Page 364 - From GMS to LTE

350 From GSM to LTE-Advanced Pro and 5G

Internet traffic. The sleep time before the radio has to wake up to see if there is data
waiting in the downlink direction has been set to 40 ms in this example, which is also
much shorter than for default bearers carrying Internet traffic. Setting the DRX parame-
ters to such low values is essential as the gap between two IP packets carrying speech data
is rather short but periodic and predictable. As a consequence DRX mode can be entered
very quickly. As it is also essential to deliver the packets as quickly as possible to minimize
the ‘mouth‐to‐ear’ delay, it is necessary to wake up from DRX mode quite often.
rrcConnectionReconfiguration
[…]
pdcp-Config
[…]
headerCompression: RoHC
maxCID: 2
profile 1: Used [Not used]
profile 2: Used [Not used]
profile 3: Not Used [Not used]
[…]
rlc-Config: um-Bi-Directional
um-Bi-Directional
ul-Unacknowledged-RLC [ul-Acknowledged-RLC]
dl-Unacknowledged-RLC [dl-Acknowledged-RLC]
[…]
drx-Config
onDurationTimer: psf6 [psf4]
drx-InactivityTimer: psf4 [psf200]
drx-RetransmissionTimer: psf4 [psf16]
longDRX-CycleStartOffset: sf40 [sf80]
To the voice service at the application layer of the protocol stack, the voice optimization
of the bearer is transparent. All that is required is that the voice service in the network
requests appropriate QoS parameters to be used for a data stream via a network interface.
In theory, this interface could also be used by Internet‐based voice services if offered to
external services by network operators.

5.3.7 Speech Codec and Bandwidth Negotiation

In the past, circuit‐switched fixed‐line networks used a single speech codec so no codec
negotiation was necessary during call setup. In fixed‐line SIP networks, devices support
different speech codecs with various speech qualities and bandwidth requirements
(codec rates). As a consequence, devices establishing a connection need to exchange
information about which codecs each side supports and then pick a commonly sup-
ported codec. In VoLTE networks, codecs are additionally rate adaptive and bandwidth
for the data stream can be limited by the mobile network to a value that is lower than
the highest datarate of a codec family.

Codecs – Wide and Narrow
In VoLTE the two most‐used codecs are Adaptive Multi‐Rate Narrowband (AMR‐NB)
and AMR‐Wideband (AMR‐WB). In addition high‐end models also support the