Internet Standards/CD2.mdf/ccitt/1992/q/q723_1.asc

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All drawings contained in this Recommendation have been done in
Autocad.

Recommendation Q.723

FORMATS AND CODES

1    Basic format characteristics

1.1    General

    The telephone user messages are carried on the signalling data link by
means of signal units, the format of which is described in Recommendation
Q.703, º 2.2.

    The signalling information of each message constitutes the signalling
information field of the corresponding signal unit and consists of an integral
number of octets. It basically contains the label, the heading code and one or
more signals and/or indications. Structure and function of the label are
described in º 2; the heading codes and detailed message formats are
described in º 3.

1.2    The service     information octet

    The service information octet comprises the service indicator and the
subservice field.

    The service indicator is used to associate signalling information with
a particular User Part and is only used with message signal units (see Rec-
ommendation Q.704, º 12.2).

    The information in the subservice field permits a distinction to be
made between national and international signalling messages. In national
applications when this discrimination is not required possibly for certain
national User Parts only, the subservice field can be used independently for
different User Parts.

    The format of the service information octet is shown in Figure 1/
Q.723.

Figure 1/Q.723 - CCITT 35510



    The following codes are used in the fields of the service information octet:

a)    The service indicator is coded 0100.

b)    Subservice field.

bits    B    A    Spare (see Note)

bits    D    C    Network indicator

0    0    International network

0    1    Spare (for international use only)

1    0    National network

1    1    Reserved for national use

    Note ù The two unused bits in the service information octet are spare for
possible future needs that may require a common solution for all interna-
tional User Parts and Message Transfer Part level 3. The bits are coded 00.

1.3    Format principles

    The user generated information in the signalling information field is,
in general, divided into a number of subfields which may be either of fixed
or variable length. For a given message type identified by a unique message
heading, the presence of a given subfield may be either mandatory or
optional. The various types of subfields are further defined below.

1.3.1    Mandatory subfields

    Subfields which have been declared mandatory for a given message
type appear in all messages of that type.

1.3.2    Optional subfields

    Subfields which have been declared optional for a given message type
only appear when required in messages of that type. The presence or
absence of each optional field is indicated by the state of a field indicator
located in an indicator field, which in this case is a mandatory subfield.

1.3.3    Fixed length subfields

    Subfields which have been declared fixed length for a given message
type, contain the same number of bits in all messages of that type.

1.3.4    Variable length subfields

    For subfields which have been declared variable length for a given
message type, the number of bits may vary between messages of that type.
The size of a variable length subfield is indicated in an immediately preced-
ing fixed length subfield in terms of a predefined unit such as bits, octets or
halfùoctets.

1.3.5    Order of subfield transmission

    For a given type of message the various types of subfields are trans-
mitted in the following order:

a)    mandatory subfields,

b)    optional subfields.

    Within each of these two classes, the order of subfield transmission is, in
general, as follows:

1)    fixed length subfields (with the exception of the indicator field and
subfields indicating the size of a variable length subfield),

2)    variable length subfields.

1.3.6    Order of bit transmission

    Within each defined subfield the information is transmitted least sig-
nificant bit first.

1.3.7    Coding of spare bits

    Spare bits are coded 0 unless indicated otherwise.

2        Label

2.1    General

    The label is an item of information which forms part of every signal-
ling message and is used by the message routing function at Message Trans-
fer Part level 3 to select the appropriate signalling route and by the User Part
function to identify the particular transaction (e.g. the call) to which the
message pertains.

    In general, label information encompasses an explicit or implicit indi-
cation of the message source and destination and, depending on the applica-
tion, various forms of transaction identification.

    For messages which are related to circuits or calls, the transaction is
conveniently identified by including the corresponding circuit identity in the
label. This technique applies to messages which pass between adjacent
nodes, and to messages which pass between nodes which are not adjacent;
in this case the technique is known as the passùalong method. In future, the
introduction of new subscriber services may require the transfer of call
related messages between exchanges at a time when no circuit is associated
with the call. Such messages could be carried using the services of the Sig-
nalling Connection Control Part SCCP [6]. In this case the standard access
to the Signalling Connection Control Part is used.

    Note ù The service information octet, the routing label and the cir-
cuit identification code are not included in the information transferred
between the Telephone User Part and the Signalling Connection Control
Part.

    One standard label format is specified (º 2.2) for international use.
The same standard label is applicable for national use; admitted deviations
from the format of the standard label are described in º 2.3.

