PATHS.TXT DIGIPEATING PATHS IN APRS Document version: 8.7.3 of 6 FEB 2005. (previous was 8.6.7 24 July 2004) Author(s): Bob Bruninga, WB4APR (updated 10 Sep 2005) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ This PATH information used to be in DIGIS.TXT from 1994 to 2004. But was recently split out because there was a need to separate the PATH info that users need from the DIGI setup info that DIGIpeater owners needed. Understanding APRS paths is absolutely critical for understanding APRS. Both PATHS.TXT and DIGIS.TXT are a "must read" for anyone who seriously uses APRS. Further, users must also read the on-line NEW N-N PARADIGM concept initiated in 2004 that drastically simplifies the APRS digipeater system, discourages the multiple dupes and QRM that can be caused by continued use of RELAY,WIDE and WIDE,WIDE,WIDE type paths and tries to limit the abuse caused by large values of WIDEn-N. OBSOLETE: In summary, RELAY, WIDE, TRACE, SS, and TRACEn-N are obsolete. APRS now only uses WIDEn-N since it has perfect dupe-elimination system. Further, under the New-N Paradigm, WIDEn-N is now traceable like TRACEn-N used to be and it Traps large abusive values of N. These changes can improve capacity or reliability by a factor of 2 to 5 ore more in some areas. Lastly, the New-N Paradigm also introduces State or ARRL Section paths of SSn-N for non routine state and section wide nets. See all about the New-N Paradigm: http://www.ew.usna.edu/~bruninga/aprs/fix14439.html FUNDAMENTALS: Universal PATH: Under the New n-N Paradigm (2005) the universal APRS path is WIDEn-N with small values of N. This should work everywhere, even in areas with old PacComm (used to be called TRACE "T" digis). For areas that used to use RELAY,WIDEn-N, the new equivalent is WIDE1-1,WIDE2-1, because "WIDE1-1" is the modern replacement for "RELAY". APRS is built on these fundamentals: 1) A 1200 baud system operating as an ALOHA random access channel 2) The desire to deliver new data to all local users in SECONDS 3) A net cycle time of 10 minutes for older local data 4) A net cycle time of 30 minutes for older area wide data (>1 hop) 5) The statistics of these fundamentals means about 60 stations constitutes a 100% full channel. This can be computed as X users transmitting Y packets per hour at the standard rates for, mobile, fixed and weather stations. And this is independent of terrain! 6) Any more traffic simply cuts the reliabilty of everyone. DIGIPEATERS: Digipeaters extend the range of your packets, but at a 50% loss of channel capacity for each hop. These days, anything more than 2 hops in the dense metro areas or 3 hops in most other areas just adds too much QRM to everyone else. Where every digi hears 4 others, a 1 hop path generates 4 copies, but 2 hops generates 12 copies and 3 hops generates as many as 24 COPIES of each packet! (7 hops can generate 200 copies!) Thus hops beyond your local 60 to 100 or so users is BAD and is just QRM to everyone else! ALOHA CIRCLE: The magic number of surrounding stations that can be supported reliably on your local RF LAN is the ALOHA limit of about 60 to 100 as noted above. Looking at a map, draw a circle around your 60 to 100 nearest users and that is your reliable APRS network range for that area. It might be only 15 miles in the big cities or it might be over 150 miles in Wyoming. Topology doesn't matter. What matters is the number of users on the same 1200 baud channel that equals a 100% full channel. Any more users guarantees collisions and loss of reliability. All APRS software should calculate the local ALOHA circle range and always display that circle on all maps so the user is always aware of the limits of his RF network. APRS871 (the APRSmax version) includes this. Keep your packets in your own ALOHA circle. Here is an example of how how the ALOHA limit is calculated... TYPE STATION PKTS/30m Total % LOAD HOPS COPIES TOTAL ------------------- -------- ----- ------ ---- ------ ----- 30 Home stations 2 60 27 % 2 8 480 2 LOCAL digis (evy 10m) 2 6 3 % 2 8 48 8 2nd-tier digis (evy 30m) 1 8 3 % 2 8 64 5 area WX stations 6 30 12 % 2 8 240 4 Mobiles every 5m 6 24 11 % 2 8 196 4 Mobiles every 3m 10 40 16 % 2 8 320 4 Mobiles every 2m 15 60 27 % 2 8 480 100 % 1828 Since this represents the most number of packets that your local network can handle in 30 minutes, this is what drives then how many digipeaters and how many HOPS users must use to be able to communicate within this network of local users. In APRSmax, hit the OPS-ShowALOHA command and MAPS-PLOTS-HOPS command to see these effects on your map. NETWORK OVERLOAD AND QRM: These days, too many users are totally blind to the limitations of the 1200 baud APRS national network and are not satisfied unless they can see hundreds and hundreds of users from the 5 surrounding states covering millions of square miles. This is just impossible and we must get users to only use the path necessary to cover their ALOHA radius as noted above.. Trying to propogate packets too far just KILLs the APRS network for others due to lost reliability. UNPROTO PATH: The UNPROTO command in the TNC sets the digipeaters that will be used for all packets. The packet will be relayed by each such digipeater in the path 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. For conventinoal packet, this requires users to know the complete intended path for their packets... But not in APRS. GENERIC ALIASES: What makes APRS