Page 284 - From GMS to LTE

270 From GSM to LTE-Advanced Pro and 5G

consumption even further, the network can additionally configure a timing alignment
timer. Once this timer expires the mobile device no longer needs to transmit any reports.
Once data has to be transmitted again, however, the device needs to get time aligned
with the network again by performing a random access procedure to get a new timing
advance value.

4.7.2 Mobility Management in RRC Idle State
During long times of inactivity, it is advantageous for both the network and the mobile
device to put the air interface connection into the RRC idle state. This reduces the
amount of signaling and the amount of power required for the mobile device to main-
tain the connection. In this state, the mobile device autonomously performs cell rese-
lections, that is, it changes on its own from cell to cell whenever required by signal
conditions. The network is contacted only when a cell is in a new tracking area. As a
consequence, the MME in the core network is aware only of the tracking area, which
usually comprises many cells. The LTE Tracking Area concept is hence similar to the
concept of location and routing areas in GPRS and UMTS and reduces the location
signaling, which helps to reduce the mobile device’s power consumption.
In RRC idle state, no user data tunnel is present on the S1 interface between the
eNode‐B and the Serving‐GW. The user data tunnel between the Serving‐GW and
the PDN‐GW, however, remains in place. From a logical point of view, the connection
is still established and all logical bearers remain in place. This means that the IP
address or addresses assigned to the mobile device remain in place. Whenever there is
renewed activity, the physical radio bearer and the S1 user data tunnel have to be
reestablished.
When the mobile device wants to send a new IP packet to a server on the Internet, the
tunnel reestablishment is straightforward. After the mobile device has connected to the
eNode‐B, the S1 tunnel is recreated and the device enters RRC connected mode again.
In case an IP packet arrives from the Internet while the mobile device is in RRC idle
state, it can be routed through the core network up to the Serving‐GW. As the Serving‐
GW has no S1 tunnel for the user, it contacts the MME and requests it to reestablish the
tunnel. As the MME is only aware of the tracking area and not the individual cell in
which the mobile is currently located, it sends a Paging message to all eNode‐Bs that the
belong to tracking area. The eNode‐Bs in turn forward the Paging message over the air
interface to inform the mobile device that data is waiting in the network.
When in RRC idle state, the mobile device deactivates the radio module for most of
the time. Only at the end of the paging interval, usually in the range of 1–2 seconds,
does it temporarily activate the radio receiver to check if the eNode‐B is transmitting a
Paging message with its identity. This behavior is also referred to as DRX in RRC idle
state, which is different from the DRX mode in the RRC connected state that was
described earlier.
If the mobile device finds a Paging message addressed to itself, it reestablishes a con-
nection with the eNode‐B with a random access procedure and requests the reestab-
lishment of the connection. The eNode‐B that receives the mobile’s request then
answers the Paging message from the MME and both the air interface connection and
the S1 tunnel are reestablished. Once both are in place, the MME contacts the Serving‐
GW, which then forwards the waiting IP packets to the mobile device. At this point, the