Signal Modes And Timing - Cisco 7000 Hardware Installation And Maintenance

Preparing Network Connections
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 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 less retransmits.
Both the sender and the receiver must use the same setting. (See the section "Configuring 32-Bit
Cyclic Redundancy Check (CRC)" in the chapter "Maintenance" for configuration instructions.)

Signal Modes and Timing

DCE is the default mode for all serial ports. You normally use DCE when connecting the router to a
DTE device such as a PC, host, or another router. DCE mode requires an internal clock signal. To
set up a port as a DCE interface, you must use a DCE adapter cable and use the clockrate
configuration command to set the internal clock speed.
Note
attached. When using DCE mode interfaces, you must still specify the internal clock rate.
DTE is the most commonly used mode. You normally use DTE when connecting the router to a DCE
device such as a modem or CSU/DSU. Most DTE interfaces require an external clock signal, which
the remote DCE device provides. Although DCE is the default mode, you do not need to specify the
mode of the port when configuring DTE interfaces. When the port recognizes the DTE interface
cable, it automatically uses the clock signal from the remote DCE device.
Occasionally, systems with long cables may experience high error rates when operating at the higher
transmission speeds. Slight variances in cable construction, temperature, and other factors can cause
the clock and data signals to shift out of phase. Inverting the clock can often correct this shift. If an
FSIP port is reporting a high number of error packets, suspect a phase shift and invert the clock with
the invert-transmit-clock command.
For brief descriptions of the clockrate and invert-transmit-clock commands, refer to the section
"Configuring Timing (Clock) Signals" in the chapter "Maintenance." For complete command
descriptions and instructions, refer to the related software configuration and command reference
documentation on UniverCD or in the printed manuals.
All serial signals are subject to distance limits, beyond which a signal degrades significantly or is
completely lost. For specific cabling distance limitations refer to the section "Distance Limitations"
earlier in this chapter. The distance and rate limits in these descriptions are the IEEE-recommended
maximum speeds and distances for signaling; however, you can usually get good results at speeds
and distances far greater than these. For instance, the recommended maximum rate for V.35 is
2 Mbps, but 4 Mbps is commonly used without any problems. If you understand the electrical
problems that might arise and can compensate for them, you should get good results with rates and
distances greater than those shown here. However, do so at your own risk. We do not recommend
that you exceed published data rates and transmission distances.
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DCE is the default mode because it allows you to perform loopback tests without a cable