<s>
JMODEM	B-Protocol
is	O
a	O
file	B-Protocol
transfer	I-Protocol
protocol	O
developed	O
by	O
Richard	O
Johnson	O
in	O
1988	O
.	O
</s>
<s>
It	O
is	O
similar	O
to	O
the	O
seminal	O
XMODEM	B-Protocol
in	O
most	O
ways	O
,	O
but	O
uses	O
a	O
variable-size	O
packet	O
in	O
order	O
to	O
make	O
better	O
use	O
of	O
the	O
available	O
bandwidth	O
on	O
high-speed	O
modems	B-Application
.	O
</s>
<s>
JMODEM	B-Protocol
uses	O
variable-length	O
records	O
called	O
blocks	O
.	O
</s>
<s>
JMODEM	B-Protocol
also	O
included	O
a	O
basic	O
RLE	B-Algorithm
data	B-General_Concept
compression	I-General_Concept
system	O
,	O
which	O
replaces	O
strings	O
of	O
repeated	O
characters	O
with	O
a	O
counter	O
.	O
</s>
<s>
If	O
a	O
string	O
of	O
many	O
similar	O
characters	O
are	O
found	O
,	O
JMODEM	B-Protocol
sends	O
a	O
"	O
sentinel	O
byte	O
"	O
(	O
hexadecimal	O
0xBB	O
)	O
followed	O
by	O
a	O
two-byte	O
number	O
,	O
followed	O
by	O
the	O
byte	O
to	O
be	O
repeated	O
.	O
</s>
<s>
JMODEM	B-Protocol
applied	O
RLE	B-Algorithm
on	O
a	O
block-by-block	O
basis	O
,	O
as	O
opposed	O
to	O
the	O
file	O
as	O
a	O
whole	O
.	O
</s>
<s>
Since	O
many	O
files	O
were	O
already	O
compressed	O
with	O
systems	O
like	O
.zip	B-General_Concept
,	O
JMODEM	B-Protocol
only	O
used	O
RLE	B-Algorithm
on	O
blocks	O
where	O
it	O
actually	O
reduced	O
the	O
size	O
of	O
the	O
block	O
.	O
</s>
<s>
JMODEM	B-Protocol
is	O
explained	O
in	O
some	O
detail	O
in	O
the	O
John	O
Dvorak	O
book	O
Dvorak	O
's	O
Guide	O
to	O
PC	O
Telecommunications	O
.	O
</s>
