Hi Treg Hi Joe. I can see the evolution of PSTN in most countries moving towards CC feed of typically 25 to 35 mA. Standards will probably keep higher limits in place until such time that it can be guaranteed that no old fashioned analogue bits or strange loading coils in lines etc remain. A PSTN CC feed will still have an OC voltage of 48 - 50 v for idle state conditions. This type of arrangement is available on AXE-10 and System X Network exchanges.
To think we still talk about the longevity of our PSTN! Shame we haven't all moved to the great pan-European ISDN that everybody was seeing as the future 10 (or more) years ago but we don't want to open a political debate now do we! Bill Ellingford PS I know ISDN was first mentioned in the Seventies & evolved in the1984 (red) & 1988 (blue) CCITT books (now ITU-T) ---------- From: [email protected][SMTP:[email protected]] Sent: 15 March 1999 16:54 To: [email protected] Cc: [email protected] Subject: Re: TBR 21- 60 mA max. loop current In a message dated 3/15/99, [email protected] writes: > I Understand that the French PTT say that the Feed inductors / relay coils > used in their network saturate at 60mA +, when saturated no ac is passed so > transmission becomes highly attenuated. This was too powerful an argument > for allowing current to exceed 60mA in the TBR / CTR. Bill: Thanks for shedding some light on the rationale that the French used to defend the inclusion of the 60 mA requirement. I have heard various explanations, but this is the most believable one yet. It seems to me that eventually, the French network (already claimed to be one of the most modern in the world) will be fully upgraded to solid state feed circuits which contain their own current limiting. I would think that the feed coils which are in danger of saturating would only be associated with older, electro-mechanical central offices. Has there been any discussion of phasing out the 60 mA requirement at some point in the future? I recall that in their formal response to the proposed TBR 21, the UK included a comment that perhaps the 60 mA current limit could eventually be phased out. It seems to me that if enough vendors of terminal equipment complained about the 60 mA requirement, France might be persuaded to agree to a phase-out date. I might also add that while the 2 watt dissipation may be tolerable in most types of TE, it is particularly onerous for designers of PCMCIA cards and high density, multi-port cards. I think that most manufacturers of TE (including manufacturers located in France) would have an interest in eliminating this requirement. Joe Randolph
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