<s>
XMODEM	B-Protocol
is	O
a	O
simple	O
file	B-Protocol
transfer	I-Protocol
protocol	O
developed	O
as	O
a	O
quick	O
hack	O
by	O
Ward	O
Christensen	O
for	O
use	O
in	O
his	O
1977	O
MODEM.ASM	O
terminal	B-Application
program	I-Application
.	O
</s>
<s>
It	O
allowed	O
users	O
to	O
transmit	O
files	O
between	O
their	O
computers	O
when	O
both	O
sides	O
used	O
MODEM	B-Application
.	O
</s>
<s>
Keith	O
Petersen	O
made	O
a	O
minor	O
update	O
to	O
always	O
turn	O
on	O
"	O
quiet	O
mode	O
"	O
,	O
and	O
called	O
the	O
result	O
XMODEM	B-Protocol
.	O
</s>
<s>
XMODEM	B-Protocol
,	O
like	O
most	O
file	B-Protocol
transfer	I-Protocol
protocols	O
,	O
breaks	O
up	O
the	O
original	O
data	O
into	O
a	O
series	O
of	O
"	O
packets	B-Protocol
"	O
that	O
are	O
sent	O
to	O
the	O
receiver	O
,	O
along	O
with	O
additional	O
information	O
allowing	O
the	O
receiver	O
to	O
determine	O
whether	O
that	O
packet	B-Protocol
was	O
correctly	O
received	O
.	O
</s>
<s>
If	O
an	O
error	O
is	O
detected	O
,	O
the	O
receiver	O
requests	O
that	O
the	O
packet	B-Protocol
be	O
re-sent	O
.	O
</s>
<s>
A	O
string	O
of	O
bad	O
packets	B-Protocol
causes	O
the	O
transfer	O
to	O
abort	O
.	O
</s>
<s>
XMODEM	B-Protocol
became	O
extremely	O
popular	O
in	O
the	O
early	O
bulletin	B-Application
board	I-Application
system	I-Application
(	O
BBS	O
)	O
market	O
,	O
largely	O
because	O
it	O
was	O
simple	O
to	O
implement	O
.	O
</s>
<s>
It	O
was	O
also	O
fairly	O
inefficient	O
,	O
and	O
as	O
modem	B-Application
speeds	O
increased	O
,	O
this	O
problem	O
led	O
to	O
the	O
development	O
of	O
a	O
number	O
of	O
modified	O
versions	O
of	O
XMODEM	B-Protocol
to	O
improve	O
performance	O
or	O
address	O
other	O
problems	O
with	O
the	O
protocol	O
.	O
</s>
<s>
Christensen	O
believed	O
his	O
original	O
XMODEM	B-Protocol
to	O
be	O
"	O
the	O
single	O
most	O
modified	O
program	O
in	O
computing	O
history	O
"	O
.	O
</s>
<s>
Chuck	O
Forsberg	O
collected	O
a	O
number	O
of	O
common	O
modifications	O
into	O
his	O
YMODEM	B-Protocol
protocol	O
,	O
but	O
poor	O
implementation	O
led	O
to	O
a	O
further	O
fracturing	O
before	O
they	O
were	O
re-unified	O
by	O
his	O
later	O
ZMODEM	B-Device
protocol	O
.	O
</s>
<s>
ZMODEM	B-Device
became	O
very	O
popular	O
,	O
but	O
never	O
completely	O
replaced	O
XMODEM	B-Protocol
in	O
the	O
BBS	O
market	O
.	O
</s>
<s>
The	O
original	O
XMODEM	B-Protocol
used	O
a	O
128-byte	O
data	B-Protocol
packet	I-Protocol
,	O
the	O
basic	O
block	O
size	O
used	O
on	O
CP/M	B-Application
floppy	B-Device
disks	I-Device
.	O
</s>
<s>
The	O
packet	B-Protocol
was	O
prefixed	O
by	O
a	O
simple	O
3-byte	O
header	O
containing	O
a	O
<SOH>	O
character	O
,	O
a	O
"	O
block	O
number	O
"	O
from	O
1-255	O
,	O
and	O
the	O
"	O
inverse	O
"	O
block	O
number	O
—	O
255	O
minus	O
the	O
block	O
number	O
.	O
</s>
<s>
The	O
header	O
was	O
followed	O
by	O
the	O
128	O
bytes	O
of	O
data	O
,	O
and	O
then	O
a	O
single-byte	O
checksum	B-Algorithm
.	O
</s>
<s>
The	O
checksum	B-Algorithm
was	O
the	O
sum	O
of	O
all	O
128	O
data	O
bytes	O
in	O
the	O
packet	B-Protocol
modulo	O
256	O
.	O
</s>
<s>
The	O
complete	O
packet	B-Protocol
was	O
thus	O
132	O
bytes	O
long	O
,	O
containing	O
128	O
bytes	O
of	O
payload	O
data	O
,	O
for	O
a	O
total	O
channel	O
efficiency	O
of	O
about	O
97%	O
.	O
</s>
<s>
