The network knows when to expect RACH from the UE because RACH locations are configured in SIB2, and defined in 36.331, (simply search for prach-Config).
For FDD there are 2 basic configuration parameters, first is the prach-ConfigIndex, which indicates which subframes are viable for transmission of RACH preambles.
A second paramameter, prach-FreqOffset, which basically indicates the number of RB up from the 0 RB index where the 6 RB of the PRACH are allotted in the time-frequency grid defined in 36.211.
Note that the PRACH itself does not have exactly the same OFDM signal structure of the rest of the resource grid, but does fit within a 6 RB window in frequency.
In FDD, only one PRACH opportunity is available per subframe, and thus FDD only employs 10 RA-RTNIs.
For TDD multiple PRACH channels may exist per subframe, see 36.211 section 5.7
Hope that helps.