We also observe the acknowledgment of this frame: Prepare the testbed First, load the wifi-experiment. From the WITest console, run: When this process finishes successfully, turn the nodes on with omf tell -a on -t omf. Set up the access point One node is designated to act as the wireless access point. On the terminal of this node, run ifconfig wlan0 up to make sure the wireless interface is up. Set up STA connectivity Two of the nodes are designated as wireless station devices.
802.11 MAC Header Breakdown
First, run ifconfig wlan0 up to bring up the wireless interface. Managed Frequency: Finally, set an IP address on the wireless interface with ifconfig wlan0 X where "X" is the node number e. Set up monitoring Next, we will set up the fourth node I used node18 for this to monitor First, make sure the wireless interface is down: Here, -c 11 says to capture on channel 11, --bssid E4: Use the BSSID of your network that you observed, which may be different from mine , --output-format pcap -w wlan-capture specifies the format to capture in, and a prefix to use for the output file name, mon0 is the interface that we will listen on.
Leave that running.
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Send traffic over the network Now, we will send some traffic over the network. Here's a video of this part of the experiment: Analyze captures Next, we will analyze our packet capture using Wireshark. You'll need to transfer the capture file from the testbed to your laptop. Assuming the file is "wlan-capture Once you have it open, make some changes to the way it is displayed: Also under "View", turn off "Colorize Packet List". Enter the filter wlan. Then hit "Enter" or "Return" to filter your capture to the frames of interest: The filter bar should turn green if your syntax is correct: You should be able to actually see the text of your message: We are going to trace this message from the time that it is transmitted, until it is received.
Frame control field The "type" in the "Frame Control" field indicates that the frame is a "Data" frame as opposed to "Control" or "Management". Frame Control Field: Data frame 2 This is the last fragment Frame is not being retransmitted STA will stay up.. No data buffered. Data is not protected In data frames, and when the network is in infrastructure mode, the address fields depend on the value of the DS flags in the frame control field: The destination address is the MAC address of the wireless station to which this frame should eventually be delivered: Receiver address: It should be the next frame in the capture: ACK frames have only three fields: Frame Control bits are set to to indicate an ACK frame.
Duration field is set to 0 if acknowledging a complete data frame or the final fragment in a fragment burst. Receiver address field is set to the Address 2 field transmitter address of the frame that is being acknowledged. Retransmission from AP to destination Next, in our capture, we can observe that the same data payload is transmitted from the AP to the destination STA: Here, we see some changes in the headers: The DS bits have changed, to indicate that this frame is sent from the AP to the distribution system.
The duration is different. The addresses are different. We are now following the scheme in the second row in the table in the Addresses section, and the receiver and transmitter addresses are different now because the frame is going from the AP to the destination STA, rather than from the source STA to the AP.
- Understanding the Wireless LAN MAC frame format!
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The sequence number is different. This number indicates where a frame belongs in the sequence of frames sent from a transmitter address, to a receiver address. But, the data payload of the frame, including all the higher-layer headers and the text of the message, are the same: Notes Exercise Annotate the packets you captured following the tutorial above, as I have done in the Results section. Show four annotated packets: The first frame carrying the message you typed into netcat its Then, modify the experiment so that the network operates in ad-hoc mode, rather than in infrastructure mode, as follows: First, reconfigure each of the two wireless station nodes so that they are in ad-hoc mode: Ad-Hoc Frequency: The format of a data frame is shown in the below diagram.
The format of a management frame is shown in the below. You can find more details almost all the detail from the below IEEE standard document which I used as reference. IEEE Also below Cisco support forum document also help you to understand field of these different type of wireless frames. A starter guide to learn wireless sniffer traces.
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You are commenting using your WordPress. You are commenting using your Twitter account. The Power Management field is reserved in all frames transmitted by the AP. A value of 0 indicates that the STA will be in active mode. The Power Management field is reserved in all management frames that are not bufferable management frames and that are not individually addressed Probe Request frames. A value of 0 in group addressed frames, in management frames transmitted to nonpeer STAs, and in Probe Response frames indicates that the mesh STA will be in active mode towards all neighbor mesh STAs.
A value of 1 in group addressed frames, in management frames transmitted to nonpeer STAs, and in Probe Response frames indicates that the mesh STA will be in deep sleep mode towards all nonpeer mesh power STAs. A value of 0 in individually addressed frames transmitted to a peer mesh STA indicates that the mesh STA will be in active mode towards this peer mesh STA A value of 1 in individually addressed frames transmitted to a peer mesh STA, except Probe Response frames, indicates that the mesh STA will be in either light sleep mode or deep sleep mode towards this peer mesh STA.
The mesh power mode transition rules are described in This flag is probably the most viewed flag in the mac header.
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Many things can cause a frame to be retransmitted. High retransmission rates cause poor WiFi performance. If this bit is flipped to 0 it indicates no retransmission. If this bit is flipped to 1 it indicates a retransmission. George HInt: This is one of the more important indicators related to network performance in a mac header for troubleshooting. The Retry field is 1 bit in length and is set to 1 in any data or management type frame that is a retransmission of an earlier frame. It is set to 0 in all other frames.
A receiving STA uses this indication to aid in the process of eliminating duplicate frames. Alerting the receiving station that a fragmented frame exist. If this bit is flipped to 0 it indicates no more pending fragmented frames. If this bit is flipped to 1 indicates pending fragmented frames.
There are a number of scenarios where both of these bits in combination have a variety of meanings.
Understanding the 802.11 Wireless LAN MAC frame format
Note Omnipeek is playing with wording of this field. The duration field in a mac header has a two different purposes. This field is used to reset NAV timers for devices on channel. It is also used for legacy ps-polling and the use of the AID number. This is another field that can have an impact on performance. Check out my blog post: The contents of this field vary with frame type and subtype, with whether the frame is transmitted during the CFP, and with the QoS capabilities of the sending STA.
The contents of the field are defined as follows: The value of the AID is in the range 1— See 9. WiFi can have 4 mac address fields. There are four address fields in the MAC frame format. Certain frames may not contain some of the address fields. Certain address field usage is specified by the relative position of the address field 1—4 within the MAC header, independent of the type of address present in that field.