This	O
character	O
was	O
not	O
in	O
a	O
packet	B-Protocol
,	O
but	O
sent	O
alone	O
as	O
a	O
single	O
byte	O
.	O
</s>
<s>
Since	O
the	O
file	O
length	O
was	O
not	O
sent	O
as	O
part	O
of	O
the	O
protocol	O
,	O
the	O
last	O
packet	B-Protocol
was	O
padded	O
out	O
with	O
a	O
"	O
known	O
character	O
"	O
that	O
could	O
be	O
dropped	O
.	O
</s>
<s>
In	O
the	O
original	O
specification	O
,	O
this	O
defaulted	O
to	O
<SUB>	O
or	O
26	O
decimal	O
,	O
which	O
CP/M	B-Application
used	O
as	O
the	O
end-of-file	O
marker	O
inside	O
its	O
own	O
disk	O
format	O
.	O
</s>
<s>
Files	O
were	O
transferred	O
one	O
packet	B-Protocol
at	O
a	O
time	O
.	O
</s>
<s>
When	O
received	O
,	O
the	O
packet	B-Protocol
's	O
checksum	B-Algorithm
was	O
calculated	O
by	O
the	O
receiver	O
and	O
compared	O
to	O
the	O
one	O
received	O
from	O
the	O
sender	O
at	O
the	O
end	O
of	O
the	O
packet	B-Protocol
.	O
</s>
<s>
If	O
the	O
two	O
matched	O
,	O
the	O
receiver	O
sent	O
an	O
<ACK>	O
message	O
back	O
to	O
the	O
sender	O
,	O
which	O
then	O
sent	O
the	O
next	O
packet	B-Protocol
in	O
sequence	O
.	O
</s>
<s>
If	O
there	O
was	O
a	O
problem	O
with	O
the	O
checksum	B-Algorithm
,	O
the	O
receiver	O
instead	O
sent	O
a	O
<NAK>	O
.	O
</s>
<s>
If	O
a	O
<NAK>	O
was	O
received	O
,	O
the	O
sender	O
would	O
re-send	O
the	O
packet	B-Protocol
,	O
and	O
continued	O
to	O
try	O
several	O
times	O
,	O
normally	O
ten	O
,	O
before	O
aborting	O
the	O
transfer	O
.	O
</s>
<s>
A	O
<NAK> was also sent if the receiver did not receive a valid packet within ten seconds while still expecting data due to the lack of a <EOT>	O
character	O
.	O
</s>
<s>
A	O
seven-second	O
timeout	O
was	O
also	O
used	O
within	O
a	O
packet	B-Protocol
,	O
guarding	O
against	O
dropped	O
connections	O
in	O
mid-packet	O
.	O
</s>
<s>
After	O
receiving	O
a	O
packet	B-Protocol
successfully	O
,	O
the	O
next	O
packet	B-Protocol
should	O
have	O
a	O
one-higher	O
number	O
.	O
</s>
<s>
If	O
it	O
instead	O
received	O
the	O
same	O
block	O
number	O
this	O
was	O
not	O
considered	O
serious	O
,	O
it	O
was	O
implied	O
that	O
the	O
<ACK>	O
had	O
not	O
been	O
received	O
by	O
the	O
sender	O
,	O
which	O
had	O
then	O
re-sent	O
the	O
packet	B-Protocol
.	O
</s>
<s>
Any	O
other	O
packet	B-Protocol
number	O
signalled	O
that	O
packets	B-Protocol
had	O
been	O
lost	O
.	O
</s>
<s>
Transfers	O
were	O
receiver-driven	O
;	O
the	O
transmitter	O
would	O
not	O
send	O
any	O
data	O
until	O
an	O
initial	O
<NAK>	O
was	O
sent	O
by	O
the	O
receiver	O
.	O
</s>
<s>
Since	O
the	O
delay	O
between	O
asking	O
the	O
remote	O
system	O
for	O
the	O
file	O
and	O
issuing	O
a	O
local	O
command	O
to	O
receive	O
was	O
unknown	O
,	O
XMODEM	B-Protocol
allowed	O
up	O
to	O
90	O
seconds	O
for	O
the	O
receiver	O
to	O
begin	O
issuing	O
requests	O
for	O
data	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
Although	O
XMODEM	B-Protocol
was	O
robust	O
enough	O
for	O
a	O
journalist	O
in	O
1982	O
to	O
transmit	O
stories	O
from	O
Pakistan	O
to	O
the	O
United	O
States	O
with	O
an	O
Osborne	B-Device
1	I-Device
and	O
acoustic	B-Device
coupler	I-Device
over	O
poor-quality	O
telephone	O
lines	O
,	O
the	O
protocol	O
had	O
several	O
flaws	O
.	O
</s>
<s>
XMODEM	B-Protocol
was	O
written	O
for	O
CP/M	B-Application
machines	O
,	O
and	O
bears	O
several	O
