How does Firstnet work?

[CanesFan95]

What frequencies does all this use?

 

[buddrousa]

AT&T FIRSTNET LTE 5G AND 4G

 

[CanesFan95]

Do we know what the actual frequencies are?

 

[nd5y]

Do we know what the actual frequencies are?

FirstNet is allocated LTE band 14 nationwide.
758-768 MHz Base
788-798 MHz Mobile
It can also use other bands that AT&T (and maybe other carriers?) owns in different areas.
LTE normally uses 5 or 10 MHz wide channels. There are no "actual frequencies" that you can recevie on a scanner.

700 MHz Public Safety Spectrum

The 700 MHz Band is an important swathe of spectrum available for both commercial wireless and public safety communications. The Band consists of 108 megahertz of spectrum running from 698-806 MHz and was freed up as a result of the Digital Television Transition. The location of the 700 MHz...

www.fcc.gov

LTE frequency bands - Wikipedia

en.wikipedia.org

 

[CanesFan95]

So if there's no actual frequency, how do the radios transmit and receive audio? And how do all the radios know the right encryption key?

 

[rescue161]

The same way a cell phone knows which frequency to use. It is an LTE device. Most of them are cellular, like iPhones, etc. Some devices, like the L3Harris XL-200P, can have a SIM slot to accept the FirstNet SIM.

 

[mmckenna]

So if there's no actual frequency, how do the radios transmit and receive audio? And how do all the radios know the right encryption key?

It's broadband. The LTE radios work between a set of limits and utilize the entire band. There is no 'one' frequency. LTE usually runs in 5MHz or 10MHz bands. One band is used for uplink, one is used for downlink. On a spectrum analyzer, it'll look sort of like this:


That's showing two separate bands of downlink from a cell site.

 

[buddrousa]

In plain English FIRSTNET is a CELLPHONE SYSTEM

 

[CanesFan95]

It's definitely confusing but also interesting. I don't seem to grasp how RF signals are transmitted and received on this whole thing.

 

[CanesFan95]

So from ND5Y's post, it looks like there's a +30 MHz offset input / output on the repeater, just like the +5 on UHF?

 

[nd5y]

So from ND5Y's post, it looks like there's a +30 MHz offset input / output on the repeater, just like the +5 on UHF?

The 746-806 MHz band is split in two. The low half is base and the high half is mobile. +30 MHz offset.
As far as FirstNet goes there are no repeaters, just base stations like a cellular system.

 

[CanesFan95]

So if there's no repeaters, how can it work?

That spectrum analyzer photo looks like there's 11 MHz used up per frequency? I don't understand this. I mean, people bicker about 25 kHz being inefficient, or even 12.5.

 

[buddrousa]

It is a cellphone that works as a radio.
It is a cellphonE

 

[Floridarailfanning]

FirstNet is based on the same cellular system that your phone uses. Some of FirstNet has dedicated bands but most uses the same bands as civilians just with priority preemption.

 

[CanesFan95]

OK, so it's a cell phone. But I guess I still don't understand the mechanics how/where it transmit and receieve. With traditional scanning, we have repeaters, conventional, trunking, control channels, slots, and so on. But I'm not grasping how all this cell phone stuff works.

 

[buddrousa]

It is a new unmonitorable world

 

[radiomankc]

FirstNet is pushing Band 14 which is dedicated to FirstNet.

FirstNet Increases Network Coverage as Band 14 is Added to 2,500 Sites

Band 14 deployments are underway in 51 states and territories and it’s already live in 40 states. Band 14 will continue to be rapidly deployed nationwide.

about.att.com

Band 14 is in the 700MHz range.

 

[mmckenna]

OK, so it's a cell phone. But I guess I still don't understand the mechanics how/where it transmit and receieve. With traditional scanning, we have repeaters, conventional, trunking, control channels, slots, and so on. But I'm not grasping how all this cell phone stuff works.

It's like any other cellular network.
It uses strategically located base stations that handle the traffic to/from the hand held/mobile devices. The base stations use 'backhaul' to connect back to the network. There really isn't much in the way of smarts at the cell site. It's just transmitters and receivers connected back to the network over the backhaul.

It's not a repeater, since traffic doesn't go between the cell phones like a radio does. All the traffic to/from the devices goes back into the network for handling. Even if two phones are on the same tower, the signals go back to the Mobile Switching Office/network.

All the voice traffic is just voice packets on a big IP network. The days of channelized voice traffic are long gone. Too inefficient. Voice packets are easier to handle/compress, and it can all be mixed together with Youtube videos of cats.

FirstNet is just part of the larger AT&T network. FirstNet has some prioritization features, and has the dedicated "Band 14" in the 700MHz band, but FirsNet can use other parts of the AT&T network.

Do some research on LTE networks, and it'll explain it well.

 

[cg]

My understanding is that Band 14 is NOT reserved for the exclusive use of FirstNet however the prioritization and preemption given to FirstNet users can result in them being the only users on it during an emergency.
One of the benefits to ATT is shared (but prioritized) use with regular ATT customers.

chris

 

[mmckenna]

My understanding is that Band 14 is NOT reserved for the exclusive use of FirstNet however the prioritization and preemption given to FirstNet users can result in them being the only users on it during an emergency.
One of the benefits to ATT is shared (but prioritized) use with regular ATT customers.

chris

You would be correct.