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CPE Signaling Model
November 2009 340
Information passed from the inbound side to the outbound side is:
Called line condition (for example, free with change, busy, etc.)
Network congestion
ITU recommendations specify a set of 6 frequencies to be used for
forward signals, and another 6 for backward signals. Each signal
consists of 2 frequencies, thus providing 15 forward signals and
15 backward signals. The recommendations allow countries to use
only 5 frequencies (10 signals) for forward signaling and 4
frequencies (6 signals) for backward signaling. Both the forward and
backward signals can assume a number of meanings depending on
the signaling state. Transition from one set of meanings (called a
'group') to another is controlled by transmission of specific signals.
ITU recommendations define 2 groups for forward signals (groups I
and II) and 2 for backward signals (groups A and B). Some countries
chose to define 3 groups in each direction (I, II and III for forward
signals and A, B and C for backward signals). The concept of 'groups'
allows more information to be conveyed (2 groups containing up to
15 signals results in 30 signals in each direction) without having to
increase the number of signal generators and detectors, which are
expensive resources.
The forward channel initiates inter-register signaling with the
transmission of the first DNIS digit. Subsequent forward
transmission is controlled by the last backward channel signal
response.
1. DNIS transmission processing
2. ANI transmission processing
3. Called line condition processing
The backward channel controls inter-register signaling in the sense
that after the first DNIS digit transmission, each subsequent forward
channel signal is a response to the last backward channel signal.
Following the first DNIS digit, the backward channel directs the
collection of the address information according to the inbound
application parameters associated with the call. The backward
protocol is abstracted into four processing states that correspond to
the four blocks of information that are sent by the forward side:
1. DNIS number collection
2. DNIS category
3. ANI number collection
4. ANI category