UK Telecom FAQ, Part 3/3 - Technical matters Q: What do those wires do? [last update 27/3/95; answers courtesy Alan J Flavell] (Unless otherwise stated, this answer only deals with ordinary BT telephone lines using the British-type modular socket system. NB: the wiring up to and including your master socket is the property of BT and the rules say you mustn't interfere with it. Therefore these details are here for educational value only. British regulations prohibit the use of equipment that has not been type-approved for use in Britain. Therefore, any mention in here of equipment brought from elsewhere must be treated as purely hypothetical and for educational value only...) At the point where your telephone line comes into the house, only two wires (1 pair) are used. If you have more than that, the remainder are spare. The wire costs almost nothing, compared to the cost of installing it, so the wire that they lay often has several pairs in it. The wire pair does several things: it carries speech and bell-ringing current etc., and it brings a limited amount of DC power from the exchange; see the "How phones work" writeup for a general description, which is, however, expressed in US terminology. The two wires of a telephone line pair are called A and B. These are connected to your master socket or linebox (everything said in here about a master socket is also true of a linebox, unless a specific difference is mentioned). Nominally, line A is at 0V and line B at -50V. The 50V comes from a big battery at the exchange, which is kept charged from the public electricity supply ("the mains"). This is used to power ordinary telephones, thus allowing calls to be made even during mains failures. More sophisticated phones (e.g cordless) need a local mains supply, but the customer is advised to own at least one telephone that does not. Secondary sockets are connected to the master socket via two-pair or three-pair cable, in accordance with the instructions that accompany the kits. However, only three of these 4 or 6 wires are really used: namely, the two lines, and the bell wire (also known as "shunt", see later point). The British telephone plug (type 431A) is rectangular: it has ways for six contacts along one side, and a latch (and keyway) on one end. Only the middle four contacts are actually present, with one blank position at each end, but the contact ways are numbered the same irrespective of whether pins are actually present. However, bizarre as it may seem, it turns out that there are two different schemes of numbering the pins: the original BT one, starting with pin 1 at the latch end, and the "BS" one, with pin 6 at the latch end. The physical wiring, thankfully, is the same in any event - it is only the contact numbering convention that is different. BT plug, contact face up, pin number installation wire with cable to the left (BT) (BS) colour (Body/stripe) no pin not used 6 1 (White/green) .......... --- line A 5 2 White/blue .......... --- not used(E) 4 3 White/orange .......... --- bell shunt 3 4 Orange/white .......... --- line B 2 5 Blue/white no pin not used 1 6 (Green/white) latch/ There are some situations not covered by this FAQ (PABX Earth Recall for example) in which the pin marked (E) would be connected to a local earth. The master socket contains the following circuit (SP=Surge Protection) ------------------------------o pin BT2 (BS5) -> line B | | | = 1.8uF from | | SP X |----------o pin BT3 (BS4) -> to secondary socket(s) exchange | > bell wire | < 470kohms line A | > ------------------------------o pin BT5 (BS2) -> In the normal UK telephone, the bell is connected between the bell wire and line A. (There are some types of telephone approved for UK use in which the ringer is connected between the two lines, just as it would be elsewhere.) (Many modems will be like this too, detecting ringing current from the line pair and disregarding the bell wire. If the modem also offers a socket for connecting a telephone, UK modems can be expected to supply the telephone with bell current correctly; but foreign modems might not do so, with the result that the phone, if it uses the "British" bell arrangement, will not ring.) (There exists also a plug which is the mirror-image of the 431A, and which, of course, won't work in this socket: thankfully, most people will never meet one.) Q: Why a third wire for the bell? The system was designed at a time when pulse (loop disconnect) dialling was common, and telephones still had real bells. During pulse dialling, there is a tendency for the bells on the associated telephones to tinkle. By using a separate wire for the bell, it's possible for the telephone that's doing the dialling to shunt out the bells, preventing them from tinkling. Nowadays this is much less