P25 and NXDN are digital voice standards created for professional land mobile radio: public safety, utilities, transport. A few amateurs use them because commercial radios are widely available second-hand, and because many multi-mode hotspots can carry them. Neither was designed for amateur radio, so some features do not fit it.
Sharing the spectrum
Digital voice squeezes more calls into the same spectrum in different ways. The two standards take different routes.
Frequency runs up, time runs right. Colours show different calls. NXDN also has a 12.5 kHz option.
P25 Phase 1 sends one call per 12.5 kHz channel. Phase 2 adds two time slots in the same 12.5 kHz, as DMR does.
NXDN can use channels of 6.25 kHz, so two calls fit in the space of one 12.5 kHz channel, or 12.5 kHz where wider is wanted.
P25 Phase 1 and NXDN use four-level frequency shifts (see Yaesu System Fusion). All of them rely on a vocoder, as in D-STAR.
Getting in
P25
NXDN
Access code
NAC, a code in the signal; a receiver set to one NAC ignores others
RAN, similar idea
Who to hear
a talkgroup ID
a talkgroup or unit ID
Origin
Public-safety standard
Commercial two-way radio
Amateur networks that carry these modes work much like the DMR ones: repeaters or hotspots link over the internet and talkgroups pick the conversation. See EchoLink, IRLP and AllStar.
Amateur-radio cautions
No encryption. Commercial P25 and NXDN radios can scramble audio. Part 97 forbids encoding messages to obscure their meaning, so any such feature must be off.
Check the band. Professional radios are made for other services and may not cover or be programmable on amateur channels.
Other digital modes cannot decode them. A receiver must understand the same standard and vocoder.