Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!ucbcad!ucbvax!kitty.UUCP!larry From: larry@kitty.UUCP.UUCP Newsgroups: comp.dcom.telecom Subject: Submission for comp.dcom.telecom (Transmission Quality; Answer Detection) Message-ID: <8705102110.AA00258@seismo.CSS.GOV> Date: Sun, 10-May-87 17:10:46 EDT Article-I.D.: seismo.8705102110.AA00258 Posted: Sun May 10 17:10:46 1987 Date-Received: Thu, 14-May-87 00:46:13 EDT Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 194 Approved: telecom@xx.lcs.mit.edu I thought the following two articles might be of interest to those who do not read or archive the groups to which they were posted: >From postnews Sat May 9 00:15:58 1987 Subject: Re: Phone Line Quality Newsgroups: comp.dcom.modems Summary: More on noise and other topics... References: <9728@decwrl.DEC.COM> In article <9728@decwrl.DEC.COM>, nourse@nac.dec.com (Andy Nourse) writes: > > ... the transmission loss on most interoffice trunks originating at End > > Offices (Class 5) trunks is carefully kept below 4.0 dB... > > Toll Center (Class 4) and up... below 2.6 dB > > a reasonably flat transmission characteristic between 300 and 3,000 Hz. > What are the specs for signal-to-noise ratio? If I want to call out of > the three-small-town local calling area, I have to dial 1 > which routes the call out to Leominster, 15 miles away. Voice communication channels usually have a minimum of 30 dB as a signal-to-noise ratio. This may be surprising, but data channels can usually get away with a _lower_ S/N ratio minimum, typically 15 dB. Other parameters, such as phase shift (envelope delay distortion) are far more critical on data circuits than S/N ratio. It sounds to me like your central office has some bad N-carrier circuits (analog FDM), even more likely of the N1-variety which still uses vacuum tubes. N1 carrier requires continual maintenance to keep its circuits "in tune", and it sounds like someone has been remiss in this area. There is still an amazing amount of vacuum tube FDM carrier in service today. > > ... Trunks which fail to pass these automatic tests are disabled > > until repair is effected. > That must be why I sometimes get a fast busy, or drop back to a dial tone, > when i dial 1 (this is before I get to dial any other digits). I suppose > it can thus disable ALL the trunks. That does, of course, also affect > 0 (for Operator) and all paths to Repair Service. I did not mean to imply that automatic trunk testing is in use everywhere, although it is extensively implemented by AT&T Communications and the BOC's. Losing a large number of trunks though intentional disabling by automatic trunk test apparatus would be an unusual event, and would be rare and quickly attended to. It sounds more like switching apparatus trouble or traffic overload. Switching apparatus trouble which limits access to less than the allocated number of trunks is an insidious problem which is not easily diagnosed until customer complaints reach a level where a repairperson is eventually dispatched the CO and to get to the bottom of the problem. Automatic trunk test apparatus does NOT test connectivity to the trunks by the switching apparatus. If you are plagued with all trunk busy signals, complain to repair service - otherwise, they may never know about the problem! > > So the point is: under virtually all circumstances, > > you should have little concern about the transmission quality of interoffice > > trunks, as compared to your own subscriber loop. > Are old, rural, step-by-step exchanges an exception to this? > The noise starts when I dial 1, so I don't think the subscriber loop > has anything to do with it. Well, unfortunately older SxS offices usually have older carrier of the FDM variety (see above). FDM carrier requires continual maintenance. Well-maintained carrier should not present a noise problem (even N1 carrier); it sounds like your CO may be the victim of reduced maintenance manpower (to save money, of course!). > > ...some of this standardization has gone to hell with the advent of > > Alternate Long Distance carrier... > Some local operating companies (including ours) are using that as an excuse to > let line quality deteriorate, even for calls within the LATA. That's true. The telephone companies are also getting killed by increasing labor costs, so they reduce preventative maintenance, resulting in - voila! - poor transmission in some areas. Reduced maintenance is more common with independent operating telephone companies; AT&T and the BOC's predicted rising labor costs some years ago, and therefore developed and installed an extensive variety of automatic trunk test apparatus. The independent operating telephone companies have much less automatic trunk test apparatus than the BOC's and AT&T. > > the comparatively small additional monthly and installation charge is well > > worth it to get a better subscriber loop. > I just checked with New England Telephone. They said there is no such option. > In 1980, when I had a Bell 212a modem installed (with an RJ41 jack), the > installer fiddled around with his test gear on the phone line for a couple of > hours, but I can detect no difference in quality from my other phone line. It > is