of an issue, what with tone dialling and electronic ringers. Q: What's this about master sockets and lineboxes? A: The original design of "master socket" was wired by the installer, and the customer was not allowed to interfere with it. The approved way to wire a secondary socket, if you still have such an installation, is to purchase an approved extension kit which comes equipped with an adapter. After fitting the extension socket, you unplug the phone from the master socket, plug the adapter into the master socket in its place, and, assuming you still want to have a telephone at the master socket location, you plug it into the adapter. The "linebox" is a much neater solution. It can be recognised by having a sub-panel on the front, which the customer can remove by undoing two screws. On removing it, one finds that the sub-panel is, in effect, an adapter, but with the advantage that the customer is allowed to punch down their own wiring to it. When investigating a fault, the engineer can remove the panel, together with the customer's wiring, and test the internal socket, free of confusion from customer additions. It is a good idea, before reporting a fault, to take off the subpanel yourself, and try a phone plugged into the internal socket in order to eliminate your own wiring - also try a second phone in case the first is defective - both kinds of fault would incur a charge if you call for a repair (line rental covers free repair only of BT's line). There are, consequently, two different kinds of kit for installing a secondary socket. The one with an adapter is meant for the original "master socket"; it can be used with a linebox too, but it would be pointless to do so. The other kit has no adapter - only a length of telephone wire (two or three pair, it doesn't matter) and a secondary socket: this type of kit is for use only with a linebox. Q: My equipment (telephone, modem, computer...) has an RJ11 socket for connecting the line cord. What are the connections? It is clear from the answers given by readers that there ~IS NOT ONE UNIQUE STANDARD~. The two situations most likely to be met in practice are these - The middle pair of the RJ11 (pins 3,4) carries the phone line. The line cord must connect that pair to pins 2 and 5 of the BT socket. This would also be the situation with equipment brought from the USA. - The line cord between the RJ11 and the BT plug is a straight cord. In this case, the manufacturer has arranged to put the line onto pins 2 and 5 of the RJ11. It follows that UK models of this type of equipment necessarily differ from the US models (in the USA, it is standard to use pins 3,4 of the RJ11 for the phone line). Some contributors asserted that one specific arrangement was ALWAYS used. But on the evidence, they must be mistaken. Some contributors say that the first is more common, while others have more often met the second: so one can only guess what the statistics are. At least one contributor implied that there are yet other pin assignments that have been used at the equipment end. (There is, however, only one arrangement at the BT plug end. Phew!). Note that in the USA, the middle pair of the RJ11 (pins 3 and 4) carries the phone line. Therefore, equipment that has been brought from the USA will assuredly be like that, and will need a line cord that connects the RJ11 middle pair to BT pins 2 and 5. It does not follow, however, that equipment supplied in the UK by US companies will necessarily be like this. Several readers have UK versions of gear made by US companies, that definitely has the phone line on pins 2 and 5 of its RJ11 socket. Specialist shops such as Maplin or Tandy should be able to supply either kind of RJ11-to-BT line cord. A US traveller unable to get hold of a correct cord could, say, bring a US phone cord with them, buy a British-style extension cord (readily available here), cut both cords in half and splice the appropriate halves together. Q: does it matter if A and B are interchanged? Interchanging A and B on the incoming line will merely reverse the polarity of the lines. Since normal telephone equipment is designed to work on either polarity, this would not matter. (But the installer **should** get it right, even so.) (One contributor asserts that ALL phone equipment will work irrespective of polarity. Reference was made to the UK (BABT) approvals procedure, which lays down that equipment must be unaffected by polarity in order to be approved for UK use. Some informants seem to know of equipment that will only work on one polarity, although it was not claimed that this equipment had been approved for UK use: the best that can be said with confidence is that such gear would be unusual.) Interchanging A and B between the master socket and a secondary socket, or on