Page 361 - From GMS to LTE

VoLTE, VoWifi and Mission Critical Communication 347

In addition, the UM DRB bearer for voice is configured with a guaranteed bit
rate and network resources are permanently allocated to the user during the call.
One of the options to achieve this is to use semi‐persistent air interface schedul-
ing as described in Chapter 4, which enables the mobile device to periodically
send and receive data without requiring bandwidth assignments. This guarantees
the bandwidth for the call and also saves the overhead of dynamic bandwidth
assignments.
For a VoLTE call a dedicated bearer establishment procedure is always initiated and
controlled by the network by sending an Activate Dedicated EPS Bearer Context
Request message to the VoLTE device. The message excerpt below shows the most
important parameters and their values. Values used in a default bearer for Internet
traffic are shown in brackets as comparison. Dedicated Bearers for the VoLTE speech
path use Quality of Service Class Identifier (QCI) 1 instead of QCI 9, which is used for
the bearer for Internet traffic. QCI 1 is standardized in 3GPP and instructs the radio
base station (eNodeB) to queue such packets for a maximum time of 100 ms instead
of 300 ms for a QCI 9 bearer in case of congestion. As described above, no acknowl-
edgment shall be performed on the RLC layer of the protocol stack. In addition the
dedicated bearer setup instructs the base station and the mobile device to guarantee
a bit rate of 40 kbit/s and to limit the traffic flow through the dedicated bearer to the
same value. As the dedicated bearer shall be limited to voice packets, four traffic‐flow
templates (filter rules) are part of the dedicated bearer activation, two for the down-
link and two for the uplink. These rules limit the use of the dedicated bearer to and
from a single IP address on the network side and for two UDP port combinations. One
port combination is for the RTP (Real‐time Transport Protocol) voice packet stream,
the other is for RTCP (Real Time Control Protocol) packets that are used for manag-
ing the RTP stream.

EPS quality of service
Quality of Service Class Identifier: QCI 1 [QCI 9]
Maximum bit rate for uplink: 40 kbps [default = 0]
Maximum bit rate for downlink: 40 kbps [default = 0]
Guaranteed bit rate for uplink: 40 kbps [default = 0]
Guaranteed bit rate for downlink: 40 kbps [default = 0]
Traffic Flow Templates for the Downlink
Remote IPv6 address: 2304:724:610:4221::8
Local UDP Port: 1254, Remote Port: 60002
Local UDP Port: 1255, Remote Port: 60003
Traffic Flow Templates for the Uplink
Remote IPv6 address: 2304:724:610:4221::8
Remote UDP Port: 60002
Remote UDP Port: 60003
Negotiated QoS
Precedence Class: Normal priority [default = low priority]
Mean Throughput: Best effort
Traffic Class: Conversational [default = background]
Transfer Delay: 100 ms [default = 300 ms]
Reliability: Unacknowledged GTP/LLC/RLC
[default = Unack. GTP/LLC, Ack RLC]