home
***
CD-ROM
|
disk
|
FTP
|
other
***
search
/
InfoMagic Standards 1994 January
/
InfoMagic Standards - January 1994.iso
/
ccitt
/
1988
/
troff
/
7_5_01.tro
< prev
next >
Wrap
Text File
|
1991-12-13
|
68KB
|
3,075 lines
.rs
.\" Troff code generated by TPS Convert from ITU Original Files
.\" Not Copyright ( c) 1991
.\"
.\" Assumes tbl, eqn, MS macros, and lots of luck.
.TA 1c 2c 3c 4c 5c 6c 7c 8c
.ds CH
.ds CF
.EQ
delim @@
.EN
.nr LL 40.5P
.nr ll 40.5P
.nr HM 3P
.nr FM 6P
.nr PO 4P
.nr PD 9p
.po 4P
.rs
\v | 5i'
.sp 1P
.ce 1000
\v'12P'
\s12FASCICLE\ VII.5
\v'4P'
.RT
.ce 0
.sp 1P
.ce 1000
\fBRecommendations T.65\(hyT.101, T.150\(hyT.390\fR \v'2P'
.ce 0
.sp 1P
.ce 1000
\fBTERMINAL\ EQUIPMENT\ AND\ PROTOCOLS\fR
.EF '% \ \ \ ^''
.OF ''' \ \ \ ^ %'
.ce 0
.sp 1P
.ce 1000
\fBFOR\ TELEMATIC\ SERVICES\fR
.ce 0
.sp 1P
.LP
.rs
.sp 29P
.ad r
BLANC
.EF '% \ \ \ ^''
.OF ''' \ \ \ ^ %'
.ad b
.RT
.LP
.bp
.LP
\fBMONTAGE:\fR p. 2 = PAGE BLANCHE
.sp 1P
.RT
.LP
.bp
.sp 2P
.LP
\fBRecommendation\ T.65\fR
.RT
.sp 2P
.ce 1000
\fBAPPLICABILITY\ OF\ TELEMATIC\ PROTOCOLS\ AND\ TERMINAL\ CHARACTERISTICS\fR
.EF '% Fascicle\ VII.5\ \(em\ Rec.\ T.65''
.OF '''Fascicle\ VII.5\ \(em\ Rec.\ T.65 %'
.ce 0
.sp 1P
.ce 1000
\fBTO\ \fR \fBCOMPUTERIZED\ COMMUNICATION\ TERMINALS\ (CCTs)\fR
.ce 0
.sp 1P
.ce 1000
\fI(Melbourne, 1988)\fR
.sp 9p
.RT
.ce 0
.sp 1P
.LP
The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR
.sp 9p
.RT
.PP
(a)
that there is an increasingly growing base of
computerized communication terminals, such as communicating personal
computers;
.PP
(b)
that Administrations will require provisions to enable these devices to
access CCITT\(hydefined services, such as telematic services;
.PP
(c)
that communication of these devices with each other
may use provisions specified for communication within telematic services;
.PP
(d)
that such devices may, due to their adaptive nature, require, in some areas,
different protocols and terminal characteristics than existing telematic
terminals;
.PP
(e)
that the various telematic services are defined in the F\(hySeries of Recommendations;
.PP
(
f
)
that the reference model for open systems
interconnection is defined in the X\(hy200\(hySeries of Recommendations;
.PP
(g)
that the various telematic protocols and terminal
characteristics are defined in the T\(hySeries of Recommendations;
.PP
(h)
that there is a requirement to assess the
applicability of the protocols and terminal characteristics defined in the
CCITT telematic recommendations to computerized communication terminals;
.sp 1P
.LP
\fIunanimously declares the view\fR
.sp 9p
.RT
.PP
that the following technical provisions determine the
applicability of protocols and terminal characteristics specified in CCITT
Telematic Recommendations to Computerized Communication Terminals.
\v'1P'
.sp 1P
.ce 1000
CONTENTS
.ce 0
.sp 1P
.sp 2P
.LP
1
\fIScope\fR
.sp 1P
.RT
.sp 1P
.LP
2
\fICharacteristics and model\fR
.sp 9p
.RT
.LP
2.1
Definition
.LP
2.2
Characteristics
.LP
2.3
General model
.LP
2.4
Minimum capability
.sp 1P
.LP
3
\fIAccess to the Teletex service\fR
.sp 9p
.RT
.LP
3.1
General
.LP
3.2
Characteristics
.LP
3.3
Applicability of the relevant CCITT Recommendations
.LP
3.4
Access methods
.bp
.sp 1P
.LP
4
\fIAccess to the Group 3 facsimile service\fR
.sp 9p
.RT
.LP
4.1
General
.LP
4.2
Characteristics
.LP
4.3
Applicability of the relevant CCITT Recommendations
.sp 1P
.LP
5
\fIAccess to the Group 4 facsimile service\fR
.sp 9p
.RT
.sp 1P
.LP
6
\fIAccess to the mixed\(hymode option of the Teletex service\fR
.sp 9p
.RT
.sp 1P
.LP
7
\fIAccess to the videotex service\fR
.sp 9p
.RT
.LP
7.1
General
.LP
7.2
Characteristics
.LP
7.3
Applicability of the relevant CCITT Recommendations
.sp 1P
.LP
8
\fIAccess to MHS\fR
.sp 9p
.RT
.LP
8.1
General
.LP
8.2
Characteristics
.LP
8.3
Applicability of the relevant CCITT Recommendations
.sp 1P
.LP
9
\fIAccess to the directory service\fR
.sp 9p
.RT
.LP
9.1
General
.LP
9.2
Characteristics
.LP
9.3
Applicability of the relevant CCITT
Recommendations
\v'6p'
.sp 2P
.LP
\fB1\fR \fBScope\fR
.sp 1P
.RT
.PP
1.1
This Recommendation addresses the applicability of the protocol and terminal
characteristics specified in CCITT\(hydefined Recommendations to
Computerized Communication Terminals (CCTs). It should be observed that the
\*Qadaptive\*U (as opposed to dedicated) nature of CCTs calls for, in certain
areas, more flexibility, but without undue degradation of capabilities. The
issues of flexibility versus degradation of capabilities strongly influenced
the proposals made in this Recommendation.
.sp 9p
.RT
.PP
1.2
This Recommendation specifies how the various telematic
Recommendations may be used, and any additional requirements, to enable
computerized communication terminals to access the various telematic services.
It is noted that while this Recommendation is applicable to CCTs only when
accessing telematic services, consideration may be given to the use of the
technical aspects of this Recommendation if CCTs communicate with each other
utilizing the telematic protocols.
.PP
1.3
Section 2 describes the characteristics of computerized
communication terminals. The remaining sections define how the relevant
telematic Recommendations may be used to enable CCTs to access the telematic
services.
.PP
1.4
Figure 1/T.65 shows various methods for CCTs to access the
telematic services which are described
in \(sc\(sc\ 3 to\ 9.
.PP
Three methods are proposed:
.LP
i)
access to and from a telematic service via service access
facility (SAF) (see \(sc\ 3.4.2, for example);
.LP
ii)
direct access from and to a telematic service;
.LP
iii)
direct access from a CCT to a telematic service, reverse access via SAF
(see \(sc\ 3.4.3, for example).
.sp 2P
.LP
\fB2\fR \fBCharacteristics and model\fR
.sp 1P
.RT
.sp 1P
.LP
2.1
\fIDefinition\fR
.sp 9p
.RT
.PP
The term Computerized Communication Terminal (CCT) refers to a
device or equipment, which may be portable, with a processor and communication
facility, typically a user work station, which permits entry of various
applications and which can access CCITT\(hydefined services, such as telematics,
as prescribed in this Recommendation.
.bp
.RT
.LP
.rs
.sp 31P
.ad r
\fBFigure 1/T.65, p.1\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
2.2
\fICharacteristics\fR
.sp 9p
.RT
.PP
Computerized communication terminals differ in certain
characteristics from telematic terminals. The following subsections identify
the characteristics of CCTs. Characteristics specific to each case of the
access to telematic services are given in \(sc\(sc\ 3 to\ 9.
.RT
.sp 1P
.LP
2.2.1
\fICapability\fR
.sp 9p
.RT
.PP
A CCT maybe used to access the telematic services. The provisions in this
Recommendation provide a basic level of compatibility between CCTs and
the telematic services.
.RT
.sp 1P
.LP
2.2.2
\fIProtocols\fR
.sp 9p
.RT
.PP
In general, CCTs will use OSI protocols defined in the X.200\(hySeries
of Recommendations, but configured to meet the requirements defined in
the
relevant T\(hySeries of Recommendations. Exceptions include the cases of
access to the non\(hyOSI\(hybased telematic services, where the relevant
T\(hySeries of
Recommendations apply.
.RT
.sp 1P
.LP
2.2.3
\fITerminal requirements\fR
.sp 9p
.RT
.PP
In general, the relevant T\(hySeries Recommendations for terminal
requirements apply. The details specified to each access to telematic services,
and any additional (or relaxed) requirements are specified in \(sc\(sc\
3
to\ 9.