2.2        Standard telephone label

2.2.1    Label format

    The standard label has a length of 40 bits and is placed at the begin-
ning of the signalling information field. The label structure is as shown in
Figure 2/Q.723.

Figure 2/Q.723 - CCITT 35520



    The destination point code (DPC) indicates the signalling point for which
the message is intended, while the originating point code (OPC) indicates
the signalling point which is the source of the message. The circuit identifi-
cation code (CIC) indicates one speech circuit among those directly inter-
connecting the destination and the originating points.

    The portion of the label that consists of the destination point code and origi-
nating point code fields and of the four least significant bits of the circuit
identification code field corresponds to the standard routing label specified
in Recommendation Q.704, º 13.2.

2.2.2        Destination and originating point codes

    The standard label structure requires that each telephone exchange in
its role as signalling point is allocated a code from code plans established
for the purpose of unambiguous identification of signalling points.

    Separate code plans will be used for the international signalling net-
work and for different national signalling networks.

    The principles of code allocation which apply to the international sig-
nalling network should be in accordance with Recommendation Q.708.

    The destination point code will be the code applicable to the tele-
phone exchange to which the message is sent. The originating point code
will be the code applicable to the telephone exchange from which the mes-
sage is sent.

2.2.3        Circuit identification code

    The allocation of circuit identification codes to individual telephone
circuits is determined by bilateral agreement and/or in accordance with
applicable predetermined rules.

    Allocation rules for certain applications are defined below:

a)    2048 kbit/s digital path

    For circuits which are derived from a 2048ùkbit/s digital path (Rec-
ommendations G.732 [1] andG.734[2]) the circuit identification
code contains in the 5 least significant bits a binary representation
of the actual number of the time slot which is assigned to the
speech circuit. The remaining bits in the circuit identification code
are used where necessary, to identify one among several systems
interconnecting an originating and destination point.

b)    8448 kbit/s digital path

    For circuits which are derived from a 8448ùkbit/s digital path (Rec-
ommendation G.744 [3] andG.746[4]) the circuit identification
code contains in the 7 least significant bits an identification of the
channel which is assigned to the speech circuit. The codes in Table
1/Q.723 are used.

    The remaining bits are used, where necessary, to identify one among
several systems interconnecting an originating and destination
point.

c)    Frequency division multiplex (FDM) systems in networks using the
2048ùkbit/s pulse code modulation standard

    For FDM systems existing in networks that also use the 2048ùkbit/s
pulse code modulation standard, the circuit identification code
contains in the 6 least significant bits the identification of a chan-
nel within a group of 60 channels carried by 5 basic FDM groups
which may or may not be part of the same supergroup.

The codes in Table 2/Q.723 are used.



TABLE 1/Q.723



0000000

channel
1

0000001

channel
2

|

|

0011111

channel
32

0100000

channel
33

|

|

1111110

channel
1127

|

|

1111111

channel
128

TABLE 2/Q.723



000000

unallocat
ed

000001

channel
1

|

|

1st basic
(FDM) group

001100

channel
12

001101

channel
1

001110

channel
2

001111

channel
3

010000

unallocat
ed

2nd basic
(FDM) group

010001

channel
4

|

|

011001

channel
12

011010

channel
1

|

|

011111

channel
6

100000

unallocat
ed

3rd basic
(FDM) group

100001

channel
7

|

|

100110

channel
12

100111

channel
1

|

|

101111

channel
9

110000

unallocat
ed

4th basic
(FDM) group

110001

channel
10

110010

channel
11

110011

channel
12

110100

channel
1

|

|

5th basic
(FDM) group

111111

channel
12

2.3    Optional national labels

    For the purpose of satisfying the requirements imposed by specific
characteristics of some national signalling networks, field sizes different
from those specified for the standard label are admitted for the destination
point code, originating point code and circuit identification code fields in
national labels.

3        Telephone signal message formats and codes

3.1    General

    All telephone signal messages contain a heading consisting of two
parts, heading code H0 and heading code H1. Code H0 identifies a specific
message group (see Recommendation Q.722, º 3.2.1) while H1 either con-
tains a signal code or in case of more complex messages, identifies the for-
mat of these messages. The allocation of the H0 and H1 code is summarized
in Table 3/Q.723.

