3.5	Backward set–up request message
	The following type of backward set–up request message is specified and is identified by one of the heading codes H1. The other H1 codes in this message group are spare.
3.5.1	General request message
	The basic format of the general request message is shown in Figure 10/Q.723.
Figure 10/Q.723 - CCITT 35570

	The following codes are used in the fields of the general request message:
a)	Label: see § 2
b)	Heading code H0 is coded 0011
c)	Heading code H1 is coded 0001
d)	Request type indicators
bit	A:	calling party category request indicator
0	no calling party category request
1	calling party category request
bit	B:	calling line identity request indicator
0	no calling line identity request
1	calling line identity request
bit	C:	original called address request
0	no original called address request
1	original called address request
bit	D:	malicious call identification indicator (national option)
0	no malicious call identification encountered
1	malicious call identification encountered
bit	E:	hold request indicator
0	hold not requested
1	hold requested
bit	F:	echo suppressor request indicator
0	no outgoing half echo suppressor requested
1	outgoing half echo suppressor requested
bit	GH:	spare
3.6	Successful backward set–up information messages
	The following types of successful backward set–up information messages are specified and are each identified by a different heading code H1:
–	address–complete message
–	charging message.
3.6.1	Address–complete message
	The basic format of the address–complete message is shown in Figure 11/Q.723.
Figure 11/Q.723 - CCITT 35580

	The following codes are used in the fields of the address–complete message:
a)	Label: see § 2
b)	Heading code H0 is coded 0100
c)	Heading code H1 is coded 0001
d)	Message indicators
bits	B	A:	type of address–complete signal indicators
0	0	address–complete signal
0	1	address–complete signal, charge
1	0	address–complete signal, no charge
1	1	address–complete signal, payphone
bit	C:		subscriber–free indicator
0		no indication
1		subscriber–free
bit	D:		incoming echo suppressor indicator
0		no incoming half echo suppressor included
1		incoming half echo suppressor included
bit	E:		call forwarding indicator
0		call not forwarded
1		call forwarded
bit	F:		signalling path indicator
0		any path
1		all signalling system No. 7 path
bits	G	H:	spare, for national use (may be used to indicate call redirection, holding of the connection or the end–to–end signalling method to be used).
	Note – The address–complete signal without qualification is classified for the time being in the basic national category of signals. The use of this signal in the international network is for further study.
3.6.2	Charging message (see Note)
	The basic format of the charging message is shown in Figure 12/Q.723.
Figure 12/Q.723 - CCITT 35585

	The following codes are used in the fields of the charging message:
a)	Label: see § 2
b)	Heading code H0 is coded 0100
c)	Heading code H1 is coded 0010
d)	Charging information
	(Possible formats and codes of the charging information field are shown in Annex A.)
	Note – The charging message is classified, for the time being, in the basic national category of messages. The use of this message in the international network is for further study.
3.7	Unsuccessful backward set–up information messages
3.7.1	Simple unsuccessful backward set–up information message
	The basic format of the simple unsuccessful backward set–up information message is shown in Figure 13/Q.723.
Figure 13/Q.723 - CCITT 35570

	The following codes are used in the fields of the simple unsuccessful backward set–up information message.
a)	Label: see § 2
b)	Heading code H0 is coded 0101

c)	Heading code H1 contains signal codes as follows:
0000		spare
0001		switching–equipment–congestion signal
0010		circuit–group–congestion signal
0011		national–network–congestion signal
0100		address–incomplete signal
0101		call–failure signal
0110		subscriber–busy signal (electrical)
0111		unallocated–number signal
1000		line–out–of–service signal
1001		send–special–information–tone signal
1010		access barred signal
1011		digital path not provided signal
1100		misdialled trunk prefix signal (for national use)
1101	ü
 to		ý	spare
1110	þ
3.7.2	Extended unsuccessful backward set–up information message
	The basic format of the extended unsuccessful backward set–up information message is shown in Figure 13a/Q.723.
Figure 13a/Q.723 - CCITT 85960

	The following codes are used in the fields of the extended unsuccessful backward set–up information message:
a)	Label: see § 2
b)	Heading code H0 is coded 0101
c)	Heading code H1 contains signal code 1111
d)	Octet indicator
bits	D	C	B	A	unsuccessful indicator	
	0	0	0	0	spare	
	0	0	0	1	subscriber busy	
	0	0	1	0	ü	
	to				ý spare	
	1	1	1	1	þ	
	G	H	F	E:	spare	
e)	Signalling point code
The point code of the signalling point in which the message is originated.
3.8	Call supervision message
	The basic format of the call supervision message is shown in Figure 14/Q.723.
Figure 14/Q.723 - CCITT 35570

