MagTek MCP - Manual (Page 15)

Section 3. Transport Frame Types
9
R-FRAMES
This section defines the R-frames. An R-frame (receipt frame) is used for the following
purposes:
• for conveying positive or negative acknowledgement about received I-frames
• for requesting a receipt from another node (used for error recovery)
R-frames must always use the 8-bit LRC error detection method for the entire frame.
This section defines the PCB and data fields of the R-frame.
PCB of R-frames
Below is the diagram of the PCB field in the frame header of R-frames:
7 6 5 4 3 2 1 0
FT FT
1 1
POLL RFU RFU RFU RFU N(R)
MSB LSB
• FT
Frame Type (2 bits) = 11
• POLL Poll indicator (1 bit)
• N(R)
Receive sequence number (1 bit)
Frame Type (FT)
The R-frame is designated as frame type 11 (two most significant bits in PCB).
Poll Indicator (POLL)
The poll indicator specifies whether the sender of the R-frame is requesting a receipt or just
receipting a previously received I-frame.
When the poll indicator bit is clear, the R-frame indicates what the expected sequence number is
for the next frame. These are used to acknowledge received I-frames.
When the poll indicator bit is set, the R-frame in addition requests that the recipient send what its
next expected sequence number is. R-frames with poll indicator are used for error recovery
when the sender’s receipt timeout expires. When a node receives an R-frame with the poll
indicator set, it should respond with an R-frame (usually with poll indicator clear); the node can
also respond with an I-frame. In both cases the node includes its next expected sequence
number, which allows the sender to decide whether it needs to resend its last I-frame.