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.\" Troff code generated by TPS Convert from ITU Original Files
.\"                 Not Copyright ( c) 1991 
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.EN
.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
.nr FM 6P
.nr PO 4P
.nr PD 9p
.po 4P

.rs
\v | 5i'
.sp 1P
.ce 1000
\v'12P'
\s12PART\ I
\v'4P'
.RT
.ce 0
.sp 1P
.ce 1000
\fBRecommendations Q.251 to Q.297\fR \v'2P'
.EF '%	 \ \ \ ^''
.OF ''' \ \ \ ^	%'
.ce 0
.sp 1P
.ce 1000
\fBSPECIFICATIONS\ OF\ SIGNALLING\ SYSTEM\ No.\ 6\fR 
.ce 0
.sp 1P
.LP
.rs
.sp 29P
.LP
.bp
.LP
\fBMONTAGE:\fR PAGE 2 = PAGE BLANCHE
.sp 1P
.RT
.LP
.bp
.sp 1P
.ce 1000
\v'4P'
\fBSIGNALLING\ SYSTEM\ No.\ 6\fR \v'4p'
.ce 0
.sp 1P
.LP
.EF '%	Fascicle\ VI.3\ \(em\ Introduction''
.OF '''Fascicle\ VI.3\ \(em\ Introduction	%'
.LP
\fIPreamble\fR 
.sp 1P
.RT
.PP
This specification of Signalling System No.\ 6 represents an
evolution originating with the \fIGreen Book\fR text and proceeding through the
\fIYellow Book\fR revision to the present text. It is intended that new or
modernized applications of System\ No.\ 6 should be based on this text. 
Updating of earlier versions is encouraged with the caution that careful 
coordination is called for. 
\v'1P'
.RT
.sp 1P
.ce 1000
\fBINTRODUCTION\fR \v'4p'
.ce 0
.sp 1P
.sp 2P
.LP
\fIGeneral\fR 
.sp 1P
.RT
.PP
Signalling System No.\ 6
can be used to control the
switching of all types of international circuits to be used in a worldwide
connection, including TASI\(hyderived circuits and satellite circuits.
.PP
The system meets all requirements defined by the CCITT concerning the service 
features for world\(hywide international semi\(hyautomatic and automatic 
telephone traffic. It is designed for both\(hyway operation of the speech
circuits.
.PP
The system can also be used for regional and national applications,
and a large part of the signal code capacity is reserved for this purpose.
.PP
Moreover, a large unused signal code capacity will allow the addition of 
new signals to cater for some unknown future requirements. This spare 
capacity may be used for increasing the number of telephone signals as 
well as for introducing other signals, e.g. network\(hymanagement signals 
and 
network\(hymaintenance signals.
.PP
The system features are obtained by entirely removing the signalling from 
the speech paths and introducing the concept of a separate common 
signalling link over which all signals for a number of speech circuits are
transferred. A number of these common signalling links interconnected by a
number of transit centres and signal transfer points will form a coherent
signalling network which can transfer all signals for all speech circuit 
groups within that network area. 
.RT
.sp 1P
.LP
\fIModes of operation\fR 
.sp 9p
.RT
.PP
The signalling system may be operated both in an 
\fIassociated\fR \fImode\fR and in a 
\fInon\(hyassociated mode\fR . In the associated mode of 
.PP
operation, the signals are transferred between the two exchanges which 
are the end points of a group of speech circuits over a common signalling 
link 
terminating at the same exchanges. In the non\(hyassociated mode of operation, 
the signals are transferred via two or more common signalling links in 
tandem 
associated with other groups of circuits, the signals being processed and
forwarded through one or more intermediate exchanges acting only as signal
transfer point.
.PP
The associated mode of operation is suited for use with large circuit groups, 
while a non\(hyassociated mode makes the signalling system economically 
suitable for use with small circuit groups by sharing the capacity of a
signalling link among several groups.
.PP
A signalling link may be operated in the associated mode for one
circuit group and in a non\(hyassociated mode for other circuit groups, either
under normal or under breakdown conditions.
.bp
.RT
.sp 1P
.LP
\fICommon signalling link\fR 
.sp 9p
.RT
.PP
The separate common signalling link is capable of operation over
both analogue and digital circuits. Signalling information is transmitted in  
.PP
the serial data mode on a link\(hyby\(hylink basis \(em i.e.\ the signals 
are transferred from one link to the next only after processing. 
.PP
Analogue signalling links are capable of operations over standard
international voice bandwidth channels including the\ 3\(hykHz spaced telephone
channels used for some intercontinental circuits. Over voice\(hyfrequency 
channels the stream of pulses is normally transmitted at a rate of 2400 
bit/s using the four\(hyphase modulation method. 
.PP
With respect to digital signalling links, the 1544\ kbit/s and
2048\ kbit/s internationally standardized PCM primary multiplexes
(Recommendation\ Q.47 and Recommendation\ Q.46) are treated differently. 
In the case of 1544\ kbit/s a channel is derived over which the stream 
of pulses is 
transmitted at 4\ kbit/s. Signalling information is also transmitted at
4\ kbit/s. In the case of 2048\ kbit/s, a channel is derived over which the
stream of pulses is transmitted at 64\ kbit/s. Signalling information may be
sent over such a channel at specified rates of either 4\ kbit/s or 56\ kbit/s.
Other bit rates may have application in the future and other provisions for
channel derivation may also prove useful, but neither are included in the
present specification.
.PP
In both analogue and digital channels the pulse stream is divided into 
signal units of 28\ bits each and into blocks of 12\ signal units each. 
