Q.921/931 ITU-TS (formerly CCITT) Q Series recommendations describing LAP-D, the Layer Two protocol for an ISDN D-channel.

QAM (Quadrature Amplitude Modulation). A modulation method where the carrier amplitude is modulated, together with two carriers of 90-degree phase difference. The information signal is divided into two independent components, termed I and Q, so each carrier describes the value of alternate bits in a bit stream.

Qbit (Qualifier bit). In X.25 networks, bit 8 in the packet header octet which is used to indicate if a packet contains control information.

QLLC (Qualified Logical Link Control) . A datalink layer protocol defined by IBM to make SNA networks compatible with X.25 networks.

QoS (Quality of Service). In telecommunications - the idea that transmission rates, error rates, and other characteristics of the service can be improved upon and to some extent, guaranteed in advance.

QoS (Quality of Service) for ATM. Defined on an end-to-end basis in terms of the following attributes of the end-to-end ATM connection: Cell Loss Ratio, Cell Transfer Delay and Cell Delay Variation.

QPSK (Quadrature Phase Shift Keying). A modulation method where the carrier can assume 4 possible phase states, each 90 degrees apart.

Queuing. Holding an incoming call until the resources become available to connect the call.

QD (Queuing Delay). The delay imposed on a cell due to the unavailability of resources to provide the connection the subscriber desires. This can be caused by congestion or the connection of a higher priority service using the available resources.

QUIPU. A database of electronic mail addresses, full names, and telephone numbers. This software allows someone to connect with X.500 directories around the world to find a particular e-mail address.