High layer characteristics identification \(em octet 4
(e.g.,\ Facsimile Group\ 4, Teletex).
.PP
To maximize the usefulness of HLC checking:
.LP
1)
the calling telematic terminal shall select the HLC element according
to the type of document to be tranferred;
.LP
2)
the called terminal holds a list of HLC elements describing its receiving
capabilities. It will accept an HLC element corresponding to any one of
these.
.PP
This scheme is illustrated in Table 1/T.90.
.sp 1P
.LP
2.2.5
\fILayer 3 \(em virtual connection control and information transfer\fR
.sp 9p
.RT
.PP
ISO 8208 (1987) shall apply.
.PP
\fINote\fR \ \(em\ This protocol, based on the l984 version of
Recommendation\ X.25, is partially expanded in order to include DTE\(hyDTE
application. In particular, the following sections of ISO\ 8208 are
referred:
.RT
.LP
\(em
\(sc 3.2: Differences in DTE/DTE and DTE/DCE operation
.LP
\(em
\(sc 3.3: Operating over circuit\(hyswitched connections
.LP
\(em
\(sc 4.5: Determining \*QDTE\*U or \*QDCE\*U characteristics
.PP
In addition, the following points should be noted when using this protocol.
.LP
a)
Calling DTE shall send a RESTART REQUEST packet, begin the restart procedure,
and establish virtual circuits. See \(sc\ 3.3 of ISO\ 8208.
.LP
b)
The qualifier bit in data packets should always be set to
\*Q0\*U.
.LP
c)
The delivery confirmation bits in all packets should be set to \*Q0\*U.
.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.
.bp
.LP
f
)
The terminal should not send a DTE REJECT packet.
.LP
g)
In case of Group 4 facsimile and Teletex, terminals shall
use a specific protocol identifier within CALL REQUEST/INCOMING CALL packets.
This identifier is represented by the first octet of the call user data
field (remaining octets, if any should be ignored) as shown below:
\v'6p'
.LP
bit
87654321
\fBoctet\fR \fB00000010\fR
octet
00000010
.PP
.sp 1
The use of this protocol identifier for Videotex is for further
study.
.ce
\fBH.T. [T2.90]\fR
.ce
TABLE\ 1/T.90
.ce
\fBUse of HLC codes by various Telematic terminals\fR
.ps 9
.vs 11
.nr VS 11
.nr PS 9
.TS
center box;
cw(48p) | cw(90p) sw(90p) , ^ | c | c.
Telematic service terminals HLC codes
{
Sent from calling terminals
(Notes 2, 3)
} {
Accepted by receiving terminals
(Note 4)
}
_
.T&
lw(48p) | lw(90p) | lw(90p) .
Teletex basic Basic Teletex Basic Teletex
_
.T&
lw(48p) | lw(90p) | lw(90p) .
Teletex mixed mode {
Basic Teletex
Mixed mode
(Note 1)
} Basic Teletex Mixed mode
_
.T&
lw(48p) | lw(90p) | lw(90p) .
Group 4 facsimile class 1 Group 4 facsimile Group 4 facsimile
_
.T&
lw(48p) | lw(90p) | lw(90p) .
Group 4 facsimile class 2 Group 4 facsimile {
Group 4 facsimile
Mixed mode
Basic Teletex
}
_
.T&
lw(48p) | lw(90p) | lw(90p) .
Group 4 facsimile class 3 {
Group 4 facsimile
Mixed mode
Basic Teletex
(Note 1)
} {
Group 4 facsimile
Mixed mode
Basic Teletex
}
.TE
.LP
\fINote\ 1\fR
\ \(em\ In case that the calling terminal is Teletex, Mixed mode or group\ 4 facsimile class\ 3, only one element shall be sent depending on originating
document type.
.LP
\fINote\ 2\fR
\ \(em\ For multi\(hyservice telematic terminals sending more than one document in the same call, the HLC shall indicate the maximum requirement for that
call.
.LP
For example, when sending a Teletex and a Mixed mode document, the Mixed mode HLC element shall be sent.
.LP
\fINote\ 3\fR
\ \(em\ When the calling terminal only wishes to receive a document from a called terminal (polling), it shall know in advance the type of document it
expects to receive in order to send the appropriate HLC element.
.LP
\fINote\ 4\fR
\ \(em\ Appendix I provides additional information in order to cater for
cases where calls for facsimile equipment are incoming from networks not able to convey HLC information.
