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System Information Block 2 specifies the resources reserved for the PRACH transmissions. The resource may be specified as 1, 2, 3, 5, ... sub-frames within the frame (the set of options depending on frame type and PRACH preamble type). Initially this will be 6 contiguous resource blocks but additional frequencies could be specified once all the time resource has been allocated.
The mobile sends a PRACH preamble in a randomly chosen PRACH resource and waits for the Random Access Response (RAR).
The Random Access Response comprises a resource assignment which is implicitly addressed to the mobile by scrambling the CRC with the RA-RNTI that was used by the mobile in the PRACH transmission. The Downlink resource is used for transmission of the RAR which is addressed to a number of mobiles which have sent PRACH preambles. For each mobile a grant is given which the mobiles use to send their first RRC message.