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Almost ever since the telephone was established as an essential part of modern life there have always been situations where the demand for telephone service in a particular road or district outweighs the available line plant - the copper cables that connect a subscriber to the telephone exchange. It can be a costly and time consuming task to draw more cable through, often over several miles.

Over the years telecomms companies have sought solutions to this problem. Firstly, there was shared service, more commonly known as the party line. Secondly, there was a 1 + 1 Subscribers Carrier system known as WB900. Thirdly, there was a system called DACS (Digital Access Carrier System) based on ISDN technology.

All these solutions enable two telephone lines to be carried over one line pair. I shall now explain a little more about these different technologies and show pictures of the equipment that was used. I am indebted to the various contributors who have assisted me in compiling this topic. WB900 and DACS are known as Pair Gain Devices (PGD).


Shared Service (Party Line)

Party line diagram used for 700 series telephonesParty lines are where two or more subscribers are connected to one telephone line pair. The most popular party line used was the two-party line, which as it name implies provides for two subscribers on the same pair of wires, one party was the A-line and the other was the B-line. Party lines do not provide secrecy from the other party, but they use earthed ringing circuits so that only the intended recipient's telephone bell rang. When a subscriber wished to make a call they had to operate a push-to-make switch on their telephone to signal to the exchange that they wished to make a call, this ensured that the exchange metering billed the correct subscriber. A subscriber wishing to make a call had to wait for the other party to finish their call, if they were already on a call. If the call was an emergency they had to request the other party to conclude their call. Shared service was introduced on automatic exchanges in 1942 but ended in the early 1980's as the new system X and system Y digital exchanges did not have the facility for party lines.

Ivory 312L [Courtesy of Dave Hayball] Ivory 706F
Black 312F Black 706R Mk.I
300 series telephones used for shared service 700 series telephones used for shared service
Further information on the early days of Shared Service

1 + 1 Subscribers Carrier System (WB900)

WB900 Block DiagramWB900 was first introduced in the late 1970's and this new technology meant that the two subscribers had complete privacy and could use their telephones simultaneously. One subscriber was known as the audio subscriber and the other subscriber was known as the carrier subscriber.

The audio subscriber has a normal line using DC signalling, the only difference is that they have a filter unit (No: 8) fitted at the Distribution Point (DP) usually on the telegraph pole which fed both telephones, in underground installations a filter unit (No: 10) was fitted and another filter unit in the exchange. These filters were to eliminate the 40 kHz carrier signal from the audio signal.

The carrier subscriber is separated from the audio subscriber by using a high frequency carrier of 40 kHz over the cable pair. When the user lifts their handset the loop condition signals the carrier equipment at the exchange to send a 40 kHz high frequency (H.F.) signal to the exchange where it is recognised by the carrier equipment and is converted to a DC signal to operate the exchange equipment. The same H.F. signal is modulated with speech signals from the carrier subscriber, is interrupted at 10 pulses per second during dialling and is also used to signal the exchange when the telephone is answered on incoming calls. 

Incoming signals to the carrier subscriber from the exchange use a 64 kHz carrier, this is modulated with 25 Hz during ringing. The ringing current is then demodulated in the subscribers house and amplified to ring the telephone bell.

The subscribers carrier equipment which is installed in the carrier subscriber's home is powered by a 10 volt Ni-Cad battery, which is charged from the line current, when neither telephone is in use. However, problems were prone to occur when the battery was exhausted due to prolonged use of the line

Subscribers Carrier Equipment
Subscribers Unit WB900 Adaptor No: 1A; Item Code: 374696; Made by Elremco
WB900 Subscribers Unit
 Made by STC
WB900 Subscribers Power Unit
WB900 Filter No: 15A WB900 Filter
Exchange Carrier Equipment
Racks of WB900 Cards
Racks of WB900 Equipment Each card has two lines
  Racks of WB900 Cards  

Digital Access Carrier System (DACS)

DACS Block DiagramLike WB900 DACS provides two direct exchange lines over one cable pair, without either subscriber's line affecting or overhearing each other. It works in a similar way except it send both lines down the cable pair between the exchange and the subscriber as a digital signal. DACS is known as a 0 + 2 Carrier System. The DACS equipment at the exchange monitors the line and an alerts if errors occur or the connection is lost. DACS consists of Exchange Units (EU) which are housed in racks, the cable pair and the Remote Unit (RU). DACS equipment is made by Telspec and ECI.

The RU can be an internal type fitted in the subscribers premises, an external type fitted to the pole or to a wall or an underground type for use in a footwell or joint box. The Telspec underground and pole top, look exactly the same. The differences are that the underground one employs a re-enforced plastic case that is resistant to oil, petrol, insects and impact. The pole top one is hermetically sealed and has resistance to ultra violet light . They are also different in the results they give back to the EU under fault conditions. An underground one tells the EU that it is a contact battery fault, so a Jointer is required, and a pole top sends a loop or earth fault so a Customer Apparatus and Line man is required. The line voltage for DACS is 140V.

Each Telspec DACS EU rack has eight pairs of cards, eight Analogue Line Cards (ALC), eight Digital Line Cards (DLC) and one Synchronisation, Maintenance & Alarm Card (SMAC) card, providing 80 POTS lines (40 digital trunks) in total. Each ECI EU rack has 15 identical cards each card has four POTS lines (2 digital trunks) providing 60 POTS lines (30 digital trunks) in total. See photos below.

The copper pair between the EU and the RU uses 2B1Q signalling similar to ISDN technology. The first generation was DACS1, introduced around 1990, did not support CLI (Caller Line Identification) and the data rate was only fast enough for Fax machines, not modems. DACS2, the second generation, was introduced around 1995, this supports CLI and modems up to 33.6 kbits/s. Almost all DACS installations are now DACS2.

DACS Subscribers Equipment
For installation in Manholes and on Poles
DACS 2B External Pole RU Mk.V DACS 2B External Underground RU Mk.V DACS 2A Underground Remote Unit
Made by Telspec in 2001 Made by Telspec in 2000 Made by ECI in 2001
Item Code: 379930 Item Code: 379932 Item Code: 379884

For installation in Subscribers Premises

DACS 2B Internal Remote Unit Mk:IV DACS 2B Internal Remote Unit Mk.V Internal DACS without top cover Inside of Telspec DACS2 Remote Unit
Made by Telspec in 1999

Made by Telspec in 2000

Item Code: 379929 Item Code: 379929  
External DACS Unit
External and Internal DACS2 DACS2 External Remote Unit External DACS2 Remote Unit (on right)
DACS Exchange Equipment
DACS EU Rack made by ECI DACS EU Rack made by ECI DACS EU Rack made by ECI
DACS EU Rack made by Telspec DACS EU Rack made by ECI DACS EU Rack made by Telspec
DACS EU Rack made by Telspec DACS EU Rack made by Telspec DACS EU Rack made by Telspec
Telspec Analogue Line Card Telspec Digital Line Card Telspec SMAC Card
Telspec DACS Line Locator, Part No: 600-0676

Many thanks to Adrian Rodsett, Andy Blofield, Chris Hall, Derrick Mulvana, Hywel Clatworthy, Ian Jolly, John Griffiths and Kevin Sanderson who have all very kindly contributed material to this page.

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