MagTek MCP - Manual (Page 14)

MCP Serial Transport Protocol
8
Send and receive sequence numbers (N(S), N(R))
The send sequence number, N(S), is the sequence number of the current I-frame sent by a node.
The receive sequence number, N(R), is the sequence number expected in the next received I-
frame. The N(R) sequence number allows a node to embed an acknowledgement (receipt) in an
I-frame.
One bit is sufficient for sequence numbers because a node cannot send a second I-frame until the
first one is acknowledged (the sliding window size is one frame).
Data Field
The data field is not interpreted by the physical transport layer. The contents of this field
depends upon the application message carried within. Application messages are outside the
scope of this document.