System information

Troubleshooting ATM Switching Environments
Book Title
21-442
Checking Bit Rates
This procedure outlines the steps for determining whether the bit rate for a port is correctly
configured. This procedure applies only to low-speed line cards:
Step 1 Use the show port port-number all command to display information about a port.
Step 2 Check the Measured Bit Rate field to ensure that the specified bit rate is legal. If the bit
rate is not legal, use the set port c.p characteristics dce-bitrate-bps or set port c.p
characteristics dte-bitrate-bps command, as appropriate, to configure a legal bit rate for
the port. The following is the syntax for the set port command:
characteristics {dce-bitrate Kbits | dte-bitrate bits}
Set the DCE or DTE bit rate for the specified port, depending on the dce-dte-type value
described below. The value of Kbits for the DCE bit rate may be 56, 64, 128, 192, 256,
384, 448, 512, 768, 896, 1344, 1536, 1792, 2688, 3584, 4000, or 5376. The value of bits
for the DTE bit rate is unrestricted in the range of decimal integers 9,000—6,000,000.
Step 3 Compare the Measured Bit Rate with the Admin DCE Rcv Bit Rate field. If the value
shown in the Measured Bit Rate field is significantly different from that shown in the
Admin DCE Rcv Bit Rate field, a problem exists.
Step 4 If the port is DCE, it provides the clocking function. Make sure that the cabling is correct
and that the configured bit rate is valid. If an attempt is made to activate the port when an
invalid bit rate is configured, problems will occur. The value of Kbits for the DCE bit rate
may be 56, 64, 128, 192, 256, 384, 448, 512, 768, 896, 1344, 1536, 1792, 2688, 3584,
4000, or 5376.
Step 5 If the port is DTE, it uses the clock supplied by the attached device (such as a CSU/DSU
or a router). If the correct clock is not being detected, make sure that the correct cable type
is used to connect the port to the attached device and that the attached device is providing
a clock function.
Performing Loopback Tests
Loopback tests can help you pinpoint faults by looping a signal at various points in the network. The
LightStream 2020 ATM switch provides the following two types of loopback tests:
• Remote loopback test—The remote loopback test loops data from an external device through the
I/O module and back. This test verifies that the data sent from the remote end can cross the
telephone company line or cable, pass through the I/O module, and return to the remote end.
• Internal loopback test—The internal loopback test loops data from the line card to the line chip
or to the physical layer protocol processor (PLPP) I/O module to see whether the I/O module is
able to receive data intact.
If the test is successful, data is reaching the I/O module properly. However, a successful test does
not verify whether the I/O module correctly encodes the data that will be sent onto the line.
Note You can loop any port. However, only trunk ports and Frame Relay ports have active port
management protocols that automatically verify the port’s ability to process data.