System information

X.25 Technology Basics
Book Title
19-410
Figure 19-1 The X.25 Model
A DTE can be a terminal that does not implement the complete X.25 functionality. A DTE is
connected to a DCE through a translation device called a packet assembler/disassembler (PAD). The
operation of the terminal-to-PAD interface, the services offered by the PAD, and the interaction
between the PAD and the host are defined by ITU-T Recommendations X.28, X.3, and X.29,
respectively.
The X.25 specification maps to Layers 1 through 3 of the OSI reference model. Layer 3 X.25
describes packet formats and packet exchange procedures between peer Layer 3 entities. Layer 2
X.25 is implemented by Link Access Procedure, Balanced (LAPB). LAPB defines packet framing
for the DTE/DCE link. Layer 1 X.25 defines the electrical and mechanical procedures for activating
and deactivating the physical medium connecting the DTE and the DCE. This relationship is shown
in Figure 19–2. Note that Layers 2 and 3 are also referred to as the ISO standards ISO 7776 (LAPB)
and ISO 8208 (X.25 packet layer).
DTE DCE PSE
DCE DTE
PSE
PSE
PSN
PSN
DCE DTE