“DID YOU HEAR THE LATEST NEWS ABOUT PAGING?” PROBABLY NOT.
|Article by Tait System Engineers Jerry Vargas and Frans Andrea|
We tend to think of paging as very “last millennium.” Yet while cell phones and text messaging have effectively terminated two-way paging, one-way paging remains a prevalent, critical communications tool in sectors such as EMS, hospitals and fire fighting. With the advent of digital radio technologies such as APCO P25, what options do emergency responders have to continue support for paging users?
UNDERSTANDING PAGING TECHNOLOGY
Paging, or alerting, is a highly reliable and very low cost method for providing one-way-short communications to low-complexity radio devices. It can flow from a dedicated radio dispatcher, over the telephone system, even from your computer via the Internet.
The transmissions within a paging system can be analog or binary. Analog pagers receive audio tones or voice transmissions within a carrier frequency. With binary transmission, the receiver looks for frequency shifts above and below a carrier frequency. This is referred to as FSK-NRZ (Frequency Shift Keying Non Return to Zero). Shifts above and below the carrier are identified as binary data, ones and zeros. Binary data transmission rates (baud rates) are defined by bits per second, where each bit is a binary one or zero. The greater the baud rate, the faster the transmission speed and the less air time needed for each transmission.
Paging systems can be simulcast or non-simulcast. Non-simulcast implies a single base station providing coverage over a building, hospital, or village. A simulcast system with multiple base stations can provide coverage over much larger areas such as counties. This is the case with Tait QS2 communications.
COMMONLY-USED PAGING TECHNOLOGY
Paging transmissions consist of the address of the recipient pager (CAP code) and the message. There are multiple analog and binary coding formats used for addressing pagers. One analog address format is known as “Two-Tone” and uses 110 tones ranging from 67 Hz to 2468.5 Hz to provide up to 3,540 unique address codes.
An alternative address format is 5-tone/6-tone. It consists of 12 frequency tones used to represent the digits 0 thru 9, a repeat number tone, and a special tone. The address of the pager is represented by five digits, or tones. This format is capable of multiple addressing; using the sixth tone and capacity is around 100,000 addresses.
There are two important binary addressing formats. Golay Sequential Coding (GSC) allows the pager to have up to four different addresses and provides a system capacity of 400,000 users. More popular is the Post Office Code Standardization Advisory Group (POCSAG) format which uses up to four addresses per pager and with capacity for two million addresses.
Originally, the POCSAG data delivery speed was defined as 512 baud. On-going developments of the protocol and hardware have doubled the speed with POCSAG 1200, and then doubled again with POCSAG 2400.
THE FUTURE OF PAGING
With text messaging and smart phones, do we still need paging? The answer for many agencies remains “yes.”
Many Public Safety agencies are located in rural communities. Many of them, particularly rural fire departments, depend heavily on volunteers. They use paging as a reliable and affordable means of communication with volunteers. Pagers are small, non-intrusive, and usually silent until an alert is received.
"ONE-WAY-PAGING REMAINS A PREVALENT CRITICAL COMMUNICATIONS TOOL"
For P25 digital communications, conventional wisdom suggests that paging is not supported. This is one reason why some cities or counties that implement a digital radio system have an additional analog network to support paging. However, this may not be the case in the future. The innovative deployment of paging on a P25 Simulcast network in North Dakota illustrates this point.
PAGING IN A DIGITAL WORLD
The city required a secure and resilient system capable of alerting (paging) each Fire Station from within the Sheriff's Department Dispatch Center's existing Centracom Elite console. This would run over a new P25 Fire Department channel. The alerting function had to be capable of alerting individual or multiple Fire Stations. The alert also had to be broadcast over the receiving Fire Station's public address system.
The city also required the alert to activate station lighting and to open the main garage doors to minimize fire unit response times. A custom alert solution was implemented through the Call-Alert and Talk-group functions within the P25 standard.
This highly customized solution required:
|A mobile remote unit, set up as an Alert Base Station, at each of the City's fire stations|
|Custom radio programming|
|Localized Fire Station interface wiring|
|Custom programming of the Dispatch Console|
Each fire station was assigned a talk group with an additional talk-group designated as “All-Call Fire Alert.” This is used to alert all of the Fire Stations at once.
In an emergency, the dispatch operators can now alert each or any combination of Fire Stations. A message describing the emergency can be heard over the PA system while the station's lights are switched on and the main doors opened. This combination of functions saves precious time during an emergency and ensures the highest possible level of service to the city. The solution allowed the fire department to move onto the fully digital P25 radio system, offering seamless interoperability with other public safety agencies, while continuing to support the voice and paging traffic that its fire fighters rely on.