Q.931

Q.931 and TCP/IP work at different layers and share few features. Q.931 has reliable layers but does not provide retransmission or flow control. Q.931 frame elements are as follows:

• Protocol discriminator (PD): Specifies connection signaling protocol.
• Call reference value (CR): Addresses concurrent connections.
• Message type (MT): Specifies three-message layer type (i.e., call setup, release and feature control) from Q.931 call control message set.
• Information elements (IE): Specifies additional message details, that is, name, length and variable content field.