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The drawings contained in this Recommendation have been done in Autocad
Recommendation X.29
PROCEDURES FOR THE EXCHANGE OF CONTROL INFORMATION
AND USER DATA BETWEEN A PACKET
ASSEMBLY/DISASSEMBLY (PAD) FACILITY
AND A PACKET MODE DTE OR ANOTHER PAD
(provisional, Geneva, 1977; amended, Geneva, 1980,
Malaga-Torremolinos, 1984 and Melbourne, 1988)
Preface
The establishment in various countries of public data networks providing
packet-switched data transmission services creates a need to produce standards to
facilitate international interworking.
The CCITT,
considering
(a) that Recommendations X.1 and X.2 define the user classes of service and
facilities in a public data network, and Recommendation X.96 defines call
progress signals;
(b) that Recommendation X.3 defines the PAD in a public data network;
(c) that Recommendation X.28 defines the DTE/DCE interface for a start-stop
mode DTE accessing the PAD in a public data network;
(d) that Recommendation X.25 defines the interface between the DTE and the
DCE for DTEs operating in the packet mode in public data networks;
(e) the need to allow interworking between a packet mode DTE and a
non-packet mode DTE in the packet-switched transmission service;
(f) the urgent need to allow interworking between a start-stop mode DTE in
a public switched telephone network, public switched data network or a leased
line and a packet mode DTE using the virtual call facility of the packet-switched
transmission service;
(g) the need to allow interworking between PADs;
(h) that the packet mode DTE shall not be obliged to use the control
procedures for PAD functions, but that some packet mode DTEs may wish to control
specific functions of the PAD,
unanimously recommends that
(1) the Recommendation X.29 procedures shall apply to the Recommendation
X.25 interface between the DCE and the packet mode DTE;
(2) the Recommendation X.29 procedures may be applied for interworking
between PADs;
(3) the procedures be as specified below in S 1 Procedures for the exchange
of PAD control information and user data;
(4) the manner in which user data is transferred be as specified below in S
2 User data transfer;
(5) the procedures for the control of the PAD via PAD messages be as
specified below in S 3 Procedures for the use of PAD messages;
(6) the formats of the data fields which are transferable on a virtual call
be as specified below in S 4 Formats.
i
is considered within a national network these packet types or procedures may have
a different form from those used in Recommendation X.25 but will have the same
operational meaning.
Note 2 V The following items are for further study:
V the use of the permanent virtual circuit service;
V interworking between DTEs having interfaces to different data
transmission services;
V operation of nonVpacket mode DTEs in other than startVstop mode.
1 Procedures for the exchange of PAD control information and
user data
1.1 The exchange of control information and user data between a PAD and a
packet mode DTE or between PADs is performed by using user data fields defined in
Recommendation X.25.
1.2 Annex A describes some of the characteristics of virtual calls as defined
in Recommendation X.25, as related to the PAD representation of a startVstop mode
DTE to a packet mode DTE. The characteristics described in Annex A also apply for
interworking between PADs.
1.3 Call user data
The call user data field call user data field of incoming call or
Fascicle VIII.2 - Rec. X.29 PAGE1
call request packets to or from the packet mode DTE or the PAD is
comprised of two fields:
a) the protocol identifier field protocol identifier field, and
b) the call data field call data field.
The protocol identifier field is used for protocol identification purposes
and the call data field contains user data.
A call request packet received by the PAD, containing no call user data
field, will be accepted by the PAD.
If a call data field is present, the PAD will send it, unchanged, to the
startVstop mode DTE, using the call data block of the incoming call PAD service
signal (see ' 3.5.22, Recommendation X.28).
1.4 User sequences
1.4.1 User sequences are used to exchange user data between the PAD and the
packet mode DTE or a PAD.
1.4.2 User sequences are conveyed in the user data fields of complete packet
sequences with Q = 0, and in both directions on a virtual call. (See
Recommendation X.25.)
1.4.3 There will be only one user sequence in a complete packet sequence.
1.4.4 The PAD will transmit all data packets with the D bit set to 0.
On reception of a data packet with the D bit set to 1, the PAD will
transmit the corresponding acknowledgement as soon as possible.
virt
virtual call.
As no error correction procedure is in place from the PAD to the
start-stop mode DTE, no guarantee of delivery can be implied from the
acknowledgement.
