Automatic Packet Reporting System Digipeaters

As of version 787, your maximum reporting period is dependent on the length of your digipath. This is so that at a special event or local area, using direct or one hop, then your net-cycle time is 10 minutes. Two hops is 20 minutes, 3 hops is 30 minutes etc up to the maximum value in your CFIGxxx.APR file. Currently the MaxTime defaults to 30 minutes.

WHERE ARE THE DIGIS? Use the MAPS-OVERLAY-DIGIS command to see the location and range of all APRS digis no matter where you are. Please send me data on any new digis so we can keep this DIGIS.POS file up to date.

DIGI-CALL-REPLACEMENT: At Dayton 97, PacComm has introduced their new digipeater ROM for all of their TNC's including any TAPR-2 clone which will substitute its callsign in place of WIDE when a multiple WIDE,WIDE packet is digipeated. It will also IGNORE the packet from then on completely eliminating looping duplicate packets. This is the greatest new capability in four years since APRS itself was introduced. See the section below on TRACE DIGIS.

BACKGROUND: The range of any AX.25 packet may be extended by specifying one or more digipeater callsigns. The packet will be relayed by each such digipeater in turn. After each such digipeat, that callsign is marked as used up so that at any instant, only the "next" digipeater in the list has the potential to digipeat the packet. Normally this requires users to know the complete intended path for their packets.

APRS, however, satisfies its real-time, emergency tactical needs without prior knowledge by using generic callsigns. ALL APRS stations are given the generic digipeater callsign of RELAY. This way any station can use any other station as a digipeater by simply addressing the packet VIA RELAY. With this form of generic digipeating (RELAY), a mobile, or new station does not have to know anything about the network in advance in order to be seen by adjacent nodes. After 10 minutes and his map begins to show the location of all stations and digipeaters on frequency, he can then customize his outgoing Unproto path to specific digipeater callsigns to cover his intended area without as much QRM. Similarly all wide area digipeaters have the generic digipeater alias of WIDE.

CAUTION: Since the preferred APRS frequency of 145.79 is adjacent to the SATELLITE band at 145.8, all APRS users should run minimum power. High and WIDE digipeaters in residential areas or near other satellite ops should probably operate at 10 watts or less to minimimze QRM. The purpose of APRS digipeaters is to HEAR mobiles, NOT to be an aligator!

ROUTES: It is important that as APRS networks mature with fixed, known digipeaters, that users at FIXED stations should avoid using the generic RELAY or WIDE addressing. Although it still makes sense for mobiles to use the path of RELAY,WIDE, the path of RELAY should rarely be used after the first hop by ANYONE, and never after a WIDE. Remember, every packet addressed via RELAY will key up EVERY APRS station that hears it. In any but the sparsest areas, the result is total congestion and collisions which block anyone from copying the packet. The D-list command lets you see what digipeater paths other stations are using and it also marks stations that you can hear direct. Also under the OPS-DIGI command, users can save up to 12 different DIGIpeater paths. Users can select any given path that is optimum for their present application with a single key stroke. The MAPS-PLOTS-POWER command will display a range circle around all stations proportional to their power, and antenna. Users can use these plots to estimate what paths, through what stations, might be useful.

Although digipeating is generally not good practice for AX.25 level 2 connections, it is ideal for APRS operation using UI frames only. The low duty cycle of APRS allows everyone to share the channel without anyone able to totally hog the frequency. Even Personal mail boxes are acceptible on the APRS frequency, since mail is posted at keyboard rates and is NOT read back by radio. Users are NOT welcome to READ PBBS mail on the frequency. Other CONNECTED mode operation of BBS's, mail forwarding, file transfers, TCP-IP and DX clusters are discouraged.

APRS DIGIPEATERS: Wide area APRS digipeaters should be widely separated to provide long distance coverage with the minimum of hops. But this does not preclude the need for many interim RELAY digipeaters to fill in weak signal areas or valleys. These sites provide the first hop (via RELAY) for all mobiles, which inturn then relay the packets to the main WIDE digipeater. These WIDE sites link to other cities and provide a backbone for wide area coverage.

WIDE DIGIPEATING: Since these WIDE area digipeaters are located at excellent locations, they should not only provide the WIDE backbone function for the long-haul, but should also have the RELAY alias as well for nearby mobiles and new stations. Many recent TNCs can be set up to digipeat either of these generic RELAY/WIDE packets. The new PacComm TNC's can support up to 4 aliases!. Set MYAlias to WIDE and set one of the other TNC functional callsigns to RELAY (such as MYnode, or MYhost, etc). If you use the KPC-3 and the MYnode call, be sure to set USERS to 1 and NUMNODES to 1. These WIDE-RELAY backbone Digi's should be spaced 50 miles or more apart depending on topology so that they are as widely separated while still being able to hit each other. All mobiles typically use the RELAY,WIDE path so it does not matter whether they are near a DIGI or someone's home station to be digipeated.

