X.25 (3)
X.25 protocol suite
- maps to the lowest three layers of the OSI RM
- PLP : Packet Layer Protocol (network layer); PLP modes: call setup, data transfer, idle, call cleaning, restoring
- LAPB : Link-Access Procedure, Balanced (data-link layer); LAPB use I-frame (information), S-frame (supervision), U-frame (unnumbered)
- X.21bis, EIA/TIA-232, EIA/TIA-229, EIA-530, G.703 (physical layer)
X.121 addresses are used by X.25 PLP in call-setup mode to establish SVCs