Page 164 - From GMS to LTE

150 From GSM to LTE-Advanced Pro and 5G

no longer aware of the cell in which a subscriber is currently located. This change was
made mainly for the following two reasons:
The SGSN has been logically separated from the radio network and its cell‐based
●
architecture. It merely needs to forward GTP packets to the RNC, which then pro-
cesses the packets and decides which cell(s) to forward them to. This change is espe-
cially important for the implementation of the soft handover mechanism, which is
further described in Section 3.7.1, as the packet can be sent to a subscriber via several
Node‐Bs simultaneously. This complexity, however, is concealed from the SGSN as it
is a pure radio network issue that is outside of the scope of a core network node. As a
consequence, a UMTS SGSN is only aware of the current Serving RNC (S‐RNC) of a
subscriber.
Using GTP and IP on the Iu(ps) interface on top of ATM or an Ethernet transport
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layer significantly simplifies the protocol stack when compared to GSM/GPRS. The
use of GTP and IP via ATM Adaptation Layer (AAL) 5 or directly over Ethernet is also
shown in Figure 3.18.

The SGSN is still responsible for the Mobility and Session Management (GMM/SM)
of the subscribers as described in Chapter 2. Only a few changes were made to the
protocol to address the specific needs of UMTS. One of those was made to allow the
SGSN to request the setup of a radio bearer when a PDP context is established. This
concept is not known in GSM/GPRS as 2G subscribers do not have any dedicated
resources on the air interface. As described in Chapter 2, GPRS users are only assigned
a certain number of timeslots for a short time, which are shared or immediately reused
for other subscribers once there is no more data to transmit.
In Release 99 networks a different concept was used at first. Here, the RNC assigned
a dedicated radio bearer (RAB) for a packet‐switched connection in a manner very
similar to circuit‐switched voice calls. On the physical layer, this meant that the user got
their own PDTCH and PDCCH for the packet connection. The bandwidth of the chan-
nel remained assigned to the subscriber even if not fully used for some time. When no
data had to be sent in the downlink direction, DTX was used, as described in
Section 3.5.4. This reduced interference in the cell and helped the mobile device to save
energy. The RNC could then select from different spreading factors during the setup of
the connection to establish bearers with a guaranteed bandwidth of 8, 32, 64, 128 or 384
kbit/s. Later on, the RNC could change the bandwidth at any time by assigning a differ-
ent spreading factor to the connection, which was useful, for example, if the provided
bandwidth was not sufficient or not fully used by the subscriber for some time. As the
standard is very flexible in this regard, different network vendors had implemented dif-
ferent strategies for radio resource management.
With 3GPP Release 5, the introduction of HSDPA has fundamentally changed this
behavior for packet‐switched connections as dedicated channels on the air interface
proved to be inflexible in practice. Instead, data packets for several users can be sent
over the air interface on a very fast shared channel. Mobile devices assigned to the
shared channel continuously listen to shared control channels and when they receive
the information that packets will be scheduled for them on the high‐speed channel,
they retrieve them from there.
Packet‐switched data on a shared or dedicated physical channel and a circuit‐switched
voice or video call can be transmitted simultaneously over the air interface. Hence, one