Troubleshooting T1/T3/Serial Circuits

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Initial Steps

Looping a circuit is a test in and of itself, as well as a tool to enable and enhance other testing methods. There are several types of loops available that can be broken down in to two groups - local and remote. Typically you will only loop from core equipment (A-end) to the remote location (Z-end) in order to verify whether a connectivity loss is on the carrier side or customer side. This article will cover the most commonly used local and remote loopbacks.

Cisco

The most common core equipment you will be dealing with will be Cisco based. Core routers have several different types of controllers that may be used, each requiring a slightly different command set to loop the circuit.

Multichannel T1

1. Enter configuration mode using the configure terminal command:
   1. Router#configure terminal
2. Once the device presents the configuration prompt, designate the applicable T1 to configure using the controller t1 [s/p] command:
   1. Router(config)#controller t1 3/4
3. The device should present the controller configuration prompt. At this point you can put the controller in to loopback mode.
   • To apply a local loop to the circuit, enter the loopback local line command:
     • Router(config-controller)#loopback local line
   • To apply a remote loop to the smartjack, enter the loopback remote iboc command:
     • Router(config-controller)#loopback remote iboc
4. To verify that the loop has applied correctly, press Ctrl-Z to exit configuration mode, and use the show controller t1 [s/p] brief command. The resulting output should state that the controller is in loopback configuration:
   1. Router#sh controller t1 3/4 brief
   2. T1 3/4 is up. (Local Line Loopback)
   3. Applique type is Channelized T1
   4. Cablelength is long gain36 0db
   5. Receiver has no alarms.
5. To remove the loop, navigate back to the controller configuration prompt as outlined in steps 1 & 2 above and enter the no loopback command:
   1. Router(config-controller)#no loopback
6. Verify that the loop has been removed as outlined in step 4.

PA-MC-T3 / PA-MC-2T3+ / SPA-2XCT3 / 2CT3+

1. Enter configuration mode using the configure terminal command:
   1. Router#configure terminal
2. Once the device presents the configuration prompt, designate the applicable T3 controller card and slot and port using the controller t3 [c/s] command:
   1. Router(config)#controller t3 1/0
3. The device should present the controller configuration prompt. At this point you can put the controller in to loopback mode by specifying the T1 port that you want to loop, and the type of loopback you want to apply.

• To apply a local loop to the circuit, enter the t1 [p] loopback local command:
  • Router(config-controller)#t1 1 loopback local
• To apply a remote loop to the smartjack, enter the t1 [p] loopback remote line fdl bellcore command:
  • Router(config-controller)#t1 1 loopback remote line fdl bellcore
1. To verify that the loop has applied correctly, press Ctrl-Z to exit configuration mode, and use the show controller t3 [c/s/p] brief command. The resulting output should state that the controller is in loopback configuration:
   1. Router#sh controller t3 1/0/1 brief
   2. T3 1/0 is up.
   3. T1 1 is down timeslots: 1-24
   4. FDL per AT&T 54016 spec.
   5. Configured for locally looped
   6. Currently in locally looped
   7. Transmitter is sending AIS.
   8. Receiver is getting AIS.
   9. Framing is ESF, Clock Source is Internal
2. For a remote loop to the smartjack, also verify if the smartjack successfully entered loopback mode using the show interface serial [c/s/p:x] command:
   1. Router#show interface serial 1/0/1:0
   2. Serial1/0/1:0 is up, line protocol is up (looped)
   3. Hardware is PA-MC-T3
   4. MTU 1500 bytes, BW 1536 Kbit, DLY 20000 usec,
   5. reliability 255/255, txload 1/255, rxload 1/255
3. To remove the loop, navigate back to the controller configuration prompt as outlined in steps 1 & 2 above and enter the no t1 [p] loopback command:
   1. Router(config-controller)#no t1 1 loopback
4. Verify the loop has been removed as outline in steps 4 & 5 above.

