Page 367 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 353

b=AS:49

The datarate signaled includes the overhead of the IP, UDP and RTP protocols.
Therefore, signaling a maximum datarate of 49 kbit/s limits an AMR‐WB data stream
to 23.85 kbit/s if IPv6 was used as transport protocol. In VoLTE networks that are not
quite state of the art and still use IPv4, 41 kbit/s is signaled as the limit. While this might
look like a significant difference it does not matter much from a bandwidth perspective
as typically Robust Header Compression (RoHC) is used on the air interface in practice.
Further details on bandwidth requirements for different codecs and datarates can be
found 3GPP TS 26.114 [12]. Table K.6 for example shows bandwidth requirements for
different AMR‐WB codec rates based on IPv6 in bandwidth‐efficient mode (not octet‐
aligned) and a frame length of 20 ms per IP packet.
During call establishment the network then assigns an LTE dedicated bearer with
this or a lower maximum bandwidth. If the network wants to limit the speech band-
width to 12.65 kbit/s, it only assigns a maximum bit rate of around 40 kbit/s for the
dedicated bearer. This is sufficient for AMR‐WB with 12.65 kbit/s, which requires at
least 38 kbit/s including IPv6, UDP and RTP overhead. The device is informed of the
decision in the LTE ‘Establish Dedicated Bearer’ NAS message as shown in the mes-
sage excerpt below.
Quality of Service Class Identifier: QCI 1
Maximum bit rate for uplink: 40 kbps
Maximum bit rate for downlink: 40 kbps
Guaranteed bit rate for uplink: 40 kbps
Guaranteed bit rate for downlink: 40 kbps
The originating mobile then has to send a SIP ‘Update’ message with a ‘b = AS:38’ line
in the SDP part to indicate to the other end that this is the maximum speed that can be
used. The other end performs the same actions on its end and finally the smallest value
signaled has to be used by both sides.

5.3.8 Alerting Tone, Ringback Tone and Early Media
As VoLTE wants to replicate the circuit‐switched telephony service another question
that presents itself is how a media stream can already be sent before the receiving party
answers the call. This is necessary for the caller to hear an alerting (ringing) tone or a
ringback tone (personalized music) as offered by some wireless network operators. This
is what is known as early media in VoLTE and there are two ways this is achieved in
practice.
The first option is that the originating device plays an internal alerting tone to the
user once it receives a SIP ‘180 RINGING’ from the other side. The interesting chal-
lenge for this solution is that the alerting tone is country specific so the mobile device
needs to have different sound files and needs to select the correct one, e.g. based on the
home country of the subscriber. The home country can, for example, be deduced from
the International Mobile Subscriber Identity (IMSI) on the SIM card.
The second option is that the network streams the alerting tone to the calling user
until the SIP ‘200 OK’ message is received from the called party. This is referred to as
‘early media’. While this wastes resources on the air interface it is the only option when
the network supports ringback tones.