There are three main types of 802.11 frames:
Data Frame--like 802.11 of workhorse, responsible for transferring data between workstations.
Control Frame--the control frame is usually used in conjunction with the data frame, which is responsible for the emptying of the area, the acquisition of the channel and the maintenance of the carrier monitor, and the positive response when the data is received, thereby promoting the reliability of data transmission between workstations.
Manage Frames--management frames are supervised, mainly used to join or exit wireless networks, and to handle the transfer of connections between base stations.
4.1 Data frame
The data frame transmits the data of the upper layer protocol to the frame body. Which bits will be used, depending on the type to which the data frame belongs.
A differentiated approach: competitive services and competition-free services two data frames differentiate between frames that carry data and provide management functions table 4-1: Various classification methods of data frames
| Frame type |
Competitive-based services |
No competitive service |
Carry data |
Not carrying data |
| Data |
√ |
|
√ |
|
| Data+cf-ack |
|
√ |
√ |
|
| Data+cf-poll |
|
AP only |
√ |
|
| Data+cf-ack+cf-poll |
|
AP only |
√ |
|
| Null |
√ |
√ |
|
√ |
| Cf-ack |
|
√ |
|
√ |
| Cf-poll |
|
AP only |
|
√ |
| Cf-ack+cf-poll |
|
AP only |
|
√ |
4.1.1Frame Control (framebuffer)
The most notable are those address bits, whose meaning will vary depending on the value of ToDS and Fromdsbit.
4.1.2Duration (Duration)
The Duration (duration) bit is used to record the value of the network allocation vector (NAV).
The Duration bits of the data frame must be set in accordance with four specifications:
1. The Duration bit must be set to 32768 for any frames that are passed during the race-free period. This specification applies to any data frame that is passed during a race-free period. 2. The destination is a broadcast or multicast address frame (address 1 bit sets the group bit) with a duration of 0. This type of frame is not part of the basic exchange process and the receiving side does not answer, so competitive media access can begin immediately after the broadcast or multicast data frame has ended. NAV is used to protect the transmission medium during frame exchange. Since there is no response from the link layer after the broadcast or multicast frame, there is no need to lock the media rights for subsequent frames. 3. If the more fragments bit in the frame Control bit is 0, it indicates that the frame has no remaining fragments. The final frame fragment only needs to be used for its own answer subscription media, after which the competing access can be resumed. The duration bit is set to send a short frame interval and the time required for the fragment to be answered. The entire procedure is shown in 4-2. The Duration bit of the penultimate fragment will lock the media usage for the last fragment. 4. If the more fragmentsbit of the frame Control bit is set to 1, then there is a frame fragment after that. Therefore, the Duration bit is set to send two responses, plus three short frame intervals and the time required for the next frame fragment. The way to set NAN for a non-final fragment is essentially the same as RTS, so it is also called a virtual RTS.
4.1.3
Address and
DS Bit
The number and function of address bits depends on which DS (transfer system) bit is set, so the type of network used will indirectly affect the usage of the address bit. Table 4-2 lists the various uses of address bits in data frames. Only the wireless bridge will use the fourth address bit, so it is relatively rare. Table 4-2: Usage of address bits in data frames
| Function |
ToDS |
Fromds |
Address 1 (receiver) |
Address 2 (transmitter) |
Address 3 |
Address 4 |
| IBSS |
0 |
0 |
DA |
Sa |
BSSID |
Not used |
| To AP |
1 |
0 |
BSSID |
Sa |
DA |
Not used |
| From AP |
0 |
1 |
DA |
BSSID |
Sa |
Not used |
| WDS (bridge) |
1 |
1 |
Ra |
Ta |
DA |
Sa |
Attention
Receiving EndAnd
Destinationis not exactly the same.
DestinationRefers to the workstation responsible for processing the network layer packets in the frame;
Receiving Endis the workstation responsible for decoding the radio to 802.11 frames.
Source AddressRefers to a workstation that produces a Network Layer protocol packet in a frame
Send Sideis responsible for sending the frame to the wireless link. The Address 3 bit is for the base station and the transmission system filtering, but the use of this bit depends on the type of network used.
4.1.4
the secondary type of the data frame
the secondary type is Data frames, which are transmitted only during competitive access.
