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Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in 1975, which is used to set up and tear down most of the world's public switched telephone network (PSTN) telephone calls. It also performs number translation, local number portability, prepaid billing, Short Message Service (SMS), and other mass market services. In North America it is often referred to as ''CCSS7'', abbreviated for ''Common Channel Signalling System 7''. In the United Kingdom, it is called ''C7'' (CCITT number 7), ''number 7'' and ''CCIS7'' (Common Channel Interoffice Signaling 7). In Germany it is often called N7 (''Signalisierungssystem Nummer 7''). The only international SS7 protocol is defined by ITU-T's Q.700-series recommendations in 1988.〔(ITU-T Recommendation Q.700 )〕 Of the many national variants of the SS7 protocols, most are based on variants of the international protocol as standardized by ANSI and ETSI. National variants with striking characteristics are the Chinese and Japanese (TTC) national variants. The Internet Engineering Task Force (IETF) has defined level 2, 3, and 4 protocols compatible with SS7 which use the Stream Control Transmission Protocol (SCTP) transport mechanism. This suite of protocols is called SIGTRAN. ==History== SS5 and earlier systems used in-band signaling, in which the call-setup information was sent by playing special multi-frequency tones into the telephone lines, known as ''bearer channels''. As the bearer channel was directly accessible by users, it was exploited with devices such as the blue box, which ''played'' the tones required for call control and routing. As a remedy, SS6 and SS7 implemented out-of-band signaling, carried in a separate signaling channel,〔 thus keeping the speech path separate. SS6 and SS7 are referred to as common-channel signaling (CCS) protocols, or ''Common Channel Interoffice Signalling'' (CCIS) systems. Since 1975, CCS protocols have been developed by major telephone companies and the International Telecommunication Union Telecommunication Standardization Sector (ITU-T); in 1977 the ITU-T defined the first international CCS protocol as Signalling System No. 6 (SS6). In its 1980 Yellow Book Q.7XX-series recommendations ITU-T defined the Signalling System No. 7 as an international standard.〔 SS7 replaced SS6 with its restricted 28-bit signal unit that was both limited in function and not amenable to digital systems.〔 SS7 also replaced Signalling System No. 5 (SS5), while R1 and R2 variants are still used in numerous countries. The Internet Engineering Task Force (IETF) defined SIGTRAN protocols so the common channel signaling paradigm could be translated to IP Message Transfer Part (MTP) level 2 (M2UA and M2PA), Message Transfer Part (MTP) level 3 (M3UA) and Signalling Connection Control Part (SCCP) (SUA). While running on a transport based upon IP, the SIGTRAN protocols are not an SS7 variant, but simply transport existing national and international variants of SS7.〔(RFC 2719 ) - Framework Architecture for Signaling Transport〕 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「Signalling System No. 7」の詳細全文を読む スポンサード リンク
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