	The following codes are used in the fields of the call supervision message:
a)	Label: see § 2
b)	Heading code H0 is coded 0110
c)	Heading code H1 contains signal codes as follows:
0000		answer signal, unqualified
0001		answer signal, charge
0010		answer signal, no charge
0011		clear–back signal
0100		clear–forward signal
0101		re–answer signal
0110		forward–transfer signal
0111		calling party clear signal (national option)
1000	ü
to		ý	 spare
1111	þ
3.9	Circuit supervision message
	The basic format of the circuit supervision message is shown in Figure 15/Q.723.
Figure 15/Q.723 - CCITT 35570

	The following codes are used in the fields of the circuit supervision message:
a)	Label: see § 2
b)	Heading code H0 is coded 0111
c)	Heading code H1 contains signal codes as follows:
0000	spare
0001	release–guard signal
0010	blocking signal
0011	blocking–acknowledgement signal
0100	unblocking signal
0101	unblocking–acknowledgement signal
0110	continuity–check–request signal
0111	reset–circuit signal
1000	ü
to		ý spare
1111	þ
3.10	Circuit group supervision message
	The basic format of the circuit group supervision message is shown in Figure 16/Q.723:
Figure 16/Q.723 - CCITT 85970

	The following codes are used in the fields of the circuit group supervision message:
a)	Label: see § 2
The following interpretations apply to the CIC given in the label:
i)	If the range field is not coded all zero the CIC given in the label is the first CIC within the circuit group or the first CIC within that part of the circuit group.
ii)	If the range field is coded all zero (national option) the CIC given in the label is a representative CIC within the circuit group.
b)	Heading code H0 is coded 1000
c)	Heading code H1 contains message codes as follows:
0000	spare
0001	Maintenance oriented group blocking message
0010	Maintenance oriented group blocking–acknowledging message
0011	Maintenance oriented group unblocking message
0100	Maintenance oriented group unblocking–acknowledgement message
0101	Hardware failure oriented group blocking message
0110	Hardware failure oriented group blocking–acknowledge message
0111	Hardware failure oriented group unblocking message
1000	Hardware failure oriented group unblocking–acknowledgement message
1001	Circuit group reset message
1010	Circuit group reset–acknowledgement message
1011	Software generated group blocking message (national option)
1100	Software generated group blocking–acknowledgement message (national option)
1101	Software generated group unblocking message (national option)
1110	Software generated group unblocking–acknowledgement message (national option)
1111	spare
d)	Range: in principle, two different codings are possible:
i)	not all zero: The message is related to a whole circuit group or a part thereof, and includes a status field unless the message is the circuit group reset message. The number of consecutive circuits to be handled is indicated by the value contained in the range field increased by 1. The CIC of the first circuit to be handled is given in the label. The number of circuits to be indicated is 2 (range value 1) to 256 (range value 255).
ii)	all zero1) (national option): The message is related to a pre–determined circuit group. No status field is included. In this case the circuit group is addressed by means of a representative CIC within the circuit group.
	Note – In national networks, the range field may not be used if only the concept of pre–determined circuit group applies.
e)	Status field
	All circuit group supervision messages except the circuit group reset message include a status field containing status indicator bits when the range field is not coded all zero. The number of status indicator bits is indicated by the value given in the range field increased by one.
	The status field contains up to 256 one bit status indicators. The first status indicator bit is related to the circuit indicated by the CIC contained within the label, the second one is related to the circuit address by the CIC contained in the label increased by 1.
Figure 17/Q.723 - CCITT 73570

	The CIC of the last circuit concerned is obtained by adding the value given in the range field to the CIC in the label. The status field consists of an integral number of octets. Bits within the last octet that are not used as status indicators are filled with zeros.
	The status indicator bits are coded as follows:
–	in all group blocking messages (MGB, HGB, SGB)
1	blocking
0	no blocking

