Tag Archives: AMR

Tales from the Trenches: mode-set in AMR

IMS / VoLTE, Mobile Networks, VoIP, , , , ,

This one was a bit of a head scratcher for me, but I’m always glad to learn something new.

The handset made a VoLTE call, and it’s SDP offer shows it can support AMR and AMR-WB:

        Media Attribute (a): rtpmap:116 AMR-WB/16000/1
        Media Attribute (a): fmtp:116 mode-set=0,1,2,3,4,5,6,7,8;mode-change-capability=2;max-red=220
        Media Attribute (a): rtpmap:118 AMR/8000/1
        Media Attribute (a): fmtp:118 mode-set=0,1,2,3,4,5,6,7;mode-change-capability=2;max-red=220
        Media Attribute (a): rtpmap:111 telephone-event/16000
        Media Attribute (a): fmtp:111 0-15

Okay, that’s pretty normal, I can see we have the mode-set parameter defined, which indicates what modes the handset supports for each codec.

In our problem scenario, the Media Gateway that the call was sent to responded with this SDP answer:

        Media Description, name and address (m): audio 24504 RTP/AVP 118 110
        Media Attribute (a): rtpmap:118 AMR/8000
        Media Attribute (a): fmtp:118 mode-set=7
        Media Attribute (a): rtpmap:110 telephone-event/8000
        Media Attribute (a): fmtp:110 0-15
        Media Attribute (a): ptime:20
        Media Attribute (a): sendrecv
        [Generated Call-ID: FA163E564B37-f4d-98f56700-735d25-65357ee0-9c488]

But we got an error about not available codecs and the call drops, what gives?

Both sides support AMR (Only the phone supports AMR-WB), and the Media Gateway, as the answerer, supports mode-set 7, which is supported by the UE, so we should be good?

Well, not quite:

If mode-set is specified, it MUST be abided, and frames encoded with modes outside of the subset MUST NOT be sent in any RTP payload or used in codec mode requests. If not present, all codec modes are allowed for the payload type.

RFC 4867 – RTP Payload Format for AMR and AMR-WB

Okay, I get it, the answerer (media gateway) only supports mode 7, but the UE supports all the modes, so we should be fine right?

Well, no.

Section 8.3.1 in the RFC goes on to say in the Offer-Answer Model Considerations:

The parameter [mode-set] is bi-directional, i.e., the restricted set applies to media both to be received and sent by the declaring entity. If a mode set was supplied in the offer, the answerer SHALL return the mode-set unmodified or reject the payload type. However, the answerer is free to choose a mode-set in the answer only if no mode-set was supplied in the offer for a unicast two-peer session.

And there is our problem, and why the call is getting rejected.

The Media Gateway (the answerer in this scenario) is sending back the mode-set it supports (7) but as the UE / handset (offerer) included the mode-set, the Media Gateway should either respond with the same mode set (if it supported all the requested modes) or reject it.

Instead we’re seeing the Media Gateway repond with the mode set, which it supports, which it should not do: The Media Gateway should either return the same mode-set (unmodified / unchanged) or reject it.

And boom, another ticket to another vendor…

Playing back AMR streams from Packet Captures

IMS / VoLTE, Mobile Networks, VoIP, , , ,

The other day I found myself banging my head on the table to diagnose an issue with Ringback tone on an SS7 link and the IMS.

On the IMS side, no RBT was heard, but I could see the Media Gateway was sending RTP packets to the TAS, and the TAS was sending it to the UE, but was there actual content in the RTP packets or was it just silence?

If this was PCM / G711 we’d be able to just playback in Wireshark, but alas we can’t do this for the AMR codec.

Filter the RTP stream out in Wireshark

Inside Wireshark I filtered each of the audio streams in one direction (one for the A-Party audio and one for the B-Party audio)

Then I needed to save each of the streams as a separate PCAP file (Not PCAPng).

Turn into AMR File

With the audio stream for one direction saved, we can turn it into an AMR file, using Juan Noguera (Spinlogic)’s AMR Extractor tool.

Clone the Repo from git, and then in the same directory run:

python3 pcap_parser.py -i AMR_B_Leg.pcap -o AMR_B_Leg.3ga

Playback with VLC / Audacity

I was able to play the file with VLC, and load it into Audacity to easily see that yes, the Ringback Tone was present in the AMR stream!

Adding support for AMR Codec in FreeSWITCH

FreeSWITCH, IMS / VoLTE, Mobile Networks, VoIP, , , ,

If you’re building IMS Networks, the AMR config is a must, but FreeSWITCH does not ship with AMR due to licencing constraints, but has all the hard work done, you just need to add the headers for AMR support and compile.

LibOpenCore has support for AMR which we build, and then with a few minor tweaks to copy the C++ header files over to the FreeSWITCH source directory, and enable support in modules.conf.

Then when building FreeSWITCH you’ve got the AMR Codec to enable you to manage IMS / VoLTE media streams from mobile devices.

Instead of copying and pasting a list of commands to do this, I’ve published a Dockerfile here you can use to build a Docker image, or on a straight Debian Buster machine if you’re working on VMs or Bare Metal if you run the commands from the Dockerfile on the VM / bare metal.

You can find the Dockerfile on my Github here,