billed as a standard business phone line. I believe that every BOC has some type of "conditioned" central office loop for use with dial-up data. The additional charge may not be a tariffed item, but is instead a non-tariffed charge, which is sometimes referred to as a "special assembly charge". It sounds like the person to whom you spoke had little technical knowledge or experience with data communications - which unfortunately is par for the course. :-( > What does the switch, and the collection of resistors and capacitors inside an > RJ41 jack do? The resistors provide an attenuation for the transmitted signal from the modem, such that the signal arrives at the central office between -8 and -9 dBm. ---------------------------------------- >From postnews Sun May 10 16:51:48 1987 Subject: Re: Telephone answer detection Newsgroups: sci.electronics Distribution: usa Summary: Not that simple... References: <813@bgsuvax.UUCP> <837@killer.UUCP> In article <837@killer.UUCP>, royf@killer.UUCP (Roy Frederick) writes: > Detection of answer and hangup depends on the type of CO you are connected > to. Well-designed telephone apparatus should NEVER be dependent upon the characteristics of one particular type of central office switching apparatus. Those characteristics of any central office switching apparatus which might permit the detection of answer supervision by DC current sensing are unreliable, and are subject to change without notice by the operating telephone company. In North America, it is possible to design telephone station apparatus which will reliably work in any central office. Answer supervision, however, cannot be reliably detected though DC sensing means over a subscriber loop. Answer supervision must be detected by inference using call-progress tone detection, possibly in conjunction with a voice-operated switch circuit. > In some older type offices the line polarity is reversed while > you are connected to the called party. It goes back to normal when the > called party hangs up. There are in fact numerous variations and exceptions to this observation: 1. Some SxS offices have provide a polarity reversal for answer supervision only on calls made within the _same_ central office; non-toll calls made to other central offices may exhibit no such polarity reversal upon answer. 2. Some SxS offices have their "connectors" wired NOT to provide polarity reversal on calls within the same central office. 3. Some SxS offices provide a polarity reversal immediately upon the seizure of an interoffice toll trunk - regardless of whether the called party answers. This may also include calls to the Operator, directory assistance, etc. One reason for such polarity reversal is to facilitate "toll diversion" or "toll denial" for business lines and PBX trunks. Such polarity reversal may be restricted to "linefinder" groups dedicated just to business lines, or it may apply to all lines in the SxS office. The variations are numerous. > In newer offices, the line voltage drops to zero > momentarily when the party answers and again when they hang up. My CO > was a step-by-step with a common-control and used the polarity reversal > scheme. It was recently replaced by an electronic (type ?) office - it > now drops the voltage to zero very briefly. This is a dangerous assumption. Electromechanical switching apparatus (#1 crossbar, #5 crossbar, etc.) will exhibit a momentary switching open (lasting << 100 ms) as the marker switches the line from an originating register to an intraoffice trunk, or to an interoffice trunk. This time period is _extremely_ variable, and depends upon both the office traffic load and certain relay adjustment characteristics which are NOT subject to standardization. In analog ESS offices (#1 ESS, #2 ESS, etc.) there is also a switching open (lasting << 100 ms) as the line is switched between dial register, ringback tone trunk, and intraoffice trunk; or between dial register and interoffice trunk. This time period is also _extremely_ variable, and depends upon both the traffic load of the office and the operate-release time variations of bistable magnetic latching relays. In digital ESS offices (#5E ESS, DMS100, etc.) there is generally NO DC line current open until a call disconnects. > Busy (and for that matter all other call progress) tone are usually > detected by duty cycle. However, specific tone frequencies are used > for each signal. Teltone makes a nice chip that detects the tone > by frequency - I can't seem to find the reference to it (which contains > the actual tones and duty cycles) - but I'll post them later. Not all central offices use CCITT standard tones; this is especially true of SxS, Stromberg-Carson XY, AE/Leich, etc. electromechanical offices which are not equipped provide touch-tone (DTMF) service. CCITT standard (or "precise") tones primarily came into being with the implementation of DTMF service. Detection of call-progress tones MUST be made upon detected pulse shape (i.e., duty cycle and repitition rate) of any tone energy in the frequency range of 305 to 640 Hz; this covers "all bases". The _specific_ CCITT frequencies are never detected. <> 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?"