Periodic RA increase is due to timer change only.
Check out 23.060
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6.2.2.
The Periodic RA Update Timer function monitors the periodic RA update procedure in the MS. The length of the periodic RA update timer is sent in the Routeing Area Update Accept or Attach Accept message. The periodic RA update timer is unique within an RA. Upon expiry of the periodic RA update timer, the MS shall start a periodic routeing area update procedure.
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and also 24.008
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4.7.5.1
Periodic routing area updating is used to periodically notify the availability of the MS to the network. The value of the update type IE in the ROUTING AREA UPDATE REQUEST message shall indicate “periodic updating”. The procedure is controlled in the MS by timer T3312. When timer T3312 expires, the periodic routing area updating procedure is started. Start and reset of timer T3312 is described in subclause 4.7.2.2.
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So the timer is called T3312 ! Default value is 54mn.
The value of this timer is used if the network does not indicate another value in a GMM signalling procedure. For example in the Routing Area Update Accept : the SGSN can indicate a different timer.
Cheers,
Pix