There are many aspects of GSM that provide useful guidance to understand processes that are performed regarding cell selection. These processes provide useful background help for cell site analysis. There is discussion at this blog about C1 and C2, but here are some other processes that rarely get a mention:
C1 Normal Cell Selection This is the process of initial cell selection, searching all RF channels.
C2 Stored List Cell Selection This is the process of initial cell selection where BCCH carrier information (e.g. a BA list) for the selected PLMN is stored in the MS.
C3 Camped Normally This is where the MS is camped on a cell of the registered PLMN and may be able to make and receive calls. (Whether or not the MS can make and receive calls depends on the state within the location registration process). The MS monitors received level and the system information and checks whether cell reselection is needed.
C4 Normal Cell Reselection This is where the MS has determined that cell reselection is needed and an attempt is being made to reselect a new cell.
C5 Choose Cell This is where the MS has returned to idle mode from "connected mode" and is choosing a suitable cell to camp on.
C6 Any Cell Selection This is where the MS is unable to camp normally on any cell of the selected PLMN, or cannot obtain service because of certain responses to a location registration (LR) attempt. It is searching for a cell of any PLMN to camp on (so that emergency calls can be made and warning notifications can be received).
C7 Camped on any Cell This is where the MS has camped on a cell irrespective of its PLMN identity, so that emergency calls can be made and warning notifications can be received.
C8 Any Cell Reselection This is where the MS is attempting to reselect a cell, irrespective of PLMN identity.
C9 Choose Any Cell This is where the MS is returning to idle mode, after having entered "connected mode" from the "camped on any cell" state to make an emergency call. It is attempting to find an acceptable cell to camp on.