.rs
.\" Troff code generated by TPS Convert from ITU Original Files
.\"                 Not Copyright ( c) 1991 
.\"
.\" Assumes tbl, eqn, MS macros, and lots of luck.
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.ds CH
.ds CF
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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'
.LP
\fBMONTAGE:\ \ 
FIN DE LA RECOMMANDATION I.515 EN\(hyT\* | TE DE CETTE PAGE\fR 
.sp 2P
.LP
\v'30P'
\fBRecommendation\ I.520\fR 
.RT
.sp 2P
.ce 1000
\fBGENERAL\ ARRANGEMENTS\ FOR\ NETWORK\ INTERWORKING\fR 
.EF '%	Fascicle\ III.9\ \(em\ Rec.\ I.520''
.OF '''Fascicle\ III.9\ \(em\ Rec.\ I.520	%'
.ce 0
.sp 1P
.ce 1000
\fBBETWEEN\ ISDNs\fR 
.ce 0
.sp 1P
.ce 1000
\fI(Melbourne, 1988)\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fB1\fR 	\fBIntroduction\fR 
.sp 1P
.RT
.PP
The number of ISDNs existing in the world is increasing and more
than one ISDN may exist even within a single country. Therefore, ISDN\(hyISDN
network interfaces should be standardized to facilitate the interworking
between ISDNs and to extend connectivity world\(hywide.
.RT
.sp 2P
.LP
\fB2\fR 	\fBScope\fR 
.sp 1P
.RT
.PP
The purposes of this Recommendation are:
.RT
.LP
	1)
	to identify the general arrangements for ISDN\(hyISDN
interworking, and
.LP
	2)
	to define the functions and other requirements for the
ISDN\(hyISDN interface.
.PP
Recommendation I.324 defines the reference point between two
interconnected ISDNs
to be the N\dx\ureference point. This Recommendation (I.520) identifies 
other Recommendations which should be applied to the N\dx\ureference point 
and 
clarifies the functions and requirements for interworking at the N\dx\ureference 
point. 
.bp
.sp 2P
.LP
\fB3\fR 	\fBRequired information and information handling\fR 
.sp 1P
.RT
.PP
Figure 1/I.520 illustrates the general configuration
for interworking between
two ISDNs. The information given in Tables\ 1/I.520,
2/I.520 and 3/I.520, when required, has to be
carried by Signalling System\ No.\ 7 (SS\ No.\ 7)
ISUP and X.75, and is handled at
the IWF in one of the following ways:
.RT
.LP
	i)
	information is terminated at the IWF and is not transferred  to other ISDNs;
.LP
	ii)
	information is interpreted at the IWF and is transferred to other ISDNs;
.LP
	iii)
	information is transferred through the IWF transparently;
.LP
	iv)
	information is newly generated at the IWF.
.PP
Tables 1/I.520, 2/I.520 and 3/I.520 also show the classification of information 
into the above four categories for circuit mode bearer services, circuit 
mode supplementary services and packet mode bearer services 
respectively.
.PP
Additional information required specifically for OAM (Operational,
Administrative and Maintenance) functions is for further study.
.RT
.LP
.rs
.sp 31P
.ad r
\fBFigure 1/I.520, p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T1.520]\fR 
.ce
TABLE\ 1/I.520
.ce
\fBInformation required for IWF between ISDNs for circuit
.ce
mode bearer services\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(36p) | cw(68p) | cw(62p) | cw(62p) .
Category	Required information	Q.931 information element	Q.763 parameter name
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
i	 {
First transit network subsequent to IWF
 }	Transit network selection	Transit network selection
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Called party number  (Note 1)	Called party number/Key pad	 {
Called party number/Subsequent number
 }
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	 {
Calling party's category
(Note 2)
 }	(Unnecessary)	Calling party's category
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Bearer capability	Bearer capability	 {
Transmission medium requirements
 }
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
ii			User service information
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Call indicators  (Note 3)	(Unnecessary)	 {
Forward call indicators
Backward call indicators
 }
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Use of satellite  (Note 4)	(Unnecessary)	 {
Nature of connection indicators
 }
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Calling party number	Calling party number	Calling party number
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Subaddress	Subaddress	Access transport
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Calling party's category	(Unnecessary)	Calling party's category
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
iii  (Note 8)	 {
Terminal compatibility
(Note 5)
 }	 {
Low layer compatibility
High layer compatibility
 }	Access transport
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	User\(hyto\(hyuser signalling	 {
User\(hyto\(hyuser information element
 }	 {
User\(hyto\(hyuser information
 }
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Cause	Cause	Cause indicator
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Charge	(Unnecessary)	Charge information
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Cause for interworking	Cause	Cause indicator
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
iv	 {
Charging information
(Note 6)
 }	(Unnecessary)	Charge information
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Change of services  (Note 7)	(Should be defined)	(Should be defined)
.TE
.LP
\fINote\ 1\fR
\ \(em\ For charging use.
.LP
\fINote\ 2\fR
\ \(em\ For discrimination of priority call/ordinary call.
.LP
\fINote\ 3\fR
\ \(em\ These indicators are used to identify:
.LP
1)
international incoming call,
.LP
2)
available end\(hyto\(hyend signalling system,
.LP
3)
charged call/noncharged call.
.LP
\fINote\ 4\fR
\ \(em\ When a satellite circuit is employed for an interworking call at the interworking point, this information is processed at the IWF. If a satellite
circuit is not employed for a call, this information is transferred through the IWF transparently.
.LP
\fINote\ 5\fR
\ \(em\ There may be cases where the terminal compatibility information
is processed (see \(sc\ 5.4).
.LP
\fINote\ 6\fR
\ \(em\ This information is used only when access charging is necessary.
.LP
\fINote\ 7\fR
\ \(em\ All ISDNs do not necessarily provide identical services (or
connection types). When a change of services occurs at the IWF, the network
should send the indication for change of services and may solicit acceptance of change of services to a calling user in certain cases (see \(sc\ 5.3.1 of this
Recommendation).
.LP
\fINote\ 8\fR
\ \(em\ The information in this category is transferred through the IWF
transparently.
.nr PS 9
.RT
.ad r
\fBTableau 1/I.520 [T1.520], p. 2\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.ce
\fBH.T. [T2.520]\fR 
.ce
TABLE\ 2/I.520
.ce
\fBInformation required for IWF between ISDNs for circuit
.ce
mode supplementary services\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(36p) | cw(68p) | cw(62p) | cw(62p) .
Category	Required information	Q.931 information element	Q.763 parameter name
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
ii	Supplementary service request	 {
Network specific facility
Key pad facility
Feature activation
Feature indication
 }	(Should be defined)
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Progress indicator	Progress indicator	Access transport
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
iii	Suspend/Resume indicator	Notification indicator	Suspend/Resume indicator
_
.TE
.nr PS 9
.RT
.ad r
\fBTableau 2/I.520 [T2.520], p. 3\fR 
.ad b
.RT
.ce
\fBH.T. [T3.520]\fR 
.ce
TABLE\ 3/I.520
.ce
\fBInformation required for IWF between ISDNs for packet mode
.ce
\fBbearer services (in\(hyband signalling)\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(36p) | cw(68p) | cw(62p) | cw(62p) .
