Line Signalling Systems Book

A Review of UK Signalling Systems
by Peter Walker

Throughout my 22 years with the PO and BT, I had a good deal of involvement with signalling, introducing R2, CCITT No. 6 and CCITT No. 7 to the international network, while trying to persuade the ITU to update its antique CCITT No. 1 manual ring-down signalling system from its original 500/20 frequency to 2280Hz.  However, all international exchanges also needed connections to the national network, which brought me into contact with a wide variety of signalling systems, including loop disconnect, DC2, AC9 and AC11/MF2.  Through folk such as John Maurer, "Bubble" Hubbard, Ken Fretten, Peter Steele and Charlie Miller, I was educated in the principles and arcane details of the various types of signalling, including wisdom from experts from a previous generation, such as Sammy Welch and Brian Horsfield.

Signalling is a very broad subject - indeed several textbooks have been written on the topic - so I can only skate over the surface of the issues.

As well as varying by type, signalling systems also tend to be designed for a particular network application, such as Access (subscriber signalling), Trunk & Junction, International and Private Network signalling.

Types of Signalling
The first major division amongst signalling systems is between Common Channel Signalling and Channel Associated Signalling.  Today, common channel systems rule the roost: CCITT No. 7, DASS2 and DPNSS, but the technique is by no means new.  In manual days, Order Wire working was a form of common channel signalling, where the operators spoke over one circuit for calls on a larger group. Modern common channel signalling systems use a message based system, with messages for a given voice channel distinguished by a header address.  The only major variant within common channel systems might be considered to be the nature of the common channel path itself.  The CCITT No. 6 system mainly used an analogue bearer with a 2400 baud modem, whereas today, a 64kbit/s digital bearer (usually Time Slot 16) is almost always employed.

Manual Signalling Systems
Manual signalling systems, of all types, provide a simple calling signal to the next operator in the call path.  These comprise systems using DC (loop or battery), AC (essentially magneto ringing, previously known as generator ringing) and Voice Frequency (VF).  The latter include the aforementioned 500/20 system: a 2s pulse of 500Hz interrupted at 20Hz, to emulate the magneto system with its 'modulation' at 16 or 25Hz.  A range of specialised signalling systems also developed for interworking manual to automatic exchanges and vice versa.

This is the application of a low frequency AC ringing current of 17, 25 or 50 Hz to the A and B wire of a 2 wire circuit or the phantom of a 4 wire circuit by a manual operation, such as the operation of a ring key. The signal is only present on the circuit while the manual operation persists. Where M/AC is used in both directions a signal has to be given to call the distant PBX and at the termination of the call a manual clear signal has to be given. M/AC may be used in conjunction with loop signalling e.g. on extensions of a PMBX, "M/AC OUT AUTOMATIC LOOP IN" to the PMBX.

This is the application of similar DC potentials to the A and B wire of a 2 wire circuit or each wire of the phantom of a 4 wire circuit, by a manual operation, such as the operation of a ring key. The signal is only present on the circuit while the manual operation persists. M/BB is generally used in both directions and manual call and clear signals have to be given in both directions.

NOTE (a) M/AC and M/BB both require physical circuits when used exclusively.
NOTE (b) Improvements in certain PBX equipments designed to speed up operating procedures have produced some difficulties in differentiating between manual and automatic signalling.

e.g. PMBX 4 and 11. There is no ring key associated with the cord circuit on a PMBX 4 or 11. A short period of ringing is applied to the circuit on the insertion and withdrawal of the plug from the PBX jack when manual signalling is employed over the inter-PBX circuit. This is not considered to be automatic signalling but manual signalling applied automatically.

Automatic Signalling Systems
Amongst automatic systems, the most familiar signalling system must be loop-disconnect, widely used in junction signalling (and even in some trunk systems).  It uses the same dialling principle as on subscriber lines, but over the years many additional features were added.  With loop disconnect, the loop current performs both line signalling (supervisory functions) and inter-register signalling (address digits) - though with Non Director Strowger, of course, there were no registers! Over the years, many extra signals were added to loop disconnect signalling to provide extra functions, including features to provide backward busying, busy flash, trunk offering, manual hold, UAX discrimination signals for level 9, ORD and CCB, Coin & Fee Check control and metering over junction.  (Another textbook would be required to describe all the functions -  but I'm not going to duplicate Atkinson!).

