<s>
YMODEM	B-Protocol
is	O
a	O
file	B-Protocol
transfer	I-Protocol
protocol	I-Protocol
used	O
between	O
microcomputers	B-Architecture
connected	O
together	O
using	O
modems	B-Application
.	O
</s>
<s>
It	O
was	O
primarily	O
used	O
to	O
transfer	O
files	O
to	O
and	O
from	O
bulletin	B-Application
board	I-Application
systems	I-Application
.	O
</s>
<s>
YMODEM	B-Protocol
was	O
developed	O
by	O
Chuck	O
Forsberg	O
as	O
an	O
expansion	O
of	O
XMODEM	B-Protocol
and	O
was	O
first	O
implemented	O
in	O
his	O
CP/M	B-Application
YAM	O
program	O
.	O
</s>
<s>
Initially	O
also	O
known	O
as	O
YAM	O
,	O
it	O
was	O
formally	O
given	O
the	O
name	O
"	O
YMODEM	B-Protocol
"	O
in	O
1985	O
by	O
Ward	O
Christensen	O
,	O
author	O
of	O
the	O
original	O
XMODEM	B-Protocol
.	O
</s>
<s>
YMODEM	B-Protocol
extended	O
XMODEM	B-Protocol
in	O
three	O
ways	O
,	O
combining	O
features	O
found	O
in	O
other	O
extended	O
XMODEM	B-Protocol
varieties	O
.	O
</s>
<s>
Like	O
XMODEM-CRC	O
,	O
YMODEM	B-Protocol
replaced	O
the	O
8-bit	O
checksum	B-Algorithm
with	O
a	O
16-bit	O
cyclic	O
redundancy	O
check	O
(	O
CRC	O
)	O
,	O
but	O
made	O
it	O
the	O
default	O
form	O
of	O
correction	O
instead	O
of	O
optional	O
.	O
</s>
<s>
From	O
TeLink	O
it	O
added	O
the	O
"	O
block	O
0	O
"	O
header	O
that	O
sent	O
the	O
filename	O
and	O
size	O
,	O
which	O
allowed	O
batch	O
transfers	O
(	O
multiple	O
files	O
in	O
a	O
single	O
session	O
)	O
and	O
eliminated	O
the	O
need	O
to	O
add	O
padding	B-Application
at	O
the	O
end	O
of	O
the	O
file	O
.	O
</s>
<s>
Finally	O
,	O
YMODEM	B-Protocol
allowed	O
the	O
block	O
size	O
to	O
be	O
increased	O
from	O
the	O
original	O
128	O
bytes	O
of	O
data	O
to	O
1024	O
,	O
as	O
in	O
XMODEM-1k	B-Protocol
,	O
which	O
greatly	O
improved	O
throughput	O
on	O
faster	O
modems	B-Application
.	O
</s>
<s>
Forsberg	O
built	O
the	O
standard	O
with	O
all	O
of	O
these	O
features	O
as	O
runtime	O
options	O
,	O
allowing	O
a	O
single	O
protocol	O
driver	O
to	O
fall	O
back	O
to	O
XMODEM-CRC	O
or	O
even	O
XMODEM	B-Protocol
when	O
connecting	O
to	O
non-YAM	O
systems	O
.	O
</s>
<s>
He	O
was	O
dismayed	O
to	O
find	O
that	O
the	O
majority	O
of	O
implementations	O
were	O
actually	O
providing	O
nothing	O
more	O
than	O
1k	O
block	O
size	O
with	O
CRC-16	O
,	O
failing	O
to	O
implement	O
the	O
"	O
block	O
0	O
"	O
while	O
continuing	O
to	O
use	O
the	O
YMODEM	B-Protocol
name	O
.	O
</s>
<s>
The	O
result	O
was	O
the	O
release	O
of	O
many	O
mutually	O
incompatible	O
YMODEM	B-Protocol
implementations	O
,	O
and	O
the	O
use	O
of	O
the	O
name	O
YMODEM	B-Protocol
Batch	O
to	O
clearly	O
indicate	O
those	O
versions	O
that	O
did	O
support	O
the	O
complete	O
standard	O
.	O
</s>
<s>
The	O
original	O
XMODEM	B-Protocol
was	O
a	O
very	O
simple	O
protocol	O
and	O
that	O
is	O
the	O
reason	O
for	O
its	O
success	O
;	O