the line cord between socket and equipment, will prevent the bell from working (if the ringer is connected in the "British" fashion), and can cause other problems. Q: Why that 470 kohm resistor in the master socket? This allows the exchange's tester or "routiner" to test the line, even if all the customer's telephones are unplugged. Subject: Exchange features Q: How does Caller Display ('Caller ID') work? [For a more detailed discussion see 'Caller Display and Call Return' by William Dangerfield, Simon Garrett and Melv Bond in British Telecommuncations Engineering; Volume 12 part 3 (October 1993). Also See Supplies Information Note (SIN) 227, available on 0800 318601. BABT have issued a specification, BABT/SITS/94/53 to replace the draft specifications BABT/TC/128 and BABT/TC/131. This specification is identical to the 131 draft and is shorter and easier to meet than the 128 draft. A copy can be obtained from BABT on (01932) 222289.] The system described here is that developed by BT for use on the UK PSTN. It is based on the Bellcore 'CLASS' standard. This has the benefit of allowing CPE manuafacturers to base their UK models on those developed for the North American Market. Most of BT's customers are connected to System X, AXE 10 or TXE4 exchanges and these exchanges are digitally interlinked using CCITT [now ITU] C7 signalling. C7 provides a way of passing the number of the calling number to the distant exchange (this information is used during call tracing). When a call is made to a customer with Caller Display the distant exchange requests the number of the caller originating the call from the exchange at the other end of the C7 link. If the call is not routed totally over C7 links (e.g. the caller is on an old analogue exchange), or the caller is on an interconnected network for which no agreement for the exchange of additional call information is in force, the number will not be complete. In this case customer with Caller Display will get a 'Number Incomplete Message' If on the other hand the caller has deliberately withheld the number, by use of the 141 prefix the Caller Display Customer will get a 'Number Withheld' Message. If the number is complete, and not withheld by the caller the number is routed on to the Caller Display customer over the local access network. For this purpose a V.23 sender has to be installed at every exchange concentrator. When a line is about to receive a call the polarity of the line is reversed prior to the ringing current being applied. If the customer has Caller Display additional messages are interspersed between the polarity reversal and the application of the ringing current. First a tone alert signal is sent and then an alternating series of '0's and '1's lasting 250ms is sent by the V.23 sender to assist the CPE in detecting the imminent arrival of the Caller Display message. The Caller Display message itself contains the following information: - The number of the caller - Reason for absence of number (e.g. number withheld) - Time and Date (Can be used to auto-set CPE clocks) - Caller/Name Text (Intially only used for designating calls from payphones) - Reason for absence of caller name - Call type The Caller Display message takes roughly 0.75 seconds to send, after which the normal ringing current is applied to the line. Subject: Telephones Q: Why does my pre-socket phone 'tinkle' in the night ? There have been postings regarding telephones 'tinkling'. It has also been posted in reply that this is attributable to line testing. They are tested by detecting the capacitor in the phone socket. Pre-socket phones are still tested OK, but the capacitor is actually in the phone. Hence it tinkles when tested. Phones are **not** tested every hour, the current target is once a day, but in most places it'll be once every few days. New systems coming on stream in the future will permit the lines of people who complain about tinling to be excluded from the test cycle. Q: What is the difference between Timed Break Recall (the Recall button) and flashing the hook? Conceptually Timed Break Recall (TBR) and Flash Hook are very similar; Flash Hook is used worldwide, TBR was invented by BT (GPT) for use in the UK. The difference is in the duration. TBR is 90ms, Flash Hook is anywhere between 200ms and a couple of seconds. Cable telephones tend to use Flash Hook as they are using North American exchange equipment (for example, Cambridge Cable use the NT DMS100). Subject: Telephone exchanges Q: What types of exchange are there, and which can be digital ? The types are: - Strowger - rotary switches, etc (UAXs, SAX, TXS) - Electromechanical crossbar (TXKs) - Electronic-control reed-relay switches (TXE2) - SPC non-digital (TXE4*) - Digital (Systems X and Y - AXE10: may be further categorised