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X.25 Layered Model 229
Figure 8-2
X.25 DTE/DCE
X.25 Layered Model
X.25 employs a three-layered model. Figure 8-3 compares the model to the OSI model.
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. LAPB
defines Layer 2 of the X.25 layered model and performs packet framing for the DTE/DCE link.
Layer 1 of the X.25 layered model defines the electrical and mechanical procedures for
activating and deactivating the physical connection between the DTE and the DCE.
X.25 Layer
Layer 3 of the X.25 layered model utilizes virtual circuits (VCs). These VCs fall into two
categories, permanent and switched. Permanent virtual circuits (PVCs) are nailed up
connections that are always active. Switched virtual circuits (SVCs) are set up and torn down
as needed.
PVC deployments are very common across many of the WAN technologies in use today. They
are nailed up connections and are always active. PVCs require an extremely high level of
maintenance due to their static nature.
PVCs can be equated to static routes in an IP environment. Imagine maintaining every route in
every router manually. With that in mind, it is very easy to imagine the issues that arise in a PVC
environment. They are essentially, after all, static routes. Any failure in the pathway causes the
entire circuit to come down. There is no dynamic rerouting. The circuits must be manually
reconfigured to go around a failure.
DTE
DTE
PDN
DCE
DCE