Call Processing and System Logic
In addition to the user interface found on most PBX systems, there is also logic
that controls the flow of calls.The basic process is based on dialing plans, which
compare the DTMF tones to the route plans and paths configured on the PBX.
These tones represent the numeric values of the buttons, in addition to the
asterisk (*) and pound (#) keys. Using the phone number or extension dialed,
the PBX routes the call either to the external trunk (the link to the public network),
to another PBX within the company (which is carried on an internal
trunk), or to another extension within the PBX.This addressing is signaled using
the DTMF tones.
The PBX can also make decisions based on its static tables in a dynamic
fashion.You’re probably thinking this doesn’t make sense, but it does. Recall that
a PBX route plan specifies the path an outbound call should take.What would
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Old World Technologies • Chapter 1 9
happen if that path failed? Simply, the administrator would specify an alternate
path—analogous to a floating static route in Cisco routing.These less-preferred
routes could be configured for call overflow (where insufficient capacity exists on
the primary link) or trunk failure (where the link must completely fail before
taking an alternate path).This decision adds a dynamic to the typically static limitations
of the PBX forwarding system.
NOTE
Most PBX phones are digitally connected to the PBX and do not send
the actual DTMF tones from the phone to the switch. Traditional analog
phones and some PBX phones will send the actual tones to be interpreted
by the switch. However, the call routing is still based on the
numbers pressed and received, and the non-Signaling System 7 (SS7)
signaling is either proprietary or DTMF.
As a designer, you may specify that long-distance calls (indicated with a 9, followed
by a ten-digit number, for example) should use a trunk to long-distance
provider A, which also provides the lowest cost per minute to the company.The
alternate path, configured for overflow calls, might go to long-distance company
B, which may also charge more per call. A backup path, using the local exchange
carrier, may be configured in the event the first two paths are unusable.
The system logic and call processing functions typically include collections of
billing information and other call accounting data that can be used for capacity
planning and charge-back services.These functions are independent of the final
PBX functional area: switching.