Advance Information
CNAM (Calling Name Delivery), VMWI (Visual
Message Waiting Indicator) and CIDCW (Calling
Identity Delivery on Call Waiting).
In Europe, Caller ID requirements are defined by
ETSI. The CPE documents are ETS 300 778-1 for
on-hook, ETS 300 778-2 for off-hook. The end office
requirements are ETS 300 659-1 (on-hook) and ETS
300 659-2 (off-hook). ETSI has defined services
such as CLIP and CLIP with Call Waiting which are
similar to those of Bellcore. Some European
countries produce their own national specifications.
For example, in the UK BT’s standards are SIN227
and SIN242, the UK CCA (Cable Communications
Association) standard is TW/P&E/312.
In on-hook Caller ID, such as CND, CNAM and CLIP,
the information is typically transmitted (in FSK) from
the end office before the subscriber picks up the
phone. There are various methods such as between
the first and second rings (North America), between
an abbreviated ring and the first true ring (Japan,
France and Germany). On-hook Caller ID can also
occur without ringing for services such as VMWI. In
BT’s on-hook CLIP, the signalling begins with a line
polarity reversal, followed by CAS and then FSK.
Bellcore calls an on-hook capable Caller ID CPE a
‘Type 1 CPE’.
In off-hook Caller ID, such as CIDCW and CLIP with
Call Waiting, information about a new calling party is
sent to the subscriber who is already engaged in a
call. Bellcore’s method uses CAS to alert the CPE.
When the CPE detects CAS and there are no off-
hook extensions, the CPE should mute its
transmission path and send an acknowledgment to
the end office via a DTMF digit called ACK. Upon
receiving ACK, the end office will send the FSK data.
Bellcore calls an off-hook capable CPE a ‘Type 2
CPE’. A Type 2 CPE is capable of off-hook and Type
1 functionalities and should ACK with a DTMF ‘D’.
The ETSI and BT off-hook signalling protocols are
similar to Bellcore’s but with timing and signal
parametric differences. ETSI has no requirement for
off-hook extension checking before ACK.
One factor affecting the quality of the CIDCW service
is the CPE’s CAS speech immunity. Although the end
office has muted the far end party before and after it
sends CAS, the near end (the end which is to receive
the information) user may be still talking. Therefore
the CPE must be able to detect CAS successfully in
the presence of near end speech. This is called the
talkdown immunity. The CPE must also be immune
to imitation of CAS by speech from both ends of the
connection because the CAS detector is
continuously exposed to speech throughout the call.
This is called the talkoff immunity.
MT88E45
If the CPE is a telephone, one way to achieve good
CAS speech immunity is to put CAS detection on the
telephone hybrid or speech IC receive pair instead of
on Tip and Ring. Talkdown immunity improves
because the near end speech has been attenuated
while the CAS level is the same as on Tip/Ring,
resulting in improved signal to speech ratio. Talkoff
immunity is also improved because the near end
speech has been attenuated.
In the present Bellcore off-hook protocol, the CPE
should not ACK if it detected an off-hook extension.
The FSK will not be sent and the customer will not
receive a paid for service. Bellcore, in conjunction
with the TIA (Telecommunications Industry
Association) TR41.3.1 working group, has defined a
CPE
capability
called
Multiple
Extension
Interworking (MEI) which will overcome this problem.
In the MEI scheme, all MEI compatible CPE’s must
be capable of detecting CAS when the line is off-
hook, even though the CPE itself may be on-hook.
This is because under some conditions an on-hook
CPE may become the ACK sender. Another reason
for the on-hook CPE to detect CAS is to maintain
synchronous call logs between on and off-hook
CPEs. When CAS is received and all off-hook CPEs
are MEI compatible, one of the CPEs will ACK and
all compatible sets will receive FSK.
A problem arises in a CPE where the CAS detector
is connected only to the hybrid or speech IC receive
pair: it cannot detect CAS when it is on-hook. The
reason is that when the CPE is on-hook either the
hybrid/speech IC is non functional or the signal level
is severely attenuated. Therefore an on-hook Type 2
CPE must be capable of detecting CAS from Tip/
Ring, in addition to detecting CAS from the hybrid/
speech IC receive signal when it is off-hook.
The MT88E45 offers an optimal solution which
combines good speech immunity and MEI
compatibility. Two input op-amps allow the MT88E45
to be connected both to Tip/Ring and to the hybrid/
speech IC receive pair. Both connections can be
differential or single ended. FSK demodulation is
always on the Tip/Ring signal. CAS detection can be
from the Tip/Ring or hybrid/speech IC receive signal.
Being able to detect CAS on Tip/Ring also makes the
MT88E45 suitable for BT on-hook CLIP applications.
For applications such as those in most European
countries where Tip/Ring CAS detection is not
needed, then the Tip/Ring and Hybrid op-amp gains
can be tailored independently to meet country
specific FSK and CAS signal level requirements
respectively. Note that since the Hybrid op-amp is for
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