unique to conventional packet is that it satisfies its real-time, emergency tactical needs without prior knowledge by using generic TO and DIGI callsigns. The most fundamental generic digi aliases were RELAY and WIDE. But the problem with multi- hop WIDE,WIDE or RELAY,WIDE paths is that packets ping-pong back and forth between digis generating extra dupes. The Callsign Substituting digis introduced by PacComm in 1997 (called TRACE digis and marked with a "T" overlay) helped reduce this, but it still can generate 3 to 5 times to many dupes. In 1994 I proposed the WIDEn-N system which would go N hops and with perfect dupe elimination. ROUTES: Using generic aliases is good for interoperability everywhere and covering your ALOHA area in all directions, but using generic all-direction hops beyond your local area are inconsiderate. To communicate beyond your ALOHA range to a specific station, users should use a specific path so that only selected digis are invloved. Note that the hop of RELAY should NEVER be used after the first hop by ANYONE, and never after another hop because it keys up everyone with great irritation! WIDEn-N DIGIPEATING: In 1998, Kantronics finally implemented my WIDEn-N algorithm which further improved long multi-hop effeciency. And it does excellent dupe-checking on the content of the packet so all dupes are eliminated. One shortcoming was that the packet was not traceable. So we encouraged using the path of WIDE,WIDEn-N to force a callsign substitution on the first hop.. But this use of the old WIDE still generated extra dupes. Finally this was eliminated in the New-N Paradigm in 2005 and the tracing problem was solved by moving the WIDEn-N support to the UITRACE parameter to replce the old TRACEn-N system. Old WIDEn-N digis have an "N" overlay and need to be updated to the New n-N Paradigm digis marked with an "S" which also support the new SSn-N path and limit abuse of large values of N. MULTI-ALIAS DIGIPEATING: Both the PacComm and Kantronics ROMS support up to 4 UIDIG aliases. Originally, the four generic aliases of these digis were recommended to be RELAY, WIDE, TRACE and SS. But now we have dropped all these and instead use these callsign-substituting aliases as N-N traps. THus the typical UIDIGI Alias list is now UIDIGI ON WIDE4-4, WIDE5-5, WIDE6-6, WIDE7-7 Which will Trap all large n-N values. TRACE - This was a generic alias that worked just like WIDE, and was simply a distinction to discriminate between WIDE-only digis and the better TRACE digis that did callsign substitution. It is now obsolete. THe TRACE digis (PacComm roms) can now be made somewhat compatibile with the New n-N Paradigm by setting the 4 aliases so that they also support local WIDE2-2 and WIDE3-3 mobiles: WIDE1-1, WIDE2-1, WIDE2-2, and WIDE3-3 HOME STATIONS should no longer set their alias to RELAY by default so that they digipeat every mobile they hear direct. Since the new universal path is WIDEn-N, the way to help out a passing mobile in a blind hole is to support the alias of WIDE1-1. This will let your station support mobiles using the special WIDE1-1,WIDE2-1 path to get out of the RF hole to a WIDEn-N digipeater. WIDEn-N FLOODING: WIDEn-N was conceived back in 1994 as the most efficient way to FLOOD APRS packets out multiple hops in all directions to everyone in the LAN. It also had perfect dupe elimination so that each digi only digipeated it once no matter how many times it heard the packet. But, these days, we cannot afford the multi-hop long distance flooding of large values of N, but the dupe elimination is still the best algorithm available. Thus, the network now concentrates on encouraging WIDEn-N to replace generic RELAY and WIDE paths while Trapping excessive values of N and preventing further propogation of these abusive user paths. MOBILES: Under the original plan, Mobiles typically used the path of RELAY,WIDE because they may be out of range of a WIDE digipeater but be near someone's home station acting as a RELAY. But this had the multi-dupe problem. This is solved by changing "RELAY" to "WIDE1-1" so that such mobiles can use a path of WIDE1-1,WIDE2-1. THen the fill-in-digis can support only WIDE1-1 just like they used to support RELAY. But the other digis will still use the perfect dupe ignore function and no extra dupes result. Notice the second hop is WIDE2-1 so that it only goes 1 hop. If you use WIDE1-1,WIDE2-2, then 3 hops actually result (which may be OK in some areas). NEW-N USER RECDOMMENDATIONS: We have greatly simplified APRS paths: WIDE2-2 all major metro areas. See map on fix14439.html page WIDE3-3 in rural areas more than 150 miles from big city areas WIDE1-1,WIDE2-1 for mobiles to replace the old RELAY,WIDEn-N path SSn-N STATE and ARRL SECTION NETS: Most amateur radio communications is organized at the county and state level. For some activities and nets it is necessry for everyone in the state (or section) to check into a net. Central stations can usually cover most states with WIDE3-3, but the stations in the remote reaches of the state may have to use WIDE5-5 or 6-6 to get to everyone. This generates way too much QRM outside of the state with as many as 144 copies of every packet. But by adding SSn-N to all state digipeaters, then a SS6-6 packet in Maryland for example will not go out side the state AND only about 10 total copies are produced by the 10 major digis that cover the state. This is a 14-to-1 reduction in