Role and elements of the APRS network, basic operation

APRS forms a distributed information exchange network. Data can be received directly over radio, retransmitted by digipeaters, and transferred between the radio part of the network and APRS-IS.

The network does not require a central intermediary. Two stations within direct radio range can exchange information directly. Digipeaters extend the usable range of the radio network, while iGates connect RF segments with APRS-IS.

Different network elements perform different functions. A single device may perform several of them at the same time, but each role has its own scope of operation.

APRS network elements

The basic roles found in an APRS network are:

end station,
digipeater,
iGate,
APRS-IS,
applications and services using APRS data.

These roles describe a function performed in the network, not a specific type of device.

A single installation may, for example, generate its own APRS packets, retransmit traffic as a digipeater, and forward received packets to APRS-IS as an iGate. These functions nevertheless remain separate.

End station

An end station is a source or recipient of APRS information.

It may generate, among other things, position, status, weather, telemetry, object, or message information. It may also receive and interpret data generated by other stations.

Communication does not have to pass through intermediary infrastructure. If two stations are within direct radio range of each other, a packet can be received without a digipeater, iGate, or APRS-IS.

Digipeater

A digipeater is an element of the radio part of the APRS network. Its task is to retransmit packets according to the APRS path mechanism.

This allows a packet to reach beyond the direct radio range of the source station.

During retransmission, the digipeater performs the changes defined for the packet path. Retransmission must not be used to arbitrarily modify the remaining packet data. Source data remains data originating from the station that generated the packet.

If a device operating as a digipeater generates its own beacon, status, or another APRS packet, it is a new packet originating from that station, not an addition to the contents of another station’s packet.

The details of path processing and digipeating algorithms are described separately.

iGate

An iGate is a gateway between the radio part of the APRS network and APRS-IS.

The basic direction of operation is:

RF -> APRS-IS

An iGate may also provide controlled forwarding of selected traffic in the direction:

APRS-IS -> RF

The two directions are governed by separate rules.

Like a digipeater, an iGate must not arbitrarily modify the data of a forwarded packet. Information associated with introducing a packet into APRS-IS is placed in the part of the path intended for this purpose, including through the use of a q construct. It is not appended to the source station’s data.

Detailed gating rules, q constructs, and APRS-IS -> RF traffic are described separately.

APRS-IS

APRS-IS is the Internet part of the APRS infrastructure. It connects servers, clients, applications, and iGates, allowing packets to be distributed beyond local radio coverage.

A packet received locally on RF can be introduced into APRS-IS by an iGate and made available to systems in other locations.

APRS-IS is not a separate system unrelated to the radio part of APRS. Its operation is tied to RF traffic, and data originating from APRS-IS may, in defined cases, be transmitted back onto radio.

Applications and services

APRS data can be received and used by applications and specialized services.

They may, among other things:

present the current state of the network,
display positions and objects,
handle messages,
record data,
process weather and telemetry,
provide other functions based on APRS data.

Not every application using APRS-IS is part of the infrastructure that transports packets. A system consuming data should be distinguished from a digipeater, iGate, or APRS-IS server directly participating in packet distribution.

Information flow

The simplest communication is direct:

station A -> RF -> station B

With a digipeater:

station A -> RF -> digipeater -> RF -> station B

A packet received by an iGate can be forwarded to APRS-IS:

station A -> RF -> iGate -> APRS-IS

Within APRS-IS, the same packet can be available to many clients and services:

                         -> client
                        /
RF -> iGate -> APRS-IS ---> service
                        \
                         -> another APRS system

In defined cases, traffic may also be forwarded from APRS-IS to RF:

APRS-IS -> iGate -> RF -> station

Not every packet passes through every network element. These are different possible paths for information distribution.

Information source and intermediary elements

The source of information must be distinguished from the elements participating in its forwarding.

A packet may pass through a digipeater, iGate, and APRS-IS, but these elements do not thereby become the source of the data contained in the packet.

Information required for network operation, for example information related to the radio path used or the point of entry into APRS-IS, has its own mechanisms and designated locations.

An intermediary performs only the changes resulting from the function it implements. Its own technical information must not be moved into another station’s source data.

Forwarding and generating packets

Forwarding an existing packet and generating a new packet are two different operations.

forwarding an existing packet

is not the same as:

generating its own packet

A digipeater retransmitting a packet or an iGate forwarding it to APRS-IS acts as an intermediary.

If a device generates its own APRS information, a new packet originating from that device is created.

The distinction remains important even when several functions are implemented by the same software, computer, or radio device.

Roles and limits of operation

An end station, digipeater, iGate, and APRS-IS perform different functions.

An end station generates or receives APRS information.

A digipeater retransmits packets within the radio network and performs the changes defined for the retransmission path.

An iGate forwards packets between RF and APRS-IS according to the rules defined for that function. Information concerning transport within APRS-IS is stored in the path elements intended for this purpose, including through q constructs.

APRS-IS distributes packets within the Internet part of the system.

The fact that one device can technically perform several functions does not merge their operating rules. For each packet, the role in which the device processes that packet is relevant.

In particular, intermediary infrastructure must not arbitrarily modify data originating from the source station. Changes made while forwarding a packet are limited to the fields and mechanisms defined for the network function being performed.

Shared radio channel

The radio part of APRS uses a shared medium with limited capacity.

Every transmission occupies the channel and affects the ability of other stations to transmit and receive. This applies to packets generated by stations, retransmissions performed by digipeaters, and traffic introduced onto RF by iGates.

The configuration of individual elements therefore affects not only the range of a single packet but also the operation of the entire local network.

Detailed traffic-limiting mechanisms and infrastructure design principles are described in later articles in the Network section.

The APRS network as a whole

One possible packet path can be represented as follows:

                     +-> station
                     |
station -> digipeater +-> iGate -> APRS-IS -> applications and services
                     |
                     +-> other RF stations

This is not a required topology.

APRS can operate locally on RF only, can use one or more digipeaters, can be connected to APRS-IS, or can use several of these mechanisms simultaneously.

The basic division of functions remains:

a station creates or receives information,
a digipeater extends its distribution over RF,
an iGate connects RF with APRS-IS,
APRS-IS distributes information in the Internet part of the network,
applications and services use the available data.

Each of these elements performs operations defined for its role. Detailed rules for digipeaters, iGates, and APRS-IS are developed in later parts of the Network section.