Page 168 - From GMS to LTE

154 From GSM to LTE-Advanced Pro and 5G

advantageous to lock a device to ‘UMTS‐only’ mode in areas well covered by a UMTS
network to prevent such an intersystem handover from taking place.
Another situation in which a transcoder has to be deactivated during an ongoing
connection is when an end‐to‐end connection is extended into a conference call with
several parties. The conference call is established in the MGW and therefore requires a
transcoder, which is implemented today with the AMR‐NB codec.
As can be seen from these scenarios, the introduction of AMR‐WB and overcoming
the frequency limit of PCM of 3400 Hz required much more than just the support of a
new codec in the UTRAN radio network.
In practice, there are currently a number of additional limitations as to when AMR‐
WB can be used. In case several network operators in a country have an AMR‐WB‐
capable network, calls between the two networks are still established with a narrowband
codec. This is because the interconnection between the two networks is still based on
E‐1 links, which are only PCM‐capable and over which the AMR‐WB codec cannot be
transported. Over the coming years, it is expected that network operators will replace
their PCM interconnectivity with IP connections between their MGWs, thus removing
this limitation.
Fixed‐line networks that have been migrated to using the Internet Protocol and the IP
Multimedia Subsystem (IMS) in recent years are also often wideband speech codec
compatible. Unfortunately, fixed‐line networks use the G.722 codec with a higher
bitrate than the G.722.2 codec that is used in wireless networks. As a consequence, a
transcoder is needed between networks to convert between the two wideband codecs.
While some network operators have put such transcoders in place, the majority of net-
work operators still transcode voice calls to a narrowband PCM signal at the border
between the two networks.

3.5.4 Radio Resource Control (RRC) States
The activity of a subscriber determines in which way data is transferred over the air
interface between the mobile device and the network. In UMTS, a mobile device can
therefore be in one of five RRC states as shown in Figure 3.21.

Connected Connected
Not connected Data transfer Paging required
possible at before data can
any time be sent in downlink
direction
Cell-FACH
Cell-PCH
Idle

Cell-DCH
URA-PCH

Figure 3.21 Radio Resource Control (RRC) states.