<s>
The	O
B	B-Protocol
protocol	I-Protocol
,	O
or	O
CIS	O
B	O
,	O
is	O
a	O
file	B-Protocol
transfer	I-Protocol
protocol	I-Protocol
developed	O
for	O
the	O
CompuServe	O
Information	O
Service	O
,	O
and	O
implemented	O
in	O
1981	O
.	O
</s>
<s>
Since	O
B	B-Protocol
protocol	I-Protocol
was	O
designed	O
only	O
to	O
work	O
within	O
the	O
CompuServe	O
,	O
most	O
third-party	O
communications	O
clients	O
of	O
the	O
day	O
were	O
not	O
compatible	O
with	O
it	O
.	O
</s>
<s>
Notable	O
exceptions	O
were	O
Tera	B-Protocol
Term	I-Protocol
and	O
Datastorm	B-Protocol
's	O
ProComm	B-Protocol
Plus	I-Protocol
on	O
the	O
PC	O
which	O
featured	O
the	O
ability	O
to	O
listen	O
for	O
the	O
command	O
on	O
the	O
active	O
communications	O
port	O
,	O
and	O
ZTerm	B-Operating_System
on	O
the	O
Mac	B-Device
which	O
allowed	O
auto-starting	O
transfers	O
.	O
</s>
<s>
The	O
original	O
version	O
of	O
the	O
B	B-Protocol
Protocol	I-Protocol
was	O
an	O
outgrowth	O
of	O
an	O
earlier	O
bi-directional	O
protocol	O
introduced	O
in	O
1979	O
,	O
adding	O
options	O
for	O
including	O
a	O
standardized	O
command	O
structure	O
in	O
the	O
stream	O
.	O
</s>
<s>
The	O
AgVision	O
system	O
,	O
with	O
the	O
modem	B-Application
removed	O
,	O
became	O
the	O
basis	O
of	O
the	O
TRS-80	B-Device
Color	I-Device
Computer	I-Device
.	O
</s>
<s>
All	O
potential	O
problematic	O
control	O
characters	O
were	O
always	O
quoted	B-Algorithm
,	O
a	O
requirement	O
because	O
many	O
people	O
accessed	O
CompuServe	O
over	O
non-8-bit-clean	O
packet	O
services	O
such	O
as	O
Tymnet	B-Application
.	O
</s>
<s>
Characters	O
in	O
the	O
Body	O
or	O
Trailer	O
are	O
"	O
quoted	B-Algorithm
"	O
.	O
</s>
<s>
Officially	O
only	O
a	O
few	O
characters	O
are	O
quoted	B-Algorithm
,	O
,	O
,	O
,	O
(	O
XON	O
)	O
,	O
(	O
XOFF	O
)	O
and	O
.	O
</s>
<s>
Typically	O
three	O
other	O
characters	O
are	O
quoted	B-Algorithm
as	O
well	O
,	O
,	O
+	O
0x80	O
and	O
+	O
0x80	O
.	O
</s>
<s>
Characters	O
are	O
quoted	B-Algorithm
by	O
adding	O
0x40	O
to	O
their	O
ordinal	O
value	O
,	O
and	O
prefixing	O
them	O
with	O
the	O
character	O
.	O
</s>
<s>
The	O
Check	O
Value	O
was	O
quoted	B-Algorithm
,	O
as	O
were	O
the	O
contents	O
it	O
checked	O
against	O
,	O
but	O
the	O
value	O
inside	O
was	O
the	O
check	O
of	O
the	O
unquoted	O
values	O
.	O
</s>
<s>
Four	O
types	O
of	O
Check	O
Values	O
were	O
allowed	O
,	O
the	O
original	O
XMODEM	B-Protocol
checksum	B-Algorithm
,	O
a	O
slightly	O
modified	O
version	O
of	O
the	O
cyclic	O
redundancy	O
check	O
(	O
CRC	O
)	O
used	O
in	O
XMODEM-CRC	O
,	O
or	O
the	O
CCITT	O
CRC-16	O
or	O
CRC-32	O
.	O
</s>
<s>
The	O
most	O
common	O
packets	O
,	O
in	O
terms	O
of	O
overall	O
number	O
transferred	O
,	O
are	O
T	O
packets	O
carrying	O
the	O
data	O
for	O
a	O
file	B-Protocol
transfer	I-Protocol
.	O
</s>
<s>
This	O
had	O
to	O
be	O
indicated	O
in	O
this	O
fashion	O
because	O
,	O
unlike	O
a	O
file	B-Protocol
transfer	I-Protocol
,	O
the	O
amount	O
of	O
data	O
being	O
sent	O
would	O
not	O
be	O
known	O
in	O
advance	O
.	O
</s>
<s>
Possibly	O
the	O
only	O
user	O
of	O
the	O
Transport	O
Layer	O
was	O
CompuServe	O
's	O
own	O
Host-Micro	O
Interface	O
(	O
HMI	O
)	O
API	B-Application
.	O
</s>
<s>
HMI	O
defined	O
a	O
number	O
of	O
commands	O
that	O
could	O
be	O
used	O
to	O
drive	O
CIS	O
,	O
along	O
with	O
the	O
possible	O
responses	O
to	O
them	O
,	O
bypassing	O
the	O
command	B-Application
line	I-Application
interface	I-Application
.	O
</s>
<s>
CIS	O
expanded	O
HMI	O
to	O
allow	O
control	O
of	O
most	O
of	O
the	O
batch-oriented	O
interface	O
,	O
including	O
functions	O
for	O
e-mail	O
,	O
conferences	O
and	O
file	B-Protocol
transfers	I-Protocol
.	O
</s>
<s>
Transport	O
Layer	O
streams	O
could	O
not	O
take	O
place	O
at	O
the	O
same	O
time	O
as	O
file	B-Protocol
transfers	I-Protocol
,	O
so	O
in	O
general	O
terms	O
the	O
applications	O
using	O
the	O
Transport	O
Layer	O
were	O
fairly	O
modal	O
.	O
</s>
<s>
For	O
instance	O
,	O
CIS	O
Navigator	O
for	O
the	O
Mac	B-Device
,	O
which	O
was	O
HMI	O
based	O
,	O
allowed	O
users	O
to	O
navigate	O
CIS	O
offline	O
,	O
setting	O
up	O
various	O
e-mail	O
and	O
file	B-Protocol
transfers	I-Protocol
which	O
would	O
then	O
be	O
carried	O
out	O
in	O
a	O
single	O
batch	O
in	O
order	O
to	O
reduce	O
online	O
time	O
.	O
</s>
