Conﬁguring CRCs

VIP Port Adapter Functions

CRC is an error-checking technique that uses a calculated numeric value to detect errors in transmitted data. All interfaces use a 16-bit CRC by default, but also support a 32-bit CRC. 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 designators 16 and 32 indicate 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. CRC-32, which transmits streams of 16-bit characters, generates a 32-bit FCS. CRC-32 transmits longer streams at faster rates and, therefore, provides better ongoing error correction with fewer retransmissions. Both the sender and the receiver must use the same setting.

The default for all serial interfaces is for 16-bit CRC. To enable 32-bit CRC on an interface, specify the slot and port address of the interface followed by the command crc32. In the example that follows, the ﬁrst serial port on an 4T port adapter on a VIP in interface processor slot 3 is conﬁgured for 32-bit CRC:

Router# configure terminal

Enter configuration commands, one per line. End with CNTL/Z.

Router(config)# interface serial 3/1/0

Router(config-int)# crc32

Router(config-int)# ^Z

To disable CRC-32 and return to the default CRC-16 setting, specify the slot and port address and use the no crc32 command. For command descriptions, refer to the related software documentation.

Replacing Port Adapter Cables

The port adapter cable connected to each port determines the electrical interface type and mode of the port. The default mode of the ports is DCE, which allows you to perform a loopback test on any port without having to attach a port adapter cable. Although DCE is the default, there is no default clock rate set on the interfaces. When there is no cable attached to a port, the software actually identiﬁes the port as Universal, Cable Unattached rather than either a DTE or DCE interface.

Following is an example of the show controller cbus command that shows an interface port (2/1/0) that has an EIA/TIA-232 DTE cable attached, and a second port (2/1/1) that does not have a cable attached:

Router# show controller cbus

slot2: VIP, hw 2.1, sw 200.03, ccb 5800FF50, cmdq 48000090, vps 8192 software loaded from system

FLASH ROM version 255.255, VPLD version 20.1

4T HW Revision 121, SW Revision 216, Unresponsive 0 Interface 24- Serial2/1/0, electrical interface is RS-232 DTE

31 buffer RX queue threshold, 101 buffer TX queue limit, buffer size 1520 Transmitter delay is 0 microseconds

Interface 24- Serial2/1/1, electrical interface is Universal (cable unattached) 31 buffer RX queue threshold, 101 buffer TX queue limit, buffer size 1520

To change the electrical interface type or mode of a port online, you replace the serial adapter cable and use software commands to restart the interface and, if necessary, reconﬁgure the port for the new interface. At system startup or restart, the VIP polls the interfaces and determines the electrical interface type of each port (according to the type of port adapter cable attached).

VIP-4R/4T Installation and Configuration 67