.bp
.RT
.sp 1P
.LP
2.3
\fIGeneral model\fR
.sp 9p
.RT
.PP
A model for CCTs accessing telematic services based on OSI is given in
Figure\ 2/T.65. The model identifies the relevant Recommendations applicable
to each level in the OSI layers, for each case of access to telematic services.
In particular, two sets of protocols are identified for access to OSI\(hybased
telematic services:
.RT
.LP
a)
A set of OSI protocols common to most accesses to telematic services
is identified for the lower layers up to and including the session
kernel in the session layer. The corresponding CCITT Recommendations required
are identified.
.LP
b)
Above the common set of protocols, additional session layer functional
units based on Recommendations\ X.215/X.225 are identified, together with
any Recommendations required for each of the cases of the access to
telematic services.
.PP
There are telematic services which require the use of
non\(hyOSI\(hybased protocols. In these cases, the common set of protocols
may not be applicable and the relevant T\(hySeries Recommendations must
be used.
.LP
.rs
.sp 39P
.ad r
\fBFigure 2/T.65, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
2.4
\fIMinimum capability\fR
.sp 9p
.RT
.PP
For a CCT to access an OSI\(hybased telematic service it must support all
the following capabilities, and any additional capability required for
each case of access to telematic service as prescribed in \(sc\(sc\ 3,
5, 6, 7, 8
and\ 9:
.RT
.LP
a)
The appropriate network capability as prescribed in \(sc 3 of Recommendation\
T.70.
.LP
b)
X.214/X.224 Class 0 Transport procedure.
.LP
c)
X.215/X.225 Kernel; together with half\(hyduplex, or
full\(hyduplex functional units.
.PP
\fINote\fR \ \(em\ The applicability of the minimum capability to videotex
access requires further study.
.sp 2P
.LP
\fB3\fR \fBAccess to the Teletex service\fR
.sp 1P
.RT
.sp 1P
.LP
3.1
\fIGeneral\fR
.sp 9p
.RT
.PP
The
access of CCTs to the Teletex service
is a common case of communication with an OSI\(hybased telematic service
due to the well defined
nature of Teletex. The following sections describe the characteristics
of such an access and specify how the various Teletex\(hyrelated Recommendations
may be
used.
.RT
.sp 1P
.LP
3.2
\fICharacteristics\fR
.sp 9p
.RT
.PP
3.2.1
From the technical point of view, CCTs will be able to
establish communications directly with a Teletex device and exchange documents
on a real\(hytime, end\(hyto\(hyend basis without the use of conversion
facilities.
.sp 9p
.RT
.PP
3.2.2
As far as possible, CCT access to the Teletex service should be
done via message handling systems. The technical implementation is a national
matter.
.PP
3.2.3
CCTs may not necessarily be available continuously to receive
incoming calls. However, when a CCT is available it will be technically
able to receive calls directly from and exchange documents with other Teletex
devices.
.PP
3.2.4
CCTs may technically use the Teletex protocol and terminal
characteristics as prescribed in \(sc\ 3.3 of this Recommendation to exchange
Teletex documents with each other.
.PP
3.2.5
If a Teletex device communicates with a CCT, it must be made aware of that
fact. How this information is conveyed within the Teletex terminal
identification with a specific value for Part\ 3 is described in \(sc\ 3.4.
.sp 2P
.LP
3.3
\fIApplicability of the relevant CCITT Recommendations\fR
.sp 1P
.RT
.sp 1P
.LP
3.3.1
\fIProtocols\fR
.sp 9p
.RT
.LP
a)
The network capabilities are in accordance with \(sc 3 of
Recommendation\ T.70.
.LP
b)
The transport procedure is in accordance with either:
.LP
\(em
Class 0 of the OSI transport protocol, as specified in Recommendations\
X.214/X.224, together with application rules to be compatible with and
conform to the \(sc\ 5 and Annexes of Recommendation\ T.70; or
.LP
\(em
Paragraph 5 and annexes of Recommendation T.70.
.LP
c)
The session layer procedure is in accordance with either:
.LP
\(em
Kernel with the functional units minor sync,
half\(hyduplex, capability data, activity management, and exceptions specified
in Recommendations\ X.215/X.225 together with application rules to be compatible
with and conform to Recommendation\ T.62; or
.LP
\(em
Recommendation T.62.
.LP
d)
The applicability of higher\(hylayer Recommendations, such as T.300 and
T.400, requires further study.
.sp 1P
.LP
3.3.2
\fITerminal requirements and character repertoire\fR
.sp 9p
.RT
.PP
The terminal requirements and character repertoire specified in
Recommendations\ T.60 and\ T.61 will apply except for the following:
.RT
.LP
a)
A CCT may or may not support full automatic operation.
.LP
b)
A CCT must be able to receive and store all characters
belonging to the basic Teletex character repertoire. However, only those
graphic characters which form the primary character set of the basic Teletex
character set as defined in Recommendation\ T.61 need to be presented.
.bp
.LP
c)
A CCT may require a different terminal identification from that of a
Teletex terminal. The format of this identification is defined in
\(sc\ 3.4.3.1.
.LP
d)
Other items require further study.
.sp 2P
.LP
3.4
\fIAccess methods\fR
.sp 1P
.RT
.sp 1P
.LP
3.4.1
\fIIntroduction\fR
.sp 9p
.RT
.PP
This paragraph describes a technical method for CCT access to and from
the Teletex service. This access method is based on the assumption that
CCTs should enjoy a maximum flexibility and that the service characteristics
of Teletex should not be degraded.
.PP
These prerequisites imply that the CCT must be supported by a service access
facility (SAF) which emulates the Teletex service characteristics and
provide for the handling of messages.
.RT
.sp 1P
.LP
3.4.2
\fIDescription of the access method\fR
.sp 9p
.RT
.PP
A CCT may establish a connection to the SAF at any time, from any network
and from any access point within these networks. If a CCT wants to
transmit a message but does not wish to receive a message, it need not be
identified. The message will be received by the SAF and forwarded immediately
to the Teletex destination. The SAF must add information which will indicate
to the Teletex destination that this message was originated by an unidentified
CCT.
.PP
If a CCT is to receive an answer to its previously transmitted
message, it should be able to register itself temporarily using a password.
The password will be provided by the CCT user. The message from the CCT
will be
forwarded immediately to the Teletex destination including information
that the answer may be placed in the SAF under the given password. Provisions
to allow positive or negative acknowledgements to the Teletex source and
to allow
control of the status of messages sent by the Teletex source are technically
feasible.
.PP
In the following, the functions of the SAF are described which are
needed to support CCTs for access to/from the Teletex service.
.RT
.sp 1P
.LP
3.4.3
\fIModel\fR (see Figure 3/T.65)
.sp 9p
.RT
.LP
.rs
.sp 11P
.ad r
\fBFigure 3/T.65, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.4.3.1
\fICCT to Teletex\fR
.sp 9p
.RT
.PP
The following functions will be provided by the SAF in order to
enable a CCT to access the Teletex service:
.RT
.LP
a)
insertion of an appropriate information from which the
Teletex subscribes can identify that the message is being sent from a CCT
(e.g.,\ the letters \*QCCT\*U into Part\ 3 of the Teletex\(hyTID);
.LP
b)
temporary registration on an optional basis (to allow
messages to be sent back to the CCT by a Teletex terminal, see
\(sc\ 3.4.3.2).
.bp
.sp 1P
.LP
3.4.3.2
\fITeletex to CCT\fR
.sp 9p
.RT
.PP
The following functions will be provided by the SAF in order to
enable a Teletex terminal to send documents to a CCT:
.RT
.LP
a)
memory for storing messages sent by the Teletex terminal;
.LP
b)
allocation of stored messages to registration numbers to
allow their retrieval by the CCT;
.LP
c)
means for a delivery notification call to the Teletex
terminal to indicate that the CCT has retrieved the message;
.LP
d)
a time\(hyout mechanism for deleting a message if not retrieved within
a certain period of time;
.LP
e)
additional notification calls (e.g., status of stored
messages) are for further study.
.sp 2P
.LP
\fB4\fR \fBAccess to the Group 3 facsimile service\fR
.sp 1P
.RT
.sp 1P
.LP
4.1
\fIGeneral\fR
.sp 9p
.RT
.PP
A CCT may be used to access the Group 3 facsimile service.
.RT
.sp 1P
.LP
4.2
\fICharacteristics\fR
.sp 9p
.RT
.PP
A CCT accessing the Group 3 facsimile service will operate in
accordance with the CCITT Recommendations\ T.4 and\ T.30.
.RT
.sp 2P
.LP
4.3
\fIApplicability of the relevant CCITT Recommendations\fR
.sp 1P
.RT
.sp 1P
.LP
4.3.1
\fIProtocols\fR
.sp 9p
.RT
.PP
The requirements defined in the CCITT Recommendation T.30
apply.
.RT
.sp 1P
.LP
4.3.2
\fIModulation systems\fR
.sp 9p
.RT
.PP
The requirements defined in the CCITT Recommendation T.4
apply.
.RT
.sp 2P
.LP
\fB5\fR \fBAccess to the Group 4 facsimile service\fR
.sp 1P
.RT
.PP
(For further study.)
.RT
.sp 2P
.LP
\fB6\fR \fBAccess to the mixed\(hymode option of the Teletex service\fR
.sp 1P
.RT
.PP
(For further study.)
