
		*******************************************
		* PHED-ONE's ULTIMATE GUIDE TO BLUEBOXING *
		*	   IN THE (LATE) 90s		  *
		*	      [FULL STOP]		  *
                *******************************************

				 PART-3
		  CCITT 5/ITU-T 5 in Depth, and Signalling

This part of the "Ultimate Guide" will discuss CCITT5/ITU-T 5 (they're the same),
the way the system works in-depth and variations on it in todays telephone network.

By now, you should know the basic principals of blueboxing and how to seize a 
trunk and route a call. This section will discuss CCITT5 and advanced signalling 
and routing issues. This whole guide was written by a CCITT5 junkie (me) and
so you'll have to forgive me if I get carried away. But, this section is just
as important as 1 and 2 because if you want to crack a system you have to know
the system better than the system itself. You'll find a lot of the information
here in various manuals and it is very useful when social engineering international
exchange personnel because you'll know CCITT 5 better than they do after this.

System 5 was specified in 1964 by the CCITT. An analogue system, it has seperate line and
interregister signalling. The line signalling is 2 v.f (Two Voice Frequency) and uses two 
main tones, 2600hz and 2400hz, inband, compound, and simple frequency continious signalling 
and link-by-link. The interregister signalling is 2/6m.f, link-by-link in the forward 
direction only. No digital version of the system existed, but recently a digitally encoded
system was devised, using E1 circuits and is used extensively.

CCITT5 is a versitile system, many operations may apply, unidirectional, bothway, terminal,
transit, automatic and semiautomatic. The system is suitable for 3khz and 4khz spaced 
submarine, land cable, microwave radio and satellite circuits, with or without t.a.si 
(when you're not talking, it uses the trunk for another call to use, when you start 
talking again, it re-assigns you a trunk. This takes about 0.5 seconds and sometimes
you get 0.5 seconds "clipped" off the voice answering when making international calls).

To begin with, the system was intended as a joint development between the GPO and Bell Labs 
(AT&T USA) for dialling over t.a.s.i equipped transoceanic Atlantic cables, the first 
application of intercontinental dialling and the t.a.s.i technology. The system was then 
specified by the  CCITT and since then was applied to other intercontinental circuits, at one 
point being the main system for international calling. In the early nineties, one estimate 
stated that 80% of the  international infrustructure used system 5, in my opinion that 
estimate was a little generous. I'd say that about 65% of international trunks at the most 
are CCITT5 at present (1999) from my experience. 

[Afterall, if you consider 70% of African countries, 50% of Latin American Countries and 
the Caribbean, 60% of Asian Countries and 25% of European Countries (Russia Included) use
CCITT 5 for international dialling you get a very high figure for the amount of C5 
infrustructure used today. But if you check out the 0800 89 and 96 ranges you'll
find a lot of C7 stuff, so it would be fair to say that most C5 countries are less
developed and make less use of international communications. This isn't always true,
though, as Malta and Iceland use C5 for some of their international connections. 
Don't get me wrong, I'm not "dissing" C5 as a system for poor countries, I think
that C5 is a great system. But, C7 is more efficient and is a much better system when 
employed on fibre optic cabling, as it has greater compression and can accomodate more 
calls.]

As this was the first system used in intercontinental dialling, the aim was to create a 
simple and robust system for the new service. The TAT-1 cable from London to New York was 
the first application of CCITT 5. As transoceanic cables were and still are expensive, 
t.a.s.i dictated the features of the system. As a result, the signals and facilites of 
CCITT5 were kept to a minimum consistent with the intercontinental dialling service.
In fact, you could call the US via a CCITT5 connection up until 1995, until the TAT-6 and
TAT-7 were retired. The longevity of CCITT5 is much longer than most signalling systems
to date, this is testament to the flexible, reliable, and robust system that the CCITT
specified over 30 years ago. 

All line signals in system 5 are continuous-compelled, trunk-channel association always being
assured in the actual time required for this function. The acknowledgement signalling in 
continuous-compelled increases the t.a.s.i signalling time and activity, but some signals 
normally give rise to return signals (seizure/proceed-to-send, release guard/clear forward 
etc) and no penalty results from the adoption of continuous-compelled for these signals. 
Other  signals (for instance, answer) do not normally require a return signal and the 
adoption of continuous-compelled, instead of pulse, could be a penalty when t.a.s.i os 
heavily loaded, but an advantage when light. So, on balance, continuous compelled was 
preferred for these signals.

