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Q5.  Which standard defines Layer 2 hardware addressing for D-channel in ISDN?

A. Q.931

B. Q.921

C. Q.431

D. DSS1

Correct Answer: B

Explanation: ISDN can be mapped to bottom 3 layers of OSI reference model.
Layer 1 is the physical layer and the following protocols are defined at this level:
1. I.430: Defines the communication across S/T reference point.
2. I.431: Defines a full-duplex, point-to-point, serial, synchronous connection and applicable for ISDN PRI.
3. ANSI T1.601: Defines the communication across the U interface (North America only).
Layer 2 and layer 3 work for D channel only. 
Layer 2 of ISDN uses LAPD (Q.921), and Layer 3 of D channel uses Q.931 which is a subset of DSS1.
B channel uses IP/IPX with HDLC/PPP/FR/LAPB protocol.

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