telecom voltages

[jasneetsingh]

in most of the telecom infrastructure the negative DC voltages are used.
Can anybody explain the reason why ?

 

[Nigel Goodwin]

in most of the telecom infrastructure the negative DC voltages are used.
Can anybody explain the reason why ?

I wasn't aware it was? - I thought that they used a balanced line, with the polarity connected either way?. We used to have a leased telephone line, for a radio telephone base station - it reversed the polarity between transmit and receive. Every time BT worked on the phone lines they used to reconnect it the wrong way :lol:

 

[samcheetah]

ive also heard that the telephone lines use 48VDC and if im not forgetting it was -48VDC and for the ringing there is a signal of about 50-100Vp-p.

but telephone systems are country dependent. the voltages and the ringing frequencies are different for each country. im not sure what the values are for india. u should contact your local exchange for more info.

 

[Russlk]

The reason is probably lost in antiquity. Some vehicles used positive ground until, finally, everyone standardized on negative ground. In early systems, the earth was used as the return (the resistance of ground is nearly independent of distance). Electrolysis of the ground rod may be different for negative bias versus positive.

 

[Dialtone]

Most current telephone systems have standardized to use -48VDC as their power source. But this was not always the entire picture. Older systems also had +24VDC power systems in addition to the -48. The two power plants had the +48 and -24 sides tied common to earth ground, and as such were capable of providing a 72 VDC differential voltage if needed. In addition to these, some offices also had + and - 130VDC power plants.
One such application of using the 72 volts was to drive long telephone line cable loops for rural customers (range extenders), and 130VDC was used to power digital line repeaters housed along cable routes between offices, that had no local power source. New technologies have pretty much done away with these needs, and in many cases the telco's have dropped the +24VDC and +-130VDC systems from existance.
As to the original question of WHY the standard of - voltage, it is anyones guess. As you can see from the above, either polarity is and was used as needed. I worked in telecom 30 years and never really heard anyone give a definate reason as to the why, but I assume it is because of the theory that electrons flow from negative pole to a positive pole.
Dialtone

 

[samcheetah]

thanx for the info dialtone. what about the voltages for the ring. how much is that and what is the frequency. i know that its not standard but as u have seen this technology being developed over the years u could give a small history lecture :lol:

 

[Sebi]

In Hungary the ringing voltage 100V 16Hz. This frequency need for mechanical ringer, same as old handy-generator freq.

 

[Dialtone]

US ringing voltage varies somewhat but generally ranges from 86VAC to 105VAC 20 hz, again depending on the distance of the line from the central office. With new technologies available now which extend the virtual central office presence out farther (Subscriber loop carriers), the ringing voltage is generally kept at the low end, as the need to drive the long loops is not as much of a factor anymore.
In the US, the 2 line leads are referred to as "Tip" (voltage return), and "Ring" (-48vdc). These terms have their origins in the old cord board operator consoles of days gone by. The patch cords used were simular in size and shape to the large stereo hedphone jacks today. If you look at the jack, you see it is divided into 3 sections. First is the tip of the jack, next is a section that resembles a finger ring, and third is what resembles a long shirt sleeve. Thus the terms "Tip", "Ring" and "Sleeve" were used to refer to the 3 main leads used in early electro-mechanical switching systems.
Ringing voltage is generally applied to the "ring" lead of the line pair (-48vdc side) with the following exceptions.
In the case of 2, 4, and 8 party lines, the parties ringing voltage was applied to either lead depending on the party designation of the number. Parties 1,3,5, and 7 were designated "Ring" parties and parties 2,4,6,8 were "Tip" parties, with the ringing cadence applied to the appropriate line lead. The telephone sets for party lines were modified by the installer to connect the internal ringer circuit to the correct side of the line for ringing detection. This modification allowed only 1/2 of the parties on any line to hear the ringing cadence at any given time. The pre-determined cadence of short and long ringing bursts (4 and 8 party only) was applied to the line and the users had to determine which party should answer the phone. I do not think there are any more multi-party lines (with the possible exception of 2 party lines) available in the US anymore, but it makes a good history lesson anyway.
Dialtone

 

[samcheetah]

I do not think there are any more multi-party lines (with the possible exception of 2 party lines) available in the US anymore, but it makes a good history lesson anyway.

it sure did!!!!!!!!!!

thanx