ATM Call Signaling : Understanding PNNI, UNI and Message Flow
ATM (Asynchronous Transfer Mode) networks rely on sophisticated signaling mechanisms to establish, maintain, and release connections. Two key signaling protocols in ATM networks are Private Network-to-Network Interface (PNNI) and User-Network Interface (UNI). These protocols handle the exchange of control messages, ensuring efficient connection management. In this guide, we will explore the message flow in PNNI and UNI signaling, how these protocols facilitate ATM connections, and their roles in call setup, maintenance, and teardown processes.
ATM UNI Signaling

The figure-1 depicts UNI signaling messages exchanged between user and ATM network.
Following messages are exchanged between source host, network and destination host:
• Setup
• Call Proceeding
• Connect
• Connect ACK
• Release
• Release Complete
ATM PNNI Signaling

The figure-2 depicts PNNI signaling messages. This signaling type lies between private network node interface and between private network to network interface. Here routing protocol specifies selection of routes as per QoS need. PNNI is a signaling protocol for the exchange of messages between switches and private networks.
Following messages are exchanged between source machine, switches (source and transit) and destination machine.
• Setup
• Call proceeding
• Connect
• Connect ACK
• Release
• Release Complete
Conclusion
ATM call signaling, particularly PNNI and UNI protocols, plays a critical role in the efficient management of ATM connections. Understanding how these protocols function and the flow of signaling messages allows for better network management and optimization. By mastering ATM call signaling, network administrators can ensure seamless and reliable communication across ATM networks, delivering on the promise of high-speed, connection-oriented data transfer.
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