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APPENDIX I
(to Recommendation X.25)
Examples of data link layer transmitted bit patterns
by the DCE and the DTE
This appendix is provided for explanatory purposes and indicates the bit
patterns that will exist in the physical layer for some of the unnumbered frames.
It is included for the purpose of furthering the understanding of the
transparency mechanism and the frame check sequence implementation.
I.1 The following are examples of the bit patterns that will be transmitted by
a DCE for some unnumbered frames.
Example 1 : SABM command frame with address = A, P = 1
First bit Last bit
transmitted transmitted
» »
0111 1110 1100 0000 1111 1(0 3))100 1101 1010 0011 0111 0111 1110
Flag Address = A SABM(P = 1) Frame check sequence Flag
Example 2 : UA response frame with address = B, F = 1
First bit Last bit
transmitted transmitted
» »
3) Zero inserted for transparency.
PAGE142 Fascicle VIII.2 - Rec. X.25
0111 1110 1000 0000 1100 1110 1100 0001 1110 1010 0111 1110
Flag Address = B UA(F = 1) Frame check sequence Flag
I.2 The following are examples of the bit patterns that should be transmitted
by a DTE for some unnumbered frames:
Example 1 : SABM command frame with address = B, P = 1
First bit Last bit
transmitted transmitted
» »
0111 1110 1000 0000 1111 1(0 3))100 1101 0111 11(0 3))11 0111 1110
1011
Flag Address = B SABM(P = 1) Frame check sequence Flag
Example 2 : UA response frame with address = A, F = 1
First bit
Fascicle VIII.2 - Rec. X.25 PAGE1
transmitted Last bit
transmitted
» »
0111 1110 1100 0000 1100 1110 1100 1100 0010 0110 0111 1110
Flag Address = A UA(F = 1) Frame check sequence Flag 3)
PAGE142 Fascicle VIII.2 - Rec. X.25
APPENDIX II
(to Recommendation X.25)
An explanation of how the values for N1
in Section 2.4.8.5 are derived
Introduction
This appendix provides a description of how the values given for the link
level parameter N1 in S 2.4.8.5 are derived.
DTE N1
Section 2.4.8.5 states that for universal operation a DTE should support a
value of DTE N1 which is not less than 1080 bits (135 octets).
For universal operation, a DTE must be capable of accepting at least the
largest packet that can be transmitted across a DTE/DCE interface when no options
apply. This implies that the DTE may choose not to support, for example, any
optional facilities for universal operations, but must support, for example, a
data packet using the standard default packet size. Therefore, the determining
factor for the maximum value of N1 that a DTE must support is the standard
default packet size of a data packet rather than the size of a call setup packet.
Thus, for universal operation a DTE should support a value of DTE N1 which is not
less than 135 octets, derived as shown in the following table.
TABLE II-1/X.25
Derivation of the maximum value of N1 for a DTE
Name of the field Length of the field
(octets)
Packet header (Layer 3) 3
User data (Layer 3) 128
Address (Layer 2) 1
Control (Layer 2) 1
FCS (Layer 2) 2
TOTAL 135
Note - A DTE will need to support larger values of N1 when optional facilities will
apply.
DCE N1
Section 2.4.8.5 also states that all network shall offer to a DTE which
requires it a value of DCE N1 which is greater than or equal to 2072 bits (259
octets) plus the length of the address field plus the length of the control field
and plus the length of the FCS field.
When the maximum length of the data field of a data packet supported is
less than or equal to the standard default value of 128 octets, the determining
factor (for the value of DCE N1) is the clear request packet rather than the data
packet. Therefore, the network shall offer to a DTE, a value of DCE N1 which is
not less than 263 or 264 octets, derived as shown in the following table.
TABLE II-2/X.25
Derivation of the minimum value of N1 for a DCE
Name of the field Length of the field
(octets)
Header (Layer 3) 3
Clearing cause (Layer 3) 1
Diagnostic code (Layer 3) 1
DTE address length (Layer 3) 1
DTE address(es) (Layer 3) 15
Facility length (Layer 3)
Fascicle VIII.2 - Rec. X.25 PAGE1
1
Facilities (Layer 3) 109
Clear user data (Layer 3) 128
Layer 3 - TOTAL 259
Address (Layer 2) 1
Control (Layer 2) 1 or 2*
Multilink procedure 2**
FCS (Layer 2) 2
263 or 264*
TOTAL or 265**
or 266*, **
* If level 2 modulo 128 is supported.
** Multilink procedures (MLP) are supported.
When the maximum length of the user data field of a data packet supported
is greater than the standard default value of 128 octets, the determining factor
(for the value of DCE N1) is the data packet rather than the clear request
packet. Therefore, the network shall offer to a DTE, a value of DCE N1 which is
greater than or equal to:
[the maximum length of the data packet +
the length of the address field (Layer 2) +
the length of the control field (Layer 2) +
the length of the FCS field (Layer 2)].
General DCE N1 calculations
The following table indicates the value of DCE N1 for each possible case.
