What does the phrase “The layer 3 protocol at the Q reference point is visible at an interface at the C reference point” mean?
The QSIG protocol is used across a virtual or logical interface between 2 Call Control entities in different PINXs. The reference point describing this virtual or logical interface is the Q reference point. If it exists at all it is usually internal to a PINX. The only point at which you can physically see or “measure” the QSIG protocol is at the C reference point, which is usually a real interface between a PINX and, for example, a leased circuit. Thus, we say it is visible at an interface at the C reference point.