The system 5 interregister m.f (multi-frequency) signalling is pulse forward only, in order 
to minimise the t.a.s.i signalling time and activity. A small amount of backward signalling 
is used. The arrangement requires that t.a.s.i channel be prior-associated for address 
signalling in order to ensure that the first address signal is not clipped. Because sytem 5 
was the first  intercontinental signalling system in use, they wanted the cicuits to be used 
in the most efficent way possible and in order to ensure that circuits were not being used on 
incomplete dialled calls as much validity checking of the address information as possible 
was used at the outgoing international exchange before seizing an intercontinental circuit. 
The complete en bloc of the address information to enable a measure of 
validity-checking to be performed, and thus en bloc transmission. En-bloc means that all the 
signalling information for the call is send in one burst: Kp2-44-0-181-000-1212-ST. 
In this mode, the t.a.s.i speech detector stays on the trunk during the gaps between 
successive address signals so that clips do not occur and mess up the signals. 
If an alternative mode was used, where clipping could occur after recieving line signals, 
a t.a.s.i locking tone would be required to avoid clipping of pulse-address signals. 
In actual fact, a t.a.s.i lock tone is used in some cases, for the  transfer of data etc 
- 1887hz and disables t.a.s.i clipping.

CCITT system 5 line signals
~~~~~~~~~~~~~~~~~~~~~~~~~~~
---------------------------------------------------------------------------------------------
SIGNAL		DIRECTION		FREQ	      SEND DURATION	     RECOGNITION TIME
										     ms

SEIZURE		 --->			f1	       Continuous		 40 +/- 10
PROCEED-TO-SEND  <---			f2	       Continuous		 40 +/- 10
BUSY FLASH	 <---			f2	       Continuous	         125+/- 25
ACKNOWLEDGEMENT	 --->			f1	       Continuous		 125+/- 25
ANSWER		 <---			f1	       Continuous		 125+/- 25
ACKNOWLEDGEMENT	 --->			f1	       Continuous		 125+/- 25
CLEAR BACK	 <---			f2	       Continuous		 125+/- 25
ACKNOWLEDGEMENT  --->			f1	       Continuous		 125+/- 25
FORWARD TRANSFER --->			f2	      850ms+/-200ms		 125+/- 25
							 (pulse)
CLEAR FORWARD	 --->			f1+f2	       Continuous		 125+/- 25
				      (compound)
RELEASE GUARD    <---			f1+f2	       Continuous		 125+/- 25
				      (compound)
---------------------------------------------------------------------------------------------
f1 = 2400 Hz	f2 = 2600 Hz

---> forward signal
<--- backward signal, continuous-compelled mode

Notes on the table
~~~~~~~~~~~~~~~~~~
(1) By taking advantage of the fixed order of ocurrence of specific signals, signals of the 
same frequency are used to characterise different functions, e.g in the backward direction 
f2 is used to indicate proceed-to-send, busy flash and clear back without conflict. The 
signalling equipment must operate in a sequential manner, retaining memory of the precedung 
signalling states and the direction of signalling to differentiate between signals of the 
same frequency. All signals except the forward transfer are acknowledged in continuous
-compelled manner. The order of transmission of backward signals is subject to the 
following:

(i) busy flash; only after a proceed-to-send signal after an answer signal.
(ii) answer: never after a busy flash signal
(iii) clear back: only after an answer
The receipt of the answer signal (f1) permits discrimination between the busy flash and 
clear-back signals (both f2).


(2) Except for the recognition time (40ms) of the seizure/proceed-to-send signal sequence, 
which can be short as this sequence is not subject to signal imitation by speech and a 
rapid seizure is desired to minimise the postdialling delay and the probability of double 
seizure on bothway working, all recognition times are the same (125 ms). This simplifies 
terminal design.

(3) The use of compound for the clear forward/release guard sequence improves the immunity 
to false release by signal imitation (signal imitation, hmmmmm!), this being particularly
necessary as the recognition time (125 ms) of these important signals is relatively short 
for uniformity. The compound clear forward, which must always be acknowledged by a release
-guard signal under all conditions, is completely overriding.