.PP
The error control necessary for a common signalling link is based on error 
detection by coding and on error correction by retransmission. Error 
detection is based on decoding of checking bits included in each signal unit
and on data carrier failure detection. This provides the desired system
reliability. Error\(hyfree signal messages are used without delay. Provision is
made for automatic transfer to an alternative link in the event of failure
caused by breakdown or excessive error rate.
.RT
.sp 1P
.LP
\fISignal messages\fR 
.sp 9p
.RT
.PP
Signal messages carry information to identify the telephone circuit concerned. 
Since the circuit identity, i.e. the \fIlabel\fR , requires a large 
proportion of the bits (11\ out of the\ 20 available information bits), 
provision is made for sending \fImulti\(hyunit messages\fR consisting of 
several signal units 
under one label. A single digit or a random telephone signal will normally 
be transferred in a one\(hyunit message while several or even all digits 
may be 
transferred in a multi\(hyunit message.
.RT
.LP
.sp 1P
.LP
\fISignal processing\fR 
.sp 9p
.RT
.PP
All signals are processed at each transit centre or signal transfer point 
which has to be passed. 
.PP
The processing of messages at a signal transfer point is minimal and includes 
label translation, if necessary, and the sending of signal messages 
within the proper priority category. In addition to the processing required 
at a signal point, a transit centre examines sufficient signal information 
to 
perform proper switching action.
.RT
.sp 1P
.LP
\fISignalling equipment\fR 
.sp 9p
.RT
.PP
Because of the new technique based on a separate common signalling link, 
on data type transmission and on central processing of the signalling 
information, Signalling System\ No.\ 6 will be used in general between 
exchanges of the stored programme control type. 
.RT
.LP
.rs
.sp 5P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.sp 1P
.ce 1000
\v'3P'
SECTION\ 1
.ce 0
.sp 1P
.ce 1000
\fBFUNCTIONAL\ DESCRIPTION\ OF\ THE\ SIGNALLING\ SYSTEM\fR \v'2P'
.ce 0
.sp 1P
.sp 2P
.LP
\fBRecommendation\ Q.251\fR 
.RT
.sp 2P
.sp 1P
.ce 1000
\fB1.1\ GENERAL\fR \v'4p'
.EF '%	Fascicle\ VI.3\ \(em\ Rec.\ Q.251''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.251	%'
.ce 0
.sp 1P
.LP
1.1.1 
	\fIBlock diagrams\fR 
.sp 1P
.RT
.PP
Because 
common channel signalling
, used in conjunction with exchanges having 
stored programme control
, allows a wide latitude in
the distribution of signalling functions between the processor and peripheral 
equipment, and because common channel signalling is not limited to exchanges 
.PP
of this type, it is not practicable to specify well\(hydefined equipment
interfaces.
.PP
The major signal transfer functions are shown in Figures\ 1/Q.251,
2/Q.251 and Table\ 1/Q.251 for both the analogue version and the digital
version. The blocks are functional blocks and should not be construed as
depicting equipment arrangements.
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure 1/Q.251, p. 1\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 21P
.ad r
\fBFigure 2/Q.251, p. 2\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 27P
.ad r
\fBTable [1/Q.251], p. 3\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
1.1.2
	\fISignal unit and block structure\fR 
.sp 9p
.RT
.PP
Each 
signalling channel
of the system (shown in
Figure\ 2/Q.251) is operated synchronously: that is, a continuous stream of  
.PP
data flows in both directions. The data stream is divided into signal units 
of 28\ bits each, of which the last 8 are check bits, and these signal 
units in 
turn are grouped into \fIblocks\fR of 12\ signal units. The 12th and last 
signal 
unit of each 
block
is an acknowledgement signal unit coded to indicate the number of the block 
being transmitted, the number of the block being 
acknowledged and whether or not each of the 11\ signal units of the block 
being acknowledged was received without detected errors. 
.PP
Eight consecutive blocks form a 
\fImulti\(hyblock\fR . Since the
system allows for up to 32\ multi\(hyblocks, the maximum number of blocks 
in the 
error control loop is\ 256.
.PP
In normal operation, the first 11\ signal units within a block will
consist of signal units carrying either telephone signals or management
signals, or of synchronization signal units. Synchronization signal units,
which are transmitted only in the absence of other signalling traffic, are
coded to indicate the number of the position they occupy within the block to
facilitate locating the acknowledgement signal unit. Their format has been
chosen to produce a large number of dibit transitions to facilitate achieving 
or maintaining bit synchronism in the analogue version. 
.PP
During system\(hysynchronizing procedures, only synchronization and
acknowledgement signal units are transmitted until bit, signal unit, and 
block synchronism has been achieved at both ends of the signalling system. 
.RT
.sp 1P
.LP
1.1.3 
	\fITransmitting terminal\fR 
.sp 9p
.RT
.PP
The transmission of a signal in System No.\ 6 starts in the
processor
as shown in Figure\ 1/Q.251. Signals corresponding to the
information to be transmitted are formed in accordance with the format
specified and delivered to the output buffer. These signals, which may be
one\(hyunit messages
or 
multi\(hyunit messages
, are stored in this
buffer
according to their priority level. The output buffer delivers
the highest priority signal awaiting transmission to the coder in serial 
form in the next available time slot. In the 
coder
, each signal unit is
encoded by the addition of 
check bits
in accordance with the 
check  bit polynomial
.
.PP
In the analogue version of the signalling system the signal is then
modulated and delivered to the outgoing voice frequency channel for
transmission to the distant receiving terminal. In the digital version of the 
.PP
signalling system the signal is passed through the interface adaptor before
entering the outgoing digital channel.