TABLE 3/Q.723

Heading code allocation



Me
s-
sag
e
gro
up

H1

H0

00
00

00
01

00
10

00
11

01
00

01
01

01
10

01
11

10
00

10
01

10
10

10
11

11
00

11
01

11
10

11
11



00
00

Spare, reserved for national use



FA
M

00
01

IA
M

IAI

SA
M

SA
O

FS
M

00
10

GS
M

CO
T

CC
F

BS
M

00
11

GR
Q

SB
M

01
00

AC
M

CH
G

UB
M

01
01

SE
C

CG
C

NN
C

AD
I

CF
L

SS
B

UN
N

LO
S

SS
T

AC
B

DP
N

MP
R

EU
M

CS
M

01
10

AN
U

AN
C

AN
N

CB
K

CL
F

RA
N

FO
T

CC
L

CC
M

01
11

RL
G

BL
O

BL
A

UB
L

UB
A

CC
R

RS
C

GR
M

10
00

M
GB

M
BA

M
GU

M
UA

HG
B

HB
A

HG
U

HU
A

GR
S

GR
A

SG
Ba)

SB
Aa
)

SG
Ua
)

SU
Aa
)



10
01

RESERVED

CN
M

10
10

AC
C

Spare reserved for international

10
11

and basic national use

11
00

11
01

11
10

Spare, reserved for national use

11
11

a)    National option.

Abbreviations used in Table 3/Q.723

ACB    Access barred signal

ACC    Automatic congestion control information message

ACM    Address complete message (note)

ADI    Address incomplete signal

ANC    Answer signal, charge

ANN    Answer signal, no charge

ANU    Answer signal, unqualified

BLA    Blockingùacknowledgement signal

BLO    Blocking signal

BSM    Backward setùup message

CBK    Clearùback signal

CCF    Continuityùfailure signal

CCL    Calling party clear signal

CCM    Circuit supervision message

CCR    Continuityùcheckùrequest signal

CFL    Callùfailure signal

CGC    Circuitùgroupùcongestion signal

CHG    Charging message

CLF    Clearùforward signal

CNM    Circuit network management message group

COT    Continuity signal

CSM    Call supervision message

DPN    Digital path not provided signal

EUM    Extended unsuccessful backward setùup information message

FAM    Forward address message

FOT    Forwardùtransfer signal

FSM    Forward setùup message

GRA    Circuit group resetùacknowledgement message

GRM    Circuit group supervision messages

GRQ    General request message

GRS    Circuit group reset message

GSM    General forward setùup information message

HBA    Hardware failure oriented group blockingùacknowledgement mes-
sage

HGB    Hardware failure oriented group blocking message

HGU    Hardware failure oriented group unblocking message

HUA    Hardware failure oriented group unblockingùacknowledgement mes-
sage

IAI    Initial address message with additional information

IAM    Initial address message

LOS    Lineùoutùofùservice signal

MBA    Maintenance oriented group blockingùacknowledgement message

MGB    Maintenance oriented group blocking message

MGU    Maintenance oriented group unblocking message

MPR    Misdialled trunk prefix

MUA    Maintenance oriented group unblockingùacknowledgement message

NNC    Nationalùnetworkùcongestion signal

RAN    Reanswer signal

RLG    Releaseùguard signal

RSC    Resetùcircuit signal

SAM    Subsequent address message

SAO    Subsequent address message with one signal

SBA    Software generated group blockingùacknowledgement message

SBM    Successful backward setùup information message

SEC    Switchingùequipmentùcongestion signal

SGB    Software generated group blocking message

SGU    Software generated group unblocking message

SSB    Subscriberùbusy signal (electrical)

SST    Sendùspecialùinformation tone signal

SUA    Software generated group unblockingùacknowledgement

UBA    Unblockingùacknowledgement signal

UBL    Unblocking signal

UBM    Unsuccessful backward setùup information message

UNN    Unallocatedùnumber signal

Note ù Each address complete message contains one of the following sig-
nals:

ù ADC    Addressùcomplete, charge

ù ADN    Addressùcomplete, no charge

ù ADX    Addressùcomplete, coin box

ù AFC    Addressùcomplete, charge subscriber free

ù AFN    Addressùcomplete, no charge, subscriber free

ù AFX    Addressùcomplete, coin box, subscriber free

3.2        Heading code H0

    The heading code H0 occupies the 4ùbit field following the label and
is coded as follows:



0000

    spare, reserved for national use

0001

    forward address messages

0010

    forward setùup messages

0011

    backward setùup request messages

0100

    successful backward setùup information messages

0101

    unsuccessful backward setùup information messages

0110

    call supervision messages

0111

    circuit supervision messages

1000

    circuit group supervision messages

1001

    reserved

1010

    circuit network management messages

1011

    reserved for international and basic national use

1100

ⁿ

to

²    reserved for national use

1111

■

3.3        Forward address messages

    The following types of forward address messages are specified and
are each identified by a different heading code H1:

ù    Initial address message.

