[Asterisk-Users] Re: TDM users: modified zttest.c for testing

[Tony Mountifield]

I wrote:
> In article <Chameleon.1115133799.adar0 at vegas>,
> Rich Adamson <radamson at routers.com> wrote:
> > 
> > It would be very interesting to see everyone's results in running
> > this, and even more interesting to report the results with the OS
> > distro in use, mobo in use (if known), etc. If anyone actually
> > get's a result that is very close to 1.000 seconds, I'd really
> > like to know more about those systems. (email off list is fine
> > if you want.)
> 
> --- Results after 20 passes ---
> Best: 1.024003 -- Worst: 1.023981 -- Average: 1.023993
> 
> This looks very close to 1024ms instead of 1000ms. That got me thinking:
> 
> I believe your premise is wrong. The sample rate of telephony audio
> is 8kHz. With 8-bit samples (uLaw or aLaw), that means 8000 bytes
> should be supplied in 1 second, not 8192.
> 
> At a rate of 8000 bytes/sec, 8192 bytes will arrive in 1.024 seconds.

I've done a few more tests and think I may have uncovered a problem in
the pseudo-driver. Whether it's relevant to Rich's problem I don't know,
but it might have something to do with MeetMe drift on SIP channels.

I modified Rich's program as follows:

a) Changed "char buf[8192];" to "char buf[SIZE];" (SIZE is 8000).

b) Changed the objective line to:

printf("Objective: to read %d bytes from TDM card in 1.000000 seconds.\n", sizeof(buf));

On running it again I was surprised to find it STILL showing times for
8192 bytes instead of 8000.

I added the following line just after the read() in the main loop:

printf("\nread(fd, buf, %d) returns %d", sizeof(buf), res);

That showed me that read(fd,buf,8000) was returning 1024 bytes.

Aha, so by the time count >= SIZE, it had read 8 blocks totalling 8192.

I changed the read to res=read(fd,buf,sizeof(buf)-count) so it would stop
at 8000 bytes, and got the following results:

[root at softins zaptel]# ./zttest-mod -v
Objective: to read 8000 bytes from TDM card in 1.000000 seconds.
Opened pseudo zap interface, measuring accuracy...

read(fd, buf, 8000) returns 1024
read(fd, buf, 6976) returns 1024
read(fd, buf, 5952) returns 1024
read(fd, buf, 4928) returns 1024
read(fd, buf, 3904) returns 1024
read(fd, buf, 2880) returns 1024
read(fd, buf, 1856) returns 1024
read(fd, buf, 832) returns 832
8000 bytes in 1.023988 seconds
read(fd, buf, 8000) returns 1024
read(fd, buf, 6976) returns 1024
read(fd, buf, 5952) returns 1024
read(fd, buf, 4928) returns 1024
read(fd, buf, 3904) returns 1024
read(fd, buf, 2880) returns 1024
read(fd, buf, 1856) returns 1024
read(fd, buf, 832) returns 832
8000 bytes in 1.023998 seconds
--- Results after 2 passes ---
Best: 1.023998 -- Worst: 1.023988 -- Average: 1.023993
[root at softins zaptel]# 

So it looks like the pseudo driver is always handling 1024 byte chunks,
and even if you ask it for fewer bytes, it takes 1024 bytes' worth of
time.

I think it should really be handling 1000-byte chunks in 125ms rather
than 1024-byte chunks in 128ms, if it is supposed to be emulating
telephony channels.

But zaptel.c is Deep Magic, and I'd be interested in comments from those
who are famliar with it in detail.

Cheers
Tony
-- 
Tony Mountifield
Work: tony at softins.co.uk - http://www.softins.co.uk
Play: tony at mountifield.org - http://tony.mountifield.org