Bluetooth SPP throughput

Question

I am trying to figure out what the maximum throughput of a Bluetooth 2.1 SPP connection is.

I found 2 publications concerned with the topic (1, 2) and they both show diagrams, which show the throughput as a function of the Signal to noise ratio (that I can assume to be perfect for my concideration) and the kind of ACL package used. My problem is, I have no Idea which ACL packets are used. How is this decision made? Is it made on the fly, like "what's needed to transfer the current data is used"?

Furthermore, in the Serial Port Profile specification (chapter 2.3) I found this sentence:

This profile requires support for one-slot packets only. This means that this profile ensures that data rates up to 128 kbps can be used. Support for higher rates is optional.

The last sentence realy confuses me. How do I find out whether this "option" applies in my case?

Answer 1

This means that in SPP mode, all bluetooth modules should work up to 128kbps, and some modules may work even faster.

Under SPP is RFCOMM, which tries to deliver the packets as quickly as possible. If only one packet is sent in one timeslot, you get the 128kbps. The firmware of the bluetooth module, or the HCI driver however can do things differently.

There are SPP speeds up to 480kbps reported - however this requires that both SPP modules are from the same vendor (e.g. BlueGiga iWrap modules can do this speed).

On the other end, if you're connecting to an unknown device, for example a BT112, or an RN41 module to an Android device, the actual usable SPP bandwidth can be much lower than 128 kbps (I have measurements as low as 10kbps).

In case of some old generation iPhones, the usable SPP bandwidth is around 8 kbps.

It is a wise idea to treat "standards" and "datasheets" very conservative, and do your own measurements if actual net data bandwidth is critical.

Even though BT, BT+EDR has theoretical on-the-air bitrates of 3Mbps, the actual usable net data bandwidth is a way smaller.