Re: [Bluez-devel] Thoughts about AVDTP support

Re: [Bluez-devel] Thoughts about AVDTP support

[Marcel Holtmann]

Hi Aarti,

> I agree that we have to keep the provision of recovery
> and reporting functionality from the start.

and there are some more, like multiplexing etc.

> The PSM value for AVDTP is 17 and the order of
> creation will determine the channel type -signal or
> media transfer. 

The PSM for AVDTP is 0x19 and this is 25 decimal ;)

> For the socket type I think it should be
> SOCK_SEQPACKET.

Any other comments about the socket type?

> Yes I do have initial part of the code that I can
> share but have nothing with SBC codec.

Please send it over, so I can see if I can integrate it or not.

Regards

Marcel




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[Bluez-devel] Thoughts about AVDTP support

[Marcel Holtmann]

Hi Folks,

I think one of the next major goals of BlueZ should be the support of
the Audio/Video Distribution Transport Protocol (AVDTP). So I spent some
time in reading the protocol specification. The specification is very
complex and I think most of the products will only support the basic
mode with signal and media channel. However it took me some time to
realize that they are using multiple L2CAP channels between two devices
with the same PSM. I never thought that this was possible, but our L2CAP
core already supports such configuration. Now I asked myself, why HID
and HCRP needed two different PSM values?

To be capable of handling multiple audio and video streams nicely, I
think we need to implement AVDTP as a kernel module. In that case we can
make use of two different interfaces to AVDTP. One for the A2DP support
that is directly connected to ALSA and the second should be a simple
socket interface. I am no audio/video streaming expert, so what socket
type is the best for this case?

Before I start writing any source code, I would like to get my hands on
a product that already uses AVDTP. I need to know what parts of this
protocol are in use and how they handle it. Searching for an end user
product I found one from Air2U (www.air2u.com.tw). The german reseller
is Aiptek and they call it BT MusiCool 300

	http://www.aiptek.de/index.php?mapid=31&katid1=4&detailid=11

I asked them to donate one of these to me, but they said no. Actually
not a nice company. Searching a little bit more I found out that Amazon
is selling it for 99 EUR. This is fair price for two Bluetooth devices
(SNK and SRC), but I decided not to buy them. My conclusion was that if
I should really start working on AVDTP, someone else should buy me an
end user device for testing any of my code. However there is always the
possibility to donate money (www.bluez.org/sponsors.html) which I can
use to buy unsupported hardware.

Regards

Marcel




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Re: [Bluez-devel] Thoughts about AVDTP support

[Aarti Kumar]

Hi Marcel,

The bluetooth avdtp specification is complex but the
if we are dealing with the basic functionality part
itself and it reduces a lot of complexities. 

What makes you think that avdtp is using Multiple
l2cap channels between same devices with same PSM - is
it beacuse of the siganlling link and data transfer
link ?

Using AVDTP as kernel module is good idea (much better
than using it at user space).

I think the connection oriented socket is required for
avdtp. Its also mentioned in the specification.

I would like to help with coding and testing if it
fine with you. I had started with the AVDTP
implementation, but it was at the user level only and
I have not much experience coding at kernel level.

Thats it for now. 

Aarti.

--- Marcel Holtmann <marcel@holtmann.org> wrote:
> Hi Folks,
> 
> I think one of the next major goals of BlueZ should
> be the support of
> the Audio/Video Distribution Transport Protocol
> (AVDTP). So I spent some
> time in reading the protocol specification. The
> specification is very
> complex and I think most of the products will only
> support the basic
> mode with signal and media channel. However it took
> me some time to
> realize that they are using multiple L2CAP channels
> between two devices
> with the same PSM. I never thought that this was
> possible, but our L2CAP
> core already supports such configuration. Now I
> asked myself, why HID
> and HCRP needed two different PSM values?
> 
> To be capable of handling multiple audio and video
> streams nicely, I
> think we need to implement AVDTP as a kernel module.
> In that case we can
> make use of two different interfaces to AVDTP. One
> for the A2DP support
> that is directly connected to ALSA and the second
> should be a simple
> socket interface. I am no audio/video streaming
> expert, so what socket
> type is the best for this case?
> 
> Before I start writing any source code, I would like
> to get my hands on
> a product that already uses AVDTP. I need to know
> what parts of this
> protocol are in use and how they handle it.
> Searching for an end user
> product I found one from Air2U (www.air2u.com.tw).
> The german reseller
> is Aiptek and they call it BT MusiCool 300
> 
> 
>
http://www.aiptek.de/index.php?mapid=31&katid1=4&detailid=11
> 
> I asked them to donate one of these to me, but they
> said no. Actually
> not a nice company. Searching a little bit more I
> found out that Amazon
> is selling it for 99 EUR. This is fair price for two
> Bluetooth devices
> (SNK and SRC), but I decided not to buy them. My
> conclusion was that if
> I should really start working on AVDTP, someone else
> should buy me an
> end user device for testing any of my code. However
> there is always the
> possibility to donate money
> (www.bluez.org/sponsors.html) which I can
> use to buy unsupported hardware.
> 
> Regards
> 
> Marcel
> 
> 
> 
> 
>
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>
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Re: [Bluez-devel] Thoughts about AVDTP support

[Marcel Holtmann]

Hi Aarti,

> The bluetooth avdtp specification is complex but the
> if we are dealing with the basic functionality part
> itself and it reduces a lot of complexities. 

you can leave reporting and recovery out, but it will be still complex
and at some point you must include these features. So we must think
about them from the start.

> What makes you think that avdtp is using Multiple
> l2cap channels between same devices with same PSM - is
> it beacuse of the siganlling link and data transfer
> link ?

As I read the specification, between two devices there is always one
signal channel (and only one). This is the first L2CAP channel opened on
the PSM 25. It also assigns the roles of initiator and acceptor. For the
streaming between two endpoints it has to open at least another L2CAP
channel for the media transport. This channel uses the same PSM and so
the order of the L2CAP channel creation tells us what they are for.

> Using AVDTP as kernel module is good idea (much better
> than using it at user space).

No. Doing something in userspace is almost better and easier, but with
the definition of the streaming endpoints a kernel based socket design
makes sense.

> I think the connection oriented socket is required for
> avdtp. Its also mentioned in the specification.

For the L2CAP connection, yes, but I was asking for the socket type of
our AVDTP socket. I think it should be SOCK_DGRAM or SOCK_SEQPACKET.

> I would like to help with coding and testing if it
> fine with you. I had started with the AVDTP
> implementation, but it was at the user level only and
> I have not much experience coding at kernel level.

How far did you came? Any code that you can share with us?

Do anyone started implementing the SBC audio codec? The specification of
this codec is in the A2DP document and the test document contains sample
*.sbc files and a DOS decoder and encoder (works fine with wine). I like
to see an open source implementation of SBC for FFMPEG or a separate
library.

Regards

Marcel




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