SDP media field format

Question

Hey,

I would like to create a SDP media field with its attributes, and there are a few things I don't understand. I've skimmed and read the relevant RFC and I understand most of what each field means, but what I don't understand is how do I derive from the Audio/Video Format of the JMF, which parameters of the format compose the rtpmap registry entries I need to use. I see many times the fields

m=audio 12548 RTP/AVP 0 8 101
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-16
a=silenceSupp:off - - - -
a=ptime:20
a=sendrecv

these are received from the pbx server I'm connecting to, what do they mean in the terms of the JMF audio format properties. (I do understand these are standard audio format commonly used in telecommunication)

UPDATE:

I was more wondering about the format parameter '0 8 101' at the end of

m=audio 12548 RTP/AVP 0 8 101

I know they are referenced from this list, but how do I determine according to the JMF media format which one to use?

Thanks in advance,

Adam Zehavi.

Answer 1

I'm not sure if this is what you asked for, but:

• PCMU/8000: 1-channel, 8000 Hz, m-Law encoded format
• PCMA/8000: 1-channel, 8000 Hz, A-Law encoded format
• telephone-event: DTMF digits, telephone tones

Answer 2

You can use any of the codecs listed in the SDP. The agent that sent the SDP is stating that it supports all of the codecs listed.

In the SDP example you've provided you could start sending RTP encoded with either G711 ULAW (PCMU) or G711 ALAW (PCMA).

Answer 3

Well after long while of searching and not really understanding, I can answer my question. In my eyes, the only use for SDP would have been for each side to state to the other his media capabilities, I did not realize it was in the form of negotiation, I didn't understand the need for such a deep negotiation about media, I thought Client 1 could supply X,Y,Z,W, client 2 would response I can only get X,W and then Client one would say ok I send you W format...

don't know why this made perfect sense to me, and I'm going to design my SDP wrapper of my application in this manner, and only to use JMF formats as a comparison instead of dealing with the gutter of the SDP, over and over, I would try to design a general template that would perform all these annoying text generating methods, using JMF format array, just like I think it should be, the only thing I'm surprised is that I didn't find anything like this already made...

Thanks for all your help, and if anyone ever wonder about this subject again, just start reading this RFC