================================================================ D A S (DTMF ACCESSORY SQUELCH) APPLICATION NOTE 2 SELECTIVE GROUP CALLING 951105/Iss-1 ================================================================ Paul Newland, ad7i Post Office Box 205 Holmdel, NJ 07733 ad7i@tapr.org ---------------------------------------------------------- | Copyright (c) 1995 by Paul Newland, P.E. | | All Rights Reserved. This guide may be reproduced | | by radio amateurs for their own personal use, provided | | this copyright notice is included in any reproduction. | ---------------------------------------------------------- ******************************************************************* * NOTES * * * * SPECIAL TYPESETTING FUNCTIONS (I.E., FOOTNOTES, ITALICS, * * ETC.) ARE MARKED WITHIN THE TEXT USING A DOUBLE SLASH (LIKE * * THIS //). * * * ******************************************************************* INTRODUCTION ------------------------------------ DAS (DTMF Accessory Squelch) acts like a switch connected in series between the speaker output of your VHF or UHF transceiver and an external speaker. DAS will monitor a radio channel for you, with the speaker switch open so the speaker remains silent, until someone sends one of the DTMF sequences you have selected. When DAS hears your your personal Touch-Tone ID sequence on the radio channel it will light a LED, sound an buzzer and close the series speaker switch so that you can hear the audio of the calling station. An introductory discussion of DAS was published in //QST//, December, 1995, pages 25-31. The "DAS Configuration Reference Guide", available from the TAPR FTP file server as well as the ARRL BBS//1, provides a more complete description of each programming option, the purpose of the option and how the various options may interact. This document, DAS Application Note 2, "Selective Group Calling", describes how select PER ID sequences for various stations that DAS-AN2: SELECTIVE GRP CALLING -2- 951105/Iss-1 might be used in some sort of network, with DAS serving as the selective calling decoder. WILD CARD CHARACTER ------------------------------------ Most hams are now familiar with PCs and computer operating systems. Many of these systems allow the use of star (*) and query (?) as wild card characters. In computer operating systems * is often used to represent one or more positions of any symbol while ? is often used to represent just one position of any symbol. For example, consider a computer file system that contained the six files: DOG DOGBARK, DOGBONE, DOGHAIR, DOGGONE & DOGPARK. Issuing the command "SELECT DOG*" would select all six files. Issuing the command "SELECT DOGB*" would select only DOGBARK and DOGBONE. Issuing the command "SELECT DOG?" would select no files while issuing the command "SELECT DOG?ARK" would select only DOGBARK and DOGPARK. In these computer operating system cases ? means the system will accept only one character at the site of the ? but that character can be any symbol, while * means the system will accept any number of characters at the site of the * and those characters can be any symbol. In the DAS system, the * symbol can serve as a wild card symbol in the same way that the ? symbols serves as a wild card for computer operating systems -- it represents a single character position of any value. DAS does NOT consider a DTMF * to be like the wild card * as used in a computer system. For example, if a DAS decoder was set up to make use of Wild Cards (STAR IS WILD CARD CHARACTER set to Active) then a decoder programmed for a sequence of "123456" will respond to "12*456", "123**6", "1***56", or even "******". However, it will not respond to "123*", or "1*", etc. SELECTING ID SEQUENCES ------------------------------------ When selecting ID sequences for selective group calling applications, it's important to determine the different dimensions you may want to use for selective group calling. For example, geographical location is one example. Perhaps you have stations in several townships or counties. Further, you might want to divide each of of those areas into sub areas (like a tic-tac-toe board: NW, NC, NE, WC, CC, EC, SW, SC, SE, where NW stands for northwest, etc., and C stands for central). Another dimension might be functional purpose of station. In the case of communications backup or auxiliary order-wire of a RACES group, there might be shadow stations at law enforcement DAS-AN2: SELECTIVE GRP CALLING -3- 951105/Iss-1 locations, fire fighter stations, paramedic stations, transportation depots, material/supply depots, etc. Yet another dimension might be type of station. You might have fixed stations, portable stations (mobile trailers that can be placed for several days but could be moved on 30 minutes notice), automobile stations, highly mobile stations (motorcycle, bicycle) and even pedestrian stations. So, taking these factors into account, consider how a useful, workable system might be developed. Below is a concrete, although hypothetical, example