.RT
.sp 2P
.LP
\fB7\fR \fBAccess to the videotex service\fR
.sp 1P
.RT
.sp 1P
.LP
7.1
\fIGeneral\fR
.sp 9p
.RT
.PP
A CCT may be used to access the videotex service. Since a videotex service
will not distinguish between what type of terminal is connected to it,
there are no special requirements for CCTs above those which apply to dedicated
videotex terminals.
.RT
.sp 1P
.LP
7.2
\fICharacteristics\fR
.sp 9p
.RT
.PP
7.2.1
CCTs accessing the videotex service should emulate videotex
terminal characteristics. In the emulation, attention should be given to the
profiles, ranks or service reference modes of the videotex terminals concerned
as used in the various videotex services. Where insufficient display
capabilities are available, a CCT should provide fall\(hyback by graceful
degradation of capabilities so that the integrity of the information content
is preserved. For example, a wide range of colours may fall back to fewer
related colours, or to grey scales, or an accented character may fall back
to a
character without accent.
.sp 9p
.RT
.PP
7.2.2
Videotex services are interactive and CCTs should be able to
transmit and receive data interactively.
.bp
.sp 2P
.LP
7.3
\fIApplicability of the relevant CCITT Recommendations\fR
.sp 1P
.RT
.sp 1P
.LP
7.3.1
\fIProtocols\fR
.sp 9p
.RT
.PP
To be defined.
.RT
.sp 1P
.LP
7.3.2
\fIData syntax and terminal requirements\fR
.sp 9p
.RT
.PP
The requirements defined in CCITT Recommendation T.101 (Annexes B, C and
D) apply.
.RT
.sp 2P
.LP
\fB8\fR \fBAccess to MHS\fR
.sp 1P
.RT
.sp 1P
.LP
8.1
\fIGeneral\fR
.sp 9p
.RT
.PP
This paragraph describes the characteristics of CCTs to access MHS and
specifies how the various related Recommendations may be used.
.RT
.sp 1P
.LP
8.2
\fICharacteristics\fR
.sp 9p
.RT
.PP
In its present form, the message handling system has as its
fundamental component the message transfer system (MTS), which comprises a
number of message transfer agents (MTAs). A CCT can then access MHS in
two ways as described in Figure\ 4/T.65 and the text below.
.RT
.LP
.rs
.sp 7P
.ad r
\fBFigure 4/T.65, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
i)
The CCT can access MHS through a telematic interworking
facility (TIF) as defined in Recommendations\ T.300\(hySeries.
.LP
ii)
The CCT can support the MHS user agent functions to access the MTS directly.
.sp 1P
.LP
8.3
\fIApplicability of the relevant CCITT Recommendations\fR
.sp 9p
.RT
.PP
When a CCT does not support the MHS user agent functions it shall access
MHS through a TIF, which provides for interworking between Telematic
services and MHS. In this case the relevant sections of
Recommendations\ T.300\(hySeries and\ T.65 apply, depending on the choice of
protocols and terminal characteristics.
.PP
When a CCT supports the MHS user agent functions in addition to the
Telematic protocols and terminal characteristics, it will use the relevant
sections in the series of Recommendations\ X.400.
.RT
.sp 2P
.LP
\fB9\fR \fBAccess to the directory service\fR
.sp 1P
.RT
.sp 1P
.LP
9.1
\fIGeneral\fR
.sp 9p
.RT
.PP
The access of CCTs to the directory service will often precede the other
CCITT\(hydefined services such as MHS, Teletex, or telephony, in order
to
determine or ascertain the address of a user or service. This section describes
the characteristics of such an access and specifies how the various related
Recommendations may be used.
.bp
.RT
.sp 1P
.LP
9.2
\fICharacteristics\fR
.sp 9p
.RT
.PP
In its present form, the directory system has two fundamental
components: the directory user agent (DUA) and the directory (see
Figure\ 5/T.65).
.RT
.LP
.rs
.sp 10P
.ad r
\fBFigure 5/T.65, p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
In terms of this model two ways of CCT access are possible:
.LP
i)
The CCT can access the DUA using suitable telematic
protocols and terminal characteristics defined in the T\(hySeries of
Recommendations.
.LP
ii)
The CCT can support DUA functions to access the directory directly.
.PP
It should be noted that directory access is essentially an
interactive application. Therefore, this interactive nature influences the
protocol and terminal requirements.
.sp 1P
.LP
9.3
\fIApplicability of the relevant CCITT Recommendations\fR
.sp 9p
.RT
.PP
When a CCT does not support DUA functions, it shall access the
directory through a DUA. In this case the relevant sections of the
Recommendations\ X.500 and\ T.65 apply, depending on the choice of protocols
and terminal characteristics.
.PP
When a CCT supports DUA functions in addition to the Telematic
protocols and terminal characteristics it will use the relevant sections
in the series of Recommendations\ X.500.
\v'1P'
.RT
.sp 2P
.LP
\fBRecommendation\ T.70\fR
.RT
.sp 2P
.sp 1P
.ce 1000
\fBNETWORK\(hyINDEPENDENT\ BASIC\ TRANSPORT\ SERVICE\fR |
\fBFOR\ THE\ TELEMATIC\ SERVICES\fR
.EF '% Fascicle\ VII.5\ \(em\ Rec.\ T.70''
.OF '''Fascicle\ VII.5\ \(em\ Rec.\ T.70 %'
.ce 0
.sp 1P
.ce 1000
\fI(Geneva, 1980, amended at Malaga\(hyTorremolinos, 1984,\fR
.sp 9p
.RT
.ce 0
.sp 1P
.ce 1000
\fIand Melbourne, 1988)\fR
.ce 0
.sp 1P
.LP
The\ CCITT,
.sp 1P
.RT
.sp 1P
.LP
\fIconsidering\fR
.sp 9p
.RT
.PP
(a)
that the Teletex service will be introduced in different types of network,
i.e.\ circuit\(hyswitched public data networks (CSPDN),
packet\(hyswitched public data networks (PSPDN) and the public switched
telephone network\ (PSTN);
.PP
(b)
that there is a need for international
interworking \ between terminals
belonging to the same or different types of
Telematic services
;
.bp
.LP
\fIunanimously declares the following view\fR
.sp 1P
.RT
.sp 2P
.LP
\fB1\fR \fBScope\fR
.sp 1P
.RT
.PP
1.1
This Recommendation defines the \fInetwork\(hyindependent basic\fR
\fItransport service\fR applicable to Teletex and Group\ 4 facsimile terminals
connected to the types of network mentioned in\ (a) above in terms of:
.sp 9p
.RT
.LP
a)
the transport services provided to the higher layer [the
transport services are provided by the transport layer (layer\ 4)
in association with the underlying services provided by the
supporting layers\ 1 to\ 3];
.LP
b)
the transport layer procedure (see \(sc\ 5 below).
.PP
1.2
Paragraph 2 describes the transport service. Paragraph\ 3
describes the transport service implementation for different types of networks.
Paragraph\ 4 outlines the guidelines for interworking between networks.
Paragraph\ 5 specifies the transport layer procedure, and Annexes\ A and\ B
provide associated state transition diagrams and tables respectively.
.LP
\fB2\fR \fBTransport service\fR
.sp 1P
.RT
.sp 2P
.LP
2.1
\fITransport service objectives\fR
.sp 1P
.RT
.PP
2.1.1
The purpose of the transport service is to provide two
communicating session entities within two terminals with transport services,
i.e.\ the means for transparent and reliable end\(hyto\(hyend transfer
of data between them irrespective of the particular type of network used.
.sp 9p
.RT
.PP
2.1.2
The main requirements of the transport service to be provided
by a transport entity to the local transport user, i.e.\ the session entity,
are:
.LP
a)
\fINetwork independence\fR . The transport service shall be
homogeneous, while allowing a suitable wide variety of
underlying communications media, protocols and mechanisms.
.LP
b)
\fIEnd\(hyto\(hyend significance\fR . The transport service shall have
end\(hyto\(hyend significance, connecting the end users irrespective
of the number of individual communication links used.
.LP
c)
\fITransparency\fR . The transport service shall be octet
transparent, i.e.\ not restrict the content, format or coding of
the information (data or control) received from or delivered to
the transport user.
.LP
d)
\fIError\(hyfree delivery\fR . The transport service shall assure
error\(hyfree delivery. Non\(hyrecoverable errors are to be visible to
the transport service user.
.LP
e)
\fICost efficiency\fR . The transport service shall optimize the
use of the available communication resources to provide the
performance required by each communicating transport user at
maximum efficiency.
.sp 2P
.LP
2.2
\fIGeneral structure of the transport service\fR
.sp 1P
.RT
.PP
2.2.1
The general structure of the transport service is shown in
Figure\ 1/T.70.
.sp 9p
.RT
.sp 2P
.LP
\fB3\fR \fBTransport service implementation for different types of\fR
\fBnetworks\fR
.sp 1P
.RT
.PP
\fINote\fR \ \(em\ The
transport layer
procedure on all types of
networks is defined in \(sc\ 5. The network dependent control procedures of the
underlying layers are described in the following.
