Firstnet – Some Technical Details

When I was in the US recently, I noticed that AT&T now broadcasts 2 Mobile Country Codes / Mobile Network codes from their LTE base stations in the places in Illinois and Ohio I checked. One is their own, MCC/MNC 310/410, and the other is MCC/MNC 313/100. A quick search revealed that this is the code assigned to Firstnet, a network for emergency services and first responders such as police, fire departments, ambulances and other public functions that require high priority access in congestion situations. Wikipedia has an article about Firstnet here. Up to now I always thought that the US wanted to establish a separate network but it looks like I was either wrong or that they have changed their mind over the years.

Another clue I should have noticed straight away is that SIB5 in the places I checked had band 14 in its list of bands which is in the 700 MHz band and has been allocated the lowest possible priority of 1. In other words, devices only camp on this band if there is nothing else around. Makes sense for a low band, you don’t want devices to camp there while channels on higher bands are available as well. However, band 14 is special, as its bandwidth of 10 MHz (or 20 MHz if you count the US way) has been set aside for a first responder network. By today’s standards, 10 MHz is not much anymore but especially in the lower bands between 600-900 MHz, broader channels are usually not available.

But as I said before, Firstnet is not a separate radio network. Instead, it piggy-backs on AT&Ts existing LTE radio network by using 3GPP’s Multi-Operator Core Network (MOCN) features to offer voice and data services to first responders. I will leave the financial and political issues aside in this article and only focus on the technical aspects. However, if you care to find out more about those, have a look here.

According to this technical description (see SD-1.2), Firstnet subscribers can not only use band 14, but all bands deployed at a cell site. AT&T subscribers in turn also have access to band 14 spectrum as part of Carrier Aggregation (CA) or also as a single channel in case nothing else is found. Firstnet subscribers, however, have precedence over AT&T subscribers. And inside Firstnet, some subscribers can have a higher priority than others. One document linked to below has a description which 3GPP features are used to ensure Firstnet users can preempt traffic in congested areas so I won’t go into the details here.

As per the MOCN approach, FirstNet shares the RAN with AT&T subscribers but uses a separate LTE core network which seems to managed by AT&T a well. Part of this core network seems to be an IMS VoLTE network including ePDGs for VoWifi access. One feature any network for first responders must have is being able to make group calls to replace Walkie Talkies. At the moment, Firstnet states that they have deployed a proprietary Push To Talk solution they refer to as EPTT but are in the process of installing two independent 3GPP compliant Mission Critical Push to Talk (MCPTT) IMS based services. These are expected to be available towards the end of 2019. Tightly integrating them into devices is going to be a challenge unless of course, it’s done directly by the device manufacturers. Time will tell. Also, without specialized hardware like external microphones and firefighter grade speakers so they can leave the device on their belt while talking or waiting for a response will be a make or break usability requirement.

Another thing a network stands and falls with are the devices that are supported. As Firstnet subscribers can use all of AT&T bands, no special device is required in theory. In practice a device has to support FirstNet’s VoLTE parameters, band 14 should be supported for rural coverage (which seems to be optional), and 3GPP’s High Priority Access (HPA) features for preemption in congestion situations. Fortunately, a lot of Samsung and LG smartphones declare support already and recent iPhones do as well. Done deal I would say. Here’s a link to a recent device list to give you an impression.

There are a number of features to ensure preemption and priority in congestion situations and there is what is referred to as an ‘uplift’ mechanism (see SD- to increase the priority of a subscriber or a group of subscribers for a minimum of 1 hour to a maximum of 24 hours. Again, I didn’t have a closer look yet as to which 3GPP features this mechanism interacts with but support for this feature seems a good idea.

There we go, this is the result of my first high level collection of technical information that nobody on the net seems to have done so far. If you have further technical details that you think are useful to a more technically oriented readership, please consider leaving a comment below. And for some further information here’s a link to an article on Fiercewireless about the status of the network in summer 2019.