[TheList] Phone Jacks

Tue Oct 30 20:25:26 AEDT 2012

Very interesting stuff!

-----Original Message-----
From: TheList [mailto:thelist-bounces at radiowiki.org.nz] On Behalf Of Neill Ellis
Sent: Tuesday, 30 October 2012 3:13 PM
To: thelist at radiowiki.org.nz
Subject: Re: [TheList] Phone Jacks

Hi Guys,

Whilst I've been out of Telecom some 17 years and left that side of the industry some 15 years ago it's been an interesting read this topic.

The days of POTS is almost over! POTS meaning Plain Old Telephone Service, some people newer to the industry may never have heard that term before.

POTS has a DC feed of nominally 48V (often 54.5V) with a current limit of around 25 mA to power the phone. During ringing a 25 Hz 90 VAC ringing signal is superimposed on the DC feed. This voltage can be as low as 12V on some more modern pair gain devices with ringing being quite bizarre waveforms, but they achieve the same result.

Capacitor are used to strip the 25 Hz 90 VAC ringing from the line for the ringers. Drawing DC will answer the call indicating the off hook condition to the line equipment.

In the old days when BT jacks were first installed (1983?), a master and secondaries were the norm, with a third "Anti tinkle" wire between the jacks that shared the ringing current.

The line from the exchange (or roadside cabinet these days) is a balanced pair of wires. They would feed into your master jack and then any secondary jacks would be wired off that single master jack.

The reasons?

There were a couple, line tests in the old days measured capacitance on your line. Seeing approximately 2 uF of capacitance indicated a intact line to the phone. If more than 2 uF was observed it usually meant an illegally wired extension phone was installed at the "Subscribers". In those days you rented your phone and paid extra for any additional phones installed. Hence you had to make sure any acquired extensions were wired by someone who knew what they were doing, some less scrupulous staff did well out of these "Homers".

Also modern telephone equipment was coming into use that included electronic line cards, be it SPC (Stored Program Controlled or
Computerised) exchanges or pair gain devices. These miniature relays for ringing they used had small unforgiving contacts, excess capacitance would stress those contacts as high current during ringing or alternatively back EMF (ElectroMotive Force) from inductive bell coils could generate sparks that might pit the contacts when they opened.

The 470 k resistor included in the master jack served two purposes, it would allow test staff to see the capacitor should no phone be plugged into the jack(s), also it reduced back EMF from any inductive bell coils as the ringing relay opened (not normally an issue unless close to the line equipment as line capacitance would do that).

The surge arrestor fitted is obvious, to minimise damage should a surge occur.

A big change that occurred with BT jacks was the way bells were wired in the old non electronic phones. Previously the bells were wired in series, but on BT jacks they effectively became paralleled up. Sometimes the Master Secondary system was called the parallel wired system for that reason.

The term anti-tinkle comes from decadic or rotary dialling days were a series of pulses to the DC feed to the telephone were used to dial numbers, before DTMF (Dual Tone Multi Frequency) Dialling was widely adopted. These pulses would cause bells to tinkle with dial pulses, so the wiring of the phones ensured that the phone off hook would effectively shunt the ringing circuit to prevent it happening.

Going back to this balanced pair of wires from the exchange (or cabinet) feeding your house. These wires carry signals well, but they get unbalance inside your house by a third wire that's capacitively coupled to one of the pair coming in. Whilst that was ok with POTS, it's not with the likes of ADSL. This unbalancing can increase crosstalk to any adjacent line or increase noise etc.

In the early 90's Telecom made the move away from Master/Secondary to 2 Wire Jacks. These 2W jacks just had a single capacitor, 1 uF from memory, but I had little contact with them. By this time decadic dialling was a thing of the past and electronic ringers meant lower current draw and no back EMF issues. Best of all the balanced pair could remain balanced with no third wire between jacks (important with ADSL). 
Important to note for those who aren't familiar with 2W jacks, when installed a termination unit is put at the demarcation point, usually at the point of entry into the building, it has a resistor & capacitor (presumably 1.8 uF & 470k).

Locating an ADSL modem as close as practical to the demarcation point where the phone line comes into your home is always a good idea. So is upgrading any cabling prior to the ADSL filter that isn't of twisted pair design.

The original NZPO/Telecom 6 wire cable (initially grey sheath, later
white) or original 4 wire (grey sheath) cabling wasn't twisted into pairs at all and as such isn't a proper balanced pair, this isn't good at the frequencies ADSL relies on. Hence upgrading to Cat 5 or similar twisted pair inside the house as well as 2 wire jacks is an excellent idea.

Now I must say I was originally trained as an exchange (& transmission) tech in NZPO so my knowledge of this "Linesman" equipment isn't as good as others, but hopefully it will fill in some gaps in some others knowledge.

How much longer will POTS exist? ADSL and FTTH has bought high speed internet into our homes. VOIP is now so common and works so well that a lot of people don't notice the difference between a good solid 64 kbps PCM connection and it. VOIP is the way of the future and is here now, how long till POTS disappears altogether?

I believe in time the big cellsites we see today will largely disappear in general suburbia. They'll still exist to serve large coverage requirements in the rural area for example, or in areas where there is likely to be high traffic. But around the home, what's to stop your phone from automatically connecting to the wireless router of a nearby house or business instead of a cellsite. That way could provide even better coverage with less infrastructure costs. We live in interesting times, technology advances so fast these days and I'm keen to watch the next step in the evolution of the technology. In time will the police still have their own network? Maybe it'll just be IP based and encrypted over the public network like everyone else, fine if all is ok, but potentially bad in an emergency scenario.

Food for thought? Thanks for letting me say my 20c. My apologies if grey matter is rusty and I got a bit wrong. Enjoy your day.

Regards,

Neill.

On 29/10/2012 11:14 p.m., Denis Dawson wrote:
>
> If you have broadband and have a number of jacks daisy chain connected 
> downstream from the master then your broadband could be degraded 
> especially if you use the old thin white phone cable.
>
> Daisy chained jacks and old cable are not good for broadband. If you 
> have to have multiple phone outlets wire them all in a star 
> configuration using CAT 5 cable.
>
> My advice is to take heed of what Neil said about the master jack but 
> put your ADSL filter right at that first jack and buy a multihandset 
> cordless phone so you only have one phone connected and use CAT 5 
> cable if possible.  If you can rewire back from the demark point to 
> the first phone jack use CAT5 and install the filter there and run 
> separate cables for the ADSL and the phones.
>
> Denis
>
>

_______________________________________________
TheList mailing list
TheList at radiowiki.org.nz
http://radiowiki.org.nz/mailman/listinfo/thelist_radiowiki.org.nz