.RT
.sp 2P
.LP
3.1
\fITerminals connected to a PSPDN\fR
.sp 1P
.RT
.sp 1P
.LP
3.1.1
\fIPhysical layer\fR \fIDTE/DCE interface characteristics\fR
.sp 9p
.RT
.PP
The physical layer of Recommendation\ X.25 applies.
.RT
.sp 1P
.LP
3.1.2
\fILink layer procedure\fR
.sp 9p
.RT
.PP
The
link layer
procedure shall, unless otherwise specified, be the symmetrical procedures
as specified in Recommendation\ X.25, LAPB (Link Access Procedure\ B).
.bp
.RT
.LP
.rs
.sp 47P
.ad r
\fBFigure 1/T.70 TO803680\(hy89, p.6\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
3.1.3
\fINetwork layer procedure\fR
.sp 9p
.RT
.PP
Recommendation X.25 Virtual Call procedures apply. However the
following points should be noted when using this
transport
protocol
:
.RT
.LP
a)
The
qualifier bit
in data packets should always be set to 0.
.LP
b)
The
delivery confirmation bits
in all packets should
be set to\ 0.
.LP
c)
The terminal should not send an \fIinterrupt request\fR packet.
.LP
d)
Normal X.25 reset procedures will apply.
.LP
e)
Each control block or data block of the
transport
layer
shall be carried in a complete data packet sequence.
.LP
f
)
The terminal should not send a \fIDTE\ REJ packet\fR .
.LP
g)
Terminals shall use a specific
protocol identifier
within
call request/incoming call packets for the Teletex service and
Group\ 4 facsimile apparatus. This identifier is represented
by the first octet of the call user data field (remaining
octets, if any, should be ignored) as shown below:
.LP
bit
87654321
.LP
octet\ 1
00000010
.LP
In the case of CSPDN/PSPDN interworking the functional
mapping of this protocol identifier requires further study.
.LP
h)
Terminals shall not use the fast select facility.
.sp 2P
.LP
3.2
\fITerminals connected to the PSTN\fR
.sp 1P
.RT
.sp 1P
.LP
3.2.1
\fIPhysical layer DTE/DCE interface characteristics\fR
.sp 9p
.RT
.PP
The DTE/DCE physical layer element shall be in accordance with
existing Series\ V Recommendations. The physical layer may provide for
half\(hyduplex or full\(hyduplex transmission depending on the modem standard.
.PP
\fINote\fR \ \(em\ The PSTN modem standards are discussed in Study Group\
XVII. Furthermore, in the case of a modem integrated in the terminal, the
interface may only be functionally equivalent to a Series\ V Recommendation.
This is also for further consideration in Study Group\ XVII.
.RT
.sp 2P
.LP
3.2.2
\fILink layer procedure\fR
.sp 1P
.RT
.PP
3.2.2.1
Depending on the service provided by the physical layer, the
link layer procedures over a single physical circuit between two terminals
have to cater for a half\(hyduplex or full\(hyduplex transmission facility
to provide a
full\(hyduplex service to the network layer. For full\(hyduplex physical layer
service, the link layer procedure shall conform to the
Link Access
Procedure
described in Recommendation\ X.75, for single link operation. For addressing
assignments and the system parameters see \(sc\(sc\ 3.2.2.2 and\ 3.2.2.3
respectively. For half\(hyduplex physical layer service the link layer
procedure is as defined in Recommendation\ T.71. This is a half\(hyduplex
Link Access Procedure
,
based on Recommendation\ X.75 for single link operation.
.sp 9p
.RT
.PP
3.2.2.2
The following describes the application of the
link
addressing
procedure
of Recommendation\ X.75. Link addresses (A\ and\ B) shall be
assigned dynamically or on a per\(hycall basis according to the following
rules:
.LP
a)
the calling terminal shall take Address\ A;
.LP
b)
the called terminal shall take Address\ B;
.LP
c)
commands and responses shall be transferred as shown in
Figure\ 2/T.70;
.LP
d)
A and B addresses are coded as follows:
.LP
Address
12345678
.LP
\ \ A
11000000
.LP
\ \ B
10000000
.PP
\fINote\fR \ \(em\ The terminal will discard all frames received with an
address other than\ A and\ B.
.bp
.LP
.rs
.sp 9P
.ad r
\fBFigure 2/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
3.2.2.3
System parameters are:
.LP
a)
timer, T1;
.LP
b)
maximum number of retransmissions, N2;
.LP
c)
maximum number of bits in an I frame, N1;
.LP
d)
maximum number of outstanding I frames, \fIk\fR .
.PP
The above system parameters are to be specified by the
Administration.
However, the possible range of values that may be attributed to each parameter
is to be standardized. Such values are for further study.
.sp 2P
.LP
3.2.3
\fINetwork layer procedure\fR
.sp 1P
.RT
.PP
3.2.3.1
See \(sc\ 3.1.3. In addition, for all calls (PSTN only, PSTN\(hyPSPDN,
PSTN\(hyPSPDN\(hyPSTN)
second stage addressing
will apply using X.25 virtual call procedures. The calling terminal should
include the called address and the calling address (see Note\ 2) in call
request packets. The format of the called address shall conform to:
.sp 9p
.RT
.LP
a)
the telephone network addressing scheme for PSTN only
calls;
.LP
b)
the telephone network addressing scheme with an X.121 DNIC
for PSTN\(hyPSPDN calls (see Note\ 3);
.LP
c)
the X.121 addressing scheme for PSTN\(hyPSPDN calls (see
Note\ 1).
.PP
\fINote\ 1\fR \ \(em\ For other cases of internetworking the above rule
shall apply.
.PP
\fINote\ 2\fR \ \(em\ In the case of PSTN\(hyPSPDN calls the verification
of the
calling address by the network requires further study. The format of the
calling address is for further study.
.PP
\fINote\ 3\fR \ \(em\ The feasibility of such connections is for further
study.
.RT
.sp 2P
.LP
3.3
\fITerminals connected to a CSPDN\fR
.sp 1P
.RT
.sp 1P
.LP
3.3.1
\fIPhysical layer DTE/DCE interface characteristics\fR
.sp 9p
.RT
.PP
The DTE/DCE physical interface characteristics shall be in
accordance with Recommendation\ X.21, or as an option, Recommendation\
X.22 for multi\(hycall operation.
.RT
.sp 2P
.LP
3.3.2
\fILink layer procedure\fR
.sp 1P
.RT
.sp 1P
.LP
3.3.2.1
\fIGeneral\fR
.sp 9p
.RT
.PP
The link layer procedure shall be used during the data phase of
Recommendation\ X.21 (or\ X.22) for data interchange over a single physical
circuit between two terminals operating in User Classes of Services\ 3 to\ 7
and\ 30 as defined in Recommendation\ X.1. The link layer procedure shall
consist of a fully symmetrical HDLC procedure as defined in Recommendation\
X.75 for
single link operation.
.bp
.RT
.sp 1P
.LP
3.3.2.2
\fILink layer address procedure\fR
.sp 9p
.RT
.PP
The following describes the application of the link addressing
procedures of Recommendation\ X.75. Link addresses (A\ and\ B) shall be
assigned dynamically on a per\(hycall basis according to the following
rules:
.RT
.LP
a)
the calling terminal shall take address\ A;
.LP
b)
the called terminal shall take address\ B;
.LP
c)
commands and responses shall be transferred as shown in
Figure\ 3/T.70;
.LP
d)
A and B addresses are coded as follows:
.LP
Address
12345678
.LP
\ \ A
11000000
.LP
\ \ B
10000000
.PP
\fINote\fR \ \(em\ The terminal will discard all frames received with an
address other than\ A and\ B.
.LP
.rs
.sp 9P
.ad r
\fBFigure 3/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
3.3.2.3
\fILink layer implementation rules\fR
.sp 9p
.RT
.PP
In order to achieve full compatibility between different
implementations, the rules below for the implementation of Recommendation\
X.75 shall be followed.
.RT
.sp 1P
.LP
3.3.2.3.1
\fIGeneral rules\fR
.sp 9p
.RT
.LP
a)
The 1984 version (\fIRed Book\fR ) of CCITT Recommendation\ X.75, \(sc\
2, shall be used as the reference specification.
.LP
b)
The term \*QSTE\*U shall be read as \*QDTE\*U.
.LP
c)
The Non\(hyExtended mode of operation (modulo 8) shall be
used.
.LP
d)
Only the single link procedure (SLP) shall be used.
.sp 1P
.LP
3.3.2.3.2
\fISpecific rules\fR
.sp 9p
.RT
.PP
The following rules refer to the indicated sections and tables of Recommendation\
X.75.
.RT
.LP
a)
\fITable 1/X.75\fR (see Note 1)
.LP
I\(hyframe must not be sent with an empty I\(hyfield.
.LP
N\ \(>="\ 0
.LP
N\ \(=\ N1 \(em 32
.LP
A received empty I\(hyframe shall be treated as a valid
I\(hyframe.
.LP
b)
\fI\(sc 2.3.4.9\fR
.LP
Subparagraphs 5), 6) and 7) are not valid (shall not result in the sending
of a FRMR). Instead the following actions shall be implemented:
.LP
\(em
Not expected supervisory frames with the F bit set to 1 shall be ignored.