1.5 PAD messages
1.5.1 PAD messages are used to exchange control information between the PAD and
the packet mode DTE (or remote PAD). A PAD message consists of a control
identifier field and a message code field possibly followed by a parameter field (see
S 4.4 below).
1.5.2 PAD messages are conveyed in the user data fields of complete
packet sequences with Q = 1 and in both directions on a virtual
call. (See Recommendation X.25.)
1.5.3 There will be only one PAD message in a complete packet
sequence.
1.5.4 The PAD will take into consideration a PAD message only when
it has been completely received.
1.5.5 In the case where a parameter reference (see S 3 below)
appears more than once in a PAD message, only the last appearance
is taken into account.
1.5.6 The PAD will transmit all data packets with the D bit set to
0.
On reception of a data packet with both the Q bit and the D
bit set to 1, the PAD will transmit the corresponding
acknowledgement as soon as possible.
If the PAD does not support the D bit procedure, the PAD may
reset the virtual call.
2 User data transfer
2.1 Data packets will be forwarded by the PAD when a set, read, or set and
read PAD message is received, or under any of the other data forwarding
conditions provided by the PAD (see Recommendation X.28, S 4.4).
2.2 The occurrence of a data forwarding condition will not cause the PAD to
transmit empty data packets.
3 Procedures for the use of PAD messages
3.1 Procedures for reading, setting, and reading and setting of PAD parameters
3.1.1 The current values of PAD parameters may be changed and read by
transmitting to the PAD a set, read, or set and read PAD message.
me
message as a data forwarding condition.
3.1.3 The PAD will respond to a valid read or set and read PAD message by
transmitting a parameter indication PAD message. This PAD message will have a
parameter field containing a list of parameter references and current values
(after any necessary modification) of the PAD parameters to which the received
PAD message referred.
PAGE14 Fascicle VIII.2 - Rec. X.29
3.1.4 The PAD will not return a parameter indication PAD message in response to
a valid set PAD message received.
3.1.5 Table 1/X.29 specifies the PAD's response of the PAD to set, set and read,
and read PAD messages.
3.1.6 If the function of a character is duplicated by the selection of parameter
values by use of the set or set and read PAD message, the PAD will consider these
parameter changes as valid, and will respond as described in this Recommendation.
After these changes are invoked, the PAD will follow the procedure described in
Recommendation X.28, ' 3.3.2.
3.2 Procedures for inviting the PAD to clear
3.2.1 The invitation to clear PAD message is used to request that
the PAD clears the virtual call, after transmission of all data
previously transmitted to the startVstop mode DTE.
Note V The clear indication packet, which is transmitted by
the PAD after delivery of the last character to the startVstop mode
DTE, will have a clearing cause field set to DTE clearing.
3.3 Interrupt and discard procedures
3.3.1 If parameter 7 is set to 21, the PAD will transmit an interrupt packet
with all bits of the interrupt user data field set to 0 followed by an indication
of break PAD message to indicate that the PAD, at the request of the startVstop
mode DTE, is discarding the user sequences received. The PAD message will contain
an indication in its parameter field that parameter 8 has been set to 1 (discard
output).
3.3.2 Before resuming data transmission to the PAD, the response to the
indication of break PAD message shall be a set or set and read PAD
message, indicating that parameter 8 should be set to 0 (normal
data delivery).
Prior to sending this PAD message, any inVprogress complete
packet sequence being transmitted to the PAD must be terminated
(with a packet that will be discarded by the PAD) in accordance
with Recommendation X.25 procedures.
Fascicle VIII.2 - Rec. X.29 PAGE1
TABLE 1/X.29
PAD messages transmitted by the PAD in response to set, set and read, and read PAD
messages
PAD message received by Corresponding parameter
the PAD Action upon PAD indication PAD message
parameters transmitted to the
Type Parameter packet mode DTE
field
None Reset all implemented None
Recommendation X.3
parameters to their
Set initial values
corresponding to the
initial profile
List of Set the selected a) None
selected parameters to the given b) List of these invalid
parameters values: parameters
with the a)if no error is (see Note)
desired encountered
values b)if the PAD fails to
modify the values of some
parameters
None Reset all implemented List all implemented
Recommendation X.3 Recommendation X.3
Set and parameters to their parameters, and their
read initial values initial values
corresponding to the
initial profile
List of Set the selected List of these parameters
selected parameters to the given with their new current
parameters values values (see Note)
with the
desired
values
None None List all implemented
Recommendation X.3
Read parameters with their
current values
List of None List of these parameters
selected with their current values
parameters (see Note)
Note - If any of the parameters contain an error, then the error bit is set and
PAGE14 Fascicle VIII.2 - Rec. X.29
the value field is coded as described in Table 3/X.29.