These signals variously comprised earth, battery or positive battery applied to one or both legs of the junction with earth return.  The exception, of course, is backward busy which used a high impedance relay to monitor for loop potential from the next exchange. But loop disconnect was not the only combined line and register signalling system.  Exchanges of the Rotary or Ericsson 500-point system types used Revertive Signalling. With this system, the distant selectors sent back loop pulses to the originating register as the selector was driven forward by its motor.  When the selector was correctly positioned, the register sent forward a Stop signal, which caused the motor driven selector to halt. Hunting for the next selector would then take place and when this was found it would restart the backward pulsing.

With register controlled exchanges, signalling systems developed which separated the line signalling and inter-register signalling functions.  Only with the modern Common Channel signalling systems have these been recombined.

This is an automatic signalling system in which a low resistance path is automatically applied across the A and B wire of a circuit or phantom to give a call signal which is detected by a battery and earth backed double coil relay at the distant end. An automatic answer signal is normally given from the distant end by a reversal of the battery and earth condition. When loop signalling is applied in both directions, a clear signal from either end is given by reverting to a normal battery and earth condition through a double coil relay. Dialling can be used with loop signalling by disconnection and reconnection of the loop normally at the rate of 10 pulses per second with a 66.66 break to 33.33 make ratio. Loop signalling is used in both directions or it may be used in conjunction with 25 Hz ringing current, as on a normal PBX telephone extension i.e. LOOP OUT - AC IN.

An earth is automatically applied to one wire phantom to call the distant end. An earth is automatically applied to the other wire of the pair as an answer signal from the distant end. Removal of the earth at either end constitutes a clear signal. A/WE is generally used in both directions.

Dialling can be used with earth signalling by disconnection of the earth and reconnection of the earth from the wire of the pair, normally at 10 pulses per second with 66.66 break to 33.33 make ratio. Although it is technically feasible to get bothway dialling, in practice it is generally applied in one direction only. The use of A/WE signalling is now discouraged because of the noise it can induce into other cable pairs.

Balanced battery is generally used in both directions. Dialling of the pair or is not available with this system. To call a distant PBX, similar or balanced DC potentials are applied automatically to the A and B wire of the line or phantom. When the distant end answers, balanced battery is returned to the originating end automatically to provide an answer supervisory. When either end clears, the balanced battery is removed automatically from the circuit by the PBX which clears first and the distant end receives a clear supervisory.

An earth is automatically applied to the A wire and a battery to the B wire of the pair or phantom to constitute a call signal. The answer signal from the far end is an earth automatically applied to the A and B wires of the circuit. A clear signal from the far end is the automatic replacement of the earth on the A and B wires by a relay loop. A clear from the originating end is the automatic application of an earth on the B wire and a battery on the A wire. This condition is subsequently replaced by a relay loop when the PBX is ready to accept incoming calls. Dialling is effected by the commutation of the calling battery and earth at the originating end. This signalling system provides automatic bothway signalling with or without dialling in either or both directions.

Channel-Associated Signalling
Channel-Associated line signalling systems can be divided between In Band, Out Band and Separate Channel systems.  In the former, signalling is sent within the voice channel - so all these are voice frequency systems.  Outband systems signal outside the voice frequency band (300-3400Hz), usually using VF signals at the top of the channel at 3825Hz, or lower on older 3kHz channels.  However, we might also categorise DC systems as Outband, since the direct current is outside of the speech band.  The same is true of systems which used 50Hz or magneto signalling.

Separate Channel Signalling
Separate Channel systems use a discrete signalling path for each circuit, but these paths are grouped together in a separate channel from the voice circuits. Early examples of Separate Channel signalling systems were AC2x and AC6, described later on, though the former was then described as a Common Channel Signalling System.  Modern day digital 30 channel PCM systems use Time Slot 16 as a separate signalling channel, though this is usually known simply as Channel-Associated Signalling  (CAS) to distinguish from Common Channel systems which also use TS16.  This leads to the conclusion that the term 'separate channel signalling' has never been well defined or consistently used.  Where separate line and inter-register signalling is used, they may use different forms of signalling, for example, a DC (outband) line system combined with a  multifrequency VF (inband) inter-register system.

Line Signalling Systems
Line signalling systems usually take one of three forms: DC, VF (voice frequency) and Digital.  DC line signalling systems follow many of the concepts described above for loop-disconnect, but without the digit pulsing.

Voice Frequency Signalling
VF systems come in various types. The VF may be pulsed, pulsed and acknowledged or compelled.  Where two frequencies are used, these are normally termed 2VF systems.  In a simple pulsed system, a VF pulse is sent to indicate some new line state. With pulsed and acknowledged systems, the pulse is acknowledged by the other exchange by a reverse VF pulse.  With compelled signalling, the first VF signal remains on until the next exchange returns an acknowledgement VF signal.  The latter also stays on until the original signal ceases.

