P25 control channel

When the live radio is in P25 mode it reads the local trunked system's control channel: which site this is, how its channel numbers map to frequencies, and which talkgroups are being given a channel right now. It also plays the voice of calls that are in the clear on the live stream, following each call to its channel; encrypted calls are only counted, never tuned, and radio ids are never kept. Public safety traffic isn't ours to publish (47 U.S.C. 605), so this page and the audio are for the station owner only.

Other digital and trunked systems the two radios found nearby: Digital and trunked systems.

Loading…

Playing now

Nothing: the P25 preset isn't on, or no call in the clear right now.

Calls played

Each call the radio followed in P25 mode, newest first, with how many seconds of speech it decoded. Most of a call is silence between transmissions, so a call with a few seconds of speech is normal. Calls with at least a second of speech are kept for 7 days; press play to hear one.

On a channel now

Talkgroups heard

Every talkgroup heard on this site. A ★ marks one set to follow (follow: true in apps/sdr/config/p25_systems.yml): it's played ahead of other calls while it's in the clear and in range. A call is one session: the first message for a talkgroup after more than 5 seconds of nothing. The control channel repeats each grant, so grant msgs (raw grant messages) runs higher. ENC and EMERG count calls whose grant said so. A call heard only in grant updates has no grant, so its encryption is unknown. Names are from a public listing (source and date on hover), not confirmed from audio.

Site and channel plans

How a channel number (plan id in the top 4 bits) turns into a frequency, exactly as the site sends it, in Hz.

Neighbour sites (advertised)

Sites this site advertises as neighbours. That they're listed doesn't mean this receiver hears them, and one site doesn't list the whole statewide system.

Messages not decoded yet

Control channel messages this decoder doesn't interpret, counted by manufacturer id (MFID) and opcode, both hex. Only the counts are kept: payloads can carry radio ids. MFID 0x90 is Motorola.

Control channel messages

Radios active (counts only)

How many distinct radios key up on each talkgroup, to show how busy a group is and roughly how many units are on it. No radio id is ever stored: each id is hashed with a key that is thrown away every window, so the same radio can't be linked from one window to the next. Only these counts are kept.

What the control channel tells us

P25 sends all of this in the clear on the control channel, even for a system whose voice is encrypted — the point the 2011 "Why (Special Agent) Johnny (Still) Can't Encrypt" paper (Clark, Blaze et al.) makes. A receiver that never touches the voice still learns the shape of the system.

NAC

Network Access Code, 12 bits on every frame. Like a CTCSS tone for digital: it says which system a transmission belongs to, so a receiver ignores other systems on the same frequency.

WACN + System ID

Together these name the trunked network uniquely (WACN is the wide-area network, the System ID the system within it). Matching them to a public listing is how we know this is Michigan's MPSCS.

RFSS / Site

Which radio sub-system and which tower site we're hearing. A system has many sites; this is the one in range.

Channel plan

The formula that turns a 16-bit channel number into a frequency (a base, a spacing and a mobile transmit offset). Without it the grants are just numbers; with it a receiver can follow a call to its voice channel, which is how the P25 mode plays clear calls.

Talkgroup

The group a call is for (a dispatch, a tactical net, a fireground), not a person. Counting grants per talkgroup shows which groups are busy and when, which is most of what traffic analysis of a trunked system is.

Service options

A byte on each grant, in the clear: whether the call is an emergency, whether its voice is encrypted, and its priority (0–7). So even on an encrypted talkgroup we can see that an emergency call just happened.

Adjacent sites

Each site advertises its neighbours' control channels, so over time the whole system's map of sites can be drawn from one receiver.

Receive only. Only calls the control channel marks as in the clear are played, decoded with our own P25 receiver and the open mbelib vocoder; encrypted calls are never tuned or decrypted. The radio ids that ride on the grants are parsed off and thrown away, not kept.