Data Sheet MT88E46
5
Functional Overview
In the Calling Identity Delivery on Call Waiting (CIDCW)
and Call Waiting Deluxe (CWD) services offered by
North American telephone operating companies, a dual
tone known as CAS (CPE Alerting Signal) is sent from
the central office to notify the near end CPE, which is
already engaged in an established call, that the central
office wishes to deliver calling identity information of a
waited call. The signalling protocol is specified in
Bellcore GR-30-CORE, the CPE (Customer Premises
Equipment) requirements in SR-TSV-002476.
In the GR-30-CORE off hook protocol, the central office
mutes the far end connection (the other end of the
established call) just before CAS is transmitted. When the
near end CPE detects the CAS, it mutes the handset and
checks whether there is any parallel off hook CPE. If there
is no parallel off hook CPE, it acknowledges CAS
reception by sending ACK, which is a predefined DTMF
digit, back to the central office. When the central office
receives ACK, it transmits the calling party information in
1200 baud Bell 202 format FSK to the near end CPE
which then typically displays the information to the user.
When CAS is transmitted from the central office, even
though the far end has been muted the near end user (the
end which is to receive the caller ID information) may be
speaking. Therefore, the CAS must be detected in the
presence of near end speech, noise or music. Failure to
detect the CAS and reply with ACK within a defined
interval is known as ‘talkdown’. Talkdown is undesirable
because the central office will not deliver the calling
information, hence the quality of the CIDCW or CWD
service will be degraded.
Since CAS can be transmitted anytime during an
established call, the CAS detector is therefore subjected to
speech, noise or music - which can imitate CAS - from
both the near end and the far end throughout the call.
False detection followed by ACK is known as ‘talkoff’.
Talkoff is undesirable because it annoys the far end user
by the near end CPE’s sending ACK and because the near
end CPE is muted in anticipation of the FSK signal.
Bellcore has specified talkdown and talkoff immunity
performance requirements in SR-TSV-002476. If the CPE
is a telephone, one way to achieve good CAS speech
immunity is to put CAS detection on the receive pair of
the telephone hybrid or speech IC instead of on Tip and
Ring. Compared to a Tip/Ring connection, talkdown
immunity improves because the near end speech is
attenuated on the hybrid / speech IC receive pair while the
CAS level is the same as on Tip/Ring. Talkoff immunity is
also better because the near end speech is attenuated.
In the GR-30-CORE issue 1 off hook protocol, the near
end CPE must not ACK if there is a parallel off hook
CPE. Otherwise the ACK will not be detected reliably at
the central office. This restriction is modified by a
protocol known as MEI (Multiple Extension
Interworking) developed by the TIA
(Telecommunications Industry Association) in
conjunction with Bellcore. MEI allows a CPE to ACK if
all off hook CPEs are MEI compatible. MEI is described
in the TIA/EIA-777 standard.
MEI introduces the concept of the ACK-Sender and the
Backup ACK-Sender.
• On a per call basis, the ACK-Sender is the first CPE
to go off hook for the call. It retains its status even if
it returned on hook while the line remains in use.
The ACK-Sender must give up its status if a Type 3
(Analog Display Services Interface) CPE asserts its
ACK-Sender status.
• The Backup ACK-Sender is the CPE to last respond
to CAS with an ACK and successfully received FSK
data. It retains its status from call to call but must
give up its Backup ACK-Sender status when another
CPE successfully completes the CAS-ACK-FSK
sequence.
When CAS is sent from the central office, all MEI
compatible off hook CPEs detect CAS and go back on
hook. After the ACK-Sender detected CAS, it monitors
the line voltage. When the line voltage has returned to the
HIGH state (the voltage when the line is not terminated by
any CPE), it goes off hook and sends the ACK. If there is
no ACK-Sender because the first CPE to go off hook is
not MEI compatible, the Backup ACK-Sender takes over
and sends the ACK. Note that both the ACK-Sender and
the Backup ACK-Sender can be on hook or off hook.
Because it may be the ACK-Sender or Backup ACK-
Sender, an MEI compatible on hook state CPE must be
able to detect CAS when the line is in use. Additionally,
the TIA/EIA-777 standard requires an MEI on hook state
CPE to detect CAS during a call so that it can listen in on
the FSK to keep its call log consistent with the off hook
CPEs. However, a CAS detector connected only to the
hybrid / speech IC cannot detect CAS when the CPE itself
is on hook because either the hybrid / speech IC is not
operational or the signal level is severely attenuated.
Therefore an MEI compatible CPE must be able to detect
CAS from Tip/Ring when the CPE is on hook, and be able
to detect CAS from the hybrid / speech IC when the CPE
is off hook.
The MT88E46 offers an optimal solution which combines
Bellcore compliant speech immunity and MEI
compatibility. Two input op-amps allow the MT88E46 to
be connected to both Tip/Ring and to the receive pair of
the telephone hybrid or speech IC (the 4-wire side). Each
connection can be differential or single ended. FSK