(4) The use of different frequencies of the seizure (f1) and the proceed-to-send (f2) 
facilitates double seizure detection on bothway working. The seizure signal continues until
acknowledged by the proceed-to-send, which is returned when an incoming register is 
associated and continues until acknowledged by the cessation of the seizure signal. As there 
is no backward interregister signalling, the proceed-to-send signal must be a line signal, 
which is convenient in system 5 acknowledge the seizure signal.

(5) There is a frequency discrimination between all the line signals except busy flash and 
clear back, which are both f2 acknowledged by f1. As the busy flash will be recieved without 
an answer signal, the answer signal is used to bring about a change of condition in the 
outgoing equipment to permit the discrimination.

(6) The forward transfer signal f2 (a rarely used signal and used in semiautomatic operation
only) is an unacknowledged pulse. The nonacknowledgment avoids possible confusion with the 
f1 answer and the f2 busy flash and clear back.

(7) The proceed-to-send ceases the seizure signal. The outgoing register pulses out the 
address information en bloc and a silent period of 80 +/- 20ms is arranged between cessation 
of the seizure signal and the beginning of the register pulse out. The tunk-channel 
association is maintained by the t.a.s.i speech-detector hangover during this interval. 
This, together with the speech-detector hangover maintaining trunk-channel association 
during the gaps between successive interregister address signals, avoids the requirement 
for locking trunk-channel association (lock tone etc) during interregister signalling, 
although, as mentioned earlier a t.a.s.i lock tone is available, useful for data 
transmission - 1887hz.

(8) In transit (international) operation, the line equipment at the transit exchange is 
informed by the interregister KP signal that the condition is transit (KP2). This 
descriminates between terminal (local) calls and transit calls.

(9) Should the called party flash his switchhook faster than the equipment can transmit a 
succession if clear-back (on-hook - hangup) and answer (off-hook) signals, the correct 
indication of the final position of the switchhook must always be given by the appropriate 
signal.

(10) If, after 1-2min of receipt of the clear-back signal there is no clear-forward signal, 
the international connection is released by a system-manufactured clear-forward signal, 
and the measurement of call charge ceased.

(11) The clear-forward signal continues until acknowledged by the release guard, which, 
depending on the administration's choice, may be sent:

(a) on recognition of the clear-forward signal, and continues until acknowledged by the
cessation of the clear-forward signal, or until the relevant incoming equipment at the 
international exchange has released, whichever occurs later,
(b) in response to the clear-forward signal, to indicate that this signal has brought 
about the release of the relevant incoming equipment at the international exchange, 
the release-guard signal continuing until cessation of the clear-forward is recognised/

On a bothway circuit, the outgoing access at the incoming end is maintained busy for 
200-300ms after the end of transmission of the release-guard signal to cover the 
responses to the cessation of the release-guard at the outgoing end.

(12) Busy flash is transmitted for any of the following reasons:
(This bit could be useful if you get a busy flash during boxing attempts:)

(i) congestion at a transit or at an incoming international (terminal) exchange
(ii) error detected in the receipt of register signals
(iii) busy flash, if recieved, from the interworking international signalling 
system(s) or from the incoming national network.
(iv) time out of an incoming international register

Receipt of the busy-flash signal at the outgoing international exchange causes appropriate
indication (for instance, tone) to be sent to the caller and the release of the 
international connection using a system manufactured clear-forward signal.

(13) In inband v.f signalling such as C5, a quick verbal answer may be clipped, partially 
or completely, by the line splitting on transmission of the electrical-answer signal. 
Failing to repeat the verbal answer could end up with each party expecting the other to 
respond and hanging each other up (hehehehe). Multilink connections increase the danger. 
So, it is preferred that the transmission of the electrical-answer signal should be as fast 
as possible to ensure that line splits are terminated prior to the person answering the call. 
As a contribution to this, the answer signal of system 5 is trasmitted in the overlap-
compelled, instead of normal-compelled mode at transit exchanges. In this technique, the 
process of transmitting the answer signal from the transit exchnage to a preceding exchange 
is initiated as soon as the transit signal receiver response to the incoming answer signal 
has caused the receive line split (35ms maximum), the  onward transmission not awaiting 
the full signal recognition time of the answer signal as would be the case in normal
-compelled. The normal signal recognition is complete. The answer signal on each link is 
ceased by its acknowledgment on that link. If the incoming answer signal duration is less 
than the signal recognition time, the transmission of the ongoing answer signal recognition 
time, the transmission of the ongoing answer signal already insituted from that transit 
exchange is ceased. After signal recognition of the incoming answer, there is no control
at the transit exchange of the ongoing answer signal by the incoming answer signal.