marks	O
of	O
that	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Notably	O
,	O
files	O
on	O
CP/M	B-Application
were	O
always	O
multiples	O
of	O
128	O
bytes	O
,	O
and	O
their	O
end	O
was	O
marked	O
within	O
a	O
block	O
with	O
the	O
<EOT>	O
character	O
.	O
</s>
<s>
These	O
characteristics	O
were	O
transplanted	O
directly	O
into	O
XMODEM	B-Protocol
.	O
</s>
<s>
However	O
,	O
other	O
operating	B-General_Concept
systems	I-General_Concept
did	O
not	O
feature	O
either	O
of	O
these	O
peculiarities	O
,	O
and	O
the	O
widespread	O
introduction	O
of	O
MS-DOS	B-Application
in	O
the	O
early	O
1980s	O
led	O
to	O
XMODEM	B-Protocol
having	O
to	O
be	O
updated	O
to	O
notice	O
either	O
a	O
<EOT> or <EOF>	O
as	O
the	O
end-of-file	O
marker	O
.	O
</s>
<s>
For	O
some	O
time	O
it	O
was	O
suggested	O
that	O
sending	O
a	O
<CAN> character instead of an <ACK> or <NAK>	O
should	O
be	O
supported	O
in	O
order	O
to	O
easily	O
abort	O
the	O
transfer	O
from	O
the	O
receiving	O
end	O
.	O
</s>
<s>
However	O
,	O
this	O
character	O
could	O
be	O
easily	O
"	O
created	O
"	O
via	O
simple	O
noise-related	O
errors	O
of	O
what	O
was	O
meant	O
to	O
be	O
an	O
<ACK> or <NAK>	O
.	O
</s>
<s>
XMODEM	B-Protocol
was	O
designed	O
for	O
simplicity	O
,	O
without	O
much	O
knowledge	O
of	O
other	O
file	B-Protocol
transfer	I-Protocol
protocols	O
–	O
which	O
were	O
fairly	O
rare	O
anyway	O
.	O
</s>
<s>
Most	O
of	O
this	O
was	O
due	O
to	O
the	O
use	O
of	O
a	O
simple	O
checksum	B-Algorithm
for	O
error	O
correction	O
,	O
which	O
is	O
susceptible	O
to	O
missing	O
errors	O
in	O
the	O
data	O
if	O
two	O
bits	O
are	O
reversed	O
,	O
which	O
can	O
happen	O
with	O
a	O
suitably	O
short	O
burst	O
of	O
noise	O
.	O
</s>
<s>
Additionally	O
,	O
similar	O
damage	O
to	O
the	O
header	O
or	O
checksum	B-Algorithm
could	O
lead	O
to	O
a	O
failed	O
transfer	O
in	O
cases	O
where	O
the	O
data	O
itself	O
was	O
undamaged	O
.	O
</s>
<s>
Many	O
authors	O
introduced	O
extensions	O
to	O
XMODEM	B-Protocol
to	O
address	O
these	O
and	O
other	O
problems	O
.	O
</s>
<s>
Many	O
asked	O
for	O
these	O
extensions	O
to	O
be	O
included	O
as	O
part	O
of	O
a	O
new	O
XMODEM	B-Protocol
standard	O
.	O
</s>
<s>
However	O
,	O
Ward	O
Christensen	O
refused	O
to	O
do	O
this	O
,	O
as	O
it	O
was	O
precisely	O
the	O
lack	O
of	O
these	O
features	O
,	O
and	O
the	O
associated	O
coding	O
needed	O
to	O
support	O
them	O
,	O
which	O
led	O
to	O
XMODEM	B-Protocol
's	O
widespread	O
use	O
.	O
</s>
<s>
Another	O
problem	O
with	O
XMODEM	B-Protocol
was	O
that	O
it	O
required	O
the	O
transfer	O
to	O
be	O
user-driven	O
rather	O
than	O
automated	O
.	O
</s>
<s>
They	O
would	O
then	O
trigger	O
the	O
transfer	O
from	O
their	O
end	O
using	O
a	O
command	O
in	O
their	O
terminal	B-Application
emulator	I-Application
.	O
</s>
<s>
For	O
automated	O
transfers	O
between	O
two	O
sites	O
,	O
a	O
number	O
of	O
add-ons	O
to	O
the	O
XMODEM	B-Protocol
protocol	O
were	O
implemented	O
over	O
time	O
.	O
</s>
<s>
These	O
generally	O
assumed	O
the	O
sender	O
would	O
continue	O
sending	O
file	O
after	O
file	O
,	O
with	O
the	O
receiver	O
attempting	O
to	O
trigger	O
the	O
next	O
file	O
by	O
sending	O
a	O
NAK	B-Protocol
as	O
normal	O
at	O
the	O
start	O
of	O
a	O
transfer	O
.	O
</s>
<s>
When	O
the	O
NAKs	B-Protocol
timed	O