Category	Required information	Rec. X.25 information	Rec. X.75 information
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
i	 {
Transit network identification
 }	RPOA selection	 {
Transit network identification
 }
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Packet type	Packet type identifier	Packet type identifier
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Logical channel number	Logical channel number	Logical channel number
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Called party number	Called DTE address	Called DTE address
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Throughput class	Throughput class negotiation	Throughput class indication
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
ii	Window size	 {
Flow control parameter negotiation
 }	Window size indication
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Packet size	 {
Flow control parameter negotiation
 }	Packet size indication
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Call identifier	(Unnecessary)	Call identifier
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Transit delay selection	 {
Transit delay indication/selection
 }	Transit delay indication
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	 {
User\(hyto\(hyuser information
 }	Fast select identifier	Fast select indication
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Calling party number	Calling DTE address	Calling DTE address
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Terminal compatibility	(Call user data)	(Should be defined)
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
iii	Subaddress	 {
Calling address extension
Called address extension
 }	 {
Calling address extension
Called address extension
 }
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Cause	Diagnostic code	Diagnostic code
_
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
	Cause for interworking	(Should be defined)	(Should be defined)
.T&
cw(36p) | lw(68p) | lw(62p) | lw(62p) .
iv	Charging	Charging information	(Should be defined)
.TE
.LP
\fINote\fR
\ \(em\ The relationship between X.25 facilities and ISDN supplementary
services is for further study.
.nr PS 9
.RT
.ad r
\fBTableau 3/I.520 [T3.520], p. 4\fR 
.ad b
.RT
.LP
.bp
.sp 2P
.LP
\fB4\fR 	\fBDescription of \fR \fBISDN\(hyISDN interworking configurations\fR 
.sp 1P
.RT
.sp 1P
.LP
4.1
	 \fIISDN\(hyISDN interface where circuit mode bearer services are provided\fR 
\fIby both ISDNs\fR 
.sp 9p
.RT
.PP
See Figure 2/I.520.
.RT
.LP
.rs
.sp 8P
.ad r
\fBFigure 2/I.520, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.1.1
	\fIBearer services\fR 
.sp 9p
.RT
.PP
Individual bearer service categories are defined in the
I.230\(hySeries of Recommendations.
.PP
Layer 1 interworking specifications are recommended in
Recommendation\ I.511. Layers\ 2 and 3 in the U\(hyplane are passed
transparently.
.RT
.sp 2P
.LP
4.1.2
	\fISupplementary services\fR 
.sp 1P
.RT
.sp 1P
.LP
4.1.2.1
	\fIOther than user\(hyto\(hyuser signalling\fR 
.sp 9p
.RT
.PP
For supplementary services other than user\(hyto\(hyuser signalling, call 
control information is transferred via Signalling System\ No.\ 7 across 
the N\dx\ureference point. The interface for user information transfer 
is not different from that of basic bearer services. 
.RT
.sp 1P
.LP
4.1.2.2
	\fIUser\(hyto\(hyuser signalling services\fR 
.sp 9p
.RT
.PP
There are two methods of transferring user\(hyto\(hyuser signalling. One 
is transfer of user\(hyto\(hyuser signalling within Q.931 call control 
messages which have been mapped into Signalling System\ No.\ 7 messages 
and then are conveyed 
via the Signalling System\ No.\ 7 network. The other is transfer of user\(hyto\(hyuser 
signalling within stand alone USER INFO messages (which have been mapped 
into Signalling System\ No.\ 7 messages and then are conveyed via the Signalling 
System\ No.\ 7 network), or optionally may be transferred via packet handlers
(PHs) in some ISDNs. In the case where user\(hyto\(hyuser signalling is 
transferred between packet handlers (PHs) in both ISDNs, the X.75 protocol 
may be applied to the internetwork interface to transfer user\(hyto\(hyuser 
signalling. In the case where user\(hyto\(hyuser signalling is transferred 
via Signalling System\ No.\ 7 
networks in both ISDNs or at least in one ISDN, the Signalling System\ No.\ 7
protocol should be applied to the internetwork interface for user\(hyto\(hyuser
signalling.
.RT
.sp 1P
.LP
4.1.3
	 \fISignalling System No. 7 for the control of circuit mode services\fR 
\fIat the \fR \fIN\fR 
.sp 9p
.RT
.EF '%	\fIx\ reference\ point''
.OF '''\fIx\ reference\ point	%'
.PP
For the control of circuit mode services in the long term,
Signalling System\ No.\ 7 with ISUP will be used at the N\dx\ureference
point.
.RT
.sp 1P
.LP
4.2
	\fIISDN\(hyISDN Interface where both ISDNs provide X.31 case B based\fR 
\fIpacket mode bearer services\fR 
.sp 9p
.RT
.PP
See Figure 3/I.520.
.PP
The X.75 protocol is used to transfer X.31 based packet mode
services at the N\dx\ureference point. Layers\ 1, 2, and 3 for this interface
are specified in X.75.
.bp
.RT
.LP
.rs
.sp 7P
.ad r
\fBFigure 3/I.520, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.3
	 \fIISDN\(hyISDN interface where a circuit mode bearer service is provided\fR 
\fIby one ISDN to access either a PSPDN, or a PH and an X.31 case B packet\fR 
\fImode bearer service provided by another ISDN\fR 
.sp 9p
.RT
.PP
With this type of interworking, two different configurations are
considered, I and II. In configuration I, interworking between the two ISDNs
utilizes X.75 interexchange signalling. See Figure\ 4/I.520.
.PP
In configuration II, a circuit switched access to the PH in the
ISDN\d(\\dp\\ds\\d)\uis provided, and the interworking between the two ISDNs
utilizes a Signalling System\ No.\ 7 protocol.
.PP
This interworking arrangement applies for data transmission services. General 
arrangements are covered in \(sc\ 6.3 of X.320. There are two 
possibilities:
.RT
.LP
	i)
	 X.31 case A interworking with X.31 case\ B. Case\ A refers to the situation 
where a transparent circuit switched access to PSPDN is provided by ISDN. 
Case\ B refers to the situation where a packet mode bearer service is provided 
by an ISDN PH. 
.LP
	ii)
	 ISDN circuit switched access to an ISDN PH (this case may exist if the 
originating ISDN does not have PH functionality). 
.PP
Several aspects of interworking for data transmission services as well 
as their application to other transmission services are for further 
study.
.LP
.rs
.sp 9P
.ad r
\fBFigure 4a/I.520, p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigure 4b/I.520, p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
4.4
	\fIISDN\(hyISDN interworking via a transit network\fR 
.sp 9p
.RT
.PP
ISDN\(hyISDN interworking via a transit network ( see Figure 5/I.520) may 
be a useful configuration in the short term for extending specific ISDN 
services on an end\(hyto\(hyend basis. Special transmission, switching 
and signalling capabilities may have to be deployed in the transit network 
to ensure that the specific ISDN service is available end\(hyto\(hyend. 
.PP
The detailed interworking functions and interfaces for this
configuration are for further study.
.RT
.LP
.rs
.sp 7P
.ad r
\fBFigure 5/I.520, p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
4.5
	\fIISDN\(hyISDN interface for additional packet mode bearer services\fR 
.sp 9p
.RT
.PP
For packet mode services that are currently under study, out\(hyband call 
control signalling is used. The same out\(hyband call control is used for 
circuit mode services. Two alternatives can be considered for this out\(hyband
call control: enhancement of Signalling System\ No.\ 7 and enhancement of the
D\(hychannel protocol. The choice between the two alternatives is for further
study.