The busy flash and clear-back signals are returned in the normal compelled manner mode at 
transit points, onward transmission not commencing until the incoming signal recognition is 
complete.

(14) The acknowledgments of the busy flash, answer and clear-back signals are sent after 
signal recognition (125 +/- 25ms) of the relevant primary signal. The primary signal is not 
ceased  until the recognition time of the acknowledgment is complete (125 +/- 25ms). 
Signal cessation recognition time, primary or acknowledgment, is at least 40ms.


Bothway Operation
~~~~~~~~~~~~~~~~~
The extreme of t.a.s.i trunk-channel association time (about 500ms) combined with the
long reaction and propagation times of the equipment, there can be a long unguarded
interval in bothway operation. Double seizure is when the same frequency  (f1 seizure 
signal) is recieved as it is being transmitted. On detection of this double seizure,
the transmitted signal is ceased 850 +/- 200ms after commencement, this makes sure that
both ends of the t.a.s.i equipped bothway circuit will detect the double seizure.

The signalling equipment is released on cessation of both the outgoing and incoming
seizure signals. A clear-forward signal is not sent. Either of these conditions may
apply on detection of double seizure, depending on the telco's choice:

(a) an automatic repeat attempt to set up the call,
(b) a reorder tone given to the callers (reorder - similar to an engaged tone)

"(a)" is the preferred option. But is flexible as to allow the repeat attempt to be
made on a different circuit than that used for the first attempt. If the first circuit
is seized on the second search over the circuits, a minimum time of 100ms elapses between
the termination of the first-attempt outgoing seizure signal (or recognition of the cessation
of the incoming signal, whichever occurs later) and the commencement of the second-attempt
seizure signal.

To minimise the probability of double seizure (and thats a bad thing, believe me), the 
cicuit selection at the two ends should be such that, as far as possible, double seizure
can occur only when a single circuit remains free (woohoo!) (e.g by selection of circuits 
in opposite order at the two ends).

Transmit line split
~~~~~~~~~~~~~~~~~~~
The exchange side of the circuit is disconnected 30 - 50ms before a v.f signal is 
transmitted. The split persists with signal and is terminated 30 - 50ms following the end of 
sending of the v.f signal.

