Caller identity delivery (CID) is a member of the custom local-area signaling services (CLASS) family of telephone services offered by local telephone companies. CLASS services vary by the service provider. Caller ID is one of the most popular call services. Caller ID, which is also known as calling number delivery (CND), is a method of transmitting telephone caller information, such as telephone number and/or caller name. In a public switched telephone network (PSTN), the terminal receiving the call features is referred to as terminal equipment (TE). In most PSTN applications, the telephone is the TE. The customer premises equipment (CPE) name is also used in place of TE. In general, the call features description makes use of the names TE and CPE from the PSTN service. In VoIP, the CPE name is used for the VoIP adapter, Internet Protocol (IP) phone, wireless fidelity (WiFi) handset, and so on. In VoIP, deployment infrastructure, VoIP CPEs, and telephones used with CPE to establish call features. Two primary methods of caller ID delivery services are normal caller ID and call wait ID (CWCID). Normal caller ID happens during the on-hook phase of the phone. It is also known as Type-1 caller ID. Call wait ID or call wait caller ID is displayed in the middle of the on-going call. It happens in the off-hook state of the phone. Call wait caller ID is known as Type -2 caller ID. Depending on the country, governing local standards, and central office (CO) switch capabilities serving a given area, the caller ID data are transmitted in the voice band as frequency-shift keying (FSK) or dual-tone multi frequency (DTMF). FSK-based caller ID follows two types of
recommendations outlined by Telcordia [GR-30-CORE (1998), GR-31-CORE (2000)] and ETSI [ETSI ETS 300 659-1 (2001), ETSI ETS 300 659-2 (2001)]. DTMF caller ID makes use of DTMF digits. Initial versions of the caller ID documents are provided by Bellcore, but currently Bellcore is part of Telcordia. Therefore, some previous versions of documents may appear under Bellcore and later versions may appear under Telcordia .
Caller ID and many other call features make use of direct current (DC) line conditions of off-hook, on-hook, flash-hook, open switch interval (OSI), and line reversal. In this topic, different methods for caller ID, call wait ID in PSTN, and the mapping of these features to VoIP foreign exchange subscriber (FXS) and foreign exchange office (FXO) interfaces are presented. VoIP-based FXO caller ID requires additional call progress tone detections (CPTDs) and incoming caller ID detection. In PSTN, caller ID data are transmitted as a voice band signal from the PSTN central office. In VoIP, IP packets carry caller ID data and VoIP end gateways generate voice band caller ID signals. VoIP gateway emulates functions of the PSTN central office, and no distinction is made between the caller ID coming from PSTN or VoIP.
The caller ID parameters such as frequency, timing, and signal characteristics are country dependent. Country-dependent information is not provided in this topic. For most countries, the caller ID service is based on either Telcordia or ETSI recommendations with slight deviations in signal characteristics and data formats. The TE alerting signal differs between the different recommendations. In this topic, FSK and DTMF caller ID parameters, timing intervals, and FSK and alerting signal characteristics are presented. For converging on the overview on caller ID, some relevant parameters are included with tolerances from the referenced ETSI and Telcordia documents. For complete information on caller ID service and signaling specifications, refer to the relevant ETSI and Telcordia recommendations and to the country-specific local PSTN standard documents.

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