3.3.3 If a PAD receives an indication of break PAD message which contains a
parameter field as described in S 3.3.1 above, it will respond by transmitting a
set PAD message as described in S 3.3.2 above and will transmit a break signal to
the start-stop mode DTE. If a PAD receives an indication of break PAD message
which does not contain a parameter field, it will not respond to the packet mode
DTE or PAD but it will transmit a break signal to the start-stop mode DTE.
3.3.4 When the PAD transmits an interrupt packet after the receipt from the
start-stop mode DTE of an interrupt PAD command signal or a break signal, when
parameter 7 is set to 1, the interrupt user data field is coded in bits 8 to 1 as
00000001.
3.3.5 If the PAD receives an interrupt packet it will confirm it in accordance
with Recommendation X.25 procedures. The PAD will not transmit the contents of
the interrupt user data field to the start-stop DTE. The PAD will ignore the
values of the interrupt user data field. It is for further study whether the
coding of this field given in S 3.3.4 above causes a different response.
3.3.6 If parameter 7 is set to 5, the PAD will transmit an interrupt packet with
all bits of the interrupt packet set to 0, followed by an indication of break PAD
message. The PAD message will not contain a parameter field as described in S
4.4.7.
3.3.7 Some PADs may always send the break signal to the start-stop mode DTE upon
receipt of an interrupt packet rather than upon receipt of an indication of break
PAD message.
3.4 Procedure for resets
Virtual calls may be reset according to the procedures defined in
Recommendation X.25. The effect of the resetting procedure on the value of PAD
parameter 8 is to reset its value to 0 (normal data delivery). The current values
of all other PAD parameters are not affected.
3.5 Error handling procedures by the PAD
3.5.1 If the PAD receives a set, read or set and read PAD message containing an
invalid reference to a PAD parameter, the parameter field within the parameter
indication PAD message transmitted by the PAD will contain an indication that
this has occurred. The remaining valid references to PAD parameters are processed
by the PAD.
Possible reasons for an invalid access to a PAD parameter are:
a) the parameter reference has not been implemented in the PAD;
b) the parameter value has not been implemented in the PAD or cannot be
altered from the current setting;
c) the parameter is a read-only one (set and set and read PAD messages
only);
d) the parameter follows an invalid parameter separator (see S 4.4.5.4
below).
3.5.2 The PAD will transmit an error PAD message containing the message code of
an invalid PAD message received under the following conditions:
a) if the PAD receives an unrecognizable message code;
b) if the parameter field following a recognizable message code is
incorrect or incompatible with the message code;
c) if the parameter field following a recognizable message code has an
invalid format;
d) if the PAD receives an unsolicited parameter indication PAD message;
e) if the PAD receives a PAD message that is too long.
3.5.3 The PAD will transmit an error PAD messag D message
containing less than 8 bits is received.
3.5.4 If the PAD receives an error PAD message it will not respond
with a PAD message of any type. Subsequent action is for further
study.
3.6 Procedures for inviting the PAD to reselect the called DTE
The reselection or reselection with TOA/NPI PAD message (Type of
address/Numbering Plan Indicator) used by a packet mode DTE to request that the
PAD clear the virtual call, after transmission to the start-stop mode DTE of all
the previously transmitted data. Then, the PAD will establish a call to the
reselected DTE.
Note - The TAO/NPI address subscription facility is designated in
Recommendation X.2 for further study.
When the reselection PAD message is received, the PAD will transmit an
Fascicle VIII.2 - Rec. X.29 PAGE1
error PAD message with an error type unauthorized reselection PAD message
(00000110) under the following conditions:
a) the virtual call has been established by the packet-mode DTE;
b) the called DTE reselection prevention facility has been requested by
the start-stop mode DTE;
c) the reselection PAD message has been already received more than N times
(where N is for further study).
The format of the reselection PAD message is given in S 4.4.9 below. The
format of the reselection with TOA/NPI PAD message is given in S 4.4.10 below.
These messages contain the information needed by the PAD to establish the new
virtual call.
