User`s manual
BB
COUNT
FLAG
RESPONSE
SEQUENCE
ADDRESS
CRC-1
DATA
CRC-2
SOH
(ANY
NUMBER
OF
8-BIT
14
BITS
2BITS
8 BITS
8 BITS
8 BITS
16
BITS
CHARACTERS UP
TO
214 )
16
BITS
11-2897
Figure B-4
DDCMP
Data
Message
Format
Figure
B-5
shows a simple example
of
data
exchange
between the D V
III
PD
P-II
and a
data
terminal.
More efficient procedures can be derived after a
study
of
DDCMP.
B.4.2 Error Checking and Recovery
DDCMP
uses CRC-16 for detecting transmIssIon
errors. When an
error
occurs,
DDCMP
sends a sepa-
rate
NAK
message.
DDCMP
does
not
require an
acknowledgment message for all
data
messages.
The
number
in
the response field
of
a normal header
or
in
either the special
NAK
or
ACK
message, specifies
the sequence number
of
the last good message
received.
For
example, if messages 4,
5,
and
6 have
been received since the last time an acknowledgment
was sent and message 6
is
bad, the
NAK
message
specifies number 5 which says "message 4 and 5 are
good
and
6
is
bad."
When
DDCMP
operates in full-
duplex mode, the line does not have
to
be turned
around; the
NAK
is
simply added to the sequence
of
messages for the transmitter.
When a sequence
error
occurs in
DDCMP,
the
receiving station does not respond to the message.
The
transmitting station detects, from the response
field
of
the messages it receives (or via timeout),
that
the receiving station
is
still looking for a certain mes-
sage
and
sends it again.
For
example,
if
the next mes-
sage the receiver expects
to
receive
is
5,
but
6
is
received, the receiver will not change the response
field
of
its
data
messages, which contains a 4. This
says:
"I
accept all messages up through message 4
and I'm still looking for message 5."
B.4.3 Character Coding
DDCMP
uses ASCII control characters for SYN,
SOH,
ENQ
and DLE.
The
remainder
of
the message,
including the header,
is
transparent.
B.4.4
Data
Transparency
DDCMP
defines transparency by use
of
a
count
field
in the header.
The
header
is
of
fixed length.
The
count
in
the header determines the length
of
the
transparent
information field, which can be zero
to
16,383 bytes long.
To
validate the header and count
field, it
is
followed by a 16-bit CRC-16 field; all head-
er characters are included
in
the
CRC
calculation.
Once validated, the count
is
used to receive the
data
and to locate the second CRC-16, which
is
calculated
on the data field. Thus, character stuffing
is
avoided.
B.4.5
Data
Channel Utilization
DDCMP
uses either full-
or
half-duplex circuits at
optim
urn
efficiency.
In
the
full-duplex
mode,
DDCMP
operates as two dependent one-way chan-
nels, each containing its own data stream.
The
only
dependency are the acknowledgments which must be
sent
in
the
data
stream
in
the opposite direction.
Separate
ACK
messages are unnecessary, reducing
the control overhead. Acknowledgments are simply
placed
in
the response field
of
the next message for
the opposite direction.
If
several messages are
received correctly before the terminal
is
able
to
send a
message, all
of
them can be acknowledged by one
response. Only when a transmission
error
occurs
or
when traffic in the opposite direction
is
light (no
data
message to send)
is
it necessary to send a special
NAK
or
ACK
message, respectively.
In summary,
DDCMP
data channel utilization fea-
tures include:
I.
The
ability to run on full-
or
half-duplex
data
channel
facilities.
2.
Low control character overhead.
3.
No
"character
stuffing."
4.
5.
6.
No
separate ACKs when traffic
is
heavy;
this saves on extra SYN characters and
inter-message gaps.
Multiple acknowledgments (up to 255)
with one ACK.
The ability to
support
point-to-point and
multipoint lines.