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Which ISDN device converts the four-wire BRI signals from an S/T interface into
the two-wire signals of a U interface?
A. TE1
B. NT-2
C. TA
D. TE2
E. NT-1

Answer: E

Explanation:
When using a router BRI card with an S/T reference point, the router must be cabled to an
external NT1, which in turn is plugged into the line from the telco (the U interface)
Reference: Cisco CCNA ICND p.331