Specifications

Chapter A Networking Concepts
WAN Technologies
A-6
Cisco 1700 Series Router Software Configuration Guide
78-5407-03
X.25
X.25 is a method of packet switching that is used for communication between user
devices (such as routers, bridges, and host machines) and network devices (such
as switching nodes and modems). User devices are called data terminal
equipment (DTE), and network devices are called data circuit-terminating
equipment (DCE).
With X.25, one computer calls another to request a communication session. The
called computer can accept or refuse the connection. If the call is accepted, the
two computers begin full-duplex information transfer. Either computer can
terminate the connection at any time.
User devices communicate with a bidirectional association called a virtual circuit.
Devices on a network use virtual circuits to communicate through intermediate
nodes without being directly, physically connected to each other. Virtual circuits
are permanent or switched (temporary). PVCs are typically used for the
most-often-used data transfers, and SVCs are used for sporadic data transfers.
BRI is an ISDN interface consisting of two B channels (B1 and B2) and one D
channel. The B channels are used to transfer data, voice, and video. The D channel
carries signal and call setup information. IPX, AppleTalk, transparent bridging,
Xerox Network Systems (XNS), DECnet, and IP can all be encapsulated as X.25
over the ISDN B channels.
ISDN uses the D channel to carry signal information. ISDN can also use the D
channel in a BRI to carry X.25 packets. The D channel has a capacity of 16 kbps;
the X.25 over D channel can use up to 9.6 kbps.
You can set the parameters of the X.25-over-D-channel interface without
disrupting the original ISDN interface configuration. In a normal ISDN BRI
interface, the D and B channels are bundled together and represented as a single
interface. The original BRI interface continues to represent the D, B1, and B2
channels.
Because some end-user equipment uses static terminal endpoint identifiers (TEIs)
to access this feature, X.25 supports static TEIs. The dialer recognizes the
X.25-over-D-channel calls and initiates them on a new interface.
X.25 traffic over the D channel can be used as a primary interface when
low-volume, sporadic interactive traffic is the normal mode of operation.
Supported traffic includes IP, IPX, AppleTalk, and transparent bridging.