SPAC-4XT3/E3 / M1T-T3+ pa

1. Enter configuration mode using the configure terminal command:
   1. Router#configure terminal
2. Once the device presents the configuration prompt, designate the applicable T3 to configure using the controller t3 [s/p] command:
   1. Router(config)#interface serial [s/p]
3. The device should present the controller configuration prompt. At this point you can put the controller in to loopback mode.
   • In order to apply a local loop, enter the loopback local command:
     • Router(config-if)#loopback local
   • In order to apply a remote loop, enter the loopback remote command:
     • Router(config-if)#loopback remote
4. To verify that the loop has applied correctly, press Ctrl-Z to exit configuration mode, and use the show controller t3 [s/p] brief command. The resulting output should state that the controller is in loopback configuration.
5. Also verify that the loop has been successful on the serial interface using the show interface serial [s/p] command.
6. To remove the loop, navigate back to the controller configuration prompt as outlined in steps and enter the no loopback command:
   1. Router(config-if)#no loopback
1. Verify the loop as been removed as outlined in step 4 & 5.

Adtran

1. Enter configuration mode using the configure terminal command:
   1. Router#conf terminal
2. Designate the applicable T1 to configure using the interface t1 [s/p] command:
   1. Router(config)#int t1 1/1
3. The device should present the interface configuration prompt. At this point you can put the interface in to loopback mode.
   • In order to apply a remote loop, enter the loopback remote line fdl command:
     • Router(config-t1 [x])#loopback remote line fdl
4. To remove the loop, enter the no loopback command command:
   1. Router(config-t1 1/1)#no loopback

Samsung

1. Enter test mode:
   1. Router#test
2. Designate the applicable T1 controller to place in to test mode:
   1. Router/test#t1 0/0
3. The device should present the test prompt for that T1. To loop the circuit, enter the loopback remote line ansi fdl command:
   1. Router/test/t1 (p/s)#loopback remote line ansi_fdl
4. To remove the loop, enter the following command:
   1. Router/test/t1 (0/0)#no loopback

Juniper

Currently we do not have access to any Juniper equipment that carries T1/T3 interfaces. For reference, information on monitoring and troubleshooting can be found here: https://www.juniper.net/documentation/en_US/junos/topics/topic-map/interfaces-investigate-interface-steps-and-commands.html

Follow Up

If a circuit goes down and you are able to loop to the smartjack, the problem most likely lies with the customer premise equipment.

If a circuit goes down and you are unable to loop to the smartjack, the problem most likely lies with the line carrier.

Additional Information / Testing

Interface Indications

The state of the interface once a loop has been applied can be used as an additional indicator for troubleshooting.

• Circuit Looped (Up/Up)
  • If the circuit is configured for HDLC, the interface will show the circuit as up/up (looped) when a loop is applied.
• Circuit Looped (Up/Down)
  • If the circuit is configured for PPP, the interface will show the circuit as up/down (looped) when a loop is applied)

Advanced Testing

There are several follow up troubleshooting steps that can be taken while the circuit is still looped. See Advanced T1 Troubleshooting for in-depth instructions.

• Hard/Physical Loopback Test
  • This test involves using a loopback plug/connector to physically loop the circuit. This is typically done at the Z-end/customer premises in conjunction with a technician on the network side who can verify the loop on the A-end equipment.
• Extended Ping Testing
  • This test involves pinging the local interface with varying data patterns while a loop is applied in an attempt to generate errors.
• Bit Error Rate Testing (BERT)
  • This test uses an in-built testing module to send traffic through the circuit while a loop is applied in an attempt to generate errors.

Reference Material

• Cisco - Troubleshooting Serial Lines

====[[Troubleshooting T3 Circuits]]====

http://www.cisco.com/c/en/us/support/docs/asynchronous-transfer-mode-atm/permanent-virtual-circuits-pvc-switched-virtual-circuits-svc/10435-d3e3-tshoot.html

http://en.wikipedia.org/wiki/Remote_error_indication