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348 From GSM to LTE-Advanced Pro and 5G

5.3.5 Dedicated Bearer Setup with Preconditions
One way of configuring a dedicated bearer for the VoLTE speech packets is to use the
SIP precondition mechanism during call establishment. 3GPP TS 23.490, chapter 5.3.1
[9], shows a SIP message flow and how ‘preconditions’ work in practice. RFC 3312 [10]
describes the precondition mechanism in general. The following abbreviated message
flow already shown in more detail in Figures 5.7 and 5.8 above shows which SIP mes-
sages contain ‘Precondition’ information using an asterisk (*). The numbers at the
beginning of each line are the message numbers as used in 3GPP TS 23.490.
--> UE to network
<-- network to UE
1 --> * Invite
2 <-- 100 Trying
16 <-- * 183 Session Progress
17 --> PRACK (Provisional Acknowledgement)
24 <-- 200 OK
25 --> * Update
32 <-- * 200 OK
36 <-- 180 Ringing
40 <-- 200 OK
41 --> ACK
Precondition setup starts with the first message (Invite) in which the mobile tells the
network and the terminating side that it supports the precondition mechanism for the
speech path before the call can proceed. In practice this is done with the following
header statement and attributes (a=) in the Session Description (SDP) part of the SIP
message:
Supported: precondition
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos none remote sendrecv
a=sendrecv
At this point in time there is no resource reservation (Quality of Service, QoS) either
locally nor at the terminating side (remote). The UE tells the network and the other
party, however, that QoS is ‘mandatory’ on its side. Concerning the terminating side it
does not care (none).
Next is message number 16, which is a SIP Session Progress message with infor-
mation from the network and the terminator with the following header and
QoS lines:
Require: precondition
[…]
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv
a=conf:qos remote sendrecv
a=sendrec