In simplest terms: P25 Phase 1 is use of FDMA which allows only one pathway ( Time Slot) on an TG, or Frequency. P25 Phase II is use of TDMA which (in current form) allows 2 pathways (Time Slots) on an TG or Frequency.
EX: On an P25 Phase I TRS, per an site lets say there are 5 freqs. 1 Control, and 4 Voice/Data with 1 slot each. This would allow up to 4 TG's active on that site at the same time, or 3 Voice TG's and an Data TG. Now lets upgrade it to P25 Phase II. Now you have TDMA which multplies the capacity. Now you can have up to 8 user groups active. 1 CC, and 4 Voice/Data with 2 slots each.
On the MSWIN, all sites can carry FDMA, or TDMA traffic depending on the user. If an user using TDMA affiliates with an site, it will automatically switch to TDMA as long as that user is active. Once the user is inactive, the site reverts to FDMA traffic. Eventually, FDMA will be discontinued and the MSWIN will carry only TDMA but this is till a few years away.
Also as technology improves, TDMA will grow to 4 Slots, then 6 Slots. 4 Slot TDMA for P25 is actually not too far off as for the technology timescale. Eventually they will have to create an TDMA P25 standard for conventional radio systems or lose more and more to DMR on conventional systems. TDMA allows far more flexibility for multiple use in the digital world.
