<s>
ZMODEM	B-Device
is	O
an	O
inline	O
file	B-Protocol
transfer	I-Protocol
protocol	I-Protocol
developed	O
by	O
Chuck	O
Forsberg	O
in	O
1986	O
,	O
in	O
a	O
project	O
funded	O
by	O
Telenet	O
in	O
order	O
to	O
improve	O
file	O
transfers	O
on	O
their	O
X.25	B-Protocol
network	O
.	O
</s>
<s>
In	O
addition	O
to	O
dramatically	O
improved	O
performance	O
compared	O
to	O
older	O
protocols	O
,	O
ZMODEM	B-Device
offered	O
restartable	O
transfers	O
,	O
auto-start	O
by	O
the	O
sender	O
,	O
an	O
expanded	O
32-bit	O
CRC	O
,	O
and	O
control	B-Algorithm
character	I-Algorithm
quoting	I-Algorithm
supporting	O
8-bit	O
clean	O
transfers	O
,	O
allowing	O
it	O
to	O
be	O
used	O
on	O
networks	O
that	O
would	O
not	O
pass	O
control	O
characters	O
.	O
</s>
<s>
In	O
contrast	O
to	O
most	O
transfer	B-Protocol
protocols	I-Protocol
developed	O
for	O
bulletin	B-Application
board	I-Application
systems	I-Application
(	O
BBSs	O
)	O
,	O
ZMODEM	B-Device
was	O
not	O
directly	O
based	O
on	O
,	O
nor	O
compatible	O
with	O
,	O
the	O
seminal	O
XMODEM	B-Protocol
.	O
</s>
<s>
Many	O
variants	O
of	O
XMODEM	B-Protocol
had	O
been	O
developed	O
in	O
order	O
to	O
address	O
one	O
or	O
more	O
of	O
its	O
shortcomings	O
,	O
and	O
most	O
remained	O
backward	B-General_Concept
compatible	I-General_Concept
and	O
would	O
successfully	O
complete	O
transfers	O
with	O
"	O
classic	O
"	O
XMODEM	B-Protocol
implementations	O
.	O
</s>
<s>
This	O
list	O
includes	O
Forsberg	O
's	O
own	O
YMODEM	B-Protocol
.	O
</s>
<s>
ZMODEM	B-Device
eschewed	O
backward	B-General_Concept
compatibility	I-General_Concept
in	O
favor	O
of	O
producing	O
a	O
radically	O
improved	O
protocol	O
.	O
</s>
<s>
It	O
performed	O
as	O
well	O
or	O
better	O
than	O
any	O
of	O
the	O
high-performance	O
varieties	O
of	O
XMODEM	B-Protocol
,	O
did	O
so	O
over	O
links	O
that	O
previously	O
did	O
n't	O
work	O
at	O
all	O
,	O
like	O
X.25	B-Protocol
,	O
or	O
had	O
poor	O
performance	O
,	O
like	O
Telebit	O
modems	B-Application
,	O
and	O
included	O
useful	O
features	O
found	O
in	O
few	O
or	O
no	O
other	O
protocols	O
.	O
</s>
<s>
ZMODEM	B-Device
became	O
extremely	O
popular	O
on	O
bulletin	B-Application
board	I-Application
systems	I-Application
(	O
BBS	B-Application
)	O
in	O
the	O
early	O
1990s	O
,	O
becoming	O
a	O
standard	O
as	O
widespread	O
as	O
XMODEM	B-Protocol
had	O
been	O
before	O
it	O
.	O
</s>
<s>
Generally	O
,	O
file	B-Protocol
transfer	I-Protocol
protocols	I-Protocol
break	O
down	O
a	O
file	O
into	O
a	O
series	O
of	O
packets	O
,	O
and	O
then	O
send	O
them	O
one-at-a-time	O
to	O
the	O
receiver	O
.	O
</s>
<s>
After	O
the	O
payload	O
comes	O
a	O
checksum	B-Algorithm
or	O
cyclic	O
redundancy	O
check	O
(	O
CRC	O
)	O
that	O
can	O
be	O
used	O
to	O
determine	O
if	O
the	O
payload	O
was	O
received	O
correctly	O
.	O
</s>
<s>
As	O
modem	B-Application
speeds	O
increase	O
,	O
this	O
delay	O
represents	O
a	O
larger	O
and	O
larger	O
number	O
of	O
packets	O
that	O
could	O
have	O
been	O
sent	O
during	O
the	O
delay	O
,	O
