Q signaling (QSIG) is a protocol used to carry data over integrated services high-speed digital network (ISDN) lines. QSIG is used for signaling across these lines to connect to private branch exchanges (PBXs). Signaling is the transfer of information to points on a network. A PBX is a privately-owned telephone network inside a business, school, or other large entity, like a government office.
PBXs switch calls between lines inside the company and also allow users to make external calls. QSIG is used for ISDNs that use the Q.931 standard to transport information between an individual user and a PBX. QSIG allows equipment from different vendors to work together access a PBX network via the ISDN line.
Networks that include the Q.931 standard use Q signaling to process calls coming in to a network. Q signaling supports the essential network functions of the Q.931 standard. These functions include alerting the network that a call is coming in, establishing a connection, and ending the connection when the call is complete.
QSIG is a common channel signaling protocol (CCS). CCS creates a digital link that is based on packet switching. Packet switching breaks up the information being sent into packets.
These packets can then travel throughout the network on the ISDN line using different paths. The packets are reassembled at the destination point. Since packets can travel separately using the fastest routes currently available, ISDN lines can handle more traffic with increased efficiency.
CCS uses either a subnetwork or a separate network to support communication with the main network. The subnetwork connects all network communication points to a central computer and its databases. This allows the CCS to control and monitor all communications throughout the network. This process is part of the overall network management.
Common uses of Q signaling include telephone services that use an Internet connection instead of phone lines, better known as voiceover IP. QSIG is also used by companies that allow staff to use a private, off-site connection to the company's network. These connections are called Virtual Private Networks (VPNs). QSIG can also be used to transport data throughout a company's private network.
Q signaling works using two layers of communication — basic call (BC) and general function (GF) — to accomplish different tasks. BC allows successful signal transfer throughout a network between different vendors. GF supports enhanced functions inside a company's private network, such as call diversion, and provides support for multiple applications.