Page 366 - From GMS to LTE

352 From GSM to LTE-Advanced Pro and 5G

Figure 5.9 AMR‐WB codec in an RTP packet. Source: Gerald Combs / Wireshark.

The first line (media, m=) indicates that the device supports two types of media
streams to which it assigns the identification numbers 116 and 118. These are then
described in the lines that follow. Another important parameter given in the first line is
the local UDP port number (42888), to which the incoming audio stream should be sent
later on. The attribute (a=) lines that follow then describe the codecs behind IDs 116
and 118 which are AMR‐WB and AMR‐NB in this example. The other side of the con-
nection then selects one of the two codecs and informs the originator which it has
chosen in a 183 Session Progress message. It should be noted at this point that the net-
work also looks at the codec list and can remove any entries which it does not want to
be used, e.g. due to bandwidth requirements, before the message is forwarded to the
other client device.

Bandwidth Negotiation
Today, VoLTE uses the AMR, AMR‐WB and EVS codecs, which are adaptive and can
encode the voice stream to several datarates and speech qualities. In the case of AMR‐
WB, voice streams can be sent with datarates between 6.6 and 23.65 kbit/s. In practice,
many networks limit the codec rate to 12.65 kbit/s in the case of AMR‐WB and to 12.2
kbit/s for the narrowband AMR codec as speech quality improves only a little with even
higher datarates. Thus, network operators save capacity in the network and the codecs
are also compatible with legacy networks that also support only 12.2 kbit/s for AMR
and 12.65 kbit/s if they also support AMR‐WB. This is done as follows. When establish-
ing a call the device signals its speech codec bandwidth capabilities in the SDP part of
the SIP Invite message by including a ‘bandwidth information’ parameter. The following
example shows the parameter signaling an application‐specific (AS) maximum datarate
of 49 kbit/s.