Recieve line split
~~~~~~~~~~~~~~~~~~
The circuit is split at the international exchange when either a single or compound frequency
is recieved so that the spillover does not exceed 35ms. The split persists with signal and is
terminated within 25ms following the end of the signal. The splitting device may be a physical
line disconnection, high-impedence device, insertion of signal frequency bandstop filter, etc.
Leak current in the split condition should be at least 40dB below the recieved signal level.


Relevant data: Line signalling
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

TRANSMIT:
	
		f1 2400 +/- 6Hz		f2 2600 +/- 6Hz
		Transmitted level - 9dBm0 +/1 1dB per frequency
		For compound the difference in transmitted level
		between f1 and f2 not to exceed 1dB.
		The difference in the time between f1 and f2 of a
		compound signal on sending and ceasing not to exceed 5ms

RECIEVER:
 Operate:	f1 2400 +/- 15Hz	f2 2600 +/- 15Hz
		The absolute power level 'N' of each recieved signal
		to be within the limits (-16 + n < N < (- 2 + n)dBm,
		where 'n' is the relative power level at the reciever
		input. The limits give a margin of +/- 7dB on the nominal
		absolute level of recieved signal at the reciever input.

		The absolute level of the two frequencies of a compound
		signal not different by more than 5dB

 Nonoperate:	Reciever not to operate outside:
		f1 2400 +100/-150Hz	f2 2600 +150/-100Hz
		Reciever not to operate on signal:
		2400 +/-15Hz or 2600 +/-15Hz whose absolute power level
		at the point of reciever input is (-17 -9 + n)dBm. This
		limit is 17dB below nominal absolute level of signal at 
		reciever input.

SIGNALS:	a) Should the transmission of any seizure, busy flash, answer
		answer, clear back or clear-forward signal persist beyond a 
		maximum of 10 - 20s, the signal is terminated and the condition
		alarmed.
		b) If the transmission of any proceed-to-send, release guard, or
		any other acknowledgment signal persist beyond a maximum of 4-9s
		the signal is terminated and the condition alarmed.
		c) After signal recognition, interruptions of up to 15ms in a
		signal or acknowledgement are to be ignored. Interruptions of
		more than 40ms are recognised as the end of the appropriate signal
		(primary or acknowledgment) in the compelled sequence.
		d) Once the sending of a signal, pulse, or continuous, has begun,
		it should be completed except when the clear-forward signal 
		overrides.
		e) An interval of at least 100ms should seperate two successive 
		signals in the same direction.

[Side note: see point (e), on some trunks, the clear-forward and seize are sent
successively and the two then acknowledged....

Clear forward
Seize
PLEEP...PLEEP

In the case below, according to this manual, 100ms should be the delay. This has been true
in some cases. Generally, the two are acknowledged seperately...

Clear foward
PLEEP
Seize
PLEEP

But this is something to consider anyway, despite the fact that many C5 systems don't
adhere to the standard spec.]


TABLE	-	CCITT system 5 interregister signals	
-----------------------------------------------------------------
SIGNAL				PULSE FREQs		DURATION		
				(compound)		   ms
				    Hz

KP1 (terminal traffic)		1100+1700		100
KP2 (transit traffic)		1300+1700		100
Digit	1			700+900			 55
	2			700+1100		 55
	3			900+1100		 55
	4			700+1300		 55
	5			900+1300		 55
	6			1100+1300		 55
	7			700+1500		 55
	8			900+1500		 55
	9			1100+1500		 55
	0			1300+1500		 55		
Code 11 Operator		700+1700		100
Code 12 Operator		900+1700		100
ST (end of pulsing)		1500+1700		100
------------------------------------------------------------------

Interregister Signals:
Automatic access to the international circuits is used for outgoing traffic, the address
signals from the operator or subscriber being stored is an outgoing system 5 register
before the international circuit is seized. As soon as ST (end-of-pulsing) is available
to the outgoing register, a free international circuit on the appropriate route is 
selected and a seizure signal sent. The seizure signal is terminated on receipt of
a proceed-to-send, and KP (start of pulsing), address signals, and ST, transmitted
by the register.

Both forward and backward interregister signalling is normally preferred in networks
This in turn, implies end-to-end signalling for reasons of reduced register holding time,
reduced register provision, and leading register control. Backward interregister 
signalling, however, would increase t.a.s.i signalling time and activity and would
require some arrangement to assure t.a.s.i trunk-channel association during the
interregister signalling. It was assessed that t.a.s.i efficiency was the first 
priority for system 5 and backward interregister signalling would just reduce this
efficiency and therefore hasn't been adopted. This means that compelled signalling and
end-to-end signalling are not used in system 5, because there is no backward interregister
signal to release the registers. The line answer signal can't be used to release 
registers and set-up speech conditions because an answer signal is not always given, and
further, the transmission path is required prior to answer to pass the ring tone.

For those reasons, the system 5 interregister signalling in link-by-link, pulse, forward
signalling only, the registers releasing in sequence after transmission of ST. The 