out	O
,	O
it	O
could	O
be	O
assumed	O
that	O
either	O
there	O
were	O
no	O
more	O
files	O
,	O
or	O
the	O
link	O
was	O
broken	O
anyway	O
.	O
</s>
<s>
MODEM7	O
,	O
also	O
known	O
as	O
MODEM7	O
batch	O
or	O
Batch	O
XMODEM	B-Protocol
,	O
was	O
the	O
first	O
known	O
extension	O
of	O
the	O
XMODEM	B-Protocol
protocol	O
.	O
</s>
<s>
A	O
normal	O
XMODEM	B-Protocol
file	B-Protocol
transfer	I-Protocol
starts	O
with	O
the	O
receiver	O
sending	O
a	O
single	O
NAK	B-Protocol
character	O
to	O
the	O
sender	O
,	O
which	O
then	O
starts	O
sending	O
a	O
single	O
SOH	O
to	O
indicate	O
the	O
start	O
of	O
the	O
data	O
,	O
and	O
then	O
packets	B-Protocol
of	O
data	O
.	O
</s>
<s>
For	O
a	O
non-aware	O
XMODEM	B-Protocol
implementation	O
,	O
this	O
data	O
would	O
simply	O
be	O
ignored	O
while	O
it	O
waited	O
for	O
the	O
SOH	O
to	O
arrive	O
,	O
so	O
the	O
characters	O
would	O
not	O
be	O
echoed	O
and	O
the	O
implementation	O
could	O
fall	O
back	O
to	O
conventional	O
XMODEM	B-Protocol
.	O
</s>
<s>
Transfers	O
could	O
continue	O
with	O
another	O
<NAK>	O
,	O
each	O
file	O
is	O
saved	O
under	O
the	O
name	O
being	O
sent	O
to	O
the	O
receiver	O
.	O
</s>
<s>
MODEM7	O
sent	O
the	O
filename	O
as	O
normal	O
text	O
,	O
which	O
meant	O
it	O
could	O
be	O
corrupted	O
by	O
the	O
same	O
problems	O
that	O
XMODEM	B-Protocol
was	O
attempting	O
to	O
avoid	O
.	O
</s>
<s>
TeLink	O
avoided	O
MODEM7	O
's	O
problems	O
by	O
standardizing	O
a	O
new	O
"	O
zero	O
packet	B-Protocol
"	O
containing	O
information	O
about	O
the	O
original	O
file	O
.	O
</s>
<s>
This	O
included	O
the	O
file	O
's	O
name	O
,	O
size	O
,	O
and	O
timestamp	O
,	O
which	O
were	O
placed	O
in	O
a	O
regular	O
128	O
byte	O
XMODEM	B-Protocol
block	O
.	O
</s>
<s>
Whereas	O
a	O
normal	O
XMODEM	B-Protocol
transfer	O
would	O
start	O
with	O
the	O
sender	O
sending	O
"	O
block	O
1	O
"	O
with	O
a	O
<SOH> header, the TeLink header packet was labeled "block 0" and began with a <SYN>	O
.	O
</s>
<s>
The	O
packet	B-Protocol
contained	O
the	O
file	O
creation	O
date	O
and	O
time	O
,	O
filename	O
up	O
to	O
16	O
characters	O
,	O
the	O
file	O
size	O
as	O
a	O
4-byte	O
value	O
,	O
and	O
the	O
name	O
of	O
the	O
program	O
sending	O
the	O
file	O
.	O
</s>
<s>
A	O
normal	O
XMODEM	B-Protocol
implementation	O
would	O
simply	O
discard	O
the	O
packet	B-Protocol
,	O
the	O
assumption	O
being	O
that	O
the	O
packet	B-Protocol
number	O
had	O
been	O
corrupted	O
.	O
</s>
<s>
But	O
this	O
led	O
to	O
a	O
potential	O
time	O
delay	O
if	O
the	O
packet	B-Protocol
were	O
discarded	O
,	O
as	O
the	O
sender	O
could	O
not	O
tell	O
whether	O
the	O
receiver	O
had	O
responded	O
with	O
a	O
<NAK>	O
because	O
it	O
did	O
not	O
understand	O
the	O
zero	O
packet	B-Protocol
or	O
because	O
there	O
was	O
a	O
transmission	O
error	O
.	O
</s>
<s>
The	O
basic	O
"	O
block	O
0	O
"	O
system	O
became	O
a	O
standard	O
in	O
the	O
FidoNet	O
community	O
,	O
and	O
was	O
re-used	O
by	O
a	O
number	O
of	O
future	O
protocols	O
like	O
SEAlink	B-Protocol
and	O
YMODEM	B-Protocol
.	O
</s>
<s>
The	O
checksum	B-Algorithm
used	O
in	O
the	O
original	O
protocol	O
was	O
extremely	O
simple	O
,	O
and	O
errors	O
within	O
the	O
packet	B-Protocol
could	O
go	O
unnoticed	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
introduction	O
of	O
XMODEM-CRC	O
