。
（A） ：在设备尝试传输（transmit）任何数据之前，它会启动载波感应。
（B） ：如果设备检测到任何载波信号carrier signal（即其他设备正在使用该信道channel），它只会返回“等待”模式，并且在退避期（BackOff Period）内不会尝试进一步的载波感测。
（C） ：即使载波（carrier）不忙（即，即使没有人使用信道），设备也会在一定的持续时间（duration）内保持对载波的感测（DIFS：DCF Interframe Space）。如果在此期间检测到任何载波信号，则返回“等待”模式，并且在退避期间（BackOff Period）内不会尝试进一步的载波感测（carrier sensing）。
（D） ：如果在一段时间内没有检测到载波信号（DIFS），则设备假设信道是畅通的（没有人使用信道），并发送自己的数据。
注意：图（illustration）中的术语（term）“睡眠”并不意味着用于省电（ power saving）的睡眠模式。它只是意味着“等待（没有数据传输和载波感应活动）”。
参考文献：
[1] How WLAN CSMA/CA Works
[2] 802.11n Tutorial
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