decreasing	O
the	O
channel	O
efficiency	O
.	O
</s>
<s>
XMODEM	B-Protocol
used	O
128-bytes	O
payloads	O
with	O
a	O
three-byte	O
header	O
and	O
one-byte	O
checksum	B-Algorithm
for	O
a	O
total	O
of	O
132	O
bytes	O
per	O
packet	O
.	O
</s>
<s>
In	O
the	O
era	O
of	O
300bit/s	O
modems	B-Application
,	O
a	O
packet	O
took	O
about	O
four	O
seconds	O
to	O
send	O
,	O
and	O
typical	O
latencies	O
were	O
on	O
the	O
order	O
of	O
of	O
a	O
second	O
,	O
so	O
the	O
performance	O
overhead	O
was	O
not	O
significant	O
.	O
</s>
<s>
A	O
number	O
of	O
new	O
versions	O
of	O
XMODEM	B-Protocol
with	O
sliding	O
window	O
support	O
appeared	O
in	O
the	O
early	O
1980s	O
.	O
</s>
<s>
Sliding	O
windows	O
are	O
useful	O
for	O
latencies	O
on	O
the	O
order	O
of	O
several	O
packet	O
lengths	O
,	O
which	O
is	O
the	O
case	O
for	O
XMODEM	B-Protocol
on	O
conventional	O
phone	O
lines	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
not	O
enough	O
to	O
address	O
longer	O
latencies	O
found	O
on	O
overseas	O
phone	O
calls	O
or	O
X.25	B-Protocol
services	O
such	O
as	O
PC	O
Pursuit	O
,	O
where	O
the	O
latencies	O
are	O
on	O
the	O
order	O
of	O
a	O
second	O
or	O
longer	O
.	O
</s>
<s>
In	O
other	O
cases	O
,	O
where	O
the	O
reverse	O
channel	O
was	O
much	O
slower	O
than	O
the	O
sending	O
one	O
,	O
as	O
was	O
the	O
case	O
for	O
Telebit	O
or	O
US	O
Robotics	O
modems	B-Application
,	O
even	O
the	O
small	O
number	O
of	O
s	O
might	O
overwhelm	O
the	O
return	O
channel	O
and	O
cause	O
the	O
transfer	O
to	O
pause	O
.	O
</s>
<s>
ZMODEM	B-Device
addressed	O
these	O
problems	O
by	O
removing	O
the	O
need	O
for	O
s	O
at	O
all	O
,	O
allowing	O
the	O
sender	O
to	O
send	O
data	O
continually	O
as	O
long	O
as	O
the	O
receiver	O
detected	O
no	O
errors	O
.	O
</s>
<s>
Since	O
ZMODEM	B-Device
was	O
often	O
used	O
on	O
links	O
with	O
built-in	O
error	O
correction	O
,	O
like	O
X.25	B-Protocol
,	O
the	O
receiver	O
would	O
often	O
not	O
send	O
a	O
single	O
message	O
back	O
to	O
the	O
sender	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
system	O
would	O
send	O
the	O
entire	O
file	O
in	O
a	O
continual	O
stream	O
,	O
and	O
ZMODEM	B-Device
referred	O
to	O
itself	O
as	O
a	O
"	O
streaming	O
protocol	O
"	O
.	O
</s>
<s>
ZMODEM	B-Device
's	O
performance	O
was	O
so	O
improved	O
over	O
previous	O
common	O
protocols	O
that	O
it	O
generally	O
replaced	O
even	O
special	O
protocols	O
such	O
as	O
YMODEM-g	B-Protocol
,	O
which	O
included	O
no	O
error	O
correction	O
at	O
all	O
and	O
instead	O
relied	O
on	O
error-free	O
links	O
maintained	O
by	O
the	O
modems	B-Application
.	O
</s>
<s>
Although	O
YMODEM-g	B-Protocol
was	O
faster	O
,	O
the	O
lack	O
of	O
other	O
features	O
such	O
as	O
restartable	O
transfers	O
made	O
it	O
less	O
appealing	O
.	O
</s>
<s>
XMODEM	B-Protocol
,	O
and	O
most	O
protocols	O
based	O
on	O
it	O
,	O
managed	O
packet	O
order	O
by	O
prefixing	O
the	O
data	O
with	O
a	O
packet	O
number	O
from	O
1	O
to	O
255	O
.	O
</s>
<s>
ZMODEM	B-Device
replaced	O
the	O
packet	O
number	O
with	O
