Q.931 is a signaling protocol used primarily in ISDN (Integrated Services Digital Network) networks for establishing, managing, and terminating calls. It’s one of the ITU-T recommendations and plays a role similar to other signaling protocols in different types of networks. Here’s a brief overview:
Purpose: Q.931 is responsible for setting up and tearing down connections in ISDN networks. This includes call establishment, maintenance, and call termination.
Protocol Stack: In the ISDN protocol stack:
- Q.931 resides in the D-channel and is responsible for signaling.
- B-channels (Bearer channels) are used for the actual voice or data communication.
Message Types: Q.931 defines several message types that control the call process, including:
- SETUP: Initiates a call.
- CONNECT: Confirms call establishment.
- ALERTING: Indicates the call is ringing at the destination.
- DISCONNECT: Indicates one end wishes to end the call.
- RELEASE: Confirms the disconnection.
- RELEASE COMPLETE: Finalizes the call teardown. (Among several other message types for various scenarios.)
IE (Information Elements): Messages in Q.931 contain different Information Elements, which provide further details about the call, such as the called and calling party numbers, reason for call termination, etc.
Relationship with H.323: When H.323 was developed as a protocol for multimedia communication over packet-switched networks, it borrowed some aspects of Q.931 for call signaling in the early versions. This is why some familiarities exist between H.323 call signaling and Q.931 procedures.
Modern Use: As newer technologies and protocols, like VoIP using SIP, have become more widespread, the prevalence of ISDN and Q.931 has decreased. However, they still find use in certain legacy systems and scenarios where ISDN’s deterministic quality of service is required.
In essence, Q.931 is a fundamental protocol for call signaling in ISDN environments, ensuring that calls are set up, managed, and terminated efficiently.