–	in all group blocking–acknowledgement messages (MGB, HBA, SBA)
1	blocking acknowledgement
0	no blocking acknowledgement
–	in all unblocking messages (MGU, HGU, SGU)
1	unblocking
0	no unblocking
–	in all group unblocking–acknowledgement messages (MUA, HUA, SUA)
1	unblocking acknowledgement
0	no unblocking acknowledgement
–	in the circuit group reset–acknowledgement message (GRA)
1	blocking for maintenance reasons
0	no blocking for maintenance reasons
3.11	Circuit network management messages
	The following type of circuit network management message is specified and identified by one of the heading codes H1. Unallocated H1 codes in this message group are spare.
3.11.1	Automatic congestion control information message
	The basic format of the automatic congestion control (ACC) information message is shown in Figure 18/Q.723:
Figure 18/Q.723 - T1109900-88

	The following codes are used in the fields of the automatic congestion control information message.
a)	Label: see § 2
b)	Heading code H0 is coded 1001
c)	Heading code H1 is coded 0001
d)	Message indicators
bits	B	A	ACC information	
	0	0	spare	
	0	1	congestion level 1	
	1	0	congestion level 2	
	1	1	spare	
bits	HGFEDC	spare
ANNEX A
(to Recommendation Q.723)
Charging messages
A.1	Introduction
	The application of Signalling System No. 7 in national networks was recognized from the beginning of the discussions about the signalling system. The result of this can be found throughout the specifications especially in those Recommendations dealing with the TUP. One of the points which is particularly of interest for an Administration is the possibility of transfer of charging information. Signalling System No. 7 allows for such a feature for charging a calling subscriber by defining a specific charging message as indicated in § 3.6.2. However, the detailed format, coding and related procedures are not given, mostly because this matter is very dependent on the circumstances within a specific national network. The following examples illustrate a particular implementation in a national network for telephony without exclusion of other possible solutions.
A.2	Starting points
	Before describing in detail the messages involved, a number of starting points have to be adopted.
a)	The first No. 7 exchange performs metering according to all possible tariffs.
b)	The determination of a particular tariff is performed in a point somewhere in the network.
c)	The receipt of messages containing charging information should be acknowledged within the call control procedures.
d)	At dedicated moments the actual charging should be adapted.
e)	A variety of charging possibilities should be available.
	The effect of these starting points is:
a)	the actual generation of charging units according to a particular tariff is always performed at the lowest level of the national public telephone network (local exchange);
b)	the determination of tariffs for local and trunk calls is carried out in the local exchange and for international calls in the international exchange; however, also the use of a centre for determination of all kinds of tariffs is possible;
c)	the transmission of charging information is assured at the highest level of the call control procedures and possibly inhibits call completion without receipt of charging information;
d)	calls of long duration can be subject to different charging rates;
e)	the application of charge free calls, specific charge on answer, time dependent charging during a call, additional (specific) charge during a call and a combination of these.
A.3	Messages and procedures
	To meet all the above mentioned requirements a number of messages are defined.
A.3.1	Charging message
	This message has to be sent for any call, charge free or not. In the procedure this is covered by the fact that the charging message has to be received during call set up before receipt of the address complete message.
	If not, then the call should be cleared immediately.
	The content of the message will vary depending on the actual tariff and this is indicated by a number of indicators indicating the presence of certain fields in the message.
	Possible contents:
a)	charge band
	The indication of a certain charge band should allow the receiving exchange to charge a call according to a certain tariff including possible switchover times to higher or lower rates. This method results in a simple message but requires the receiving exchange to have all the information available related to all possible charge bands, national and international.

b)	explicit charging indication
In this case the message contains explicit indications of details of the tariff viz.
–	number of charging units on answer (packet)
–	time dependent tariff(s)
–	possible switchover time.
	This method results in a more complex message but does not require the permanent storage of any charging information.
A.3.2	Change message
	A consequence of the adoption of the method with explicit charging indication (§ A.3.1 b)) is the necessity to allow for tariff switchover for calls of very long duration or for calls which are answered just after the switchover time given in the message described in § A.3.1 b). The content of such at message is rather simple because it only contains the new applicable tariff and the actual switch–over time.
	The procedure to acknowledge the receipt of the message cannot be found in the normal call control procedure, therefore an acknowledgement message (see § A.3.5) in the forward direction is used. If this acknowledgement message is not received within a certain time, the change message has to be repeated.
A.3.3	Collection charging
	For a variety of reasons it might be necessary to charge a subscriber during the call a certain amount. For this purpose a message is used indicating the number of charging units related to the amount for which the subscriber has to be charged.
	The procedure to assure the receipt of this message is the same as described in § A.3.2 above. A possible further collection charging message should not be sent before receipt of the acknowledgement message and the charging confirmation message (see § A.3.4 charging confirmation).
A.3.4	Charging confirmation
	In relation with the message described in § A.3.3 a message in the forward direction is required indicating how many charging units actually are charged to the subscriber. This number should match to the number given in the collection charging message, otherwise it must be concluded that for some reason the order is not executed, e.g., a certain service should now be withheld to be furnished to the subscriber.
	Again the procedure is the one as described in § A.3.2 above but in the opposite direction.
A.3.5	Acknowledgement
	To acknowledge the receipt of the messages described in §§ A.3.2, A.3.3 and A.3.4, an acknowledgement message is used in both directions only indicating the receipt of the related message.
A.4	Formats and codes
A.4.1	Charging messages
A.4.1.1	Charge band
TABLEAU CCITT 86010

