Page 54 - From GMS to LTE

40 From GSM to LTE-Advanced Pro and 5G

64 kbit/s PCM-coded speech signal

MSC

64 kbit/s Speech compression and decompression
TRAU
12.2 kbit/s in the TRAU and the MS

BSC

12.2 kbit/s coded FR, EFR
speech signal for the air interface

Figure 1.31 GSM speech compression.

In the mobile network, the compression and decompression of the voice data stream
is performed in the Transcoding and Rate Adaptation Unit (TRAU), which is located
between the MSC and a BSC and controlled by the BSC (see Figure 1.31). During an
ongoing call, the MSC sends the 64 kbit/s PCM‐encoded voice signal toward the radio
network and the TRAU converts the voice stream in real‐time into a 13 kbit/s com-
pressed data stream, which is transmitted over the air interface. In the other direction,
the BSC sends a continuous stream of compressed voice data toward the core network
and the TRAU converts the stream into a 64 kbit/s coded PCM signal. In the mobile
device, the same algorithms are implemented as in the TRAU to compress and decom-
press the speech signal (see Figure 1.32).
While the TRAU is a logical component of the BSS, it is most often installed next to
an MSC in practice. This has the advantage that four compressed voice channels can be
transmitted in a single E‐1 timeslot. After compression, each voice channel uses a 16
kbit/s sub‐timeslot. Thus, only one‐quarter of the transmission capacity between an
MSC and BSC is needed in comparison to an uncompressed transmission. As the BSCs
of a network are usually located in the field and not close to an MSC, this helps to
reduce transmission costs for the network operator substantially as shown in Figure 1.32.
The TRAU offers a number of different algorithms for speech compression. These
algorithms are called speech codecs or simply codecs. The first codec that was stand-
ardized for GSM is the full‐rate (FR) codec, which reduces the 64 kbit/s voice stream to
about 13 kbit/s.
At the end of the 1990s, the enhanced full‐rate (EFR) codec was introduced. The EFR
codec not only compresses the speech signal to about 13 kbit/s but also offers superior
voice quality compared to the FR codec. The disadvantage of the EFR codec is the
higher complexity of the compression algorithm, which requires more processing
power. However, the processing power available in mobile devices has increased signifi-
cantly since the 1990s, and thus modern GSM phones easily cope with the additional
complexity.
Besides those two codecs, a half‐rate (HR) codec has been defined for GSM that only
requires a bandwidth of 7 kbit/s. While there is almost no audible difference between