Binary Synchronous Communications - Link Control

Link Control

The link control protocol is similar to STR. The designers attempted to protect against simple transmission errors. The protocol requires that every message be acknowledged (ACK0/ACK1) or negatively acknowledged (NAK), so transmission of small packets has high transmission overhead. The protocol can recover from a corrupted data frame, a lost data frame, and a lost acknowledgment.

Error recovery is by retransmission of the corrupted frame. Since Bisync data packets are not serial-numbered, it's considered possible for a data frame to go missing without the receiver realizing it. Therefore, alternating ACK0s and ACK1s are deployed; if the transmitter receives the wrong ACK, it can assume a data packet (or an ACK) went missing. A potential flaw is that corruption of ACK0 into ACK1 could result in duplication of a data frame.

Error protection for ACK0 and ACK1 is weak. The Hamming distance between the two messages is only two bits.

The protocol is half-duplex (2-wire). In this environment, packets or frames of transmission are strictly unidirectional, necessitating 'turn-around' for even the simplest purposes, such as acknowledgments. Turn-around involves

  • the reversal of transmission direction,
  • quiescing of line echo,
  • resyncing.

In a 2-wire environment, this causes a noticeable round-trip delay and reduces performance.

Some datasets support full-duplex operation, and full-duplex (4-wire) can be used in many circumstances to improve performance by eliminating the turn-around time, at the added expense of 4-wire installation and support. In typical full-duplex, data packets are transmitted along one wire pair while the acknowledgements are returned along the other.

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