T1: A Survival Guide

T1 CSU/DSUs

Figure 6-4 shows a typical CSU/DSU. It has a series of lights and a switch that is used to control the loopback test mode. The meanings of various common indicator lights are discussed in Table 6-2.

Figure 6-4. Typical CSU/DSU

Table 6-2. Common T1 CSU/DSU indicators and their meanings

Indicator	Common labels	Meaning
Clear to send	CTS, CS	The CSU/DSU is ready to receive data.
Request to send	RTS, RS	The DTE is ready to send data.
Carrier detect	DCD, CD	The CSU/DSU is generating carrier signal.
Send data	TX, TXD, SD	The CSU/DSU is transmitting pulses to the telconetwork.
Receive data	RX, RXD, RD	The CSU/DSU is receiving pulses from the telconetwork.
Loss of signal	LOS	When no pulses arrive within 100 to 250 bit times, LOS is declared. Under normal conditions, at least a few pulses would be present in that interval.
Out of frame	OOF	The CSU/DSU triggers this when framing bits are in error and clears it when frame synchronization is regained.
Loss of frame(red alarm)	LOF	The CSU/DSU asserts this when OOF has persisted continuously for 2.5 seconds. This alarm is usually cleared when frame synchronization has been obtained for at least one second, but some CO hardware may not clear the alarm for much longer (15 seconds or more).
Keepalive transmission	KA	When framing is lost, the CSU/DSU transmits a keepalive signal to other network components.(See ...