C H A P T E R
6
Troubleshooting Serial Lines
This chapter describes methods for detecting and correcting data errors on the Cisco 10000 series router
serial interfaces.
Optical Signal Input/Output Problems
Signal input and output problems can occur at any point in the network and can be caused by mechanical
defects in cables or fiber, poor connections, or loss of signal caused by other equipment failures. Refer
to your site log and other facility records to isolate signal connections for your facility.
Fiber-Optic Connections
An optical signal I/O problem can be caused by:
•
Incorrect type of fiber
•
Defective fiber
•
Transmit (TX) and Receive (RX) fibers that are reversed
•
Insufficient power budget on the optical link
•
Receiver overload on the optical link
Be sure to use single-mode fiber for a single-mode interface and multimode fiber for a multimode
interface. Table 6-1 describes the fiber types appropriate for each Cisco 10000 series router line card.
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Optical Signal Input/Output Problems
Table 6-1
Optical Fiber Types for Cisco 10000 Series Router Line Cards
Card Type
Appropriate Fiber Type
OC-12 Packet Over SONET line card
Single mode.
Gigabit Ethernet line card
The appropriate fiber type for the gigabit Ethernet
line card is dependent upon the installed GBIC.
1.
1000BaseSX, multimode.
2.
1000BaseLX/LH, single mode and
multimode1.
3.
1000BaseZX, single mode.
1. Mode-conditioning patch cord (CAB-GELX-625 or equivalent) is required. If you use an ordinary patch cord with MMF,
1000BaseLX/LH GBICs, and a short link distance (10s of meters), you can cause transceiver saturation, resulting in an
elevated bit error rate (BER). In addition, if you use the LX/LH GBIC with 62.5-micron diameter MMF, you must install a
mode-conditioning patch cord between the GBIC and the MMF cable on both the transmit and receive ends of the link. The
mode-conditioning patch cord is required for link distances greater than 984 ft. (300 m).
Evaluating the Power Budget
Use the following equation to ensure that an appropriate power budget has been allotted to optical links
terminating at the Cisco 10000 series router.
The power budget (PB) is the maximum possible amount of power transmitted. The following equation
shows the calculation of the power budget:
PB = PTmin – PRmin
Where:
PTmin = Minimum transmitter power
PRmin = Minimum receiver sensitivity
Insufficient power budget occurs when the power margin (PM) is less than 0. PM is equal to the power
budget minus the link loss (LL).
PM = PB – LL
Three factors contribute to link loss:
Note
•
Fiber attenuation (single mode) 0.5 dB/km
•
Connector 0.5 dB
•
Splice 0.5 dB
These are typical values; refer to the manufacturer for the actual values.
Managing Receiver Overload
Receiver overload can occur when (PRmax – (PTmax – LL)) is less than 0, where PRmax is maximum
receiver power and PTmax is maximum transmitter power. To prevent overloading the receiver, you can
use an attenuator on the link between any singlemode SONET transmitter and the receiver. Doing so
increases the value of LL.
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Using Bit Error Rate Tests
Note
For the gigabit Ethernet line card, PRmax is greater than or equal to PTmax, so an attenuator is unnecessary.
Using Bit Error Rate Tests
This section discusses problem isolation using bit error rate (BER) tests. The topics discussed are:
•
Configuring a BER Test on a T1 Line, page 6-3
•
Sending a BER Test Pattern on a T1 Line, page 6-4
•
Viewing the Results of a BER Test, page 6-5
•
Terminating a BER test, page 6-6
Configuring a BER Test on a T1 Line
BER test circuitry is built into the CT3 line card. With BER tests, you can test cables and signal problems
in the field. You can configure individual T1 lines to run BER tests, but only one BER test circuit exists
for all 28 T1 lines. Hence, only one BER test can be run on a single T3 port at any given time.
There are two categories of test patterns that can be generated by the onboard BER test circuitry:
pseudorandom and repetitive. Pseudorandom test patterns are exponential numbers and conform to the
CCITT/ITU O.151 and O.153 specifications; repetitive test patterns are all zeros, all ones, or alternating
zeros and ones.
A description of each type of test pattern follows:
•
Pseudorandom test patterns:
– 2^11 (per CCITT/ITU O.151)
– 2^15 (per CCITT/ITU O.151)
– 2^20 (per CCITT/ITU O.153)
– 2^20 QRSS (per CCITT/ITU O.151)
– 2^23 (per CCITT/ITU O.151)
•
Repetitive test patterns:
– All zeros (0s)
– All ones (1s)
– Alternating zeros (0s) and ones (1s)
Both the total number of error bits received and the total number of bits received are available for
analysis. You can set the testing period from 1 minute to 14,400 minutes (240 hours), and you can also
retrieve the error statistics anytime during the BER test.
