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British telephone sockets were introduced in their current plug and socket form on 19 November 1981 by British Telecom to allow subscribers to connect their own telephones. They are governed by British Standard BS6312.
They are similar to modular connectors (as used in RJ11), but feature a side-mounted hook, rather than a bottom-mounted one, and are physically incompatible.
Contents |
History
Standard sockets were introduced to allow customers to easily purchase their own telephones, as required by Oftel, the phone regulator. Thus any phone whose plug conformed to BS6312 and met certain other regulatory standards, such as BABT, could be connected to the network, rather than British Telecom controlling the market.
The new system replaced the older hard-wired system which came in many 'flavours', e.g. Plan 1A, 1B, 1C, 2, 2A, 105, 107 etc, which could be very complicated and required the attendance at the premises of a GPO telephone-engineer, who needed a complete set of 'N' (wiring) Diagrams, which was very extensive and ran to over 15 volumes. N diagrams also had their own numbering system e.g. a Plan 1A had an N diagram of N4502, and were frequently updated.
From at least 1920, the GPO (subsequently British Telecom) did have a plug and socket system available for rent. It was called a Plan 4 system (N4306) and employed a much heavier-duty plug (circular in cross-section) than the present-day ones. It also had to have a separate Bell-Set No. 26 which was permanently in-circuit to provide ringing if there were no telephones plugged in; in these circumstances, the bell-set also provided a testing circuit for remote engineers. Rental had to be paid on the phones and on all the sockets, so the system was hardly a fore-runner of the present system.
Sockets
Master socket
A domestic single British telephone line installation will have a single master socket in the premises, which is provided by BT or another service provider: this socket is the demarcation point between the customer-owned and maintained on-premises wiring, and the telephone network. The demarcation point is actually within the socket: the front wiring is the customer's, while the back wiring is the telco's.
This master socket contains a high voltage surge protection (SP1) to suppress high voltage spikes etc, a 1.8 µF capacitor (Bell Circuit) to feed the AC ringing and a 470 kΩ resistor (R1 Out of Service Resistor) to provide remote testing when no telephones are plugged into any sockets. Additional internal extension (secondary) sockets are wired off the master socket (connected in parallel using the IDC system) and do not contain the surge protector, bell circuit capacitor and the out-of-service resistor.
Old style: Fixed plate
The 'old style' fixed master socket (see image at right) had only one set of terminals on the back and customers were supposed to use extension kits plugged into the front socket, however many customers hard-wired their own extensions anyway for neatness and robustness reasons which was a poor arrangement since it provided no way to isolate the customer's internal extension wiring from BT's wiring.
New style: Removable plate (NTE5)
In recent years NTE5 sockets (see image at left) have been fitted in place of master sockets. These have a front plate where the lower-half is removable so allowing customer's access to the terminals required for connecting internal extension sockets; it also provides access to a test jack, to determine if line faults are due to the customer's wiring or BT's. The removable panel also allows the external telephone line to be easily disconnected from the internal wiring, provided the wiring of the premises has been correctly carried out. The terminals on the back part are large screw terminals allowing direct connection to many types of external phone cable; cable companies use a similar socket with insulation-displacement connectors (IDCs) instead of screw terminals on the back piece, known as a CTE5.
Now that BT does not have a monopoly of internal wiring, they make a substantial charge if a fault reported to them turns out to be in the customer's internal/domestic wiring. It is therefore important for the customer to have the facility to check whether any problem or fault is in their internal wiring/equipment or externally in BT's cabling or systems. Since the NTE5 socket represents the official demarcation point between the internal/domestic wiring (at the removable front of the socket which is the customer's responsibility) and the external telephone line/cabling fixed at the rear (which is BT's responsibility) the physical disconnection of the two wirings (made possible by the NTE5's removable front plate) is crucial in identifying faults and allocating responsibility for their rectification.
Plugs
There are two types of modern British Telecom plugs – 431A and 631A.
