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Figure 6.52
Frame transmission due to CRC error detection.
last correctly received frame. The transmitting station must retransmit the last data frame, as shown in Figure 6.54.
The receiving station never responds. It is possible that the receiving station is momentarily off-line or simply too busy to respond to
the poll. To accommodate situations such as these, the transmitting station will wait for 3 seconds and poll again. If, after three such
cycles, no response is obtained, the transmitting station aborts the transmission.
There are a number of variations and exceptions to the time and number of repeats the transmitting station performs. In some
installations, for example, the number of polls can be greater to accommodate transmission delays, or the time between polls can be
increased.
Other responses. There are other responses allowed by Bisync. For example, should the receiving station be unable to accept more data
frames because of problems such as no paper in the line printer, the communication channel can be kept active by the receiving station
responding with WACKs (wait acknowledgments). The transmitting station continues polling (with ENQs) until an acknowledgment is
received.
Although there are exceptions (for example, a station with only a line printer or a station with only a card reader), most Bisync
implementations allow either station to transmit and receive information. Bisync requires the station about to transmit to send an ENQ
character, and the potential receiving station is to respond with an ACK 0, after which the data transfer may proceed. In Figure 6.55,
station 1 is bidding for the line.
Figure 6.53
The correct acknowledgment signals transmission of the next frame.
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