TRACE DIGIPEATERS: Since the new PacComm ROMS can support 4 aliases and have the Callsign-substitution algorithm, these new digipeaters should also be given the alias of TRACE. Although all 4 aliases are treated equally, using the TRACE call has some important advantages:

  1. The callsign substitution algorithm not only solves the looping duplication of packets, but it also provides a "TRACE" capability since packets arrive showing the actual path of all the digis that were used to propogate the packet.

  2. It allows stations to use long generic paths such as TRACE,TRACE, TRACE... without dupes. This is because only the new TRACE digis will respond and not the old WIDES. YOu may mix TRACES and WIDES in a path if you know the locations of the WIDES. The TRACE digis will still work on this path too.

  3. DIGI operators should use the Numeric Overlay symbol "T" for their new TRACE digis so that users know where they are.

Even if these WIDE/RELAY backbone nodes are 30 to 50 miles apart, as long as every home station and local RELAY digipeater can hit at least one WIDE, then the mobile path of RELAY,WIDE can cover as far as 100 miles! Wider ranging mobiles can use the RELAY,WIDE,WIDE path without causing too much QRM because of their low antennas. BUT CONVERSLY, RELAY,WIDE,WIDE should NEVER be used by a home station since he will undoubtly hit many home RELAYS all at the same time and therefore generate numerous dupes with every packet.

CAUTION: Fixed stations that can hit 2 or more WIDES should NEVER use three generic RELAY/WIDE callsigns in a row, and RELAY should NEVER be anywhere except the FIRST in the list. Multiple TRACE hops are fine but you shouldnt plan on QRMING beyond your immediate area except as needed. Although generic paths for mobiles are the normal, special consideration must be given whenever there will be a great convergence of generic mobiles using RELAY,WIDE paths, since each of them will repeat each other! In this case, they should change the path to not begin with RELAY. Here are the typical recommended settings for ALIASES and PATHS:

           (25-50 w  ):  UN APRS V WIDE,WIDE        MYAlias = RELAY


   To set up a WIDE area APRS digi, simply connect a TNC to a radio, and put
it as high as you can get it.  Set the following minimum commands:

MYCall W3XYZ-x (the digipeater call and SSID) MYAlias WIDE (this makes it digipeat WIDE packets) RELAY,TRACE,XX (Other aliases. XX may be your state) UNPROTO APRS VIA WIDE,TRACE,WIDE,TRACE (you want its own BText posit (to go as far as reasonable. THis (requires creativity where TRACE is not (yet fully used. Do not use W,W,W if (this digi can hit more than one other For KPC-3's you may use MYnode as a second alias if you set (NUMNODES and MAXUSERS=1) B E 90 (Sets Beacon to once every 15 minutes) (or B E 15 for Kantronics = 15 mins) BText (This is very important! BT !DDMM.mmN/DDDMM.mmW#PHG5360/W-R.... (identifying comments) | | | | | |||| LAT | LONG | | ||||________ Omni (Direction of max gain) | | | |||_________ Ant gain in dB | | | ||__________ Height = log2(HAAT/10) | | | |___________ Power = SQR(P) | | |_____________ Power-Height-Gain PHG indicator | |_______________ # is symbol for digipeater |_________________________ / for WIDE, for WIDE-RELAYS OR put the characters W-R in the comment field. Use T for TRACE digis you can see by the integers in the POWER-HEIGHT-GAIN (PHG) string, there are only 9 plus 0 possible values for each of these fields as follows:

DIGITS 0 1 2 3 4 5 6 7 8 9 as used in the Pwr field ------------------------------------------------------------------------- POWER 0, 1, 4, 9, 16, 25, 36, 49, 64, 81 watts SQR(P) HEIGHT 10,20,40, 80,160,320,640,1280,2560,5120 feet LOG2(H/10) GAIN 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 dB DIR 0,45,90,135,180,225,270, 315, 360, . deg D/45 This offsets * the range circle in the * 0 means OMNI indicated direction

HEIGHT ABOVE AVERAGE TERRAIN: Going out 10 miles in all directions, write down the elevation every mile or so. Average all of these points and compare your elevation to the average. You may be at 2000 feet above sea level and have a 150 foot tower, but if the ground around you is at 2200 feet, then your HAAT is -50 feet!!! Be honest! Your circle should go no further than the distance to which you can reliably copy an HT! Even though you have an OMNI antenna, if the terrain favors a certain dierction, then put that in for your directivity.


Although the GENERIC WIDE/RELAY digipeating works well to get an APRS net going, once you have more than two WIDES, the generic calls should be avoided by all fixed stations to minimize unnecessary duplicates and collisions. Or use TRACE. Using SPECIFIC callsigns significantly reduces QRM. A path of WIDE,WIDE,DIGI3,DIGI4 will get you out 2 hops in all directions and 4 more hops in the direction of DIGI3 and DIGI4. If you want to go long distance in two directions, save another long path in the other direction using the OPS-DIGi-SAVE comamnd and then activate it 50% of the time usign the OPS-DIGI-ALT command.