it	O
could	O
be	O
implemented	O
on	O
practically	O
any	O
machine	O
of	O
the	O
era	O
,	O
even	O
those	O
with	O
very	O
limited	O
processors	O
and	O
storage	O
.	O
</s>
<s>
It	O
worked	O
by	O
breaking	O
up	O
the	O
data	O
to	O
be	O
sent	O
into	O
128-byte	O
packets	B-Protocol
,	O
adding	O
a	O
3-byte	O
header	O
and	O
1-byte	O
checksum	B-Algorithm
footer	O
,	O
and	O
sending	O
the	O
resulting	O
132-byte	O
packets	B-Protocol
out	O
in	O
order	O
.	O
</s>
<s>
The	O
receiving	O
computer	O
recalculated	O
the	O
checksum	B-Algorithm
from	O
the	O
128	O
bytes	O
of	O
data	O
,	O
and	O
if	O
it	O
matched	O
the	O
checksum	B-Algorithm
sent	O
in	O
the	O
footer	O
it	O
sent	O
back	O
an	O
,	O
and	O
if	O
it	O
did	O
not	O
match	O
,	O
a	O
.	O
</s>
<s>
When	O
the	O
sender	O
received	O
an	O
it	O
sent	O
the	O
next	O
packet	B-Protocol
,	O
while	O
a	O
caused	O
it	O
to	O
re-send	O
the	O
last	O
one	O
.	O
</s>
<s>
The	O
use	O
of	O
a	O
simple	O
checksum	B-Algorithm
meant	O
some	O
common	O
errors	O
could	O
go	O
unnoticed	O
.	O
</s>
<s>
The	O
small	O
packet	B-Protocol
size	O
and	O
requirement	O
to	O
wait	O
for	O
the	O
or	O
led	O
to	O
slow	O
performance	O
on	O
higher	O
speed	O
links	O
or	O
ones	O
with	O
significant	O
latency	O
.	O
</s>
<s>
XMODEM-CRC	O
replaced	O
the	O
checksum	B-Algorithm
with	O
a	O
16-bit	O
cyclic	O
redundancy	O
check	O
(	O
CRC	O
)	O
,	O
which	O
was	O
much	O
more	O
resistant	O
to	O
common	O
errors	O
.	O
</s>
<s>
XMODEM-1k	B-Protocol
expanded	O
the	O
packet	B-Protocol
size	O
from	O
128	O
bytes	O
to	O
1024	O
,	O
improving	O
performance	O
on	O
higher-speed	O
connections	O
,	O
while	O
others	O
,	O
like	O
WXMODEM	O
and	O
SEAlink	O
instead	O
introduced	O
sliding	O
window	O
systems	O
to	O
combat	O
both	O
performance	O
and	O
latency	O
,	O
at	O
the	O
cost	O
of	O
some	O
complexity	O
.	O
</s>
<s>
Chuck	O
Forsberg	O
,	O
author	O
of	O
the	O
CP/M	B-Application
"	O
Yet	O
Another	O
Modem	B-Application
program	O
"	O
,	O
or	O
YAM	O
,	O
decided	O
to	O
write	O
a	O
single	O
protocol	O
driver	O
that	O
supported	O
many	O
features	O
compared	O
to	O
XMODEM	B-Protocol
and	O
called	O
it	O
YMODEM	B-Protocol
.	O
</s>
<s>
One	O
problem	O
with	O
the	O
original	O
XMODEM	B-Protocol
was	O
that	O
there	O
was	O
no	O
defined	O
way	O
to	O
abort	O
the	O
transfer	O
once	O
started	O
.	O
</s>
<s>
The	O
normal	O
solution	O
was	O
to	O
send	O
s	O
to	O
every	O
subsequent	O
packet	B-Protocol
if	O
the	O
user	O
requested	O
it	O
.	O
</s>
<s>
Since	O
the	O
XMODEM	B-Protocol
protocol	O
defined	O
a	O
limit	O
of	O
ten	O
s	O
to	O
abort	O
a	O
send	O
,	O
and	O
each	O
packet	B-Protocol
might	O
take	O
a	O
second	O
to	O
send	O
,	O
this	O
meant	O
there	O
was	O
a	O
ten	O
second	O
delay	O
where	O
the	O
sender	O
continually	O
sent	O
data	O
that	O
was	O
simply	O
ignored	O
.	O
</s>
<s>
Some	O
implementations	O
