DUAL - An Unreliable Amateur Link Format

DUAL, the Digital Unreliable Amateur Link format, provides an amateur link layer with:

• a simple logical link control mechanism for the unreliable transmission and reception of link layer packets, and

• a standard but flexible format for these packets.

DUAL sits as the basic Logical Link Control layer in the overall Link Layer, as shown in the following diagram.

Figure : DUAL within the Link Layer

Below DUAL is the Medium Access Layer, which controls access to the link and performs other tasks such as bit/byte-stuffing. At the bottom is the Physical Layer, which transmits the data over the medium using some form of modulation, and perhaps with some data encoding.

DUAL is designed to accept packets from higher layers, and to pass them on to the appropriate MAC and Physical layers for encoding and eventual transmission. Similarly, it accepts packets from MAC layers and passes them on to the appropriate higher layer. Although DUAL will most often be used with a single higher layer and MAC layer, the following scenario is quite possible.

Figure : An Example Protocol Stack with DUAL

Here DUAL has access to two MAC layers, a CSMA-style one like that used in AX.25, and ALOHA. Below that are three physical interfaces which can transmit data. Above DUAL are three Network layers, IP, NETROM and a layer which provides AX.25-style virtual circuit connections.