.RT
.sp 1P
.LP
1.1.4
	\fIReceiving terminal\fR 
.sp 9p
.RT
.PP
The receiving function starts with the acceptance of the serial
data from the transmission path. The output of the demodulator or the
interface adaptor
is delivered to the decoder where each signal unit is checked for errors 
on the basis of the associated check bits. Signal units 
received with detected errors are discarded. Signal units carrying telephone
signals or management signals which are error\(hyfree are transferred to 
the input buffer after deletion of the check bits. The input buffer delivers 
the signal units to the processor where the processor analyzes the signals 
and takes 
appropriate action.
.RT
.sp 1P
.LP
1.1.5
	\fIError control\fR 
.sp 9p
.RT
.PP
Error control is based on error detection by redundant coding and on error 
correction by retransmission of those signal messages found to be in error. 
This procedure requires that each transmitted signal message be stored 
until acknowledged as being received correctly. In the case of multi\(hyunit 
.PP
messages, each signal unit of the message must be stored until all units 
of the message are acknowledged as being received correctly. When an acknowledgement 
signal unit is received, it is analyzed in the box marked \fIerror control\fR 
in 
Figure\ 1/Q.251. If an acknowledgement bit indicates that a signal unit being
acknowledged was received in error, the retransmission process is started.
Request for retransmission of a synchronization signal unit is ignored. 
If any unit of a multi\(hyunit message is in error, the entire multi\(hyunit 
message must be retransmitted in its initial order. 
.PP
The data channel failure detector complements the decoder for longer error 
bursts. When activated it gives an indication to the box marked \fIerror\fR 
\fIcontrol\fR in Figure\ 1/Q.251. An error indication from either the 
decoder
or 
the\ 
data\ channel failure detector
is associated with the position of the erroneous signal unit(s) within 
the block. 
.bp
.PP
This information is used by the acknowledgement signal unit generator to
control the marking of the acknow
ledgement\ bits.
.PP
As shown in Figure\ 1/Q.251, the processor may also be notified whenever 
an error is detected in a signal unit. This information may be used by 
the 
processor to erase the memory of any signal unit(s) of a multi\(hyunit message
received which is associated with the one found in error, since this entire
message will be retransmitted.
\v'1P'
.RT
.sp 2P
.LP
\fBRecommendation\ Q.252\fR 
.RT
.sp 2P
.sp 1P
.ce 1000
\fB1.2\ \fR \fBSIGNAL\ TRANSFER\ TIME\fR \fB\ DEFINITIONS\fR \v'4p'
.EF '%	Fascicle\ VI.3\ \(em\ Rec.\ Q.252''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.252	%'
.ce 0
.sp 1P
.LP
1.2.1 
	\fIFunctional reference points\fR 
.sp 1P
.RT
.PP
The major 
functional reference points
are as indicated in Figure\ 3/Q.252, i.e. points\ A, B, C and\ D, which 
are defined below: 
.PP
\fIPoint\ A.\fR \ \(em\ That point in a switching centre where the signal as a
signal unit, before being coded (check bits added), is delivered from the
processor to an 
output buffer store
.
.PP
\fIPoint\ B.\fR \ \(em\ That point where the signal unit (check bits included) 
in serial form will be delivered to the transmission path. 
.PP
\fIPoint\ C.\fR \ \(em\ That point where the signal unit (check bits included) 
in serial form will be delivered to the demodulator or interface adaptor. 
.PP
\fIPoint\ D.\fR \ \(em\ That point in a switching centre where the signal 
unit, after being decoded (check bits deleted), will be presented from 
an 
input   buffer store
to the processor.
.PP
The functional reference points\ B\ and\ C are typically those points
which define the transmission path used for common channel signalling. 
In the analogue version this transmission path is provided by a voice frequency 
channel and in the digital version by a digital channel.
.RT
.LP
.rs
.sp 22P
.ad r
\fBFigure 3/Q.252, p. 4\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
1.2.2
	\fISignal transfer time components\fR 
.sp 9p
.RT
.PP
The various components of signal transfer time between two
switching centres are defined as follows:
.RT
.LP
	\fIT\fR\d\fIc\fR\u	=\ 
cross\(hyoffice transfer time
,
.LP
	\fIT\fR\d\fIe\fR\u	=\ 
emission time of a signal unit
(included in
\fIT\fR\d\fIs\fR\u),
.LP
	\fIT\fR\d\fIh\fR\u	=\ 
processing (handling) time
,
.LP
	\fIT\fR\d\fIp\fR\u	=\ 
transfer channel propagation time
,
.LP
	\fIT\fR\d\fIq\fR\u	=\ 
queueing delay in the output buffer
store
(included in \fIT\fR\d\fIs\fR\u),
.LP
	\fIT\fR\d\fIr\fR\u	=\ 
receiver transfer time
,
.LP
	\fIT\fR\d\fIs\fR\u	=\ 
sender transfer time
,
.LP
	\fIT\fR\d\fIt\fR\u	=\ 
total signal transfer time
.
.PP
\fB\fIT\fR\d\fIh\fR\uis that period from the moment the signal is available
for acceptance by the processor to the moment the signal is placed in the
output buffer and is available for transmission.
.PP
\fB\fIT\fR\d\fIr\fR\uis that period of time from the moment when the last 
bit of the signal unit leaves the transfer channel to that time when the 
signal is 
completely in the input buffer and is available for acceptance by the
processor. \fIT\fR\d\fIr\fR\u\ thus includes the following actions: demodulation, 
decoding (error detection) and, where present, serial to parallel conversion. 
