SSID in AX.25 and APRS

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SSID, Secondary Station Identifier, is part of an AX.25 address. APRS uses this field according to its protocol function and also applies conventions that indicate typical station roles.

The following must be distinguished:

SSID as an AX.25 address field,
SSID recommendations used in APRS,
historical uses of SSID,
identifiers existing only on APRS-IS or in other systems.

SSID in AX.25

An AX.25 address consists of six callsign octets and a seventh octet containing, among other fields, the four-bit SSID.

For a source or destination address:

bit:   7 6 5 4 3 2 1 0
       C R R S S S S E

For a digipeater address:

bit:   7 6 5 4 3 2 1 0
       H R R S S S S E

Meaning:

SSSS - SSID,
R - reserved bits,
C - Command/Response,
H - Has Been Repeated,
E - Extension Bit.

The SSID occupies four bits, so its value range is 0..15.

This limitation applies to every actual AX.25 address, regardless of whether it is used as a source, destination, or digipeater address.

SSID in APRS

APRS does not change the AX.25 SSID format. However, conventions were adopted for source stations to suggest the intended role of a station.

These are recommendations, not APRS syntax and not a validation rule.

Classic APRS recommendations:

SSID	Typical use
`-0`	operator’s primary station, usually fixed and message capable
`-1`	generic additional station
`-2`	generic additional station
`-3`	generic additional station
`-4`	generic additional station
`-5`	other networks or systems
`-6`	special activity, satellites, other bands, etc.
`-7`	handheld radio or other human-portable station
`-8`	boat, RV, or second primary mobile
`-9`	primary mobile station, usually communication capable
`-10`	Internet, iGate, EchoLink, Winlink, and similar uses
`-11`	balloon, aircraft, spacecraft
`-12`	APRStt, DTMF, RFID, devices, one-way trackers
`-13`	weather station
`-14`	trucker or other full-time driver
`-15`	generic additional station

At the AX.25 level, CALL-9 is simply a station with SSID 9. Interpreting it as a primary mobile station comes from APRS convention.

Software should not use this table as a basis for rejecting packets or forcing a station type.

`-9` and `-12`

In the classic APRS recommendations, -9 is intended primarily for an operator’s main mobile station, usually capable of APRS or voice communication.

-12 is appropriate, among other uses, for one-way trackers.

A device installed in a vehicle therefore does not automatically need to use -9. If it only transmits position and has no return communication capability, -12 may better describe its role.

Historical use of SSID for symbol selection

In early APRS, SSID was also used to select a symbol for trackers transmitting raw NMEA-0183 data.

This mechanism was later replaced by other symbol encoding methods, including the GPSxyz destination-address scheme and APRS position formats that carry the symbol directly.

A modern source SSID should therefore not be used to determine the station symbol.

Source, destination, and digipeater SSID

The frame:

SQ9MDD-9>APRS-2,WIDE1-1:

contains three independent SSID values:

SQ9MDD-9     source SSID = 9
APRS-2       destination SSID = 2
WIDE1-1      digipeater SSID = 1

All are encoded in the same way at the AX.25 level, but their APRS meaning depends on where they occur.

Recommendations such as -7 handheld, -9 mobile, or -13 weather apply primarily to the source SSID. Destination SSIDs and SSIDs in the path can be used by other APRS mechanisms and should not be interpreted using the station-role table.

APRS-IS and AX.25 limitations

APRS-IS uses a textual packet representation and permits a suffix after the hyphen consisting of one or two alphanumeric characters, subject to the remaining APRS-IS format restrictions.

For example, the following identifier is possible:

SQ9MDD-D

Such an identifier can be used by a station that exists on APRS-IS, for example a gateway or a system integrating APRS with another network.

It cannot, however, be encoded as a native AX.25 source address on RF because the AX.25 SSID field contains only the four-bit value 0..15.

An APRS-IS -> RF gateway therefore cannot assume that every valid APRS-IS identifier can be copied directly into an AX.25 address field.

Third-party traffic

APRS defines a third-party traffic mechanism in which the original header and data can be carried inside the information field of another valid AX.25 frame.

This can preserve information about a station originating in another network, but it does not extend the AX.25 address space. An identifier such as SQ9MDD-D can occur inside third-party data, but it does not become a native AX.25 source address.

Identifiers outside the `0..15` range

APRS documentation contains historical examples of suffixes such as:

-63
-tt
-ID
-A ... -Z

They were used for objects or stations generated by Internet-based and other systems. They are not additional values of the four-bit AX.25 SSID.

For a native RF address, the range remains 0..15.

APRS objects

An APRS object name is carried in the information field, not in the AX.25 address field, and can contain up to nine characters.

Names such as:

TEST-63
TEST-ID
WX-A

can therefore contain endings that resemble SSIDs, but those endings are part of the object name and are not subject to the four-bit AX.25 SSID limitation.

Consequences for gateways and software

An APRS implementation should distinguish validation of a native AX.25 address from validation of an identifier used on APRS-IS.

For an address transmitted directly over RF, the SSID must be within 0..15.

Software must not automatically assume that every identifier present on APRS-IS can be:

converted into an AX.25 address,
retransmitted over RF,
used as a source address,
passed through KISS to a TNC as a native frame address.

This is especially important for APRS-IS -> RF gateways, DMR/APRS and D-STAR/APRS integrations, hotspots, telemetry systems, and applications generating Internet-only APRS stations.

SSID interpretation in software

SSID can be used as auxiliary information about a station’s intended role, but it should not override data carried in the packet.

If CALL-9 transmits a specific APRS symbol, software should use the symbol from the packet. If CALL-13 does not transmit weather data, the SSID alone does not make the packet a weather packet.

SSID conventions are primarily intended for station organization and human recognition.

Summary

Key rules:

AX.25 SSID
    4 bits
    range 0..15

APRS source SSID
    uses the AX.25 SSID
    may additionally follow 0..15 recommendations

APRS-IS
    may contain identifiers with no
    direct AX.25 representation

APRS object
    name is carried in the information field
    a suffix after a hyphen does not have to be an SSID

third-party traffic
    may carry a textual representation
    of a station from outside RF, but does not extend AX.25

When interpreting a value written after a hyphen, first determine whether it is part of an actual AX.25 address, an APRS-IS identifier, or part of a name carried in the information field.