Page 135 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 121

3.2.3 Common Transport Protocols for CS and PS
In GSM networks, data is transferred between the different nodes of the radio network
with three different protocols. The most important task of these protocols is to split
incoming data into smaller frames which can be transferred over the air interface. While
these protocols are described in more detail in Chapters 1 (GSM) and 2 (GPRS), a short
overview is given below:
Circuit‐switched data (e.g. voice calls). The Transcoding and Rate Adaptation Unit
●
(TRAU) converts the pulse code modulated (PCM)‐coded voice data, which it receives
from the MSC, via optimized codecs like enhanced full‐rate (EFR), half‐rate (HR) or
Adaptive Multirate (AMR). These codecs are much more suitable for data transmis-
sion over the air interface as they compress voice data much better than PCM. This
data is then sent transparently through the radio network to the BTS. Before the data
is sent over the air interface, the BTS only has to perform some additional channel
coding (e.g. increase of redundancy by adding error detection and correction bits).
● Signaling data (circuit‐switched signaling as well as some GPRS channel request
messaging and paging). This data is transferred via the Link Access Protocol (LAPD)
protocol, which is already known from the Integrated Services Digital Network
(ISDN) world and which has been extended for GSM.
Packet‐switched user and signaling data for GPRS. While user and signaling data
●
are separated in GSM, GPRS combines the two data streams into a single lower‐layer
protocol called Release Complete RLC/MAC.
In UMTS, these different kinds of data streams are combined into a single lower‐layer
protocol called the RLC/MAC protocol. Giving this protocol the same name as a protocol
in the GPRS network was intentional. Both protocols work quite similarly in areas like
breaking up large data packets from higher layers into smaller chunks for transmission
over the air interface. However, due to the completely different transmission methods
of the UMTS air interface compared to GSM/GPRS, there are also big differences,
which are discussed in the next section.

3.3 Code Division Multiple Access (CDMA)

To show the differences between the UMTS radio access network and its predecessors,
the next paragraph gives another short overview of the basic principles of the GSM/
GPRS network and its limitations at the time Release 99 UMTS networks were rolled
out. As discussed in Chapter 2, some of those limitations have been reduced or over-
come in the meantime and are now not as severe as in the description below.
In GSM, data for different users is simultaneously transferred by multiplexing them
on different frequencies and timeslots (Frequency and Time Division Multiple Access,
FTDMA). A user is assigned one of eight timeslots on a specific frequency. To increase
the number of users that can simultaneously communicate with a base station the num-
ber of simultaneously used frequencies can be increased. However, it must be ensured
that two neighboring base stations do not use the same frequencies as they would otherwise
interfere with each other. As the achievable speed with only a single timeslot is limited,
GPRS introduced the concept of timeslot bundling on the same carrier frequency.