A number of peculiar challenges face the designer of a VF signalling system. Because the signalling uses the same path as the speech, it must be arranged as far as possible that the speech cannot interfere with the signalling or be mistaken for signalling.  Equally the signalling must not interfere with the speech, especially the Answer signal which is sent just at the point that conversation may commence.  VF line systems therefore tend to use frequencies above 2000Hz, where speech energy is less.  2280Hz was a popular choice, as this seems to match a minimum energy spot in English speech, except apparently for female Scots speakers!

One way to minimise misoperation by speech is to keep the bandwidth of the signalling receiver as small as possible, but this tended to make the receiver more expensive and lengthen tone recognition times.  A more reliable technique is to use a guard band, from around 1000-1900Hz, so that no signalling is recognised in the presence of energy in the guard band.  This prevents speech and especially modem data from misoperating the 2280Hz detector.  This is important, not only to avoid the speech being mistaken for signalling, but also to prevent the speech being interrupted by the line splitting function.  Line splitting is employed to break the forward transmission path on receipt of a VF signal, so as to prevent the VF signal spilling over onto a subsequent circuit in the call path.  It is important to ensure the line splitting is set at the right timing.  Too quick and it may react to some speech syllables and interfere with the speech.  Too long and an excessive spill-over can occur. Line splitting is also employed just before and after the transmission of a VF signal, so that any speech on the circuit is separated by a quiet period just before and after the actual signal.  This ensures that the VF and guard band filters operate correctly to the VF signal.

To ensure that the Answer signal does not interfere with the start of the conversation, it is advisable that the VF pulse is fairly short. AC9 used 300ms.  It would be recognised after a persistence check of 150-200ms, which would then trigger the answer on any preceding circuit.  With some connections in the Transit Network, up to five tandem VF links could occur and the answer signal on each link would not start until it had been recognised on the former. This would be noticeable at the start of the conversation - the caller would hear five sets of pips as each signal started before the line splitting operated.  With international networks, this effect could be even worse and various strategies were employed to minimise the problem.  With CCITT4, the line signalling (as well as the inter-register) used the end to end system, rather than link by link - effective as long as two or more contiguous links used CCITT4.

With CCITT5 the answer signal would start on one link even before it was recognised on the previous.  Another problem which taxed the ingenuity of designers was to avoid VF systems being deliberately misoperated by so-called 'phone phreakers' with accurately tuned VF senders.  They would attempt to clear a circuit, so that a new call could be established.  The first call would be a no-charge call, then attempts would be made to set a call to a chargeable destination.  This became a major epidemic in the USA, where the Bell SF (single frequency VF) used a tone-on-idle 2600Hz signal, so it was only necessary to transmit a continuous 2600Hz tone to clear the call. British designers learnt from this experience.  AC9 and AC11 used an 800ms forward pulse as the clearing signal.  On receipt at the incoming exchange, a 'check tone' would be returned in acknowledgement.  When the check tone was received at the outgoing exchange, it not only operated the normal incoming line splitting, but also operated the line splitting on the outgoing side.  If the clearforward signal was genuine, such line splitting would be superfluous, as the line would already be split as part of the sending sequence.  However, if the 'clearforward' was a tone generated by the caller, the line splitting would interrupt the signal, so that the incoming exchange would not recognise the signal.

Because of this system, 'phone phreaking' was never as commonplace here as it was in the USA.  There were a few weaknesses however.  I myself discovered that on many Strowger GSCs, the check tone would not be sent while the incoming MF2 register was still connected. This left a small window between the end of the MF signalling and the completion of forward loop pulsing when the check tone system was ineffective.  This loophole was later closed.  CCITT No. 4 and CCITT No. 5 were open to abuse.  The latter is still in use to many third world countries, but the specification has now been changed so that a circuit is cleared if the Release Guard (sent in acknowledgement to the Clearforward) is recognised during the speech state.  A further refinement, necessary because CCITT No5 is usually a compelled system, is that any Release Guard signal has a minimum duration so that it will always be recognised.

