Page 363 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 349

In the message the network now informs the device in the header and ‘remote’ lines
that it requires the use of preconditions. At this point in time, neither side has QoS in
place. The (a = conf:…) line is also of importance as the terminator requests a confirma-
tion (conf) from the originator once their resources have been granted. Only then will
the terminator start ringing.
In message number 25 the local device signals to the other end that it has received an
LTE RRC message for a dedicated bearer assignment (not shown in the flow above as it
is not a SIP message) and thus tells the other side that QoS on its side is now in place (as
requested in the conf(irm) line in message 16):
a=curr:qos local sendrecv
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
a=sendrecv
And finally, in message number 32 the other end also returns the information that
resources are in place, which means the call can go forward.
a=curr:qos local sendrecv
a=curr:qos remote sendrecv
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
The other side then starts alerting the user, which it informs the originator of with
message number 36 ‘180 Ringing’. When the user accepts the call the remote end then
sends a ‘200 OK’ message and the two parties can speak to each over a QoS‐enforced
dedicated bearer on both sides.
If the network does not include any precondition information in message 16 ‘183
Session Progress’, the mobile device continues the call establishment procedures with-
out the precondition mechanism. This means that message 25 and 32 are not sent once
the dedicated bearer for the speech path is in place.

5.3.6 Header Compression and DRX
The main inefficiency of VoIP data streams is the overhead from the IP headers of each
packet. To compensate, RoHC can be used between the base station and the mobile
device as described in more detail in Section 4.3.11 on PDCP header compression.
In addition, reducing power consumption during a voice call during the times when
no voice data is sent or received is also important. This can be achieved by using DRX
(Discontinuous Reception). During the DRX period, the UE’s transceiver is put into a
sleep state. This is especially important for voice sessions as the bandwidth required is
so small that the time between two IP packets containing voice data is very long. Keeping
the receiver constantly switched on would waste a lot of energy in the mobile device.
Both RoHC and special DRX settings are activated in an RRCConnectionReconfiguration
message at the time the dedicated bearer for the speech data packets is set up, as described
above. The following message excerpt shows how parameters are set. The values in brack-
ets give an indication of how the parameters are set for a normal default bearer that is
used for Internet access. In this example, DRX is configured to activate after 4 ms, com-
pared to several hundreds of milliseconds typically used for a default bearer that carries