.RT
.sp 1P
.LP
4.6
	 \fIISDN\(hyISDN interface where an X.31 case B based packet mode bearer\fR 
\fIservice is provided on one ISDN and an additional packet mode bearer\fR 
\fIservice is requested on another ISDN\fR 
.sp 9p
.RT
.PP
Two alternatives can be considered: the first is based on in\(hyband signalling 
(X.75), and the second is based on out\(hyband signalling (Signalling System\ 
No.\ 7 or D\(hychannel protocol). The choice between the two alternatives 
is for further study. 
.RT
.sp 1P
.LP
4.7
	\fIISDN\(hyISDN interface for circuit mode to additional packet mode\fR 
\fIservice\fR 
.sp 9p
.RT
.PP
This section is for further study.
.RT
.sp 2P
.LP
\fB5\fR 	\fBInterworking functions\fR 
.sp 1P
.RT
.PP
Interworking functions commonly employed for various types of
interworking are described in Recommendation\ I.510. The interworking functions 
specific to ISDN\(hyISDN interworking are described here. 
.RT
.sp 1P
.LP
5.1
	\fIEcho control processing and speech processing\fR 
.sp 9p
.RT
.PP
Table 4/I.520 shows the permitted relationship between circuit mode bearer 
services and various forms of speech processing functionality. These 
speech processing functions include digital speech interpolation (DSI), 
low rate encoding (LRE) and digital circuit multiplication (DCM). Depending 
upon the particular relationship to the circuit mode bearer services, these
processing functions are specified as essential, optional, prohibited or
functionally disabled.
.PP
For a speech, 3.1 kHz audio, or 64 kbitB/Fs unrestricted call within an 
ISDN, appropriate network control is required to ensure that the relationship 
shown within Table\ 4/I.520 is realized. An example of this control might 
be 
routing (to exclude or include a function) or out\(hyband signalling (to 
disable a function). Further, it is to be noted that a disabling tone (see 
Recommendations\ V.25 and I.530) may be used to functionally remove echo 
control devices on a 3.1\ kHz audio bearer service connection. 
.bp
.RT
.LP
.ce
\fBH.T. [T4.520]\fR 
.ce
TABLE\ 4/I.520
.ce
\fBRelationship between speech processing and bearer services\fR 
.ce
\fBwithin an ISDN and for ISDN\(hyISDN interworking \fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(48p) | cw(36p) sw(36p) sw(36p) sw(36p) sw(36p) , ^  | c | c | c | c s 
^  | c | c | c | c s 
^  | ^  | ^  | ^  | c | c.
Speech processing functions	Bearer service
	1	2	3	4	Speech	3.1 kHz  audio | ua\d\u)\d	64 kbit/s  Unrestricted	 {
Alternate speech/64 kbit/s
Unrestricted | ub\d\u)\d
 }	Speech 	64 kbit/s
_
.T&
lw(48p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) .
Echo control | uc\d\u)\d	E | ud\d\u)\d | ue\d\u)\d	E | ud\d\u)\d | ue\d\u)\d	FD	\ue\d\u)\d	FD
.T&
lw(48p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) .
 {
A\(hy\(*m law conversion | uf\d\u)\d
 }	E	E	FD	E	FD
.T&
lw(48p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) .
DSI	O	O | ug\d\u)\d	FD	O	FD
.T&
lw(48p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) .
LRE	O	O | ug\d\u)\d	FD	O	FD
.T&
lw(48p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) .
DCM	O	O | ug\d\u)\d	FD | uh\d\u)\d	O | ui\d\u)\d	FD | ui\d\u)\d
.T&
lw(48p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) | lw(36p) .
Analog facilities	O	O | ug\d\u)\d	P	P	P
.TE
.LP
E
Essential
.LP
O
Optional
.LP
P
Prohibited
.LP
FD
Functionally disabled
.LP
DSI
Digital speech interpolation
.LP
LRE
Low rate encoding (e.g. Rec. G.721).
.LP
DCM
Digital circuit multiplication employing LRE and DSI and having
controllable flexibility in modes of operation.
.LP
\fINote\fR
\ \(em\ The bearer services in columns 1, 2 and 3 of the table permit control of speech processing devices only at call set\(hyup as required for the particular bearer service requested. The bearer service in column\ 4 requires additional
post set\(hyup user\(hyto\(hynetwork signalling (out\(hyband by D\(hychannel messages) in
order to perform the required in\(hycall service modifications between the
relevant alternative services.
.LP
\ua\d\u)\d
For the 3.1 kHz audio bearer service, echo control is included in the connection at the time of call set\(hyup. It is disabled for the transmission of voice\(hyband data by use of the disabling tone (see Recs.\ V.25 and I.\ 530).
.LP
\ub\d\u)\d
The exchange may set up a 64 kbit/s unrestricted bearer path with echo control devices and A\(hy\(*m law converters (if necessary) enabled for speech. In any case, the set up of parallel paths for speech and 64\ kbit/s unrestricted must be avoided.
.LP
\uc\d\u)\d
Echo control needs to be disabled when continuity check is
performed.
.LP
\ud\d\u)\d
Although echo control may not be required in ISDN\(hyISDN
interworking for digital telephones (for further study), its inclusion for
possible internetworking reasons for the speech bearer service is essential
(see also Rec.\ I.530).
.LP
\ue\d\u)\d
The necessity for network or terminal provided echo control in
4\(hywire end\(hyto\(hyend speech connections is for further study.
.LP
\uf\d\u)\d
The IWF converting A\(hy\(*m laws should also make the necessary bit
translation in the bearer capability information element to indicate the law
used.
.LP
\ug\d\u)\d
The network may include signal processing techniques provided they are appropriately modified or functionally removed prior to information
transfer.
.LP
\uh\d\u)\d
The 64 kbit/s transparent capability will be invoked, subject to the available transmission capacity, by the adjoining exchange over a dedicated out\(hyband signalling system.
.LP
\ui\d\u)\d
The provision of this bearer service using DCM is subject to the ability of the out\(hyband signalling system and the DCM equipment to execute
in\(hycall modifications initiated by the adjoining exchange.
.nr PS 9
.RT
.ad r
\fBTable 4/I.520 [T4.520], p.\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.PP
For a call which involves communication through different ISDNs, the network 
information regarding control of these functions needs to be 
extended across the ISDN\(hyISDN internetwork interfaces. This information
transfer is realized between the exchanges in interworking ISDNs by means
of:
.LP
	1)
	the Signalling System No. 7 ISUP bearer capability
information element, and
.LP
	2)
	the use of a disabling tone (see Recommendations\ V.25 and
I.530) by terminals, in the case of a 3.1\ kHz audio bearer service.
.PP
The control of speech processing functions (DCM, A\(hy\(*m\ law
conversion, echo control,\ etc.) by exchanges is:
.LP
	a)
	 not needed when a disabling tone (see Recommendations\ V.25 and I.530) 
is used, in conjunction with the 3.1\ kHz audio bearer service by a terminal(s), 
and 
.LP
	b)
	 to be implemented using out\(hyband call processes (currently under study) 
when needed. 
.PP
The procedures in the case of alternate speech/64 kbitB/Fs
unrestricted bearer services, are for further study.
