3
The data is passed from the terminal software
of the PC to the 3212 modem. It is processed
by the 3212 modem and then send to the CO-
DAN transceiver for transfer via HF to the
transceivers in the communication system. At
the receiving node, the data is transferred
from the transceiver to the 3212 modem,
where it is decoded, and then sent to the ter-
minal software of the PC. In order to send data
between multiple modems, the corresponding
transceivers need to be set to the same chan-
nel. A front panel LED indicator shows pres-
ence of power, modem link status and
throughput of the transmitted or received da-
ta
.
Prior to sending data between two modems, a
proprietary ALE (automatic link establishment)
will provide automatic channel selection. CO-
DAN modems operate under full-duplex mode,
so they act as RS232 (serial communication
transmission of data) link between two sta-
tions once a link is established. The data be-
tween two or more CODAN-3212 modems can
either be sent by a peer-to-peer communica-
tion (Selective Repeat ARQ protocol) or by a
broadcast communication (Non-ARQ protocol).
The Selective Repeat ARQ (Selective Automatic
Repeat Request) is a protocol for error detec-
tion and error correction in a data transmission
and is a specific instance of the ARQ protocol.
In a Selective Repeat ARQ transmission, the
transmitting node needs an explicit confirma-
tion that the data was received at the receiving
node. When the receiver detects an error in a
received frame, it automatically requests the
transmitter to resend the frame, whereas in
other forms of the ARQ protocol, every frame
from that point must be sent again. This pro-
cess repeats until the packet is error free or
the error continues beyond a predefined num-
ber of transmissions and thus the communica-
tion breaks down on poor conditions. In a Non-
ARQ transmission, the transmitting node sends
packets to the receiving node, without explicit
confirmation that the data was received at the
receiving node. In a peer-to-peer communica-
tion, only two modems are involved (sender
and receiver), whereas in a broadcast commu-
nication all modems within a certain address
range can be addressed. The setup of a peer-to
-peer and a broadcast communication is illus-
trated in Figure 2. Error-free transmission is
guaranteed for peer-to-peer communications,
but not for broadcast communications in diffi-
cult transmission conditions without the possi-
bility of repetition (ARQ). Additionally, in a
peer-to-peer communication the data can be
sent using a compression, which is not possible
in the broadcast mode because of the unrelia-
ble communication conditions. A lossless com-
pression format is used for compressing CO-
DAN-3212 data stream with arbitrarily length.
Advanced Protocols
CODAN HF Data Decoder
