Backup for a TNPP satellite-based simulcasting system for paging
Paging backup systems provide a means to maintain quality of service and retain customers.
TNPP satellite-based simulcasting In the January issue of MRT, James Taylor and I described a TNPP satellite-based simulcasting system for paging, as shown in Figure 1 below. We described simulcasting and the economic case for its use. The simulcasting system consists of a central control site and a number of remote sites, all synchronized with the transmissions of the central site. The central site consists of a paging terminal equipped with a number of TNPP input-output ports, a satellite uplink subsystem (or modem to another uplink site), a master paging controller, a satellite receiver and a paging transmitter. Each remote site consists of a satellite receiver, a slave paging controller and a paging transmitter.
Remote backup Why back up the satellite feed at remote sites? It is a question of quality of service. As with electrical, gas or water service, paging customers need their service when they need it. For paging, quality service means a high percentage of message delivery any time of the day. Quality of service may also equate to a high percentage of customer retention.
Satellite-based systems do a great job, but, as with all systems, they are subject to downtime. Lightning can knock out the low-noise receiver in a satellite dish, rain can cause fades in the signal, an uplink dish or transmitter can fail and, infrequently, a satellite itself can fail. If a backup system is effective, economical and easy to maintain, superior service can be maintained.
Ways to back up the satellite feed There are three ways to back up the TNPP satellite feed: Use a radio control link, a second satellite feed or a dedicated or dial-up phone line. A radio link or telco feed would be the least expensive, and radio might be the cheapest, assuming a control link (typically 72MHz) and radios are already available. In fact, many terrestrial systems in use today, without any satellite component, feed POCSAG directly to each remote site. These systems even simulcast by using a digital or audio delay line at each remote site to balance out terrestrial path differences from the central site.
Backup switch requirements An intelligent switch or an externally controlled electronic switch is required to provide automatic backup feed at each site. This article considers the first case only. To be automatic and adaptable (for future applications nobody’s thought of yet), the switch must be microprocessor-based. To support simulcasting with a TNPP feed while on a terrestrial backup link, a digital delay line must also be included. Because the feed is binary in nature, and is as likely to be a 2400-baud feed as a 1200-baud feed, a front-end processor is required, too. Finally, the switch must have a number of specially designed input ports to handle the types of feed anticipated.
Configuring the backup switch The block diagram of such a switch is shown in Figure 2 on page 36. The switch consists of two input ports, one full RS-232 port, an output port and two microprocessors. The primary satellite feed connects to port one. If a radio link feed or second satellite feed is used, it connects to port 2. A telephone modem feed connects to port 3. The switch output, port 4, from either the satellite or one of the secondary feeds, provides the single feed to a paging controller.
With two processors on board, secondary functions (other than just switching TNPP feeds to the output) are built into the “switch.” These secondary functions include a digital delay line for use with the backup feeds for simulcasting, A/D inputs to gather site status, firmware to send alarms under certain conditions via the modem port and firmware to loop back the TNPP feed from the satellite or radio link to the central site. Site status can be obtained by dialing the modem at the site. Alarms are sent automatically by dialing a pager number and adding alarm status.
How does the backup system work? On power up, the backup switch would accept the satellite feed and route it through a set of normally closed relay points to its output. The switch could lose power and the feed would still be active. If the satellite receiver fails to present “carrier detect,” or if the microprocessor within the switch “sees” no TNPP frames for a user-programmable duration, then the switch will change feed to either the radio port or the modem port (depending on how it was programmed or configured via its modem port).
The switch will stay with the backup feed until it “sees” good TNPP data arriving once again at the satellite port for a user-programmable duration. Then it will switch back to the primary satellite feed. If the switch is “stuck” on backup for a third user- programmable duration, then it may (if the feature is selected) dial a pager number (if the modem is attached) and send an alarm.
Alternative uses for the switch First, the switch can also be used as a stand-alone digital delay line, taking TNPP feed from the radio link or telco modem port. In this case, the satellite feed is not used at all. The radio feed input is designed to expect frequency-shift keyed (FSK) data from the discriminator of an FM radio. Pure TNPP data would modulate the transmitter at the other end of the control link. Second, if packet modems are used as a part of the radio link to transmit TNPP data, then the RS-232 port of the remote site packet modem would attach to the radio input port of the switch. Third, the feed could come via a telephone line, and, in this case, the telco modem would attach to port 3. In this configuration, the switch acts as a digital delay line and paging controller feed, with TNPP. The paging controller would encode the paging frames and drive the site transmitter.
Timing considerations Based on traffic load, site locations and potential channel sharing, each paging system may require custom timing adjustments. For this reason, we’ve set up four timers in the switch, each user-programmable at install time or remotely programmable by TNPP command pages or telco modem connect: o T1-satellite to backup, programmable 0-255 seconds, default 5 seconds. o T2-backup to satellite timer, 0-255 minutes, default 10 minutes. o T3-alarm timer, 0-255 minutes, default 20 minutes. o T4-Switch delay timer, 0-500 miles, default 0.
Experience shows us that we don’t want to “churn” the switch; that is, switch too often. If we do, then data are lost that arrive during the switching process. We want to switch quickly to backup when the satellite fails, but we want to make sure that the feed has returned solidly (as in a rain or weather fade) before returning to the primary feed. Each technician may find other settings defaults for these timers appropriate for his system.
Backup switch details We have configured the SBS-2400 backup switch with four DB-9 connectors. The primary reason for this was to make use of off-the-shelf cables when possible, i.e., for the telco modem connection. Satellite receiver pinouts are not consistent from brand to brand, so users may have to assemble some cables. We also designed the front panel to let the tech know which feed was active (satellite, radio or modem), and with green LEDs to indicate when data is present.
System installation and testing Installation is straightforward. Let’s take the primary case where we plan to use a radio link to back up the satellite feed. Wire the satellite receiver to port 1 of the switch and port 4 of the switch to the paging controller, in our case the KPC-2000. Next, wire the radio discriminator, “flat audio” to port 2 of the switch, and, if desired, connect a modem to port 3.
A “TNPP Toolbox” of DOS programs was used to bench test the set of equipment for a remote site. We attached a laptop to the output of the switch to monitor the TNPP feed, using TNPPMON. You could also simply observe or listen to the KPC-2000’s output pages. We then disconnected the satellite feed cable (or you could turn off the receiver) and watched for the feed to transfer over to the radio feed. For testing, the radio feed can come from a TNPP source (PC-generated or from the satellite) driving a “flat audio” transmitter or paging exciter with a dummy load. Finally, reconnect the satellite feed and observe that the switch returns to the primary feed after the programmed duration.
