The 64 preambles available per cell are divided into:
Random Access Preamble used for Contention Based Access.
And to Dedicated Preamble used for NonContention Based.

The ratio is defined by Random Preamble Ratio parameter.
that Indicates the proportion of the number of random preambles in a cell used for Contention based Access, to the number of preambles available in the cell.

the remaining preamble are used for Noncontention based.

The contention based Random Access preambles are grouped into 2 types:
Type A and type B.

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