Information Technology Reference
In-Depth Information
Figure19-1 The X.25 Model
PSN
DCE
DTE
PSE
DTE
DCE
PSE
PSE
DCE
DTE
PSN
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).
