Public Safety Advocate: Why Public Safety Devices Need a New SIM Card

This week’s PSA is based on a question I hear nearly every day. It started when AT&T won the FirstNet contract and offered up its own LTE networks in addition to what it will build out on FirstNet Band 14 spectrum. AT&T is offering early opt-in states and territories (at least 11 so far) the use of its AT&T network on a priority access basis with full pre-emption on the entire AT&T LTE network by the start of 2018. AT&T says it is easy to start using the AT&T network for public safety. Once a state has opted in, each public safety entity will decide if it wants to join the FirstNet system and become users on the AT&T broadband network.

If the answer is yes and the pricing is acceptable to the agency, all that is needed, according to AT&T, is to install a new SIM (Subscriber Identity Module) into the mobile device to be instantly considered a public safety user on the AT&T network and on Band 14 as it is built over time. The question about why you need to install a new SIM in your device is based on a number of factors. The most important of these is that the new SIM identifies the device (and the user) as a member of the public safety community. The network is then notified that when this device is on the network, in addition to normal AT&T capabilities, the user will have access to all additional capabilities and information being made available only to the FirstNet public safety community. The AT&T network and soon FirstNet Band 14 recognizes a user as a public safety user by the SIM in the device and the information it contains.

The SIM contains special information required by the network to identify the device as a public safety authorized device. Many readers are already conversant with SIMs because when you purchased your most recent smartphone you probably had to find the SIM card in the packaging and then, following directions, insert it into your phone. After it was properly activated by the network, your device was fully functional. It is estimated by the United Nations that of the approximately 7 billion people in the world, 6 billion have cell phones of one type or another. So how do networks keep track of this vast number of phones, any of which can easily roam from one country to another and access service?

Every SIM in the world has a different set of numbers embedded in it. These numbers are referred to as a Public Land Mobile Network Identifier or PLMN-ID. It may also be referred to as the Home Network Identifier (HNI) and the Public Mobile Network (PMN). This number is unique not only to each individual phone but to each network as well. The PLMN-ID is made up of a 3-digit Mobile Country Code (MCC) and a 3-digit Mobile Network Code (MNC). Beyond this 6-digit code, there are millions of other codes that enable identification of a specific device. Years ago, well before FirstNet, a number of organizations including the Public Safety Spectrum Trust Operator Advisory Council (PSST OAC), the Broadband Technical Opportunities Program (BTOP) users group, the National Public-Safety Telecommunications Council (NPSTC), and the APCO Broadband Committee spent hundreds of hours looking at the need for a specific PLMN-ID for the National Public Safety Broadband Network (NPSBN) that those working on the project knew would be coming in one form or another.

The timeframe for these discussions was 2010 and 2011, well before FirstNet became real in 2012. The PLMN-ID codes are coordinated around the world. Within the United States the organization in charge of assigning a new PLMN-ID is the Alliance for Telecommunications Industry Solutions (ATIS). After a great deal of work those involved in the project were successful in having ATIS establish a specific 6-digit code for the public safety network in the United States. The PLMN-ID for public safety is 313-100. The ‘313’ is the Mobile Country Code (MCC) and the ‘100’ is the Mobile Network Code (MNC) specific to the FirstNet Network.


The PLMN-ID specific to the public safety community is contained on the SIMs provided to public safety by AT&T in order for the device to distinguish a member of the FirstNet/AT&T public safety broadband network from an AT&T commercial customer. All of the AT&T LTE network, rural partner networks, and Band 14 as it is built out recognize the specific identifier for the device. The network routing and access to priority and soon pre-emptive priority will key off this special PLMN-ID number.

Beyond the PLMN-ID

Work did not stop after ATIS had assigned the PLMN-ID for public safety. Once a network recognizes which network serves as the device’s home network, additional information about the device follows so below the PLMN-ID there is a series of other numbers that further identify a specific unit. There are also numbers that can be used to place a device or group of devices within a specific area, and as the group working on the numbering system realized, it is possible to assign specific numbers to each agency in the United States and within that agency, specific devices. It was recognized that these assignments could be important to both the public safety community and the network operator. The 9 digits that make up this part form the Mobile Subscriber Identification Number (MSIN). Each network then has a pool of 1 billion individual device IDs to work with.

                               Diagrams Courtesy of SAIC Report to the Public Safety Spectrum Trust (PSST) 2011

The total number includes the 6-digit PLMN-ID plus a 9-digit Mobile Subscriber Identification Number (MSIN). This number enables the network to know exactly which device the network is communicating with and what services are permitted to be accessed by the device. During the early work on the PLMN-ID, it was recommended these numbers be preassigned in ranges to each of the states and territories.

After much consideration, this recommendation was included in a paper by SAIC that was written for the Public Safety Spectrum Trust (PSST), which held the broadband license at the time, and was approved by the body as far as I know.

The result was that 8,000 blocks (400,000,000) of International Mobile Subscriber Identity Numbers (MSINs) were distributed based on population percentages to the 56 states and territories. Each block contains 50,000 unique identifiers. The proposal also stated that once (and if) 70% of these identifiers had been issued, another 20% of the remaining 60% of unallocated numbers would be made available.

Some examples of assignments are Arizona with a state population in 2012 of 6,392,017 was pre-assigned a pool containing 163 blocks of 50,000 individual IDs each. California was assigned 953 blocks, New York received 496 blocks, and the U.S. Virgin Islands was assigned 3 blocks. The hope was that each state would assign the blocks in order to specific agencies. The committee thought that by doing this if there was an illegal or unauthorized device on the network the IMSI number would enable the device to be tracked not only to the device itself but back to the specific agency that originally authorized its use.

FirstNet/AT&T Today

Current thinking is that since there will only be one nationwide system and one logical Home Subscriber Server (HSS), preallocated blocks of IDs are no longer necessary. Hundreds of volunteer hours were spent on this project, led by Cynthia Wenzel Cole who owns the consulting company Cynergyye and is currently a contractor to the Texas Department of Public Safety, Bill Schier who is now with FirstNet, Dr. Michael Britt, myself, and a large number of others. At the time it seemed as though the best of both worlds would be to make use of these individual device numbers to track them in event of them being stolen or going rogue.

I had hoped AT&/FirstNet would follow the lead for IMSIs that were developed well before FirstNet was born. It made sense at the time and it still makes sense today. The IMSI is primarily a tool for the network to use to identify devices, which cell they are using, which services they are authorized to use, and other attributes of the device. However, the way the IMSI was envisioned by the committee more than 5 years ago still makes a lot of sense, especially since the object of this network is nationwide interoperability and there are some real advantages to knowing where a unit originated and if it is still permitted to be active on the network. But then things change over time. The original vision of what is now FirstNet is a perfect example of such changes, so far, all for the best.

Andrew M. Seybold

©Andrew Seybold, Inc.


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