The table shows for each case, whether
a) Layer 2 Modulo 128 is used,
b) Multilink Procedures are used,
c) Layer 3 Modulo 128 is used, and/or
d) the maximum length of the data field (p) in a data packet is greater
than or equal to 256 octets.
TABLE II-3/X.25
Various cases and corresponding minimum N1 values for a DCE
Layer 2 MLP Layer 3 p │ 256
Modulo 128 Modulo
128
PAGE142 Fascicle VIII.2 - Rec. X.25
DCE N1 (octets)
259 + 4*
X 259 + 4* + 2*****
X p + 3** + 4*
X X p + 3** + 4* + 2*****
X 259 + 4*
X X 259 + 4* + 2*****
Fascicle VIII.2 - Rec. X.25 PAGE1
X X p + 3** + 1*** + 4*
X X X p + 3** + 1*** + 4* + 2*****
X 259 + 4* + 1****
X X 259 + 4* + 1**** + 2*****
X X p + 3** + 1**** + 4*
X X X p + 3** + 1**** + 4* + 2*****
X X 259 + 4* + 1****
PAGE142 Fascicle VIII.2 - Rec. X.25
X X X 259 + 4* + 1**** + 2*****
X X X p + 3** + 1*** + 4* + 1****
X X X X p + 3** + 1*** + 4* + 1**** + 2*****
* The number of octets for modulo 128 layer 2 frame fields.
** The number of octets for layer 3 packet header fields.
*** Additional octet for layer 3 modulo 128 operations.
**** Additional octet for layer 2 modulo 128 operations.
***** Additional octets for MLP support.
Fascicle VIII.2 - Rec. X.25 PAGE1
APPENDIX III
(to Recommendation X.25)
Examples of multilink resetting procedures
III.1 Introduction
The following examples illustrate application of the multilink resetting
procedures in the case of:
a) MLP reset initiated by either the DCE or the DTE; and
b) MLP reset initiated by both the DCE and the DTE simultaneously.
III.2 MLP reset initiated by either the DCE or the DTE
Figure III-1/X.25 - T0702241-87
III.3 MLP reset initiated by both the DCE and the DTE simultaneously
Figure III-2/X.25 - T0702250-87
APPENDIX IV
(to Recommendation X.25)
Information on addresses in call set-up
and clearing packets
IV.1 Main address and complementary address
A DTE address may include two components: a main address and a
complementary address.
IV.1.1 Main address
When the A bit is set to 0, the main address is conforming to formats
described in Recommendations X.121 and X.301 (including possible prefixes and/or
escape codes).
When the A bit is set to 1, the main address is as described in Figure
IV-1/X.25.
Type of address Numbering plan Address digits
identification
<--- 1 semi-octet <--- 1 semi-octet
---> --->
FIGURE IV-1/X.25
Format of the main address when the A bit is set to 1
The possible values and the semantic of these subfields are described in S
5.2.1.2.2.
IV.1.2 Complementary address
A complementary address is an address information additional to that
defined in X.121 (see S 6.8.1 of Recommendation X.301).
Some networks allow the DTE to include a complementary address. When a
complementary address is permitted by the network, the DTE is not obliged to use
this complementary address. The complementary address may be as long as possible
in considering the maximum value of the DTE address length fields defined in SS
5.2.1.1.1 and 5.2.1.2.1.
When a complementary address is contained in a DTE address field of a
packet transmitted by the network to the DTE, this complementary address is
always passed transparently from the remote DTE: it means that the network never
creates a complementary address from itself.
When a complementary address is invoked in the following sections, it is
supposed that the network supports the use of complementary addresses.
When the A bit is set to 1 and a complementary address is present alone
(i.e., without main address) in DTE address field, it is preceded by the type of
address and numbering plan identification subfields.
IV.2 Addresses in call request packet
In call request packet, the called DTE address should be provided by the
DTE except when the bilateral closed user group selection is provided in the
facility field (see S 6.15.3). Depending on the called network and the DTE, this
called DTE address may be made of a main address then a complementary address, or
of a main address alone.
Depending on the network, the DTE may have the following possibilities for
the called DTE address:
i) The DTE may include either no calling DTE address, or a main address
optionally followed by a complementary address. When a calling DTE
address is provided by the DTE, the network is required to check its
validity. If the calling DTE address is not valid, the network may
PAGE142 Fascicle VIII.2 - Rec. X.25
either replace this invalid calling DTE address by a valid one, or
clear the call. If the hunt group facility has been subscribed to by
the calling DTE (see S 6.24) and a specific address has been assigned
to the calling DTE/DCE interface, the main address provided by the
calling DTE may be the hunt group address or the specific address.
Note - In this later case, some networks do not allow the calling DTE
to indicate the hunt group address, but only the specific address.
ii) The DTE may include either no calling DTE address, or a calling
complementary address. In this last case, when the A bit is set to 1,
this complementary address shall be preceded by the type of address and
numbering plan identification subfields.
