The Signaling System No. 5 (SS5) is a multi-frequency (MF) telephone signaling system that was in use from the 1970s for International Direct Distance Dialing (IDDD). Internationally it became known as CCITT5 or CC5. [1] It was also nicknamed Atlantic Code because it was used for the first IDDD connections between Europe and North America.
Signaling systems in use at the time were designed for in-band signaling, meaning they used the same channel as the media that they controlled. SS5 was designed for inter-continental traffic for which many transmission paths were long terrestrial, often submarine cable, and geostationary satellite links. Trunks using satellite links also had echo suppressors connected at their end points. SS5 was specifically designed to work within those links.
Based on the Bell System multi-frequency (MF) signaling system known by CCITT as Regional System R1, it used six signaling frequencies: 700 Hz (A), 900 Hz (B), 1100 Hz (C), 1300 Hz (D), 1500 Hz (E) and 1700 Hz (F). The first five frequencies were used in a two-out-of-five code to represent decimal numbers (telephone numbers), and the last frequency in combination with one of the others represented the beginning or end of a sequence of digits. These frequencies were combined to encode the following signals:
| Frequency pair | Code | Signal | |||||
|---|---|---|---|---|---|---|---|
| A | B | C | D | E | F | ||
| X | X | 1 | digit 1 | ||||
| X | X | 2 | digit 2 | ||||
| X | X | 3 | digit 3 | ||||
| X | X | 4 | digit 4 | ||||
| X | X | 5 | digit 5 | ||||
| X | X | 6 | digit 6 | ||||
| X | X | 7 | digit 7 | ||||
| X | X | 8 | digit 8 | ||||
| X | X | 9 | digit 9 | ||||
| X | X | 0 | digit 0 | ||||
| X | X | 11 | code 11, or a prefix to reach any international operator in the country | ||||
| X | X | 12 | code 12, or a prefix to reach an individual international operator in the country | ||||
| X | X | 13 | code 13, the keying prefix (KP1) where the following digits do not contain a country code (Terminal working) | ||||
| X | X | 14 | code 14, the keying prefix (KP2) where the following digits do contain a country code (Transit working) | ||||
| X | X | 15 | code 15, the keying finish (KF or End-of-keying) code instructing the register not to wait for any more digits | ||||
The 2 out of 6 frequency code was used to pass digits forward between Registers in traditional International Switching Centres. Each digit took 55 ms with a 55 ms Inter-digital pause (IDP) and the sequence was sent 'en-bloc' to ensure that Echo Suppressors would not switch out the forward path as the links tended often to be satellite channels. The first digit was a keying Prefix (a KP) to indicate Terminal or Transit working and the last digit was the digit 15 or Keying Finish (KF).
In addition, for line signaling a 2 frequency (2VF) code using compelled sequences of 2400 Hz and 2600 Hz rather than continuous SF tone was used to seize the line at the beginning of a call over that trunk and clear it at the end of the call - long after the valuable register had sent 15 or KF and dissociated from the call. This Line signalling element was 2VF to introduce the possibility of more meanings and tone-off idle in order that hundreds of channels in transmission media would not be transmitting standing tones simultaneously.
The deficiencies of SS5 are widely known as the root cause for "Blue box fraud" that enabled phone phreaks, such as Captain Crunch (John Draper), to abuse the telephone system and place telephone calls for free, among exploiting other capabilities. SS5 was abused by sending a "seize" signal to simulate and pose as a telephone switching system from a subscriber telephone line. This enabled the perpetrator to control the remote switch that had no means for distinguishing the signaling from that of another legitimate switch.
This fraud was a factor in the widespread adoption of Signalling System No. 7 (SS7) in replacement of SS5.
Dual-tone multi-frequency signaling (DTMF) is a telecommunication signaling system using the voice-frequency band over telephone lines between telephone equipment and other communications devices and switching centers. DTMF was first developed in the Bell System in the United States, and became known under the trademark Touch-Tone for use in push-button telephones supplied to telephone customers, starting in 1963. DTMF is standardized as ITU-T Recommendation Q.23. It is also known in the UK as MF4.
Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work on the standard began in 1980 at Bell Labs and was formally standardized in 1988 in the CCITT "Red Book". By the time the standard was released, newer networking systems with much greater speeds were available, and ISDN saw relatively little uptake in the wider market. One estimate suggests ISDN use peaked at a worldwide total of 25 million subscribers at a time when 1.3 billion analog lines were in use. ISDN has largely been replaced with digital subscriber line (DSL) systems of much higher performance.
In telecommunications and computer networking, multiplexing is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a physical transmission medium. For example, in telecommunications, several telephone calls may be carried using one wire. Multiplexing originated in telegraphy in the 1870s, and is now widely applied in communications. In telephony, George Owen Squier is credited with the development of telephone carrier multiplexing in 1910.
In telecommunication, signaling is the use of signals for controlling communications. This may constitute an information exchange concerning the establishment and control of a telecommunication circuit and the management of the network.
In telecommunications, a transmission system is a system that transmits a signal from one place to another. The signal can be an electrical, optical or radio signal. The goal of a transmission system is to transmit data accurately and efficiently from point A to point B over a distance, using a variety of technologies such as copper cable and fiber optic cables, satellite links, and wireless communication technologies.
