RFC 3551 chapter 4.5 and RFC 4856 chapter 2.1.17 mention it briefly: The rate of PCMA is not fixed at 8000 samples per minute but can vary. In the year 2007, the German company AVM added PCMA/16000 to their Internet access device FRITZ!Box 7170 to enable HD Voice for their Wi-Fi phone FRITZ!Mini. AVM called this ‘PCMA 16’. The same was done for PCMU, which was called ‘Linear PCM 16’. In the log at ‘fritz.box → Telefonie → Eigene Rufnummern → Sprachübertragung’, AVM calls this G.711-HD.
This is not G.711-WB (G.711.1), also called PCMA-WB (RFC 5391). Instead, the 16000 samples of PCMA 16 simply double the bit-rate of PCMx/8000. Still, AVM did not increase the size of each data packet, because AVM uses 10 milliseconds as packetization time (ptime) always – even when ptime=20 was negotiated via SIP/SDP.
This is an implementation of PCMA 16 and allows pass-through between two FRITZ!Boxes and optionally transcoding to/from other audio formats (which is done by a FRITZ!Box normally).
In the same year, G.722 was chosen in DECT/CAT-iq to replace G.726-32 and enable HD Voice. Wi-Fi handsets got replaced by DECT handsets. AVM went for DECT and G.722 in their FRITZ!Box 7270, just one year later. Nevertheless even today, AVM products offer PCMA/16000 and PCMU/16000; why ever.
This implementation is for testing purposes, whether your network allows a RTP stream of 128 kbit per seconds (plus the RTP, UDP, and IP overhead) and no traffic shaping kicks in. Furthermore, it can be used for interoperability testing because some media gateways are known to ignore the sampling rate and threat PCMA/16000 like PCMA/8000. Beside that, I am not aware of any practical use, because all HD-Voice enabled FRITZ!Boxes transcode G.722 as well. If you know an additional use-case, please, drop me an E-mail – I am curious.
At least Asterisk 13 is required. These changes were last tested with Asterisk 13.22 (and Asterisk 16.0). If you use a newer version and the patch fails, please, report!
cd /usr/src/
wget downloads.asterisk.org/pub/telephony/asterisk/asterisk-13-current.tar.gz
tar zxf ./asterisk*
cd ./asterisk*
sudo apt --no-install-recommends --assume-yes install build-essential pkg-config libedit-dev libjansson-dev libsqlite3-dev uuid-dev libxslt1-dev xmlstarlet
Apply the patch:
wget github.com/traud/asterisk-alaw16/archive/master.zip
unzip -qq master.zip
rm master.zip
cp --verbose --recursive ./asterisk-alaw16*/* ./
patch -p0 <./alaw16_pass_through.patch
patch -p0 <./alaw16_transcoding.patch
Configure your patched Asterisk:
./configure
Enable slin16 in menuselect for transcoding, for example via:
make menuselect.makeopts
./menuselect/menuselect --enable-category MENUSELECT_CORE_SOUNDS
Compile and install:
make
sudo make install
- Guido Reismüller (at that time with AVM) who explained PCMA 16 in the Asterisk bug tracker…
- Asterisk team: Thanks to their efforts and architecture, I was able to add this format to Asterisk in less than three hours. Writing this Read Me took longer!