for demonstration purposes. Consider a multi-township RACES network with DAS servicing as a selective calling device. The selective calling feature may not always be needed at all stations (stations with only one communications channel may want to monitor all voice traffic on that channel, all the time) but for some stations, perhaps where one or two operators are responsible for operating 10 communications devices, selective calling will be a must in order to keep the background "din" to a reasonable and workable level. PER ID sequences for stations on this RACES net will be constructed as follows: The first digit is always 2 (an arbitrary choice) to differentiate this system from any others (which might use 3-8 is the first digit of their sequences). The second digit will be 1-5 and will designate township. The third digit will be 1-9 and will designate location within the township by means of 9 sector (tic-tac-toe) grid. The fourth digit will be 1-9 and will indicate the function performed by that station's location. The fifth and final digit will be 1-6 and will indicate the type of station. Putting this all together, assign digits as follows: Second Digit, Townships: Name Symbol -------- ------ Washington 1 Lincoln 2 Hamilton 3 Jackson 4 Franklin 5 Third Digit, Sectors within Townships: DAS-AN2: SELECTIVE GRP CALLING -4- 951105/Iss-1 Name Symbol -------- ------ North-West 1 | | North-Central 2 NW-1 | NC-2 | NE-3 North-East 3 ------------------ West-Central 4 | | Central-Central 5 WC-4 | CC-5 | EC-6 East-Central 6 | | South-West 7 ------------------ South-Central 8 SW-7 | SC-8 | SE-9 South-East 9 | | Fourth Digit, Functional Operation: Name Symbol -------- ------ Law Enforcement 1 Fire Fighters 2 Paramedics 3 Materials & Supplies 4 Shelter 5 Transportation 6 Communications 7 Fifth Digit, Mobility of Station Name Symbol -------- ------- Fixed 1 Portable 2 Automobile 3 Motorcycle 4 Bicycle 5 Pedestrian 6 Now for actual sequence assignments. For the station covering the fire department in Hamilton township in the east-central sector by means of a ham operating out of his car in the parking lot, that station would be assigned PER ID sequence 23623. For the station covering the City of Lincoln shelter in the south east sector that is using an HT and was dropped there by the police (so he's on foot, at best), that station would be assigned PER ID sequence 22756. There are a couple of things to note at this point. First, note that there is a good possibility of repeated digits. Don't worry about that -- the problems caused by repeated digits, if any, are infrequent problems. Second, note that under these assignment tables it's possible that two or more stations could be assigned the same code. This isn't a problem either. The goal is not to DAS-AN2: SELECTIVE GRP CALLING -5- 951105/Iss-1 ensure that everyone has a unique ID sequence -- the goal is to allow any station to open up groups of speakers with just one command sequence. It's better to have a stations SELCAL device be activated, determine by monitoring that the call is not of interest and press the CLR button than it is to miss a call. It's better to skew the system performance toward false positive detection than toward missing calls altogether. CONFIGURING DAS FOR WILD CARD ------------------------------------ First and foremost, any DAS decoder that's used as part of this selective group calling system must have the STAR IS WILD CARD CHARACTER option set to ACTIVE. To do this, press and hold the CLR button until the STA lamp is winking. This indicates that you are in the CONFIG mode. Send DTMF "*4041#". Observe the command accepted flash code (GRP lamp flashing). Press and release CLR. You have now configured DAS to accept the * (star) as a wild card character. OPERATING THE SYSTEM ------------------------------------ Net control would have a list of the PER ID sequences used for each station. To send a bulletin to several stations net control could send the PER ID sequence for each station, voice the bulletin and then as for a QSL from each station. In the group calling scenario only one DTMF sequence would be needed. For example, let's say that there has been an earthquake in Hamilton. Standard telecommunications facilities are out of service or overloaded. The local RACES group has been called out and are on site at their respective posts. The water department has notified authorities that water pressure to fire hydrants in the northern sectors of Hamilton is likely to be interrupted for several hours. This will affect the ability of fire fighters to contend with fire. Net control has been asked for forward a bulletin to all fire stations. In this hypothetical situation, net control needs to contact all stations shadowing fire fighters in all sectors of Hamilton (central and southern sectors need to have this information because they could be called north to help with fire fighting because of low water resources). Net control could send the DTMF code for every station