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Preparing
Network
Connections
NRZ
and NRZI
Formats
All data
communications
interfaces
support
both
nonreturn
to
zero
NRZ
and
nonreturn
to
zero
inverted
NRZI
formats
Both
formats
use
two
different
voltage
levels
for transmission
NRZ
signals
maintain
constant
voltage
levels
with
no
signal
transitions
no
return
to
zero voltage
level
during
bit
interval
and
are
decoded
using
absolute
values
and
NRZI
uses
the
same
constant
signal
levels
but
interprets
the presence
of data
at
the beginning
of
bit
interval
as
signal
transition
and
the absence
of data
as
no
transition
NRZI
uses
differential
encoding
to
decode
signals
rather
than
determining
absolute
values
NRZ
the factory
default
on
all
interface
types
is
most
common
All interface
types
also
support
NRZI
format
which
is
commonly
used
with
EIA/TIA-232
connections
in
IBM
environments
See
the section
Configuring
NRZI
Format
in
the chapter
Maintaining
the
Router
for
configuration
instructions
Cyclic
Redundancy
Checks
All data
communications
interfaces
use
16-bit
cyclic
redundancy
check
CRC
by
default
but
also
support
32-bit
CRC
CRC
is
an error-checking
technique
that
uses
calculated
numeric
value
to
detect
errors
in
transmitted
data
The
sender
of
data
frame
divides
the
bits
in
the
frame message
by
predetermined
number
to
calculate
remainder
or
frame
check
sequence
FCS
Before
it
sends
the
frame
the sender
appends
the
FCS
value
to
the
message
so
that
the
frame
contents
are exactly
divisible
by
the predetermined
number
The
receiver
divides
the
frame
contents
by
the
same
predetermined
number
that
the
sender
used
to
calculate
the
FCS
If
the
result
is
not
the receiver
assumes
that
transmission
error
occurred
and
sends
request
to
the sender
to
resend
the
frame
The
designators
16 and
32
indicate
the
number
of
check
digits
per
frame
that
are
used
to
calculate
the
FCS
Sixteen-bit
CRC
which
transmits
streams
of
8-bit
characters
generates
16-bit
FCS
Thirty-two
bit
CRC
which
transmits
streams
of
16-bit
characters
generates
32-bit
FCS
Because
32-bit
CRC
transmits
longer
streams
at faster
rates
it
provides
better
ongoing
error
detection
with
less
retransmits
Both
the sender
and
the receiver
must
use the
same
setting
Sixteen-bit
CRC
is
the
most
widely
used
method
throughout
the United
States
and Europe
and
it
is
used
extensively
with
wide-area
networks
WANs
IEEE-802
specifies
32-bit
CRC
and some
point-to-point transmission
standards
specify
it
as
an option
It
is
often
used
on
SMDS
networks
and
LANs
See
the section
Configuring
32-Bit
Cyclic
Redundancy
Check
in
the chapter
Maintaining
the
Router
for configuration
instructions
Signal
Modes
and Timing
Data communications
ports
operate
in
either
DCE
or
DTE mode
the
mode
depends on
the source
of the clock
signal
that
the port
uses and
the
mode
of the
remote
device
to
which
the port
is
connected
In
DCE mode
the port supplies
its
own
clock
signal
You
normally
use
DCE
when
connecting
the router
to
DTE
device
such
as
PC
host
or another
router
To
set
up
port
as
DCE
interface
you must
use
DCE
adapter
cable
and
use
the clockrate
configuration
command
to
set
the
internal
clock
speed
In
DTE mode
which
is
the
most
commonly
used
mode
the port uses
clock
signal
supplied
by
remote
or external
device
You
normally
use
DTE
when
connecting
the router
to
DCE
device
such
as
modem
or
CSUIDSU
Most
DTE
interfaces
require
an
external
clock
signal
which
the
remote
DCE
device
provides
To
set
up
port
as
DTE
interface
you need
only
connect
DTE
adapter
cables
When
the port recognizes
the
DTE
cable
it
automatically
uses
the clock
signal
from
the
remote
DCE
device
Preparing
for
Installation
2-43
Содержание 7513 Series
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