Minimum Requirements and Best Practices

Requirements for Devices on System:

• Project 25 Phase II (TDMA) compliant: more information found here: www.cisa.gov/safecom/project-25
• Hardware System Key is required for all radios coming onto TACN
• Ability to Inhibit: A lost or stolen radio is a security risk to the network. Radios may be remotely inhibited (renders the radio inoperable until verified by a system administrator). Some brands allow the inhibit function to be turned off, or the feature is off by manufacturer default and must be turned on by an administrator. A lost or stolen radio ID may be prohibited if the ability to inhibit has been turned off. This will only limit the radio’s ability to function on TACN or networks that have limited the access of that radio’s ID. It does not fully inhibit the radio’s ability to function.

Recommendations and Best Practices:

• TACN recommends partner agencies have clear policy for immediately reporting lost or stolen radios and that TACN administration is also notified as soon as the agency is notified by one of their endusers.
• Encryption is optional, however, if used, ADP/ARC4 and DES algorithms will only be used for backwards compatibility and interoperability with legacy equipment and only when required. AES-256 is the recommended best practice (some Federal grants may require it, see more info here: www.dhs.gov/science-and-technology/approved-grant-eligible-equipment)
• Multi-key encryption: if using encryption-best practice indicates use of multi-key to avoid limiting interoperability with other agencies

Approved Vendor Devices, P-25 CAP Testing and Grants Eligibility

TACN defers to the approved vendor devices list meeting P-25 Standards as defined by the Project 25 Compliance Assessment Program (P25 CAP), a partnership of the U.S. Department of Homeland Security’s Science and Technology Directorate (DHS S&T) and SAFECOM. DHS-recognized laboratories provide P25 CAP testing for compliance to P25 CAP standards. The DHS S&T’s “Approved (Grant-Eligible) Equipment” is the definitive list of products that meet the requirements and are allowable for purchase by FEMA and other federal entities.

Information on the P25 CAP program can be found here: www.dhs.gov/science-and-technology/p25-cap

Federal grants have specific requirements for interoperability and encryption. Information on Federal grant-approved radio brand/devices can also be found here: www.dhs.gov/science-and-technology/approved-grant-eligible-equipment

The TACN system is built around a Motorola core, therefore Motorola devices will have optimal performance while some of the proprietary features or functionality in other brands may not be fully compatible or do not operate as intended unless operating on a same-brand network core.

TACN Capacity

Capacity is the radio system’s ability to support multiple conversations simultaneously. Several factors influence system capacity, including the number of “channels” or talk paths available at each tower site, the radio system protocol used (FDMA, TDMA, or DCA), and overall system design. Other external variables may include end users’ peak talk times, number of end users affiliated to sites at a given time, and number of system-wide talk groups affiliated to sites.

TDMA or time division multiple access involves splitting talk time between two users, in milliseconds. This allows two conversations to take place seemingly simultaneously, on a single channel, effectively boosting the capacity of the radio channel.

FDMA or frequency division multiple access, entails dividing conversations by frequency. Only one conversation takes place at a time on the channel because the conversation is occurring on a specifically allocated part of the spectrum (that frequency assigned to the end user for that talk group at that moment). It is less efficient since only one talk group can utilize it at a time.

DCA or dynamic channel assignment, assigns end users based on demand and availability, enhancing overall system capacity by responding dynamically to varying usage scenarios, based on real-time demand and network conditions. This maximizes available capacity, making the system even more efficient. TACN has converted all sites to DCA; however, the system still must accommodate the least capable device on the system when it affiliates to a site (reverts to FDMA).

Sufficient capacity is at or below 30% utilization, depending on number of users during normal daily operations. The system more easily handles radio traffic during emergencies, spikes in activity, or other high-volume events.

Network congestion may occur when a tower site is consistently using around 60% of the available capacity during normal daily operations. Peak times, spikes in usage, or high-volume events may result in end users getting a “system busy” tone (sounds like a bonk), or channel not available (the number of end users and talk groups affiliating to the site in that moment is exceeding the system’s available capacity).

The standard minimum number of channels at any tower site is 6 (often referred to as a 6-pack), while some sites have 12 or more. Each channel is housed in a server (sometimes referred to as a blade), inserted into a rack at the tower site equipment shelter.

A 6-channel site can support 10 conversations on 10 different talk groups simultaneously if they are all TDMA (one channel is always reserved for the control channel, which steers users to the available channels). When adding a new agency onto the system, it is necessary to assess the impact the agency’s talk groups will have on the overall capacity at the tower site or sites to which the agency will affiliate. The assessment is an estimate, based on current radio traffic trends, number of existing talk groups already affiliated to the site, and additional talk groups to be added.