as to whether they're ISDN-capable, I suppose; 5ESS; DMS100; UXD5.) The last crossbar and Strowger exchanges have now been removed from service. The UXD5, a public-exchange variant of the Monarch digital PABX, probably remains only in the Highlands of Scotland and rural Wales. You can tell whether you have an electronic exchange by trying to dial a number with DTMF dialling. If you get unobtainable when you dial * or # then you have an electronic one, but not digital. On TXE4(RD) exchanges (the original design), a change to the class-of-service threading is needed to enable DTMF. The TXE4A (with a newer design of processor and register) has DTMF enabled for all lines. Dial *#001#. If you get "No services are in operation on this line" or a list of services, you're System-X. If you get "Sorry, you have dialled an invalid service code" it's AXE10. Also, *55* will prompt you for a time on System-X; it won't on AXE10. If you get Number Unobtainable, you're not on a digital exchange. If you still have dial tone, you're on an "old technology" exchange. If you hear nothing (except crackling in your case) you're probably on an Ericsson (System-Y) Now, type (a) accepts your dialling at the point in the switching matrix that your call has so far reached. Every switch has its own dial pulse decoding and routing intelligence. Short of generating the dial pulses at the MDF (where the lines come into the exchange), there's no way to convert them to DTMF. And what would be the point? - pulsing dial pads are _so_ cheap that pretty much everyone's telephone can do the job. All but type (a) accept the digits you dial into a register, and make the routing decisions centrally based on the contents of the register. In principle, any such exchange can easily be converted to accept DTMF: it simply requires a different input box for the register. (Historical note: the first electronic research exchange to hit public service in the mid-60s, the TXE1 at Leighton Buzzard (RIP) was DTMF capable. As a person involved with fiddling 'phones at the time, I arranged to go round it with some like-minded friends: we were shown a DTMF 'phone in the exchange manager's office, and were told that there was precisely one `public' user of the service.) Pretty much any SPC exchange (types (d) and (e)) can in principle offer network services. We know that type (d) can in some cases, since we've had a post to that effect from someone at STC (was it - sorry, I've forgotten your name). We know that type (e) can do it, since they all do... Five designs of digital exchange are being installed. System X, AXE 10, UXD 5, 5ESS PRX and DMS100. System X is an all-British family of digital exchanges manufactured by GEC Plessey Telecommunications (GPT); AXE 10 is a Swedish design and manufactured in the UK by Ericsson Telecommunications Ltd; UXD 5 is a small system specifically devleoped by BT for rural areas, and 5ESS PRX is the European version of a system developed by the American company AT&T and widely used in North America. The DMS100, designed and built by Northern Telecom (NT) is being used for Featurenet services. The last analogue switch was removed from the trunk network in June 1990. All traffic on the trunk network is now handled by 59 fully interconnected Digital Main Switching Units (DMSUs) and four partially connected Digital Switching Units (DSUs) which help handle high call volumes from between London and the Home Counties. International traffic is handled by 4 dedicated exchanges. At the local level, around 97% of customers are connected to digital or modern electronic exchanges. At 30th September 1993, BT operated 7,537 local exchanges in the UK, comprising 5,532 digital, 1,301 electronic, 38 crossbar and 666 Strowger (electromechanical). By March 1996 all customers will be served by modern electronic exchanges. Q: What is the difference between a System X and System Y exchange ? System X was a co-operative development by Post Office Telephones, GEC, Plessey and STC. There was a big bust-up in which STC were removed (and given exclusive rights to supply TXE4) and GEC and Plessey were given all the rights and obligations to develop System X as a commercial system which BT would then buy by competitive tender. It was decided that there should be at least TWO suppliers to the PO/BT, and there was a large international tender to decide on a second, competitive system. Ericsson made the AXE10 which offered practically the same flexibility in a smaller box (mainly because System X was designed by committee: it was already dated before the first release model became available). The AXE10 was chosen and was then (and still is) bought by BT in competition with System X. The AXE10 became BT's 'System Y' just to keep things 'simple'. If anything the