Cisco EF4116 Hardware Installation And Maintenance Manual page 45

To turn off this command, use the no dce-terminal-timing-enable command.
When the serial port is a DTE, the invert-txc command inverts the TXC clock signal it receives from
the remote DCE. When the serial port is a DCE, this command inverts the clock signal to the remote
DTE port. Use the no invert-txc command to change the clock signal back to its original phase. The
no invert-txc command is redundant with the four-port serial module because the module will
automatically discover the polarity of the clock and invert the signal.
Configuring NRZI Format on the Four-Port Serial Module
All 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 a zero voltage level) during a bit interval and
are decoded using absolute values (0 and 1). NRZI uses the same constant signal levels but interprets
the presence of data at the beginning of a bit interval as a signal transition and the absence of data as
no transition. NRZI uses differential encoding to decode signals, rather than determining absolute
values.
NRZ format, which is the factory default on all interfaces, is commonly used for EIA/TIA-232
connections in IBM environments. To enable NRZI encoding on any interface, specify the port
address of the interface followed by the command nrzi-encoding. In the example that follows, serial
port 0 is configured for NRZI encoding:
To disable NRZI encoding on a port, specify the port and use the no nrzi-encoding command. For
complete command descriptions and instructions, refer to the related software documentation.
Calculating CRCs on the Four-Port Serial Module
All Cisco 4000 series router serial interfaces support CRC-CCITT, a 16-bit cyclic redundancy check
(CRC). CRC is an error-checking technique that uses a calculated numeric value to detect errors in
transmitted data. The sender of a data frame divides the bits in the frame message by a predetermined
number to calculate a 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 0, the receiver assumes that a
transmission error occurred and sends a request to the sender to resend the frame.
The designator 16 indicates the number of check digits per frame that are used to calculate the FCS.
CRC-16, which transmits streams of 8-bit characters, generates a 16-bit FCS. Both the sender and
the receiver must use the same setting of 16.
The default for all serial interfaces is for 16-bit CRC.
Checking the Configuration of Serial Interfaces
After configuring your serial interfaces, use the show interface command to check the network
interface statistics. Options to the show interface command include interface, the type of interface
(for example, serial), and unit, the unit number of the interface. The following example shows the
output of show interface serial 0:
router# configure terminal
interface serial 0
nrzi-encoding
^Z
Network Connection Considerations
Preparing for Installation 2-23