Upon receipt of the reselection or reselection with TOA/NPI PAD message,
the PAD will:
- transmit to the start-stop mode DTE all previously received data;
- clear the virtual call that is established;
- after having made the appropriate state changes as described in
Recommendation X.28, S 3.2.5, establish a virtual call to the
reselected DTE. The call request packet sent by the PAD, will contain
only the facilities subscribed by the start-stop mode DTE and/or
assigned by default. Any other facilities contained in the reselection
PAD message will be ignored. In particular:
i) Closed User Group Signals - Independently by the CUG indicated in
the reselection PAD message, the PAD will use the same CUG of the
original call.
message
message contains the reverse charging facility.
iii) Charging information:
V facility assigned for an agreed contractual period: The
information will be sent to the startVstop mode DTE at the
clearing of each call (original and reselected), or at the
clearing of the last reselected call. If the later procedure was
selected, the PAD will send the total charging information,
without sending the charge of the individual calls (original and
reselected).
V facility on a per call basis: The PAD follows the procedure
indicated above, starting from the first charging information
facility request (by the startVstop mode DTE or packet mode
DTE).
iv) RPOA selection: for further study
Note V The other facilities indicated in Table 4/X.28 with Note 2 are for
further study.
Note V This procedure is an optional feature of the PAD. PADs which do not
implement this feature will consider reselection and reselection with TOA/NPI PAD
messages as invalid. PADs may implement this feature either by accepting (1)
reselection PAD messages or (2) reselection and reselection with TOA/NPI PAD
messages. The sending of reselection or reselection with TOA/NPI PAD messages by
a PAD is for further study.
4 Formats
4.1 Introduction
Bits of octets are numbered 8 to 1 where bit 1 is the low order bit and is
transmitted first. Octets of the call user data, of user sequences, of PAD
messages and of interrupt user data are consecutively numbered starting from 1
and are transmitted in this order.
4.2 Call user data format (see Figure 1/X.29)
4.2.1 Protocol identifier format
The protocol identifier field protocol identifier field standardized by
CCITT consists of four octets.
The first octet is coded as follows:
bits 8 and 7 = 00 for CCITT use
= 01 for national use
= 10 reserved for international user bodies
= 11 for DTEVDTE use.
C
CCITT, subject to the rules of Recommendation X.244. All bits of
octets 2, 3 and 4 are set to 0. These octets are reserved as a
PAGE14 Fascicle VIII.2 - Rec. X.29
future mechanism for providing the called PAD or packet mode DTE
with additional information pertinent to the calling party.
Fascicle VIII.2 - Rec. X.29 PAGE1
Fig. 1/X.29 = 6 cm
4.2.2 Call data format
Octets of the a field will contain the user
characters received by the PAD from the start-stop mode DTE during
the call establishment phase. The coding of these octets is similar
to that of user sequences (see S 4.3 below). The call data field is
limited to 12 octets (see Figure 1/X.29).
4.3 User sequence format
4.3.1 The order of bit transmission from the PAD is the same as the order that
bits are received from the start-stop mode DTE. The order of bit transmission to
the start-stop mode DTE is the same as the order that bits are received.
4.3.2 No maximum is specified for the length of a user sequence.
4.4 Control message format
4.4.1 Bits 8, 7, 6, 5 of octet 1 of a user data field of complete packet
sequences with Q = 1 are defined as the control identifier field, used to
identify the facility, such as PAD, to be controlled. The control identifier
fiel coding for PAD messages to control a PAD
for a start-stop mode DTE is 0000. Other codings of the control
identifier field are reserved for future standardization.
Note - The possibility of extending the control identifier
field is for further study.
4.4.2 When the control identifier field (see S 4.4.1 above) is set
to 0000, bits 4, 3, 2, 1 of octet 1 are defined as the message code
field. The message code field is used to identify
specific types of PAD messages, as given in Table 2/X.29.
PAGE14 Fascicle VIII.2 - Rec. X.29
TABLE 2/X.29
Type and coding of octet 1 of PAD messages
Message code
Type Bit 4 3 2 1
s
Set PAD message 0 0 1 0
Read PAD message 0 1 0 0
Set and read PAD message 0 1 1 0
Parameter indication PAD message 0 0 0
Fascicle VIII.2 - Rec. X.29 PAGE1
0
Invitation to clear PAD message 0 0 0 1
Indication of break PAD message 0 0 1 1
Reselection PAD message 0 1 1 1
Error PAD message 0 1 0 1
Reselection with TOA/NPI 1 0 0 0
Note - The possibility of extending the message code field is for further study.
