Bluetooth

Inside bluetooth

Specifications are available via bluetooth org, however registration is required for full access. White-papers as https://www.bluetooth.org/Technical/Specifications/whitepapers.htm can be downloaded and give a good overview. Many different protocol layers exist, and therefore bluetooth is not bluetooth, more has to be specified to get something working. Additionally different implementation exist. The HCI (Host controller interface) is the link to the hardware. Unfortunately it is not clear what additional core protocols are included in HCI. Some of those core protocols might be below HCI some above. The core protocols are:

Baseband to access the hardware, asynchronous and synchronous link features are supported

LMP (Link Manager Protocol)

Bluetooth logical link control and adaptation protocol (L2CAP)

SDP (Service Discovery Protocol)

On Top of the core protocols there is the Telephony Control Protocol layer containing RFCOMM the Cable Replacement Protocol that allows receiving the AT commands from the AT command protocol to setup a serial link, as in the old days using a modem. But there is also TCS Binary protocol.

Finally on top of those protocols there can be the adopted protocols as TCP/IP, PPP, OBEX, and many more.

The blueZ stack in Linux supports the L2CAP, RFCOMM and SDP protocols.

Working with bluetooth means therefore knowing with what layer to deal with.

However this is often not enough, since the protocols do not describe a behavior of a device. To get such devices with a standard interface and allow them to be exchanged by other devices, bluetooth profiles have been defined that make use of the mentioned protocols. Interoperability is the key word.

Profiles might use and include other profiles. Generic Access Profile (GAP) is a fundamental profile that is used by many other profiles.

The Serial Port Profile (SSP) is used to replace a point to point RS232 cable with a bluetooth link. It makes use of GAP to make the link and the RFCOMM to set the parameters as baud date as well as for the data exchange. Obviously it makes also use of the low layer core protocols.

The protocols can be found at https://www.bluetooth.org/Technical/Specifications/adopted.htm

Adeunis ARF7678AA

The device is small but still hand solder-able even it is SMD. It works well for cable replacements but there is nothing built in to ask for PIN for pairing. Therefore if security is an issue, it has to be done on top of it or use an other approach as autoconnect to a given bluetooth mode.

After power up it is in transparent mode so no AT command will be accepted. To go in command mode the send the A+++ sequence. Unfortunately this has also some timing constraints so when typing in using the keyboard it might fail (as when it takes longer than 1 second that is the default time out). gtkterm has the feature to send hex strings that can be edited and the send out in a canonical way so the hex sequence is 41 2b 2b 2b (there is no need to write 0x41). If successful the module returns 0.

An other hurdle is that the AT commands must be upper case and AT will return an E indicating error and not OK. A useful command is AT&V that pops out all registers.

Default settings : 9600, 8bits, 1 stop, no parity, HW flow control Enabled (connect RTS and CTS on the module if connecting device has no hardware handshake.

The register S201 holds Serial Port Device and is the name that appears on bluetooth. It is advised to change it so the device can better recognized ATS201=<new name> and verify it ATS201? Finally store it in non volatile memory AT&W

Laird BTM411

Laird modules have the SPP (Serial Port Profile) profile implemented. The development board contain an FT232 chip and can therefore be plugged into a PC. A program as gtkterm can be started at /dev/ttyUSB0. Different ways exist how to make the connection

In general ATI<n> are used to get information back from the device.

The S registers can be used to configure the device ATS<n>? reads them ATS<n>=<nn> sets them. ATIS512? shows the status after power up.

Connecting two modules using the bluetooth address

Preparation of the server

AT&f* restore factory settings

ATS102=1 enable Serial Port Profile (SPP)

ATS0=1 automatic response after ring

AT&W store setting

ATZ reset

AT+BTP make device connect-able until next power up happens

ATI4 returns the bluetooth address of the server

To not have to set AT+BTP at each power up the S512 register can be set to 4 via ATS512=4

Preparation of the client module

AT&f* restore factory settings

ATS102=1 enable Serial Port Profile (SPP)

ATS0=1 automatic response after ring

AT&W store setting

ATZ reset

Connection android app SENA BTerm

Pair the device using android bluetooth. Start SENA BTerm connect via ATD<address of server bluetooth address>. Click ok on android and send AT+BTBY on server module. To not require sending AT+BTBY the pairing security level can be lowered in register 321 to 3 via ATS321=3.

Communication

Making the connection using the servers bluetooth address

AT+SPD<servers bluetooth address>

after that the connection is established and data can be exchanged. Both client and server can go back to command mode entering a ^^^ sequence (instead of the +++ used for phones) (additional there might be a timing constraint) or via DTR->DSR line. Once in command mode AT+SPH disconnects the connection.

Pairing

Useful commands are ATI4 to see its own bluetooth address.

AT+BTT? List the trusted devices, the ones that have been paired in the past.

AT+BTD* Deletes the trusted devices ATZ should follow

AT+BTW<bluetooth address of server>makes a pair request on the server and if successful enters the requesters bluetooth address in the AT+BTT? list.

Pairing can be more or less safe. Register S321 sets the different safety levels. Some of them require a user interface (keys and display). ATS321? shows the level and ATS321=3 set level 3 that does not require a user interface.

Other AT Commands

AT+BTK=”1234” set the pin code

After checking if the passkey matches AT+BTBY accepts the incoming pairing request and AT+BTBY. More secure confirmations require AT+BTB<6 digit key> to enter the key.

The profiles have uuid numbers assigned as 1101 for the SPP. This number pops up during connection requests.

On the same or on an other PC that has Bluetooth running, using Blueman Device Manager it can be scanned for a new device and then paired with the given pin, in the sample above 1234. After that the device can be connected using the SPP profile. The device /dev/rfcomm0 will be created and an other gtkterm can be started.

Since many devices do not have a keyboard and display or just make pairing more user friendly, SSP (secure simple pairing) got introduced with bluetooth version 2.1. SSP allows to different user interfaces as: nothing, yes/no button, display, ... . SSP has level 0 to 3 depending on the desired security as Man In The Middle attack MITM.


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