It is composed of the MAC header and the FCS Mark tail. In traditional Ethernet, NULL frames are nothing more than an extra burden; in 802.11 networks, mobile workstations use null frames to notify the base station of the change in power saving state.
Change.
4.1.5
Encapsulation of data frames
4.1.5.1 IBSS Frame IBSS The secondary type of the data frame is not either data or Null, which is only used to inform the current power management state. 4.1.5.2 frames sent from the base station (from AP)
It is necessary to differentiate between the source and the sender, because the 802.11 MAC will reply to the transmitter (send-side AP) of the frame, and the higher-level protocol will reply to the source of the frame (source). 4.1.5.3 the frame sent to the base station (to AP)
The ToDS bit of the frame sent to the transmission system (DS) is set to 1, and the Fromos bit is set to 0.
4.1.5.4 a frame in WDS
4.1.5.5 Encrypted Frames
Frames control (Frame control) bit Protected frame bit will be set to 1
4.2 Control Frame
Control frames are primarily assisted in the transfer of data frames.
4.2.1General Frame Control bits
ToDS
with the
Fromds bitThe control frame is responsible for handling the access of the wireless media, so it can only be generated by the wireless workstation. The transmission system does not receive control frames, so these two bits must be 0.
4.2.2RTS (Request send)
RTS frames can be used to gain control of the media in order to transfer "large" frames. As to how large the large: is defined by the RTS threshold (threshold) in the NIC driver. Media access can only be reserved for single-point propagation (unicast) frames, while broadcast (broadcast) and multicast (multicast) frames are only sent.
Duration
(Duration)
RTS frames attempt to reserve media usage for use by the frame Exchange program, so the RTS frame sender must calculate how much time is required after the RTS frame has ended.
4.2.3
CTS
(allow send)
There are two purposes of CTS frames: 1.for answering RTS frame; 2.Was 802. 11g protection mechanism to avoid interfering with older workstations
4.2.4
ACK
(Answer)
An ACK frame is a front-end response (positive acknowledgment) required by the MAC and any data transfer, including frames and frame fragments before the general transfer Rts/cts interchange.
4.2.5
Ps-poll
(Power saving Mode one poll)
When a mobile workstation wakes up from power-down mode, a ps-poll frame is sent to the base station to obtain any staging frame. AID (Connection ID): The Ps-poll frame will represent the connection Identifier (association ID) with the third and fourth bit of the MAC header. The connection identifier is a value specified by the base station to differentiate the connections. Placing this identification code in a frame allows the base station to identify the frames that are staged for its (mobile workstation).
4.3
Manage Frames
4.3.1Structure of the management frame
4.3.2Fixed-length management frame elements
Authentication Algorithm number-bit
authentication Transaction Sequence numberThe bit authentication program is divided into several steps, including a cross-examination password (challenge) issued by the base station and a response from the mobile workstation attempting to connect.
Beacon interval
bit
Beacons are signaled every once in a while to announce the existence of 802.11 networks. In addition to information that contains the BSS parameters, the beacon frame also contains information about the base station staging frame, so the mobile workstation must listen carefully to the beacons signal. The Beacon interval (beacon interval) bit has a length of 16 bits, which is used to set how many time units are spaced between beacon signals.
Capability Information
bit
The Capability information performance information bit of length 16 bit, when sending beacon signal, it is used to inform the parties, which performance of the network.
- ESS/IBSS (extended service portfolio/standalone Plug-in service portfolio)
of these two bit flags are mutually exclusive ( mutually exclusive). The base station will ESS bit is set to 1 , but will IBSS the cloth bit is set to 0, which indicates that the base station is part of the underlying network. IBSS the workstations in the ESS bit is set to 0, but willIBSSThe bit is set to 1.
- Privacy (Private ring sex)
Setting the Privacy bit to 1 indicates the need to use WEP to maintain confidentiality. In the underlying network, the sending side is the base station. In IBSS, beacon signals must be handled by one of the workstations in IBSS.
- Short preamble (Fast Sync signal)
This bit is added to the 802.11B specification to support high-speed direct sequence spread spectrum physical layer (high-rate DSSS PHY). Setting it to 1 indicates that the network is currently using short sync signals (preamble), and this tenth chapter will be further explained. 0 means that this option is not used and a short sync signal is not used in the BSS. The 802.11g specifies the use of short-sync signals, so this bit must be set to 1 in the network built with the 802.11G standard.
