Q.920 / Q.921
ISDN specifications for the user-network interface - UNI - data-link layer

Q.2931
An ITU-T specification defining user-to-network interface signaling for broadband ISDN

Q.922A
An ITU-T recommendation for data link layer specification for frame mode bearer services. 

Q.922 A Frame
A variable-length unit of data, formatted in standard frame relay format, transmitted as pure data.

Q.931
CCITT recommendation. The standard for signaling to set up ISDN connections.

Q.93B
CCITT recommendation that is a standard for signaling to set up ATM virtual connections.

QOS
Quality Of Service 
A measurement of the service provided to a user. In ATM networks, QoS measurements related to traffic can be divided into two categories: cell level and connection level. In ST2 (Internet Stream) Protocol, the QoS must satisfy data rate and delay characteristics of specific packet streams.

QPSK
Quadrature Phase-Shift Keying
A digital modulation technique used in modems and wireless networks, allowing the transmission of 2 bits per symbol. Provides a 2:1 compression ratio, resulting in double efficiency for the circuit being used.

QSIG
A global signaling and control standard for PINX to PINX (Private Inter Network exchange) applications, intended for use in private corporate ISDN networks to link multiple vendors' PBX's while retaining feature transparency.