players in any data communication. They identify the role and responsibilities of two X.25
communicating devices. However, the roles are not necessarily defined by physical
characteristics as is traditionally thought. The X.25 DTE/DCE (that is, the Layer 2 DTE/DCE)
relationship has nothing to do with the electrical characteristics of the interface (that is, the
Layer 1 DTE/DCE). Although it is possible that a device can simultaneously play the role of
the physical DTE or DCE while performing the role of X.25 DTE/DCE, the two are
independent of each other.
to the PDN within a switch. In a lab situation, the setup might have two routers in a back-to-
back configuration; one router must be DTE and the other DCE. This configuration is in
addition to the setting of the clock rate on one router and the utilization of a back-to-back serial
cable with specific electrical characteristics. When the clock rate is set, that device becomes the
DCE physically. It is also possible for the other router to be specified as the X.25 DCE.
packet form. At the same time, the PAD receives transmissions from the X.25 network and
forwards them on to data terminals. The communications between the PAD and the X.25
network edge device are defined by ITU specifications X.3, X.28, X.29, and X.75. Table 8-2
describes each specification.
the device. Typically the DCE refers to a switch or concentrator within the PDN. Figure 8-2
depicts the DTE/DCE relationship.
baud rate, and so on, used in X.25 host connections.
locally. This specification defines the keystrokes for PAD setup similar to modem
AT commands.
configuration through network connections. Upon connect, a remote host can
request changes in terminal configuration parameters.
PDNs. Essentially, this is a type of inter-carrier interconnect (ICI) also known as a
public Network to Network Interface (NNI).