Page 146 - From GMS to LTE

132 From GSM to LTE-Advanced Pro and 5G

The assignment of dedicated channels for both circuit‐ and packet‐switched connections
in UMTS has a big advantage for mobile users compared to GPRS. In the GPRS net-
work, the mobile device has sole responsibility for performing a cell change. Once the
cell has been changed, the mobile device first needs to listen to the broadcast channel
before the connection to the network can be reestablished. Therefore, in a real network
environment, a cell change interrupts an ongoing data transmission for about one to
three seconds. A handover, which is controlled by the network and thus results in no or
only a minimal interruption of data transmission, is not foreseen for GPRS. Hence,
GPRS devices frequently experience interruptions in data transmission during cell
changes while traveling in cars or trains. With UMTS there are no interruptions of an
ongoing data transfer when changing cells due to a process called ‘soft handover’, which
makes data transfers while on the move much more efficient.
Another problem with GSM is the historical dimensioning of the transmission channel
for narrow band voice telephony. This limitation was overcome for GPRS by combining
several timeslots for the time of the data transfer. The maximum possible datarate, how-
ever, was still limited by the overall capacity of the 200 kHz carrier. For UMTS Release
99, what were considered to be high bandwidth applications at the time were taken into
consideration for the overall system design from the beginning. Owing to this, a maximum
data‐transfer rate of 384 kbit/s could be achieved in early networks with a spreading
factor of eight in the downlink direction. In the uplink direction, datarates of 64–384
kbit/s could be reached.
UMTS can also react very flexibly to the current signal quality of the user. If the user
moves away from the center of the cell, the network can react by increasing the spread-
ing factor of the connection. This reduces the maximum transmission speed of the
channel, which is usually preferable to losing the connection entirely.
The UMTS network is also able to react very flexibly to changing load conditions on
the air interface. If the overall interference reaches an upper limit or if a cell runs out of
available codes owing to a high number of users in the cell, the network can react and
assign longer spreading factors to new or ongoing connections.

3.4 UMTS Channel Structure on the Air Interface

3.4.1 User Plane and Control Plane
GSM, UMTS and other fixed and wireless communication systems differentiate
between two kinds of data flow. In UMTS, these are referred to as two different planes.
Data flowing in the user plane is data which is directly and transparently exchanged
between the users of a connection, such as voice data or IP packets. The control plane
is responsible for all signaling data exchanged between the users and the network.
The control plane is thus used for signaling data to exchange messages for call estab-
lishment or messages, for example, for a location update. Figure 3.10 shows the sepa-
ration of user and control planes as well as some examples of protocols that are used
in the different planes.

3.4.2 Common and Dedicated Channels
Both user plane data and control plane data is transferred over the UMTS air interface
in so‐called ‘channels’. Three different kinds of channels exist: