MagTek MCP - Manual (Page 31)

Section 4. Protocol
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GENERAL PROCEDURES
During the protocol, any node may send a frame at any time, regardless of whether it has
received a frame or not. The frame may either be an I-frame, R-frame or S-frame.
Every I-frame carries its send sequence number (N(S)). The I-frames sent by any two nodes are
counted independently of each other. N(S) is counted modulo 2 and is coded by one bit. At the
start of the protocol or after resynchronization, the initial value is N(S) = 0; then the value
alternates after sending each I-frame.
Every I-frame and R-frame carries N(R) which is the send sequence number N(S) of the next
expected I-frame. I-frames and R-frames are used to acknowledge that a frame has been
received and indicates readiness to receive the next frame.
A sent I-frame is acknowledged by receiving:
• either an I-frame where N(R) is different than N(S) of the previously received I-frame
• or an R-frame where N(R) is different than N(S) of the sent I-frame
S-frames carry no sequence numbers; an S-frame request (S(… request)) is acknowledged by
receiving an S-frame response (S(… response)).
ERROR HANDLING
The transport layer should be able to handle the following errors:
• Block Wait Timeout (BWT)
• Character Wait Timeout (CWT)
• Reception of a damaged frame due to an EDC error
• Reception of an invalid frame, examples are: unsupported EDC type, unsupported
frame type, etc.