.PP
\fB\fIT\fR\d\fIs\fR\uis that period of time from the moment when the signal 
enters the output buffer store to that time when the last bit of the signal 
unit 
passes into the transfer channel. \fIT\fR\d\fIs\fR\uthus includes the following 
times and actions: emission time of signal unit(s) (one\(hyunit or multi\(hyunit 
message), 
queueing delay in the output buffer store, encoding (adding check bits),
parallel to serial conversion where present, modulation in the analogue 
version and clock and data rate conversion where applicable in the digital 
version. 
.PP
The definitions of signal transfer times give rise to the following
time relationships:
.RT
.ce 1000
\fIT\fR\d\fIc\fR\u= \fIT\fR\d\fIr\fR\u+ \fIT\fR\d\fIh\fR\u+ \fIT\fR\d\fIs\fR\u
.ce 0
.sp 1P
.ce 1000
\fI\fI\fR \fIT\fR\d\fIt\fR\u= \fIT\fR\d\fIs\fR\u+ \fIT\fR\d\fIp\fR\u+ \fIT\fR\d\fIr\fR\u 
.ce 0
.sp 1P
.PP
In the case when an error is detected, retransmission will occur and the 
above time relationships are not valid. Rather, the time involved in 
retransmission and the extra queueing delays, which may occur on a
retransmitted signal, must be taken into consideration.
\v'1P'
.LP
.sp 2P
.LP
\fBRecommendation\ Q.253\fR 
.RT
.sp 2P
.sp 1P
.ce 1000
\fB1.3\ ASSOCIATION\ BETWEEN\ SIGNALLING\ AND\ SPEECH\ NETWORKS\fR \v'4p'
.EF '%	Fascicle\ VI.3\ \(em\ Rec.\ Q.253''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.253	%'
.ce 0
.sp 1P
.LP
1.3.1
	\fIDefinitions\fR 
.sp 1P
.RT
.PP
The signals pertaining to a given group of speech circuits between two 
exchanges utilizing a common channel signalling system can be transferred 
in the following ways: 
.RT
.sp 1P
.LP
1.3.1.1
	\fBassociated mode of operation\fR 
.sp 9p
.RT
.PP
In the associated mode of operation, the signals are transferred between 
the two exchanges over a common signalling link which terminates at the 
same exchanges as the group of speech circuits to which the signalling 
link has been assigned. 
.RT
.sp 1P
.LP
1.3.1.2
	\fBnon\(hyassociated mode of operation\fR 
.sp 9p
.RT
.LP
.PP
In a non\(hyassociated mode of operation, the signals are transferred between 
the two exchanges over two or more common signalling links in tandem, the 
signals being processed and forwarded through one or more intermediate 
\fIsignal transfer points\fR (see \(sc\ 1.3.3 below). Following this
.bp
.PP
definition, there may be a range of non\(hyassociated modes of operation which
vary in the degree of rigidity imposed on the choice of the path utilized by
the signals pertaining to the speech circuit. The ends of this range can be
described as fully dissociated mode and quasi\(hyassociated mode of
operation.
.RT
.LP
	a)
	\fBfully dissociated mode of operation\fR 
.PP
The fully dissociated mode of operation is the extreme case of the non\(hyassociated 
mode. It is assumed that there is an established network of 
common signalling links and signal transfer points which may have its own
routing principles.
.PP
In the fully dissociated mode of operation, the signals are
transferred between the two exchanges via any available path in the signalling 
network according to the rules of that network. 
.RT
.LP
	b)
	\fBquasi\(hyassociated mode of operation\fR 
.PP
The quasi\(hyassociated mode of operation is the limited form of the non\(hyassociated 
mode. The common signalling links to be used are generally each operating 
in the associated mode with a group of circuits. 
.PP
In the quasi\(hyassociated mode of operation the signals are transferred 
between the two exchanges over two or more common signalling links in tandem, 
but only over certain predetermined paths and through predetermined signal 
transfer points.
.RT
.sp 1P
.LP
1.3.2
	\fIAssociation methods provided by the System No.\ 6\fR 
.sp 9p
.RT
.PP
Signalling System No.\ 6 is designed to provide associated and
quasi\(hyassociated modes of operation as defined in \(sc\(sc\ 1.3.1.1 
and\ 1.3.1.2,\ b) 
above, e.g.\ as shown in Figure\ 4/Q.253.
.RT
.LP
.rs
.sp 22P
.ad r
\fBFigure 4/Q.253, p. 5\fR 
.sp 1P
.RT
.ad b
.RT
.PP
As far as quasi\(hyassociated structures are concerned, the number of signal 
transfer points in the signalling path for a group of speech circuits 
.PP
between the two System\ No.\ 6 exchanges should be kept as low as practicable.
Normally, one such signal transfer point should suffice. However, there 
may be groups of circuits without associated common signalling links which 
will need more than one signal transfer point to handle the signalling 
traffic.
.bp
.PP
Attention is drawn to the fact that the addition of a signal transfer point 
involves the handling time at that point and one additional signal 
transfer time. Extensive use of signal transfer points will reduce some 
of the advantages of the signalling speed of System\ No.\ 6. 
.PP
\fINote\fR \ \(em\ It should be noted that where a speech circuit group 
has an associated signalling link, dependability requirements may be met 
with 
economically, by using quasi\(hyassociated operation under breakdown condition
when the associated signalling link is non\(hyoperative.
