Which OSI/OSI layer defines the X.24, V.35, X.21 and HSSI standard interfaces?

A.
Transport layer

B.
Network layer

C.
Data link layer

D.
Physical layer

Explanation:
X.25, V.35, X21 and HSSI all work at the physical layer in the OSI model.
X.25 is an older WAN protocol that defines how devices and networks establish and maintain connections.
V.35 is the interface standard used by most routers and DSUs that connect to T-1 carriers.
X21 is a physical and electrical interface.High-Speed Serial Interface (HSSI) is a short-distance communications interface.
Incorrect Answers:
A: X.25, V.35, X21 and HSSI all work at the physical layer, not the transport layer.
B: X.25, V.35, X21 and HSSI all work at the physical layer, not the network layer.
C: X.25, V.35, X21 and HSSI all work at the physical layer, not the data link layer.

Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, New York, 2013, p. 679