Digital line signalling systems
Digital line signalling systems were first introduced in the 1960s with the first PCM systems.  With the original 24-channel system in the UK, a single signalling bit was transmitted with the eight speech bits.  Three states were created in each direction by using the coding 00000.. , 11111..., and 101010.... 
In the USA, a different 24-channel system was deployed.  With this, the 8th bit of the speech channel is 'stolen' for signalling purposes once each multiframe.  This is why in the USA until quite recently, ISDN calls could only deliver 56kbit/s connectivity, as compared to our 64kbit/s, because the 8th bit could not be relied on for end to end transmission.  This is sometimes referred to as 'In Slot' signalling, as compared to the UK 'Out Slot' system and can be compared to In Band and Out Band analogue signalling.
Outside North America, the CCITT standard 30 channel PCM system is now the predominant digital system.  This separates the signalling from the speech slots.  Signalling uses Time Slot 16 and is divided so that signalling for each channel is sent once every multiframe and four bits per channel are provided.  This provides for a wide range of signals, including Trunk Offer, Back Busy, Manual Hold and C&FC.

Inter-register signalling
We now turn to inter-register signalling systems.  We have already noted that loop disconnect provided both line and inter-register functions.  Pure inter-register signalling systems tended to use either VF or MF (multifrequency) systems.  Examples of VF systems were CCITT No. 3 and No. 4, which used 1VF and 2VF binary coding respectively.  Four binary 'pulses' allowed 16 different signals to be sent.  With international signalling systems, address digits above 10 were used, such as Code 11 and Code 12 for operator access and Code 15 for 'end of pulsing'.  With CCITT No. 3, the signal was either a tone or absence of tone, similar in many respects to telegraph systems with mark/space signals.  With CCITT No. 4, the two VF tones represented binary 1 or 0. 

More modern systems employed MF, such as MF2 used in the UK transit network and on routes to the London Sector Switching Centres and International Exchanges.  MF2 and the international R2 system used the '2 out of 6' coding scheme, where 15 signals in the forward and backward direction were obtained by transmitting two from the range of six frequencies.  R2 uses fully compelled signalling, while MF2 used a peculiar combination of compelled prefix signals followed by pulsed information signals.  This was to allow MF2 to be reliably used in the face of 'Strowger noise' at GSCs.  Compelled signalling was not always an advantage. 

For signalling over satellite circuits, pulsed MF systems, such as CCITT No. 5 are far faster, as the compelled method takes around 1 second per digit - as slow as loop disconnect! Inter-register signalling systems could be link-by-link or end-to-end.  With link-by-link, each exchange register forwards the address signals to the next exchange.  With end-to-end, the first exchange register signals in turn to each downstream register. As each transit exchange completes the switching of the call, the register disconnects and the VF or MF signals pass transparently to the next exchange.  The advantage of the latter was that intermediate transit registers could be simpler, with no forward transmitting functions and shorter holding times could be obtained. 

List of UK signalling systems
Having reviewed signalling systems in general, I will now give a short description of the various signalling systems as used in the UK.

DC systems
Loop disconnect was never given a number, but note that loop disconnect, when deployed over the phantoms of a four-wire junctions to International Exchanges was often known as LD4, but the "4" referred to four-wire.  In favourable conditions Loop Disconnect could be used up to 45 miles in length, but in practice it was more like 20 miles.

DC1 - this was a system used over long trunk lines. It employed battery, rather than loop, dialling and used rather expensive valve detectors to regenerate the pulses at the incoming end.  It was introduced in the early 1940s in a number of locations, principally Northern Ireland, but was superseded by DC2.  Digits are sent at 10PPS.

DC2 - this was a system of employing "single commutation DC pulsing" (SCDC) and could be used up 100 miles in length (as long as a metallic path can be provided for the DC signals).  By using symmetrical double currents and sensitive Carpenter relays, longer distance routes could be used than ordinary loop disconnect, which suffered from pulse distortion.  Three versions were designed, one for trunk routes, one for local junctions and a special version for four-wire junctions to and from international manual boards, which had an additional Forward Transfer signal, often known as DC2/FT.

DC3 - 4 wire long distance DC line system used on the Transit Network, mainly between GSCs and the first transit exchange.  It could work over 5000-ohm routes. The trunk was 'seized' by a forward balanced battery which was detected by a high impedance relay at the incoming end. The answer condition was signalled by inserting a much lower impedance relay, causing the loop connected relay at the outgoing end to operate.  It was therefore like a loop junction but operated in reverse.

DC4 - this was a special 4 wire signalling system, used with MF3 for signalling on fairly short routes in London between international manual boards and international exchanges.  It features the Forward Transfer signal for recalling downstream operators.

DC5 - This signalling system is used for signalling over short distances and generally used between a PBX and co-sited equipment. The system operates over two discrete signalling wires which are electrically separated from the associated speech circuit. The two signalling wires are known as E&M. DC signals are sent on the M wire and are received on the E wire. Although the system is often referred to as E&M signalling it should be noted that it is not compatible with some other forms of E&M systems. In general use as it is an international standard.  Generically called DC5 even though there are 4 variants.

