A Data Service Unit/Channel Service Unit (DSU/CSU) WAN Interface Card ( WIC) these days is usually a blade on a router. In the past, these were separate. The CSU originated at AT&T as an interface to their non-switched digital data system. The DSU provides an interface to the data terminal equipment (DTE) using a standard (EIA/CCITT) interface. It also provides testing capabilities. They evolved from standalone hardware, to shelf type systems and are now just a blade or Wan Interfacde Card (WIC) in a router.
The functions of the LEDs
| LED | Description |
| TD | Data is being transmitted to the DTE interface. |
| RD | Data is being received from the DTE interface. |
| LP | Internal DSU/CSU is in loopback mode. |
| AL | One of these alarm conditions is present: no receive signal, loss of frame signal from the remote station, or out of service signal from the remote station. This LED is off during normal operation. |
| CD | Internal DSU/CSU in the WIC is communicating with another DSU/CSU. This LED is on during normal operation |
There are many manufactures, each with their own ideas of abbreviations so TD, TX, or TXD all mean you’re transmitting data. You may not have every LED but in general…
| Power, PWR | Power |
| ERR, ER | Error indicator |
| AL, ALARM | Critical alarm indicator |
| Loop, LP | Diagnostic loopback indicator |
| SYNC, RS | DTE sync indicator (Receive signa from telco) |
| TD, TX, or TXD | Transmit data |
| RD, RX, or RXD | Receive data |
| CTS | Clear to send (per flow control) |
| CLOS | Carrier loss of signal |
| RLOS | Receiver loss of signal |
What you could see
| Scenario | Power | Err | Alarm | Loop | Sync | TD | RD | CTS | CLOS | RLOS | Description |
| 1 | flash | flash | Normal – up and passing traffic | ||||||||
| 2 | Loopback mode detected from telco or configured in CPE | ||||||||||
| 3 | flash | flash | flash | Circuit is experiencing errors, but still passing traffic. | |||||||
| 4 | CSU detects a total disconnect. Circuit disconnected/no cable. | ||||||||||
| 5 | Carrier loss of signal. Possible timing, switch misconfiguration, or circuit degradation | ||||||||||
| 6 | Receiver loss of signal. Possible timing, switch misconfiguration | ||||||||||
| 11 | No Power |
See Also:
DataSMART-558 DataSMART_558 DataSmart_500_Series
http://www.cisco.com/en/US/products/hw/modules/ps3129/products_tech_note09186a00800b1501.shtml
http://www.cisco.com/en/US/products/hw/routers/ps233/products_tech_note09186a0080093c56.shtml
http://www.hypersurf.com/blog/t1-line/how-to-connect-a-business-t1-line/