Understanding the network.pdf - Back to Home

X.25 networks are comprised of four components: Data Terminal Equipment (DTE),
Packet Assembler/Disassembler (PAD), Data Circuit-terminating Equipment (DCE),
and Packet-Switching Exchanges (PSEs). The DTE is the device that sends and
receives the network data over the packet-switched network. This can be a
computer, point of sale/credit card processing system, or even a data router. This
device sends data to the PAD (or might incorporate this service). The PAD operates
on OSI-RM Layers 2 and 3, and provides the buffering and X.25 packet
assembly/disassembly of the DTE data. At Layer 3, this process includes creation
and removal of the X.25 Packet Layer Protocol (PLP) header. X.25 PLP provides
packet delivery services over the X.25. Layer 2 services needed for formatting the
data are provided by an HDLC derivative protocol called Link-Access Procedure,
Balanced (LAPB), which is also similar to IEEE 802.2. From the PAD, the X.25
packets are handed to the DCE. The DCE provides the OSI-RM Layer 1 facilities,
which are provisioned using a TIA/EIA RS232 interface on a modem or TIA/EIA V.35
on a DSU/CSU. This provides the PSTN circuit termination and bitstream conversion.
The type of DCE termination device depends largely on the type of VC used for
transporting the DTE data.
The PSEs (also OSI-RM Layer 1) are what make up the actual PSN. These devices
are located at the carrier facilities. Different PSEs are interconnected over standard
PSTN trunk facilities. Figure 5.15 illustrates the relation between the different X.25
network elements.
NOTE
Figure 5.15. X.25 network elements.