QRM while still providing state wide coverage for those stations far from the center. SHORT RANGE FOR MOBILES! Mobiles have less than HALF the range of what you expect compared to your normal experince with VOICE repeaters. The human ear can copy a voice down into the noise, but a packet signal will FAIL at the first even ONE millisecond of a single fade. And SINCE mobiles ALWAYS pass through 6 to 30 dB fades due to multipath, then the effective range of a mobile is much less than half of what it is for voice. SO be sure to take this into consideration in your PATH planning. Since July 2002 we changed the APRS SPec to reduce all PHG circles by half to compensate. FEWER HOPS: Although you are tempted to set a LONG path so everyone can see you, remember that to them, you are just QRM. Especially since the amount of QRM you generate grows geometrically with the number of hops as shown in the following table. ALSO the probability of a successful packet also goes down greatly. The following table shows the decreasing probability of a successful packet if we assume a 50% probability of collision and each WIDE can hit 4 other WIDES. HOPS SUCCESS COPIES COMMENTS ---- ------- ------ -------------------------------------- 1 50% 1 For local ops & special events 2 25% 5 Routine ops 3 12% 13 extended ops 4 6% 25 statewide ops Heavy QRM 5 3% 41 Nothing gets through. Too much QRM 6 1% 61 Useless AND totally boggs down network PLEASE limit your hops to just your local region. APRS *IS* designed to assure delivery to EVERYONE in a 1 or 2 hop area for REAL-TIME nets or events. It was NOT designed for large extended area nets with thousands on line... These days you CAN QSO great distances via the IGates, but this is still a LOCAL process with only 1 hop to get to your IGate in dense areas. As MORE people come to APRS, we must begin thinking SMALLER areas... In general, the digipeating function should be turned OFF in all mobile TNC's except in very specific instances of special roving digipeaters. You can imagine what it would be like at a Hamfest if as the mobiles converged they all began digipeating each other... SEE HF.TXT for setting up your UNPROTO path for HF and HF/VHF gateways. HOW WIDEn-N WORKS: Since 1994 I had asked for this capability, and it was finally implemented in 1998 Kantronics TNC's. The WIDEn-n digi simply repeats ANY packet with the VIA address of WIDEn-n; but ONLY ONCE and then decrements the N. 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 eliminates the multiple looping of packets caused by multiple generic paths such as WIDE,WIDE or more. The little "n" keeps a copy of the original N so that when the packet arrives we can tell how many hops it took to get here. SOURCE IDENTIFICATION: The UIFLOOD n-N function does not trace packets and for years we receommended WIDE,WIDEn-N to force a callsign substitution to identify the first hop. But this generated multiple dupes. It also required all digis to have UIFLOOD set to NOID. This was hard to enforce, since it ws impossible by looking at a packet to know if the first (NOID) or last digi (ID) was the one inserted. Under the new n-N paradigm, WIDEn-N was moved to the UITRACE function where the path is fully traced and this ID/NOID problem goes away. The new KPC-3+ 9.0 ROM supports a special option of FIRST. So UIFLOOD WIDE,30,FIRST will cause the FIRST digi only to insert its call. VICINITY PLOTTING: The advantage of knowing the initial digipeater is for locating the source of a packet even if no position is transmitted. This is very powerful since it lets recepients estimate the position of a station based on what initial digi he is hitting no matter what his packet contains. This is automatic in APRSdos which will plot the station as a QUESTION mark ICON within about 1 mile of the first DIGI. These are called "vicinity plots" and give you an approximate area until you finally get a real position packet later.. HOME STATIONS: The powerful WIDEn-N, and SSn-N systems should not be enabled at homes, but ONLY at HIGH digipeater sites. If it is enabled at any home stations, this will SEVERLY QRM the network... Only the UIDIGI alias of WIDE1-1 should be supported at user stations and only if it does callsign substitution. SPECIFIC NOTES FOR APRSdos USERS: NOTE: In APRSdos 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 or more will cause your station to drop to the 30 minute default net-cycle time for old data. These times only come into play for data that is old. Since APRSdos transmits new information immediately and then on a decaying algorithm starting at 8 seconds and doubling every transmission these net cycle times only apply to old data that needs to be periodically refresehd for new users joining the net. 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 NOTIFY Jeff of dididahdahdidit.com of any new digipeaters so that he can update the original DIGIS.POS file. The DIGIpath page in APRSdos lets you see what digi paths other stations are using and it also marks stations that you can hear direct. Use the powerful JUST XXXX to sort and search for various paths. 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. APRS870 introduced a very powerful MAPS-PLOTS-HOPS command in APRSmax that shows the number of hops to each digi that you hear.. and then in APRS872, the MAPS-PLOTS-USERS command to let you see how many hops were used for each stations packets to get to you. de WB4APR, Bob