by	O
John	O
Byrns	O
,	O
which	O
used	O
a	O
16-bit	O
CRC	O
in	O
place	O
of	O
the	O
8-bit	O
checksum	B-Algorithm
.	O
</s>
<s>
CRCs	O
encode	O
not	O
only	O
the	O
data	O
in	O
the	O
packet	B-Protocol
,	O
but	O
its	O
location	O
as	O
well	O
,	O
allowing	O
it	O
to	O
notice	O
the	O
bit-replacement	O
errors	O
that	O
a	O
checksum	B-Algorithm
would	O
miss	O
.	O
</s>
<s>
XMODEM-CRC	O
was	O
designed	O
to	O
be	O
backwardly	O
compatible	O
with	O
XMODEM	B-Protocol
.	O
</s>
<s>
To	O
do	O
this	O
,	O
the	O
receiver	O
sent	O
a	O
C	O
(	O
capital	O
C	O
)	O
character	O
instead	O
of	O
a	O
<NAK>	O
to	O
start	O
the	O
transfer	O
.	O
</s>
<s>
If	O
the	O
sender	O
responded	O
by	O
sending	O
a	O
packet	B-Protocol
,	O
it	O
was	O
assumed	O
the	O
sender	O
"	O
knew	O
"	O
XMODEM-CRC	O
,	O
and	O
the	O
receiver	O
continued	O
sending	O
C	O
's	O
.	O
</s>
<s>
If	O
no	O
packet	B-Protocol
was	O
forthcoming	O
,	O
the	O
receiver	O
assumed	O
the	O
sender	O
did	O
not	O
know	O
the	O
protocol	O
,	O
and	O
sent	O
an	O
<NAK>	O
to	O
start	O
a	O
"	O
traditional	O
"	O
XMODEM	B-Protocol
transfer	O
.	O
</s>
<s>
Since	O
it	O
was	O
possible	O
that	O
the	O
initial	O
C	O
character	O
would	O
be	O
lost	O
or	O
corrupted	O
,	O
it	O
could	O
not	O
be	O
assumed	O
that	O
the	O
receiver	O
did	O
not	O
support	O
XMODEM-CRC	O
if	O
the	O
first	O
attempt	O
to	O
trigger	O
the	O
transfer	O
failed	O
.	O
</s>
<s>
This	O
meant	O
that	O
if	O
the	O
user	O
selected	O
XMODEM-CRC	O
while	O
attempting	O
to	O
talk	O
to	O
any	O
XMODEM	B-Protocol
,	O
as	O
it	O
was	O
intended	O
,	O
there	O
was	O
a	O
potential	O
10	O
second	O
delay	O
before	O
the	O
transfer	O
started	O
.	O
</s>
<s>
To	O
avoid	O
the	O
delay	O
,	O
the	O
sender	O
and	O
receiver	O
would	O
generally	O
list	O
XMODEM-CRC	O
separately	O
from	O
XMODEM	B-Protocol
,	O
allowing	O
the	O
user	O
to	O
select	O
"	O
basic	O
"	O
XMODEM	B-Protocol
if	O
the	O
sender	O
did	O
n't	O
explicitly	O
list	O
it	O
.	O
</s>
<s>
To	O
the	O
average	O
user	O
,	O
XMODEM-CRC	O
was	O
essentially	O
a	O
"	O
second	O
protocol	O
"	O
,	O
and	O
treated	O
as	O
such	O
.	O
</s>
<s>
Since	O
the	O
XMODEM	B-Protocol
protocol	O
required	O
the	O
sender	O
to	O
stop	O
and	O
wait	O
for	O
an	O
<ACK> or <NAK>	O
message	O
from	O
the	O
receiver	O
,	O
it	O
tended	O
to	O
be	O
quite	O
slow	O
.	O
</s>
<s>
In	O
the	O
era	O
of	O
300bit/s	O
modems	B-Application
,	O
the	O
entire	O
132-byte	O
packet	B-Protocol
required	O
just	O
over	O
3.5seconds	O
to	O
send	O
(	O
132	O
bytes	O
*	O
(	O
8	O
bits	O
per	O
byte	O
+	O
1	O
start	O
bit	O
+	O
1	O
stop	O
bit	O
)	O
/	O
300	O
bits	O
per	O
second	O
)	O
.	O
</s>
<s>
Assuming	O
it	O
takes	O
0.2seconds	O
for	O
the	O
receiver	O
's	O
<ACK>	O
to	O
make	O
it	O
back	O
to	O
the	O
sender	O
and	O
the	O
next	O
packet	B-Protocol
to	O
start	O
hitting	O
the	O
receiver	O
(	O
0.1seconds	O
in	O
both	O
directions	O
)	O
,	O
the	O
overall	O
time	O
for	O
one	O
packet	B-Protocol
would	O
be	O
3.7seconds	O
,	O
just	O
over	O
92%	O
throughput	O
.	O
</s>
<s>
The	O
time	O
for	O
the	O
<ACK>/<NAK>	O
process	O
was	O
a	O
fixed	O
function	O
of	O
the	O
underlying	O
communications	O
network	O
,	O
not	O
of	O
the	O
performance	O
of	O
the	O
modems	B-Application
.	O
</s>
<s>
As	O
modem	B-Application
speeds	O
increased	O
,	O
the	O
fixed	O
delay	O
grew	O
in	O
