Re: [Bluez-users] how to get maximum throughput with Bluetooth 2.0?

[Steven Singer <steven.singer@csr.com>]

Loreno Oliveira wrote:
> USB-250). For my tests I wrote a program that opens a l2cap socket with a
> second host and continuously writes a message of 1200 bytes.

1200 is a bad number.

1200 byte L2CAP payloads become 1204 bytes over the air. Multiple L2CAP
PDUs can not be aggregated into an air packet. 3-DH5 packets can hold
only 1021 bytes.

This means that, even in ideal circumstances, your payload will split into
one 1021 byte 3-DH5 packet and one 183 byte 2-DH3 packet. This will
take 10 slots to send (5+1+3+1).

So, the absolute best you could hope for is to send 1200 bytes in
6.25 ms. That's 1.5 Mbps.

>                                                              Befores opening
> the socket connection I change the connection packet type to 3-DH5 with
> hcitool utility.

This is generally a bad idea and is extremely bad in this case.

Now, even under ideal circumstances, the 183 byte trailer has to go into
a 3-DH5. You're now looking at 12 slots or 7.5 ms per L2CAP PDU. This
reduces your theoretical maximum data rate to 1.3 Mbps.

Make the following changes to your test.

  * Increase your L2CAP PDU size to 2033 bytes (that's 2037 over the
    air).

  * Don't restrict the choice of packet types.

This should be optimal for real world throughput as 2037 not only splits
nicely into 1021 byte 3-DH5 packets but also splits nicely into 679 byte
2-DH5 packets. This means that when your link starts to degrade - as it
will do in the real world - your bandwidth will fall to 1.4 Mbps. If you
used, say, 1017 byte packets to get optimal 3-DH5 performance your 2-DH5
bandwidth would be worse.

Restricting to 3-DH5 in the real world is bad. 2-DH5 packets are much
more robust than 3-DH5 packets so when the link starts to degrade,
2-DH5 packets will give much higher bandwidth than 3-DH5. Note also that
you can't prevent the baseband from using DM1 packets so restricting you
may get worse results than you expect if the baseband decides that 3-DH5
packets aren't getting through and it needs to fall back. It'll have to
go all the way to DM1 packets which would be bad.

	- Steven
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