X.25 Physical Layer

As with any protocol stack, the physical layer specifies the mechanical, electrical, functional, and procedural characteristics that are necessary to activate, maintain, and terminate a physical connection between the DTE and DCE.

The ITU-T X.25 Recommendation does not itself specify how the physical layer should work. Instead, it refers to other ITU-T Recommendations, and specifies which of these may be used. The Recommendations specified are: X.21, X.21 bis, X.31 and the V-series interfaces, that is V11, V24 and V25bis.

Electronic Industries Association (EIA) specifications are often compatible with the V-series interfaces and can be used with X.25. The chart below lists equivalent standards.

Table 2-1 ITU-T V-series and Equivalent EIA Inferfaces

ITU-T	Equivalent EIA
V24, V28	RS232, RS423
V11	RS449