Sending TNPP over paging control links

Multiple paging providers can merge their traffic, uplinking it to a satellite and collecting the combined traffic using satellite receivers. Paging controllers at remote sites, which receive and encode TNPP data, and link modems make the system work.

Many paging providers exchange pages to earn more revenue and to provide for a greater coverage area. Pages are delivered between their paging systems via satellite, radio link or leased telephone lines. Most systems in current use are networked using the Telocator Network Paging Protocol (TNPP). This network for paging terminals is similar in many ways to the Internet, which links computers and servers. A group of connected paging terminals, running TNPP, will send, route and receive pages among the group. Once a terminal has collected pages addressed to it, it must encode and transmit them within its coverage area. Until recently, telephone circuits have dominated as the means for exchanging batches of pages.

Now many associations of paging providers are merging their traffic, uplinking it to a satellite and collecting the combined traffic using satellite receivers. It’s a natural and easy way for all in a group to receive all of the pages of the group. One motivation for using a satellite feed is that the feed is available at once at all sites; there is no need to collect the traffic at each provider’s central site, encode it there to a standard page format and ship the pages out via telco lines or control links to the remote sites. This system does require that a paging controller be installed at each remote site to receive TNPP and encode it into whatever paging format the traffic requires.

Systems that are not using a satellite feed (yet) but that have control links or telco lines available to their remote sites can take advantage of the methods described above. You could take an area-wide feed from another provider via a telco line, a control link, or a single satellite receiver and then deliver those pages through your existing system from your central site. You could also upgrade your feeds to the remote sites by sending TNPP traffic to each and by adding a TNPP input-encoder output (paging controller) at each site.

There are a number of good reasons for sending pages in TNPP format directly to your remote sites. * Mixing paging protocols (such as POCSAG and Flex) is easier. * Simulcasting is simplified. * Control-link feeds can serve as a backup feed when SAT feed is added. Sending TNPP over control links-to reach your remote sites-is an emerging art. You can use standard FM transceivers at 1200 baud, which is satisfactory for systems with low-to-medium traffic. The TNPP feed is RS-232, so you’ll hav e to add a TNPP link modem at the central site and each remote site. Link modems are reasonable in cost and are available for 1200-baud or 2400-baud to 9600-baud service. Link modems running 2400 baud and faster use direct frequency-shift keying (DFSK) (flat audio) modulation and require a data-ready radio.

Historic use of control links Figure 1 on page 61 shows the traditional use of paging control links (typically at 72MHz, but sometimes at VHF or UHF frequencies). Pages that are delivered over the link are already encoded. The job of the remote site equipment is simply to key the paging frequency transmitter and present the encoded pages. Simulcast remote sites include an audio delay line between the link receiver and the receive modem to delay the tones. Presentation of the pages to the transmitter is usually in digital format, taken from the serial port of the modem. Push-to-talk (PTT) is provided by the modem carrier detector or by additional circuitry. Sometimes all of these functions are combined in a manufactured “paging controller” unit.

Standard 202 modems have been used for a long time in control links, and they have worked well. However, these 1200-baud modems cannot carry 2400-baud POCSAG or future Flex traffic unless that traffic is in TNPP format. In addition, these modems don’t do anything but modulate or demodulate; that is, they don’t contain PTT controls, any kind of TNPP filtering or a digital delay line for simulcasting. All those features have been added as extra equipment. However, link modems and remote site paging controllers do include these requirements, allowing you to go digital-send TNPP-over your control links.

Use of modern digital control links Figure 2 on page 62 presents two “digital” uses of modem control links: exchanging TNPP traffic between paging terminals and delivering traffic in TNPP format from a terminal to remote sites. In the later case, the link must contain modems capable of buffering and processing the digital paging traffic, provide for CW ID and include PTT controls. In addition, each remote site must contain a paging controller capable of encoding the TNPP traffic in whatever paging format is required, add a delay for simulcasting, control access to the paging channel and include keying controls for the transmitter.

