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BOF A Chapter 4 / Infrared Communications Overview EOF
C
FCS
I
SIR Coding
This topic describes how IrDA miniport drivers or their IR NICs code
frames for transmission at Serial IrDA (SIR) link speeds. The SIR spec-
ification defines a short-range IR asynchronous serial transmission
mode with one start bit, eight data bits, and one stop bit. The maxi-
mum data rate is 115.2 Kb/s (half duplex). This SIR coding scheme is
called return to zero, inverted (RZI). The primary benefit of coding
frames for SIR speeds is that existing serial hardware can be used very
cheaply. The low cost of using serial hardware is one of the reasons for
the widespread availability of IR SIR devices.
The BOF flag for SIR speeds is defined as 0xC0. The EOF value is
defined as 0xC1. To avoid ambiguity in a frame that contains BOF and
EOF, an escape sequence is defined for values of 0xC0 and 0xC1 that
occur in other parts of the frame. The escape character is defined as
0x7D.
For each byte that the transmitter encounters that is the same as a BOF,
EOF, or the escape character, the transmitter performs the following
steps:
1. Inserts a control-escape byte (0x7D) preceding such a byte.

2. Complements bit five of each byte that is the same as the BOF,
EOF, or escape character (i.e., performs an exclusive OR opera-
tion on such a byte with 0x20).

MIR Coding

This topic describes how IrDA miniport drivers and their IR NICs code
frames for transmission at Medium IrDA (MIR) link speeds. The MIR
data rates are 0.576 Mb/s and 1.152 Mb/s (half duplex).
For MIR link speeds, definitions for BOF and EOF values are the same;
both BOF and EOF are defined as 0x7E. To avoid ambiguity in the
frame with BOF and EOF, rather than using an escape sequence as is
done at SIR rates, a zero is inserted at MIR rates after any five consec-
utive one bits in all members, except BOF and EOF. Because the
process of inserting and stripping zeros at the bit level is highly

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