Introduction... This is a list of frequently asked questions made in the TELECOM Digest. New versions of the list are occasionally made available to deal with new, corrected or updated questions. Many contributors have made the FAQ what it is today (those listed in the "Who contributed to this FAQ?" question are hereby thanked). Check the Archives... Much of the telecom information that is requested can be found in the TELECOM Digest Home Page, via WWW at: http://hyperarchive.lcs.mit.edu/telecom-archives This web page should have links to this FAQ file, as well as various informational files on telecom subjects and the Digest itself. The Archives for TELECOM Digest should also be available via the Home Page. The TELECOM Digest Archives are also available through anonymous FTP at massis.lcs.mit.edu (login as anonymous, mail address for password, cd to telecom-archives). If possible, try to access the information via the TELECOM Digest Home Page first. You can also access the Archives using anonymous ftp at the mirror site: ftp.epix.net/pub/telecom-archives. Also, there is an email <--> ftp service operating called the Telecom Archives Email Information Service. Anything in the archives can be obtained automatically by email. Send mail addressed to: tel-archives@massis.lcs.mit.edu The subject does not matter. Include no text, sending just a blank letter. You will get a help file by return mail and should use that as a guide to ordering stuff by email from the archives. A list of terms commonly used in TELECOM Digest is contained in a "Glossary" section on the Home Page or in the Archives. Try direct inquiries... Direct netmail requests to persons posting on topics of interest to you may also be helpful. In fact, doing things "behind the scenes" can be more productive as the Digest Moderator is frequently swamped with other items. Future editions of this list could include netmail addresses of contacts for certain topics (say for ISDN, cellular, area codes/numbering plan, consumer protection matters, etc.); offers to that end would be appreciated. Where to contact the FAQ maintainer... Suggestions for other common questions, or corrections or other amendments to this file may be made to Telecom.FAQ@superctl.tor250.org (Fido 1:259/730) or dleibold@else.net or aa070@freenet.toronto.on.ca. Note that any or all of these addresses are subject to change or discontinuance. This file is updated approximately annually; special updates may be made as time and circumstances permit. Disclaimer Type Stuff... All information herein should be considered subject to correction or change. No endorsements or promotions of specific products or companies are intended. Any specific references are made for example only, or in order to adequately deal with certain subjects. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - List of subjects and questions covered as they appear in this list: Technical - How do phones work? - What is a Central Office? What is a switch? What roles do Central Offices and switches play in the telephone network? - How many different types of switches are there, how do they differ, and what switches are most commonly found in use? - When did the first ESS (electronic) switch go into service? - What frequencies do touch tones use for which numbers? - What are the A, B, C and D touch tone keys used for? Why are they not found on touch tone phone sets? - The # key - what is it officially called? How many different names are there for it? Why is it sometimes called a "pound" key? - Can tone-operated services (voice mail, interactive voice response sytems, etc.) be used by customers with dial pulse lines? - What is call supervision? - How can I find out what my own phone number is? - Are there other kinds of test numbers used? - Can a US modem or phone work in the UK, or some other European country? (Or vice versa, or in general for international substitution of phone equipment) - What do "tip" and "ring" mean? - Why use a negative charge (-48 volts) for Ring instead of a positive charge (such as +48 volts)? - What is "Caller ID" (or Call Display, or CNID (Caller Number Identification))? - How can I get specifications on how Caller ID service works? - What is the best way to busy a phone line? I have a bank of modems which are set up as a hunt group. When a modem dies I would like to be able to busy out the line that is disconnected, so that one of the other modems in the hunt group will take the call. - What telephone wiring should be used? - What is the difference between Caller ID/CNID and ANI? - What is Switched 56? - How does caller number delivery work on toll-free or extra-charge services? - Is there some way I can use a modem to send text messages to my alphanumeric pager? - How can I prevent an extension from interfering with a modem call? Numbering - What is a numbering plan? - Who has authority over numbering plans? - How was the country code system developed? - What is the correct way to write a telephone number for international use? - What are the access codes used in international dialing? - What does NPA, NNX, or NXX mean? - What happens when all the telephone numbers run out in a given area? - How long can a phone number be (internationally speaking)? - What is Time T? - What is an overlay area code? - How was extra numbering capacity achieved in North America? - In North America, why did long distance dialing within area codes change so that 1 + home area code + number has to be dialed, or change to just seven digits (like a local call)? - What is an "interchangeable" area code? - Why were "interchangeable" area codes introduced? - Why not increase the number of digits in the North American Numbering Plan? - How will we make room if North American area codes are running out? - What "interchangeable" area codes been assigned? - What about expanding