Page 171 - From GMS to LTE

Universal Mobile Telecommunications System (UMTS) and High-Speed Packet Access (HSPA) 157

the FACH usually uses a high spreading factor, which limits the total available bandwidth
for subscribers on this channel. Typically, the FACH is configured as a 32 kbit/s channel.
Compared to the Cell‐DCH state in which the mobility of the subscriber is controlled
by the network, no such control has been foreseen for the Cell‐FACH state. In the Cell‐
FACH state the mobile device itself is responsible for changing cells and this is called
cell update instead of handover. As the network does not control the cell update it is also
not possible to ensure an uninterrupted data transfer during the procedure. For these
reasons the Cell‐FACH RRC state is not suitable for real‐time or streaming applications.
In practice, it can be observed that a dedicated connection is established, even for
small‐screen web browsing that requires only a little data, but is released again very
quickly after the data transfer. More about the use of the different RRC states in opera-
tional networks can be found in Section 3.13.2.
The Cell‐FACH state is also suitable for the transmission of MM and PMM signaling
messages between the mobile device and the MSC or SGSN. As the mobile device
already indicates the reason for initiating the connection to the network in the RRC
Connection Setup message, the network can flexibly decide if a DCH is to be used for
the requested connection or not. In practice, it can be observed that this flexibility is
not always used as some networks always establish a DCH independently of the reason
for the connection setup.
If the mobile device is in Cell‐FACH state, uplink data frames are sent via the RACH,
whose primary task is to forward RRC Connection Setup Request messages. As
described in Section 3.4.5, access to the RACH is a time‐intensive procedure that causes
some delay before the actual data frame can be sent.
There are two possibilities for a mobile device to change to the Cell‐FACH state. As
already discussed, the network can decide during the RRC connection setup phase to
use the FACH for MM/PMM signaling or user data traffic. In addition, it is possible
to enter the Cell‐FACH state from the Cell‐DCH state. The RNC can decide to modify
the radio bearer this way if, for example, no data has been sent or received by the mobile
device for some time. This reduces the power consumption of the mobile device. As
long as only small amounts of data are exchanged, the Cell‐FACH state is usually main-
tained. If the data volume increases again, the network can immediately establish a new
dedicated bearer and instruct the mobile device to enter Cell‐DCH state to be able to
transfer data more quickly.

Cell‐PCH and URA‐PCH States
The optional Cell‐Paging Channel (Cell‐PCH) RRC state and the UTRAN Registration
Area – Paging Channel (URA‐PCH) RRC state can be used to reduce the power con-
sumption of the mobile device during extended times of inactivity. Similar to the idle
state, no resources are assigned to the mobile device. If data arrives for a subscriber
from the network, the mobile device needs to be paged first. The mobile device then
responds and implicitly changes back to Cell‐FACH state.
As the name Cell‐PCH already indicates, the subscriber is only paged in a single cell
if new data from the core network arrives. This means that the mobile device has to
send a Cell Update message to the RNC whenever it selects a new cell. In the URA‐PCH
state, the mobile only informs the RNC whenever it enters a new URA. Consequently,
the Paging message needs to be sent to all cells of the URA in case of incoming data (see
Section 3.7.3).