They are similar to the American registered jack standard, in that they have a number of positions, some or all of which have contacts (pins).
431A is 4-way and 631A 6-way. They fit a standard "Type 600" telephone socket. There are also plugs with only two contacts commonly seen on modem leads. These are a recent introduction and do not seem to be easily available as separate parts.
The Sinclair QL and 128k Spectrums used type 630W connectors for their serial ports. These closely resemble standard 631A connectors but the keying slots on the back of the plug have a different size and position.
The 431B and 631B plugs had the latch on the opposite side of the plug, and were used as headset plugs on some switchboards and as handset connectors on some telephones, eg Ambassador.
The 631A and 631B plugs are also used for connecting sensors to interfaces for computer based measurements in educational environments, the former for connecting analogue sensors and the latter for digital sensors. Companies using these plugs include Vernier, TI and Casio, for interfaces connecting to their graphical calculators, and in the Netherlands CMA. They all use the same pinning.
Connector on phone
The connector on the phone is not standardized: the connector at the wall is standardized by regulation, to allow individuals to use their own phones (interconnection), but the wire from the phone to the wall may be hard-wired to the phone, or use various connectors.
Typically it will have a 6P4C or 6P2C modular connector at the telephone end: this latter may be wired as per the RJ11 standard (with pins 3 and 4), or it may be wired with pins 2 and 5, as a straight through cable from the BT plug (which uses pins 2 and 5 for the line, unlike RJ11, which uses pins 3 and 4). Thus cables are not in general compatible between different phones, as the phone base may have a socket with pins 2 and 5 (requiring a straight through cable), or have an RJ11 socket (requiring a crossover cable).
Use in other countries
The BS 6312 jack has been used in New Zealand since the 1980s, replacing a number of other connectors and hard-wired connections. The "BT Connector" is still the most common phone jack in use, although many installations in business use structured cabling with 8P8C ("RJ45") connectors for telephone as well as data services.
It is also used in Bahrain, Bangladesh, Belize, Botswana, Brunei, Cyprus, Eritrea, the Falkland Islands, Ghana, Gibraltar, Israel, Jordan, Kenya, Kuwait, Lesotho, Malawi, Malta, Myanmar (Burma) , Nigeria, Oman, Qatar, Saudi Arabia, Swaziland, Tanzania, the United Arab Emirates, Zambia, and Zimbabwe. The jack is still found in Hong Kong, where new installations ceased in 1997, while in Saint Vincent and the Grenadines, new installations ceased in 2001, with RJ11 now used instead.
Making the connections
As previously mentioned the actual connections are made using Insulation-displacement connectors (IDC). A punch down tool is required to do this and two sorts are available. One is of plastic construction and only intended for occasional use. The other is a tool manufactured by Krone and is of more robust construction : an example is shown in the photograph. It also comes with a tool for removing wires from sockets. The outer sheath of the cable is removed but the insulation from each wire is left and just placed in the connector. The Krone tool both inserts the wire into the connector and cuts off the excess wire in one action. The action of pushing the wire into the connector cuts into the insulation and makes contact with the wire. A maximum of three wires can be attached to each connector but it is best to stick to two if possible as the third is not usually a good connection.
 Krone IDC Tool, showing the wire removal tool open officially known as Inserter Wire No. 2a |
 Plastic IDC Tool, for occasional use only |
 4 wire cable the old sort, but much thinner |
 6 wire cable the current type |
Cabling arrangements
Shown below are the cabling arrangements for both 4-wire and 6-wire cable. The 6-wire is now standard but the 4-wire, if it can be obtained, does have the advantage of being thinner and it is therefore easier to get it into small spaces. Note that the wires in the 6-wire cable are coloured with two colours in a ratio of four to one in length, with the first colour mentioned being the predominant colour, e.g. if the colour of the wire is W-B then the wire will be coloured White for 12mm, then Blue for 3mm and so on. In other words it looks like a white wire with blue patches on it.