4.4.3 All PAD messages consist of a control identifier field (bits 8, 7, 6, 5 of
octet 1 equal to 0000) and a message code field (bits 4, 3, 2, 1 of octet 1).
Set, read, set and read and parameter indication PAD messages consist of
octet 1 which may be followed by one or more parameter fields. Each parameter
field c a parameter reference octet and a
parameter value octet.
The parameter value octets of the read PAD message contain the
value 0.
The error PAD message consists of octet 1 and one or two
octets giving the reason for the error.
The indication of break PAD message consists of octet 1 which
may be followed by a parameter field.
The invitation to clear PAD message consists of octet 1 only.
4.4.4 The maximum length of PAD message is network dependent, but
will be at least 128 octets.
4.4.5 Parameter field for set, read, set and rea , and parameter
PAGE14 Fascicle VIII.2 - Rec. X.29
indication PAD messages (see Figure 2/X.29)
A parameter field contained in one of these PAD messages consists of a
reference f d and a value field. A parameter
field is two octets in length, except when the extension mechanism
is used (see S 4.4.5.1 below).
c
coded in bits 7 to 1, where bit 1 is the low order bit. Reference
fields need not be ordered by increasing parameter reference
numbers.
The code 1111111 (decimal 127) in bits 7 to 1 of the reference
field will be used for the extension of this field. Such coding
will indicate that there is another octet following. The following
octet is coded with the parameter reference of Recommendation X.3
minus 127.
4.4.5.2 In PAD messages received by the PAD, bit 8 of each octet
will be ignored. In parameter indication PAD messages, bit 8 of
each reference field set to 1 will indicate an invalid access to
the referred parameter as described in S 3.5 above.
Fascicle VIII.2 - Rec. X.29 PAGE1
Fig. 2/X.29 = 13 cm
4.4.5.3 A parameter value field consists of a value of the parameter reference,
identified as a decimal number in Recommendation X.3, and is binary coded in bits
8 to 1, where bit 1 is the low order bit. Value fields in read PAD messages are
coded as all binary 0s. In set and set and read PAD messages, they will indicate
the requested values of parameters. In parameter indication PAD messages, they
will indicate the current values of PAD parameters, after modification if any. If
bit 8 (error bit) is set to 1 in the preceding octet (i.e. the parameter
reference field), the parameter value field will indicate the reason for the
error, as given in Table 3/X.29.
4.4.5.4 Parameters not standardized by CCITT may be supported. The parameter
separator is used in PAD messages to indicate the separation between parameters
specified in Recommendation X.3 and any others implemented nationally or locally.
The parameter separator consists of a parameter field which contains a
reference field set to 00000000 and a value field set to 00000000.
When present, the parameter separator and the national or local parameter
fields must be placed after any CCITT standardized parameter fields in PAD
messages.
Note - It is recommended that only the parameters defined in
Recommendation X.3 are used when communicating with a PAD in a different country
or network.
PAGE14 Fascicle VIII.2 - Rec. X.29
TABLE 3/X.29
Coding of parameter value field in case of error
Parameter value field code
Error type bits Decimal
8 7 6 5 4 3 2 1
No additional information 0 0 0 0 0 0 0 0 0
The parameter reference does not exist or 0 0 0 0 0
has not been implemented in the PAD
Fascicle VIII.2 - Rec. X.29 PAGE1
0 0 1 1
The parameter value is invalid or has not 0 0 0 0 0 0 1 0 2
been implemented in the PAD
The parameter value cannot be altered from 0 0 0 0 0 0 1 1 3
the current setting
The parameter is read-only 0 0 0 0 0 1 0 0
PAGE14 Fascicle VIII.2 - Rec. X.29
4
The parameter follows an invalid parameter 0 0 0 0 0 1 0 1 5
separator
Note - The value 0 is mandatory. Other values are optional.
4.4.6 Format of error PAD messages (see Figure 3/X.29)
Fig. 3/X.29 = 7 cm
4.4.6.1 Octet 2 of the error PAD message will be coded as shown in Table
4/X.29.
4.4.6.2 In cases b, c, d, e and f in Table 4/X.29, octet 3 of an error PAD
message will contain the message code of the received PAD message.
