Joe, The current limiting requirement was introduced in TBR21 in a plain oldfashioned horse-trade with the French. This was one of the ways to have them vote yes in ETSI.
The official "technical" reason is that there are some quite new Alcatel PBX:es out there that need this feature i.e. not old stuff. However, the requirement will eventually be removed (e.g. the limiter function could be implemented in a separate unit instead) but as the current political approach is to persuade the no voters using the existing text, it will be in there when TBR21 becomes a CTR. It would be interesting to hear other comments from manufacturers who had the opportunity to implement the current TBR21 standard in actual designs. Our experience so far (after two TBR21 approvals) is that when you actually have a design on the table and you start pre-testing it, some very strange requirements pop up. One example is the Loop current characteristics (ยง4.6.2 in prTBR 21: June 1996), which is measured down to extremely low currents (0.3 mA!!!). These currents simply don't exist in the real world and this requirement shows that the standard was not prepared with semi-conductor based designs in mind. I find it quite amazing.......... Can someone explain why this requirement is needed (not to mention why it should be justifed by Art 4(f) in the Directive)???? Roger Magnuson TAD GRUBER COMMUNICATION AB ---------- From: [email protected] Sent: Wednesday, April 23, 1997 06:58 To: [email protected]; GECAB/EST/ROGER Subject: Re: PABXs & TBR21 current limiting In a message dated 97-04-22 11:24:43 EDT, you write: << I heard one colleague changed his French DAA current limiter from 45mA (i.e right in the middle of the range of values (26-60mA) that France requires you to perform your current limiting for NET4) to 55mA (i.e dangerously close to the high end of the scale using low tolerance components), because some PABXs tried to force >45mA into this DAA and therefore wouldn't work (No Off-hook detect). >> John: This story is an interesting one, but I am surprised at the reported problem with off-hook detect. Most solid-state SLIC circuits these days (PBX and central office) incorporate current limiters. The reasons are twofold: 1) Conserve battery power 2) Prevent excessive heat dissipation in the SLIC (self protection) This function is a current limiter, not strictly a current source. That is, the current is not allowed to exceed a specified maximum limit. However, there is no problem if the loop current is below the limiter threshold. The SLICs that I have worked with tend to have off-hook detection thresholds around 10 mA, although this parameter is programmable in many devices. Maybe there was something strange about the specific PBX in the story you heard. I would like to hear more if this is a real problem. And while I'm on this subject......... The whole issue regarding current limiting in TBR 21 really annoys me. Clearly, there is no need for current limiting in the TE in any country except France. Even in France, which supposedly has a modern infrastructure that is 100% ISDN-ready, I would expect that the vast majority (perhaps all?) of the PSTN lines are fed with modern, solid state SLIC circuits that incorporate current limiting at the central office. Maybe, somewhere in the south of France, there is a rural central office that still relies on the TE to limit the loop current. This would probably have to be an old electro-mechanical switch that has not yet been changed out. I question whether this is sufficient justification to force all of Europe to adopt a TE design that contains a current limiter which must be capable of dissipating 2 watts. Surely, if there really are some old central offices remaining in France that might actually be damaged by a non-current-limited TE, the French might be able to tell us when these last few offices will be replaced. Then, we could contemplate a CTR 21 that includes a phase-out period for the current limiter requirement. My experience suggests that in regulatory matters, reason usually prevails in the end. Thus, I continue to hope (wish?) that the next version of TBR 21 will do a better job of dealing with the current limiter issue. I noticed in an earlier posting that the second attempt at TBR 21 has gone out for voting. Does anyone on treg have a copy of the version that went out? If so, can you tell us whether there is any relief on the current limiter requirement? Joe Randolph Telecom Design Consultant Randolph Telecom, Inc. <<File: UUCP_ENV.TXT>>
