sr-ProhibitTimer was introduced in Rel-9. This is to prevent the UE from sending a SR within a short duration of having already sent a SR, as there will be a round-trip-delay (typically about 4ms) since the SR is made and Grant is sent, UE will not be allowed to do a SR during the time this timer is active.
sr-ProhibitTimer is defined in multiples of sr-periodicity.