.sp 1P
.LP
5.2
	\fIGeneration of in\(hyband tones and announcements for speech and\fR 
\fI3.1 kHz audio bearer services\fR 
.sp 9p
.RT
.PP
(\fINote\fR \ \(em\ This function is also necessary for a call within one
ISDN, which does not involve network interworking nor internal ISDN
interworking.)
.RT
.sp 1P
.LP
5.2.1
	\fIUnsuccessful call delivery\fR 
.sp 9p
.RT
.PP
The point of call failure (i.e. the point at which the connection cannot 
proceed further) should generate the appropriate out\(hyband clearing 
message toward the calling exchange. In response to this message, the calling 
exchange should send the appropriate out\(hyband message to the calling 
user. 
However, for speech and 3.1\ kHz audio bearer services, the network must be
capable of generating the appropriate in\(hyband tones or announcements. 
In this case, the clearing message should not be sent prior to the completion 
of the 
announcements.
.RT
.sp 1P
.LP
5.2.2
	\fISuccessful call delivery\fR 
.sp 9p
.RT
.PP
For speech and 3.1 kHz audio bearer services, the terminating
exchange should generate in\(hyband ring back tone towards the calling 
user upon successful delivery of the call. 
.RT
.sp 1P
.LP
5.3
	\fICall negotiation between ISDNs\fR 
.sp 9p
.RT
.PP
There are two aspects of call negotiation between ISDNs: service
agreement and connection agreement.
.RT
.sp 1P
.LP
5.3.1
	\fIService agreement between ISDNs\fR 
.sp 9p
.RT
.PP
Service agreement between ISDNs is defined as established
compatibility between the two networks on a requested service.  The service
agreement does not necessarily occur on a call\(hyby\(hycall basis, but in a
pre\(hydetermined way which has been agreed by bilateral negotiation between 
the two ISDNs. If the service agreement is established, connection agreement 
then begins between the two ISDNs. 
.PP
If the service agreement is not established, procedures are for
further study, including the following four alternatives.  Additionally, the
impact of these alternatives on user\(hyto\(hynetwork protocols or internetwork
protocols is for further study.
.RT
.LP
	1)
	The call may be established without the service
compatibility (e.g.\ in the case of a supplementary service request).
.LP
	2)
	The call may be cleared.
.LP
	3)
	 Either of the ISDNs may negotiate with the originating user to change 
or abandon the user's service request. 
.LP
	4)
	Another alternative may be selected from the originating
user's service profile.
.bp
.sp 1P
.LP
5.3.2
	\fIConnection agreement between ISDNs\fR 
.sp 9p
.RT
.PP
Connection agreement between ISDNs is defined as negotiation on the connection 
element between the two networks. Connection agreement is required when 
the connection elements employed in each ISDN are different, even if 
service agreement exists. (For example, see Appendix\ I.) The use of call
progress indicators for this purpose is for further study.
.PP
In a speech bearer service, the objects for connection agreement might 
be the use of one of the following: UDI (unrestricted digital information)/RDI 
(restricted digital information), satellite circuits, DSI circuits, the 
difference of PCM coding rules, circuit selection between digital networks
having different hierarchical structures,\ etc. Parameter exchange, if 
required, are executed by the two networks. 
.PP
The connection agreement does not necessarily occur on a call\(hyby\(hycall 
basis, but in a pre\(hydetermined way which has been established by other 
Recommendations (e.g.\ Recommendation\ G.802 for interworking between hierarchies 
and Recommendation\ G.711 for A\(hy\(*m\ law conversion) or agreed between 
two 
ISDNs.
.RT
.sp 1P
.LP
5.4
	\fICompatibility checking between end users of different\fR 
\fIISDNs\fR 
.sp 9p
.RT
.PP
When the connection path between two terminals on different ISDNs is established, 
low level compatibility (LLC), high layer compatibility (HLC) or user defined 
compatibility may be examined on an end\(hyto\(hyend basis. 
.PP
Compatibility checking items between end users are as
follows:
.RT
.LP
	1)
	\fILow layer compatibility\fR 
.LP
	LLC information would normally be used for user\(hyto\(hyuser call
negotiation and would be passed transparently through the networks. The IWF
may, where required, examine and act on LLC information (see
Recommendation\ I.515, \(sc\ 2.2.1.3) in the cases where the LLC checking 
lists (see Recommendation\ Q.931) employed by the relevant ISDNs are different. 
.LP
	2)
	\fIHigh layer compatibility\fR 
.LP
	The HLC is to be conveyed transparently and the networks
need not operate on it. The examination and action on HLC information by the
IWF is for further study, in the case where the HLC checking lists employed 
by the relevant ISDNs are different. 
.LP
	3)
	\fIUser defined compatibility checking\fR 
.LP
	User defined compatibility
checking is the user responsibility. The network does not participate in 
this compatibility checking. 
.sp 2P
.LP
\fB6\fR 	\fBFunctional interworking requirements for data transmission\fR 
\fBservices\fR 
.sp 1P
.RT
.PP
See Recommendation X.320 on general arrangements for interworking between 
ISDNs for the provision of data transmission services. 
.PP
Network interworking requirements for the case where an X.31 based
packet mode bearer service is requested on one ISDN and a new packet mode
bearer service is requested on another ISDN will be provided when new packet
mode bearer services are defined.
.RT
.sp 2P
.LP
\fB7\fR 	\fBReferences\fR 
.sp 1P
.RT
.PP
See Recommendation I.500.
\v'1P'
.RT
.ce 1000
APPENDIX\ I
.ce 0
.ce 1000
(to Recommendation I.520)
.sp 9p
.RT
.ce 0
.ce 1000
\fBISDN connections involving\fR 
.sp 1P
.RT
.ce 0
.ce 1000
\fBrestricted 64 kbit/s transfer capability\fR 
.ce 0
.LP
I.1
	\fIGeneral\fR 
.sp 1P
.RT
.PP
During an interim period the existence of networks or parts of
networks only capable of transferring 64\ kbit/s in a restricted manner
(i.e.\ 64\ kbit/s octet structured transfer capability with the all\(hyzero not
permitted) will have to be taken into account for international
intercommunication purposes.
.bp
.PP
For those networks, or parts thereof, the rules described hereafter
have to be followed in order to allow communication with networks, or parts
thereof, that already provide unrestricted 64\ kbit/s transfer capability. 
The necessary interworking functions (e.g.\ interworking units, rate adaptors) 
have to be provided by the network with restricted 64\ kbit/s transfer 
capability. 
Signalling provisions should be incorporated in Recommendation\ I.451 (Q.930). 
The network with 64\ kbit/s transfer capability will not be affected by 
this 
interworking, other than transporting the appropriate signalling across this
network to and from the terminal connected to the 64\ kbit/s network.
.RT
.sp 1P
.LP
I.2
	\fIInterworking with ISDNs providing restricted 64\ kbit/s\fR 
(see Figure I\(hy1/I.520)
.sp 9p
.RT
.LP
.rs
.sp 20P
.ad r
\fBFigure I\(hy1/I.520, p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
ISDN 1 may have some arrangements having only restricted 64\ kbit/s transfer 
capability. ISDN\ 2 is unrestricted. In both cases, at the reference 
points\ S/T, the I.412 user\(hynetwork interfaces are provided. However, where
restricted 64\ kbit/s arrangements are involved, only information streams not
having the all\(hyzero octet are possible.