had	O
added	O
the	O
ability	O
to	O
send	O
a	O
instead	O
of	O
or	O
at	O
the	O
end	O
of	O
a	O
received	O
packet	B-Protocol
to	O
indicate	O
an	O
abort	O
.	O
</s>
<s>
CRC	O
support	O
had	O
been	O
introduced	O
in	O
XMODEM-CRC	O
.	O
</s>
<s>
If	O
the	O
remote	O
sender	O
supported	O
the	O
CRC	O
option	O
,	O
it	O
would	O
begin	O
sending	O
packets	B-Protocol
as	O
normal	O
,	O
but	O
with	O
a	O
16-bit	O
CRC	O
in	O
the	O
footer	O
rather	O
than	O
the	O
1-byte	O
checksum	B-Algorithm
.	O
</s>
<s>
1024-byte	O
packets	B-Protocol
had	O
been	O
introduced	O
in	O
XMODEM-1k	B-Protocol
.	O
</s>
<s>
This	O
version	O
did	O
not	O
change	O
the	O
trigger	O
character	O
from	O
the	O
receiver	O
,	O
so	O
there	O
was	O
no	O
way	O
for	O
the	O
sender	O
to	O
know	O
whether	O
the	O
receiver	O
supported	O
larger	O
packets	B-Protocol
.	O
</s>
<s>
Instead	O
,	O
XMODEM-1k	B-Protocol
was	O
presented	O
as	O
a	O
separate	O
protocol	O
on	O
both	O
ends	O
of	O
the	O
connection	O
.	O
</s>
<s>
When	O
such	O
a	O
connection	O
was	O
started	O
,	O
the	O
sender	O
could	O
choose	O
to	O
send	O
either	O
1024	O
bytes	O
in	O
a	O
packet	B-Protocol
or	O
128	O
,	O
indicating	O
the	O
larger	O
with	O
an	O
character	O
in	O
the	O
header	O
rather	O
than	O
the	O
normal	O
.	O
</s>
<s>
Normally	O
only	O
the	O
last	O
few	O
packets	B-Protocol
would	O
use	O
the	O
smaller	O
packets	B-Protocol
,	O
to	O
avoid	O
sending	O
large	O
amounts	O
of	O
padding	B-Application
.	O
</s>
<s>
This	O
was	O
not	O
reliable	O
,	O
and	O
TeLink	O
improved	O
this	O
by	O
placing	O
the	O
filename	O
,	O
and	O
optionally	O
other	O
data	O
like	O
the	O
creation	O
date	O
and	O
file	O
length	O
,	O
in	O
a	O
complete	O
128-byte	O
packet	B-Protocol
.	O
</s>
<s>
XMODEM	B-Protocol
started	O
transfers	O
with	O
packet	B-Protocol
number	O
one	O
,	O
so	O
TeLink	O
sent	O
this	O
packet	B-Protocol
as	O
number	O
zero	O
.	O
</s>
<s>
This	O
"	O
zero	O
packet	B-Protocol
"	O
or	O
"	O
block	O
zero	O
"	O
became	O
common	O
in	O
other	O
FidoNet	O
systems	O
like	O
SEAlink	O
and	O
others	O
.	O
</s>
<s>
YAM	O
supported	O
the	O
zero	O
packet	B-Protocol
format	O
,	O
but	O
it	O
was	O
ignored	O
by	O
many	O
3rd	O
party	O
implementations	O
of	O
YMODEM	B-Protocol
.	O
</s>
<s>
When	O
one	O
implementation	O
attempted	O
to	O
send	O
the	O
zero	O
packet	B-Protocol
to	O
a	O
non-aware	O
version	O
,	O
the	O
receiver	O
would	O
naturally	O
the	O
packet	B-Protocol
,	O
as	O
packet	B-Protocol
zero	O
is	O
illegal	O
.	O
</s>
<s>
The	O
sender	O
would	O
then	O
see	O
the	O
as	O
a	O
transmission	O
error	O
and	O
try	O
to	O
send	O
the	O
packet	B-Protocol
again	O
,	O
attempting	O
this	O
ten	O
times	O
before	O
failing	O
.	O
</s>
<s>
For	O
reasons	O
that	O
are	O
not	O
entirely	O
clear	O
,	O
many	O
implementations	O
of	O
YMODEM	B-Protocol
did	O
not	O
implement	O
this	O
feature	O
