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What is Q Signaling?

By N.M. Shanley
Updated May 16, 2024
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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.

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Discussion Comments
By NathanG — On Dec 04, 2011

@Charred - I am not an expert in telecommunications, but I am going to guess that some of the more advanced communications services offered to customers using VOIP are handled by the General Function layer of Q signaling.

For example, I would have to imagine that call blocking or caller ID may be handled by this protocol as well. I don’t know this for certain, but I do know that VOIP providers offer a large bundle of features, and I assume that these features are made possible by the Q signaling protocol.

By Charred — On Dec 03, 2011

I worked in the telecommunications industry for over ten years, and Q signaling was one of the first – and most useful – concepts I learned about.

It’s used with PBXs in businesses as the article talks about, usually within VOIP (Voice Over Internet Protocol) contexts. What is fascinating is how these calls, which are packet based, get assembled and reassembled.

Your voice call is chopped up in individual packets, and there is header information attached to these packets so that when it’s reassembled, it’s in the right order.

Otherwise, you would experience “drop outs,” where parts of the call are missing. In the early days of making calls over the Internet drop outs were common but now not so much anymore.

We focused on selling the VOIP services mainly to businesses using PBXs more than we did residential customers, as I think it was easier to refine the technology for internal communications.

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