Main page This type of signal is commonly heard on the HF bands. Asynchronous burst signals generally allow data transfer to take place in either direction and they often support some form of automatic repeat request (ARQ). Synchronisation is provided by known sequences of elements at the beginning or end of each burst. The most popular system is 2-level FSK AX.25 amateur packet radio which can be heard around 14.100 MHz.
AX.25 is a packet system in which the data elements are grouped into frames which contain routing and other information so that basic networking is supported. It is possible for an information sending station (ISS) to establish a virtual circuit connection with an information receiving station (IRS), to exchange message data and to disconnect. Connections can also be established via intermediate stations and a number of data speeds are supported (from 300 bps up to 56k bps). Services provided include bulletin boards, e-mail, binary file transfers and sophisticated networking protocols (such as TCP/IP).
The message characters in each frame (or packet) are protected with a complex error correction scheme so that the system has a claimed error rate of 1 in 1000000000 bits. Data is transmitted in blocks of up to 256 bytes with header and tail flag sequences creating packets.
Each packet is constructed from a number of fields as follows:
| Field name | Length | Description |
| start flag | at least 1 byte | repeats of the binary sequence 01111110 |
| address field | at least 3 bytes | call of IRS; call of ISS; up to 8 repeater stations |
| control field | 1 byte | status of connection |
| data field | 0-256 bytes | message data (CCITT ITA No 5 characters) |
| check field | 2 bytes | frame check sequence for error detection |
| end flags | at least 1 byte | repeats of the binary sequence 01111110 |
The experienced ear can hear the start and end flags in AX.25 packets received on a shortwave radio making this digital mode relatively easy to identify. The flags cause periods of cyclic activity to be heard at the beginning and end of each burst.