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Topic Title: The longest telephone line
Topic Summary: Without using amplifiers
Created On: 22 June 2009 06:10 PM
Status: Read Only
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 22 June 2009 06:10 PM
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jencam

Posts: 608
Joined: 06 May 2007

A question from my son.

Telephone exchanges were (and still sometimes are) interconnected using twisted pair cables carrying analogue voice frequency signals. What is the maximum possible length of one of these interconnections using standard issue cables if the signal is to arrive with sufficient strength to be useful? My son is wondering whether it would have been possible for a phone call from LOWer Hook exchange to HAInault exchange, on opposite sides of the London dialling code area, to have been accomplished 50 years ago without any amplification between the two exchanges. Amplifiers were used on long distance interconnections between STD exchanges, but were they used between exchanges within dialling code areas?
 06 August 2009 02:42 AM
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ericmark

Posts: 319
Joined: 12 February 2008

On the Falkland before the conflict when mines were placed under telephone wires so they could no longer be repaired there were no amplifiers and the single wire went from Stanley to Bluff Cove then Fitzroy then Island harbour house across what is now main runway to Swan Inlet then Goose Green then to North Arm. I am not sure if the phone would work over whole distance but from Stanley to Goose Green would work about 50 miles.

When I was there Island Harbour house was the limit and it did not connect to Stanley but it was very dependent on Weather the wind would produce static and would stop the phones working. These were old phones with crank handles.

With twin wires the range was higher but to do very long distances Morse code was way to go as it could use higher currents.

Or before Morse we had the fax machine I think there is still one of the pair in a museum in Paris! I thing Paris to Lyon? I think around 1843 fax was invented around 30 years before Morse code.
 06 August 2009 03:12 PM
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normcall

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Joined: 15 January 2005

In the 70's I used to have a AC private wire from Luton to Croydon area (about 30 miles as the crow flies) and a DC private wire from Luton to Dunstable Downs (about 5 miles).
We regularly linked the two via a manual switch and also into the PSTN without amplifiers without many problems.

This was in connection with a message handling service (pre mobilephones!) using mobile radio.

-------------------------
Norman
 22 September 2009 11:56 AM
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cmap

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Joined: 22 September 2009

I am of the opinion that this is a rather interesting question. I actually never thought about it. After reading the posts in this thread I think that the maximal length of such a telephone line is 50 miles - which I think is quite long. Is there anyone who has information about a line which is longer than 50 miles?!?
 22 September 2009 12:38 PM
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seeker

Posts: 319
Joined: 10 March 2007

Strangely enough I was recently looking for my ancient Atkinsons Telephony books but seem to have lost them somewhere along the way of moving houses a couple of times.
No doubt the subject would have been considered at length in this source. Does anyone still have a copy?
 23 September 2009 11:24 AM
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AlanKay

Posts: 232
Joined: 09 July 2002

This is exactly the sort of question that the dear, departed Peter Strangman could have answered without hesitation.
For those unfamiliar with the man, he was for many years a priceless contributor to the uk.telecom newsgroup, regularly commending others to refer to Atkinson Part 1 or 2. It may still be worth posting your question on that newsgroup. It may still be inhabited by other deeply-knowledgeable types.

-------------------------
Alan Kay, CEng MIEE
 24 September 2009 09:27 PM
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alancapon

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I still have Atkinson Parts 1 & 2 in a box in the loft. I will have to dig them out and have a read.


Regards,

Alan.
 14 October 2009 04:06 PM
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gmarett

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To a large extent this would depend on the type of circuit deployed. An overhead open wire route with heavy conductors (say 200lb/mile) would produce a very long trunk route. In the early part of the 20th century such routes connected London with Edinburgh some 330 miles. This was in the days before thermionic valve amplifiers and although the signal was weak, it was nevertheless audible on a normal telephone, albeit the communicators had to shout!

Graeme
 28 October 2009 10:12 AM
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jencam

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My son managed to borrow a copy of Telephony by Atkinson. He has also found an owner of a preserved Strowger telephone exchange who let him play around with it. The owner verbally informed him that in 1990 over half of all telephone exchanges in the UK were Strowger although most of them served small villages rather than urban areas. Is this really true?
 28 October 2009 11:02 AM
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seeker

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Originally posted by: jencam

My son managed to borrow a copy of Telephony by Atkinson. He has also found an owner of a preserved Strowger telephone exchange who let him play around with it. The owner verbally informed him that in 1990 over half of all telephone exchanges in the UK were Strowger although most of them served small villages rather than urban areas. Is this really true?


Strowger was sometimes used as a generic name. There were some true Strowger exchanges but by the 1990s most had been updated to 2000 and 3000 equipment with TXE and crossbar exchanges creeping in. I worked for GEC when Llanwern and Bishopton were installed and they were GECs first venture into TXE2 as I recall. It was around the same time that the practice of running cables via planned routes was discontinued and cable grid was introduced to allow short, straight runs from rack to rack ot IDF/MDF. Many of the old style PO clerks of works abhorred what they saw as abandonment of good practice.
One of the least missed parts of exchange installation practice must be wiper setting! Mind numbing beyond belief!
 28 October 2009 11:49 AM
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jencam

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The British Post Office decided to standardise on the 2000 type Strowger exchanges in the 1930s. It was specially designed by collaboration between the Post Office and the Strowger manufacturers.
 03 November 2009 01:21 PM
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johnnmann

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Has your son discovered the Telecommunications Heritage Group (http://www.thg.org.uk/)? He might find it interesting.
 14 November 2009 08:15 PM
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CliveM

Posts: 228
Joined: 09 October 2002

Electronic and crossbar exchanges in fact were well established during the 1970s. I came across grid mesh instead of cable runways working on exchanges in the late 70s. System X followed by AXE digital exchanges started to be brought into service to replace strowger, crossbar and electrinic exchanges during the the early 80s and in fact some of the very early system X exchanges were being replaced in the late 80s.