.RT
.sp 2P
.LP
1.3.3 
	\fBsignal transfer point\fR 
.sp 1P
.RT
.sp 1P
.LP
1.3.3.1
	\fIDefinition\fR 
.sp 9p
.RT
.PP
A signal transfer point is a signal relay centre handling and
forwarding telephone signals from one signalling link to another in case of
signalling in a non\(hyassociated mode of operation as defined in \(sc\ 1.3.1.2
above.
.PP
\fINote\fR \ \(em\ Following this definition there is no need for a signal
transfer point to have any connection with, or relation to, a switching
centre.
.PP
However, in the case of a quasi\(hyassociated mode of operation as
defined in \(sc\ 1.3.1.2 | ) above, it is obvious that a signal transfer 
point may coincide with the System\ No.\ 6 exchange where the signalling 
links terminate 
and that the equipment may be incorporated into the signalling equipment of
that System\ No.\ 6 exchange.
.RT
.sp 1P
.LP
1.3.3.2 
	\fIFunctions of a signal transfer point\fR \v'3p'
.sp 9p
.RT
.PP
a)
The equipment at a signal transfer point has to analyze the label and telephone 
signal information of every telephone signal message 
received in order to offer the message to the proper outgoing signalling
channel, taking account of its priority, if any.
.PP
b)
In doing so, it may be necessary to change the label of  the
received telephone signal message according to some preset rules. However, 
the telephone signal information included in the message will never be 
changed by the equipment of a signal transfer point. 
.LP
.PP
c)
If for some reason a signal transfer point is unable to transfer signal 
messages, a procedure is provided to notify the preceding 
exchange(s) so that signal messages may be sent via reserve routes if
available.
.PP
\fINote\fR \ \(em\ The fact mentioned under b) above and the fact that 
the analysis of the received message will never be accompanied by the switching 
of speech circuits provide a distinction between a signal transfer point 
and a 
transit exchange. In general, a transit exchange will be designed to perform
both the normal transit exchange functions and the signal transfer point
functions.
.LP
.rs
.sp 15P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
\fBMONTAGE: PAGE 12 = PAGE BLANCHE\fR 
.sp 1P
.RT
.LP
.bp
.sp 1P
.ce 1000
\v'3P'
SECTION\ 2
.ce 0
.sp 1P
.ce 1000
\fBDEFINITION\ AND\ FUNCTION\ OF\ SIGNALS\fR \v'2P'
.ce 0
.sp 1P
.sp 2P
.LP
\fBRecommendation\ Q.254\fR 
.FS
Some section numbers have been reserved for  future use.
.FE
.RT
.sp 2P
.sp 1P
.ce 1000
\fB2.1\ \fR \fBTELEPHONE\ SIGNALS\fR \v'4p'
.EF '%	Fascicle\ VI.3\ \(em\ Rec.\ Q.254''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.254	%'
.ce 0
.sp 1P
.PP
Signals concerning a particular call or a particular speech
circuit.
.sp 1P
.RT
.sp 1P
.LP
2.1.1
	\fBaddress signal\fR 
.sp 9p
.RT
.PP
A call set\(hyup signal sent in the forward direction containing one element 
of information (digit\ 1, 2,\ . |  |  9 or\ 0, code\ 11 or code\ 12) about 
the called party's number or the end\(hyof\(hypulsing (ST)\ signal. 
.PP
For each call, a succession of address signals is sent.
.RT
.sp 1P
.LP
2.1.2
	\fBcountry\(hycode indicator\fR 
.sp 9p
.RT
.PP
Information sent in the forward direction indicating whether or
not the country code is included in the address information.
.RT
.sp 1P
.LP
2.1.3
	\fBnature\(hyof\(hycircuit indicator\fR 
.sp 9p
.RT
.PP
Information sent in the forward direction about the nature of the circuit 
or any preceding circuit(s) already engaged in the 
connection:
.RT
.LP
	\(em
	\fIsatellite circuit\fR , or
.LP
	\(em
	\fIno satellite circuit\fR .
.PP
An international exchange receiving this information will use it (in combination 
with the appropriate part of the address information) to 
determine the nature of the outgoing circuit to be chosen.
.sp 1P
.LP
2.1.4 
	\fBecho suppressor indicator\fR 
.sp 9p
.RT
.PP
Information sent in the forward direction indicating whether or
not an outgoing half\(hyecho suppressor is included in the connection.
.RT
.sp 1P
.LP
2.1.5
	\fBcalling\(hyparty's\(hycategory indicator\fR 
.sp 9p
.RT
.PP
Information sent in the forward direction about the \fIcategory of\fR \fIthe 
calling party\fR  | nd, in case of semi\(hyautomatic calls about the \fIservice\fR 
\fIlanguage\fR to be spoken by the incoming, delay and assistance 
operators.
.bp
.PP
The following categories are provided:
.RT
.LP
	\(em
	operator,
.LP
	\(em
	ordinary calling subscriber,
.LP
	\(em
	calling subscriber with priority,
.LP
	\(em
	data call,
.LP
	\(em
	test call.
.sp 1P
.LP
2.1.6
	\fBend\(hyof\(hypulsing (ST) signal\fR 
.sp 9p
.RT
.PP
An address signal sent in the forward direction indicating that
there are no more address signals to follow.
.RT
.sp 1P
.LP
2.1.10 
	\fBcontinuity signal\fR 
.sp 9p
.RT
.PP
A signal sent in the forward direction indicating continuity of  
.PP
the preceding No.\ 6\ speech circuit(s) as well as of the selected speech 
circuit to the following international exchange, including verification 
of the speech path across the exchange with the specified degree of reliability. 
.RT
.sp 1P
.LP
2.1.12
	\fBswitching\(hyequipment\(hycongestion signal\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating the failure of the call 
set\(hyup attempt due to congestion encountered at international 
switching equipment.