.PP
Four possible cases of interworking for circuit switched connections between 
terminals\ A and\ B are considered (UDI means unrestricted digital 
information and RDI means restricted digital information). (See
Table\ I\(hy1/I.520.)
.RT
.sp 1P
.LP
I.3
	\fIConsiderations for terminal designed to operate with restricted\fR 
\fI64 kbit/s transfer capability\fR (Figure I\(hy2/I.520)
.sp 9p
.RT
.PP
Existing terminals at rates less than 64 kbit/s will require rate adaption 
to operate with restricted 64\ kbit/s transfer capability (see 
Recommendation\ I.464).
.RT
.LP
.rs
.sp 7P
.ad r
Blanc
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 32P
.ad r
\fBTableau I\(hy1/I.520, p. 12 \ \ 
(\*`a traiter comme figure MEP)\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.sp 2
.rs
.sp 12P
.ad r
\fBFigure I\(hy2/I.520, p. 13\fR 
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
\fBRecommendation\ I.530\fR 
.RT
.sp 2P
.ce 1000
\fBNETWORK\ INTERWORKING\ BETWEEN\ AN\ ISDN\fR 
.EF '%	Fascicle\ III.9\ \(em\ Rec.\ I.530''
.OF '''Fascicle\ III.9\ \(em\ Rec.\ I.530	%'
.ce 0
.sp 1P
.ce 1000
\fBAND\ A\ PUBLIC\ SWITCHED\ TELEPHONE\ NETWORK\ (PSTN)\fR 
.ce 0
.sp 1P
.ce 1000
\fI(Melbourne, 1988)\fR 
.sp 9p
.RT
.ce 0
.sp 1P
.LP
\fB1\fR 	\fBGeneral\fR 
.sp 1P
.RT
.PP
In many countries, digitization of the existing PSTN has been
ongoing over a number of years through the implementation of digital switching 
and transmission facilities. Furthermore, common channel signalling systems 
(e.g.\ Signalling System\ No.\ 6, Signalling System\ No.\ 7) have been 
introduced or will soon be introduced in these networks. 
.PP
The digitization of the user network access is one of the steps by
which an IDN becomes an ISDN. However, it is foreseen that this will be 
a long transition period for some networks. 
.PP
Thus, the purpose of this Recommendation is to identify the
interworking functions and requirements to support interworking between 
an ISDN and a PSTN. 
.RT
.sp 2P
.LP
\fB2\fR 	\fBScope\fR 
.sp 1P
.RT
.PP
The purpose of this Recommendation is to describe the general
arrangements for interworking between ISDN and PSTN. Both the provision 
of ISDN voice transmission and data transmission services are within the 
scope of this Recommendation. 
.RT
.sp 2P
.LP
\fB3\fR 	\fBAbbreviations\fR 
.sp 1P
.RT
.PP
DP
	Dial Pulse
.RT
.LP
	DTE
	Data Terminal Equipment
.LP
	DTMF
	Dual\(hytone Multiple Frequency
.LP
	IDN
	Integrated Digital Network
.LP
	ISDN
	Integrated Services Digital Network
.LP
	ISUP
	ISDN User Part
.LP
	LE
	Local Exchange
.LP
	NT
	Network Termination
.LP
	PABX
	Private Automatic Branch Exchange
.LP
	PSTN
	Public Switched Telephone Network
.LP
	SS No. 7
	Signalling System No. 7
.LP
	TE
	Terminal Equipment
.LP
	TA
	Terminal Adaptor
.LP
	TUP
	Telephone User Part.
.sp 2P
.LP
\fB4\fR 	\fBInterworking configuration and network characteristics\fR 
.sp 1P
.RT
.sp 1P
.LP
4.1
	\fIInterworking configurations\fR 
.sp 9p
.RT
.PP
See Figure 1/I.530.
.RT
.sp 1P
.LP
4.2
	 \fIKey \fR \fIISDN and PSTN characteristics and related interworking\fR 
\fIfunctions\fR 
.sp 9p
.RT
.PP
Table 1/I.530 identifies the key characteristics of an ISDN and a PSTN, 
indicating possible interworking functions to accommodate dissimilar 
characteristics.
.RT
.sp 1P
.LP
4.2.1
	\fILocation of interworking functions\fR 
.sp 9p
.RT
.PP
Given that the transition period from a PSTN to an ISDN may occur over 
a long period of time, there will be an ongoing requirement for ISDN\(hyPSTN 
interworking. In such a situation, it is likely that interworking functions 
will be required at not just one but several locations. As the transition to
ISDN continues, interworking points will come into existence and later 
may not be required. 
.bp
.RT
.LP
.rs
.sp 17P
.ad r
\fBFigure 1/I.530, p.\fR 
.sp 1P
.RT
.ad b
.RT
.PP
Points where interworking may exist are:
.RT
.LP
	\(em
	within the local exchange;
.LP
	\(em
	at transit exchange;
.LP
	\(em
	at international gateway offices.
.PP
\fINote\fR \ \(em\ The optimum location of each interworking function may 
be specific per interworking function and dependent on the usage of the 
service, network topology,\ etc. 
.sp 2P
.LP
\fB5\fR 	\fBISDN bearer services suitable for ISDN\(hyPSTN interworking\fR 
.sp 1P
.RT
.PP
This section considers the subject of ISDN services suitable for
ISDN\(hyPSTN interworking. The discussions dealing with the ISDN to/from PSTN
direction are addressed in individual subsections, as is the subject of 
circuit mode and packet mode. 
.RT
.sp 1P
.LP
5.1
	\fIISDN bearer services suitable for ISDN to PSTN interworking\fR 
\fI(circuit)\fR 
.sp 9p
.RT
.PP
Currently, there are three identified bearer services that could be used 
within the ISDN to PSTN interworking. These are (refer to 
Recommendation\ I.211):
.RT
.LP
	i)
	circuit mode 64 kbit/s, 8 kHz structured bearer services,
usable for speech information transfer (Note\ 1);
.LP
	ii)
	 circuit\(hymode 64 kbit/s, 8 kHz structured bearer services, usable for 
3.1\ kHz audio information transfer (Note\ 2); 
.LP
	iii)
	circuit\(hymode 64 kbit/s unrestricted, 8 kHz structured
bearer service (Note\ 3).
.PP
It is recognized that the communication characteristics obtained for each 
of these three bearer services on an ISDN to PSTN basis may not be the 
same as that obtained for ISDN to ISDN configurations. 
.PP
\fINote\ 1\fR \ \(em\ This bearer service is used for the ISDN to
PSTN\(hyinterworking for the purpose of speech information transfer.
.PP
\fINote\ 2\fR \ \(em\ This bearer service is used for the ISDN to PSTN
interworking for the purpose of 3.1\ kHz audio information transfer. For 
PSTN to ISDN interworking, this bearer service will be selected at the 
boundary of the PSTN to the ISDN for the purpose of speech information 
transfer \fIand\fR for 
3.1\ kHz audio information transfer.
.PP
\fINote\ 3\fR \ \(em\ This bearer service may be required for ISDN\(hyPSTN
interworking. Refer to Recommendation\ I.231 for the 64\ kbit/s interworking
service definition.
.PP
\fIRemarks\fR \ \(em\ ISDN\(hyPSTN interworking means interworking both ways
between ISDN and PSTN, while ISDN to PSTN refers to a call initiated in the
ISDN and terminated in the PSTN, and PSTN to ISDN refers to a call initiated 
in the PSTN and terminated in ISDN. 
