My understanding is that current WiFi driver uses rate control algorithm to choose a data rate within a small set of predetermined values to send packets over the WiFi medium. Different algorithms exist for this purpose. But how does this process work when WiFi driver decides that the connection is lost and shutdown the connection all together? Which part of the code should I read in open source WiFi driver such as MadWiFi and the likes?
The WiFi driver for your hardware which runs in Linux communicates with the WiFi chip which also runs a pretty complex firmware. The interface between the driver and the firmware is hardware specific. In some hardware the detection of connection loss events is done completely by the firmware and the driver only gets a "disconnected" event while in others the driver is also involved.
Regardless of who does what disconnection usually occurs due to either
Receiving a DEAUTH frame from the AP
Detecting too many missing beacons. Beacons are WiFi frames sent periodically by the AO (for most APs every ~100ms) . If you get too far from the AP or the AP was just powered off you stop seeing the beacons in the air and usually you'll signal disconnection or try to roam to a different AP.
Too many failures on Tx of packets (i.e. not receiving ACK frames for too much traffic)
This usually indicates that you've gone too far from the AP. It could be that you can "hear" the AP but it can't hear you already. In this case it also makes sense to signal a disconnection.
For example you can look in TI wifi driver in the Linux kernel drivers/net/wireless/ti/wlcore/events.c and the function wlcore_event_beacon_loss()
In Cfg80211 architecture, assume we are station mode.
driver call kernel API cfg80211_send_disassoc() if we received a deassoc/deauth frame.this function will notify corresponding application (ex wpa_supplicant) a disconnect event.
on another hand, when we decide to disconnect with AP, applicantion (ex wpa_supplicant) can call linux kernel API cfg80211_disconnected(), it will trigger corresponding driver ioctl function to finish disconnection task.
Related
I know I can send 802.11 custom packets with wifi_send_pkt_freedom, and I'am using it without any problem.
But what about receiving?. Yes, I can enable promiscuos mode, and filter by MAC address. Doing that is perfect for sniffing, but not for communication, because the ESP8266 won't ACK the packets and if I'am not sniffing and my ESP is the only receiver, it will make the transmitter to repeat many times the packet. Yes, I can send it like broadcast or multicast to avoid waiting for the ACK, but I'll missing the ACK/re-send functionality. In short I would like to put the ESP Wifi interface in monitor mode (which is not promiscuos mode)
And yes, I can use ESPNOW, but for my application it wont suit very well my needs.
Thanks!
Román
When the device is in confirmed mode, it waits for a downlink confirmation (ACK) from the network after each uplink. If the confirmation is not received by the device, it will repeat (up to a maximum of 8 times and increasing the SF of the UL if it was lowered before) the uplink until it receives a confirmation. It may take about 30 seconds to send the 8 repetitions.
If the device does not see a confirmation and needs to send another uplink (e.g. alarm or new periodic measurement), it will send the new uplink and forget the previous one. To operate in confirmed mode, the device must be declared in confirmed mode (or ACK) on the network platform. You can activate it via the IoT configurator in the network parameters
Be aware that this operation consumes much more battery power than a traditional operation, even more, if the network quality is poor.
If the transmitter loses a lot of frames, it would be better to reposition the transmitter (if possible) or the GW (if possible) to improve the transmission rather than activating the ACK which will drain the battery faster than "expected" depending on the network condition.
Is this answer correct or not?
Is Network traffic monitoring with WireShark:
Active Monitoring
Passive Monitoring
Combinational Monitoring
Wireshark performs passive monitoring. It would likely help you to know what the differences are.
Passive monitoring indicates that a network device has been put into promiscuous mode and begins collecting network frames as they pass by, making them available for analysis.
Active monitoring indicates that some sort of stimulation is generated by the tool performing the monitoring. An example would be broadcasting out a large number of SSIDs to attempt to stimulate a response from a wireless network device.
I have a wireless sensor network deployed in a building. Each node is in a separate room. All the sensory data goes to a datastore.
The user once he/she gets to a room should be able to get the sensory data on his phone from the datastore, provided that we know in which room he/she is. GPS does not give high accurary neither infering it from the wifi signal strength. We thought of having the phone send a dummy frame through wifi that can be intercepted by the sensor node and then based on the node who gets it, or gets it first (in case many nodes intercept that frame) should give an indication to the system of what room the user is in
Wifi and Zigbee both communicate on 2.4Ghz. Is there a way I can intercept all the RF signals from the Zigbee node and entrepret the frame even if it is not a a Zigbee frame?
No, it's not possible, they use different signaling methods.
I am trying to build an app for iOS that can connect to computers running macOS or windows, and control a few stuff on those computers. Another application will be installed on those computers so that the app on iOS can connect to them. But at first I need to discover those computers in the network that has my app installed and running. What is a good way of doing that? I thought about using broadcasting, multicasting or bonjour. Are there any other options? Which one is best for my situation?
I am planning on doing two different applications for macOS and windows, one with objective c and other with c#, so the networking stuff should be available for both of those. Thanks in advance
The simplest option by far would be to use IP/UDP broadcast packets. The application on the computers (running whatever OS) can all sit there listening on a predefined UDP port (e.g. 9999), and when the iOS device wants to 'scan' the network, it will send out an IP/UDP broadcast packet with the destination port of 9999. Upon receiving the broadcast packet(s), the application on the computers can respond since it now knows the IP address of the iOS device, and you can take things from there.
The cleanest way to handle a computer leaving the network is for the application that is running on the computer to communicate this fact to the iOS device since it already knows the IP address of the iOS device. But if keeping a current list of computers is crucial, then some sort of a polling mechanism is unavoidable because the computers may crash for whatever reason without having the chance to send the bye-bye message.
Multicasting can be utilized as follows: computers periodically send IGMP joins for a predefined multicast group (e.g. 224.1.1.1), and iOS device sends the multicast UDP packet destined to 224.1.1.1 when it wants to 'scan' the network. The multicast UDP packet(s) will be received by the computers since they have already joined the multicast group of 224.1.1.1, and then the computers can start communicating with the iOS device now that the IP address is known. However, this seems overly complex, and does not really offer any advantages. The whole point of using multicast is to save bandwidth, but the amount of bandwidth saved will be minuscule. Unless you are going to send the same data in substantial quantities from the iOS device to all the computers, there is simply no reason to go down this path.
As for Bonjour, unfortunately I am unable to comment as I have no experience with it, but I would still vote for simple broadcasting to keep things platform independent... well, at least on the computers side. :)