.LP
\(em
Not expected UA or DM response shall be ignored.
.LP
\(em
Frames with an invalid N(S) shall be responded to by sending REJECT.
.LP
Frames with a FRMR cntrol field shall not be responded by sending a FRMR.
.bp
.LP
c)
\fITable 7/X.75\fR
.LP
Bits W, X, Y and Z set to 0 indicate that no reason for
frame rejection is given.
.LP
d)
\fI\(sc 2.3.5.3\fR
.LP
The DTE and the CSPDN are not octet aligned and the last
paragraph is therefore not valid.
.LP
e)
\fI\(sc 2.3.5.5\fR
.LP
Higher layers should be notified when timer T3 expires
(excessive idle state).
.LP
f
)
\fI\(sc 2.4.3\fR
.LP
Related to the first paragraph, read instead of \*Qnext
response\*U \*Qcorresponding response\*U.
.LP
g)
\fI\(sc 2.4.4.1\fR
.LP
In the active channel state, the DTE shall transmit
contiguous flags independent of the other DTE.
.LP
The calling DTE shall initialize the link by sending a SABM command with
the P\ bit set to\ 1.
.LP
h)
\fI\(sc 2.4.4.4.1\fR
.LP
A condition for entering the disconnected phase is also that no unacknowledged
DISC command exists, because of collision cases (Ref.\ X.75, \(sc\ 2.4.4.5).
.LP
In the disconnected phase, it is the calling DTE which may initiate link
set up.
.LP
i)
\fI\(sc 2.4.5.9, fourth paragraph\fR
.LP
If a RNR is received, the DTE shall remain in the timer
recovery condition (because the other DTE is still in the busy condition).
.LP
j)
\fI\(sc 2.4.5.9, fifth paragraph\fR
.LP
If a RNR is received, the DTE shall not resume I\(hyframe
transmission or retransmission.
.LP
k)
\fI\(sc 2.4.5.9, last paragraph\fR
.LP
If the transmission attempt variable is equal to N2, the DTE shall enter
the disconnected phase.
.LP
l)
\fI\(sc 2.4.7.3\fR
.LP
In the frame rejection condition, the DTE shall only check the commands
and react with a FRMR according to the P\ bit.
.LP
The frame rejection condition is cleared when the DTE
receives a SABM, or, receives or transmits a DISC command.
.LP
m)
\fI\(sc 2.4.7.3, second paragraph\fR (see Note 2)
.LP
Only the DTE which caused the FRMR condition may try to
reset the link.
.LP
n)
\fI\(sc 2.4.7.3, third paragraph\fR (see Note 3)
.LP
After N2 attempts to get the other DTE to reset the link,
the DTE shall enter the disconnected phase.
.LP
o)
\fI\(sc 2.4.8.1\fR (see Note 4)
.LP
The timer T1 shall be started at the end of frame
transmission. The value of T1 depends on the data signalling rate, the frame
length, the value of N2 and a fixed time of 1.5\ s representing both T2
and the transmission delay [see \(sc\ 3.3.2.3.2 | )]. The recommended value
range is:
1.5\(hy15\ s.
.LP
p)
\fI\(sc 2.4.8.2\fR (see Note 4)
.LP
T1\ >\ T2
.LP
T2\ \(=\ 1\ s
.LP
Depending on the acknowledgement strategy used, the DTE
designer may regard T2 as a decision parameter only, in which case the
DTE is not obliged to implement a corresponding timer.
.LP
q)
\fI\(sc 2.4.8.3, second paragraph\fR
.LP
30\ s\ \(=\ T3\ \(=\ 60\ s
.LP
r)
\fI\(sc 2.4.8.4\fR
.LP
N2\ \(mu\ T1\ \(>="\ 60\ s
.LP
s)
\fI\(sc 2.4.8.5\fR
.LP
N1 = 1080 + (\fIn\fR \(mu 1024) bits; \fIn\fR = 0 or 1 or 3 or 7 or
15.
.LP
t)
\fI\(sc 2.4.8.6\fR (see Note 4)
.LP
\fIk\fR = 2 \(em 7 (modulo 8)
.bp
.LP
\fINote\ 1\fR \ \(em\ Terminals complying with the \fIRed Book\fR version of
Recommendation\ T.70 may react by DL Reset ind. (FRMR).
.LP
\fINote\ 2\fR \ \(em\ Terminals complying with the \fIRed Book\fR version of
Recommendation\ T.70 may react differently.
.LP
\fINote\ 3\fR \ \(em\ It is not meaningful to reset the link if the other
DTE is not responding for N2\ \(mu\ T1.
.LP
\fINote\ 4\fR \ \(em\ The acknowledgement strategy used by the receiving
DTE should be independent of any knowledge about the value of \fIk\fR used
by the
sending
DTE. This can be achieved by either acknowledging every correctly received
I\(hyframe as soon as possible or by implementing an acknowledgement timer,
i.e.,\ a T2 timer as defined above [see \(sc\ 3.3.2.3.2 | )].
.sp 2P
.LP
3.3.3
\fINetwork layer procedure\fR
.sp 1P
.RT
.sp 1P
.LP
3.3.3.1
\fICall control phase\fR
.sp 9p
.RT
.PP
The call control procedure conforms to Recommendation\ X.21, or as an option,
Recommendation\ X.22 for multi\(hycall operation.
.RT
.sp 1P
.LP
3.3.3.2
\fIData transfer phase\fR
.sp 9p
.RT
.PP
A minimal network layer is present during the data transfer phase and accommodated
through the use of a two\(hyoctet network block header. The
header comprises a one\(hyoctet length indicator followed by a network
block type code specified below. The only network block currently defined
is a
network protocol data block
as shown in Figure\ 4/T.70.
.RT
.LP
.rs
.sp 22P
.ad r
\fBFigure 4/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
3.3.3.3
\fIData transfer procedure\fR
.sp 1P
.RT
.sp 1P
.LP
3.3.3.3.1
\fIHandling of the\fR
\fIM\(hybit\fR
.sp 9p
.RT
.PP
The calling DTE shall negotiate the TPDU size with the called DTE at the
transport layer, based on either the maximum TPDU size supported or the
optimum TPDU size for the specific call, unless the default value of 128
octets is used. The agreed value will allow the sending DTE to transfer
TPDUs without the need for segmenting at the Network layer and consequently
the M\(hybit is set to zero.
.bp
.PP
However, receiving DTEs must always be capable of reassembling
segmented TPDUs by using the M\(hybit, since segmenting may take place in the
network in some interworking situations, e.g.,\ when the composite network
connection comprises a PSDN.
.RT
.sp 1P
.LP
3.3.3.3.2
\fIError procedures\fR
.sp 9p
.RT
.PP
A Data PDU with a length indicator different from hexadecimal \*Q01\*U
and/or with less than three octets shall be discarded and the physical
network connection shall be cleared.
.RT
.sp 1P
.LP
3.4
\fITerminals connected to an ISDN\fR
.sp 9p
.RT
.PP
See Recommendation T.90.
.RT
.sp 2P
.LP
\fB4\fR \fBInterworking between networks\fR
.sp 1P
.RT
.PP
4.1
It is the responsibility of Administrations to decide in which network(s)
the telematic services are to be provided.
.sp 9p
.RT
.PP
4.2
Four possibilities are considered below:
.LP
a)
Terminals connected to a circuit switched public data
network\ (CSPDN);
.LP
b)
Terminals connected to a packet switched public data
network\ (PSPDN);
.LP
c)
Terminals connected to a public switched telephone
network\ (PSTN);
.LP
d)
Terminals connected to an integrated services digital
network (ISDN).
.PP
4.3
Interworking between telematic terminals connected to any
network must be possible.
.PP
4.4
International interworking between telematic terminals shall
preferably take place between networks of the same type when these networks
are provided by both countries involved.
.PP
4.5
In the case of international interworking between telematic
terminals connected to dissimilar networks, Recommendation\ X.300 shall apply.
.PP
The interworking between CSPDNs and PSPDNs is described in
Recommendation\ X.82 (Detailed arrangements for interworking between CSPDNs
and PSPDNs based on Recommendation\ T.70).
.LP
\fB5\fR \fBTransport layer procedure\fR
.sp 1P
.RT
.sp 2P
.LP
5.1
\fITransport functions\fR
.sp 1P
.RT
.sp 1P
.LP
5.1.1
\fIGeneral\fR
.sp 9p
.RT
.PP
5.1.1.1
The transport layer will perform all those functions that are necessary
to bridge the gap between the services provided by the network layer and
the services needed by the
session layer
. Therefore, the functions performed are dependent on two criteria: the
services provided by the
underlying network layer and the services required by the session layer.
.sp 9p
.RT
.PP
5.1.1.2
It is the responsibility of the transport service user to
select a given quality of service, which may imply the use of certain
transport layer functions such as:
.LP
a)
establishment of a transport connection
.LP
\(em
transport connection
identification
.LP
\(em
transport connection multiplexing;
.LP
b)
data transfer
.LP
\(em
sequence control
.LP
\(em
error detection
.LP
\(em
error recovery
.LP
\(em
segmenting and reassembling
.LP
\(em
flow control
.LP
\(em
purge;
.LP
c)
termination of a transport connection.
