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1.3 ARQ Mode (A-Mode)
Description
The ARQ (Automatic Re-transmission request, or Automatic Re-
quest for repetition) Mode allows private communications between
any two stations using semi-duplex communication. Reception
confirmation is done to assure that each character is received cor-
rectly. Since the two stations (automatically) exchange identities,
this affords some degree of protection for confidential messages.
Traffic Exchange Sequence
In the ARQ mode two stations communicate directly with one an-
other. One station sends information and receives controls signals,
while the other station receives information and sends confirming
control signals. The first station is the ISS (Information Sending
Station), and the second is the IRS (Information Receiving Sta-
tion). These functions are interchangeable by a special control sig-
nal.
The station which initiates the call is the Master Station (MS). The
MS initiates the call by sending the selective identity code of the
called station, consisting of an RQ signal and two traffic informa-
tion signals, listening between blocks.
Example: Identity Code XQKM
X RQ Q K M RQ
Calling Block 1 Calling Block 2
Example: Identity Code PEARDBY
P RQ E RQ A R
Calling Block 1 Calling Block 2
D B Y
Calling Block 3
Figure 1-1 How identity code is transmitted
The Slave Station (SS) recognizes own identity code received and
answers it is ready by sending a control signal. The calling station
then initiates normal traffic.
The ISS sends information in blocks of three characters. Each char-
acter is sent at the rate of 100 bauds, amounting to 70 ms for one
character or 210 ms for one character block. The block repetition
cycle is 450 ms, so there is 240 ms during each cycle that the ISS is