ù    Initial address message with additional information.

ù    Subsequent address message (with one or more address signals).

ù    Subsequent address message with one (address) signal.

3.3.1        Initial address message

    The basic format of the initial address message is shown on Figure 3/
Q.723.

Figure 3/Q.723 - CCITT 35530



    The following codes are used in the fields of the initial address message.

a)    Label: see º 2

b)    Heading code H0 is coded 0001

c)    Heading code H1 is coded 0001

d)    Calling party category



bit
s

F

E

D

C

B

A

0

0

0

0

0

0

    unknown source (Note 1)

0

0

0

0

0

1

    operator, language French

0

0

0

0

1

0

    operator, language English

0

0

0

0

1

1

    operator, language German

0

0

0

1

0

0

    operator, language Russian

0

0

0

1

0

1

    operator, language Spanish

0

0

0

1

1

0

ⁿ

0

0

0

1

1

1

²    available to Administrations for select-
ing a particular language

0

0

1

0

0

0

■     provided by mutual agreement

0

0

1

0

0

1

    reserved (see Recommendation Q.104
[5]) (Note 2)

0

0

1

0

1

0

    ordinary calling subscriber

0

0

1

0

1

1

    calling subscriber with priority

0

0

1

1

0

0

    data call

0

0

1

1

0

1

    test call

0

0

1

1

1

0

    spare

0

0

1

1

1

1

    payphone

0

1

0

0

0

0

ⁿ





t
o

²    spare

1

1

1

1

1

1

■

    Note 1 ù The calling party category ôunknown sourceö is classified,
for the time being, for basic national use. The use of this category
in the international network is for further study.

    Note 2 ù In national networks, code 001001 may be used to indicate
that the calling party is a national operator.

e)    Spare

The bits in this field are spare for international allocation.

f)    Message indicators

bits    B    A:    nature of address indicator

0    0        subscriber number

0    1        spare, reserved for national use

1    0        national (significant) number

1    1        international number

bits    D    C:    natureùofùcircuit indicator

0    0        no satellite circuit in the connection

0    1        one satellite circuit in the connection

1    0        spare

1    1        spare

bits    F    E:    continuityùcheck indicator

0    0        continuityùcheck not required

0    1        continuityùcheck required on this circuit

1    0        continuityùcheck performed on a previous circuit

1    1        spare

bit    G:        echoùsuppressor indicator

0            outgoing half echo suppressor not included

1            outgoing half echo suppressor included

bit    H:        incoming international call indicator

0            call other than international incoming

1            incoming international call

bit    I:        redirected call indicator

0            not a redirected call

1            redirected call

bit    J:        allùdigitalùpathùrequired indicator

0            ordinary call

1            digital path required

bit    K:        signalling path indicator

0            any path

1            all signalling system No. 7 path

bit    L:        spare

    Note ù The spare indicator may be used, e.g., to provide the m/A law
conversion control, pending further study.

g)    Number of address signals

    A code expressing in pure binary representation the number of
address signals contained in the initial address message, except for
the code 0000 to which the meaning 16 digits including ST signal
is assigned.

h)    Address signals

0000    digit 0

0001    digit 1

0010    digit 2

0011    digit 3

0100    digit 4

0101    digit 5

0110    digit 6

0111    digit 7

1000    digit 8

1001    digit 9

1010    spare

1011    code 11

1100    code 12

1101    spare

1110    spare

1111    ST

    The most significant address signal is sent first. Subsequent address
signals are sent in successive 4ùbit fields.

i)    Filler

    In case of an odd number of address signals, the filler code 0000 is
inserted after the last address signal. This ensures that the variable
length field which contains the address signals consists of an inte-
gral number of octets.

3.3.2    Initial address message with additional information

    The basic format of the initial address message with additional infor-
mation is shown in Figure 4/Q.723.