proportion	O
to	O
time	O
needed	O
to	O
send	O
the	O
packet	B-Protocol
.	O
</s>
<s>
For	O
instance	O
,	O
at	O
2400bit/s	O
the	O
packets	B-Protocol
took	O
only	O
0.44seconds	O
to	O
send	O
,	O
so	O
if	O
the	O
<ACK>/<NAK>	O
still	O
took	O
0.2seconds	O
to	O
make	O
it	O
back	O
to	O
the	O
user	O
's	O
machine	O
,	O
the	O
throughput	O
has	O
fallen	O
to	O
under	O
60%	O
.	O
</s>
<s>
At	O
9600bit/s	O
it	O
is	O
under	O
30%	O
–	O
more	O
time	O
is	O
spent	O
waiting	O
for	O
the	O
reply	O
than	O
is	O
needed	O
to	O
send	O
the	O
packet	B-Protocol
.	O
</s>
<s>
A	O
number	O
of	O
new	O
versions	O
of	O
XMODEM	B-Protocol
were	O
introduced	O
in	O
order	O
to	O
address	O
these	O
problems	O
.	O
</s>
<s>
Like	O
earlier	O
extensions	O
,	O
these	O
versions	O
tended	O
to	O
be	O
backward-compatible	O
with	O
the	O
original	O
XMODEM	B-Protocol
,	O
and	O
like	O
those	O
extensions	O
,	O
this	O
led	O
to	O
further	O
fracturing	O
of	O
the	O
XMODEM	B-Protocol
landscape	O
in	O
the	O
user	O
's	O
terminal	B-Application
emulator	I-Application
.	O
</s>
<s>
In	O
the	O
end	O
,	O
dozens	O
of	O
versions	O
of	O
XMODEM	B-Protocol
emerged	O
.	O
</s>
<s>
WXmodem	O
,	O
short	O
for	O
"	O
Windowed	O
Xmodem	B-Protocol
"	O
,	O
is	O
a	O
variant	O
of	O
XMODEM	B-Protocol
developed	O
by	O
Peter	O
Boswell	O
in	O
1986	O
for	O
use	O
on	O
high-latency	O
lines	O
,	O
specifically	O
public	O
X.25	B-Protocol
systems	O
and	O
PC	O
Pursuit	O
.	O
</s>
<s>
These	O
have	O
latencies	O
that	O
are	O
far	O
higher	O
than	O
the	O
plain-old	O
telephone	O
service	O
,	O
which	O
leads	O
to	O
very	O
poor	O
efficiency	O
in	O
XMODEM	B-Protocol
.	O
</s>
<s>
Finally	O
,	O
in	O
the	O
case	O
of	O
an	O
error	O
that	O
required	O
a	O
resend	O
,	O
it	O
was	O
sometimes	O
difficult	O
to	O
know	O
whether	O
a	O
SOH	O
was	O
a	O
packet	B-Protocol
indicator	O
or	O
more	O
noise	O
.	O
</s>
<s>
WXmodem	O
adapted	O
XMODEM-CRC	O
to	O
address	O
these	O
problems	O
.	O
</s>
<s>
In	O
theory	O
,	O
this	O
meant	O
the	O
packet	B-Protocol
might	O
be	O
as	O
long	O
as	O
264	O
bytes	O
if	O
it	O
originally	O
consisted	O
entirely	O
of	O
characters	O
that	O
required	O
escaping	O
.	O
</s>
<s>
Additionally	O
,	O
all	O
packets	B-Protocol
were	O
prefixed	O
with	O
a	O
SYN	O
character	O
,	O
which	O
meant	O
the	O
packet	B-Protocol
lead-in	O
was	O
SYNSOH	O
,	O
reducing	O
the	O
chance	O
that	O
a	O
stray	O
SOH	O
would	O
be	O
confused	O
for	O
a	O
packet	B-Protocol
header	O
in	O
various	O
error	O
cases	O
.	O
</s>
<s>
An	O
unescaped	O
SYN	O
found	O
in	O
the	O
body	O
of	O
a	O
packet	B-Protocol
was	O
an	O
error	O
.	O
</s>
<s>
To	O
do	O
so	O
,	O
the	O
ACK	B-Protocol
messages	O
were	O
followed	O
by	O
the	O
packet	B-Protocol
number	O
they	O
were	O
ACKing	O
or	O
NAKing	O
.	O
</s>
<s>
The	O
receiver	O
does	O
not	O
have	O
to	O
ACK	B-Protocol
every	O
packet	B-Protocol
;	O
it	O
is	O
allowed	O
to	O
ACK	B-Protocol
any	O
number	O
between	O
one	O
and	O
four	O
packets	B-Protocol
.	O
</s>
<s>
An	O
ACK	B-Protocol
with	O
the	O
fourth	O
packet	B-Protocol
sequence	O
number	O
is	O
assumed	O
to	O
ACK	B-Protocol
all	O
four	O
packets	B-Protocol
.	O
</s>
<s>
An	O
error	O
causes	O
a	O
NAK	B-Protocol
to	O
be	O
sent	O
immediately	O
,	O
with	O
all	O
packets	B-Protocol
from	O
that	O
number	O
and	O
after	O
being	O
re-sent	O
.	O
</s>
<s>
