The network utilizes Motorola’s State of the Art ASTRO 25 SmartZone/SmartX Digital Trunked Radio System. SEGARRN was one of the first groups in the country to purchase Motorola’s SmartX system and the first in the country to place the system in full operation utilizing all aspects of functionality available via the system.
The SEGARRN system operates on a mixture of 800MHz and 700MHz radio frequencies. All of the radio frequencies available on the network (currently 75 with approximately 14 additional under construction) are capable of operating as Project 25 Digital Radio Channels. Project 25 is the national standard for Public Safety Digital Mobile Radio Communications.
The City of Savannah and Chatham County were the first agencies in the State of Georgia to License 700MHz radio frequencies for use in Public Safety radio networks. SEGARRN is currently the only radio network in Georgia utilizing the 700MHz radio frequencies. (As of August, 2010).
A unique tie between the 700MHz frequencies and the SEGARRN network exists. The 700MHz frequencies were created as a result of the nation’s conversion to Digital Television in February of 2010. The radio frequencies in the old TV channels 61 through 69 were sold at public auction by the FCC to various commercial entities such as Verizon, Sprint and other Cellular type companies. Nearly a Billion dollars from these auctions was made available to local governments to build interoperable communications systems. SEGARRN received 7.2 Million dollars of that money and now utilizes some of the 700 MHz frequencies set aside in the old TV Bands for public Safety.
The Project 25 Trunking standard allows many radio networks to be interconnected. Since the radio network handles all of the audio and control information as IP (Internet Protocol) data the information can be passed between radio sites and systems over a standard computer network or even over the Internet. While passing the information over the internet may be convenient the security risks involved make it impractical. SEGARRN utilizes numerous “private” networks to pass all of the data necessary to control and operate the radio system. These networks consist of Microwave Radio Links, Fiber Optic connections and commercial T-1 Data lines to connect the various system components together.
The SEGARRN radio system utilizes what is known as “Trunking” to pass the radio traffic. Trunking is a demand based method of exchanging information over a radio network. A trunked radio network has assets consisting of radio sites and radio channels at each radio sites. Throughout the network are subscriber units (mobile and portable radios). The system’s assets are controlled by computer. Each subscriber radio is allowed certain access rights on the system. The computer keeps track of those rights.
Set up in what is called the Zone Manager (Master Computer) are tables of users (radios) and Talk Groups. Talk Groups are what most people think of as channels. Examples would be Fire Dispatch, Sewer Maintenance, Sheriffs Dispatch, etc. Talk Groups are virtual talk paths that are set up “on demand” by the Zone Controller every time a person pushes the push to talk button.
When a user presses the push to talk button on the radio a short burst of data is sent into the Zone controller . This takes about 1/10 of a second to transmit. The data tells the controller what radio is transmitting and what Talk Group it wants set up. Within a few milliseconds the Zone controller checks the tables to see if the radio is valid, looks to see what radios are registered on that Talk Group and sends commands to the radio sites where the radios registered on the talk group are located. The command stream tells each radio what specific radio frequency to go to to start the call and makes the radio equipment at each site operate to support the call. When the call is over everything returns to normal and the equipment waits for the next assignment. Only the equipment required to complete the call is used. Only radio sites needed to complete the call are used. This process takes place on a large system like the SEGARRN system hundreds of times every minute and millions of times every month. When the system is busy there are dozens of simultaneous calls in process at the same time.
The IP based Project 25 technology also allows systems to talk to each other. At present SEGARRN only talks within our own system but preliminary discussions have taken place with other similar systems to make the IP based interconnections to allow our systems to merge and provide access over an even greater geographical area.
A question often asked is “What is the purpose of increasing the geographical coverage of the radio network over such a vast area?”. The simple answer is disaster preparedness and a new word “Interoperability”. Interoperability is simply the ability of radio equipment assigned to one jurisdiction being able to talk to radio equipment from another jurisdiction. At the national level, the Project 25 standard has been mandated by the National Incident Management System (NIMS) as the digital radio protocol for use by all government agencies for public safety and disaster response. The SEGARRN system meets that requirement and then some. In times of catastrophe such as the Sugar refinery fire the ability to coordinate assets from many surrounding areas is critical. The SEGARRN radio network makes that coordination possible.