Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!lll-lcc!ames!ucbcad!ucbvax!kitty.UUCP!larry From: larry@kitty.UUCP Newsgroups: comp.dcom.telecom Subject: Submission for mod.telecom (Touch-tone in Older Central Offices) Message-ID: <8705040444.AA23587@seismo.CSS.GOV> Date: Mon, 4-May-87 00:44:47 EDT Article-I.D.: seismo.8705040444.AA23587 Posted: Mon May 4 00:44:47 1987 Date-Received: Tue, 5-May-87 00:44:00 EDT Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 143 Approved: telecom@xx.lcs.mit.edu In a recent article steves@tektools.TEK.COM (steve shellans) writes: > In my home I have a touchtone phone. When I press a number, such as > 7, for example, I hear 7 clicks coming back at me. Even though > I can dial a complete number, including area code in a couple of > seconds, the wait after that while I listen to the entire 'readback' > is very annoying. (The number I dial most often is 790-0000, which > is the local number for Allnet -- it seems to take forever.) > > From phones at work there is none of this, and all (outside) calls > go through very quickly. I assume that you have a "true" touch-tone (DTMF) telephone at home; not one of these touch-tone dial units which really put out dial pulses. In either case, the following is still applicable to the situation of using a touch-tone telephone in some older central offices. It sounds like your home telephone service is furnished by a "progressive control" electromechanical central office, such as step-by-step (SxS) or Stromberg-Carlson XY, that has BEEN UPDATED FOR DTMF SERVICE. Since you mention later in your article that your telephone company is GTE, I would bet money on the office having Automatic Electric 35E-type SxS apparatus. In general terms, "progressive control" central offices route the call through a series of devices known as "selectors" - one digit at a time in REAL TIME as the number is dialed ("step-by-step", as they say :-) ). This type of central office was designed around the use of a rotary dial in the subscriber telephone. Since these are electromechanical switches, there are minimum times required for the switches to operate. The dialing rate was standardized to accommodate the electromechanical response time of the switches. The call-processing speed of such offices is therefore limited by rotary dial pulsing specifications, which are typically: 1. Pulsing rate is a nominal 10 pulses/sec, which can typically vary from 8 to 12 pulses/sec. A digit "1" has one pulse, a digit "2" has two pulses, ..., and a digit "0" has ten pulses. Therefore, a digit "0" typically requires 1.0 seconds to dial. There are further specifications for the pulses within each digit. Pulses are line opens, and are generally specified as being 60 milliseconds open followed by 40 milliseconds closed; also referred to as 60% break (since each pulse interval is 100 milliseconds). 2. Inter-digit dialing interval (i.e., between dialed digits), of at least 0.25 seconds. So, to dial four "0"'s will require a minimum of 5.0 seconds (4 dialing intervals + 4 inter-digit intervals). So doing a bit of calculation, at BEST, dialing your example number of 790-0000 is going to take at least 8.35 seconds before the call can reach the 790 central office. This assumes an interoffice trunk between your office and the 790 office - if not, add another second for local tandem switching. In addition, add another 1.0 second for call processing in the 790 office. Now as I said earlier, these SxS and similar central offices respond only to dial pulses, because the pulses themselves control the switching apparatus. There is no way that the switching apparatus per se can deal with DTMF signals. However, for a number of years there have been converter circuits for use in SxS offices which receive a string of DTMF digits, decode them, store them as digital information in a register, and then outpulse the digits as rotary dial pulses at the nominal rate of 10 pulses/sec. These converter circuits are installed between the "linefinder" and the "first selector". When a subscriber line goes off hook and requests dial tone, an idle linefinder is selected by some simple relay arbitration logic. Each linefinder is a type of SxS switch which is dedicated to one particular first selector; when the linefinder completes its job and connects the tip and ring of the subscriber line to the first selector, the first selector returns dial tone. At this point, the first selector can respond in real time to the first dialed digit. Subscriber lines are typically arranged in groups of 100 or 200 lines, and each such group has access to a maximum of 10 or 20 linefinder- first selectors. The actual concentration ratio depends upon the particular type of SxS apparatus and the traffic design of the central office, but the typical concentration ratio at this point is 10:1. Typically, only ten percent of all lines in a given line group can make an outgoing call; if all linefinders in a given line group are busy, a line requesting service waits for for dial tone - since the dial tone is the indication that a first selector is available. So the point is: these DTMF converter circuits get installed before the first selector, are shared by a number of other lines in the same group, and in effect "fool" the first selector and all subsequent switches into believing that they are being controlled by a rotary dial. Since one can enter DTMF digits in a keypad at least 10 times faster than using a rotary dial, and since the SxS or other progressive control central office can only operate as fast as a rotary dial - use of the DTMF converter results in no faster switching time than using a rotary dial. The "clicks" you hear correspond to the dial pulses being generated by the DTMF converter, and are the impulse noise created by the operation of the selectors in your dialing path. If there is no faster switching time by adding DTMF converters to a SxS office, then why do telephone companies install such devices? Because customers "feel better" knowing that touch-tone is now available in their particular central office, because the telephone company can charge more for the touch-tone service, and because telephone companies don't want to spend the money to replace the SxS office with ESS! > Whenever I travel on business and need > to make calls, I always find electronic switching. You might still be going through a #1 or #5 crossbar office, or some other common-control electromechanical office; such offices when equipped with DTMF originating registers will usually switch a call in a short enough period of time as to be indistinguishable from ESS. > My question is this -- how unusual (in the U.S.) is the kind of > switching that I have from my home phone. If this is something > pretty rare, I would like to contact my phone company (GTE) and > the state utilities regulator to bring some pressure to bear to > update the equipment into the modern world. I don't think there is much pressure that can be brought to compel an operating telephone company to upgrade if they don't want to. They've given you DTMF service, they've given you DDD access, they've probably implemented coin-free operator, 800 and 911 service at coin telephones (done with a similar adapter card installed in the coin telephone linefinder group), and that's really about all the telephone service that any operating telephone company is OBLIGATED to provide. ESS features are nice, but they cost money to provide since they require total replacement of the SxS office with ESS equipment. New ESS equipment for a central office typically costs between $ 500.00 and $ 1,000.00 per subscriber line, with the higher figure being more common. That's a lot of money - that the telephone subscriber has to eventually pay for. This means that SxS and other electromechanical offices in "outlying" areas will be around for years to come. I don't disagree with your desire for better quality telephone service, but the point is: who is going to pay for the ESS to provide it? > Also, does anyone know when the heyday of this kind of equipment was? SxS and other progressive control central office equipment for smaller central offices peaked in usage during the late 1960's; common control electromechanical switches such as crossbar and its predecessor panel (ugh!) were just not economical for offices with less than say, 3000 lines. During the 1960's small "packaged" crossbar switches became available (like those from NEC), along with hybrid ESS-electromechanical switches, followed by true ESS switches. These switches made it economically feasible to replace SxS and similar progressive control central offices. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"