Page 270 - From GMS to LTE

256 From GSM to LTE-Advanced Pro and 5G

The Tracking Area Code (TAC), which corresponds to the location and routing areas
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in GSM and UMTS.
Cell barring status, that is, whether the cell can be used or not.
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Minimum reception level (q_RxLevMin) that the mobile device must receive the cell
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with. If the level is lower, the mobile device must not try to establish communication
with the cell.
A scheduling list of other SIBs that are sent and their intervals.
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With the information provided in SIB 1, the mobile device can decide if it wants to
start communicating with this cell. If so, for example, since the cell belongs to the home
network, the mobile device then continues to search for and decode further System
Information messages.
SIB 2 contains further parameters that are required to communicate with a cell,
such as:
the configuration of the RACH;
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the paging channel configuration;
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the downlink shared channel configuration;
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the PUCCH configuration;
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the SRS configuration in the uplink;
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uplink power control information;
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timers and constants (e.g. how long to wait for an answer to certain messages, etc.);
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uplink channel bandwidth.
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Further SIBs contain information that is mainly relevant for cell reselection once the
mobile device has established a connection with the network. Hence, they are discussed
in more detail in Section 4.7.2 on cell reselection and idle state procedures.
If the cell is not part of the home network or does not belong to the last used network
stored on the mobile device (e.g. during international roaming), the device then goes on
and searches other channels on the current frequency band and also on other frequency
bands. If the frequency band can be used by more than one radio technology, such as
the 1800 MHz band which can be used by GSM and LTE, the mobile device would try
to detect transmissions from different radio systems in the same band.

4.6.2 Attach and Default Bearer Activation
Once the mobile device has all the required information to access the network for the
first time after it has been powered on, it performs an attach procedure. From a higher‐
layer point of view, the attach procedure delivers an IP address and the mobile device is
then able to send and receive data from the network. In GSM and UMTS, a device can
be connected to the network without an IP address. For LTE, however, this has been
changed and a mobile device hence always has an IP address when it is connected to the
network. Further, the attach process, including the assignment of an IP address, has
been streamlined compared to GSM and UMTS to shorten the time from power on to
provision of service as much as possible.

Initial Connection Establishment
Figure 4.18 gives an overview of the first part of the attach procedure as per 3GPP TS
23.401 [18]. The first step is to find a suitable cell and detect all necessary parameters