Page 390 - From GMS to LTE

376 From GSM to LTE-Advanced Pro and 5G

An interesting codec supported by the device that has sent the SDP message above is
G.722, the major fixed‐line AMR‐Wideband codec which encodes audio at a datarate of
64 kbit/s. Like in the wireless domain, using this speech codec results in a much‐
improved sound quality compared to the traditional narrowband codec when calling
other fixed‐line IP‐based destinations that support the wideband codec. Some network
operators transcode between G.722 (64 kbit/s) and G.722.2 (mobile WB‐AMR at 12.65
kbit/s) at the interconnection between their fixed‐line and mobile networks and thus
enable fixed/mobile wideband calls. Note that G.722 is not found in the codec descrip-
tion list, as G.722 is the default RTP profile number 9 [32] and is thus only represented
by its number in the first (m=) line of the SDP (Session Description Protocol).

5.7 Mission Critical Communication (MCC)

5.7.1 Overview

In many countries public safety organizations such as police, fire departments and
medical first responders use TETRA (Terrestrial Trunked Radio) for communication.
TETRA is a circuit‐switched technology that was designed in the 1990s and thus has
many similarities with GSM. The main service on TETRA networks is push‐to‐talk
communication between two parties and push‐to‐talk group communication. Due to
its age and specialized use, equipment is expensive and it will become more and more
difficult in the future to maintain networks and to acquire network components and
mobile devices. Another reason for the TETRA community to consider a successor
system is the system’s limited datarate of a few tens of kilobits per second. Instead of
designing a new radio and core network for the purpose, it was decided to base the next
generation of professional mobile radio (PMR) systems for public safety organizations
on LTE and IMS. In 3GPP Releases 12 and 13 the following major building blocks have
been specified:
Group Communication and Push to Talk (PTT) features, referred to as ‘Mission
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Critical Push To Talk’ (MCPTT).
Ways to prioritize communication sessions over others and over less important data
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traffic.
Device to Device Communication and relaying of communication from one device to
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the other and from there to the network.
Local communication when the backhaul link of an LTE base station is offline while
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the radio base station is still operational.
The following sections will take a closer look at these new LTE functionalities. A
special emphasis is put on MCPTT as it is the most important PMR application.
In practice the United States and Great Britain are the first countries in which LTE‐
based PMR networks are to supersede current 2G voice‐centric networks. In the UK the
government has awarded a contract to Everything Everywhere, which was subsequently
acquired by British Telecom (BT). Instead of building a separate network, BT’s existing
commercial LTE network is used, extended for public safety use [33]. In the United
States a more traditional route was taken by setting up a separate LTE network for
public safety applications [34].