<s>
SEAlink	B-Protocol
is	O
a	O
file	B-Protocol
transfer	I-Protocol
protocol	I-Protocol
that	O
is	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
XMODEM	B-Protocol
but	O
features	O
a	O
sliding	O
window	O
system	O
for	O
improved	O
throughput	O
.	O
</s>
<s>
SEAlink	B-Protocol
was	O
written	O
in	O
1986	O
as	O
a	O
part	O
of	O
the	O
SEAdog	O
FidoNet	O
mailer	O
written	O
by	O
System	O
Enhancement	O
Associates	O
,	O
creators	O
of	O
the	O
famous	O
ARC	B-Application
program	O
.	O
</s>
<s>
SEAlink	B-Protocol
,	O
and	O
most	O
other	O
XMODEM	B-Protocol
enhancements	O
,	O
were	O
quickly	O
displaced	O
following	O
the	O
introduction	O
of	O
ZMODEM	B-Device
.	O
</s>
<s>
XMODEM	B-Protocol
breaks	O
down	O
the	O
file	O
to	O
be	O
sent	O
into	O
a	O
numbered	O
series	O
of	O
packets	B-Protocol
of	O
128	O
bytes	B-Application
each	O
.	O
</s>
<s>
Packets	B-Protocol
are	O
sent	O
one	O
at	O
a	O
time	O
to	O
the	O
receiver	O
,	O
who	O
either	O
acknowledges	O
the	O
proper	O
reception	O
by	O
sending	O
a	O
ACK	O
character	O
or	O
requests	O
it	O
to	O
be	O
resent	O
by	O
sending	O
a	O
NAK	O
instead	O
.	O
</s>
<s>
The	O
transfer	O
then	O
continues	O
with	O
another	O
packet	B-Protocol
.	O
</s>
<s>
That	O
means	O
that	O
the	O
maximum	O
transfer	O
rate	O
of	O
XMODEM	B-Protocol
is	O
a	O
function	O
of	O
the	O
phone	O
system	O
more	O
than	O
the	O
speed	O
of	O
the	O
modems	B-Application
themselves	O
.	O
</s>
<s>
At	O
300	O
bit/s	O
this	O
is	O
not	O
a	O
serious	O
concern	O
,	O
as	O
the	O
packet	B-Protocol
will	O
take	O
about	O
3.5	O
seconds	O
to	O
send	O
,	O
the	O
0.1	O
delay	O
being	O
insignificant	O
.	O
</s>
<s>
However	O
,	O
at	O
9600	O
bit/s	O
the	O
packet	B-Protocol
only	O
takes	O
about	O
0.1	O
seconds	O
to	O
send	O
,	O
meaning	O
that	O
at	O
least	O
half	O
of	O
the	O
potential	O
throughput	O
is	O
lost	O
while	O
the	O
sender	O
waits	O
for	O
the	O
ACK	O
or	O
NAK	O
to	O
arrive	O
.	O
</s>
<s>
SEAlink	B-Protocol
addresses	O
this	O
through	O
the	O
use	O
of	O
a	O
"	O
sliding	O
window	O
"	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
sender	O
does	O
not	O
wait	O
for	O
the	O
ACK	O
or	O
NAK	O
,	O
and	O
instead	O
simply	O
sends	O
out	O
its	O
packets	B-Protocol
in	O
a	O
continuous	O
stream	O
.	O
</s>
<s>
The	O
receiver	O
still	O
has	O
to	O
ACK	O
or	O
NAK	O
the	O
packets	B-Protocol
,	O
but	O
does	O
so	O
by	O
attaching	O
a	O
packet	B-Protocol
number	O
to	O
each	O
one	O
,	O
allowing	O
the	O
sender	O
to	O
identify	O
which	O
packets	B-Protocol
worked	O
or	O
failed	O
.	O
</s>
<s>
The	O
"	O
window	O
"	O
refers	O
to	O
the	O
number	O
of	O
packets	B-Protocol
that	O
can	O
be	O
sent	O
before	O
an	O
ACK	O
or	O
NAK	O
is	O
required	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
SEAlink	B-Protocol
,	O
the	O
window	O
