BT specifications SIN227 and SIN242 specify the
signalling mechanism between a network and the
Terminal Equipment (TE) providing Caller Display
Service (CDS). CDS provides Calling Line Identity
Presentation (CLIP), that is, delivery of the identity of
the caller when a telephone call arrives, before the
start of ringing (in the Idle State).
An incoming CDS call is indicated by a polarity
reversal on the A and B wires (see figure 3), followed
by an Idle State Tone Alert Signal. Caller ID FSK
information is then transmitted in CCITT V.23 format.
CNIC2 has the capability to detect the line reversal,
tone alert signal, and demodulate the incoming
CCITT V.23 FSK signals.

The U.K.’s CCA specification TW/P&E/312 proposes
an alternate CDS TE interface. According to TW/
P&E/312, data is transmitted after a single burst of
ringing rather than before the first ringing cycle (as
specified in the BT standards). The Idle State Tone
Alert Signal is not required as it is replaced by a
single ring burst. CNIC2 has the capability to detect
the ring burst. It is also able to demodulate either
Bell-202 or CCITT V.23 FSK data following the ring
burst. The U.K.’s CCA specifies that data can be
transmitted in either format.