- PBCC (packet binary convolutional encoding)
This bit is added to the 802.11B specification to support high-speed direct sequence spread spectrum physical layer (high-rate DSSS PHY).
- Channel Agility (motorized channel conversion)
This one was added to the 802.11B specification to support the high-speed direct sequence spread spectrum physical layer (high-rate DSSS PHY).
- Short Slot Time (802.LLG)
This bit, if set to 1, represents a shorter time slot supported by 802.11, which is discussed in chapter 14th.
The bit is set to 1, which represents the DSSS-OFDM frame build (frame construction) option using 802.11G.
- Contention-free Polling (free competition poll) bit
Workstations and base stations use these two bits (cf-ppllable and Cf-poll Request) as a label.
Current AP Address
bit
The mobile workstation can indicate the MAC address of the currently connected base station using the current AP address (the location of the base station) shown in Figure 4-25. The purpose of this bit is to facilitate connection (association) with reconnection (reassociation). The workstation will take this to send the address of the last connected base station. When the workstation intends to establish a connection with a different base station, this bit can be used to convert the connection and retrieve all staged frames.
Listen interval bitThe so-called Listen Interval, in fact, is the Beacon Interval (Beacon interval) for the calculation of the sleep time.
Association ID
bit
Length 16bit
Timestamp
bit
The primary timer used to synchronize workstation BSS in BSS periodically sends the number of microseconds currently in effect. When the counter reaches the maximum value, it is counted from the beginning.
Reason Code
bit
When the other party is not suitable for joining the network, the workstation sends out disassociation (de-connect) or Deauthentication (de-authentication) frames as a response. These frames contain a Reason code (reason code) bit length of 16bit, which indicates that the other party is doing the wrong thing,
| Code |
Meaning |
| 0 |
Reserved, not used (Reserved; Unused |
| 1 |
Unspecified (Unspecified) |
| 2 |
Previous identity authentication is invalid (Prior authentication is not valid) |
| 3 |
The workstation has left the base service area or extended service area and is now de-authenticated |
| 4 |
Idle timer timed out and the workstation has been disconnected |
| 5 |
The base station resources are insufficient, so the connection is unblocked |
| 6 |
Incorrect frame type or sub-type received from a workstation that has not been certified |
| 7 |
Incorrect frame type or sub-type received from a workstation that has not been connected |
| 8 |
The workstation has left the base service area or extended service area and has now been disconnected |
| 9 |
Ask to connect or reconnect before authentication is complete |
| Ten (802.11h) |
Unable to accept setpoint for Power Capability information element, so the connection is unblocked |
| One (802.11h) |
Unable to accept setpoint for supported Channels information element, so the connection is unblocked |
| 12 |
Keep |
| (802.11i) |
The information element is incorrect (the reason code added by 802.11i, so it should refer to the information element of 802.11i) |
| (802.11i) |
Data integrity check failed |
| (802.11i) |
Four key negotiation timeout |
| (802.11i) |
Group Key Negotiation Timeout |
| (802.11i) |
The security parameters of the four-channel consultation information element do not match the original parameter combination |
| (802.11i) |
Group password lock is not correct |
| (802.11i) |
Incorrect pair password lock |
| (802.11i) |
Identity authentication and Key Management protocol incorrect |
| (802.11i) |
Unsupported Gu ' an network information element version |
| (802.11i) |
RSN IE's performance item is incorrect |
| (802.11i) |
802.1X Identity authentication failed |
| (802.11i) |
The set of usage policy rejects the proposed key lock group |
| 25-65535 |
Keep |
Status Code
bit
A status code is used to indicate that a process has succeeded or failed.