Requiring	O
an	O
ACK	B-Protocol
every	O
four	O
packets	B-Protocol
makes	O
the	O
system	O
work	O
like	O
it	O
has	O
a	O
packet	B-Protocol
size	O
of	O
512bytes	O
,	O
but	O
in	O
the	O
case	O
of	O
an	O
error	O
,	O
typically	O
only	O
requires	O
128bytes	O
to	O
be	O
re-sent	O
.	O
</s>
<s>
This	O
is	O
of	O
little	O
interest	O
in	O
the	O
typical	O
modem	B-Application
's	O
full	O
duplex	O
operation	O
,	O
but	O
is	O
important	O
in	O
half	O
duplex	O
systems	O
like	O
Telebit	O
models	O
which	O
have	O
19kB	O
speed	O
in	O
one	O
direction	O
and	O
75bits/s	O
in	O
the	O
return	O
channel	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
third-party	O
mailers	O
for	O
the	O
FidoNet	O
system	O
was	O
SEAdog	O
,	O
written	O
by	O
the	O
same	O
author	O
as	O
the	O
then-popular	O
.arc	B-Application
data	B-General_Concept
compression	I-General_Concept
format	O
.	O
</s>
<s>
SEAdog	O
included	O
a	O
wide	O
variety	O
of	O
improvements	O
,	O
including	O
SEAlink	B-Protocol
,	O
an	O
improved	O
transfer	O
protocol	O
based	O
on	O
the	O
same	O
sliding	O
window	O
concept	O
as	O
WXmodem	O
.	O
</s>
<s>
One	O
difference	O
is	O
that	O
SEAlink	B-Protocol
supported	O
the	O
"	O
zero	O
packet	B-Protocol
"	O
introduced	O
by	O
TeLink	O
,	O
which	O
is	O
needed	O
in	O
order	O
to	O
operate	O
as	O
a	O
drop-in	O
replacement	O
for	O
TeLink	O
in	O
FidoNet	O
systems	O
where	O
the	O
header	O
was	O
expected	O
.	O
</s>
<s>
ACKs	B-Protocol
and	O
NAKs	B-Protocol
were	O
extended	O
to	O
three-byte	O
"	O
packets	B-Protocol
"	O
,	O
starting	O
with	O
the	O
ACK	B-Protocol
or	O
NAK	B-Protocol
,	O
then	O
the	O
packet	B-Protocol
number	O
,	O
then	O
the	O
complement	O
of	O
the	O
packet	B-Protocol
number	O
,	O
in	O
the	O
same	O
fashion	O
as	O
the	O
original	O
XMODEM	B-Protocol
packet	B-Protocol
header	O
.	O
</s>
<s>
The	O
window	O
size	O
was	O
normally	O
set	O
to	O
six	O
packets	B-Protocol
.	O
</s>
<s>
SEAlink	B-Protocol
was	O
not	O
expected	O
to	O
operate	O
over	O
X.25	B-Protocol
or	O
similar	O
links	O
,	O
and	O
thus	O
did	O
not	O
perform	O
escaping	O
.	O
</s>
<s>
This	O
was	O
also	O
needed	O
so	O
the	O
zero	O
packet	B-Protocol
would	O
work	O
properly	O
,	O
as	O
this	O
standard	O
used	O
the	O
SYN	O
character	O
that	O
WXmodem	O
had	O
re-purposed	O
.	O
</s>
<s>
This	O
suppressed	O
ACKs	B-Protocol
for	O
packets	B-Protocol
that	O
were	O
successfully	O
transferred	O
,	O
in	O
effect	O
making	O
the	O
window	O
of	O
infinite	O
size	O
.	O
</s>
<s>
SEAlink	B-Protocol
later	O
added	O
a	O
number	O
of	O
other	O
improvements	O
and	O
was	O
a	O
useful	O
general-purpose	O
protocol	O
.	O
</s>
<s>
Another	O
way	O
to	O
solve	O
the	O
throughput	O
problem	O
is	O
to	O
increase	O
the	O
packet	B-Protocol
size	O
.	O
</s>
<s>
XMODEM-1K	B-Protocol
with	O
1024-byte	O
packets	B-Protocol
was	O
the	O
most	O
popular	O
such	O
solution	O
.	O
</s>
<s>
XMODEM-1K	B-Protocol
was	O
an	O
expanded	O
version	O
of	O
XMODEM-CRC	O
,	O
which	O
indicated	O
the	O
longer	O
block	O
size	O
in	O
the	O
sender	O
by	O
starting	O
a	O
packet	B-Protocol
with	O
the	O
<STX> character instead of <SOH>	O
.	O
</s>
<s>
Like	O
other	O
backward-compatible	O
XMODEM	B-Protocol
extensions	O
,	O
it	O
was	O
intended	O
that	O
a	O
-1K	O
transfer	O
could	O
be	O
started	O
with	O
any	O
implementation	O
of	O