.PP
\fINote\fR \ \(em\ Not all of the above functions will be available in
the basic transport service (see \(sc\ 5.1.3).
.bp
.sp 2P
.LP
5.1.2
\fITransport protocol classes\fR
.sp 1P
.RT
.PP
5.1.2.1
Transport layer functions are grouped (for ease of negotiation) into a
hierarchical system of transport protocol classes whereby classes
occupying superior positions in the hierarchy implement functions of the
lower classes together with the optional functions identified for their own
class.
.sp 9p
.RT
.PP
5.1.2.2
During transport connection establishment the use of a given
transport protocol and optional functions should be negotiated according to
the following rules:
.LP
\(em
the calling terminal indicates the transport protocol class
and (if applicable) the optional functions required;
.LP
\(em
the called terminal indicates the transport protocol class
and (if applicable) the optional functions that it is willing to
support;
.LP
\(em
all parameters to be used in the transport connection must be
explicity indicated, otherwise default values will apply.
.PP
5.1.2.3
The basic transport service described here is fulfilled by a
protocol denoted in Recommendation\ X.224 as transport protocol class\ 0.
That protocol class is compatible with Recommendation\ T.70. In the event
of a discrepancy between transport protocol class\ 0 as described in
Recommendation\ X.224 and Recommendation\ T.70, the latter takes precedence.
.sp 2P
.LP
5.1.3
\fIThe basic transport service (TS)\fR
.sp 1P
.RT
.PP
5.1.3.1
A limited set of transport layer functions is defined for a
basic transport service. The basic transport service is provided by transport
layer functions which are performed by
\fItransport layer protocol\fR
\fIelements\fR .
.sp 9p
.RT
.PP
5.1.3.2
Transport protocol data units (TPDUs) carrying transport
service (TS) user information or control information are called \fIblocks\fR .
.PP
5.1.3.3
Transport layer block types are as follows:
.LP
a)
transport connection request (TCR) block;
.LP
b)
transport connection accept\fR (TCA) block;
.LP
c)
transport connection clear (TCC) block;
.LP
d)
transport data (TDT) block;
.LP
e)
transport block reject (TBR) block.
.PP
5.1.3.4
The TCR and TCA blocks are used to indicate the protocol class,
and optional functions, applying to a transport connection. The TCC\ block is
used to indicate the reason for refusing a connection establishment. The TDT
block carries information of the transport service user. The TBR block
is used to report procedure errors to the remote terminal.
.sp 2P
.LP
5.1.4
\fITransport layer functions\fR
.sp 1P
.RT
.PP
5.1.4.1
Basic class functions and associated transport layer protocol elements,
i.e.\ blocks, include:
.sp 9p
.RT
.LP
a)
transport connection establishment, transport connection
identification, optional extended addressing and optional
transport data block size negotiation (TCR, TCA and\ TCC
blocks);
.LP
b)
data delimitation, segmentation/reassembling of arbitrarily
long transport service data units (TSDU). These are contained
within TDT\ blocks. The end of a TSDU is indicated by a TSDU end
mark in the last data block;
.LP
c)
detection and indication of procedural errors
(TBR\ block).
.PP
5.1.4.2
Other characteristics of the basic transport service are:
.LP
a)
maintenance of TSDU integrity;
.LP
b)
overflow: if the user cannot absorb new data and if the
appropriate buffers are not available, flow control is performed
at the network/link layer as appropriate;
.LP
c)
error: no mechanism is provided within the transport layer
to facilitate recovery from detected errors. Where such errors
are detected the user of the transport service should be
informed so that appropriate recovery action may be taken.
.bp
.LP
5.2
\fIDescription of connection establishment and termination\fR
\fIprocedures\fR
.sp 1P
.RT
.sp 2P
.LP
5.2.1
\fIGeneral\fR
.sp 1P
.RT
.PP
5.2.1.1
The transport layer connection establishment and termination
procedures shall also be used for negotiating transport protocol class
and, if applicable, optional transport connection functions.
.sp 9p
.RT
.PP
5.2.1.2
For the basic transport service, means are provided to
establish
a transport connection using a TCR\ block and a TCA\ block. This exchange
provides:
.LP
a)
a way to negotiate options;
.LP
b)
a transport connection identification. The transport
connection is identified by use of cross\(hyreferences. Each end of
the connection is responsible for selecting a suitable transport
connection identifier.
.PP
5.2.1.3
This mechanism also provides an identification of the transport
connection independent of any network connection identification and therefore
provides independence from the life of the network connection. The binary
value\ 0 should not be used as an identifier. The use of such references for
reconnection requires further definition.
.sp 2P
.LP
5.2.2
\fITransport connection request (TCR) block\fR
.sp 1P
.RT
.PP
5.2.2.1
The calling terminal shall indicate a transport connection
request by transferring a TCR\ block to the remote terminal. The TCR\ block
includes the transport functions (e.g.\ source reference, class, and optional
functions) for negotiation of the characteristics of the transport connection
being established.
.sp 9p
.RT
.sp 2P
.LP
5.2.3
\fITransport connection accept (TCA) block\fR
.sp 1P
.RT
.PP
5.2.3.1
The called terminal shall indicate its acceptance of the
transport connection by transferring a TCA\ block to the remote terminal. The
TCA\ block includes the transport parameters applying to the connection
and to be used by the calling terminal.
.sp 9p
.RT
.PP
5.2.3.2
If a terminal receives the request for an optional TDT block
size it may either:
.LP
\(em
indicate its support by reproducing the requested value
in the TCA\ block;
.LP
\(em
request in the TCA block the use of a shorter allowable
TDT\ block. The calling side either accepts this size by sending
the first TDT\ block or disconnects the network connection;
.LP
\(em
not accept the requested TDT block size parameter value by
sending a TCA\ block without a TDT\ block size parameter.
Therefore, the standardized TDT\ block size will apply.
.PP
A TCR requesting an optional TDT block size not supported by
the called side should not be answered with\ TBR.
.sp 2P
.LP
5.2.4
\fITransport connection clear (TCC) block\fR
.sp 1P
.RT
.PP
5.2.4.1
If a transport connection cannot be established, the called
terminal shall respond to the TCR\ block with a TCC\ block. The clearing cause
shall indicate why the connection was not accepted.
.sp 9p
.RT
.PP
It is up to the calling side whether the receipt of a TCC will
cause complete disconnection or whether a new TCR with a parameter different
from the first one will be sent (e.g.\ another extended transport layer
address). In order to allow for subsequent TCRs, the sender of TCC may
provide in the optional parameter field an appropriate parameter and associated
value to indicate that another TCR is invited. The new optional parameter
and its
associated value(s) are for further study.
.PP
\fINote\fR \ \(em\ There is no explicit transport connection termination
procedure in this Recommendation. Therefore, the lifetime of the transport
connection is directly correlated to the lifetime of the supporting network
connection.
.RT
.sp 2P
.LP
5.2.5
\fITransport connection collision\fR
.sp 1P
.RT
.PP
5.2.5.1
If the calling terminal receives a TCR block, it shall transfer a TBR\
block to notify the called terminal of the procedure error (see
Annex\ B).
.bp
.sp 9p
.RT
.sp 2P
.LP
5.2.6
\fIExtended addressing\fR
.sp 1P
.RT
.PP
5.2.6.1
The extended addressing capability may be used to address
terminals in a multiterminal configuration.
.sp 9p
.RT
.PP
The extension addresses for called and calling terminals are
optional parameters to TCR and TCA. The use of the calling extension address
is for further study.
.PP
5.2.6.2
The receiving terminal shall respond with a TCA according to
Table\ 1/T.70.
.ce
\fBH.T. [T1.70]\fR
.ce
TABLEAU\ 1/T.70
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(72p) | cw(78p) sw(78p) , ^ | c | c.
Received TCR Receiver reaction
{
Multi\(hyterminal
with extended addressing | ua\d\u)\d
} Stand\(hyalone terminal
_
.T&
lw(72p) | cw(78p) | cw(78p) .
Without extended addressing {
Send TCA
with extended addressing
} {
Send TCA
without extended addressing
}
_
.T&
lw(72p) | cw(78p) | cw(78p) .
With extended addressing {
Send TCA
with extended addressing | ub\d\u)\d
} {
Send TCA
without extended addressing
}
.TE
.LP
\ua\d\u)\d
Multi\(hyterminal configuration, with capability for extended
addressing.
.LP
\ub\d\u)\d
If the called terminal is occupied or out of order, the call
should be routed to a default terminal or mailbox. The sender will then be
informed of the routing by the extension address of the connected terminal. The receiver of TCR may also in this case react by sending TCC.
.nr PS 9
.RT
.ad r
\fBTable 1/T.70 [T1.70], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.sp 3
.PP
5.2.6.3
The calling terminal may, when receiving a called terminal
address in the TCA, act as specified in Table\ 2/T.70.
.ce
\fBH.T. [T2.70]\fR
.ce
TABLE\ 2/T.70
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(72p) | cw(56p) sw(50p) sw(50p) , ^ | c s s
^ | c | c | c.