DC5A - Inter-PBX line signalling with 10 pps (will also support MF4 and MF5).  Maximum current of 25ma and maximum DC resistance of both signal wires of 25 ohms which is equivalent to 275 metres of 0.5mm cable.

DC5B - Inter-PBX line signalling for use with SSMF5 inter-register signalling.

DC5C - Long distance extension signalling.

DC5D - Inter-PBX line signalling on International circuits with 10pps or MF inter-register signalling.

DC6-9 - I have no knowledge of what these might have been.  Indeed, they might not have been used. I would welcome any information.

DC10 - Two wire signalling system used on private networks. Still in use although BT has refused to install any more of late.

DC11 - allocated, but never used, for a DC system to be used alongside MF6. A large miscellany of other DC systems was used.  In private networks, generic systems called just SCDC and E&M were deployed.  With E&M signalling, a pair of earth return wires is used to transmit a simple DC condition.  The Mark wire was to transmit the signal and the Earth wire
(at the other end) to receive.  In between, the DC condition is often converted to outband or PCM signalling.
A number of specialised DC systems were used for 'keysending' from manual boards.  A system called DC Code C was used in private networks as a subscriber signalling system for keypad telephones.  Placing an earth or diode on one or both legs of the line unbalanced the loop.  The diode could be used in either direction, and the end result is that 12 signals could be generated.  I once built a simple home exchange using this system.  Another system, called Coded Call Indicator working, was employed between automatic and manual exchanges and allowed automatically dialled calls to be completed by operators without the caller being aware.

AC systems
AC1 - was the first trunk AC system, employing 2VF - 600 and 750Hz.  The reason for the use of such low frequencies was that at the time it was designed (late 1930s), electromechanical alternators, rather than electronic oscillators were used to generate the tones.  AC1, in its original form, was used on two-wire trunk circuits.  It had relatively poor speech immunity and was later replaced by AC9, though the last systems were not withdrawn until the 1970s.  For a long time, the existence of AC1 signalling prevented customer premises equipment from using tones in the range above 500Hz, a particular problem as AC1 did not use a guard band receiver.

AC2x - was known as the Experimental Common Channel Signalling System, though today we would describe it as a Separate Channel system.  It was designed in 1945 with the objective of avoiding the problems inherent in VF systems caused by the speech immunity problem and the presence in those days of echo suppressors on trunk circuits.  Twenty circuits shared one channel for signalling, using frequencies at 100Hz spacing between 350-2350Hz.  A field trial took place at Plymouth, Bristol, Glasgow and Oban, but the system was abandoned in 1951.

AC3 - was a 500/20Hz signalling system used in the 1940s between automatic trunk exchanges and manual boards at manual Group Centres.  A pulse of 16Hz magneto was sent from the selector on seizure and converted to 500/20Hz in the repeater station.

AC4 - was the number assigned for the 2VF CCITT No. 4 system (see below).

AC5 - was a system designed for Scottish DC junctions which suffered from high induced currents from parallel high voltage power lines.  Transformers were needed to isolate the longitudinal currents.  Though it seems odd, 50Hz AC was used as the signalling frequency, as this was cheap to provide.

AC6 - was another Separate Channel system, similar to AC2x, but employing the same MCVF signalling as used on telegraph circuits. 18 or 24 circuits could use one MCVF channel.  It was deployed in the early 1950's on some routes in Scotland and to the Channel Isles.

AC7 - was the number assigned to the CCITT No. 3 signalling system (see below).

AC8 - was an outband signalling system, using 3825Hz and E&M connections via the phantoms to the relay sets.  It was introduced in the 1950s on junction routes were 'short-haul' carrier systems were being used.  These routes included those to some outlying Scottish islands were radio was used. 

AC9 - was the major trunk VF signalling system used between Strowger GSCs. It is a 1 VF system using a single signalling frequency of 2280Hz.  It was introduced in 1959 and superseded AC1.  The first valve based VF receiver was considered very advanced at the time.  Later a transistorised version was produced.  By the early 1970's, a miniaturised version was developed (sometimes known as AC9M) which gave a six-fold improvement in packing density.  Minor differences existed between these three versions. I believe the first valve system did not support check tone and a thermal relay was used to provide the Reseize/Clearforward cycle - the signalling sequence used when a Release Guard is not received in response to a Clearforward - indicating a failed circuit.  The first transistorised system had an improved Reseize/Clearforward cycle, where the Clearforward followed quickly after the Reseize.  The miniaturised version did not support bothway working, so the Reseize signal could optionally be dispensed with and the failure sequence became a Repeat Clearforward cycle. 