.RT
.sp 1P
.LP
2.1.13
	\fBcircuit\(hygroup\(hycongestion signal\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating the failure of the call 
set\(hyup attempt due to congestion encountered on an international 
circuit group or on the outgoing links of a terminal international
exchange.
.RT
.sp 1P
.LP
2.1.14
	\fBnational\(hynetwork\(hycongestion signal\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating the failure of the call 
set\(hyup attempt due to congestion encountered in the national 
destination network (excluding the busy condition of the called party's
line(s)).
.RT
.LP
.sp 1P
.LP
2.1.15
	\fBaddress\(hyincomplete signal\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that the number of address 
signals received is not sufficient for setting up the call. This 
condition may be determined in the incoming international exchange (or 
in the national destination network): 
.RT
.LP
	\(em
	immediately after the reception of an ST signal, or
.LP
	\(em
	on time\(hyout after the latest digit received.
.sp 1P
.LP
2.1.16
	\fBaddress\(hycomplete signal, charge\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that all the
address signals required for routing the call to the called party have been
received, that no called\(hyparty's\(hyline\(hycondition signals (electrical) 
will be 
sent, and that the call should be charged on answer.
.RT
.sp 1P
.LP
2.1.17
	\fBaddress\(hycomplete signal, no charge\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that all the
address signals required for routing the call to the called party have been
received, that no called\(hyparty's\(hyline\(hycondition signals (electrical) 
will be 
sent, and that the call should not be charged on answer.
.RT
.LP
.sp 1P
.LP
2.1.18
	\fBaddress\(hycomplete signal, coin\(hybox\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that all the
address signals required for routing the call to the called party have been
received, that no called\(hyparty's\(hyline\(hycondition signals (electrical) 
will be 
sent, that the call should be charged on answer, and that the called number 
is a coin (box) station. 
.RT
.sp 1P
.LP
2.1.19
	\fBaddress\(hycomplete, subscriber\(hyfree signal, charge\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction as an alternative to the address\(hycomplete, 
charge signal indicating that the called party's line is 
free, and that the call should be charged on answer.
.bp
.RT
.sp 1P
.LP
2.1.20
	\fBaddress\(hycomplete, subscriber\(hyfree signal, no charge\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction as an alternative to the address\(hycomplete, 
no charge signal indicating that the called party's line is free, and that 
the call should not be charged on answer. 
.RT
.sp 1P
.LP
2.1.21
	\fBaddress\(hycomplete, subscriber\(hyfree signal, coin\(hybox\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction as an alternative to the address\(hycomplete, 
coin\(hybox signal indicating that the called party's line is 
.PP
free, that the call should be charged on answer, and that the called number 
is a coin (box) station. 
.RT
.sp 1P
.LP
2.1.23
	\fBunallocated\(hynumber signal\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that the
received number is not in use (for example spare level, spare code, vacant
subscriber's number).
.RT
.sp 1P
.LP
2.1.24
	\fBsubscriber\(hybusy signal (electrical)\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that the
line(s) connecting the called party with the exchange is (are) engaged. The
subscriber\(hybusy signal will also be sent in case of complete uncertainty 
about the place where the busy or congestion conditions are encountered 
and in the 
case where a discrimination between subscriber\(hybusy and national\(hynetwork
congestion is not possible.
.RT
.sp 1P
.LP
2.1.25
	\fBline\(hyout\(hyof\(hyservice signal\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that the called party's 
line is out\(hyof\(hyservice or faulty. 
.RT
.sp 1P
.LP
2.1.26
	\fBsend\(hyspecial\(hyinformation tone signal\fR 
.sp 9p
.RT
.LP
.PP
A signal sent in the backward direction indicating that the
special information tone should be returned to the calling party. This tone
indicates that the called number cannot be reached for reasons not covered 
by other specific signals and that the unavailability is of a long term 
nature. 
(See also Recommendation\ Q.35.)
.RT
.sp 1P
.LP
2.1.27
	\fBconfusion signal\fR 
.sp 9p
.RT
.PP
Signal sent in the backward direction indicating that an exchange is unable 
to act upon a message received from the preceding exchange because 
the message is considered unreasonable.
.RT
.sp 1P
.LP
2.1.28
	\fBcall\(hyfailure signal\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating the failure of a call 
set\(hyup attempt due to the lapse of a time\(hyout or a fault not covered 
by specific signals and where the congestion tone is the appropriate tone 
to be 
returned to the calling party.
.RT
.sp 1P
.LP
2.1.29
	\fBmessage\(hyrefusal signal\fR 
.sp 9p
.RT
.PP
A signal sent by a signal transfer point in response to the
reception of a telephone signal which it is unable to deal with as a
consequence of the transfer\(hyprohibited situation.
.RT
.LP
.sp 1P
.LP
2.1.31
	\fBforward\(hytransfer signal\fR 
.sp 9p
.RT
.PP
A signal sent in the forward direction on semi\(hyautomatic calls
when the outgoing international exchange operator wants the help of an 
operator at the incoming international exchange. The signal will normally 
serve to bring an assistance operator (see Recommendation\ Q.101) into 
the circuit if the call is automatically set up at that exchange. When 
a call is completed via an 
operator (incoming or delay operator) at the incoming international exchange, 
the signal should preferably cause this operator to be recalled. 
.RT
.sp 1P
.LP
2.1.32
	\fBanswer signal, charge\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that the call is answered 
and subject to charge. 