Fascicle VIII.2 - Rec. X.29 PAGE1
4.4.7 Parameter field for indication of break PAD messages (see Figure 4/X.29)
This PAD message may either not contain a parameter field, or contain a
parameter field consisting of 2 octets (i.e. one reference field and one value
field) coded as follows: the reference field will be coded 00001000 (indicating
parameter 8) and the value field will be coded 00000001 (indicating decimal 1).
TABLE 4/X.29
Coding and meaning of octet 2 of error PAD messages
Coding
Case Meaning Bits 8 7 6 5 4 3 2 1
a Received PAD message contained 0 0 0 0 0 0 0 0
less than eight bits
b Unrecognized message code in
received PAD message
PAGE14 Fascicle VIII.2 - Rec. X.29
0 0 0 0 0 0 1 0
c Parameter field format of 0 0 0 0 0 1 0 0
received PAD message was
incorrect or incompatible with
message code
d Received PAD message did not 0 0 0 0 0 1 1 0
contain an integral number of
octets
e Received parameter indication
PAD message was unsolicited
Fascicle VIII.2 - Rec. X.29 PAGE1
0 0 0 0 1 0 0 0
f Received PAD message was too 0 0 0 0 1 0 1 0
long
g Unauthorized reselection PAD 0 0 0 0 1 1 0 0
message
Fig. 4/X. 29 = 5 cm
PAGE14 Fascicle VIII.2 - Rec. X.29
4.4.8 Parameter field for invitation to clear PAD message (see Figure 5/X.29)
Fig. 5/X.29 = 5 cm
This PAD message will not contain a parameter field.
4.4.9 Reselection PAD message format
The format of this message is given in Figure 6/X.29.
Fig. 6/X.29 = 9 cm
4.4.9.1 Reselected DTE address length field
Bits 4, 3, 2 and 1 of the reselected DTE address length field indicate the
length of the reselected DTE address in semi-octets. The address length is binary
coded and bit 1 is the low order bit of the indicator.
4.4.9.2 Address field
Octet 3 and the following octets consist of the reselected DTE address.
Each digit of the address is coded in a semi-octet in binary coded decimal with
bit 5 or 1 being the low order bit of the digit.
Starting from the high order digit, the address is coded in octet 3 and
consecutive octets with two digits per octet. In each octet, the higher order
digit is coded in bits 8, 7, 6 and 5.
The address field shall be rounded up to an integral number of octets by
inserting zeros in bits 4, 3, 2 and 1 of the last octet of the field when
necessary.
The reselected DTE address field should contain the internat
number (DNIC + Network terminal number).
4.4.9.3 Facility length field
The octet following the reselected DTE address field indicates the length
of the facility field, in octets. The facility length indicator is binary coded
and bit 1 is the low order bit of the indicator.
4.4.9.4 Facility field
The facility field is present only when optional user facilities are
included by the DTE. This field indicates the facilities that must be included in
the facility field of the incoming call packet received by the reselected DTE
(see S 3.6).
The coding of the facility field is defined in S 7 of Recommendation X.25.
The facility field contains an integral number of octets, the maximum
length of the complete PAD message is restricted, as described in S 4.4.4 above.
4.4.9.5 Call user data field
Following the facility field, the call user data field may be present and
has a maximum length of 12 octets.
Call user data when present in the call user data field of the reselection
PAD message is included in the call user data field of the incoming call packet
received by the reselected DTE.
4.4.10 Reselection with TOA/NPI PAD message format
The format of this message is given in Figure 7/X.29.
Note - The TOA/NPI address subscription facility is designated in
Recommendation X.2 for further study.
Fig. 7/X.29 = 8 cm
Fascicle VIII.2 - Rec. X.29 PAGE1
4.4.10.1 Reselected DTE address length field
Octet 2 indicates the length of the reselected DTE address in semi-octets.
The address length is binary coded and bit 1 is the low order bit of the
indicator. The maximum value of the reselected DTE address length field is 17.
4.4.10.2 Reselected DTE address field
Octet 3 will consist of the TOA/NPI indication as described in
Recommendation X.25. The following octets consist of the reselected DTE address.
Each digit of the address is coded in a semi-octet in binary coded decimal with
bit 5 or 1 being the low order bit of the digit. Starting from the high order
digit, the address digits are coded in consecutive semi-octets. In each octet,
the higher order digit is coded in bits 8, 7, 6 and 5.