was	O
six	O
packets	B-Protocol
,	O
allowing	O
up	O
to	O
768	O
bytes	B-Application
of	O
data	O
to	O
be	O
sent	O
at	O
once	O
.	O
</s>
<s>
SEAlink	B-Protocol
was	O
designed	O
to	O
be	O
completely	O
backward	B-General_Concept
compatible	I-General_Concept
with	O
existing	O
XMODEM	B-Protocol
implementations	O
.	O
</s>
<s>
On	O
the	O
receiver	O
side	O
,	O
all	O
that	O
was	O
required	O
was	O
that	O
the	O
receiver	O
send	O
back	O
the	O
packet	B-Protocol
number	O
(	O
contained	O
in	O
all	O
XMODEM	B-Protocol
packets	B-Protocol
)	O
along	O
with	O
the	O
normal	O
ACK	O
or	O
NAK	O
character	O
.	O
</s>
<s>
On	O
reception	O
of	O
the	O
very	O
first	O
ACK	O
or	O
NAK	O
,	O
the	O
SEAlink	B-Protocol
sender	O
would	O
"	O
notice	O
"	O
the	O
packet	B-Protocol
number	O
and	O
continue	O
to	O
use	O
windows	O
.	O
</s>
<s>
If	O
the	O
packet	B-Protocol
number	O
was	O
missing	O
,	O
it	O
was	O
assumed	O
it	O
was	O
sending	O
to	O
a	O
non-SEAlink	O
XMODEM	B-Protocol
,	O
and	O
set	O
the	O
window	O
size	O
to	O
one	O
.	O
</s>
<s>
SEAlink	B-Protocol
also	O
included	O
a	O
number	O
of	O
features	O
included	O
in	O
other	O
XMODEM	B-Protocol
derivatives	O
.	O
</s>
<s>
Notable	O
was	O
the	O
use	O
of	O
a	O
"	O
zero	O
packet	B-Protocol
"	O
,	O
which	O
was	O
introduced	O
in	O
the	O
FidoNet-related	O
TeLink	O
protocol	O
in	O
order	O
to	O
send	O
file	O
information	O
and	O
support	O
batch	O
file	B-Protocol
transfers	I-Protocol
.	O
</s>
<s>
SEAlink	B-Protocol
made	O
one	O
minor	O
modification	O
to	O
this	O
"	O
standard	O
"	O
,	O
adding	O
a	O
byte	B-Application
in	O
position	O
40	O
of	O
the	O
packet	B-Protocol
to	O
indicate	O
whether	O
or	O
not	O
to	O
use	O
"	O
overdrive	O
"	O
.	O
</s>
<s>
When	O
turned	O
on	O
,	O
by	O
placing	O
any	O
non-zero	O
number	O
in	O
byte	B-Application
40	O
,	O
the	O
receiver	O
no	O
longer	O
expected	O
ACK	O
messages	O
,	O
but	O
only	O
NAKs	O
.	O
</s>
<s>
This	O
was	O
intended	O
to	O
reduce	O
the	O
bandwidth	O
required	O
from	O
the	O
receiver	O
back	O
to	O
the	O
sender	O
,	O
which	O
was	O
an	O
important	O
consideration	O
on	O
many	O
of	O
the	O
early	O
high-speed	O
modems	B-Application
,	O
which	O
featured	O
a	O
very	O
slow	O
backchannel	B-Protocol
(	O
75	O
bit/s	O
in	O
the	O
Telebit	O
modems	B-Application
,	O
for	O
instance	O
)	O
.	O
</s>
<s>
A	O
later	O
addition	O
was	O
"	O
RESYNC	O
"	O
,	O
allowing	O
for	O
file	B-Protocol
transfers	I-Protocol
to	O
be	O
restarted	O
from	O
where	O
they	O
left	O
off	O
.	O
</s>
<s>
RESYNC	O
added	O
another	O
indicator	O
in	O
the	O
zero	O
packet	B-Protocol
,	O
byte	B-Application
42	O
,	O
to	O
indicate	O
that	O
the	O
sender	O
supports	O
the	O
feature	O
.	O
</s>
<s>
The	O
SYN	O
is	O
followed	O
by	O
the	O
packet	B-Protocol
number	O
at	O
which	O
to	O
restart	O
.	O
</s>