.bp
.RT
.ce
\fBH.T. [T1.530]\fR 
.ce
TABLE\ 1/I.530
.ce
\fBKey ISDN and PSTN characteristics\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
lw(54p) | cw(54p) | cw(54p) | cw(48p) .
	ISDN	PSTN	Interworking functions
_
.T&
lw(54p) | lw(54p) | lw(54p) | lw(48p) .
Subscriber interface	Digital	Analog	a
.T&
lw(54p) | lw(54p) | lw(54p) | lw(48p) .
User network signalling	Out\(hyband (I.441/I.451)	 {
Mainly in\(hyband
(e.g. DTMF)
 }	b, e
.T&
lw(54p) | lw(54p) | lw(54p) | lw(48p) .
 {
User terminal equipment supported
 }	 {
Digital TE (ISDN NT, TE1 or TE2 + TA)
 }	 {
Analog TE (e.g. dial pulse telephones, PABXs, modem\(hyequipped DTEs)
 }	c
.T&
lw(54p) | lw(54p) | lw(54p) | lw(48p) .
Interexchange signalling	 {
SS No. 7 ISDN user part (ISUP)
 }	 {
In\(hyband (e.g. R1, R2, No.\ 4, No. 5) or out\(hyband (e.g. SS No. 6,
SS No.\ 7 TUP)
 }	d, e
.T&
lw(54p) | lw(54p) | lw(54p) | lw(48p) .
Transmission facilities	Digital	Analog/digital	a
.T&
lw(54p) | lw(54p) | lw(54p) | lw(48p) .
Information transfer mode	Circuit/paquet	Circuit	f
.T&
lw(54p) | lw(54p) | lw(54p) | lw(48p) .
 {
Information transfer capability
 }	 {
Speech, digital unrestricted, 3.1\ kHz audio, video, etc.
 }	 {
3.1 kHz audio (voice/voice\(hyband data)
 }	 f
.TE
.LP
\fIInterworking functions:\fR
.LP
a\ \(em
Analogue\(hyto\(hydigital and digital\(hyto\(hyanalogue conversion on
transmission facilities.
.LP
b\ \(em
Mapping between PSTN signals in the subscriber access and I.451
messages for intra\(hyexchange calls.
.LP
c\ \(em
Support of communication between modem\(hyequipped PSTN DTEs and
ISDN terminals;
.LP
d\ \(em
Conversion between the PSTN signalling system and Signalling System
No. 7 ISDN user part.
.LP
e\ \(em
Mapping between signals in the ISDN subscriber (I.441, I.451) access
and PSTN in\(hyband interexchange signalling (e.g. R1).
.LP
f\ \(em
Further study required.
.nr PS 9
.RT
.ad r
\fBTable 1/I.530 [T1.530], p.\fR 
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
5.2
	\fIISDN bearer services suitable for PSTN to ISDN interworking\fR 
\fI(circuit)\fR 
.sp 9p
.RT
.PP
Currently, there is no internationally recognized method of service differentiation 
between voice and non\(hyvoice calls originating in the PSTN. 
However, the \*Qcircuit\(hymode 64\ kbit/s, 8\ kHz structured bearer service 
for 
3.1\ kHz audio information transfer\*U provides for the capability equivalent 
to PSTN. (Reference Recommendation\ I.231.) Therefore, PSTN calls may interwork 
to this service in ISDN. 
.PP
The call progress indicator within ISUP will identify when
interworking between ISDN and PSTN occurs. This indicator will enable the 
ISDN to select a connection that would support 3.1\ kHz audio. A V\(hySeries 
terminal 
connected to the ISDN via a terminal adaptor and using the 64\ kbit/s
unrestricted bearer service requires the use of an IWF (including a modem) 
for calls from PSTN users. To effect the connection, a 64\ kbit/s connection 
to the IWF would need to be used. 
.bp
.RT
.sp 1P
.LP
5.3
	\fIISDN bearer services suitable for ISDN to PSTN interworking\fR 
\fI(packet)\fR 
.sp 9p
.RT
.PP
Currently, there are two identified bearer services that could be used 
within the ISDN, for ISDN (packet mode calls) to PSTN 
interworking:
.RT
.LP
	i)
	B\(hychannel: packet\(hymode, unrestricted digital information,
service data unit integrity, X.25 link level, X.25 packet level bearer
service;
.LP
	ii)
	 D\(hychannel: packet mode, unrestricted digital information, service 
data unit integrity, I.441 link level, X.25 packet level bearer 
service.
.PP
\fINote\fR \ \(em\ Detailed mechanisms are for further study.
.sp 1P
.LP
5.4
	\fIISDN bearer services suitable for PSTN to ISDN interworking\fR 
\fI(packet)\fR 
.sp 9p
.RT
.PP
(For further study.)
.RT
.sp 2P
.LP
\fB6\fR 	\fBConnection type suitable for ISDN\(hyPSTN interworking\fR 
.sp 1P
.RT
.PP
This section identifies the mapping of ISDN bearer services and
possible connection types for ISDN\(hyPSTN interworking. Depending on the 
specific ISDN bearer service being considered, more than one ISDN connection 
type may be applicable. However, in some cases the connection type may 
not be fully 
compatible with the requested bearer service, thereby leading to downgrading 
of service. 
.PP
The ISDN bearer services and possible connection types that may be
used are summarized in Table\ 2/I.530, under the four possible interworking
cases. Refer to Recommendation\ I.335 for more details regarding the mapping
between ISDN bearer services and ISDN connection types.
.RT
.sp 2P
.LP
\fB7\fR 	\fBFunctional requirements for ISDN\(hyPSTN interworking\fR 
.sp 1P
.RT
.sp 1P
.LP
7.1
	\fIInterworking between signalling systems\fR 
.sp 9p
.RT
.PP
Interworking between signalling systems, specifically for
interexchange calls between the PSTN signalling system (which may be in\(hyband) 
and Signalling System\ No.\ 7 (ISDN UP) on an ISDN, may be required. The 
interworking procedures are specified in Recommendation\ Q.699.
.PP
For intra\(hyexchange calls between the ISDN and PSTN subscriber,
interworking between I.451 messages and signals in the PSTN subscriber 
access may also be required. 
.RT
.sp 1P
.LP
7.2
	\fIProvision of interworking indications\fR 
.sp 9p
.RT
.PP
An interworking indication is required for the ISDN local exchange (LE) 
to know that ISDN\(hyPSTN interworking has occurred. ISUP Q.761\(hyQ.764 
and 
I.451/Q.931 protocols have the ability to identify this interworking situation 
to the ISDN LE and the ISDN terminal (call progress indicator). 
.PP
The ISDN terminal would be informed in every case that ISDN\(hyPSTN
interworking has occurred. This information is required to satisfy as a 
minimum the requirement to: 
.RT
.LP
	\(em
	tell the terminal to connect the B\(hychannel so that in\(hyband
tones and announcements can be received when ISDN\(hyto\(hyPSTN calls are
originated;
.LP
	\(em
	 tell the ISDN terminal that some or all of service selection information 
and address may be unavailable \(em\ the terminal may then be required 
to accept the call without out\(hyband compatibility checking; 
.LP
	\(em
	tell data terminal equipment to anticipate in\(hyband
handshaking signals for ISDN\(hyPSTN calls.
