Harris Corp.'s XG-75 single-band radio incorporates the Harris P25 systems Enhanced Failsoft with Security mode technology. Photo courtesy of Harris.
Public safety radio networks are built for survivability so communications systems work when needed. A concept known as "failover" is an important aspect of survivability. Failover ensures that when parts of the overall system fail—either due to component issues or natural and man-made disasters—that the system continues to operate. Traditional system design incorporates redundancy for critical components.
The term "failsoft" refers to any failover condition that causes a digital trunked radio system to not trunk. This is most commonly caused by a loss of the control channel that subscriber radios use when communicating to the towers to assign a resource. Traditionally in this state, all transmitters (channels) turn on and operate in a conventional repeater mode. Subscriber radios are able to recognize this state and switch to a predetermined frequency.
In most systems, several talkgroups share a frequency. Some talk groups may not be assigned a failsoft frequency and these talk groups will cease to operate during the failsoft period. If a particular failsoft frequency has also failed, the talk groups assigned to that frequency will be off the air.
Today, many users of modern P25 digital systems are unfamiliar with conventional analog radio systems. Using conventional systems effectively to maintain radio capabilities requires experience or training to understand differences such as:
- In an analog system, one radio user can override and talk over another, interrupting urgent messages.
- Most analog signals are not encrypted, and all digital systems that fail over to analog systems lose encryption.
- Roaming between sites is done manually with adjustment of the knob position to select new channels. The talk group does not follow the user as in a trunked radio system.
- Conventional radio systems do not support user authentication.
With current trunked digital P25 systems, the average user is unaware of how the system operates and what safety it creates in radio transmission. These include:
- Encrypted radio and data transmission are free of public and criminal scanning.
- Users are authenticated through automated Unit ID capabilities, while rogue users are denied.
- Anyone roaming between tours within a given system, or an adjoining system with interoperability gateways, can use the encrypted Unit ID capabilities.
When a trunked P25 system has a failure from a site or repeater, it goes into failsoft mode, greatly limiting user capabilities. In current failsoft mode, several limitations are placed on most radio manufacturers' systems. These include:
- The digital signal is no longer possible, forcing it into analog mode.
- Users must manually switch onto the appropriate conventional channels.
- The Unit ID feature is lost; dispatchers can no longer identify users.
- Automatic roaming is lost.
- Trunking features are lost.
- Rogue users can use and listen to the system, overriding emergency calls.
Harris Corp.'s P25 system solves these issues with its Enhance Failsoft with Security mode, resulting in:
- Any repeater in the sites can assume control channel responsibility, not just a pre-determined few, reducing the probability a failsoft event occurs.
- All sites maintain a list of authorized users. Users are authenticated even if connection to the core is lost.
- All calls remain encrypted.
- Unit ID and call priority is maintained.
- No user action is required.
- Automatic roaming is supported.
- Trunked operations maintained.
- Interference mitigation maintained—talk groups steered away from interfered channels.
- Each site automatically broadcasts on pre-defined frequencies.
Users see very little change. No rogue users can scan or use the system. Radio frequencies remain efficient, ensuring the best signal.