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Caller ID (caller identification, CID), also called calling line identification (CLID), calling number delivery (CND), calling number identification (CNID) or calling line identification presentation (CLIP), is a telephone service, available in analog and digital phone systems and most voice over Internet Protocol (VoIP) applications, that transmits a caller's number to the called party's telephone equipment during the ringing signal, or when the call is being set up but before the call is answered. Where available, caller ID can also provide a name associated with the calling telephone number. This service is called CNAM. The information made available to the called party may be displayed on a telephone's display, on a separately attached device, or personal computer. Caller ID information typically consists of the caller's telephone number while CNAM consists of the caller's name. A modem can pass CLID information to a computer for purposes of call logging or blocking, but this can be problematic as modems in different countries have different systems, causing hardware or software incompatibilities. However, many modems are designed and programmed to handle multiple signalling methods, and can be configured to use the local standard. Caller ID may be used by the recipient to avoid answering unwanted incoming calls by the concept of informed consent; however, it also poses problems for personal privacy. The possibility of caller ID spoofing may render received information unreliable. ==Calling-line identification== In some countries, the terms ''caller display'', ''calling line identification presentation (CLIP)'', ''call capture'', or just ''calling line identity'' are used; ''call display'' is the predominant marketing name used in Canada (although some customers use the phrase caller ID). The idea of CNID as a service for POTS subscribers originated from automatic number identification (ANI) as a part of toll free number service in the United States. However, CNID and ANI are not the same thing. ANI was originally a term given to a system that identified a caller placing a long distance call, in a non-electronic central office switch. Previous to this implementation, after dialling the long distance number, the caller would be intercepted by the operator to request their number before the call proceeded. Caller ID is made up of two separate pieces of information: the calling number and the billing (or subscriber) name where available. When a call is made from a given name, this name can be passed on through a number of different methods. For example, the caller's name may be datafilled in the originating switch, in which case it is sent along with the number. More commonly, a database is accessed by the receiving switch, in order to match the number to a name. If the name does not exist, then the city, State, Province, or other designation may be sent. Some of these databases may be shared among several companies, each paying every time a name is "extracted". It is for this reason that mobile phone callers appear as WIRELESS CALLER, or the location where the phone number is registered (these vary based on which company owns the block of numbers, ''not'' the provider to which a number may have been ported). Additionally, nothing ensures that the number sent by a switch is the actual number where the call originated; the telephone switch initiating the call may send any digit string desired as caller ID. As such, the telephone switch, and therefore the operating entity, must also be trusted to provide secure authentication. The displayed caller ID also depends on the equipment originating the call. If the call originates on a POTS line (a standard loop start line), then caller ID is provided by the service provider's local switch. Since the network does not connect the caller to the callee until the phone is answered, generally the caller ID signal cannot be altered by the caller. Most service providers however, allow the caller to block caller ID presentation through the vertical service code '' *67''. A call placed behind a private branch exchange (PBX) has more options. In the typical telephony environment, a PBX connects to the local service provider through Primary Rate Interface (PRI) trunks. Generally, although not absolutely, the service provider simply passes whatever calling line ID appears on those PRI access trunks transparently across the Public Switched Telephone Network (PSTN). This opens up the opportunity for the PBX administrator to program whatever number they choose in their external phone number fields. Some IP phone services (ITSPs, or Internet Telephony Service Providers) support PSTN gateway installations throughout the world. These gateways egress calls to the local calling area, thus avoiding long distance toll charges. ITSPs also allow a local user to have a number located in a "foreign" exchange; the New York caller could have a Los Angeles number, for example. When that user places a call, the calling line ID would be that of a Los Angeles number, although they are actually located in New York. This allows a call return without having to incur long distance calling charges. With cellphones, the biggest issue appears to be in the passing of calling line ID information through the network. Cellphone companies must support interconnecting trunks to a significant number of Wireline and PSTN access carriers. In order to save money, it appears that many cellphone carriers do not purchase the North American feature Group D or PRI trunks or SS7 trunks (Signalling System 7) required to pass calling line ID information across the network. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Caller ID」の詳細全文を読む スポンサード リンク
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