GTH audio streaming: why stream over TCP?

Posted March 6th 2009

GTH lets you stream audio from a TCP socket to a timeslot on an E1/T1 line. Some people are surprised by the choice to use TCP. When I added that support back in 2002, my first thought was to use RTP (RFC 1889). RTP is simple: you just dump the audio in a UDP packet with some timestamping information and shoot it out on ethernet at the right rate.

I'd worked with RTP before and I'd been at a couple of SIP interops where most of the attendees had trouble emitting audio at 'the right rate', i.e. 8000 samples/s. One manufacturer's system would emit 8007 samples/s. Another would play it back at 7999 samples/s. What do you do with the extra 8 samples per second? If you do nothing, you get endlessly growing delays and, eventually, a buffer overflow. If you come up with a strategy for throwing away samples, it's bound to interact badly with something, sooner or later.

The thing is, when you're streaming in pre-recorded audio, you don't need it to be at the right rate. You just need to make sure it doesn't overrun or underrun the GTH's internal buffer. I.e. you need flow control, not rate control. TCP has flow control, and everyone knows how to use TCP sockets. In 2002, doing things that way was right at the limit of what our 50MHz embedded CPU could keep up with. Now it's no problem at all.

Python

I'm playing around with python at the moment. Here's how to put some data on an E1 timeslot, straight from the python shell. First, set up a listening TCP socket:

import socket
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.listen(0)
addr, port = s.getsockname()

Next, open another socket to the GTH command port (2089) and tell it we want to stream in audio on the socket we opened above:

a = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
a.connect(("172.16.2.7", 2089))
my_ip, _port = a.getsockname()
command = "" % (my_ip, port)
header = "Content-type: text/xml\r\nContent-length: %d\r\n\r\n" % len(command)
a.sendall(header + command)

Finally, accept() and send the data:

d, _ = s.accept()
d.sendall("hello world")
d.close()

That looks OK to me, though I imagine the style betrays my Erlang mindset. There's a more complete python example at the bottom of the API page.