Figure 4/Q.723 - CCITT 35541



    The following codes are used in the initial address message with additional
information:

a)    Label: see º 2

b)    Heading code H0 is coded 0001

c)    Heading code H1 is coded 0010

d)    Calling party category: [see º 3.3.1 d)]

e)    Message indicators: [see º 3.3.1 f)]

f)    Number of address signals: [see º 3.3.1 g)]

g)    Address signals: [see º 3.3.1 h)]

h)    First indicator octet

bit    A:    network capability or user facility information indicator

0    network capability or user facility information not included

1    network capability or user facility information included

bit    B:    closed user group information indicator

0    closed user group information not included

1    closed user group information included

bit    C:    additional calling party information indicator

0    additional calling party information not included

1    additional calling party information included

bit    D:    additional routing information indicator

0    additional routing information not included

1    additional routing information included

bit    E:    calling line identity indicator

0    calling line identity not included

1    calling line identity included

bit    F:    original called address indicator

0    original called address not included

1    original called address included

bit    G:    charging information indicator

0    charging information not included

1    charging information included

bit    H:    spare, reserved for indicating the presence or absence of a
second indicator octet

i)    Network capability or user facility information: spare, reserved for
national use. (This optional field may be used in national applica-
tions to indicate specific network capabilities and/or user facility
information.)

j)    Closed user group (CUG) information

    The basic format of the closed user group information field is shown in Fig-
ure 4a/Q.723.





DCBA

Interlock
 code

Spare

CUG
indicator

32

4

4

FIGURE 4a/Q.723

Closed user group information field

    The following codes are used in the subfields of the closed user group infor-
mation field.

ù    bits    B    A:    CUG call indicator

0    0    ordinary call

0    1    successful check

1    0    outgoing access allowed

1    1    outgoing access not allowed

ù    bits    C    D:    spare

ù    Interlock code

    A code identifying the closed user group involved in the call. The
nature of this code is for further study.

k)    Additional calling party information: for further study. (This
optional field is of fixed length and will indicate additional infor-
mation concerning the calling party, which is not carried by the
calling party's category indicator.)

l)    Additional routing information: for further study. (This optional
field is of fixed length and will indicate that the call has to be
routed in some particular way, due for example to additional cus-
tomer services.)

m)    Calling line identity

    The basic format of the calling line identity field is shown in Figure 4b/
Q.723.





DCBA

 DCBA

Calling
line iden-
tity

Number
of
address
signals

Address
indicator

n ╫ 8

4

4

FIGURE 4b/Q.723

Calling line identity field

    The following codes are used in the subfields of the calling line identity
field.

ù    Address indicators:

bits    B    A:    nature of address indicator

0    0    subscriber number

0    1    spare, reserved for national use

1    0    national significant number

1    1    international number

bit    C:        calling line identity presentation indicator

0        calling line identity presentation not restricted

1        calling line identity presentation restricted

bit    D:        incomplete calling line identity indicator

0        no indication

1        incomplete calling line identity

ù    Number of address signals

bits    D    C    B    A

0    0    0    0        calling line identity not available indicator

0    0    0    1    ⁿ

to                ²    a code expressing in pure binary representation the num-
ber of address

1    1    1    1    ■    signals.

ù    Calling line address signals

Each signal is coded as indicated in º 3.3.1 h) as applicable.

n)    Original called address

The basic format of the original called address field is shown in
Figure 4c/Q.723.





DCBA

DCBA

Original
called
address

Number
of
address
signals

Address
indicator
s

n ╫ 8

4

4

FIGURE 4c/Q.723

Original called address field

    The following codes are used in the subfields of the original address field:

ù    Address indicator

bits    B    A:    nature of address indicator

0    0    subscriber number

0    1    spare, reserved for national use

1    0    national (significant) number

1    1    international number

bits    D    C:    spare

ù    Number of address signals

bits    D    C    B    A

0    0    0    0        original called address not available

0    0    0    1    ⁿ

to                ²    a code expressing in pure binary representation the num-
ber of address

1    1    1    1    ■    signals.

ù    Original called address signals

Each signal is coded as indicated in º 3.3.1 h) as applicable.

o)    Charging information: for further study. (This optional field will
contain information to be sent to a successive exchange for charg-
ing and/or accounting purposes.)

3.3.3        Subsequent address message

    The basic format of the subsequent address message (SAM) is shown
in Figure 5/Q.723.

Figure 5/Q.723 - CCITT 35550



    The following codes are used in the fields of the subsequent address mes-
sage:

a)    Label: see º 2

b)    Heading code H0 is coded 0001

c)    Heading code H1 is coded 0011

d)    Address signal is coded as indicated in º 3.3.1 h) as applicable

e)    Number of address signals: a code expressing in pure binary repre-
sentation the number of address signals contained in the subse-
quent address message.