4.4.10.3 Facility length field
The octet following the address field indicates the length of the facility
field, in octets. The facility length indicator is binary coded and bit 1 is the
low order bit of the indicator.
4.4.10.4 Facility field
(See S 4.4.9.4.)
4.4.10.5 Call user data field
(See S 4.4.9.5.)
ANNEX A
(to Recommendation X.29)
Characteristics of virtual calls and Recommendation X.25
as related to the PAD representation of
a start-stop mode DTE to a packet mode DTE
A.1 General interface characteristics
A.1.1 The mechanical, electrical, functional and procedural characteristics to
activate, maintain and deactivate the physical access path between the DTE and
the DCE will be in accordance with the physical level procedures of
Recommendation X.25.
A.1.2 The link access procedure for data interchange across the link between the
DTE and DCE will be in accordance with the link level procedures of
Recommendation X.25.
A.1.3 The packet format and control procedures for the exchange of packets
containing control information and user data between the DTE and the DCE will be
in accordance with the packet level procedures of Recommenda-tion X.25.
A.2 Interface procedures for virtual call control
A.2.1 Incoming calls are indicated to the packet mode DTE as specified in
Recommendation X.25. Call requests are indicated by the packet mode DTE as
specified in Recommendation X.25. Any use of optional user facilities are
indicated in accordance with SS 6 and 7 of Recommendation X.25.
A.2.2 The default throughput classes used by the PAD are determined by the data
rates of the start-stop mode DTE (where exact correspondence is not obtained, the
next higher throughput class is used).
A.2.3 The PAD and the packet mode DTE will use the clearing procedures specified
in SS 4.1.7, 4.1.8 and 4.1.9 of Recommendation X.25.
A.3 Interface procedures for data transfer
A.3.1 Data transfer on a virtual call can only take place in the data transfer
state and when flow control permits (see S 4.4 of Recommendation X.25). The same
is true for the transfer of interrupt packets (see S 4.3 of Recommendation X.25).
A.3.2 Interrupt packets transmitted by the packet mode DTE will be confirmed by
the PAD following the procedures in Recommendation X.25.
A.3.3 The reset procedure may be used by the packet mode DTE or the PAD to
re-initialize the virtual call and will conform to the procedures described in S
4.4.3 of Recommendation X.25.
A.3.4 A reset of the virtual call originated by the packet mode DTE or due to
network congestion may be indicated by the PAD to the start-stop mode DTE.
A.3.5 A reset procedure initiated by the PAD may be due either to:
a) the receipt at the PAD of a request to reset from the non-packet mode
DTE. The resetting cause contained in the reset indication packet will
be DTE reset; or
b) a PAD or network failure.
A.3.6 For calls received by the PAD with bit 7 of octet 1 in the incoming call
packet set to 0, the PAD will set bit 7 of octet 1 in the call accepted packet to
0 and will set the D bit in transmitted data packets to 0.
Pending further study, and in the absence of bilateral agreement between
PAGE14 Fascicle VIII.2 - Rec. X.29
Administrations (used in conjunction with the D bit modification facility), the
following applies:
If the incoming call packet received by the PAD has bit 7 of octet 1 set
to 1, the PAD may set bit 7 of octet 1 of the call accepted packet to 1.
Calls originated by the PAD will set bit 7 of octet 1 in call request
packets to 0. The called DTE can indicate if it requires the support of the D bit
procedure by setting bit 7 of octet 1 of call accepted packets to 1.
PAD procedures associated with the Delivery Confirmation (D) bit in data
packets (see S 4.3.3 of Recommendation X.25) are described in SS 1.4.4 and 1.5.6.
A.4 Virtual call characteristics
A.4.1 Resetting
A.4.1.1 There may be a loss of data characters in any case of reset, as stated
in Recommendation X.25. Characters generated by either of the DTEs prior to the
reset indication or confirmation will not be delivered to the other DTE after the
reset indication or confirmation.
A.4.2 Interrupt transfer
A.4.2.1 An interrupt packet is always delivered at or before the point in the
data packet stream at which it was generated.
A.4.3 Call clearing
Data packets transmitted immediately before a clear request packet is
sent, may be overtaken within the network by the clear request packet and
subsequently be destroyed, as described in S 4.5 of Recommendation X.25.
Fascicle VIII.2 - Rec. X.29 PAGE1