Party 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.

WB900 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

Like 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.

For installation in Subscribers Premises

Made by Telspec in 2000