Updated Protocol and compatibility (markdown)

Protocol-and-compatibility.md

@@ -1,3 +1,16 @@
+# Protocol compatibility
 - Make sure LoRa sync word and other LoRa parameters match
 - **Normal/Client mode** should be interoperable with other clients, which transmit raw text APRS messages if bluetooth client sends them in this format to the modem
 - **Server iGate** supports only classical `AX25` frames over LoRa (as defined in http://www.aprs.org/doc/APRS101.PDF page 12). It should enable interoperability with classical Linux APRS software, such as Xastir with `kissattach`. Some LoRa ARPS implementations transfer plain text APRS messages over LoRa, as a result **Server** will not be able to process these messages and gate them to APRS-IS, also clients won't be able to decode messages gated from APRS-IS to RF by the server
+
+# APRS AX.25 protocol
+At the link level, APRS uses the AX.25 protocol, as defined in AmateurPacket-Radio Link-Layer Protocol, utilizing Unnumbered Information (UI) frames exclusively. This means that APRS runs in connection less mode, whereby AX.25 frames are transmitted without expecting any response, and reception at the other end is not guaranteed.At a higher level, APRS supports a messaging protocol that allows users tos end short messages (one line of text) to nominated stations, and expects to receive acknowledgements from those stations.
+
+- Flag — The flag field at each end of the frame is the bit sequence 0x7ethat separates each frame.
+- Destination Address — This field can contain an APRS destination callsign or APRS data. APRS data is encoded to ensure that the fieldconforms to the standard AX.25 call sign format (i.e. 6 alphanumeric characters plus SSID). If the SSID is non-zero, it specifies a generic APRS digipeater path.
+- Source Address — This field contains the callsign and SSID of the transmitting station. In some cases, if the SSID is non-zero, the SSID may specify an APRS display Symbol Code.
+- Digipeater Addresses — From zero to 8 digipeater call signs may be included in this field. Note: These digipeater addresses may be overridden by a generic APRS digipeater path (specified in the Destination Address SSID)
+- Control Field — This field is set to 0x03 (UI-frame).
+- Protocol ID — This field is set to 0xf0 (no layer 3 protocol).
+- Information Field — This field contains more APRS data. The first character of this field is the APRS Data Type Identifier that specifies the nature of the data that follows.
+- Frame Check Sequence — The FCS is a sequence of 16 bits used for checking the integrity of a received frame.