As an earlier contributor responded to the original question on the length of circuits between telephone exchanges unamplified it depended on the size of the conductor. The move to underground cables between exchanges reduced the size of the conductors in an attempt to increase the wire count of cables which would fit into the ducts which were mainly 92mm in diameter in the days of earthenware duct, increasing to 100mm with the use of plastic duct. In order to increase the distance circuits were usable over the circuit was turned into a low pass filter, with an upper range of 4kHz, by adding 88mh induction coils every 1.83km.

Even with this the vast majority of inter exchange circuits in the 70s were amplified using 2 wire amplifiers which were known as negative impedance amplifiers.

In this day and age most exchanges will be connected by optical fibres rather than pairs of wires.

-------------------------
Clive Maude
 16 November 2009 11:04 AM
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Paul1966

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Long, non-amplified lines were far from uncommon in earlier years. Test boards were regularly fitted with an artificial cable option (to imitate a very long line with resistance and capacitance) simulating 45 miles of standard cable.

On the issue of exchange types in 1990, there were certainly still many Strowger SxS switches in use, although I think half (in terms of actual number of exchanges) might be a high estimate.

Electronic exchanges had been experimented with in the early 1960's, leading to the development of the TXE (Telephone eXchange Electronic) range, starting with the Leighton Buzzard TXE1, the only one of its type built. The TXE2 was the first British electronic exchange to be installed widely, starting in 1966, and soon becoming a common replacement for smaller Strowger UAX's (Unit Automatic eXchange) in the 1970's. Growing large villages and small towns which were rapidly approaching the capacity of their UAX13 switches were common targets for TXE2 replacements. The TXE4, intended as the electronic replacement for larger towns, started to be installed during the mid 1970's. The last Strowger SxS exchange in the PSTN left service from a Scottish Island in 1996, and the last TXE was withdrawn in 1998.

Crossbar switches, common in the U.S. since the 1930's, didn't appear in Britain until the 1960's, but were soon installed fairly widely. The sector tandems in London when the capital was sectorized in the late-1960's were all crossbar, for example. TXK1 crossbar exchanges were also quite common replacements for medium-size towns in the late 1960's/early 1970's.
 16 November 2009 10:34 PM
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Angram

Posts: 557
Joined: 23 March 2009

"standard issue cables"

To get the best distance you need to match the impedance of sending and receiving
kit. In a telephone you can't match both receiver and transmitter: So you compromise.
In 1960 a rocking armature receiver was introduced. It is so sensitive you could hear
dialing clicks with a dead short across it. This enabled perfect matching of the transmitter
impedance at the expense of the receiver. So both transmitter and receiver were able
to work over longer distances. BUT they didn't do that: The cables were made smaller
to save copper and to allow more cables per underground duct, thus saving civil engineering.

Field telephones with carbon transmitters and rocking armature receivers usually quoted 30 miles
so 50 is plausible.

There used to be local codes between adjacent exchanges as well as national STD codes.
One of the games folks used to play was hopping between several exchanges using only
the local codes. You needed to find out all the other people's local codes.
As your son suggests, the volume became increasingly faint as you hopped further afield.
Of course you only paid the very cheap local call rate
The hop you have specified would have been possible in terms of distance.
 23 June 2012 07:25 AM
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fnorman

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Joined: 25 July 2008

To do the hopping as described you not only had to know the codes you had to know which exchanges had separate incoming and local 1st selectors because if they did, hopping through was usually blocked. The smaller the exchange the likelier it was to have shared inc/local. From xxx, hopping to yyy (45 miles) was faint but audible while further connected but was too faint to be usable. I don't feel too guilty about it as it was just a temporary game to find out how far you could go, and not for making calls without paying.

Originally posted by: Angram

There used to be local codes between adjacent exchanges as well as national STD codes. One of the games folks used to play was hopping between several exchanges using only the local codes. You needed to find out all the other people's local codes. As your son suggests, the volume became increasingly faint as you hopped further afield. Of course you only paid the very cheap local call rate

The hop you have specified would have been possible in terms of distance.
 20 August 2012 08:34 AM
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electricaltestinguk1

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I will search first so I can give you the best answer. Thanks
 23 February 2014 02:27 PM
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jencam

Posts: 608
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Was there a technical reason why the London Director Area was defined as all telephone exchanges inside a circle with a 12.5 mile radius from Oxford Circus? If so, then was the decision to use 12.5 miles, rather than 12 or 13 miles, determined by the attenuation of telephone cables. The boundary of Greater London has changed since the 1930s and exchanges serving parts of outer boroughs - most notably Uxbridge, Orpington, and almost all of Havering - were outside of the LDA and are not in the 020 area today. However, Elstree, Bushey Heath, Thames Ditton, Ewell, and Loughton exchanges were included in the LDA and are in the 020 area today despite them all serving localities outside of Greater London.
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