AC10 - was the designation for the line signalling part of CCITT No. 5.

AC11 - 1VF, 4 wire, line signalling system used alongside MF2 in the transit network.  It was essentially the same as AC9, but without the digit pulsing.  It was only ever designed for unidirectional circuits.  There was a variant of AC11, known as AC11/FT, used together with MF3 on routes between international manual boards at Leicester and Glasgow and international exchanges in London.  It featured two extra signals, a proceed to send signal (needed as MF3, unlike MF2, had no backward signals) and a Forward Transfer signal used for recalling a forward operator.

AC12 - was an outband 3825Hz signalling system used in the transit network, essentially on routes where AC8 was being used and suitable outband transmission equipment was available in the repeater station.  Oban was one such place, as I recall.  Essentially AC8 without the digit pulsing.

AC13 - was a private network system, using 2280Hz and capable of supporting bothway working.  It could be considered a private version of AC9.  This system was `tone off' on idle and this could cause problems if one of the fours wires went disconnect the circuit could still seized but not connect to the distant end.

AC14 - is a 1VF system (2280 Hz) that is used for 'Out of Area' exchange lines or external extensions off PBXs. Although SSAC14 is generally 2 wire working it may be modified to provide 4 wire working. SSAC14 is a unidirectional device which will only permit dialling in one direction.

AC15 - a 2280Hz private network system, which replaced AC13 in the 1970's. This system is `tone on' on idle. It has 4 versions, known as AC15A, B, C and D.

AC15A is an in-band continuous tone line signalling system used for the transmission of digit pulses and supervisory signals on bothway inter-PBX circuits where a suitable DC path is not available. i.e. Over long distances.

AC15B is the preferred method of inter-PBX signalling on circuits over any 4 wire presented transmission path where SSMF5 inter-register signalling is employed

AC15C is used to provide external extension and out of area exchange line signalling with recall facilities.

AC15D is an in-band signalling system used on International inter-PBX circuits or appropriate inland inter- PBX circuits. It uses a single frequency of 2280 Hz in each direction of a 4 wire transmission path.

AC16 - was, like DC11, assigned for use with MF6, but never used. It would have been an inband signalling system.

AC17 - to complete the list, was similarly assigned as an outband system to be used with MF6.

Multifrequency Systems
MF1 - was the designation for the inter-register part of CCITT No. 5.

MF2 - was the MF system used on the transit network and on some other trunk routes.  It used the same forward and backward frequencies as the CCITT R2 system.  As already mentioned, it used a complex system of compelled prefixes and pulsed 80ms suffix signals.  Once signalling was established between any two MF2 registers, signals were always pulsed, but nevertheless still preceded by a pulsed prefix signal.  The signalling sequence to transit exchanges used the 'en bloc' technique and end-to-end working, whereas signalling to GSCs used the 'pulsed acknowledged' method, so all in all, MF2 was a complex system.  However, it made it relatively immune to interference by the noisy environment at Strowger GSCs.  I can recall, while testing some new MF2 equipment, a call which succeeded despite having a crossed line with another circuit on which 10pps pulsing was taking place. 

MF2 was the first UK domestic system to use backward 'state of line' signals, such as Call Line Busy, Called Line Free, Number Unobtainable etc, which allowed unsuccessful calls to drop back to the originating exchange where the relevant tone would be returned to the caller.  Transit exchanges did use the Congestion and NU signals, but as the rest of the network was largely Strowger with 10pps signalling, the other signals only ever appeared when interworking with R2 and CCITT No6 at international exchanges.  A further restriction to the use of backward signalling was caused by the fact that at GSCs, only the more modern Type 5 Register Translators could actually attach the Equipment Engaged Tone.  So at Type 2 and 3 GSC RTs, the call would be switched through to the transit exchange from where a tone would be returned.  Even at Type 5 centres, the IDD registers could not cope with the EET, so efficient interworking with ISCs was not possible.  The only routes where MF2 used its backward signalling effectively was on routes between SSCs and ISCs, as the latter could attach NU, EET and also customer Engaged Tone.  As mentioned above, ISCs might send such signals if received from abroad via
R2 or CCITT No. 6.  A curious feature of MF2 was that in the original design, only 10 signals were used in the backward direction, so only two out of five frequencies were employed. 

MF3 - simplified system, derived from MF2, used on routes between international manual boards and ISCs.  It only employed the forward six frequencies and address digits were sent 'en bloc', i.e. one stream of fixed length pulsed digits, interspersed by the Code 14 prefix signal and completed by a Code 15 end-of-pulsing signal. 

