Signalling is the process by which ATM users and the network exchange the control of information, request the use of network resources, or negotiate for the use of circuit parameters. The Q.2931is ITU version of signaling protocol. Q.2931 specifies the procedures for the establishment, maintenance and clearing of network connections at the B-ISDN user network interface. The procedures are defined in terms of messages exchanged.

The basic capabilities supported by Q.2931 Signaling are as follows:

• Demand (switched virtual) channel connections.
• Point-to-point switched channel connections.
• Connections with symmetric or asymmetric bandwidth requirements.
• Single-connection (point-to-point) calls.
• Basic signalling functions via protocol messages, information elements and procedures.
• Class X, Class A and Class C ATM transport services.
• Request and indication of signalling parameters.
• VCI negotiation.
• Out-of-band signalling for all signalling messages.
• Error recovery.
• Public UNI addressing formats for unique identification of ATM endpoints.
• End-to-end compatibility parameter identification.
• Signalling interworking with N-ISDN and provision of N-ISDN services.
• Forward compatibility

The message types for Q.2931 are the same as in UNI, with few exceptions, such as point-to-multipoint messages (which are not supported in version 3.0/3.1) and SETUP ACKNOWLEDGE and INFORMATION in version 4.0. New signaling messages specific to Q.2931are ALERTING, PROGRESS, SETUP ACKNOWLEDGE, INFORMATION, and NOTIFY.

The VPI/VCI pair and requested bandwidth are allocated as a result of a successful signalling exchange. These messages are sent over the Signalling ATM Adaptation Layer (SAAL), which ensures their reliable delivery.

• Protocol discriminator -Distinguishes Messages for user-network call control from other messages. (9 for Q.2931 messages)
• Call reference - Unique number for every ATM connection which serves to link all signalling messages relating to the same connection. It is comprised of the call reference value and the call reference flag. The call reference flag indicates who allocated the call reference value.
• Message type - Connection control message types.
• Message length - The length of the contents of a message.
• Information Elements - There are several types of information elements. Some may appear only once in the message; others may appear more than once. Depending on the message type, some information elements are mandatory and some are optional. The order of the information elements does not matter to the signalling protocol. Information elements defined in Q.2931 are as follows:
• Called party number.
• Called party sub-address.
• Transit network selection.
• Restart indicator.
• Narrow-band low layer compatibility.
• Narrow-band high layer compatibility.
• Broadband locking shift.
• Broadband non-locking shift.
• Broadband sending complete.
• Broadband repeat indicator.
• Calling party number.
• Calling party sub-address.
• ATM adaptation layer parameters.
• ATM traffic descriptor.
• Connection identifier.
• OAM traffic descriptor.
• Quality of Service parameter.
• Broadband bearer capability.
• Broadband Low Layer Information (B-LLI).
• Broadband High Layer Information (B-HLI).
• End-to-end transit delay.
• Notification indicator.
• Call state.
• Progress indicator.
• Narrow-band bearer capability.
• Cause

ITU-T Recommendation I.363, “B-ISDN ATM Adaptation Layer (AAL) Specification”
http://www.atmforum.com/standards/approved.html#uni : ATM User-Network Interface Specification
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/atm.htm : ATM Overview