Cisco WS-X45-SUP6-E Hardware Maintenance Manual - Page 45

Network Connection Considerations 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: router# configure terminal interface serial 0 nrzi-encoding ^Z 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: Preparing for Installation 2-23