signalling is 2/6 m.f in the range 700-1700Hz and 200Hz spaced. The address information is
always sent "en-bloc" from the originating system 5 register and overlap from the transit
and incoming system 5 registers.

The KP signal may be used to prepare the distant system 5 register on the link for the 
receipt of the subsequent address signals. It may also be used to discriminate between
terminal (national) and transit (international) traffic:

Terminal KP (KP1): Used to create conditions at the next exchange so that equipment used
exclusively for switching the call to the national network of the incoming country is 
brought into circuit.

Transit KP (KP2): Used to bring into circuit at the next exchange, equipment required to
switch the call to another international exchange.

In system 5, the ST signal is transmitted from the register at the end of address signalling
in both automatic and semiautomatic operation. Both outgoing and transit system 5 registers
must determine the routing, and send the appropriate KP signal, by analysis of the early 
digits of the address information. The interregister signalling information in system 5 in
the international automatic service comeprises:

	KP + country code (I digits) + characteristic digit (Z digit) + 
	national significant number (N digits) + ST

In local routing, the country code may be left out.

The characteristic digit in the discriminating digit (D) in automatic operation, or the 
language digit (I) in semiautomatic operation. The discriminating digit was mentioned
earlier with the following default assignments:

	0	-	CABLE
	1	-	SATELLITE
	2	-	OPERATOR
	3	-	MILITARY
	9	-	MICROWAVE

[Above differs from previous mentions with Military being given '3'] 

This may very from country to country, and the pattern of routing above can also vary. In
some countries, 2 is for operator routing. Other countries such as Nicaragua use a different
routing pattern:

GLOBAL
	KP2-01-country code-national significant number-ST

All calls are routed using the Westar satellite.

Analysis of the country code of the destination country is generally sufficient to determine
the forward routing, the code consisting in one, two or three digits. Exceptionally, early
digits of the national number may need to be included in the analysis to permit forward routing
to any one of a number of locations in the other country. An example of this would be the North
American numbering plan, where calls from Europe to the Caribbean can go via mainland US and 
Florida or direct via satellites or cables. Calls to mainland US from Europe generally go 
straight on one of the transatlantic cables, whereas calls to the Caribbean can go via different
and more direct routes, meaning more analysis is needed when routing to offshore areas of the
North American numbering plan.

The maximum number of digits to be analysed in a system 5 outgoing transit register would be six,
i.e,

I1 I2 I3 Z N1 N2, which assumes a maximum three digit country code.

An exception is the routing of calls the the North American Numbering Plan, where the NPA is 
taken into account.

The country code will be the first digit(s) follwing KP2 recieved by a transit register. In 
system 5, the country code is not sent to the incoming international register, and here the first
digit recieved following the KP1 will be the Z digit.

The "Code11" and "Code 12" operators (given those assignments as they are the 11th and 12th 
signals in the dialset) are special international operators accessed by interregister signals.
The Code 11 operator is known as the "Inward Operator", nearly every country has one of these, 
and her job is to assist foreign operators in making calls to the country and provide
information. Code 12 is known as the "Delay Operator" or the "International Operator" and her job
is to assist operators make international calls via her country, and to provide similar 
information to the Code11 operator. Code 12 is also used for directory assistance by many 
countries.


System 5 Register Arrangements Concerning ST	
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

 (a) Semiautomatic operation: The ST condition is determined by the reciept of the "sending 
finished" signal from the operator.

 (b) Automatic operation:
(1) When the ST signal is provided by the originating national network, this signal is 
transmitted to the outgoing system 5 register and no further arrangements are necessary.
(2) The outgoing system 5 register is required to determine the ST condition when this is not
recieved from the originating national network. This determination may be on time delay or on
digit count. When on time delay, ST is determined when the cessation of the address information
input to the register exceeds a period of 4s (5 +/- 1) in either of the following conditions as
preferred by the administration:
(i)  after the minimum number of digits in the world numbering plan, or
(ii) after the minimum number of digits in the destination country numbering plan.

In (i) and (ii), prolonged cessation of the address information input before the minimum number
of digits results in time-out release of the regitser without production of the ST condition/

An immediate ST condition may be produced by digit count to avoid the 4s delay when the 
