MT88E45
Data Sheet
to alert the terminal before FSK data transmission. BT uses CAS to alert the terminal prior to FSK in both on-hook
(Idle State) and off-hook (Loop State) signalling.
In North America, Caller ID uses the voiceband data transmission interface defined in the Bellcore document GR-
30-CORE. The terminal or CPE (Customer Premises Equipment) requirements are defined in Bellcore document
SR-TSV-002476. Typical services are CND (Calling Number Delivery), 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.
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 Bellcore SR-TSV-002476 Issue 1 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 the Call Waiting ID. Bellcore, together with the
TIA (Telecommunications Industry Association) TR41.3.1 working group, has defined a CPE capability called
Multiple Extension Interworking (MEI) which overcomes this problem.
In the MEI scheme, all MEI compatible CPEs 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 CPEs 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
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