area/STD codes in other countries? - How are area codes assigned? - What is Bellcore? - How can I contact Bellcore? - How can I get exchange/billing data? What is a V&H tape? - How are letters dialed? What was "2L5D"? What is All Number Calling? Regulatory & Tariffs - How are long distance call costs rated? - What's this about the FCC starting a modem tax for those using modems on phone lines? - Why is a touch tone line more expensive than a rotary dial line (in many places)? - How come I got charged at a hotel for a call where no one answered? Why is the timing on some of the long distance carriers inaccurate? - What is AT&T Tariff 12? - What are the ITU and CCITT? Competition - Which countries have competitive telecommunications services? - What is a COCOT? - What is an AOS? - What is "splashing"? - Where can I find a list of equal access (10XXX) codes? - How can I tell who my default carrier is (or which carrier belongs to a 10XXX+ code)? - What is a LATA? - Where can LATA maps be found? Features - What is the calling card "boing" and what is it made of? - What are codes like *70? - How can one dial *70 on a non-touch-tone (pulse/rotary) phone? - How can I prevent the call waiting tone from beeping in mid-conversation? - What is distinctive ringing? Miscellaneous - Is there a way to find someone given just a phone number? - Who are the Telephone Pioneers? - Where can a Cellular/Mobile Radio mailing list be contacted? - How are VCR+ codes generated? - What is CLLI? - Why do movies often use 555-xxxx numbers? What does 555 represent? - Are there cases of local calls across international borders? - Which countries have freephone or "800" services? - What is 'area code' 710? - How can one call freephone numbers from other countries? - Which World-Wide Web (WWW) sites have telecom information? - What's an ObTelecom? - Who contributed to this FAQ? - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - --------- Technical --------- Q: How do phones work? A: A file in the TELECOM Digest archives under the name "how.phones.work" is available and should explain some details of the workings of the common telephone. Q: What is a Central Office? What is a switch? What roles do Central Offices and switches play in the telephone network? A: A Central Office (CO) is the facility to which the telephones in a public telephone network are connected. It is the gateway to the rest of the telephone system; dial tone, telephone ringing, connection to other telephones, long distance carriers or outside trunks, is done here. Special features such as Call Waiting, Call Identification ("Caller ID") and voice mail can be provided at modern digital COs. A "switch" is a general term referring to facilities where telephone traffic is routed from one destination to another. The Central Office has a switch in a local sense; calls within a municipality can often be completed within a single switch. Beyond this, there are switches for long distance or regional traffic, many of which are not directly connected to user telephones. A hierarchy of switching centres was developed in North America. Level 5 switches are the most common and are generally the local Central Office switches. Level 4 switches are used in regional or larger local settings and occasionally are connected to customer telephones. Level 3, 2 and 1 switches serve larger regions in turn. In general, a call that cannot be handled at one level of switching (by reason of distance covered, or congestion at a given switch) is passed onto the next higher level until the connection is completed. The breakup of AT&T in the USA and the introduction of new services will no doubt have disrupted this hierarchy, but this illustrates how a call can progress from one place to another. A large city usually has many central offices, each serving a certain geographical area. These central offices are connected to other central offices for local calling, or to higher level switches, or into long distance networks. Q: How many different types of switches are there, how do they differ, and what switches are most commonly found in use? A: The original telephone switches were manual, operator-run switchboards. Today, these are generally found in developing countries or in certain remote locations as newer types of switches allow for connection to automatic telephone service. Step-by-step was the first widely-used automatic switching method. This was an electro-mechanical system which made use of rotating blades and mechanical selection of various levels. Dial pulses would be used to cause the switches to select switch groups until the whole number was dialed. Some step-by-step facilities still exist today, but will eventually be replaced by more modern forms of switching (typically a digital facility). Step-by-step, with its mechanical nature, can be difficult to troubleshoot and maintain, and does not inherently support touch tones or special calling features without special addition of equipment. Step-by-step's "progressive control" could not be rerouted to avoid points of congestion in the switches but was rather at the mercy of which numbers would be dialed by the telephone users. There was also no capability to select a variety of routes for the call. Different technologies were developed to solve the inflexible routing problem, among other things. Systems were developed where a "sender" could accept a dialed number, then interact with a distant switch to establish an appropriate route for the call to be completed. The concept of "common control" was introduced: progressive customer dialing control would be replaced by giving the central office the ability to determine the call's route. The first common-control switches were developed by Western Electric ("Panel Type") and IT&T ("Rotary"). Panel Type switches used wipers that moved vertically