Configuring Cyclic Redundancy Checks
Specifying E3 Framing
In interface configuration mode, specify E3 framing by entering the framing {g751 bypass} configuration subcommand, as in the following example:
router(config-if)# framing g751
Use the no form of this command to return to the default, which is G.751 framing.
Note If you use the bypass option, scrambling must be set to the default, disabled; the DSU mode must be set to the default, 0; and the DSU bandwidth must be set to the default, 34010.
Setting the Cable Length
In interface configuration mode, set the cable length (0 to 450 feet), as in the following example:
router(config-if)# cablelength 250
Use the no form of this command to return to the default, which is 10 feet.
Configuring Cyclic Redundancy Checks
The PA-E3 uses a 16-bit cyclic redundancy check (CRC) by default; it also supports a 32-bit CRC.
The 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 sending 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.
Note To enable a 32-bit CRC on an interface, follow Step 1 and Step 2 of the preceding procedure (enter configuration mode and specify the slot and port address of the PA-E3, and the interface port number for a VIP2), and then enter the command crc 32. Enter Ctrl-Zto exit from configuration mode.
For a Cisco 7100 series, Cisco 7200 series, or Cisco uBR7200 series router, use the following example:
router# configure terminal
Enter configuration commands, one per line. End with CNTL/Z. router(config)#
router(config)# interface serial 3/0
router(config-int)# crc 32
Ctrl-z