AXE10 is more full-featured than the 'X' but BT only took the basic software package on each, so both offer practically the same. Incidentally, Cellnet tried to use System X as their mobile switch but dumped it as it was very old tech. Vodafone tried Ericsson, and that is why Vodafone runs totally on AXE10s. Q: What is the number that will dial my 'phone back to test the ringing? Dialling 174 on modern electronic exchanges will make the exchange ring back when you put the phone on hook. On a System X/Y dialling 177 will make the exchange read out your number, and dialling 175 will produce the exchange test... A replacement for both 174 and 175 has also been mentioned, which is 17070 Ringing 17070 results in "This circuit is defined as " and then " BT line test facilities, please press 1 for ring-back test or clear down." Keying 1 then results in "BT ring-back test, please clear down." After a few seconds, phone rings, and announcement is "BT ring-back test completed." Q: When I called 175 from home there was a message telling me my phone number, then when I hung up I was called back and offered a mysterious menu. Anyone know how to work it? - 1. Dial 175 - 2. Exchange answers and says "You are connected to . Start test." - 3. Hang up - 4. Exchange calls back and says "line testing ok" (assuming it is ok, of course). Then it enters a loop, inviting you to "dial next test" and performing a test based on the single-digit code you give it. From memory, test 1 is the keypad test (dial 123456789*0#) and 3 is the coin pulse test. Exchange Test Numbers Abbreviations SPM = Subscriber Pulse Metering FRB = Ringback Test CPI = Cable Pair Indicator (Loud tones on line to identify cable) LPI = Line Pair Indicator (Gives telephone number connected to) SALT = Subscriber automatic line test TXE4 - Electronic Equipment: SALT Dial 175 and wait for "Start Test" message Replace handset. Rings Message: "Line Testing Ok" or fault if present. Continue, wait for dial tone for dial test. Dial 1 3 0 5. Rings. Message "Testing Ok" or fault if present FRB Dial 174 LPI Dial 188 gives directory number & equipment, or if not, Dial 187 gives equipment number. CPI 176 plus full national code (i.e. 176 081 553 7104) Tone is present on line when dialled. Replace receiver to cancel tone. TXD-X - System X Equipment SALT Dial 175 FRB Await ringback and listen for message. Continuation tests. Await interrupted dial tone. Dial "1" for dial test. On LD phone, dial 1-0. On MF phone, dial 1-9*0#. Dial "4" for SPM test. 10 metering pulses are sent, then NU tone, followed by 10 more pulses. CPI Dial 176 plus full national code (i.e. 176 081 553 7104) (This will only work from same processor) Wait for short burst of tone reminder, then tone is transferred to the specified line. To disconnect tone, replace handset. TXD-Y - System Y Equipment SALT Dial 175 Await ringback and listen for tone. Dialtone=OK Engaged=Suspect Faulure=Fault Dial next test if required, tests as follows: 2 Low A/B Insulation test | 3 A/B to earth | Insulation tests 4 A/B to battery | 5 A/B loop to earth | 9 Bell answer before 5th ring 11 SPM test 7 Loop Res test 8 Dial test. LD Dial 0. MF Dial 1-9 0 10 Rec all pre 6 Line reversal for diode check Dialtone=OK Engaged=Suspect Faulure=Fault FRB 174 Q: What do the various messages from the 175 tester on System X really mean, eg. "Earth A, Battery B"? A-wire (leg) - connected to earth **at the exchange** via effectively a current source. B-wire (leg) - connected to negative 50v **at the exchange** likewise. Looking from the exchange, there should be a large resistor (100k I think), across 2microfarads, and a slice of resistance in series (1k nominal at dc) - the bell. (the 100k is only on new lines) Earth or battery A, as reported by the tester is a fault. it should be isolated totally. Earth or battery B likewise. Low resistance A to B likewise again. Each indicates a weak insulation, and it depends on what path the weakness has allowed. other exchange lines around it are still connected, to earth and -50v after all ... Q: Is there a way I can pulse-dial a digital exchange number and then tone dial from there? All BT payphones switch to tone dialling as a matter of course, after receiving a metering pulse. Most dual-equipped phones, if switched to pulse dialling, will change over to tone dialling by pressing the '*' key. Operator and non-payment numbers (144, 0800) do not switch. Subject: Phone numbers/ DTMF Q: What are the frequencies for DTMF (Dual-tone multi-frequency) digits? 1209 Hz 1336 Hz 1477 Hz 1633 Hz 697 Hz 1 2 3 F0/A 770 Hz 4 5 6 F/B 852 Hz 7 8 9 I/C 941 Hz * 0 # P/D The right hand column (obviously) is not normally available but has certain applications. The functions are labelled either F0, F, I, P or A, B, C, D. [end of FAQ ]