the	O
actual	O
location	O
in	O
the	O
file	O
,	O
indicated	O
by	O
a	O
32-bit	O
number	O
.	O
</s>
<s>
A	O
number	O
of	O
modified	O
versions	O
of	O
ZMODEM	B-Device
appeared	O
.	O
</s>
<s>
ZedZap	B-Device
was	O
a	O
variant	O
of	O
ZMODEM	B-Device
with	O
8	O
kbyte	O
blocks	O
for	O
better	O
performance	O
on	O
high-speed	O
modems	B-Application
.	O
</s>
<s>
LeechZmodem	B-Protocol
was	O
a	O
mischievous	O
ZMODEM	B-Device
variant	O
(	O
among	O
similar	O
XMODEM	B-Protocol
and	O
YMODEM	B-Protocol
derivatives	O
)	O
that	O
cheated	O
BBS	B-Application
download	O
quotas	O
.	O
</s>
<s>
A	O
backwards	B-General_Concept
compatible	I-General_Concept
extension	O
of	O
ZMODEM	B-Device
with	O
32	O
kbyte	O
and	O
64	O
kbyte	O
block	O
lengths	O
was	O
created	O
by	O
ADONTEC	O
in	O
2002	O
and	O
2007	O
to	O
increase	O
performance	O
on	O
high-speed	O
error	O
free	O
connections	O
like	O
ISDN	O
or	O
TCP/IP	O
networks	O
.	O
</s>
<s>
The	O
most	O
notable	O
ZMODEM	B-Device
implementations	O
were	O
from	O
Chuck	O
Forsberg	O
's	O
Omen	O
Technology	O
,	O
Inc	O
.	O
</s>
<s>
These	O
included	O
DSZ	O
(	O
DOS	O
Send	O
ZMODEM	B-Device
)	O
,	O
GSZ	O
(	O
Graphical	O
Send	O
ZMODEM	B-Device
)	O
,	O
and	O
the	O
ubiquitous	O
(	O
l	O
)	O
rzsz	O
for	O
Unix	O
variants	O
.	O
</s>
<s>
In	O
more	O
current	O
times	O
,	O
the	O
developers	O
of	O
Synchronet	B-Protocol
have	O
created	O
a	O
modern	O
X/Y/ZMODEM	O
implementation	O
named	O
SEXYZ	O
,	O
loosely	O
based	O
on	O
the	O
zmtx/zmrx	O
package	O
,	O
which	O
runs	O
natively	O
on	O
Windows	O
and	O
Unix	O
variants	O
,	O
supports	O
long	O
filenames	O
and	O
faster	O
,	O
more	O
reliable	O
data	O
transfers	O
.	O
</s>
<s>
The	O
ZMODEM	B-Device
implementation	O
from	O
SEXYZ	O
has	O
also	O
been	O
incorporated	O
into	O
the	O
SyncTERM	B-Protocol
project	O
.	O
</s>
<s>
Synchronet	B-Protocol
,	O
SEXYZ	O
,	O
and	O
SyncTERM	B-Protocol
are	O
all	O
open-source	O
,	O
cross-platform	O
,	O
BBS-centric	O
projects	O
.	O
</s>
<s>
Forsberg	O
himself	O
collected	O
a	O
number	O
of	O
improvements	O
into	O
ZMODEM-90	O
.	O
</s>
<s>
Even	O
on	O
networks	O
that	O
"	O
eat	O
"	O
control	O
characters	O
,	O
ZMODEM-90	O
can	O
be	O
tailored	O
to	O
quote	O
only	O
those	O
characters	O
the	O
network	O
actually	O
eats	O
,	O
as	O
opposed	O
to	O
every	O
possible	O
one	O
.	O
</s>
<s>
A	O
similar	O
improvement	O
allows	O
ZMODEM-90	O
to	O
work	O
on	O
7-bit	O
networks	O
,	O
whereas	O
earlier	O
protocols	O
(	O
with	O
the	O
notable	O
exception	O
of	O
Kermit	B-Protocol
)	O
had	O
all	O
demanded	O
8-bits	O
to	O
one	O
degree	O
or	O
another	O
.	O
</s>
<s>
Finally	O
,	O
ZMODEM-90	O
includes	O
a	O
basic	O
run-length	B-Algorithm
encoding	I-Algorithm
compression	O
system	O
to	O
further	O
improve	O
performance	O
on	O
uncompressed	O
files	O
.	O
</s>
<s>
Some	O
of	O
the	O
ZMODEM	B-Device
packets	O
(	O
e.g.	O
</s>
<s>
This	O
design	O
limits	O
the	O
feasibility	O
of	O
ZMODEM	B-Device
to	O
only	O
reliably	O
transfer	O
files	O
that	O
are	O
under	O
4GB	O
in	O
size	O
.	O
</s>
