Page 170 - From GMS to LTE

156 From GSM to LTE-Advanced Pro and 5G

importance of this factor for UMTS, interference being the major limiting factor in the
number of connections that can be established simultaneously in a cell.
While in the Cell‐DCH state, the mobile continuously measures the reception quality
of the current and neighboring cells and reports the results to the network. On the basis
of these values the RNC can then decide to start a handover procedure when required.
While the GSM radio network uses a static reporting interval, a much more flexible
approach was selected for UMTS. On the one hand, the RNC can instruct the mobile
device, similar to the GSM approach, to send periodic measurement reports. The meas-
urement interval is now flexible and can be set by the network at between 0.25 and 64
seconds. On the other hand, the network can also instruct the mobile device to send
measurement reports only if certain conditions are met. Measurement reports are then
only sent to the network if the measurement values reach a certain threshold. This
removes some signaling overhead. Another advantage of this method for the RNC is
that it has to process fewer messages for each connection compared to periodic meas-
urement reports. In practice, both periodic and event‐based measurement reports are
used depending on the network vendor.

Cell‐FACH State
The Cell‐FACH state is mainly used when only a small amount of data needs to be
transferred to or from a subscriber. In this mode, the subscriber does not get a dedi-
cated channel but uses the FACH to receive data. As described in Section 3.4.5, the
FACH is also used for carrying signaling data such as RRC Connection Setup messages
for devices that have requested access to the network via the RACH. The FACH is a
‘common channel’ as it is not exclusively assigned to a single user. Therefore, the MAC
header of each FACH data frame has to contain a destination ID consisting of the S‐
RNTI (Serving – Radio Network Temporary ID) which was assigned to the mobile
device during connection establishment, and the ID of the S‐RNC. Mobile devices have
to inspect the header of each FACH data frame and only forward those frames that
contain the mobile device’s ID to higher layers of the protocol stack (see Figure 3.23).
The approach of Cell‐FACH RRC state is thus similar to Ethernet (802.11) and GSM/
GPRS for packet‐switched data transmission. If data is received in the downlink direction,
no resources have to be assigned and the data can be sent to the subscriber more or less
quickly depending on the current traffic load of the FACH. As several subscribers share
the same channel, the network cannot ensure a certain datarate and constant delay
time for any mobile device in the Cell‐FACH state. Furthermore, it should be noted that

FACH e.g. 20ms

t
Data of subscriber 1
Data of subscriber 2 Frame with a RRC connection setup
message for subscriber 4
Data of subscriber 3

Figure 3.23 Data of different subscribers is time multiplexed on the FACH.