<s>
Smodem	B-Protocol
refers	O
to	O
a	O
bidirectional	O
protocol	B-Protocol
for	I-Protocol
file	I-Protocol
transfer	I-Protocol
used	O
between	O
modems	B-Application
and	O
the	O
DOS	B-Device
program	O
in	O
which	O
the	O
protocol	O
is	O
implemented	O
,	O
both	O
of	O
which	O
were	O
developed	O
by	O
a	O
Finnish	O
company	O
named	O
Arisoft	O
.	O
</s>
<s>
It	O
was	O
mainly	O
used	O
in	O
bulletin	B-Application
board	I-Application
systems	I-Application
because	O
it	O
could	O
transfer	O
files	O
in	O
both	O
directions	O
at	O
the	O
same	O
time	O
and	O
allowed	O
users	O
to	O
chat	O
with	O
each	O
other	O
with	O
AriSoft	O
's	O
GroupChat	O
software	O
.	O
</s>
<s>
Other	O
popular	O
bidirectional	O
protocols	O
such	O
as	O
BiModem	B-Protocol
,	O
HS/Link	B-Protocol
and	O
HydraCom	O
also	O
offered	O
a	O
chat	O
option	O
with	O
the	O
operator	O
,	O
but	O
not	O
with	O
the	O
system	O
's	O
other	O
users	O
.	O
</s>
<s>
Unlike	O
ZMODEM	B-Device
and	O
other	O
older	O
protocols	O
,	O
SMODEM	B-Protocol
has	O
a	O
separate	O
low-level	O
multiplexed	O
transfer	O
layer	O
(	O
MSLP	O
)	O
and	O
a	O
high-level	O
file	O
transfer	O
layer	O
.	O
</s>
<s>
The	O
protocol	O
is	O
symmetrical	O
and	O
can	O
connect	O
with	O
itself	O
in	O
loop-back	B-Protocol
mode	O
.	O
</s>
<s>
SMODEM	B-Protocol
detects	O
DESQview	B-Operating_System
and	O
DPMI	O
(	O
Windows	B-Application
,	O
OS/2	B-Application
)	O
and	O
releases	O
all	O
unused	O
processor	O
capacity	O
to	O
the	O
host	O
system	O
.	O
</s>
<s>
It	O
is	O
written	O
in	O
the	O
C	O
language	O
and	O
compiled	O
with	O
Microsoft	O
Visual	O
C++	O
v1.0	O
compiler	O
,	O
optimized	O
for	O
an	O
80286	B-General_Concept
CPU	O
.	O
</s>
<s>
The	O
packet	O
frame	O
contains	O
one	O
byte	O
for	O
the	O
channel	O
number	O
and	O
two	O
bytes	O
for	O
a	O
16-bit	O
CCITT(ITU )	O
-CRC	O
,	O
using	O
the	O
same	O
algorithm	O
as	O
in	O
error	O
correcting	O
modems	B-Application
.	O
</s>
<s>
If	O
the	O
packet	O
size	O
is	O
limited	O
to	O
256	O
bytes	O
,	O
the	O
efficiency	O
is	O
98.1	O
%	O
,	O
which	O
is	O
still	O
better	O
than	O
original	O
Zmodem	B-Device
.	O
</s>