While building a new network, some well situated home stations may need to operate as WIDEs temporarily. To do this, simply manually configure one of your TNC's other calls to WIDE. MYAlias will not work, because APRS will always reset that to RELAY. You should not set yourself to WIDE unless you have local agreement to do so. Too many WIDE's, too close together causes too much QRM. If you are operating as a WIDE-RELAY, be sure to put the BACKSLASH character between the LATLONG so you will show up as green digi or select the DIGI# on the symbols menu.

SEE HF.HTM for setting up your UNPROTO path for HF and HF/VHF gateways..

********** WIDEn-n ALL DIRECTION GENERIC DIGIPEATING! ****************

For four years, we have been asking for someone to develop the WIDEn-n digipeater code. The WIDEn-n digi simply repeats ANY packet with the VIA address of WIDEn-n; but ONLY ONCE. It keeps a copy (or checksum) of the last 30 seconds of packets, and compares each new packet that it hears with these last ones to avoid dupes. This completely eliminates the multiple looping of packets caused by multiple generic paths such as WIDE,WIDE,WIDE. Without WIDEn-n, such a 3 hop path launched in the middle of 3 WIDE digipeaters that all hear each other can generate as many as 3x3x3 or 27 dupes! In a WIDEn-n network, there would only be three packets outward bound 3 hops.

NUMBER OF HOPS: The "n" in the WIDEn-n path indicates the number of hops. Each DIGI that repeats the packet decrements the WIDE-SSID by one. So the -n decrmenets to zero but the WIDEn portion indicates the original number of hops so that recepients know how far it traveled. A long distance traveler or special event of wide interest may use up to WIDE7-7 but a local commuter may only want to use WIDE2-2 to limit QRM.

SHORT PACKETS! THe biggest advantage of the WIDEn-n routing is that every packet still only has one DIGIpeater call. THis means only 7 bytes of overhead no matter how far the packet goes...

TRACEn-n. Notice, however, that WIDEn-n packets arrive as WIDEn-0, and the receiver has no idea how it got to him. If the WIDEn-n digis also support TRACEn-n, however, then at each hop, not only does the digi decrement the n, but it also INSERTS ITS MYCALL. This way a TRACEn-n packet arrives a DIGIa,DIGIb,DIGIc,DIGId etc. Although this is very powerful, it also makes the packets grow significanly in length as they propogate and should only be used when messaging...

NEW TNC COMMANDS REQUIRED: To make this work, there are three new TNC commands needed:

MYFlood1/2: Similar to MYAlias, sets up the callsigns to be used for WIDEn-n routing. Usually WIDE and TRACE...

HOPLimit: This is a SYSOP parameter that can be used to limit the maximum number of HOPS permitted in a network. Already the maximum number of hops is limited to 7, since the upper bit of the SSID is reserved for future use. However, some SYSOPS may feel empowered to limit the maximum number of hops to some smaller number.

AGELimit: The WIDEn-n digi has to keep a copy of all digipeated packets (or a unique checksum) for a brief period for comparison with new packets heard to assure that it does not repeat a packet more than once. This age limit determines how long packets must be kept for comaprison. If the time is too long, then the list is big, If it is too short, then packets may propogate in a circle and get repeated again. 30 seconds seems like a good starting value for 1200 baud channels.

This WIDEn-n capability would give us the ULTIMATE GENERIC MOBILE GPS NETWORK! It can handle both short and long hops with no dupes. This WIDEn-n algorithm is so powerful, there is no reason that it should not be added to ALL TNC's as a FLOODn-n mode. It would only work on UI frames and would default to the generic callsign of WIDE.


Since NODES are so much smarter than digipeating, the ultimate solutionis to have the NODES do all UI frame routing via high speed backbones. The APRS station simply sends his UI frame TO APRS VIA HOME; Any NODE hearing that transmission that has knowledge of the route to HOME, will send the single packet via the NODE network (level 4) to the HOME node! When it arrives at the HOME node, it is transmitted once as a UI frame. With this arrangement, a mobile only has to specify his one intended destination, no matter where he travels!

DIGI/NODE COMPATIBILITY: Mobiles should be able to specify a path that is compatible with both nodes and digipeaters. The nodes should only look at the LAST digi field in an UNPROTO list for the final NODE destination. Any preceeding fields are assumed to be DIGI's only. This way a path of APRS VIA WIDE,HOME would be repeated by any WIDE that heard it, but any level 4 node that heard it would forward it to the HOME NODE. If only one field is included in the digipeater string, it would be interpreted as both a digi and a HOME destination without any difficulty. Digi's and NODEs would digipeat it, and nodes (hearing it direct) would forward it at level-4.

EXAMPLE: A typical mobile just wanting to keep his spouse informed of his whereabouts might want to just use the UNPROTO path of APRS VIA HOME. Then his UI frames will be digipeated by the local HOME node or digi and will also be routed back to HOME by all NET-NODES along his travels. If he also wants to be seen by most HAMS in the areas of his travels, then he sets his path to APRS VIA WIDE,HOME. If he travels through a region that has both DIGIs and NODES, he might choose APRS VIA WIDE,WIDE,HOME. This way any areas with digis would digipeat via WIDE,WIDE and if he gets to an area with nodes which are aware of the path to HOME, then they will forward his packet there.

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