Page 172 - From GMS to LTE

158 From GSM to LTE-Advanced Pro and 5G

The difference between the Cell‐PCH and URA‐PCH states compared to the idle
state is that the network and the mobile device still maintain a logical connection and
can restart data transfers in the uplink direction much quicker as no reestablishment
of the core network connection, new authentication procedure or reactivation of
ciphering is necessary. As the RRC states are managed by the RNC, the SGSN, as a core
network component, has no information on the RRC state of the mobile device.
Therefore, the SGSN simply forwards all incoming data packets from the GGSN to the
RNC regardless of the current state of the mobile. If the mobile is currently in either
Cell‐PCH or URA‐PCH state the RNC needs to buffer the packets, page the mobile
device, wait for an answer and then establish a physical connection to the mobile
device again. If the mobile device is in Cell‐DCH or Cell‐FACH state, on the other
hand, the RNC can directly forward any incoming packets. The distinction between a
logical and a physical connection has been made to separate the connection between
the mobile device and core network (SGSN and MSC) on the one hand and the con-
nection between the mobile device and the RNC on the other. The advantage of this
concept is the decoupling of the MSC and SGSN from the properties and functionality
of the radio network. Hence, it is possible to evolve the radio network and core net-
work independently of each other.
Although early networks mainly used the Cell‐DCH, Cell‐FACH and idle states for
data connectivity, it can be observed today that most networks now also use the Cell‐
PCH and URA‐PCH states to reduce battery consumption and signaling traffic.
As described in Chapter 2, the GSM/GPRS SGSN is aware of the state of a mobile
device as the idle, ready and standby states as well as the ready timer are administered
by the SGSN. Thus, a core network component performs radio network tasks like cell
updates. This has the advantage that the SGSN is aware of the cell in which a subscriber
is currently located, which can be used for supplementary location‐dependent func-
tionalities. The advantage of implementing the UMTS state management in the RNC is
the distribution of this task over several RNCs and thus a reduction of the signaling load
of the SGSN, as well as a clear separation between core network and radio access net-
work responsibilities (Table 3.4).

Table 3.4 RNC and SGSN states.

RNC state SGSN state

Idle Not connected Not connected
Cell‐DCH Connected, data is sent via the DCH or HS‐DSCH Connected
Cell‐FACH Connected, incoming data is sent immediately via the FACH Connected
(Common Channel)
Cell‐PCH Connected, but subscriber has to be paged and needs to reply before Connected
data can be forwarded. Once the answer to the paging has been
received, the subscriber is put in either Cell‐FACH or Cell‐DCH
state
URA‐PCH Same as Cell‐PCH. Furthermore, the network only needs to be Connected
informed of a cell change if the mobile device is moved into a cell
which is part of a different UTRAN registration area