Updated Protocol and compatibility (markdown)

Protocol-and-compatibility.md

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 # 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).
+- **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 field conforms 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.