SAAL Signalling ATM Adaptation Layer

The SAAL (Service-Specific Connection-Oriented Protocol, ATM Adaptation Layer) signaling is a component of the ATM (Asynchronous Transfer Mode) protocol stack that provides support for signaling and control functions within an ATM network. It is responsible for establishing, maintaining, and releasing connections between ATM endpoints, as well as managing the ATM adaptation layer (AAL) resources.

The SAAL signaling operates at the ATM adaptation layer, specifically the AAL type 3/4. AAL is responsible for adapting different types of data, such as voice, video, and data, into the ATM format. AAL 3/4 is specifically designed for connection-oriented services, such as circuit emulation and frame relay.

The SAAL signaling protocol is used to exchange control information between the endpoints of an ATM network. It enables the establishment and teardown of virtual connections, negotiation of connection parameters, and exchange of status and error information. SAAL signaling messages are transported within the payload of ATM cells, typically using the VPI/VCI (Virtual Path Identifier/Virtual Channel Identifier) fields to identify the SAAL connection.

SAAL signaling messages are divided into two categories: management and user-to-user messages.

Management Messages:

These messages are used for control functions and network management operations. Some of the common management messages include:

• Connection control messages: Used for connection establishment, modification, and release. These messages contain information about the endpoints, desired connection parameters, and status updates.
• Resource management messages: Used to manage the allocation and deallocation of resources within the AAL layer. These messages help in efficient utilization of network resources.
• Status messages: Used to exchange status and error information between the endpoints. They provide feedback on the status of connections, availability of resources, and error conditions.

User-to-User Messages:

These messages carry user-specific information between the endpoints. The content and format of these messages depend on the specific service or application running over the SAAL connection. For example, in circuit emulation, user-to-user messages may contain voice or data samples that need to be transported over the ATM network.

SAAL signaling follows a connection-oriented approach, which means that before any data transfer can occur, a virtual connection must be established between the endpoints. The connection setup process involves a series of message exchanges between the endpoints, where they negotiate and agree upon the connection parameters, such as QoS (Quality of Service) requirements, traffic parameters, and AAL-specific settings.

Once the virtual connection is established, data transfer can occur over the connection. SAAL provides flow control mechanisms to manage the data flow and ensure that the receiving endpoint can handle the incoming data at the desired rate. It also handles connection teardown when the communication is complete or an error occurs, releasing the network resources associated with the connection.

In summary, SAAL signaling is a crucial component of the ATM protocol stack that enables the establishment, maintenance, and release of virtual connections within an ATM network. It facilitates the exchange of control information and user-specific data between endpoints, ensuring efficient and reliable communication over ATM networks.