When running a BER test, your system expects to receive the same pattern that it is transmitting. To help
ensure this:
•
Use a loopback at a location of your choice in the link or network.
•
Configure remote testing equipment to transmit the same BER test pattern at the same time.
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Sending a BER Test Pattern on a T1 Line
You can send a BER test pattern on a T1 line with the controller command. The unframed option causes
the BER test pattern to use the entire T1 bandwidth including the T1 framing as well as payload bits. If
“unframed” is omitted then the T1 will be either SF or ESF framed as configured by the T1 framing
command and the BER test pattern will occupy only the T1 payload bits.
t1 t1-line-number bert pattern pattern interval time [unframed]
where:
•
t1-line-number is 1–28.
•
time is 1–14400 minutes.
•
pattern is:
– 0s, repetitive test pattern of all zeros (as 00000...).
– 1s, repetitive test pattern of all ones (as 11111...).
– 2^11, pseudorandom test pattern (2,048 bits long).
– 2^15, pseudorandom O.151 test pattern (32,768 bits long).
– 2^20-O153, pseudorandom O.153 test pattern (1,048,575 bits long).
– 2^20-QRSS, pseudorandom QRSS O.151 test pattern (1,048,575 bits long).
– 2^23, pseudorandom O.151 test pattern (8,388,607 bits long).
– alt-0-1, repetitive alternating test pattern of zeros (0s) and ones (1s), as 01010101.
Examples:
•
Send a BER test pseudorandom pattern of 2^20 through T1 line 10 for 5 minutes.
The example that follows is for a T1, numbered 10, on a CT3 line card in slot 1:
Router(config)# controller T3 1/0/0
Router(config-controller)# t1 10 bert pattern 2^20 interval 5 unframed
•
Send a repetitive pattern of all ones through T1 line 10 for 14400 minutes (240 hours).
The example that follows is for a T1, numbered 10, on a CT3 line card in slot 1:
Router(config)# controller T3 1/0/0
Router(config-controller)# t1 10 bert pattern 1s interval 14400 unframed
Note
You can terminate a BER test during the specified test period with the no t1 bert command.
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Viewing the Results of a BER Test
You can view the results of a BER test using the controller command:
show controllers T3 slot/port-adapter/port/t1-line-number
where: t1-line-number is 1–28.
You can view the results of a BER test at the following times:
•
After you terminate the test using the no t1 bert command.
•
After the test runs completely.
•
Anytime during the test (in real time).
You can view information about a BER test using the controller command:
show controllers T3 slot/subslot/port
where: t1-line-number is 1–28.
Examples follow:
•
The example that follows is for a CT3 line card:
Router# show controllers T3 1/0/0
T3 1/0/0 is up.
C2T3 H/W Version : 3, C2T3 ROM Version : 0.79, C2T3 F/W Version : 0.29.0
T3 1/0/0 T1 1
No alarms detected.
Clock Source is internal.
BERT test result (running)
Test Pattern : 2^11, Status : Sync, Sync Detected : 1
Interval : 5 minute(s), Time Remain : 5 minute(s)
Bit Errors(Since BERT Started): 6 bits,
Bits Received(Since BERT start): 8113 Kbits
Bit Errors(Since last sync): 6 bits
Bits Received(Since last sync): 8113 Kbits
Table 6-2 explains the output of the preceding command, starting at the arrow:
Table 6-2
Interpreting BER Test Results
Output Display Line
Explanation
BERT test result (running)
Indicates the current state of the test. In this case, “running”
indicates that the BER test is still in progress. After a test is
completed, “done” is displayed.
Test Pattern : 2^11, Status : Sync,
Sync Detected : 1
Indicates the test pattern you selected for the test (2^11), the
current synchronization state (sync), and the number of
times synchronization has been detected during this test (1).
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Table 6-2
Interpreting BER Test Results (continued)
Interval : 5 minute(s), Time Remain
: 5 minute(s)
Indicates the time the test takes to run and the time
remaining for the test to run.
If you terminate a BER test, you receive a message similar
to the following:
Interval : 5 minute(s), Time Remain : 2
minute(s) (unable to complete)
“Interval: 5 minutes” indicates the configured run time for
the test. “Time Remain : 2 minutes” indicates the time
remaining in the test prior to termination. “(Unable to
complete)” signifies that you interrupted the test.
