Basic Elements

The following elements represent the main scheme approach for worldwide X.25 networks:

• Switching Packet Nodes (SPN) Performs the sole action of data transit

• Local Packet Switchers (LPS) Performs access functions for DTE X.25 and data-traffic commutation

• Packet Concentrators and Adaptors (PCA) Performs PAD functions and DTE X.28 access (PSTN dialup modem call)

• Management and Operation Centers (MOC) Performs supervisory tasks and controls network and single elements

Transmission speeds change depending on the associated element. In fact, among SPNs the average speed is 64 Kbit/s between the LPSs and the linked SPNs, whereas 9600 bit/s is the average dialogue speed between PCAs and LPSs. Figure 6-5 defines a typical X.25 network structure.

X28

Character DTE (X.28)

X25

Packet DTE (X.25)

Direct circuit access

Direct circuit access

> PSTN access

Figure 6-5 X.25 network structure

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