Networking Telecommunications


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.

Internal CSU/DSU WIC
Internal CSU/DSU WIC


External CSU/DSU Universal Shelf
External CSU/DSU Universal Shelf


External CSU/DSU
External CSU/DSU


The functions of the LEDs

WIC-1DSU-56K4 Front Panel DSU/CSU WIC to a 56/64-kbps Services Wall Jack RJ48S

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
WIC-1DSU-T1 Front Panel
Push this button to place the WIC into loopback mode. The service provider can send a signal to test the connection from your site to the central office switch. Push this button again to turn off loopback mode.

DSU/CSU WIC to a 56/64-kbps Services Wall Jack RJ48S


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

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