3.3.4    Subsequent address message with one signal

    The basic format of the subsequent address message with one signal is
shown in Figure 6/Q.723.

Figure 6/Q.723 - CCITT 35560



    The following codes are used in the fields of the subsequent address mes-
sage with one signal:

a)    Label: see º 2

b)    Heading code H0 is coded 0001

c)    Heading code H1 is coded 0100

d)    Address signal is coded as indicated in º 3.3.1 h) as applicable.

3.4        Forward setùup messages

    The following types of forward setùup messages are specified and
are each identified by a different heading code H1:

ù    general forward setùup information message,

ù    continuityùcheck message.

    Unallocated H1 codes in this message group are spare.

3.4.1    General forward setùup information message

    The basic format of the general forward setùup information message
is shown in Figure 7/Q.723.

Figure 7/Q.723 - CCITT 85940



    The following codes are used in the fields of the general forward setùup
information message:

a)    Label: see º 2

b)    Heading code H0 is coded 0010

c)    Heading code H1 is coded 0001

d)    Response type indicator

bit    A:    calling party category indicator

0    calling party category not included

1    calling party category included

bit    B:    calling line identity indicator

0    calling line identity not included

1    calling line identity included

bit    C:    incoming trunk and transit exchange: identity indicator

0    incoming trunk and transit exchange identity not included

1    incoming trunk and transit exchange identity included

bit    D:    original called address indicator

0    original called address not included

1    original called address included

bit    E:    outgoing echo suppressor indicator

0:    outgoing half echo suppressor not included

1:    outgoing half echo suppressor included

bit    F:    malicious call identification indicator

0    malicious call identification not provided

1    malicious call identification provided

bit    G:    hold indicator

0    hold not provided

1    hold provided

bit    H:    spare

e)    Calling party category:



bit
s

F

E

D

C

B

A

0

0

0

0

0

0

unknown source/calling party category
unavailable indicator

0

0

0

0

0

1

ⁿ





t
o

² (see º 3.3.1 d))

1

1

1

1

1

1

■

f)    Calling line identity:

    Format and codes are the same as used in the calling line identity con-
tained in the initial address message with additional information
(see º 3.3.2).

g)    Incoming trunk and transit exchange identity:

The basic format of the incoming trunk and transit exchange iden-
tity field is shown in Figure 8/Q.723.





DCBA

DCBA

DCBA

Incomin
g trunk
identity

Field
length
indicator

Spare

Transit
exchange
 identity

Exchang
e iden-
tity
length
indicator

Identity
type
indicator

n ╫ 8

4

4

n ╫ 8

4

4

FIGURE 8/Q.723

Incoming trunk and transit exchange identity field

    The following codes are used in the subfields of the incoming trunk and
transit exchange identity field:

ù    Identity type indicator

bits    B    A:

0    0    spare

0    1    signalling point code

1    0    available part of calling line identity

1    1    spare

bits    D    C:    spare

ù    Exchange identity length indicator

    A code expressing in pure binary representation the number of
address signals included in the transit exchange identity subfield
for the case when part of the calling line identity is used for this
purpose.

    When the transit exchange is identified by the signalling point code,
this subfield is coded 0000.

ù    Transit exchange identity

A code consisting of either:

i)    the signalling point code of the exchange, or

ii)    a part of the calling line identity, in which case each address
digit contained in this identity is coded as indicated in º 3.3.1 h)
where applicable.

ù    Field length indicator

    A code indicating in pure binary representation the number of octets
in the incoming trunk identity field.

Code 0000 indicates that the incoming trunk identity is not pro-
vided.

ù    Incoming trunk identity

    A code contained in a maximum of 15 octets, identifying the incom-
ing trunk. The encoding of the incoming trunk identity is for fur-
ther study.

h)    Original called address

See º 3.3.2 n).

3.4.2        Continuityùcheck message

    The basic format of the continuityùcheck message is shown in Fig-
ure 9/Q.723.

Figure 9/Q.723 - CCITT 35570



    The following codes are used in the fields of the continuityùcheck mes-
sage:

a)    Label: see º 2

b)    Heading code H0 is coded 0010

c)    Heading code H1 contains signal codes as follows:

0011 continuity signal

0100 continuityùfailure signal