What is the use of packet boundary flag in L2CAP in Bluetooth.
And it also has concept of automatically Flushable, Non-automatically Flushable and Point-to-point. What does it mean?
Please answer the question, It would be great help.
Thank you in advance
The packet boundary flag is part of the fragmentation and reassembly mechanism which allows L2CAP to transfer data units that are larger than the payload in the physical packets sent over the air.
I suggest you read about the HCI ACL data packet format, you can find it in the Bluetooth core specification which is freely available from the Bluetooth SIG website https://www.bluetooth.com/specifications/specs/core-specification/
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Is there an easy way to send and receive BLE raw packets on Linux, e.g. Raspberry Pi devices? Very simply, I would like to use BLE as packet radio, i.e. exchange data packets over the air among several devices. Every device knows the MAC address of all other devices. The BLE packet format is very straightforward. I would think one should be able to put user data into the PDU (2~257 bytes) then push it through a socket to be sent out through the BLE PHY, just like how one can send raw UDP packets through a NIC. But I cannot figure out a straightforward way to achieve this after quite a bit of investigation. I suppose hcitool cmd could send packets if used correctly? But its usage is very cryptic.
The node client/server functions here do exactly this: exchange raw packets via BLE over a network of Pis. See section 3.7 in the documentation for NODE connections.
https://github.com/petzval/btferret
I have a small BLE beacon which is configured to send iBeacon packets every 1000ms.
In my usecase i want to detect the signal on multiple recievers every time it is sent. However the detection is not reliable no matter which receiving device and software i use (phone, computer, raspberry). The signal is sometimes detected after 2 seconds, another time 5, 6 or whatever. It seems like there is no pattern behind it.
Also it seems that sometimes the signal is received on one receiver but not on the others while definetly being in range! Also the area i am testing in is not "problematic".
What could be the problem?
Not 100% of beacon packets transmitted are ever received. There are a number of reasons for this, including radio noise, packet collisions and channel hopping. That said, the typical detection rate in a quiet radio environment is around 90 percent.
If you are seeing a much lower rate than this on multiple receivers I would check the transmitter. First, use one of.your devices to transmit a software beacon (Android and iOS have free apps like Locate Beacon that do this.) If you get a higher detection rate with a different device transmitting, the issue may be your transmitter.
A few possible issues with it:
Bad antenna sending out a very weak signal. (Measure the received RSSI when you do get a detection and verify it is over -60 dBm)
Weak transmitter power setting. See if you can configure this higher.
Advertising on wrong channels
Advertising is stopped before packet can go out. Try to leave the advertiser on for at least 2x the transmission rate to ensure at least on packet gets out.
I assume you use a large enough scan window for your scan interval so your receiver radio is actually turned on most of the time.
You could try to send advertising packets of the type ADV_NONCONN_IND (non-connectable and no scan response packet). That way if the receiver radio scans with 100% duty it should see the packet.
Otherwise if you use normal ADV_IND packets, then at least Android always waits for the SCAN_RSP packet before it sends anything to the scanning app. But if the are multiple scanners nearby, your peripheral can only respond to a single scanner's SCAN_REQ for each advertising packet. To avoid collisions of SCAN_REQ packets in the air, Bluetooth controllers also back off if they don't get a SCAN_RSP in return. If you use a BLE sniffer you can see all three kinds of packets and what happens when you have multiple scanners nearby.
Read the BLE Link Layer part of the Bluetooth Core specification for more details.
Let's say I want to start transmitting advertising packets from a Blueooth 4 module attached to a Raspberry Pi. I am planning to use BlueZ library for the same. Have a basic questions regarding the same -
How much memory does a typical Bluetooth device contains (is it standard or something that can change from vendor to vendor) ? In both the cases, does the advertisement has to be 27 bytes (iBeacon) and 28 bytes (Altbeacon and URIBeacon) or can it be extended to any number limited by bluetooth memory size or any other guidelines? Wish to understand a little bit about this topic.
Thanks in advance!
Device memory is not what limits the Bluetooth LE advertisement size. The limitation is imposed by the Bluetooth 4.0 Core specification, which allows for a maximum of 28 bytes in a manufacturer advertisement PDU (including the one-byte PDU length field).
While you can't transmit more data in a single advertisement, it is possible to send more data using other techniques including:
Interleaving multiple advertisements from the same transmitter. You can differentiate these advertisements with a "type" byte, and then use this to stitch them together with the receiving device. Disadvantage: complex implementation.
Using a scan response packet to send additional data. Disadvantage: scan responses may not arrive in a timely manner.
Provide a connectable GATT service that can be used to fetch additional data. Disadvantage: once connected, advertising stops.
Use a web service to look up additional data based on a unique identifier in the advertisement. Disadvantage: It won't work with out an internet connection.
From what I've read from Bluetooth Essentials for Programmers, this books mentions some points that I dont quite understand.
RFCOMM is a reliable stream-based protocol.
L2CAP is a packet-based protocol that can be configured with varying levels of reliability.
L2CAP actually serves as the transport protocol for RFCOMM, so every RFCOMM connection is actually encapsulated within an L2CAP connection.
This really confuses me since these two protocol are different types of protocols (one is stream based while the other is packet based). So based on its explanation, I have some questions in the following.
Since RFCOMM connection is encapsulated, then how serial port profile(SPP) can use RFCOMM connection? It should use L2CAP as well.
A2DP uses L2CAP connection. This means the same to me since L2CAP and RFCOMM are encapsulated within ACL connection, should this mean A2DP uses ACL connection? I know this is incorrect but how to explain this here.
Thanks for any explanation here.
The encapsulation could be at the "device" driver level so it would be invisible to you, its like you never "created" an Ethernet packet, but you still able to use TCP/IP natively, how come? same thing here, RFCOMM uses L2CAP, but doesn't mean you need to know L2CAP to use.
Usually encapsulation is used when you don't want to hassle with something else, in this case they didn't want to hassle with the signalling issue, RFCOMM is used for Serial interface, so they didn't want to handle the signalling issue of Serial interfaces
Is there some way to get the Frame Check Sequence (FCS) from an ethernet frame when using Wireshark to capture packets under Linux?
The FCS is generated by the sender's ethernet device and decoded by the recipients ethernet device. If the FCS is correct, the payload is passed up to higher layers. If it's not, then the potential frame is discarded.
Because it's added for the exclusive use of the ethernet layer, there's no reason to pass the data up to the operating system. There's no reason for the hardware to include additional capabilities of moving that data out of the card. All modern ethernet devices do onboard FCS processing (and often much more). The only exception would be devices intended to analyze ethernet performance and function.
The Ethernet wiki page on Wireshark states:
Most Ethernet interfaces also either don't supply the FCS to Wireshark or other applications
so I assume the answer is a no.