through contacts. Rotary equipment used wipers that moved through contacts arranged in a circle. The two switches were conceptually similar, to the point that switches of both types could be directly connected. Most large U.S. cities used such switches. Rotary switches were particularly popular in Paris, France, among other places. Crossbar was the next step in electro-mechanical switching. Rather than the types of switches used in step-by-step or in Panel/Rotary, connections were completed by means of a matrix of connectors, arranged in a grid. The configuration of crossbar matrix elements was under "common control" for routing calls. Electronic switches (e.g. ESS) were developed in the 1960's. These were often reed relay switches with an electronic common control faster than previous crossbar systems. The fewer moving parts there were, the better. Services such as call waiting or call forwarding would eventually be possible under electronic systems. Finally, the new digital electronic systems provide for a fully- programmable telephone operation. These are all-electronic systems which would process calls without moving parts (i.e. solid-state switching) and full computerisation of control. Voice traffic would now be converted to digital format for use with digital transmission facilities. A wide variety of user services can be implemented such as sophisticated types of call forwarding or Caller ID or ISDN (Integrated Services Digital Network). Ultimately, all telephone subscribers will be served by such switches as these. Presently, various kinds of switching systems are in use, and the proportions of what technologies are in use in given regions will vary. The most common will eventually be the digital electronic systems. There are significant costs associated with upgrading the network to digital, fully-programmable switching, but the eventual goal of phone companies worldwide is to modernise Central Offices and long distance networks to such switches. In the meantime, the various switching technologies in use must provide compatibility with each other in terms of signalling among switches, call routing capabilities, and the ability to use existing types of phone equipment. Q: When did the first ESS (electronic) switch go into service? A: In the U.S., the first 1ESS switch went into service May 1965 in Succasunna, New Jersey. This was a software-controlled switch using magnetic reed relays. In Canada, the first ESS was set up in Montreal, circa 1967. While there were substantial capabilities with such ESS switches as 1ESS or SP-1, these are being replaced by modern digital switches (e.g. Northern Telecom DMS, Ericsson AXE). Any information regarding international firsts in electronic or digital switching would be welcome as a future enhancement to the FAQ. Q: What frequencies do touch tones use for which numbers? A: The touch tone system uses pairs of tones to represent the various keys. There is a "low tone" and a "high tone" associated with each button (0 through 9, plus * (star) and # (the thing that's called various names: octothorp(e), hash, number sign, "pound" symbol). The low tones vary according to what horizontal row the tone button is in, while the high tones correspond to the vertical column of the tone button. The tones and assignments are as follows: 1 2 3 A : 697 Hz 4 5 6 B : 770 Hz (low tones) 7 8 9 C : 852 Hz * 0 # D : 941 Hz ---- ---- ---- ---- 1209 1336 1477 1633 Hz (high tones) When the 4 button is pressed, the 770 Hz and 1209 Hz tones are sent together. The telephone central office will then decode the number from this pair of tones. The tone frequencies were designed to avoid harmonics and other problems that could arise when two tones are sent and received. Accurate transmission from the phone and accurate decoding on the telephone company end are important. They may sound rather musical when dialed (and representations of many popular tunes are possible), but they are not intended to be so. The tones should all be +/- 1.5% of the specified frequency. The high frequency tone should be at least as loud, and preferably louder than the low frequency. It may be as much as 4 dB louder. This factor is referred to as "twist." If a Touchtone signal has +3db of twist, then the high frequency is 3 dB louder than the low frequency. Negative twist occurs when the low frequency is louder. Q: What are the A, B, C and D touch tone keys used for? Why are they not found on most touch tone phone sets? A: These are extensions to the standard touch-tones (0-9, *, #) which originated with the U.S. military's Autovon phone network. The original names of these keys were FO (Flash Override), F (Flash), I (Immediate), and P (Priority). The various priority levels established calls with varying degrees of immediacy, terminating other conversations on the network if necessary. FO was the greatest priority, normally reserved for the President or very high ranking officials. P had a lesser priority, but still took precedence over calls that were placed without any priority established. Today, the tones are commonly referred to as the A, B, C and D tones respectively; each of these tones use 1633 Hz as their high tone. These are found mainly used in special applications such as amateur radio repeaters for their signalling and control. Modems and touch tone circuits tend to include the A, B, C and D tones as well. These tones have not been used for general public service, and it would take years before these tones could be used in such things as customer information lines; such services would have to be compatible with the existing 12-button touch tone sets in any case. Q: The # key - what is it officially called? How many different names are there for it? Why is it sometimes called a "pound" key? A: That key usually found under the 7 and to the right of 0 