Q
Q.931
ISDN connection control protocol, roughly comparable to TCP in the Internet protocol stack. Q.931 doesn't provide flow control or perform retransmission, because the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates bandwidth in fixed increments of 64 kbps. Q.931 does manage connection setup and breakdown. In H.323 scenario, this protocol is encapsulated in TCP and sent to port 1720.
QoS (Quality of Service)
Measure of performance for a transmission system that reflects it’s transmission quality and service availability. Standards based QOS for VoIP usually involves the implementation of Ethernet standards 802.1p and 802.1q at layer 2 across an Ethernet.
QSIG (Q (point of the ISDN model) Signaling)
Signaling standard. Common channel signaling protocol based on ISDN Q.931 standards and used by many digital PBXs.
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