1.4.1 ATM Metasignaling

In ATM, before data can flow, a connection must be established. Metasignaling is the process of setting up these signaling channels.

• Default Channel: Vpi=0, Vci=1 is reserved for metasignaling.
• SVC (Switched Virtual Circuit): Created on-demand using signaling protocols (Q.2931).
• PVC (Permanent Virtual Circuit): Manually configured by network administrators.

1.4.2 ATM Addressing Formats

ATM doesn't use IP addresses natively; it uses three types of private network addresses (NSAP format):

1. DCC (Data Country Code): Based on ISO country codes.
2. ICD (International Code Designator): For international organizations.
3. E.164: The standard format for public telephone networks.

1.4.3 VPI/VCI Range and Significance:

• NNI (Network-to-Network Interface): 12 bits for VPI, 16 bits for VCI.
• UNI (User-to-Network Interface): 8 bits for VPI, 16 bits for VCI.
• Total Capacity: A UNI interface can theoretically support $2^8 * 2^{16} +$ connections, though hardware usually limits this much further.

1.4.4 The ATM Cell Header Details (UNI):

1.4.5 Why fixed 48 bytes of payload?

The choice of 48 bytes was a compromise for packetization delay.

• 32 bytes: Ideal for voice (echo cancellation not needed).
• 64 bytes: Ideal for data (efficiency).
• 48 bytes: Acceptable for both, but required echo cancellation for long-haul voice.