Updated Protocol and compatibility (markdown)

Protocol-and-compatibility.md

@@ -5,8 +5,9 @@
 - **Server iGate** by default supports only classical `AX25` frames over LoRa (as defined in http://www.aprs.org/doc/APRS101.PDF page 12, see section below). It should enable interoperability with classical Linux APRS software, such as Xastir with `kissattach`. 
   - NB! Some popular LoRa ARPS implementations transfer plain text APRS messages over LoRa, for this mode operation need to set `cfg.EnableTextPackets/CFG_TEXT_PACKETS` to `true`, in this case iGate will transmit, receive and digirepeat text based APRS messages instead of classical AX25 frames.
 
-# APRS AX.25 binary protocol
-## APRS UI packet format
+# APRS protocol
+
+## APRS AX.25 UI packet format
 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.
 
 ![alt text](https://raw.githubusercontent.com/sh123/esp32_loraprs/master/images/ax25.png)
@@ -20,6 +21,12 @@ At the link level, APRS uses the AX.25 protocol, as defined in AmateurPacket-Rad
 - 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.
 
+## APRS data in AX.25 information field
+![alt text](https://raw.githubusercontent.com/sh123/esp32_loraprs/master/images/aprs_information.png)
+
+## APRS position packet
+![alt text](https://raw.githubusercontent.com/sh123/esp32_loraprs/master/images/aprs_position.png)
+
 ## APRS compressed packets
 In compressed data format, the Information field contains the station’s latitude and longitude, together with course and speed or pre-calculated radio range or altitude. This information is compressed to minimize the length of the transmitted packet (and therefore improve its chances of being received correctly under less than ideal conditions). The Information field also contains a display Symbol Code, and there may optionally be a plain text comment (uncompressed) as well.