Sent TCR Calling terminal reaction
TCA received with: No extended addressing Correct extended addressing Incorrect extended addressing
_
.T&
lw(72p) | cw(56p) | cw(100p) .
Without extended addressing OK Neglect extension (See Note)
_
.T&
lw(72p) | cw(56p) | cw(50p) | cw(50p) .
With extended addressing \ua\d\u)\d OK \ua\d\u)\d
.TE
.LP
\ua\d\u)\d\ Reaction left to the discretion of the calling terminal.
.LP
\fINote\fR
\ \(em\ Terminal complying with the 1980\(hy1984 version of Recommendation T.70 may react by releasing the network connection.
.nr PS 9
.RT
.ad r
\fBTable 2/T.70 [T2.70], p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
5.3
\fIDescription of\fR \fIdata transfer procedures\fR
.sp 1P
.RT
.sp 2P
.LP
5.3.1
\fIGeneral\fR
.sp 1P
.RT
.PP
5.3.1.1
The data transfer procedure described in the following
subsections applies only when the transport layer is in the data transfer
phase, i.e.\ after completion of transport connection establishment and
prior to clearing.
.sp 9p
.RT
.PP
\fINote\fR \ \(em\ When a connection is cleared, transport data blocks
may be discarded. Hence it is left to the transport service user to define
protocols able to cope with the various possible situations that may occur.
.sp 2P
.LP
5.3.2
\fITransport data block (TDT) length\fR
.sp 1P
.RT
.PP
5.3.2.1
The standard maximum TDT block length to be supported by all
terminals is 128\ octets including the data block header octets. However, the
TDT\ block length may be restricted to a lower value when the TDT\ block is
concatenated with other TDT\ blocks (see \(sc\ 5.5.3).
.sp 9p
.RT
.PP
5.3.2.2
Other maximum data field lengths may be supported in
conjunction with an optional TDT\ block size negotiation connection function
(see \(sc\(sc\ 5.5.4.3 and\ 5.5.5.3). Optional maximum data field lengths
shall be
chosen from the following:\ 256, 512, 1024 and\ 2048\ octets. If the requested
optional TDT\ block size cannot be supported, a shorter allowable TDT\
block size must be selected (see \(sc\ 5.2.3.2).
.PP
The agreed maximum TDT block size should be aimed at for
TDT\ blocks having the TSDU end mark set to\ 0 and a number of octets less
than the agreed maximum shall not cause the receiving transport entity
to reject
this TDT\ block.
.sp 2P
.LP
5.3.3
\fITransport service data unit (TSDU)\fR \fIend\fR
.sp 1P
.RT
.PP
5.3.3.1
The
\fITSDU end mark\fR is used to preserve the integrity of the TSDU. The
TSDU end mark is set to binary\ 1 in the last TDT\ data block
carrying information related to a certain TSDU. Exceptionally, this
TDT\ block may be sent without carrying user information in order to allow
for an immediate termination of a TSDU in certain error conditions.
.sp 9p
.RT
.PP
In case of a TSDU that comprises a single TDT\ block the TSDU end mark
is also set to\ 1. In all other cases the TSDU end mark is set to zero.
.sp 2P
.LP
5.4
\fITreatment of procedure errors\fR
.sp 1P
.RT
.PP
5.4.1
A terminal shall send a TBR block to the remote terminal to
report the receipt of an invalid or not implemented block (if not explicitly
specified otherwise in this Recommendation). During the establishment of a
transport connection, terminals shall not send a TBR\ block upon the receipt
of a TCR\ block whose parameters or parameter values are invalid or not
implemented. In this case, terminals shall act as if no errors have occurred
and send the appropriate response (if any).
.sp 9p
.RT
.PP
A terminal receiving a TBR block shall take appropriate recovery action.
.PP
\fINote\ 1\fR \ \(em\ A TBR whether invalid or valid shall not be answered by
sending a TBR\ block.
.PP
\fINote\ 2\fR \ \(em\ Terminals complying with the Recommendation\ T.70
version of the 1981\(hy1984 study period may react to all of the above
indicated
conditions by sending\ TBR.
.PP
\fINote\ 3\fR \ \(em\ The definition of invalid block/parameter, etc. is
provided by the state transition tables (see Annex\ B).
.PP
\fINote\ 4\fR \ \(em\ A TCR of which the PV of the TPDU size parameter is less
than 07 (which is the basic length of the transport block size) shall be
considered as an invalid TPDU.
.PP
\fINote\ 5\fR \ \(em\ In the states 1.1 for the calling side and 2.1 for the
calling and called side the terminal may react either by sending TBR or by
releasing the network connection.
.PP
\fIAttention\fR | \ The state tables and state transition diagrams have
to be read according to Notes\ 4 and\ 5 above.
.bp
.RT
.LP
5.5
\fIFormats\fR
.sp 1P
.RT
.sp 2P
.LP
5.5.1
\fIGeneral\fR
.sp 1P
.RT
.PP
5.5.1.1
Transport protocol data units (TPDUs)
carrying
transport service (TS) user
information or control information are called \fIblocks\fR (see \(sc\ 5.1.3).
All
blocks contain an integral number of octets.
.sp 9p
.RT
.PP
5.5.1.2
Bits of an octet are numbered 8\ to\ 1 where bit 1 is the low
order bit and is transmitted first. Octets of a block are consecutively
numbered starting from\ 1 and are transmitted in this order.
.PP
When consecutive octets are used to represent a binary number, the lower
octet has the most significant value.
.PP
5.5.1.3
\fITDT block(s)\fR are used to transfer a transport service data
unit (TSDU) transparently whilst maintaining the structure of the latter by
means of the TSDU end mark.
.PP
5.5.1.4
\fIControl blocks\fR (TCR, TCA, TCC, TBR) are used to control the
transport protocol functions, including optional functions.
.PP
5.5.1.5
A parameter field is present in all control blocks within the
basic transport service to indicate optional functions. The parameter field
contains one or more parameter elements. The first octet of each parameter
element contains a parameter code to indicate the function(s) requested.
.PP
The general coding structure is shown in Figure 5/T.70.
.LP
.rs
.sp 20P
.ad r
\fBFigure 5/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.PP
5.5.1.6
The parameter code field is binary coded and, without
extension, provides for a maximum of 255\ para
meters. Parameter code 11111111 is reserved for
extension of the parameter code
. The extension
mechanism is for further study.
.PP
Octet 2 indicates the length, in octets, of the parameter value
field. The parameter field length is binary coded and bit\ 1 is the low order
bit of this indicator.
.PP
Octet 3 and subsequent octets contain the value of the parameter
identified in the parameter code field. The coding of the parameter value
field is dependent on the function being requested.
.bp
.RT
.sp 2P
.LP
5.5.2
\fIStructure of transport control and transport data blocks\fR
.sp 1P
.RT
.PP
5.5.2.1
Figure 6/T.70 illustrates the general structure of transport
layer blocks. A summary of transport layer blocks is given in Figure\ 7/T.70.
.sp 9p
.RT
.LP
.rs
.sp 8P
.ad r
\fBFigure 6/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure 7/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
5.5.2.2
\fILength indicator (LI) field\fR
.sp 1P
.RT
.sp 1P
.LP
5.5.2.2.1\ \ Octet 1 contains the
length indicator (LI)
. The value
of this indicator is a binary number that represents the length in octets of
the control block (including parameters) and the header length in octets of
data blocks (excluding any subsequent user information). In both cases this
length does not include octet\ 1.
.sp 9p
.RT
.LP
5.5.2.2.2\ \ The basic LI value shall be restricted to\ 127 (i.e.,\ a binary
value of 01111111). The use of higher LI\ values and the use of the binary
value 11111111 for extension purposes is for further study.
.sp 2P
.LP
5.5.2.3
\fIBlock type field\fR
.sp 1P
.RT
.sp 1P
.LP
5.5.2.3.1\ \ Octet 2 contains the block type code. Bits\ 1 to 4 of
octet\ 2 are set to\ 0 for all transport layer blocks currently defined. It is
for further study to determine whether or not bits\ 1 to\ 4 are required for
future extension to the range of transport layer blocks currently defined or
are used for other functions.
.bp
.sp 9p
.RT
.sp 2P
.LP
5.5.2.4
\fIFunctional code field\fR
.sp 1P
.RT
.sp 1P
.LP
5.5.2.4.1\ \ Octet 3 and subsequent octets contain functional codes in a
fixed format as per the block type (see Figure\ 7/T.70).
.sp 9p
.RT
.sp 2P
.LP
5.5.2.5
\fIParameter or TSDU field\fR
.sp 1P
.RT
.sp 1P
.LP
5.5.2.5.1\ \ A parameter field or a data field containing transport service
(TS) user data may optionally follow the functional code field.
.sp 9p
.RT
.sp 2P
.LP
5.5.3
\fIConcatenation\fR
.sp 1P
.RT
.PP
5.5.3.1
Concatenation of transport control and/or transport data blocks is currently
not applicable to this Recommendation. However, where
concatenation is used in the future, the arrangement shown in Figure\ 8/T.70
would apply.
