HP 200 Series Services And Applications page 205

Circuit switching is a fundamentally different concept from packet
switching. In a circuit-switching network the subscribers or users must dial
a phone number to access a remote device such as a router. When the
number is dialed, a data path is established by the circuit-switching network.
Once this data path is established, communications may proceed between
the two connected devices. Users are typically charged for service on the
basis of connect time. Circuit-switched communications are thus point-to-
point and limited in duration. In this application note, "switched circuit" is
used as a generic term referring to connections established over circuit-
switching networks.
In packet-switching networks, the data path between subscribers or users is
permanently established. Users communicate with each other using data-
link-layer or network-layer addresses. The data path between any two users
in a packet-switching network is usually referred to as a virtual circuit.
Users are typically charged for service on the basis of the number of packets
transmitted. Packet-switched communications are thus multi-point and may
or may not be limited in duration.
Circuit Group Considerations
The way in which a switched circuit operates is governed primarily by its
type of circuit group membership. Switched circuits can be configured as:
Primary circuits.
Backup circuits.
Pool circuits.
Note that the circuit-switching capabilities discussed in this application note
pertain only to the HP Router ER, FR, SR, and TR. In particular, these
capabilities are not available on the HP Router CR.
Primary Circuits
A primary circuit is configured when a circuit is added to a circuit group as a
"circuit group member". This is how all circuits other than switched circuits
are usually configured. Switched circuits can be configured as primary
circuits. This is generally the way remote sites that connect to central site
pool circuits are configured; see figure 4.
Improving Network Availability
Circuit Group Considerations
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