 Four wire cabling |
 Six wire cabling |
 Phone Socket - The principle of Bells in Parallel and Opt Out of Service |
Strictly speaking, a textbook installation will only actually use pins 2, 5 (for the voice) and 3 (for the ringer). Having said this, most modern telephones contain their own ringing capacitor, to cater for badly wired extensions, which means you can usually run your extension wiring with only pins 2 and 5. Often where multi-core cable is used, the remaining cables are used for wiring extensions on additional incoming telephone lines.
Broadband
In order to provide Broadband Internet services over the telephone line it is necessary to use a DSL filter. All phones must be connected via a filter (either a separate filter for each phone or one filter covering multiple phones) to avoid interference between the phones and the DSL signal. The DSL modem connects directly to the phone line (most DSL filters have a socket marked DSL that just connects directly through to the incoming phone line). The ringer wire is unnecessary in unfiltered parts of the wiring and its removal can often improve performance and reliability of the broadband service.
When ADSL was first introduced in the UK it had to be installed by a BT engineer who would replace the front part of the NTE5 (if the property still had an old style master socket it would be replaced with an NTE5) with one containing a filter. Any hardwired phone extensions were disconnected from the original front part and connected to filtered terminals on the back of the filter. The DSL modem (which at the time was also BT supplied) and, if present, a phone or plug-in extension, could then be plugged into the front. If it was desired to locate the DSL modem away from the master socket a plug-in ADSL extension kit could be purchased.
Later BT started offering "wires only" ADSL service and promoted the technique of using a separate plug-in filter on every socket. While both technically inferior and far less tidy than the solution BT engineers had used, it was usually adequate and was simple enough for a non technical householder to understand. The more discerning customer can purchase a variety of hardwired filtering products, including replacement front plates for the NTE5, some of which have unfiltered as well as filtered terminals on the back to avoid the need to plug in the extension wiring that leads to the DSL modem. These were first independently offered by Clarity.
In 2008 BT trialed and launched their 'IPlate'; the "I" is for interstitial, as it is fitted between the socket and the front panel. This plate is fitted by the consumer inside the NTE 5 and reduces interference carried by the 3rd (bell) wire. The reduced interference allows faster broadband speeds - BT claim a speed improvement of up to 1.5 Mbit/s with a theoretical 4 Mbit/s. By November 2009 BT were calling the I-plate a "BT Broadband Accelerator".
Socket and adapter gallery
 LJ 2/1A - BT Master socket showing components - surface mounted |
 LJ 2/3A - BT Secondary socket - surface mounted |
 LJ 1/1A - Compact Master socket for use where space is at a premium - surface mounted |
 LJ 4/1A - Two Exchange Line Master socket - flush fitting. Standard BS fitting |
 LJ 3/3A - Secondary - flush fitting. Standard BS fitting |
 LJ 5/1A - Double Master - flush fitting- note straps paralleling the second socket into a master as well. Standard BS fitting |
 LJ 5/3A - Double secondary - flush fitting. Standard BS fitting |
 LJ 2/1A - BT Master socket converted to a secondary by removing components |
 Master with ADSL Filter - surface mounted |
 Adapter to provide two sockets off a single socket, and a connection for an external socket |
 Adapter to provide three sockets off a single socket |
 Adapter to provide four sockets off a single socket |
 ADSL Filter Plug |
 Adapter to allow USA-type telephone plugs to be used |
See also
External links
- NTE5 master telephone sockets - Extensive information, including legal, responsibility and ownership issues pertaining to the connection
- Wiring diagram
- UK plug and socket telephones
- The RJ System of telephone plugs and sockets is gradually replacing the BT System in Hong Kong, Office of the Telecommunications Authority (OFTA), Hong Kong Special Administrative Region Government
Notes
- "BT claims UK broadband boost breakthrough". The Register. 2008-10-01. http://www.theregister.co.uk/2008/10/01/bt_broadband_boost/. Retrieved 2008-10-01.
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
