3com 3031 Instruccion De Instalación

X.25 recommendation specifies the interface between Data Terminal Equipment 
(DTE) and Data Circuit-terminating Equipment (DCE). In 1974, CCITT issued the 
first draft of X.25, whose initial files were based on the experiences and 
recommendations of Telnet and Tymnet of USA and Datapac packet-switched 
networks of Canada. It was revised in 1976, 1978, 1980 and 1984, added many 
optional service functions and facilities.

With X.25, two DTE can communicate with each other via the existing telephone 
network. X.25 sessions are established when one DTE contacts another to request 
a communication session. The DTE that receives the request can either accept or 
refuse the connection. Once the connection is established, the devices at both 
ends can transmit information in full duplex mode, and any end has rights to 
disconnect the connection at any time.

X.25 is the protocol of point-to-point interaction between DTE and DCE. DTE 
usually refers to the host or terminal at the user side, and DCE usually refers to the 
synchronous modem. DTE is connected with DCE directly, DCE is connected to a 
port of packet switching exchange, and some connections are established 
between the packet switching exchanges, thus forming the paths between 
different DTE. In an X.25 network, the relation between entities is shown in the 
following diagram:

Figure 61   X.25 network model

The X.25 protocol suite maps to the lowest three layers of the OSI (Open System 
Interconnection) reference model. As shown in the following figure, layer 3 
(packet layer) provision of X.25 describes the packet format used by the packet 
layer and the procedure of packet switching between two layer-3 entities. Layer 2 

DTE  (Data Terminal Equipment)
DCE  (Data Circuit-terminating Equipment)
PSE  (Packet Sw itching Equipment)
PSN  (Packet Switching Netw ork)

DTE  (Data Terminal Equipment)
DCE  (Data Circuit-terminating Equipment)
PSE  (Packet Sw itching Equipment)
PSN  (Packet Switching Netw ork)