.PP
In semi\(hyautomatic working, this signal has a supervisory function. In
automatic working, the signal is used:
.RT
.LP
	\(em
	to start metering the charge to the calling subscriber
(Recommendation\ Q.28), and
.LP
	\(em
	 to start the measurement of call duration for international accounting 
purposes. 
.bp
.LP
.sp 1P
.LP
2.1.33
	\fBanswer signal, no charge\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction indicating that the call is answered 
but is not subject to charge. It is used for calls to particular 
destinations only.
.PP
In semi\(hyautomatic working, this signal has a supervisory function. In 
automatic working, the reception of this signal shall not start the metering 
to the calling subscriber. 
.RT
.sp 1P
.LP
2.1.34
	\fBclear\(hyback signals\fR 
.sp 9p
.RT
.PP
Signals sent in the backward direction, the first of which
indicates that the called party has cleared. Subsequent clear\(hyback signals
indicate that the called party has cleared following a reanswer, e.g.
switch\(hyhook flashing.
.PP
In semi\(hyautomatic working, they perform a supervisory function. In
automatic working, the arrangements specified in Recommendation\ Q.118
apply.
.RT
.sp 1P
.LP
2.1.35
	\fBreanswer signals\fR 
.sp 9p
.RT
.PP
Signals in the backward direction indicating that the called  
.PP
party, after having cleared, again lifts his receiver or in some other way
reproduces the answer condition, e.g. switch\(hyhook flashing.
.RT
.sp 1P
.LP
2.1.36
	\fBclear\(hyforward signal\fR 
.sp 9p
.RT
.PP
A signal sent in the forward direction to terminate the call or
call attempt and release the circuit concerned. This signal is normally sent
when the calling party clears but also may be a proper response in other
situations, as for example, when reset circuit is received.
.RT
.sp 1P
.LP
2.1.37
	\fBrelease\(hyguard signal\fR 
.sp 9p
.RT
.PP
A signal sent in the backward direction in response to a
clear\(hyforward signal, or if appropriate to the reset\(hycircuit signal, 
when the circuit concerned has been brought into the idle condition. 
.RT
.sp 1P
.LP
2.1.38
	\fBreset\(hycircuit signal\fR 
.sp 9p
.RT
.PP
A signal that is sent to release a circuit when, due to memory
multilation or other causes, it is unknown whether, for example, a
clear\(hyforward or clear\(hyback signal is appropriate. If at the receiving 
end the circuit is blocked, this signal should remove that condition. 
.RT
.LP
.sp 1P
.LP
2.1.41
	\fBblocking signal\fR 
.sp 9p
.RT
.PP
A signal sent for maintenance purposes to the exchange at the
other end of a circuit to cause engaged conditions of that circuit for
subsequent calls outgoing from that exchange. An exchange receiving the
blocking signal must be capable of accepting incoming calls on that circuit
unless it also has sent a blocking signal. Under conditions covered later, a
blocking signal is also a proper response to a reset\(hycircuit signal.
.RT
.sp 1P
.LP
2.1.42
	\fBunblocking signal\fR 
.sp 9p
.RT
.PP
A signal sent to the exchange at the other end of a circuit to
cancel in that exchange the engaged conditions of that circuit caused by an
earlier blocking signal.
.RT
.sp 1P
.LP
2.1.43
	\fBblocking\(hyacknowledgement signal\fR 
.sp 9p
.RT
.PP
A signal sent in response to a blocking signal indicating that the speech 
circuit has been blocked. 
.RT
.sp 1P
.LP
2.1.44
	\fBunblocking\(hyacknowledgement signal\fR 
.sp 9p
.RT
.PP
A signal sent in response to an unblocking signal indicating that the speech 
circuit has been unblocked. 
.bp
.RT
.LP
.sp 2P
.LP
\fBRecommendation\ Q.255\fR 
.RT
.sp 2P
.sp 1P
.ce 1000
\fB2.2\ SIGNALLING\(hySYSTEM\(hyCONTROL\ SIGNALS\fR \v'4p'
.EF '%	Fascicle\ VI.3\ \(em\ Rec.\ Q.255''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.255	%'
.ce 0
.sp 1P
.PP
Signals used for the proper functioning of the signalling
system via the common signalling link.
.sp 1P
.RT
.sp 1P
.LP
2.2.1
	\fBacknowledgement indicator\fR 
.sp 9p
.RT
.PP
Information indicating whether or not an error has been detected in a received 
signal unit. 
.RT
.sp 1P
.LP
2.2.2
	\fBsynchronization signal\fR 
.sp 9p
.RT
.PP
A signal sent in order to establish and maintain synchronization between 
the two ends of a signalling channel. 
.RT
.sp 2P
.LP
2.2.3
	\fISystem\(hycontrol signals\fR 
.sp 1P
.RT
.sp 1P
.LP
2.2.3.1
	\fBchangeover signal\fR 
.sp 9p
.RT
.LP
.PP
A signal sent to indicate a failure on a synchronized signalling link. 
If this signal is sent on a link carrying signalling information, it also 
indicates that a changeover to the next reserve signalling link is 
required.
.RT
.sp 1P
.LP
2.2.3.2
	\fBmanual\(hychangeover signal\fR 
.sp 9p
.RT
.PP
A signal sent to initiate a changeover to a reserve signalling
link or to initiate the removal of full\(hytime synchronized reserve link from
service availability because of need for rearrangements, changes,
maintenance,\ etc.
.RT
.sp 1P
.LP
2.2.3.3
	\fBmanual\(hychangeover\(hyacknowledgement signal\fR 
.sp 9p
.RT
.PP
A signal sent in response to a manual\(hychangeover signal to
indicate that manual changeover can take place.