MF4 - is the familiar Dual Tone Multifrequency (DTMF) system used for customer signalling, often known as Touch Tone.  Unlike the '2 out of 4' MF systems, MF4 uses '1 out of 4 - twice' method, a system intended to make it easy for keypads to generate the tones on the basis of one tone per row and one per column.  Deployment of MF4 was very slow in Britain, as MF4 receivers could not cope with the purring dial tone.  The Engineering Division stated that a new dial tone was needed, but the Traffic Division refused to change.  Of course, MF4 would not have been much use in Strowger exchanges, but the growing number of Crossbar and TXE4s could have made good use of it.  Eventually, the new dial tone was introduced, but too late to ever be deployed at anything other than digital exchanges.

MF5 - is an MF system for private networks, based on the European R2 (MFC) system, usually used with DC5, DC10 or AC15 line signalling.

MF6 - was the intended MF system to provide for MF signalling throughout the rapidly modernising network in the 1970's.  MF2 was seen as overly complicated for the non-Strowger network.  A system based on R2 MFC was the obvious way forward.  However, it took so long to agree on the design of the system and it was made so complex, with many sets of so-called 'library signals' to allow for all sorts of supplementary services, that it was abandoned without ever being implemented. Instead, it became clear that CCITT No. 7 common channel signalling would be the right way forward in the digital era.

International Signalling Systems
CCITT No. 1 - was the 500/20Hz manual ring-down system used on manual routes.  During my time at the CCITT Study Group XI, I arranged for 2280Hz and 2600Hz to be recognised as alternatives on more modern international private circuits.

CCITT No. 2 - was a proposed system for 2 wire international cross-border circuits.  It would have used 600Hz and 750Hz, with 10pps pulsing, very similar to AC1.  It was designed in the late 1930's, but by the time that international communications started again after the war, it became clear that faster four-wire systems would be preferred and better voice immunity would be needed.

CCITT No. 3 - was a 2280Hz VF system using a binary 'start-stop' system, like a telegraph system.  The line signalling also used 2280Hz.  It was never widely deployed and in the UK was only used at Faraday ISC mainly on the incoming route from France.  Elsewhere CCITT No. 4 was used.

CCITT No. 4 - was a 2VF system, using 2040Hz and 2400Hz, known as X and Y.  Voice immunity was limited to using a compound 2040+2400Hz prefix before any line signal could be recognised, though PO receivers did have a guard band receiver too.  The two frequencies allowed line signals of the form PX, PY, PXX and PYY.  Inter-register signals used the X and Y signals as binary 0 and 1.  Because both 0 and 1 could be easily recognised, there was no need for a start or stop bit, as used in CCITT No. 3.  CCITT No. 4 was widely used in Europe and North Africa, until replaced by R2 progressively from the mid 1970's.

CCITT No. 5 - is the inter-continental system, first designed for the TAT1 cable in the mid-1950s.  It uses a 2VF line signalling system, employing 2400 and 2600Hz and a '2 out of 6' MF system in the forward direction only.  The line system (AC10 in the UK) had an interim version first used for the initial launch of TAT1. It used compelled signalling for the Seize/Proceed to Send and the Clearforward/Release Guard sequences, but pulsed signals only for the Answer, Clearback and Forward Transfer signals.  The final system employed compelled sequences for all signals except the Forward Transfer.  The use of compelled signalling was important as the early TAT cables used TASI (Time Assigned Speech Interpolation) which created 60 circuits from the 36 channels.  TASI clipped the beginning of speech syllables, so pulsed VF signals could also get clipped. 

CCITT No. 5 also used 'en bloc' MF inter-register signalling (both pulses and gaps set at 55ms) which ensured that the TASI channel would stay assigned during the whole digit sending sequence.  The MF system used the same frequencies as the US Bell MF system, though this had used a more conservative 80ms for pulses and gaps.  Using 'en bloc' methods meant that long post dialling delays could result, as no forward signalling could start until all dialled digits had been received from the customer and a 5s timeout was often needed to determine when this had been achieved. CCITT No. 5 was later used on satellite circuits, over which the compelled line signalling was not ideal, as the compelled cycle took over 1s to complete and the answer signal would often interfere with the initial conversation. 

CCITT No5bis - was an upgraded form of CCITT No. 5, designed to add a backward signalling capability and therefore bring the system into line with more modern systems like No. 6 and R2.  Although Denmark ordered some CCITT No. 5bis equipment, nobody else did and the system was never used in service.

