Page 102 - From GMS to LTE

88 From GSM to LTE-Advanced Pro and 5G

quick reselection. Subsequently, the network stops the user data transfer in the down-
link direction and instructs the mobile device to switch to the new cell. The mobile
device then moves to the new cell and reestablishes the connection to the network with-
out having to read the system information messages from the broadcast channel first.
By skipping this step, the data traffic interruption is reduced to a few hundred millisec-
onds. The network can then resume data transfer in the downlink direction from the
point at which the transmission was interrupted. While there is usually some loss of
data during the standard cell change procedure in the downlink, this is not the case with
NACC. Thus, this additional benefit also contributes to speeding up cell change. To
complete the procedure, the mobile device asks the network for the remaining system
information via a provide system information message, while the user data transfer is
already ongoing again.
Although the implementation of NACC in the mobile device is quite simple, there are
a number of challenges on the network side. When the old and new cells are in the same
location area and controlled from the same radio network node, the procedure is
straightforward. If the new and old cells are in different location areas, however, they
might be controlled by different network elements. Therefore, an additional synchroni-
zation between the elements in the network is necessary to redirect the downlink data
flow to the new cell before the mobile device performs the cell reselection. Unfortunately,
this synchronization was not included when NACC was first introduced into the GPRS
standards in Release 4. As a consequence many of the NACC‐capable networks today
only support the feature for cells in the same location area.

The Standby State
In case no data is transferred for some time, the ready timer expires and the mobile
device changes into the standby state. In this state, the mobile device only informs the
network of a cell change if the new cell belongs to a routing area different from
the previous one. If data arrives in the network for the mobile device after it has entered
the standby state, the data needs to be buffered and the network has to page the sub-
scriber in the complete routing area to get the current location. Only then can the data
be forwarded as shown in Figure 2.14. A routing area is a part of a location area and thus
also consists of a number of cells. Although it would have been possible to use location
areas for GPRS as well, it was decided that splitting location areas into smaller routing

Ready state Standby state
Terminal Network Terminal Network

Data Data
Data Paging
(Routing area ID)
Answer to paging
(Cell ID)
Data

Figure 2.14 Difference between ready and standby states.