gets to be called cross-hash, cross-hatch, enter, hash, noughts-and-crosses, number-sign, octothorp(e), pound, pound-sign and no doubt other names. There is some officiality to the term "octothorp(e)", spelled with or without the 'e' depending on location or who you hear. The term "number-sign" seems to be in common use for interactive voice response systems, sometimes with an explanation that this button is to the right of the 0. As for the term "pound key", this might have something to do with the # (number sign) in data being replaced by the UK Pound symbol in some setups. This topic was discussed in 1988 on the TELECOM Digest (Vol. 8, #190 with plenty of follow-up discussion) which should be available in the FTP Archives. Q: Can tone-operated services (voice mail, interactive voice response sytems, etc.) be used by customers with dial pulse lines? A: If a phone set has a switch between tone and pulse modes, it can. Calls need to be dialed with the phone in "pulse" mode, where the line is not set for tone dialing service. When the service is called and is requesting entry of tones, switch the phone to "tone" mode and proceed to enter tones as needed. On hang-up, make sure the phone set is set back to "pulse" mode for the next call. Q: What is call supervision? A: Call supervision refers to the process by which it is determined that the called party has indeed answered. Long distance calls and payphone calls are normally charged from the time the called party answers, and no charges should be assessed where the other end doesn't answer nor where the called party is busy or blocked by network problems. Q: How can I find out what my own phone number is? A: If the operator won't read your number back to you, and if you can't phone someone with a Calling # ID box, there are special numbers available that "speaks" your number back to you when dialed. These numbers are quite different from one jurisdiction to the next. Some areas use 200 222.2222; others just require 958; still others 311 or 711 and others have a normally-formatted telephone number which can be changed on occasion (such as 997.xxxx). Such numbers exist in many countries; 175 is one number in the UK while 19123 is one in Australia. There is no general rule for the format of such numbers. These are normally assigned to codes outside normal customer number sequences, however. In areas where Caller ID is available, one could arrange to call someone with an activated display unit and have that called party read back the caller's number. Q: Are there other kinds of test numbers used? A: Yes. Again, space (and available information) does not permit a complete list of what each telephone company is up to in terms of test numbers. The most common number is a "ringback" or self-ring test number. When a two or three digit number is followed by all or the last part of your phone number, another dial tone occurs. Tests for dialing or ringing may then be done. The ringback methods in some jurisdictions will vary. Other numbers include intercom circuits for telephone company staff, or switching centre supervisors, or other interesting tests for call supervision or payphone coin tests. One famous kind of test number belongs to NYNEX, the regional Bell telephone company operating in the northeast U.S.A.. In New York at least, there are "9901" numbers, or local numbers of the form xxx.9901, which result in a recording which identifies the exchange represented by the first three digits. The 9901 numbers may not necessarily exist for all combinations of first three local number (central office code) digits. All these tests and services vary with each phone company; they are not usually found in the phone book, needless to say. Q: Can a US modem or phone work in the UK, or some other European country? (Or vice versa, or in general for international substitution of phone equipment) A: Often it can, provided that the AC Voltage and the physical jack are compatible or converted, and it can generate pulse dialing, as many exchanges are not equipped for touch tone. However, in most European countries it is illegal to fit non-approved equipment. In the UK, approving equipment is the responsibility of BABT, and the penalty is confiscation of the equipment plus a fine of up to 2000 pounds sterling. Approved equipment has a mark, usually a sticker, of a green circle with the words "APPROVED for connection to the telecommunication system specified in the instructions subject to the condition set out in them" and the number of the BABT certificate. Non-approved items, if they are sold in the UK, must have a sticker with a red triangle with similar wording except that it's saying the exact opposite. It's perfectly legal to sell non-approved equipment subject to the above, as there may be a valid reason for using it, just not on the UK network. In Canada, telephone equipment requires approval from the government department known as Industry and Science Canada, or formerly through the Department of Communications. Most equipment designed for American conditions should be acceptable in Canada, and vice versa. A small sticker indicating Canadian government approval is normally found on approved devices. Adapters have been developed to allow conversion from one national standard to another. Electronics or telephone supply stores may have such items. Whether these converters can legally be used in certain countries is another matter, so buyer beware. Q: What do "tip" and "ring" mean? A: The conductors of a wire pair to a telephone set are referred to as tip (T) and ring (R).. Tip (T) is usually positive charge with respect to the Ring (R). Ring is typically at -48 volts (subject to voltage losses). Tip (T) is then at ground when no current is flowing. The actual voltages may differ in PBX/Key system situations (where 24 volt systems can