IV.3 Addresses in incoming call packets
In incoming call packet, the calling DTE address should be provided by the
DCE except when the bilateral closed user group selection is provided in the
facility field (see S 6.15.3) or in one case described in S 6.28. This calling
DTE address always includes a main address. This main address is followed by a
calling complementary address if such a complementary address had been provided
by the calling DTE in the call request packet (see S IV.2), and the calling DTE
address was considered as valid by the network at the calling DTE side. If the
hunt group facility has been subscribed to by the calling DTE (see S 6.24) and a
specific address has been assigned to the calling DTE/DCE interface, the main
address indicated in the calling DTE address may be the hunt group address (only
if the calling DTE had indicated either its hunt group address or no main
address, in the calling DTE address field of the call request packet) or the
specific address (regardless of the contents of the calling DTE address field in
the call request packet).
Depending on the network, the called DTE address may be made of:
i) The main called address optionally followed by the called complementary
address if this complementary address had been provided by the calling
DTE. If the hunt group facility has been subscribed to by the called
DTE (see S 6.24) and a specific address has been assigned to the called
DTE/DCE interface, the main address indicated in the called DTE address
field may be the hunt group address (only if the calling DTE had
indicated this hunt group address or no main address, in the calling
DTE address field of the call request packet) or the specific address
(regardless of the contents of the calling DTE address field in the
call request packet).
ii) The called complementary address alone when provided by the calling
DTE, or nothing if the calling DTE had not provided this called
complementary address. When a called complementary address is alone and
the A bit is set to 1, the called complementary is preceded by the type
of address and numbering plan identification subfields.
IV.4 Addresses in call accepted packets
Some networks do not allow any DTE addresses in call accepted packets
except a called DTE address in conjunction with the called line address modified
notification facility when supported by the network and provided by the DTE.
Some other networks allow the DTE to include in the called accepted packet
none, one or both of the two DTE addresses. When provided by the DTE, the calling
DTE address in the call accepted packet should be the same as the calling DTE
address in the incoming call packet. When provided by the DTE, the called DTE
address in the called accepted packet should be the same as the called DTE
address in the incoming call packet, except if the called line address modified
notification facility (when supported by the network) is also provided by the
DTE.
When the called line address modified notification facility (when
supported by the network) is provided by the DTE in the call accepted packet, the
called DTE address may be made of one of the following exclusive
network-dependent possibilities:
i) A main DTE address identical to that of the incoming call packet,
followed by a called complementary address different from that of the
incoming call packet, or another main DTE address valid for the DTE/DCE
interface optionally followed by any complementary address.
ii) A called complementary address, different from that which was possibly
present in the called DTE address of the incoming call packet. In this
case, when the A bit is set to 1, the called complementary address
Fascicle VIII.2 - Rec. X.25 PAGE1
shall be preceded by the type of address and numbering plan
identification subfields.
IV.5 Addresses in call connected packets
Some networks do not provide any DTE address in call connected packets
except a called DTE address in conjunction with the called line address modified
notification facility.
Some other networks always provide both DTE addresses in call connected
packets.
Some other networks provide a DTE address in a call connected packet only
if this DTE address was present in the call ccepted packet or in conjunction with
the called line address modified notification facility.
In any case, when an address is provided by the network in the call
connected packet, this address should be the same as that in the call request
packet except when the called line address modified notification facility is
present in the facility field: in this case, the called DTE address contains
always a main address optionally followed by a complementary address.
IV.6 Addresses in clear request packets
No DTE address is permitted in clear request packets except a called DTE
address when the called line address modified notification facility (see S 6.26)
is used in this packet. In this case, the clear request packet is transmitted as
a direct response to the incoming call packet and the called DTE address may be
made of one of the following network-dependent possibilities:
i) A main DTE address identical to that of the incoming call packet,
followed by a called complementary address different from that of the
incoming call packet, or another main DTE address valid for the DTE/DCE
interface.
ii) A called complementary address, different from that which was possibly
present in the called DTE address of the incoming call packet. In this
case, when the A bit is set to 1, the called complementary address
shall be preceded by the type of address and numbering plan
identification subfields.
IV.7 Addresses in clear indication packets
No DTE address is permitted in clear indication packets except when the
called line address modified notification facility (see S 6.26) is used in this
packet. In this case, the clear indication packet is transmitted as a direct
response to the call request packet and the called DTE address contains always a
main address optionally followed by a complementary address.
IV.8 Addresses in clear confirmation packets
DTE addresses are not present in clear confirmation packets.
IV.9 Addresses in call redirection and call deflection related facilities
The alternative DTE address, indicated at subscription-time (for the call
redirection facility) or in the call deflection selection facility of the clear
request packet (see SS 6.25.1 and 6.25.2), is composed of a main address
optionally followed by a complementary address.
If a called complementary address was present in the call request packet,
some networks may add this called complementary address after the alternative DTE
address.
PAGE142 Fascicle VIII.2 - Rec. X.25
