Circuit-Switched Domains (CS)
The circuit-switched domain, as shown in Figure 2.1, consists of analog and digital
switching nodes, and while the voice may be digitized, it is not packetized. This requires
additional nodes before the voice traffic (or bearer traffic) can be routed over an IP
network. Packetizing the voice is only one requirement though.
Since the facilities within the circuit-switched domain have a different set of attributes
than the domain itself, the method of signaling in the circuit-switched domain is
not compatible with the packet network. In the circuit-switched world, Signaling System
Moving from Legacy to Convergence 41
#7 (SS7) is used to communicate from one switch to another, the facility requirements
for each call. The SS7 protocol specifically identifies the circuits to be used, the attributes
of that circuit (such as codecs to be used), and even if echo cancellation is required
(to name just a few identifiers).
More importantly, it is SS7 that allows switches to communicate with databases such
as number portability to determine how to route calls through the network. SS7 and
the IN architecture have enabled services such as wireless roaming, and many services
we take for granted today (such as Calling Name and Freephone services).
SS7 can be divided into two functions. The first function is for the setup and teardown
of voice circuits between switches in the network.
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