.	O
</s>
<s>
This	O
meant	O
that	O
if	O
the	O
user	O
chose	O
to	O
use	O
a	O
compliant	O
YMODEM	B-Protocol
with	O
a	O
non-compliant	O
version	O
,	O
the	O
transfers	O
would	O
fail	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
it	O
was	O
common	O
to	O
see	O
both	O
YMODEM	B-Protocol
and	O
YMODEM	B-Protocol
Batch	O
listed	O
as	O
two	O
separate	O
protocols	O
.	O
</s>
<s>
Further	O
confusion	O
was	O
created	O
by	O
the	O
similarity	O
between	O
XMODEM-1k	B-Protocol
and	O
these	O
non-compliant	O
YMODEMs	B-Protocol
,	O
which	O
were	O
similar	O
to	O
the	O
point	O
that	O
they	O
were	O
often	O
incorrectly	O
listed	O
as	O
the	O
same	O
.	O
</s>
<s>
YMODEM-G	B-Protocol
is	O
a	O
streaming	O
variant	O
used	O
for	O
error-free	O
connections	O
.	O
</s>
<s>
It	O
does	O
not	O
wait	O
for	O
an	O
ACK	O
to	O
be	O
received	O
before	O
sending	O
the	O
next	O
packet	B-Protocol
.	O
</s>
<s>
The	O
protocol	O
is	O
faster	O
than	O
YMODEM	B-Protocol
because	O
no	O
latency	O
is	O
introduced	O
between	O
packets	B-Protocol
,	O
but	O
has	O
no	O
capability	O
for	O
correcting	O
errors	O
.	O
</s>
<s>
It	O
is	O
reliant	O
on	O
the	O
underlying	O
connection	O
being	O
error-free	O
,	O
which	O
is	O
the	O
case	O
for	O
modems	B-Application
supporting	O
MNP	B-Device
for	O
instance	O
.	O
</s>
<s>
Normally	O
,	O
a	O
YMODEM	B-Protocol
transfer	O
would	O
be	O
started	O
by	O
the	O
receiver	O
sending	O
a	O
to	O
indicate	O
it	O
wants	O
to	O
use	O
the	O
128-byte	O
format	O
with	O
CRC	O
,	O
or	O
if	O
it	O
wishes	O
to	O
use	O
the	O
original	O
checksum	B-Algorithm
system	O
.	O
</s>
<s>
If	O
the	O
sender	O
does	O
not	O
support	O
the	O
g-protocol	O
,	O
it	O
considers	O
this	O
to	O
be	O
an	O
error	O
and	O
ignores	O
it	O
,	O
but	O
if	O
it	O
does	O
support	O
g	O
it	O
begins	O
sending	O
packets	B-Protocol
in	O
a	O
continual	O
stream	O
.	O
</s>
<s>
It	O
only	O
expects	O
a	O
single	O
after	O
the	O
final	O
packet	B-Protocol
is	O
received	O
,	O
which	O
is	O
indicated	O
by	O
the	O
presence	O
of	O
an	O
character	O
in	O
the	O
data	O
.	O
</s>
<s>
YMODEM-g	B-Protocol
assumes	O
1k	O
packets	B-Protocol
are	O
available	O
.	O
</s>
<s>
However	O
,	O
despite	O
this	O
protocol	O
potentially	O
being	O
faster	O
than	O
ZMODEM	B-Device
it	O
was	O
still	O
rarely	O
used	O
.	O
</s>
<s>
Before	O
the	O
emergence	O
of	O
the	O
16550	B-Device
UART	I-Device
,	O
there	O
was	O
a	O
substantial	O
risk	O
of	O
buffer	B-General_Concept
overrun	I-General_Concept
on	O
the	O
serial	B-Protocol
port	I-Protocol
.	O
</s>
<s>
Although	O
this	O
would	O
be	O
detected	O
by	O
YMODEM-g	B-Protocol
it	O
could	O
not	O
be	O
corrected	O
since	O
no	O
block	O
re-transmission	O
is	O
possible	O
.	O
</s>
<s>
ZMODEM	B-Device
on	O
the	O
other	O
hand	O
has	O
transfer	O
resume	O
capability	O
which	O
made	O
it	O
more	O
appealing	O
.	O
</s>