.sp 9p
.RT
.LP
.rs
.sp 36P
.ad r
\fBFigure 8/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 2P
.LP
5.5.4
\fITransport connection request (TCR) block format\fR
.sp 1P
.RT
.PP
5.5.4.1
Figure 9/T.70 illustrates the format of the TCR block.
.sp 9p
.RT
.LP
.rs
.sp 28P
.ad r
\fBFigure 9/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
5.5.4.2
\fIParameters for extended addressing\fR
.sp 9p
.RT
.PP
Separate parameters are provided for the indication of called and calling
extension addresses. The coding of these parameters is shown in
Figure\ 10/T.70. The setting of bit\ 8 for extended addressing should be
ignored by the transport layer.
.PP
The use of more than one called extension address is for further
study.
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigure 10/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
5.5.4.3
\fIParameter for transport data block size negotiation\fR
.sp 9p
.RT
.PP
This parameter defines the proposed maximum transport data block size (in
octets including the transport data block header) to be used over
the requested transport connection. The coding of this parameter is shown in
Figure\ 11/T.70.
.RT
.LP
.rs
.sp 14P
.ad r
\fBFigure 11/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
5.5.5
\fITransport connection accept (TCA) block format\fR
.sp 1P
.RT
.PP
5.5.5.1
Figure 12/T.70 illustrates the format of the TCA block.
.sp 9p
.RT
.LP
.rs
.sp 27P
.ad r
\fBFigure 12/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
5.5.5.2
\fIParameters for extended addressing\fR
.sp 9p
.RT
.PP
See \(sc\ 5.5.4.2.
.RT
.sp 1P
.LP
5.5.5.3
\fIParameter for transport data block size negotiation\fR
.sp 9p
.RT
.PP
See \(sc\ 5.5.4.3. The parameter value shall be equal to or less
than the value specified in the TCR block.
.RT
.sp 2P
.LP
5.5.6
\fITransport connection clear (TCC) block format\fR
.sp 1P
.RT
.PP
5.5.6.1
Figure 13/T.70 illustrates the format of the TCC block.
.sp 9p
.RT
.LP
.rs
.sp 43P
.ad r
\fBFigure 13/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
5.5.6.2
\fIParameter for additional clearing information\fR
.sp 9p
.RT
.PP
This parameter is provided to allow additional information
relating to the clearing of the connection. The coding of this parameter is
given in Figure\ 14/T.70 below.
.RT
.LP
.rs
.sp 12P
.ad r
\fBFigure 14/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
5.5.7
\fITransport block reject (TBR) block format\fR
.sp 1P
.RT
.PP
5.5.7.1
Figure 15/T.70 illustrates the format of the TBR block.
.sp 9p
.RT
.LP
.rs
.sp 28P
.ad r
\fBFigure 15/T.70, p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
5.5.7.2
\fIRejected block parameter\fR (mandatory)
.sp 9p
.RT
.PP
This parameter is used to indicate the bit pattern of the rejected block
up to and including the octet that caused the rejection. Only the first
detected procedural error or parameter, which cannot be acted upon, shall
be
indicated by this method. The coding of this parameter is given in
Figure\ 16/T.70.
.RT
.LP
.rs
.sp 11P
.ad r
\fBFigure 16/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 2P
.LP
5.5.8
\fITransport data block (TDT) format\fR
.sp 1P
.RT
.PP
5.5.8.1
Figure 17/T.70 illustrates the format of the TDT block.
.sp 9p
.RT
.LP
.rs
.sp 18P
.ad r
\fBFigure 17/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.ce 1000
ANNEX\ A
.ce 0
.ce 1000
(to Recommendation T.70)
.sp 9p
.RT
.ce 0
.LP
A.1
\fITransport and network service\fR
.sp 1P
.RT
.PP
The
transport service
(TS) is provided by the
transport protocol
(TP) making use of the services available from the network layer. This
Annex also defines the\ TS characteristics which the
TS\ users may exploit.
.PP
Interactions between TS users and the TS provider take place at the
two TS access points (TSAP) (see Figures\ A\(hy1/T.70 to A\(hy6/T.70).
Information is passed between a TS\ user and a TS\ provider by means of
primitives, which
may convey parameters.
.bp
.PP
Primitives are abstract representations of interactions. They are
solely descriptive and do not represent a specification or implementation.
.PP
The occurrence of a primitive is a logically instantaneous and
indivisible event. The event occurs at a logically separate instant,
which cannot be interrupted by another event. Only primitives of global
significance are mentioned (having an impact on the remote user).
.PP
The following types of primitives are defined:
.RT
.LP
a)
request primitive
.LP
b)
indication primitive
.LP
c)
response primitive
.LP
d)
confirm primitive.
.PP
The primitives a) and c) are directed from the service user to the service
provider, b) and d) are going in the opposite direction.
.PP
\*QTransport\*U is designated by T, \*QNetwork\*U is designated by N. The
terms CONNECT, DATA, DISCONNECT as part of a primitive name indicate that
the primitive is used for establishment, data transfer, release of a
transport connection
\ (TC) or
network connection
\ (NC).
.RT
.sp 1P
.LP
\fIExamples:\fR
.sp 9p
.RT
.LP
T\(hyCONNECT request
request to establish a TC
.LP
T\(hyDATA request
request to transmit TS user data
.LP
N\(hyDISCONNECT indication
indication that the NC has been released.
.PP
The relationship between valid sequences of TS primitives and the appropriate
protocol elements is shown in Figures\ A\(hy1/T.70 to A\(hy6/T.70. The
sequences of valid
network service
\ (NS) primitives are illustrated in Figures\ A\(hy7/T.70 to A\(hy12/T.70.
.sp 1P
.LP
A.1.1
\fITransport service\fR
.sp 9p
.RT
.PP
The interactions shown in Figures\ A\(hy1/T.70 to A\(hy6/T.70 are not
exhaustive.
.RT
.sp 1P
.LP
A.1.1.1\ \ \fITransport connection establishment\fR
.sp 9p
.RT
.LP
.rs
.sp 19P
.ad r
\fBFigures A\(hy1/T.70 and A\(hy2/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.1.1.2\ \ \fITransfer phase\fR
.sp 9p
.RT
.LP
.rs
.sp 23P
.ad r
\fBFigure A\(hy3/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.1.1.3\ \ \fITransport service error reporting\fR
.sp 9p
.RT
.LP
.rs
.sp 18P
.ad r
\fBFigure A\(hy4/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.1.1.4\ \ \fITC release\fR
.sp 9p
.RT
.PP
At present only the implicit release of TC is defined (see
\(sc\ 5.2.4.1 of this Recommendation).
.RT
.LP
.rs
.sp 17P
.ad r
\fBFigures A\(hy5/T.70 and A\(hy6/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.1.2
\fINetwork service\fR
.sp 9p
.RT
.PP
Figures A\(hy7/T.70 to A\(hy12/T.70 show the relationships of network
service\ (NS) primitives at both sides of an\ NC.
.RT
.sp 1P
.LP
A.1.2.1\ \ \fINetwork connection establishment\fR
.sp 9p
.RT
.LP
.rs
.sp 22P
.ad r
\fBFigures A\(hy7/T.70 and A\(hy8/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.1.2.2\ \ \fINetwork data transfer\fR
.sp 9p
.RT
.LP
.rs
.sp 12P
.ad r
\fBFigure A\(hy9/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.1.2.3\ \ \fINetwork service error reporting\fR
.sp 9p
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigure A\(hy10/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.sp 1P
.LP
A.1.2.4\ \ \fINetwork connection release\fR
.sp 9p
.RT
.LP
.rs
.sp 13P
.ad r
\fBFigures A\(hy11/T.70 and A\(hy12/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.sp 1P
.LP
A.2
\fIState transition diagrams for the basic\fR
\fItransport layer\fR
\fIprocedures\fR
.sp 9p
.RT
.PP
This part represents detailed state transition diagrams for the
basic transport procedures.
.PP
Two description levels are used:
.RT
.LP
a)
\fIProtocol level\fR
.LP
This level addresses only the peer to peer protocol activities
between two transport entities. It identifies the protocol
state, events [receipt of transport protocol data units (TPDUs)]
and actions (sending of TPDUs).
.LP
b)
\fIDetailed level\fR
.LP
This level addresses the inter\(hylayer and local activities. It
identifies the events, actions, conditions and states within
each of the protocol level states. The inter\(hylayer activities
are described using the transport service primitives defined in
the first\ part of this Annex.
.PP
\fIExample\fR (see Figure\ A\(hy13/T.70)
.PP
For pure illustrative reasons, the example shows a simplified
description of state 1 (response pending, called side) of the state transition
diagram of this Recommendation. The event R\(hyTCR may be answered either
by
sending the action S\(hyTCA or\ S\(hyTCC.
.PP
The events and actions are not interruptable. They will complete
their transfer irrespective of the occurrence of other events.
.PP
The detailed state transition diagrams are given in Figures\ A\(hy14/
and\ A\(hy15/T.70.
.RT
.LP
.rs
.sp 31P
.ad r
\fBFigure A\(hy13/T.70 p.37\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure A\(hy14/T.70 p.38\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp
.LP
.rs
.sp 47P
.ad r
\fBFigure A\(hy15/T.70 p.\fR
.sp 1P
.RT
.ad b
.RT
.LP
.bp