Troubleshooting Leased Lines Using Status Lights

CSU/DSU Status Light Troubleshooting

Obviously, there are many vendors and models of CSU/DSU (channel service unit/ data service unit) products on the market, and as you would guess, all are a bit different. The diagnositcs below represent an average CSU/DSU unit. Few, if any, will have all of the LEDs mentioned, but this should serve as a basic guide for what to look for to get a basic idea of what's going on with a circuit, particularly if you don't have access to the router for diagnostics.

POWER ERR ALARM LOOP SYNC TD RD CTS CLOS RLOS Description

flash flash Normal - Passing Traffic

CSU has detected (or been configured for) a loopback test

flash flash flash Circuit is experiencing framing errors, but still passing some traffic.

CSU detects a total disconnect. Circuit cut or disconnected.

Carrier loss of signal. Possible timing, switch misconfiguration, or circuit degradation

Receiver loss of signal. Possible timing, switch misconfiguration

Legend

POWER	Power
ERR	Error indicator
ALARM	Critical alarm indicator
LOOP	Diagnostic loopback indicator
SYNC or RS	DTE sync indicator (Receive signal)
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

POWER	ERR	ALARM	LOOP	SYNC	TD	RD	CTS	CLOS	RLOS	Description
					flash	flash				Normal - Passing Traffic
										CSU has detected (or been configured for) a loopback test
	flash				flash	flash				Circuit is experiencing framing errors, but still passing some traffic.
										CSU detects a total disconnect. Circuit cut or disconnected.
										Carrier loss of signal. Possible timing, switch misconfiguration, or circuit degradation
										Receiver loss of signal. Possible timing, switch misconfiguration