Clearly, what this accomplishes is to push TNPP traffic all the way out to the paging transmitter sites. This means that your entire network will be digital. The advantages are: * Every control link will have the same equipment. * Every control link can handle any kind of paging protocol. * Your remote sites can be converted readily to local SAT feed at any time. Why can the link handle any paging protocol, even Flex? TNPP doesn’t care what the paging format is; it just delivers the page in ASCII, and at an arbitrary baud rate. You could send the pages over the link at 600 baud, 1200 baud or even 9600 baud; TNPP doesn’t care. It simply delivers all the necessary information about the page-including paging baud rate, format, and capcode, whether it is Golay, POCSAG or Flex-in ASCII format. It is the job of the local page controller to encode that page and send it at the required baud rate.

TNPP control link details There are several pertinent details in a TNPP digital control link. A block diagram and a table denoting the link modem’s features are presented in Figure 3 on page 63. Tracing through the task of sending a frame of TNPP traffic across the link, we find that the paging terminal presents a batch of pages in TNPP frame format to the modem for transmission. Unlike an analog modem, the TNPP link modem is smart. It will: * reject TNPP frames not addressed to the transmitter site. * reject TNPP frames with a CRC error. * reject TNPP frames that are duplicates or out of order. * provide a large input buffer. * perform a periodic CW ID on the control channel when required. * send only complete TNPP frames.

There are several advantages for filtering out frames in error, or out of sequence. First, it eliminates airtime wasted on incomplete pages. Second, if frames are received out of sequence, you’ll want to reject them because they are likely to be duplicates. Third, if you are receiving a wide-area feed from an association, you may wish to reject any traffic that you don’t want to retransmit as pages. This is accomplished by using a table of allowable destinations in the transmit modem for comparison. In addition, a large transmission buffer at the input of the modem gives you time to send a CW ID when called for without losing any incoming TNPP frames. The buffer also eliminates the partial frame problem. Attempts to send TNPP using “packet modems” often resulted in a splitting of the frames, sometimes causing errors. The link modem buffers only complete frames for transmission.

Modem characteristics The necessary features of a TNPP link modem are shown in the table in Figure 3. The modulation format does not matter; the modem can use tones or DFSK. These features just determine the upper limit on the baud rate for the radio side of the modem. For speeds greater than 1200 baud, DFSK and data-ready radios must be used. The TNPP filtering features should include: * rejection for frames with a bad CRC. * rejection if the TNPP destination address does not match one of those in a table. * encapsulation of each complete TNPP frame within a packet to prevent any extra characters from being added or left out due to noise or splitting of packet frames. * a large and sufficient TNPP frame buffer. * provision for CW ID and PTT.

These features are not found in telephone or “format” modems.

Simulcasting using digital control links Figure 4 on page 64 is a block diagram of a terrestrial simulcasting system that uses TNPP delivery of pages to several remote sites. The system shown substitutes control link feeds in place of a satellite feed. [See MRT January 1999 for a description of a TNPP-fed, satellite-based, simulcasting system.] The paging terminal and central portion of the system is shown at the left in the figure; a remote site is shown at the right. The central site includes a pair of link modems, a transmit modem and a master paging controller. The controller is used to time the simulcasting process. Each remote site contains a link receiver, a link modem (for reception only), a paging controller and a paging transmitter.

TNPP pages received at the central paging terminal are fed to the link modem, whose job it is to reject all pages that have errors or that don’t contain a proper TNPP destination address. The link modem then keys up when it has complete TNPP frames to transmit and presents them to the link transmitter.

All link receivers, including the one at the central site, pick up the TNPP frames and pass them on to a paging controller. The controllers encode the pages and buffer them for transmission. Once the master controller receives any pages, it sends out a “go” command to the central paging terminal, and the terminal merges that TNPP command frame with the rest of the TNPP traffic arriving from DID circuits and other terminals nearby. Once the “go” command page reaches each controller in the system, the pages previously “buffered up” are simultaneously released for transmission.

The system described sends all pages it is capable of encoding at the paging controllers. They all work exactly the same. If the controller supports POCSAG only, then other paging formats will be rejected instead of being buffered. If additional paging formats are supported, then they, too, will be buffered and transmitted. The control link modems, because they handle TNPP frames, don’t “care” what paging format is used. This establishes the TNPP link modems as paging format-independent. Timing control and encoding is pushed into the paging controllers at the remote sites.