Bit Errors(Since BERT Started):
6 bits,
Bits Received(Since BERT start):
8113 Kbits
Bit Errors(Since last sync): 6 bits
Bits Received(Since last sync):
8113 Kbits
Note
These four lines show the bit errors that have been detected
versus the total number of test bits that have been received
since the test started and since the last synchronization was
detected.
Unless unframed is selected, the BER test runs over the configured framing option for the specified T1
line (ESF or SF). Before running a BER test, you should configure the framing option that is appropriate
to your application.
Terminating a BER test
To terminate a BER test, type:
no t1 t1-line-number bert
where: t1-line-number is 1–28.
Examples:
•
Terminate the BER test running on T1 line 10 on the CT3 line card.
Router(config)# controller T3 1/0/0
Router(config-controller)# no t1 10 bert
Using Loopback Tests
The following sections describe the configuration and use of loopback tests in problem isolation:
•
Configuring the Loopback Mode for a T3 Controller, page 6-7
•
Configuring a T3 Controller to Respond to Remote Loopback Commands, page 6-7
•
Configuring the Loopback Mode for a Gigabit Ethernet Interface, page 6-8
•
Configuring the Loopback Mode for an OC-12 POS Interface, page 6-8
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Configuring the Loopback Mode for a T3 Controller
You can configure the loopback modes for a T3 controller by using the loopback command:
loopback [local | network | remote]
The default loopback mode for the T3 controller is no loopback.
To return the T3 controller to its default condition, use the no form of the command.
Examples:
•
Configure a T3 controller for local loopback.
Router(config)# controller T3 1/0/0
Router(config-controller)# loopback local
Local loopback simultaneously loops all channels toward the router and transmits a T3 AIS to the
network. You can use local loopback to diagnose problems with the port when isolated from the
network cables.
•
Configure a T3 port for network loopback.
Router(config)# controller T3 1/0/0
Router(config-controller)# loopback network
Network loopback loops the T3 line back towards the network and can be used to diagnose problems
with cables from the central switching office to the port.
•
Configure a T3 port for remote loopback.
Router(config)# controller T3 1/0/0
Router(config-controller)# loopback remote
Remote loopback sends a command to loop the T3 line at the far end (central office). It can be used
to diagnose problems with cables from the port adapter to the switching office.
Configuring a T3 Controller to Respond to Remote Loopback Commands
The equipment customer loopback command allows a port to respond to loopback commands from
remote T3 equipment. The equipment network loopback causes a controller to ignore remote T3
loopback commands.
Syntax:
equipment [customer | network] loopback
Example:
To enable the controller’s ability to respond to remote loopback requests, type:
Router(config)# controller T3 1/0/0
Router(config-controller)# equipment customer loopback
To prevent a controller from responding to remote loopback commands, type:
Router(config)# controller T3 1/0/0
Router(config-controller)# equipment network loopback
Note
Remote loopbacks are only available when you use c-bit parity framing.
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Configuring the Loopback Mode for a Gigabit Ethernet Interface
To set loopback mode on a gigabit Ethernet interface, use the loopback command in interface
configuration mode.
loopback [internal | external]
[no] loopback [internal | external]
Where:
•
external runs a loopback that requires a loopback connector.
•
internal runs a loopback at the MAC controller using a serializing/deserializing method (SERDES).
Use the no form of the command to stop the loopback.
In the following example, an internal loopback mode is defined for a gigabit Ethernet interface:
router(config)# interface GigabitEthernet 1/0/0
router(config-if)# loopback internal
Tip
If you are performing a hard plug loopback test on a gigabit Ethernet interface, be sure to set the
loopback type for the interface to external. Otherwise, no packets are transmitted onto the fiber optic
cable.
Configuring the Loopback Mode for an OC-12 POS Interface
To enable loopback testing of data transmitted from the PRE card to the OC-12 POS card and back, use
the loopback command in interface configuration mode:
loopback [line | internal]
[no] loopback [line | internal]
Where:
Both line and internal do the following:
•
Loop any data received at the OC-12 POS card’s network interface back into the network.
•
Loop any data received at the OC-12 POS card’s network interface back into the PRE card.
Use the no form of the command to stop the loopback test.
In the following example, a loopback is set for the OC-12 POS line card in slot 5:
Router(config)# interface pos 5/0/0
Router(config-if)# loopback line
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