Phreaking is a slang term coined to describe the activity of a culture of people who study, experiment with, or explore telecommunication systems, such as equipment and systems connected to public telephone networks. The term phreak is a sensational spelling of the word freak with the ph- from phone, and may also refer to the use of various audio frequencies to manipulate a phone system. Phreak, phreaker, or phone phreak are names used for and by individuals who participate in phreaking.
Signalling System No. 7 (SS7) is a set of telephony signaling protocols developed in the 1970s, which is used to set up and tear down telephone calls in most parts of the world-wide public switched telephone network (PSTN). The protocol also performs number translation, local number portability, prepaid billing, Short Message Service (SMS), and other services.
A blue box is an electronic device that produces tones used to generate the in-band signaling tones formerly used within the North American long-distance telephone network to send line status and called number information over voice circuits. This allowed an illicit user, referred to as a "phreaker", to place long-distance calls, without using the network's user facilities, that would be billed to another number or dismissed entirely as an incomplete call. A number of similar "color boxes" were also created to control other aspects of the phone network.
A radiotelephone, abbreviated RT, is a radio communication system for conducting a conversation; radiotelephony means telephony by radio. It is in contrast to radiotelegraphy, which is radio transmission of telegrams (messages), or television, transmission of moving pictures and sound. The term is related to radio broadcasting, which transmit audio one way to listeners. Radiotelephony refers specifically to two-way radio systems for bidirectional person-to-person voice communication between separated users, such as CB radio or marine radio. In spite of the name, radiotelephony systems are not necessarily connected to or have anything to do with the telephone network, and in some radio services, including GMRS, interconnection is prohibited.
In telephony, multi-frequency signaling (MF) is a type of signaling that was introduced by the Bell System after World War II. It uses a combination of audible tones for address transport and supervision signaling on trunk lines between central offices. The signaling is sent in-band over the same channel as the bearer channel used for voice traffic.
In telecommunications, in-band signaling is the sending of control information within the same band or channel used for data such as voice or video. This is in contrast to out-of-band signaling which is sent over a different channel, or even over a separate network. In-band signals may often be heard by telephony participants, while out-of-band signals are inaccessible to the user. The term is also used more generally, for example of computer data files that include both literal data, and metadata and/or instructions for how to process the literal data.
The Improved Mobile Telephone Service (IMTS) was a pre-cellular VHF/UHF radio system which linked to the public telephone network. IMTS was the radiotelephone equivalent of land dial phone service. Introduced in 1964, it replaced Mobile Telephone Service (MTS) and improved on most MTS systems by offering direct-dial rather than connections through a live operator, and full-duplex operation so both parties could talk at the same time.
2600 hertz (2600 Hz) is a frequency in hertz that was used in telecommunication signaling in mid-20th century long-distance telephone networks using carrier systems.
The Mobile Telephone Service (MTS) was a pre-cellular VHF radio system that linked to the Public Switched Telephone Network (PSTN). MTS was the radiotelephone equivalent of land dial phone service.
In a digitally modulated signal or a line code, symbol rate, modulation rate or baud rate is the number of symbol changes, waveform changes, or signaling events across the transmission medium per unit of time. The symbol rate is measured in baud (Bd) or symbols per second. In the case of a line code, the symbol rate is the pulse rate in pulses per second. Each symbol can represent or convey one or several bits of data. The symbol rate is related to the gross bit rate, expressed in bits per second.
Signalling System R2 is a signalling protocol for telecommunications that was in use from the 1960s mostly in Europe, and later also in Latin America, Asia, and Australia, to convey exchange information between two telephone switching systems for establishing a telephone call via a telephone trunk. It is suitable for signaling on analog as well as digital circuits.
An amateur radio repeater is an electronic device that receives a weak or low-level amateur radio signal and retransmits it at a higher level or higher power, so that the signal can cover longer distances without degradation. Many repeaters are located on hilltops or on tall buildings as the higher location increases their coverage area, sometimes referred to as the radio horizon, or "footprint". Amateur radio repeaters are similar in concept to those used by public safety entities, businesses, government, military, and more. Amateur radio repeaters may even use commercially packaged repeater systems that have been adjusted to operate within amateur radio frequency bands, but more often amateur repeaters are assembled from receivers, transmitters, controllers, power supplies, antennas, and other components, from various sources.
In a conventional, analog two-way radio system, a standard radio has noise squelch or carrier squelch, which allows a radio to receive all transmissions. Selective calling is used to address a subset of all two-way radios on a single radio frequency channel. Where more than one user is on the same channel, selective calling can address a subset of all receivers or can direct a call to a single radio. Selective calling features fit into two major categories—individual calling and group calling. Individual calls generally have longer time-constants: it takes more air-time to call an individual radio unit than to call a large group of radios.
Signaling System No. 6 (SS6) was introduced in the 1970s as an early common channel signalling method for telecommunication trunks between international switching centers (ISCs). It is specified in CCITT Recommendations Q.251-Q.300.
A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital systems to establish telephone calls between subscribers.
