<s>
Janus	B-Device
is	O
a	O
file	B-Protocol
transfer	I-Protocol
protocol	I-Protocol
for	O
use	O
on	O
bulletin	B-Application
board	I-Application
systems	I-Application
(	O
BBSs	O
)	O
.	O
</s>
<s>
It	O
was	O
written	O
by	O
Rick	O
Huebner	O
in	O
1987	O
;	O
Huebner	O
had	O
previously	O
written	O
a	O
ZMODEM	B-Device
module	O
for	O
the	O
Opus-CBBS	O
system	O
.	O
</s>
<s>
Using	O
Janus	B-Device
,	O
Opus	O
BBS	O
systems	O
could	O
save	O
time	O
exchanging	O
files	O
like	O
FidoNet	O
message	O
packets	O
in	O
both	O
directions	O
at	O
the	O
same	O
time	O
,	O
which	O
Huebner	O
described	O
as	O
sending	O
the	O
shorter	O
file	O
for	O
free	O
.	O
</s>
<s>
Janus	B-Device
was	O
useful	O
in	O
settings	O
where	O
the	O
upstream	O
and	O
downstream	O
links	O
were	O
both	O
of	O
similar	O
performance	O
,	O
which	O
was	O
true	O
in	O
the	O
mid-1980s	O
when	O
most	O
high-speed	O
modems	B-Application
operated	O
at	O
2,400	O
bps	O
bidirectionally	O
.	O
</s>
<s>
However	O
,	O
the	O
protocol	O
was	O
introduced	O
almost	O
at	O
the	O
same	O
time	O
as	O
the	O
rapid	O
popularization	O
of	O
the	O
USRobotics	O
HST	O
standard	O
with	O
a	O
9,600	O
bps	O
download	O
and	O
only	O
300bps	O
upload	O
,	O
where	O
Janus	B-Device
offered	O
little	O
or	O
no	O
advantage	O
.	O
</s>
<s>
When	O
higher-speed	O
bidirectional	O
modems	B-Application
appeared	O
,	O
like	O
v.32bis	O
,	O
Janus	B-Device
had	O
already	O
largely	O
disappeared	O
.	O
</s>
<s>
Janus	B-Device
is	O
a	O
packet-oriented	O
protocol	O
like	O
XMODEM	B-Protocol
and	O
similar	O
systems	O
.	O
</s>
<s>
Janus	B-Device
is	O
typical	O
of	O
late	O
1980s	O
protocols	O
that	O
attempt	O
to	O
make	O
better	O
use	O
of	O
increasing	O
modem	B-Application
speeds	O
.	O
</s>
<s>
XMODEM	B-Protocol
transferred	O
only	O
128	O
bytes	O
of	O
data	O
per	O
packet	O
,	O
and	O
then	O
waited	O
for	O
a	O
response	O
from	O
the	O
receiver	O
before	O
sending	O
the	O
next	O
packet	O
.	O
</s>
<s>
Due	O
to	O
the	O
latency	O
of	O
the	O
phone	O
network	O
,	O
the	O
minimum	O
possible	O
time	O
to	O
receive	O
this	O
acknowledgement	O
represented	O
a	O
significant	O
percentage	O
of	O
the	O
time	O
it	O
needed	O
to	O
send	O
the	O
next	O
packet	O
,	O
leading	O
to	O
relatively	O
low	O
channel	O
usage	O
on	O
faster	O
modems	B-Application
.	O
</s>
<s>
The	O
solution	O
to	O
this	O
problem	O
used	O
in	O
Janus	B-Device
was	O
to	O
allow	O
the	O
packet	O
size	O
to	O
vary	O
,	O
from	O
0	O
to	O
2052	O
bytes	O
per	O
packet	O
.	O
</s>
<s>
Additionally	O
,	O
Janus	B-Device
did	O
not	O
stop	O
and	O
wait	O
for	O
the	O
receiver	O
to	O
acknowledge	O
the	O
reception	O
of	O
the	O
packet	O
,	O
it	O
simply	O
assumed	O
it	O
was	O
received	O
correctly	O
and	O
immediately	O
began	O
sending	O
the	O
next	O
packet	O
.	O
</s>
<s>
The	O
basic	O
Janus	B-Device
packet	O
structure	O
consisted	O
of	O
the	O
following	O
pattern	O
:	O
</s>
<s>
The	O
CRC	O
was	O
a	O
16-bit	O
cyclic	O
redundancy	O
check	O
,	O
the	O
same	O
applied	O
to	O
XMODEM-CRC	O
(	O
and	O
its	O
variations	O
)	O
.	O
</s>
<s>
In	O
contrast	O
to	O
most	O
other	O
protocols	O
,	O
Janus	B-Device
used	O
a	O
32-bit	O
offset	O
within	O
the	O
file	O
to	O
identify	O
packets	O
,	O
as	O
opposed	O
to	O
XMODEM-derived	O
protocols	O
which	O
used	O
an	O
internal	O
packet	O
number	O
which	O
was	O
typically	O
a	O
monotonically	O
increasing	O
integer	O
.	O
</s>
<s>
An	O
error	O
in	O
Janus	B-Device
would	O
cause	O
the	O
receiver	O
to	O
send	O
a	O
BADPKT	O
packet	O
with	O
the	O
file	O
offset	O
to	O
be	O
sent	O
back	O
,	O
allowing	O
the	O
system	O
to	O
send	O
any	O
packet	O
out-of-order	O
at	O
any	O
time	O
.	O
</s>
<s>
Unlike	O
other	O
protocols	O
,	O
Janus	B-Device
did	O
not	O
have	O
pre-defined	O
limits	O
on	O
the	O
number	O
of	O
failures	O
that	O
would	O
cause	O
a	O
transfer	O
to	O
fail	O
,	O
transfers	O
only	O
ended	O
when	O
one	O
side	O
or	O
the	O
other	O
explicitly	O
sent	O
a	O
HALTPKT	O
.	O
</s>
<s>
The	O
packet	O
size	O
in	O
Janus	B-Device
was	O
selected	O
dynamically	O
based	O
on	O
the	O
number	O
of	O
transmission	O
errors	O
and	O
the	O
time	O
to	O
send	O
a	O
packet	O
.	O
</s>
<s>
This	O
was	O
the	O
same	O
algorithm	O
Huebner	O
had	O
developed	O
for	O
his	O
version	O
of	O
ZMODEM	B-Device
used	O
in	O
the	O
Opus	O
BBS	O
system	O
.	O
</s>
<s>
This	O
algorithm	O
was	O
later	O
back-ported	O
into	O
the	O
ZMODEM	B-Device
standard	O
by	O
Chuck	O
Foresberg	O
.	O
</s>
