802.11 Wireless WiFi Protocol Learning notes (i)

Three types of 802.11 frames:

Data frame: 802.11 workhorse, responsible for transferring data between workstations

Control frame: Used in conjunction with the data frame, responsible for area emptying, channel acquisition and carrier monitoring maintenance, and to receive the data in a positive response, thereby promoting the reliability of inter-workstation data transmission

Manage frames: Responsible for monitoring, primarily for joining or exiting wireless networks, and for handling transfer of connections between base stations

A 802.11-frame format

1, Data frame: Depending on the frame type, not every frame will use all the address bits

(1) Frame control bit:

Protocol bit: Protocol version, currently 0

Type and subtype: Specify frame type

Management Frames (Management frame: type=00)
0000 Association Request (Connection request)
0001 Association Response (Connection answer)
0010 Reassociation request (reconnect required)
0011 reassociation Response (Reconnect answer)
0100 Probe request (probing requirements)
0101 Probe Response (probe answer)
Beacon (Guiding Signal)
1001 Announcement Traffic Indication message
(ATIM) (Data generation indication notification signal)
1010 disassociation (Release connection)
1011 Authentication (authentication)
1100 deauthentication (Release certification)

Control frames (frame: type=01)

1010 Power Save-poll (Power saving mode-polling)

1011 RTS (Request send)

1100 CTS (Allow send)

1101 ACK (Reply)

1110 Cf-end (free competition period end)

1111 Cf-end (free competition period end) +cf-ack (free competition period response)

Data frames (frame: type=10)

To DS with from Dsbit:

	To Ds=0	To Ds=1
From Ds=0	All management and control frame IBSS (non-basic	The wireless station in the base network sends
From Ds=1	Data frames received by wireless workstations in the underlying network	Data frames on a wireless bridge

Retry bit: Data retransmission flag bit

Power Management bit: Notifies whether to enter Power Save mode (turns off the network card)

More data bit: The service is in the power-saving mode of the base station, such as 1, then at least one frame to be transmitted

Protected Frame bit: If the frame is protected by the Link Layer Security protocol, this bit will be set to 1

Orderbit: Frames and frame fragments can be routed sequentially

(2) The DURATION/ID bit is immediately after the frame control bit. This bit has many functions, there are three possible forms

A. When the 15th bit is set to 0 o'clock, the DURATION/ID bit is used to set NAV. This value represents how many microseconds of the current transmission is expected to use the media. The workstation must monitor any frame headers it receives and update the NAV. Any value exceeding the expected time of use of the media will update nav, while preventing other workstations from accessing the media.

B. During the competition-free period (Contention-free period, referred to as CFP), the 14th bit is 0 and the 15th bit is 1. All other bits are 0, so the value of the DURATION/ID bit is 32768. This value is interpreted as NAV. It allows any workstation that does not receive beacon (Beacon) Frame "Note" to be advertised during the competition so that the NAV can be updated to the appropriate value to avoid interference to the competition-free transmission.

C. In the Ps-poll (power-down mode-polling) frame, the 14th and 15th bits are set to 1 simultaneously. Mobile workstations can turn off antennas for power saving purposes. The workstations in hibernation must wake up regularly. To ensure that no frames are lost, the workstation waking from hibernation must send out a ps-poll frame to obtain any previously staged frames from the base station. In addition, the waking workstation adds a connection identifier (association ID, or AID) to the ps-poll frame to show the BSS it belongs to. The AID is included in the Ps-poll frame with a value between 1-2 and 007. Values that are between 2,008-16,383 are not currently used.

(3) Address position

A 802.11 frame can contain a maximum of four address bits. These bit address bits are numbered because these bits have different effects as the frame type. Basically, address 1 is for the receiving end, address 2 for the transmitting side, address 3 is received by the receiving side to pass over addresses. For example, in the underlying network, the third address bit is used by the receiving end to determine whether the frame belongs to the network to which it is connected.

Destination Address
Like Ethernet, the destination address (Destination) is an IEEE MAC identification, code, length of 48 bits,
Represents the last receiving end, which is the workstation responsible for delivering the frame to the upper layer protocol processing.
Source Address
This is an IEEE MAC identification number with a length of 48 bits, representing the source of the transmission. Each frame can only come from a single workstation,
Therefore, the Individual/group bit must be 0, representing the source address as a single workstation.
Receive-side address
This is an IEEE MAC identification number with a length of 48 bits, representing the wireless workstation responsible for processing the frame. If you are a wireless worker,
Station, the address of the receiving end is the destination address. If the destination address of the frame is an Ethernet node connected to the base station, the receiving end is the base
Station's wireless interface, and the destination address may be a router connected to the Ethernet.
Address of the delivery port
This is an IEEE MAC identification number with a length of 48 bits, representing the wireless interface that transmits frames to wireless media. Transmitting end ground
Typically used only for wireless bridging.

(4) BSSID

Different local area networks are divided in the same region, and the number and arrangement of address bits in the data frame depend on the routing path of the frame. Most transmissions will only use three addresses.

(5) Sequential control bits

16 bit,4 bits of the fragment number, and 12-bit sequential number, if the upper packet is cut, all frame fragments will have the same sequential number. If the frame is re-transmitted,
There is no change in the sequential numbering, and the difference between the frame fragments is fragment number (fragment numbering). The first fragment has a number of 0. Each subsequent
The fragment is incremented by 1 sequentially. The retransmission fragment will retain the original sequence number to assist the reorganization.

(6) Frame body

Note that 802.11 differs from other link layer technologies in that 802.11 frames do not have any upper layer protocol markings to differentiate. The upper-layer protocol is marked with an extra header type bit and is used as a 802.11
The start of the hosted data.

(7) FCS frame detection sequence

Usually the CRC is used to verify

Second, various types of frame learning

1. Data frame

The use of address bits in data frames

From AP:

To AP: