HP 200 Series Services And Applications page 204

Improving Network Availability
Circuit-Switching Networks
Public Packet-Switching
Network
Inter-switch
trunks
Switching nodes
Figure 3. LANs Connected through a Packet-Switching Network
Although packet-switching networks are themselves typically highly reliable,
the access circuit that attaches a router to the network is subject to failure
and thus is an obvious point of vulnerability.
The switching nodes used to construct PSNs usually have fault-tolerant
features. However, connecting multiple routers to the same switch increases
the seriousness of a switching node failure if it does occur. See the two
routers in the upper right in figure 3.
Finally, when choosing a PSN to interconnect routers, ask your network
vendor for a description of the physical network to verify that the network
has the redundancy you expect.
Circuit-Switching Networks
Circuit-switching networks are those in which a physical circuit is
established after a number is dialed. Circuit-switching networks include
ISDN, Switched 56 (Kbit/s), Switched 384 (Kbit/s), and the Public Switched
Telephone Network (PSTN).
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