User`s manual
APPENDIX
B
PROTOCOLS
FOR
BINARY
SYNCHRONOUS
COMMUNICA.TIONS
A
protocol
is
a set
of
rules which
govern
the
sequenc-
ing, identification,
and
synchronization
of
data
inter-
changed
between
data
terminals.
This
appendix
describes
the
features
of
two
popular
protocols
to
enable
the
user
to
select
and
plan
for
the
implementa-
tion
of
the
protocol
best suited
to
his needs.
This
appendix
also
provides
the
necessary
background
data
for
understanding
the
data
exchange require-
ments
which
the
DV
II
was specifically designed
to
accommodate.
B.l
DATA
CHANNEL
UTILIZATION
The
DV
II
interchanges
serial.
synchronous,
bytes
or
characters
with
remote
terminals
via
data
channels
or
lines.
The
maximum
efficiency with which a
channel
may
be
utilized
is
determined
by
the
structure
of
the
protocol
being used.
Four
factors
inherent
in
any
protocol
affect
data
channel
utilization efficiency:
direction
utilization
control
overhead
acknowledgement
handling
number
of
data
terminals
or
stations
per
line.
B.1.1 Direction Utilization
A
data
channel
between
two
terminals
may
physically
permit
one-way
or
two-way
transmission,
called sim-
plex
or
duplex
operation,
respectively.
The
two-way
transmission
may
alternate
in direction
of
transmis-
sion,
called
ha/fduplex.
or
may
provide
simultaneous
two-way
transmission,
called full-duplex.
Most
phys-
ical facilities
are
full-duplex, however,
the
protocol
being used
may
not
take
advantage
of
the
physical
facility.
It
may
be a half-duplex
protocol
(alternate
data
transmissions),
although
the
physical facility
is
full-duplex.
To
make
the
most
efficient use
of
a full-
duplex
facility, a full-duplex
protocol
is
required.
B.l.2 Control Ol'erhead
Data
transferred
between
terminals
is
comprised
of
information,
control
and
error-checking
bits. All
but
the
information
bits
are
Control
Overhead
bits. A
B-1
data
terminal
is
capable
of
transmitting
a fixed
num-
ber
of
bits
per
second in
each
direction;
the
control
bits reduce
the
effective
rate
of
information
transfer.
The
ratio
of
the
information
bits
to
the
total
bits
determines
the
one-way
line utilization efficiency.
The
more
control,
header
and
error-checking
charac-
ters needed by a
protocol,
the
less efficient
the
line.
B.1.3 Acknowledgement Handling
Acknowledgement
handling
can
affect line utilization
in
two
ways. First. if
the
acknowledgement
is
a sepa-
rate
message, then
both
the
acknowledgement
and
the
gaps
between the
acknowledgement
and
the
data
blocks
are
part
of
Control
Overhead.
Second,
more
overhead
occurs
if
each message requires a
separate
acknowledgement.
Acknowledgements
within blocks
containing
information
reduce
the
first
overhead
because it usually
takes
fewer
or
no
additional
char-
acters
for
normal
conditions;
only
errors
are
indicated
by
separate
blocks.
If
the
protocol
defines a
way
to
acknowledge
multiple
blocks with
one
response,
the
number
of
overhead
bits is
further
reduced.
B.l.4
Stations
Per
Line
When
the
activity from
one
station
on
a line is below
full utilization,
the
extra
capacity
can
be utilized by
putting
additional
stations
on
the
line.
This
is
similar
to
telephone
party
lines
and
is called
"multipoint"
or
"multidrop."
When
only
two
stations
are
involved, it
is
called
··point-to-point."
Most
protocols
support
both
point-to-point
and
multipoint
arrangements.
For
multipoint
operation,
one
station
in
the
network
is
designated as
the
Control Station.
The
remaining
stations
are
designated
as Tributary Stations.
The
Control
Station
initiates
data
transfers
by
"polling"
and
"selection"
of
Tributary
Stations.
Polling
is
an
invitation
to
send
data,
transmitted
from a
Control
Station
to
a
Tributary
Station.
Selection
is
a
request
to
receive
data,
to
be sent from
the
Control
Station
to
the
Tributary
Station.