AX.25 [WB4JFI 84], the current Amateur Link Protocol, has many deficiencies, which DUAL attempts to address. 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. The receiving entity takes no remedial actions; if the frame is in error, due to transmission noise, it is discarded, and the receiver is not informed about the loss of the traffic. Error recovery is a function of upper-layer protocols, like TCP.
DUAL is configured to support connectionless-mode operations. This mode dose not set up a logical link connection; instead it manages the link layer frames as independent and separate entities. No relationship is maintained between successive data transfer, and moreover, ACKs and NACKs are not provided. Error checking is performed on the frame by the Checksum field.
Since DUAL is connectionless, there will be no requests for
retransmissions of bad frames, and collision may also occur, with the
potential of losing the frames that collided
.
Because of the connectionless nature of DUAL the following procedures used in the AX.25 protocol are not necessary:
These are part of the reliable virtual circuit capabilities found in AX.25, which have been removed to minimise the header size in DUAL. The address field operation is different in DUAL which has been explained in section 6.