shadowing fire fighters to deliver this message, but that might take several minutes of air time. Net control could also use LiTZ, but that would wake up everyone and most of the stations monitoring the channel don't need to be interrupted from their non-radio duties to determine that they don't need to copy this bulletin. Instead, net control can determine the group call sequence needed to get all stations shadowing fire fighters. To do this, form the sequence digit by digit. The first digit is hard coded at "2". DAS-AN2: SELECTIVE GRP CALLING -6- 951105/Iss-1 The second digit is township and we only want Hamilton so the second digit would be set to "3". The third digit is sector and we want all sectors so set that to "*" (star) so that all sectors will be selected. The fourth digit is fire fighters and that's all we want so that digit would be "2". The fifth digit is station mobility and we don't care about that so that digit would be "*" so that all types will be selected. Thus, the group sequence for this transmission would be "#23*2*#". Use the # to start the sequence to ensure that all decoders are ready to copy. This is only important if a decoder may have heard a DTMF signal in the last 2 seconds (it's not likely, but it's possible). End the sequence with # so that the decoder will check the data immediately, and not wait for the 2 second timeout. This avoids the problem of someone else accidently sending DTMF right after you finish your transmission and causing the decoders to get confused. After sending the DTMF code you should do whatever your standard procedure requires for delivering a bulletin to many stations. That might be a roll call before sending the bulletin, send the bulletin and then collect a "roger" from each station. Another alternative would be to simply tell everyone to get ready to copy, wait 10 seconds, send the bulletin and then collect the individual "rogers", providing fills where needed. Consider the case where you wanted to call all shadows at law enforcement as well as the fire fighters. In this case you would need to send two DTMF sequences. The first would be "#23*2*#" for the fire fighters and the second would be "#23*1*#" for law enforcement. Of course, these could be combined into one concatenated sequence as "#23*2*#23*1*#". SEMI-REMOTE CONFIGURATION ------------------------------------ Although DAS provides for remote configuration, there is another concept called semi-remote configuration that can be useful in RACES/AREAS situations. In semi-remote configuration the DAS unit is placed in the CONFIG mode by the local operator pressing and holding the CLR button. However, all configuration commands are given by another remote station over the air. If at any time there is a question regarding the configuration of particular DAS unit in the field the net control station (or any knowledgeable user on channel, with the approval of net control) should re-configure the unit himself (or herself). To do this, net control would instruct the operator at the site with the questionable DAS unit to set the monitor switch to the MON position (with the monitor switch set to MON no matter how DAS is configured the operator at that site can continue to receive verbal instructions from net control). Next, net control tells the operator to press the CLR button for 5 seconds and then release the button. After that net control confirms by voice with DAS-AN2: SELECTIVE GRP CALLING -7- 951105/Iss-1 the operator that the STA LED is winking. This confirms (well, 98% likely) that the DAS microcomputer is operating correctly and that the unit is now in the CONFIG mode. Net control can now issue the configuration command to set the unit back to defaults (*30*) and have the operator confirm that the STA LED flashed several times (command accepted flash code). Next, net control configures the PER ID sequence (*nnnnnn#) and confirms with the local operator. Next, net control sets the STAR IS WILD CARD CHARACTER option ACTIVE by sending "*4041#" and confirms with the local operator. Next, net control exits the CONFIG mode by sending the exit command (*0#) and confirms with the local operator. Finally, net control can send several PER ID sequence and have the operator confirm that the PER LED is flashing and that the horn sounded. Do this again but use a "*" at any position in the PER ID sequence to confirm that STAR IS WILD CARD CHARACTER option is ACTIVE (if * works one place in the PER ID sequence it's 99.99% likely it will work at any and all positions of the PER ID sequence). CONCLUSION ------------------------------------ My examples are strictly hypothetical and may not be useful in your application. However, I think they do serve to demonstrate the utility of using DAS for selective group calling. NOTES ------------------------------------ 1. TAPR FTP File Server ftp.tapr.org, look in /tapr/das. TAPR on the Web at "http://www.tapr.org". ARRL BBS, +1 860 594 0306, 8-N-1, look in the FILES area, search on DAS. ---ooOoo---