–	Charge band
	A charge indicates the combination of tariffs including switch–over times which is applicable for a certain period (e.g., day or week).
A.4.1.2	Explicit charging indication
tableau ccitt 86020

–	Message indicators
bit	A:		tariff indicator current tariff (A)
0		packet charging field and tariff indicators current tariff (A) not present
1		packet charging field and tariff indicators current tariff (A) present
bit	B:		tariff factor current tariff (A)
0		tariff factor field current tariff (A) not present
1		tariff factor field current tariff (A) present
bit	C:		tariff indicator next tariff (B)
0		packet charging field and tariff indicators next tariff (B) not present
1		packet charging field and tariff indicators next tariff (B) present
bit	D:		tariff factor next tariff (B)
0		tariff factor field next tariff (B) not present
1		tariff factor field next tariff (B) present
bit	H–E	spare
–	Packet charging field
0000
    |			number of charging units on answer
1111
–	Tariff indicators
0000		tariff scale 0	(no time dependent tariff)
0001		tariff scale I
   |				   |	every scale indicates a certain step in seconds or parts thereof
1111		tariff scale XV
–	Tariff factors
If a call is charge free (A = B = C = D = 0) only the message indicator octet is present.
	If a call is charge free from the start but may become chargeable (A = 1, B = 0, C = 1, D = 0/1), the packet charging field for the current tariff is 0000 and the tariff indicator for the current tariff indicates scale 0.
	If a call is chargeable from the start but may become charge free (A = 1, B = 0/1, C = 1, D = 0) the packet charging field for the next tariff is 0000 and the tariff indicator for the next tariff indicates scale 0. If a call is chargeable according to only one tariff (A = 1, B = 0/1, C = 0, D = 0), also the time indicator is not present in the message. The actual tariff is determined by multiplication of the step indicated by the tariff indicator with the tariff factor which gives then a specific charging unit interval in seconds.
–	Time indicator
000000	spare
000001	00.30 h
000010	01.00 h
     |		    |
     |		    |
110000	24.00 h
A.4.2	Tariff change message
Tableau ccitt 86030

	Message indicator
Bit	A:		tariff factor next tariff
0		tariff factor field next tariff not present
1		tariff factor field next tariff present
Bits	D–B:	spare
–	Tariff indicator, tariff factor and time indicator: see § A.4.1.2
A.4.3	Collection charging message
tableau ccitt 86040

	The collection field contains the number of charging units which are to be charged to the calling subscriber. The field has a length of 8 bits so a maximum of 256 units is possible.
A.4.4	Charging confirmation message
tableau ccitt 86040

–	Heading code H1
H1 = 0101	confirmation of packet charging
H1 = 0110	confirmation of collection charging
–	Charging unit field
Number of charging units which actually are charged to the calling party
A.4.5	Acknowledgement message
tableau ccitt 86050

–	Heading code H1
H1 = 1000	acknowledgement receipt of tariff review, collection charging or charging confirmation message
References
[1]	CCITT Recommendation Characteristics of primary PCM multiplex equipment operating at 2048 kbit/s, Rec. G.732.
[2]	CCITT Recommendation Characteristics of 2048–kbit/s frame structure for use with digital exchanges, Vol. III, Rec. G.734.
[3]	CCITT Recommendation Second order PCM multiplex equipment operating at 8448 kbit/s, Rec. G.744.
[4]	CCITT Recommendation Characteristics of 8448–kbit/s frame structure for use with digital exchanges, Rec. G.746.
[5]	CCITT Recommendation Language digit or discriminating digits, Rec. Q.104.
[6]	CCITT Recommendation Signalling Connection Control Part, Recs. Q.711–Q.714.