4.9 Power-up scenario
At this point, all of the functions, protocols and mechanisms of the GSM radio interface
have been presented which are needed to illustrate a basic power-up scenario. The following
describes the basic events that occur during a power up of the MS. The scenario can be
divided into several steps.
1. Provided that a SIM card is present, immediately after turning on power, a MS
starts the search for BCCH carriers.
(...)
5. The MS must now prepare synchronization with the six cells with the strongest signal
level (RXLEV) and read out their BCCH/SCH information, i.e. steps 1–4 above are to
be performed continuously for the six neighboring cells with the best RXLEV values.
6. If significant changes are noticed using the path loss criterion C1 and the reselection
criterion C2, the MS can start reselection of a new cell. Both criteria are determined
periodically for the current BCCH and the six strongest neighbors.
Cell reselection more detailed:
4.5.4 Cell selection and operation in power conservation mode
Cell selection and cell reselection
A MS in idle mode must periodically measure the receivable BCCH carriers of the base
stations in the area and calculate mean values RXLEV

from this data (section 4.5.1).
Based on these measurements, the MS selects a cell, namely that with the best reception, i.e.
the MS is committed to this cell. This is called ‘camping’ on this cell. In this state, accessing
a service becomes possible, and the MS listens periodically to the PCH. Two criteria are
defined for the automatic selection of cells: the path loss criterion C1 and the reselection
criterion C2. The path loss criterion serves to identify cell candidates for camping. For such
cells, C1 has to be greater than zero.
At least every 5 s, a MS has to recalculate C1 and C2
for the current and neighboring cells. If the path loss criterion of the current cell falls below
zero, the path loss to the current base station has become too large. A new cell has to be
selected, which requires use of the criterion C2. If one of the neighboring cells has a value of
C2 greater than zero, it becomes the new current cell.
(...)
Once a MS is camping on a cell and is in idle mode, it should keep observing all of
the BCCH carriers whose frequencies, the BA, are broadcast on the current BCCH. Having
left idle mode, e.g. if a TCH has been assigned,
the MS monitors only the six strongest
neighboring BCCH carriers. A list of these six strongest neighboring BCCH carriers has
already been prepared and kept up to date in idle mode. The BCCH of the camped-on cell
must be decoded at least every 30 s. At least once every 5 min, the complete set of data from
the six strongest neighboring BCCH carriers has to be decoded, and the BSIC of each of
these carriers has to be checked every 30 s. This allows the MS to stay aware of changes
in its environment and to react appropriately. In the worst case, conditions have changed so
much that a new cell to camp on needs to be selected (cell reselection)."