.bp
.ce
\fBH.T. [T2.530]\fR 
.ce
TABLE\ 2/I.530
.ce
\fBISDN bearer services and connection types suitable for
.ce
ISDN\(hyPSTN interworking\fR 
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(54p) | cw(54p) | cw(30p) sw(30p) sw(30p) sw(30p) , ^  | ^  | c | c | c | c.
Interworking	 {
ISDN bearer services categories
 }	ISDN connection types
		64 kbit/s unrestricted	Speech	3.1 kHz audio	Packet
_
.T&
lw(54p) | lw(54p) | cw(30p) | cw(30p) | cw(30p) | cw(30p) .
ISDN to PSTN (circuit)	64 kbit/s unrestricted	Y	N	N	N
.T&
lw(54p) | lw(54p) | cw(30p) | cw(30p) | cw(30p) | cw(30p) .
	Speech	R	Y	Y	N
.T&
lw(54p) | lw(54p) | cw(30p) | cw(30p) | cw(30p) | cw(30p) .
	3.1 kHz audio	R	FS	Y	N
_
.T&
lw(54p) | lw(54p) | cw(30p) | cw(30p) | cw(30p) | cw(30p) .
PSTN to ISDN (circuit)	64 kbit/s unrestricted	Y	N	N	N
.T&
lw(54p) | lw(54p) | cw(30p) | cw(30p) | cw(30p) | cw(30p) .
	3.1 kHz audio	R	N	Y	N
_
.TE
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\fBTable 2/I.530 [T2.530], p.\fR 
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The following interworking scenarios have been recognized:
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	a)
	 an ISDN\(hyPSTN call which uses a Signalling System No. 7 ISUP connection 
between the originating and terminating local exchanges; 
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	b)
	 an ISDN\(hyPSTN call which uses a non\(hySignalling System\ No.\ 7 ISUP 
connection (e.g.\ R1, Signalling System\ No.\ 7 TUP) between the originating 
and terminating local exchanges; 
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	c)
	 an ISDN\(hyPSTN call which involves a combination of Signalling System\ 
No.\ 7 ISUP and non\(hySignalling System\ No.\ 7 ISUP interexchange signalling 
connections between the originating and terminating local exchanges; 
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	d)
	an ISDN\(hyPSTN call within the same local exchange (i.e. no
interexchange signalling).
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7.2.1
	\fINetwork indication of modification of communication\fR 
\fIcharacteristics\fR 
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.PP
The network will always provide an indication to the user of
modification of communication characteristics. The modification of
communication characteristics may be due to the following reasons:
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	\(em
	interworking with another network;
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	\(em
	resource constraints in the network.
.PP
In addition to providing an indication, the network may solicit
user acceptance of the modification of communication characteristics in 
certain cases. Examples are: 
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	\(em
	downgrading of service;
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	\(em
	upgrading of service.
.PP
For most interworking cases, user acceptance is not applicable.
.PP
There may be a requirement for the resolution of information transfer capability 
requests other than speech and 3.1\ kHz audio on ISDN\(hyto\(hyPSTN calls. 
The choices of rejection (with a suitable cause indication) or negotiation 
(involving parameter exchange) are possible (Recommendation\ I.515).
.PP
There may also be a requirement for the rejection of supplementary
service requests available on an ISDN, but not supported on the PSTN. However, 
negotiation for supplementary services may be possible as well. 
.PP
The principles for call negotiation in an ISDN\(hyPSTN interworking
situation are for further study.
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7.2.2
	\fIFailure indication\fR 
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.PP
Failure indication, when carried by the I.451 and ISUP signalling messages, 
should be meaningful and give a clear indication of the reason. 
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The network failure indication should be able to identify the network where 
congestion occurred. This may be of use in networks allowing RPOA 
selection.
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7.3
	\fIGeneration of in\(hyband tones and announcements\fR 
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.PP
In\(hyband tones and announcements are provided for all speech and
3.1\ kHz audio bearer service calls between an ISDN and a PSTN (reference
Recommendation\ E.180). Within ISDN, in\(hyband tones and announcements, 
with the exception of ring\(hyback tone, should be provided at a point 
as close as possible to the calling user (i.e.\ network, PABX, or terminal). 
Whenever possible, 
out\(hyband messages should also be used within the ISDN and the local access.
.PP
The network (ISDN or PSTN) must be capable of generating in\(hyband tones 
and announcements. However, for ISDN\(hyto\(hyPSTN interworking cases, 
the ISDN 
terminals will receive the in\(hyband tones and announcements whenever 
the tones are generated within the PSTN, i.e.\ beyond the interworking 
point. 
Nevertheless, this does not preclude the terminal from providing its own 
tones and announcements. 
.PP
In\(hyband ring\(hyback tone should be generated by the terminating exchange 
(or terminating PABX). 
.PP
Furthermore, two call scenarios exist:
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	a)
	the call is unsuccessful (user busy, network
congestion,\ etc.);
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	b)
	the call is delivered successfully.
.PP
Regardless of the call type, the same in\(hyband tones and
announcements (depending on the call scenario) should be provided to the
callling user.
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7.3.1
	\fICall type 1: PSTN\(hyto\(hyISDN\fR 
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7.3.1.1
	\fIUnsuccessful call delivery\fR 
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.PP
When the point of call failure (i.e. the point at which the call
cannot proceed further) is within the PSTN or at the PSTN user, normal PSTN
procedures apply.
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.PP
When the point of call failure is within the ISDN or at the ISDN user, 
the ISDN should send the appropriate out\(hyband clearing message as far 
back 
towards the gateway exchange as possible.
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	\(em
	If the out\(hyband message can be sent all the way through to
the gateway exchange, then the gateway exchange should pass the information 
to the PSTN using the PSTN's normal procedures (i.e.\ out\(hyband if the 
PSTN supports the out\(hyband message, otherwise in\(hyband). 
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	\(em
	If the message cannot be sent out\(hyband all the way to the
gateway exchange, then the appropriate in\(hyband tone or announcement 
should be provided by the ISDN at the point where out\(hyband signalling 
is no longer 
capable of handling the message.
.PP
For the above cases, the clearing message should not be sent prior to the 
completion of the announcement. 
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7.3.1.2
	\fISuccessful call delivery\fR 
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.PP
If the call is delivered successfully to the ISDN user, then the
terminating ISDN exchange should generate in\(hyband ring\(hyback tone 
towards the 
PSTN user.
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7.3.2
	\fICall type 2: ISDN\(hyto\(hyPSTM\fR 
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7.3.2.1
	\fIUnsuccessful call delivery\fR 
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.PP
When the point of call failure is within the ISDN, the call should be handled 
as an ISDN\(hyto\(hyISDN call (see Recommendation\ I.520). 
.PP
When the point of call failure is within the PSTN, the PSTN's
procedures apply. For instance, if the PSTN supports out\(hyband signalling 
to the gateway exchange, then the gateway exchange should map the message 
to the 
appropriate out\(hyband ISDN clearing message (i.e.\ the gateway exchange 
handles the call as an ISDN\(hyto\(hyISDN call). If the PSTN does not support 
out\(hyband 
signalling, then it will generate the appropriate in\(hyband tone or
announcement.
.PP
The ISDN terminal should be alerted to the fact that interworking has occurred 
so that the user can be prepared to receive the appropriate in\(hyband 
tone or announcement. The intermediate interworking point will provide the
interworking message which will suppress, when necessary, tone generation in
the ISDN terminal, and pass through any in\(hyband tones.