XMODEM	B-Protocol
on	O
the	O
other	O
end	O
,	O
backing	O
off	O
features	O
as	O
required	O
.	O
</s>
<s>
XMODEM-1K	B-Protocol
was	O
originally	O
one	O
of	O
the	O
many	O
improvements	O
to	O
XMODEM	B-Protocol
introduced	O
by	O
Chuck	O
Forsberg	O
in	O
his	O
YMODEM	B-Protocol
protocol	O
.	O
</s>
<s>
Instead	O
,	O
they	O
generally	O
implemented	O
the	O
bare	O
minimum	O
,	O
leading	O
to	O
a	O
profusion	O
of	O
semi-compatible	O
implementations	O
,	O
and	O
eventually	O
,	O
the	O
splitting	O
out	O
of	O
the	O
name	O
"	O
YMODEM	B-Protocol
"	O
into	O
"	O
XMODEM-1K	B-Protocol
"	O
and	O
a	O
variety	O
of	O
YMODEMs	B-Protocol
.	O
</s>
<s>
Thus	O
XMODEM-1K	B-Protocol
actually	O
post-dates	O
YMODEM	B-Protocol
,	O
but	O
remained	O
fairly	O
common	O
anyway	O
.	O
</s>
<s>
NMODEM	O
is	O
a	O
file	B-Protocol
transfer	I-Protocol
protocol	O
developed	O
by	O
L	O
.	O
B	O
.	O
Neal	O
,	O
who	O
released	O
it	O
in	O
1990	O
.	O
</s>
<s>
NMODEM	O
is	O
essentially	O
a	O
version	O
of	O
XMODEM-CRC	O
using	O
larger	O
2048	O
byte	O
blocks	O
,	O
as	O
opposed	O
to	O
XMODEM	B-Protocol
's	O
128	O
byte	O
blocks	O
.	O
</s>
<s>
The	O
block	O
size	O
was	O
chosen	O
to	O
match	O
the	O
common	O
cluster	O
size	O
of	O
the	O
MS-DOS	B-Application
FAT	B-Application
file	I-Application
system	I-Application
on	O
contemporary	O
hard	B-Device
drives	I-Device
,	O
making	O
buffering	O
data	O
for	O
writing	O
simpler	O
.	O
</s>
<s>
Over	O
reliable	O
(	O
error-free	O
)	O
connections	O
,	O
it	O
is	O
possible	O
to	O
eliminate	O
latency	O
by	O
"	O
pre-acknowledging	O
"	O
the	O
packets	B-Protocol
,	O
a	O
technique	O
known	O
more	O
generally	O
as	O
"	O
protocol	B-Protocol
spoofing	I-Protocol
"	O
.	O
</s>
<s>
This	O
is	O
normally	O
accomplished	O
in	O
the	O
link	O
hardware	O
,	O
notably	O
Telebit	O
modems	B-Application
.	O
</s>
<s>
The	O
modems	B-Application
,	O
when	O
the	O
option	O
was	O
turned	O
on	O
,	O
would	O
notice	O
the	O
XMODEM	B-Protocol
header	O
and	O
immediately	O
sent	O
an	O
ACK	B-Protocol
.	O
</s>
<s>
This	O
would	O
cause	O
the	O
sending	O
XMODEM	B-Protocol
program	O
to	O
immediately	O
send	O
the	O
next	O
packet	B-Protocol
,	O
making	O
the	O
transfer	O
continuous	O
,	O
like	O
an	O
infinite-sized	O
window	O
.	O
</s>
<s>
The	O
modems	B-Application
also	O
suppress	O
the	O
ACK	B-Protocol
being	O
sent	O
by	O
the	O
XMODEM	B-Protocol
software	O
at	O
the	O
far	O
end	O
,	O
thereby	O
freeing	O
up	O
the	O
low-speed	O
return	O
channel	O
.	O
</s>
<s>
The	O
system	O
can	O
also	O
be	O
implemented	O
in	O
the	O
protocol	O
itself	O
,	O
and	O
variations	O
of	O
XMODEM	B-Protocol
offered	O
this	O
feature	O
.	O
</s>
<s>
In	O
these	O
cases	O
,	O
the	O
receiver	O
would	O
send	O
the	O
ACK	B-Protocol
as	O
soon	O
as	O
the	O
packet	B-Protocol
started	O
,	O
in	O
the	O
same	O
fashion	O
as	O
the	O
Telebit	O
modems	B-Application
.	O
</s>
<s>
YMODEM	B-Protocol
formalized	O
this	O
system	O
.	O
</s>
<s>
This	O
concept	O
should	O
be	O
contrasted	O
with	O
the	O
one	O
used	O
in	O
SEAlink	B-Protocol
,	O
which	O
changes	O
the	O
behavior	O
on	O
both	O
sides	O
of	O
the	O
link	O
.	O
</s>
<s>
In	O
SEAlink	B-Protocol
,	O
the	O
receiver	O
stops	O
sending	O
the	O
ACK	B-Protocol
entirely	O
,	O
and	O
the	O
sender	O
changes	O
its	O
behavior	O
to	O
not	O
expect	O
them	O
.	O
</s>