.RT
.sp 1P
.LP
2.2.3.4
	\fBstandby\(hyready signal\fR 
.sp 9p
.RT
.PP
A signal sent on a standby reserve link to indicate that the error rate 
on that link has met the requirements of the \fIone\(hyminute proving\fR 
\fIperiod\fR .
.RT
.sp 1P
.LP
2.2.3.5
	\fBstandby\(hyready\(hyacknowledgement signal\fR 
.sp 9p
.RT
.LP
.PP
A signal sent on the standby reserve link in response to a
standby\(hyready signal and indicating that the error rate on that link has met
the requirements of the \fIone\(hyminute proving period\fR .
.RT
.sp 1P
.LP
2.2.3.6
	\fBload transfer signal\fR 
.sp 9p
.RT
.PP
A signal sent on a link to indicate that the error rate on that
link has met the requirements of the \fIone\(hyminute proving period\fR 
and that 
signalling traffic should be transferred to that particular link.
.RT
.sp 1P
.LP
2.2.3.7
	\fBemergency\(hyload\(hytransfer signal\fR 
.sp 9p
.RT
.PP
A signal sent on as many links as possible to indicate that the
error rate on those links has met the requirements of the \fIemergency 
proving\fR \fIperiod\fR , and that emergency transfer can take place to 
one of these 
links.
.RT
.sp 1P
.LP
2.2.3.8
	\fBload\(hytransfer\(hyacknowledgement signal\fR 
.sp 9p
.RT
.PP
A signal sent on a link in response to a load\(hytransfer signal or to 
an emergency\(hyload\(hytransfer signal to indicate that the load\(hytransfer 
will 
take place to that particular link.
.bp
.RT
.LP
.sp 2P
.LP
2.2.4
	\fIMulti\(hyblock synchronization signals\fR 
.sp 1P
.RT
.sp 1P
.LP
2.2.4.1
	\fBmulti\(hyblock monitoring signal\fR 
.sp 9p
.RT
.PP
A signal, required on links where the number of blocks in the
error control loop exceeds\ 8, and sent to check multi\(hyblock synchronism.
.RT
.sp 1P
.LP
2.2.4.2
	\fBmulti\(hyblock acknowledgement signal\fR 
.sp 9p
.RT
.PP
A signal sent on a link in response to a multi\(hyblock monitoring
signal and used by the receiving terminal to verify multi\(hyblock
synchronism.
\v'1P'
.RT
.LP
.sp 2P
.LP
\fBRecommendation\ Q.256\fR 
.RT
.sp 2P
.sp 1P
.ce 1000
\fB2.3\ \fR \fBMANAGEMENT\ SIGNALS\fR \v'4p'
.EF '%	Fascicle\ VI.3\ \(em\ Rec.\ Q.256''
.OF '''Fascicle\ VI.3\ \(em\ Rec.\ Q.256	%'
.ce 0
.sp 1P
.PP
Signals concerning the management of the speech circuit
network and the signalling network. The three following categories of signals 
are distinguished: 
.sp 1P
.RT
.sp 1P
.LP
2.3.1
	\fBnetwork\(hymanagement signals\fR 
.sp 9p
.RT
.PP
Information regarding the conditions of circuit groups or
equipment sent from one point in the network to one or more other points. 
This excludes information relevant to individual calls or individual speech 
circuits.
.RT
.sp 1P
.LP
2.3.2
	\fBnetwork\(hymaintenance signals\fR 
.sp 9p
.RT
.PP
Management signals used for maintenance purposes.
.RT
.LP
.sp 1P
.LP
2.3.2.1
	\fBreset\(hyband signal\fR 
.sp 9p
.RT
.PP
A signal sent by a failed exchange during recovery to request that all 
circuits in the band be put in the idle state except those circuits at 
the receiving end that have imposed a blocked condition on the sending 
end. If at the receiving end the circuit is blocked, the reset\(hyband 
signal should remove that condition. 
.RT
.sp 1P
.LP
2.3.2.2
	\fBreset\(hyband\(hyacknowledgement signal\fR 
.sp 9p
.RT
.PP
A signal sent in response to the reset\(hyband signal to indicate
whether a circuit is available for use or should be blocked in the failed
exchange.
.RT
.sp 1P
.LP
2.3.2.3
	\fBreset\(hyband\(hyacknowledgement signal, all circuits idle\fR 
.sp 9p
.RT
.PP
A signal sent in response to the reset\(hyband signal to indicate that 
all circuits in the band are available for use. 
.RT
.sp 1P
.LP
2.3.3
	\fBsignalling\(hynetwork\(hymanagement signals\fR 
.sp 9p
.RT
.PP
Information regarding the conditions of signalling links which may be required 
to modify signal routings. This excludes information relevant to 
the signals concerned with individual calls or speech circuits.
.RT
.LP
.sp 1P
.LP
2.3.3.1
	\fBtransfer\(hyprohibited signal\fR 
.sp 9p
.RT
.PP
A signal sent by a signal transfer point when it is unable to
transfer signals for a particular group of circuits.
.RT
.sp 1P
.LP
2.3.3.2
	\fBtransfer\(hyallowed signal\fR 
.sp 9p
.RT
.PP
A signal sent by a signal transfer point when it is once again
ready to transfer signals for the particular group of circuits.
.RT
.sp 1P
.LP
2.3.3.3
	\fBtransfer\(hyallowed\(hyacknowledgement signal\fR 
.sp 9p
.RT
.PP
A signal sent in response to the reception of a transfer\(hyallowed
signal.
.RT
.LP
.bp