CCITT No. 6 - was the first international common channel signalling system. It used a 2400 baud modem for the common channel, with a 28 bit message length.  It was subject of an international trial in 1972, but was not finally implemented until 1978 (1979 in the UK).  It has now been superseded by the digital CCITT No. 7 system. It is interesting to note that a digital version of CCITT No. 6 was defined, using a 4kbit/s data channel, but by the time CCITT No. 6 came to be used on digital circuits, it was easier to leave the signalling as analogue 2400 baud over a 64kbit/s channel.

CCITT No. 7 - is the current common channel signalling system, widely used in both national and international networks around the world, but I think it's too modern to be described in a heritage journal.  Come back in ten years?

Regional Systems
As mentioned above, CCITT No. 4 was mainly used in Europe.  From the late 1950's, it became clear that an MF system would be better and allow for a wider range of backward as well as forward signals.  Ericsson of Sweden did some trials in Denmark and Netherlands with the MFC (MF Compelled) concept and later the European CEPT adopted MFC as a European standard.  CEPT lobbied the CCITT to recognise it as a full international standard to replace CCITT No4, but politics intervened.  The Americans pointed out that they already had a good regional MF system (Bell MF), so in the end, instead of assigning it a CCITT number, it became known as R2 (Regional No. 2) with Bell MF/SF becoming R1.  R1 used the Bell SF 2600Hz tone on idle line signalling.

Many smaller European countries enthusiastically adopted MFC in their national networks and R2 was designed to allow end to end working not only in the international network, but into the distant country's national network too.  Germany was too wedded to its electromechanical rotary system to adopt MFC.  The UK decided that MFC was too unreliable to work over Strowger paths and designed MF2 instead.  Similarly, the French deployed the SOCOTEL MF system, though both this and MF2 used the same forward and backward signals as R2/MFC. Ericsson was always the great innovator with R2.  In their ARF crossbar system, they used MFC to signal from the register to the individual switch markers and this too could work end to end, so these markers all appeared to act like small transit registers.

No fewer than three line signalling systems were specified for R2.

For analogue use, an outband tone-on-idle 3825Hz system was defined.  This is the reverse of AC8, which used tone-off-idle.  Consequently, if the transmission system failed, a large number of circuits might appear to be being seized.  A Group Pilot frequency was therefore specified within each 12-channel group and if it ceased then a signal was sent to the 12 affected circuits to stop any line seizure being recognised.  For digital use, a channel associated system for 30 channel PCM was defined, with 2 bits in each direction used.  The second bit performed a similar function to the group pilot in the analogue system, for when a PCM system fails, all signalling bits are set to 1, so the 'b bit' going to 1 indicated a line fault.
A third system was defined for the few analogue submarine cable systems still using 3kHz channels, on which the outband system could not be used.  In these circumstances, it was suggested that the old line signalling from CCITT No. 4 should be used, but no such system was ever deployed.

When Eutelsat launched a European satellite, it was suggested that R2 should be used and a number of trials took place. The compelled cycle meant that signalling was slowed from its normal 6 digits/sec to barely 1 - about the same speed as loop disconnect.  Nevertheless, it was used as CCITT No. 5 was not always available for use (though the PO did use No. 5 on the satellite route to Gibraltar).  CCITT No. 5 itself could have a long post dialling delay, so sometimes R2 over satellite was still satisfactory, if sandwiched between national 10pps systems.  Some timing modifications to the line signalling system were required and some restrictions were also needed on the inter-register signalling too, notably ensuring that an incoming register was always used after the satellite link.  No end-to-end signalling into the distant network was allowed, as the slow compelled signalling played havoc with national registers, especially those short holding time ARF markers!

Aside from R2, CEPT defined two more European signalling systems. L1 was a 2280Hz manual ring down system for private circuits.  As mentioned above, I was involved in trying to persuade the CCITT to adopt this as a world-wide standard, with only partial success.  L2 was an automatic system for private circuits.  Again, it used 2280Hz and was known as AC15D in the UK.

One more international signalling system should be mentioned to complete the historical record.  In the late 1960's, before the PO's second international exchange at Wood Street was opened, Faraday exchange ran short of CCITT No. 5 circuits, so a set of US Bell SF 10pps signalling equipment's were imported to the UK and modified to work with AC9 relay sets.  The circuits were only ever used for operator-controlled calls, but the capacity helped out at a difficult time.

(Peter Walker was Technical Director at OFTEL, where he influenced the development of signalling in the UK networks).  Additional information on DC, Magneto, Auto, AC15, DC5 & DC10 added by R. Freshwater.

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Last revised: October 20, 2021