Page 133 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 119

3.1.8 3GPP Releases 10 and Beyond
While 3GPP Release 10 and beyond have continued to bring in new features for UMTS,
the vast majority of network operators stopped evolving their UMTS networks after
implementing dual‐cell downlink in one frequency band as per 3GPP Release 8.
Therefore the UMTS feature overview in this book ends at this point.

3.2 Important New Concepts of UMTS

As described in the previous paragraphs, UMTS on the one hand introduces a number
of new functionalities compared to GSM and GPRS. On the other hand, many proper-
ties, procedures and methods of GSM and GPRS, which are described in Chapters 1 and
2, have been kept. Therefore, this chapter first focuses on the new functionalities and
changes that the Release 99 version of UMTS has introduced compared to its predeces-
sors. In order not to lose the end‐to‐end nature of the overview, references are made to
Chapters 1 and 2 for methods and procedures that UMTS continues to use. In the sec-
ond part of this chapter, advancements that were introduced with later releases of the
standard, such as HSPA, are discussed. These enhancements complement the Release
99 functionality but do not replace it.

3.2.1 The Radio Access Bearer (RAB)
An important new concept that is introduced with UMTS and has since also been
reused in LTE is the Radio Access Bearer (RAB), which is a description of the trans-
mission channel between the network and a user. The RAB is divided into the radio
bearer on the air interface and the Iu bearer in the radio network (UTRAN). Before
data can be exchanged between a user and the network it is necessary to establish a
RAB between them. This channel is then used for both user and signaling data. A
RAB is always established by request of the MSC or SGSN. In contrast to the estab-
lishment of a channel in GSM, the MSC and SGSN do not specify the exact proper-
ties of the channel. Instead, the RAB establishment requests contain only a
description of the required channel properties. How these properties are then
mapped to a physical connection is up to the UTRAN. The following properties are
specified for a RAB:
service class (conversational, streaming, interactive or background);
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maximum speed;
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guaranteed speed;
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delay;
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error probability.
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The UTRAN is then responsible for establishing a RAB that fits the description. The
properties have an impact not only on the bandwidth of the established RAB but also on
parameters like coding scheme and selection of a logical and physical transmission
channel, as well as the behavior of the network in the event of erroneous or missing
frames on different layers of the protocol stack. The UTRAN is free to set these param-
eters as it sees fit; the standards merely contain examples. As an example, for a voice call
(service class conversational) it does not make much sense to repeat lost frames.