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Q16. Which of the following are reference points relevant to ISDN? [Select 2].
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A. T B. U C. V D. X
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Correct answer: A,B
Explanation:
ISDN uses four different reference points to define logical interfaces:
1. R-reference point: Defines the reference point between non-ISDN compatible devices and a Terminal Adapter (TA).
2. S-reference point: Defines the reference point between user terminals and an NT2.
3. T-reference point: Defines the reference point between NT1 and NT2.
4. U-reference point: Defines the reference point between NT1 devices and line-termination equipment in a carrier network.
The flow of reference points are:
[TE2] ---R--->[TA]---S/T--->[NT1]--->U--->to carrier
[TE2]--->R--->[TA]--->S--->[NT2]--->T--->[NT2]--->to carrier
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