destination numbering plan has a fixed number of digits, or when the maximum number of digits 
in the numbering plan of the destination country has been recieved.

Under all circumstances, the outgoing international circuit is not seized until the ST condition
is available to the register. Thus, when operative, the 4s delay to determine ST, while 
increasing the post-dialling delay, does not react on t.a.s.i signalling time and activity.


RELEVANT DATA: Interregister signalling
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 

Transmit:
	Frequencies 700, 900, 1100, 1300, 1500, 1700hz
	tolerance +/- 6hz
	Transmitted level -- 7 dBm0 +/- 1 dB per frequency
	Difference in transmitted level between the two frequencies
	of a signal not to exceed 1dB
	Signal duration: KP1 and KP2 100 +/- 10ms
			 All other signals 55 +/- 5ms				
			 Interval between all signals 55 +/- 5ms
	Interval between cessation of seizure line signal and transmission
	of KP interregister signal 80 +/- 20ms.

Reciever:
 Operate
	Frequency variation +/- 15 hz of the nominal
	Recieve level. The absolute power level N of each signal
	to be within the limits (-- 14 + n) < N < n dBm, where n
	is the relative power level at the recieve input. These limits
	give a margin of +/- 7dB on the nominal absolute level of each
	signal at the reciever input.

	The absolute levels of the two frequencies comprising a signal
	not to differ from each other by more than 4 dB

	Minimum signal duration 30ms
	Minimum interval between signals 30ms

Nonoperate
	Reciever not to operate to a signal whose absolute power level
	at its input is (--17 --7 + n) dBm. This limit is 17 dB below
	the absolute power level of the signal at the reciever input.

	Reciever not to operate to a signal of 10ms duration or less.
        Interruptions to signal, and intervals between successive signals
        of 10ms or less ignored.


Release of system 5 registers

 Normal Release
(a) An outgoing system 5 register releases when it has transmitted ST.
(b) An incoming system 5 register releases in either of the two cases:
 (i) on transmitting ST, on receipt of a number recieved condition from the
 destination national network, etc, depending on the arrangement adopted by
 administration
 (ii) when the bust flash signal is returned.
(c) A transit system 5 register releases in either of the two cases:
 (i) when it has transmitted ST
 (ii) when the busy flash signal is returned.

 Abnormal release
(a) An outgoing system 5 register releases, and clears forward connection in
any one of the following cases:
 (i) after a 15 - 30s time-out, after seizure, none, or less than the minimum
 number of address signals, recieved.
 (ii) proceed-to-send signal is not recieved within 10 - 20s time-out of the
 seizure signal
 (iii) proceed-to-send recieved, but, due to fault, the outgoing register has not
 pulsed out. The outgoing register will be released by the clear forward/release 
 guard sequence prompted by the busy flash signal sent from the incoming end on
 non-reciept of register signals within 4 - 9s. This assumes that the busy flash
 signal is recieved at the outgoing end before the termination of any forced 
 release delay that administrations may wish to incorporate to the outgoing 
 register.
(b) An incoming system 5 register releases in any one of the following cases:
 (i) no interregister signals recieved within 4 - 9s after the start of sending
 the proceed-to-send signal
 (ii) ST not recieved within 20 - 40s after the start of sending the 
 proceed-to-send signal
 (iii) on return of the busy flash signal from the incoming end when an error is
 detected in the receipt of interregister signals.
(c) A transit system 5 register releases in any one of the cases stated for the
 release of the outgoing and incoming (terminal) registers.


		BRIEF GUIDE TO THE MANY DIFFERENT VERSIONS OF CCITT 5
		=====================================================

CCITT 5bis:
This system is broadly based around CCITT5. It was specified in 1968 and designed for use on
non t.a.s.i equipped transoceanic cables. It was devised so that it would operate in overlap
mode at all incoming and outgoing system 5bis registers and therefore reduce the postdialling
delay. It does not require ST as a mandatory condition in automatic operation, and incorporates
both forward and backward interregister signalling to permit greater facility exploitation.

The system was never widely adopted, but rumour has it Denmark purchased some 5bis equipment in 
the early 80s but never used it. In many ways, this was a first step in devising R2, a similar
system, which uses outband tones to signal it. 


CCITT 5 Digitally Encoded:
This is a fairly recent system. It follows the standard CCITT 5 specification although all
signals are encoded digitally. There is a lower post-dialling delay and all signalling is
handled on separate trunks.

Interworking this system has some difficulties, and in many cases it is interworked with R1
resulting in a single "pleep" on answer, recived from the national network. CCITT 5 encoded is
a formidable alternative to SS7 especially when taking into account the cost of SS7. At present
digital CCITT 5 does not carry CLI data, although I'm sure someone could invent a modification
for it to carry CLI.

		





End of Part 3




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