Page 360 - From GMS to LTE

346 From GSM to LTE-Advanced Pro and 5G

specification [8], which is also referred to as the Voice over LTE (VoLTE) IMS profile.
Like 3GPP specifications this document is freely available on the Internet. On its own it
is not suitable as an introduction to IMS and VoLTE as it mainly contains references to
details in relevant 3GPP specification documents. It is very well suited, however, as a
way to quickly discover which parts of which 3GPP documents are relevant for network
operator‐based Voice over LTE systems.

5.3.4 LTE Bearer Configurations for VoLTE
Network operator‐based voice services can directly interact with the transport network
to ensure a low latency and constant bandwidth for a connection. This also helps the
eNode‐B to optimize transmission of VoIP packets over the air interface. For VoLTE,
the following optimizations are activated during the call establishment phase:
In LTE, a dedicated bearer that is established alongside a default bearer is used to
ensure QoS for a stream of similar IP packets. In HSPA, the concept is referred to as a
secondary PDP context. Dedicated bearers or secondary PDP contexts are established
when a service in the network requests prioritization of IP packets belonging to a
specific media stream between two IP addresses and TCP/UDP ports. In practice, the
dedicated bearer then ensures the behaviors described in the following sections.

Unacknowledged Radio Bearers for Voice
For a voice stream, an Unacknowledged Mode Data Radio Bearer (UM DRB) is used
and configured in an RRCConnectionReconfiguration message as shown further below.
This refers to the configuration of the layer 3 RLC protocol on the air interface. On this
layer, lost data is detected and repeated if it is configured in Acknowledged Mode
(AM) – the default RLC operating mode for user data in HSPA and LTE. For a voice data
stream, however, lost voice packets should not be repeated as they would come too late
to be useful. This is why a UM DRB is used. In addition to the unacknowledged mode
bearer, other signaling bearers and the default bearer in acknowledged mode are also
active during a voice conversation.
The unacknowledged bearer for the voice stream is established by the voice service sending
a request to the transport network during the establishment of the call to create a dedicated
bearer for IP packets being exchanged between two particular IP addresses and two
particular UDP ports. This stream is then mapped to a radio bearer for which no RLC error
correction is used. All other IP packets not matching the IP address and UDP port combina-
tion requested above are sent over an AM DRB without guarantees for latency and bandwidth.
It should be noted at this point that an extra dedicated bearer for the voice call does
not require an additional IP address. Instead, only a single IP address is used as it is the
combination of the IP addresses and UDP ports that distinguish the packets that go
through the UM bearer from those that use an AM bearer. For the application on top,
i.e. the VoLTE client application, all of this is transparent as the radio protocol stack
below automatically decides which IP packet should be sent over which bearer.

Packet Loss and Guaranteed Bit Rate
To ensure that the packet loss in UM mode stays within reasonable limits, the radio
transmission characteristics (power output, modulation, coding, …) for the UM bearer
are configured to ensure that the packet loss rate does not exceed 1% – a value that the
voice codec can still tolerate.