Page 184 - From GMS to LTE

170 From GSM to LTE-Advanced Pro and 5G

2.5–3 seconds. Therefore, the mobile device should only be put into idle state after a
prolonged period of inactivity as this delay has a negative impact on the Quality of
Experience of the user, for example, during a web‐browsing session. Instead of an
instantaneous reaction to the user clicking on a link, there is an undesirably long delay
before the new page is presented, which is noticed by the user.
While a mobile device is in idle state, the core network is not aware of the current
location of the subscriber. The MSC is only aware of the subscriber’s current location
area. A location area usually consists of several dozen cells and therefore it is necessary
to page the subscriber for incoming calls. This is done via a Paging message that is
broadcast on the PCH in all cells of the location area. This concept has been adopted
from GSM without modification and is described in more detail in Section 1.8.1.
From the point of view of the SGSN, the same concept is used if an IP packet has to be
delivered while the mobile device is in idle state. For the packet‐switched part of the net-
work, the cells are divided into routing areas (RA). An RA is a subset of a location area but
most operators use only a single routing area per location area. Similar to the location area,
the routing area concept was adopted from the 2G network concept without modification.
In the event that the mobile device moves to a new cell that is part of a different loca-
tion or routing area, a location or a routing area update has to be performed. This is
done by establishing a signaling connection, which prompts the RNC to set the state of
the mobile device to Cell‐DCH or Cell‐FACH. Subsequently, the location or routing
area update is performed transparently over the established connection with the MSC
and the SGSN. Once the updates are performed, the mobile device returns to idle state.

3.7.3 Mobility Management in Other States
In Cell‐FACH, Cell‐PCH or URA‐PCH state, the mobile device is responsible for mobil-
ity management and thus for cell changes. The big difference between these states and
the idle state is that a logical connection exists between the mobile device and the radio
network when a packet session is active. Depending on the state, the mobile device has
to perform certain tasks after a cell change.
In Cell‐FACH state the mobile device can exchange data with the network at any time.
If the mobile device performs a cell change it has to inform the network straight away
via a Cell Update message. Subsequently, all data is exchanged via the new cell. If the
new cell is connected to a different RNC, the Cell Update message will be forwarded to
the S‐RNC of the subscriber via the Iur interface. As the mobile device has a logical
connection to the network, no location or routing area update is necessary if the new
cell is in a different area. This means that the core network is not informed that the
subscriber has moved to a new location or routing area. This is, however, not necessary
as the S‐RNC will forward any incoming data over the Iur interface via the D‐RNC to
the subscriber. In practice, changing the cell in Cell‐FACH state results in a short inter-
ruption of the connection, which is tolerable as this state is not used for real‐time or
streaming services.
If the new serving cell is connected to an RNC that does not have an Iur interface to
the S‐RNC of the subscriber, the cell update will fail. As the new RNC cannot inform the
S‐RNC of the new location of the subscriber, it will reset the connection and the mobile
device automatically defaults to idle state. To resume data transmission, the mobile
device then performs a location update with the MSC and SGSN as shown in Figure 3.33.