Dear Treggers, The following remark by Joe Randolph relates to the seemingly odd DC requirements at low DC currents, stipulated by both TBR21 and by the German Standard (BAPT 223 ZV 5):
"I too have noticed the problem that you cite regarding the DC V-I test at extremely low loop current. I have no idea what the technical justification is, but I note that this requirement is similar to one that appears in the German requirements." I believe that I can shed some light on this subject. Here in Israel, use is commonly made of a pair gain system that allows two separate telephone numbers to share a single copper pair. This system is installed when a subscriber wishes to add a second directory number, but the number of pairs to that subscriber's location is limited. The system is a 2-channel PCM coding and multiplexing device, with a demultiplexer at the subscriber's end. We happen to have such a device (manufactured by Tadiran Communications) installed in our own office. In the course of our work, we test all kind of designs on our office lines, and during these tests, we have observed that the on-hook voltage of our telephone lines is around 32 volts and the apparent on-hook line resistance is approximately 150 kilohm! This value was measured by connecting a 100 kilohm resistor in parallel with the line, which causes the line voltage to drop to around 12 volts. Naturally, if the line is terminated with a lower resistance, the demultiplexer registers a valid off-hook state, and provides a line, at which stage the apparent (off-hook) line resistance becomes a much more conventional value. If this isn't reason enough for specifying the DC characteristics down to extremely low DC currents, then I don't know what is! I have to assume that equipment of the type I have described, and possibly other devices too, are also in use in Germany, and that they are the reason for the low-current requirements. Such systems clearly work imperfectly if the subscriber's equipment fails to behave in the expected manner during the on-hook to off-hook transition at low DC currents. I would not be at all surprised if such systems are being used in countries outside Israel, and that -- apart from Germany -- the regulatory bodies are simply uninterested in specifying terminal equipment accordingly, or might even have failed to notice the need! Here in Israel, I know that most imported equipment is designed to meet FCC Part 68, and to the best of my knowledge, does not have to pass any special test for DC response at low currents (although Bezeq -- Israel's very careful and professional national telephone company -- does specify the V-I curve down to 0 mA for telephone sets that it buys for distribution to its customers). I can well imagine that some subscribers are experiencing technical difficulties because of incompatible equipment both in Israel and in other countries. This would manifest itself as a malfunction in which the user lifts the handset (or a telephone answering machine or modem, etc., attempts to signal off-hook), but no dial tone is returned, and the sidetone is negligible or non-existent. I hope that it is now clear why the Germans are probably so keen to maintain a controlled transition to the off-hook state at low DC currents. As an afterthought, I would venture to express my own opinion that all regulatory bodies should be required to justify their Standards with technical explanations, for their own and everybody else's benefit. This should be standard engineering and documentary practice, and should be part of their ISO9000 documentation. Failing to do so creates ill feeling amongst designers and manufacturers, has everyone guessing the resons for apparently idiotic requirements, and brings about the creation of often misguided lobbies that waste everyone's time. If anyone on ETSI (or indeed on any Standards-generating commission anywhere) reads this or receives this, he or she might like to address this proposal, and make it a part of the process of creating the pan-European norms and requirements. Best regards, David Drori --------------------------------------------------------------------------- ------------------ Novarex Enterprises Ltd., POB 2833, Jerusalem 91028, Israel. Tel: +972 2 581 0995 Fax: +972 2 581 3750 Mobile: +972 5 067 8686 E-mail: [email protected] --------------------------------------------------------------------------- ------------------
