British Telephone Sockets - Structured Cabling (RJ45 Wiring)

Structured Cabling (RJ45 Wiring)

Structured cabling systems are general-purpose communications wires installed in offices and increasingly also homes, which can be used for several different communication technologies (analog phone, ADSL, ISDN, Ethernet, video, etc.). The most common type uses Category 5 cables (four twisted pairs with 100 ohm impedance) between 8P8C (also known as RJ45) room sockets and a central patch panel.

The A and B wires of an analogue phone line appear in a structured cabling system usually on the centre pins of the RJ45 connector (pins 4 and 5; the blue/white TIA/EIA-568 pair 1 in Cat5 cables). In most other countries, those two wires are all that is needed to connect an analogue telephone. In the UK, however, many telephones expect the 25 Hz ring signal on a third wire. But such a 3-wire interface is not the symmetric interface needed for balanced twisted-pair transmission lines, and therefore prone to electromagnetic interference and crosstalk with nearby other wiring. Therefore, the capacitor that separates the bell signal from the A/B wires should be located close to the telephone. Any bell wire provided by the master socket is therefore not connected to the structured cabling, and the telephone is plugged at the other end into the structured cabling via a line adapter unit (LAU) that contains the capacitor needed to create the separate bell signal. Different types of LAUs are on the market:

• PSTN secondary/slave – for phones that need no bell wire
• PABX master – contains ring capacitor, to provide from the RJ45 2-wire interface the 3rd "bell wire" required by UK phones
• PSTN master – implements all the circuitry found in a BT master socket, including surge protection, ring capacitor, test resistor

There exists no well established standard in which polarity pins 2 and 5 of the phone socket are connected to pins 4 and 5 of the RJ45 connector; both alternatives are common. This polarity is not important; e.g. BT also does not specify in which polarity the A and B wire are connected to pins 2 and 5 of their master telephone socket.