Kenneth Finnegan, W6KWF - 2016
This guide is meant to assist the operators of APRS digipeaters (digis) and Internet gateways (I-gates) to select the correct contemporary symbol and symbol overlay to use for their APRS node. These symbols give other local operators a first glance overview of the topology of the local RF network, and should give them a good idea of what kind of coverage they should expect. The need for this document comes from the fact that there has been a large number of different suggested conventions for infrastructure symbology over the history of APRS, without a distilled single source of information on selecting symbols specifically for digipeater and I-gate operators.
Each suggested symbol will be bulleted, and start with the two character symbol code, which consists of the overlay character and then the symbol character. If the overlay character is either a forward-slash or back-slash, the symbol is one of the primary or secondary table APRS symbols as shown here, with no character overlay.
Of course, the large number of possible permutations on different digipeater / I-gate configurations means that not all APRS infrastructure nodes will cleanly fall into one of these categories. This document should be considered a set of suggestions and local discretion used to select the most appropriate symbol.
I-gates are APRS stations which are connected to the Internet and act as a bridge between the local RF network and the global APRS-IS (-Internet System) network. I-gates may be either unidirectional, where they only gateway APRS packets from RF to the APRS-IS network and are called Rx-only (receive-only) I-gates, or may be bidirectional, where they can additionally route select relevant packets from the APRS-IS back to the local RF in addition to gatewaying local packets to the Internet. This action of gating packets from the APRS-IS back to the local RF net is often called "RF-gating".
I-gates do not perform any APRS alias-based digipeating. If a station provides both local RF-to-RF digipeating services as well as I-gating packets, it should be considered an I-gate equipped digipeater.
- R& - Rx-only I-gate without the ability to transmit packets
- I& - VHF I-gate with the ability to RF-gate packets back to the local RF net
- /& - HF Gateways are I-gates which are operating on the 30m APRS network instead of the local VHF net
Digipeaters are the core of the RF APRS network, which allow other nodes to be able to reach the entire local RF network while operating from non-ideal locations using low power or low efficiency antennas. Digipeaters receive packets, check to see if they are requesting digipeater service from them, and then selectively repeat (or digipeat) the packet using the digipeater's higher antenna and power.
- /# - High level digipeater supporting the global WIDEn-N alias
Since high level digipeaters are unable to hear all trackers in all locations inside their coverage area, there are also what are called fill-in digipeaters, which are lower level digipeaters which are meant to only shuttle traffic from near-by trackers up to the higher level digipeater network. Fill-in digipeaters are not meant to extend the range of a packet once it has reached a high-level digipeater, and will often only cover a subset of the same area as the nearest high-level digi but with better sensitivity. Because of this, fill-in digipeaters should not be used after the first hop (which is why WIDE1-1 should only ever be used as the first hop in a path).
- 1# - Fill-in digipeater
In many high population density areas, APRS will not be limited to a single VHF frequency. For example, in some parts of the US 144.990MHz is defined as a "low power tracker" frequency, where quieter stations may beacon and be heard by what are called Alt-input digipeaters, which then digipeat these quiet stations into the local (and nosier) primary channel.
This classification does not generally include multi-port digipeaters which are used to route in both directions between either two VHF or a VHF and a UHF APRS network which are both meant for general purpose traffic. These should be considered two standard digipeaters which happen to also serve the roll of bridging these two APRS frequencies.
- A# - Alternate input digipeater
Many high level digipeaters support additional local aliases in addition to the global WIDEn-N alias. This is called the SSn-N system, since it was originally developed for serving US states. For example, every digipeater in Pennsyvania could be configured to respond to the alias PAn-N in addition to WIDEn-N. This would allow any stations in Pennsyvania with traffic relevant to others in the state to use the path PA7-7. This would cause the packet to flood the entire state without spilling out and causing undesirable traffic across the entire Eastern seaboard.
SSn-N digipeaters should note what additional aliases they support in their beacon comment or status fields.
- S# - High level digipeater supporting local SSn-N aliases
Most high level digipeaters are situated on mountain tops, which traditionally lacked reliable Internet connections. As this has changed, a new class of station known as "I-gate digipeaters" has become increasingly common. These digipeaters not only provide RF digipeater service, but also I-gate packets to and from the Internet. Since these digis are also often SSn-N digis, their local aliases should also be noted in their status or comment fields.
- I# - Digipeater providing I-gate services
Digipeaters are generally expected to be long-lived, permanent, fixtures of the local APRS network. Local APRS stations are encouraged to set their path to route APRS traffic through specific local digipeaters instead of using aliases like WIDEn-N when getting digipeated by those single specific digipeaters provides the desired coverage. When a new digipeater is built supporting unusual behavior which users might find confusing, or is set up temporarily for activities such as coverage testing from a new site, it should be labeled as an experimental digipeater to prevent confusion or issues when the digipeater is then taken offline.
- X# - Experimental digipeater