.PP
For the above cases, the clearing message should not be sent prior to the 
completion of the announcement. 
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7.3.2.2
	\fISuccessful call delivery\fR 
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.PP
If the call is delivered successfully to the PSTN user, then the
terminating PSTN exchange will provide in\(hyband ring\(hyback tone. The ISDN
terminal should be alerted to the fact that interworking has occurred so the
user can be prepared to receive the in\(hyband ring\(hyback tone.
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7.4
	\fIHandling of non\(hyvoice calls between ISDN and PSTN\fR 
\fIsubscribers\fR 
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.PP
There may be an interworking requirement for the capability to
interconnect modem\(hyequipped terminals on the PSTN and compatible terminals 
on an ISDN access. This may in the future include a means for compatibility 
checking and the provision of a modem pool to perform A/D conversion and 
rate adaption (Recommendation\ I.515). 
.PP
There are in principle two alternative approaches to provide data
communication between an ISDN customer and a PSTN customer:
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	i)
	The data terminal of the ISDN customer is connected to a
modem which in turn is connected to an A/D converter (PCM). A call will be
handled as for telephony. Further study is required to determine what
interworking functions are required in this case.
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	ii)
	The data terminal of the ISDN customer is connected to a
terminal adaptor according to, e.g.\ Recommendation\ I.463, i.e.\ the data 
flow is rate adapted to 64\ kbit/s. At a suitable interworking point, the 
original data flow (e.g.\ 1.2\ kbit/s) is extracted and converted to \*Qanalogue\*U 
form by a modem for further transfer to the remote data terminal (i.e.\ 
the usage of modem 
pools). Mechanisms for modem interworking are contained in
Recommendation\ I.515.
.PP
To handle non\(hyvoice calls in an ISDN\(hyPSTN interworking situation, 
the following interworking functions may be required. 
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	a)
	capability to distinguish a data call and its relevant
parameters when the call is coming from a PSTN;
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	b)
	capability to distinguish a data call and its relevant
parameters when the call is going to a PSTN;
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	c)
	 special routing algorithms for inclusion of proper IWFs as detected in\ 
a) and\ b); 
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	d)
	IWFs for protocol conversion as detected in a) and b).
.PP
For interworking between ISDN and PSTN, the need for in\(hyband
parameter exchange is recognized as necessary, with the understanding that
out\(hyband parameter exchange should be used whenever possible (refer to
Recommendation\ I.515).
.PP
\fINote\fR \ \(em\ When ISDN\(hyPSTN interworking, using a modem pool in
conjunction with the 64\ kbit/s unrestricted bearer service, it may not be
possible to extend PSTN supervisory tones to the ISDN user. The interworking
implications of this in the ISDN are for further study.
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7.5
	\fIControl of speech processing and echo control devices\fR 
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.PP
Connections provided for ISDN/PSTN interworking may use speech
processing techniques as long as these do not restrict the required information 
transfer. Restrictive devices should be functionally modified or removed 
using, for example, the 2.1\ kHz in\(hyband [ECD (echo control device) 
disabling] tone. 
.PP
Digital circuit multiplication equipment (DCME)
for example is designed to be compatible with the 3.1\ kHz audio transfer 
capability. Echo 
control devices and their use in the PSTN are recommended in
Recommendation\ G.131.
.PP
Similar Recommendations should apply to the ISDN/PSTN interworking
case. In particular, both echo suppressors and echo cancellers must be 
located within a range limitation of the four\(hywire/two\(hywire interface. 
These limits are mentioned in Recommendations\ G.131, \(sc\ 2.2, G.164, 
\(sc\ 1.1.3 and G.165, \(sc\ 3.2. If echo control devices are included 
in the ISDN connection, they will need to be disabled using the 2.1\ kHz 
echo control disabling tone generated by the modem 
.PP
as is current practice in the PSTN. While echo suppressors should respond 
to a 2100\ Hz tone (Recommendation\ G.164), echo cancellers should only 
respond if the tone includes phase reversals as specified in Recommendation\ 
G.165. It is 
recommended that the 2.1\ kHz tone should not be converted into an ISDN
signalling message and vice\(hyversa.
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7.6
	\fIA/\(*m law encoding\fR 
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The treatment of A/\(*m law encoding and translation in ISDN/PSTN
interworking can be based on the continuation of existing procedures whereby
appropriate A/\(*m law translation is performed by the \(*m\(hylaw network 
when crossing international boundaries. Terminals would encode speech and 
3.1\ kHz audio using the G.711\ law appropriate to the resident network. 
Unrestricted 64\ kbit/s 
services bit streams would not be manipulated in any way by the ISDN: terminals 
would be free to use any encoding (including G.711 or G.721) as deemed 
appropriate between themselves when unrestricted 64\ kbit/s capability is
requested.
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\fB8\fR 	\fBReferences\fR 
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.PP
See Recommendation I.500.
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\fBRecommendation\ I.540\fR 
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\fBGENERAL\ ARRANGEMENTS\ FOR\ INTERWORKING\fR 
.EF '%	Fascicle\ III.9\ \(em\ Rec.\ I.540''
.OF '''Fascicle\ III.9\ \(em\ Rec.\ I.540	%'
.ce 0
.ce 1000
\fBBETWEEN\ CIRCUIT\ SWITCHED\ PUBLIC\ DATA\ NETWORKS\ (CSPDNs)\fR 
.ce 0
.ce 1000
\fBAND\ INTEGRATED\ SERVICES\ DIGITAL\ NETWORKS\ (ISDNs)\fR 
.ce 0
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.ce 1000
\fBFOR\ THE\ PROVISION\ OF\ DATA\ TRANSMISSION\fR 
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\fI(Melbourne, 1988)\fR 
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.PP
See Recommendation X.321, Volume VIII, Fascicle VIII.6.
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\fBRecommendation\ I.550\fR 
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\fBGENERAL\ ARRANGEMENTS\ FOR\ INTERWORKING\fR 
.EF '%	Fascicle\ III.9\ \(em\ Rec.\ I.550''
.OF '''Fascicle\ III.9\ \(em\ Rec.\ I.550	%'
.ce 0
.ce 1000
\fBBETWEEN\ PACKET\ SWITCHED\ PUBLIC\ DATA\ NETWORKS\ (PSPDNs)\fR 
.ce 0
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\fBAND\ INTEGRATED\ SERVICES\ DIGITAL\ NETWORKS\ (ISDNs)\fR 
.ce 0
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.ce 1000
\fBFOR\ THE\ PROVISION\ OF\ DATA\ TRANSMISSION\fR 
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\fI(Melbourne, 1988)\fR 
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.PP
See Recommendation X.325, Volume VIII, Fascicle VIII.6.
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\fBRecommendation\ I.560\fR 
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\fBREQUIREMENTS\ TO\ BE\ MET\ IN\ PROVIDING\ THE\ TELEX\ SERVICE\fR 
.EF '%	Fascicle\ III.9\ \(em\ Rec.\ I.560''
.OF '''Fascicle\ III.9\ \(em\ Rec.\ I.560	%'
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\fBWITHIN\ THE\ ISDN\fR 
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\fI(Melbourne, 1988)\fR 
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.PP
See Recommendation U.202, Volume VII, Fascicle VII.2.
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.rs
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Blanc
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