4.3.3
managing information elements for frames
Information elements (information Element) are the constituent components of a managed frame and are variable in length. The information element typically contains an element ID (the identifier) bit, a length (length) bit, and an indeterminate bit, as shown in 4-31. The standard values for Element ID numbers are shown in table 4-7. Table 4-7: Information elements
| Element ID |
Name |
| 0 |
Service Set Identity (SSID) |
| 1 |
Supported rates (supported Rates) |
| 2 |
Frequency hopping parameter set (FH Parameter set) |
| 3 |
Direct sequence parameter set (DS Parameter set) |
| 4 |
Competition-Free parameter set (CF Parameter set) |
| 5 |
Data message to be transmitted (traffic indication MAP (TIM)) |
| 6 |
IBSS parameter collection (IBSS Parameter set) |
| 7 (802.11D) |
Country (country) |
| 8 (802.11D) |
Frequency hopping mode parameter (hopping pattern Parameters) |
| 9 (802.11D) |
Frequency hopping mode table (hopping pattern tables) |
| Ten (802.11D) |
Requests (Request) |
| 11-15 |
To retain or use (Reserved; Unused |
| 16 |
Interrogation Password (challenge text) |
| 17-31 |
Keep "Note a" (before 802.11 shared key authentication is deactivated, reserved for future expansion of the inventory password) |
| (802.11h) |
Power limitation (Power Constraint) |
| (802.11h) |
Power Performance (Power Capability) |
| (802.11h) |
Transmit power control requests (transmit power control (TPC) request) |
| (802.11h) |
Transmit Power Control report (TPC) |
| (802.11h) |
Supported channels (supported Channels) |
| Panax Notoginseng (802.11h) |
Channel switching announcement (channels switch announcement) |
| (802.11h) |
Measurement requests (measurement request) |
| (802.11h) |
Measurement reports (Measurement report) |
| (802.11h) |
Forbidden Sound (Quiet) |
| (802.11h) |
IBSS Dynamic Frequency selection (IBSS DFS) |
| (802.11g) |
ERP Information (ERP information) |
| 43-49 |
Reserved (Reserved) |
| (802.11i) |
Gu ' an networks (robust Security network) |
| (802.11g) |
Extended Support Rate (Extended supported Rates) |
| 32-255 |
To retain or use (Reserved; Unused |
| 221 |
Wi-Fi access Protection (Wi-Fi Protected access) |
4.3.3.1
Service Set Identity (
Service Set Identity
(
SSID
))
Some files treat the SSID as a network name because the webmaster typically specifies the SSID as a string. In fact, the SSID is simply a string of bytes formed to mark the BSSID of the owning network. Some products require that the string must be an ASCII string at the end of Null (that is, 0), although the standard does not have a specific specification for this. The SSID is between 0 and 32 bytes in length. If not specified at all, this special case, known as the broadcast Ssid;broadcast SSID, is used only for Probe Request frames, where the workstation can identify all 802.11 networks in the zone.
4.3.3.2
Support Rate (
supported Rates
)
Wireless local area networks support several standard rates. 802.11 the network can use supported Rates (the supported freeze rate) Information element to specify the rate that it supports. When a mobile workstation attempts to join the network, it first examines the data rate used by the network. Some rates are mandatory, each workstation must be supported, and some are optional. It is composed of a string of bytes. Each byte uses seven inefficient bits to represent the data rate, and the most efficient bit is used to indicate whether the data rate is mandatory. If it is a mandatory rate, the most efficient bit is 1 and the non-mandatory rate is 0.This information element can contain a maximum of eight rates.
| Binary values |
Hexadecimal |
Rate of response ( corresponding rates (Mbps)) |
| 2 |
82 |
1Mbps |
| 4 |
84 |
2 Mbps |
| One (802.11B) |
8B |
5.5 Mbps |
| (802.11g) |
0C |
6 Mbps |
| (802.11g) |
12 |
9 Mbps |
| (802.11B) |
96 |
Mbps |
| (802.11g) |
18 |
Mbps |
| (802.11g) |
24 |
Mbps |
| (802.11g) |
2 C |
Mbps (802.11g PBCC option optional 802.11g PBCC) |
| (802.11g) |
30 |
Mbps |
| (802.11g) |
42 |
Mbps (802.11g PBCC option optional 802.11g PBCC) |
| (802.11g) |
48 |
Mbps |
| (802.11g) |
60 |
Mbps |
| 108 (802.11g) |
6C |
-Mbps |
Figure 4-33 shows how tosimultaneous encoding of two data rates。 In addition to supporting mandatory 2Mbps services, optional 11Mbps services are also supported.
4.3.3.4
Direct Sequence parameter collection (
DS Parameter Set
)
802.11 direct sequence (direct-sequence) networks have only one parameter: The number of channels used by the network.
802.11 frame Package Details
