C:\WINWORD\CCITTREC.DOT_______________

The drawings contained in this Recommendation have been done in 
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Recommendation X.2

INTERNATIONAL DATA TRANSMISSION SERVICES
AND OPTIONAL USER FACILITIES IN PUBLIC DATA NETWORKS 
AND ISDNs

(Geneva, 1972; amended at Geneva, 1976, 1980;
Malaga–Torremolinos, 1984 and Melbourne, 1988)

	The CCITT,

considering

	(a)	the international user classes of service and categories of access 
defined in Recommendations X.1 and X.10;

	(b)	the need to standardize data transmission services, optional user facili-
ties and DTE services, in public data networks and ISDNs, which should be 
made available on an international basis;

	(c)	the need to standardize additional optional user facilities and DTE ser-
vices which may be provided by Administrations and which may be avail-
able on an international basis;

	(d)	that the optional user facilities indicated in this Recommendation are 
defined in other Recommendations, for example Recommendation X.301 
for network implementations, where appropriate, and Recommendations 
X.21, X.25, etc., for network procedures;

	Note – Alignment and interworking between the facilities in Recommenda-
tions X.2 and supplementary services in the I.250 series are for further 
study.

	(e)	the need to standardize the identification methods applicable to these 
DTE services, which identification methods should be made available on an 
international basis, and which identification methods may be provided by 
Administrations and may be available on an international basis;

	(f)	the impact which these optional user facilities and DTE services could 
have on tariff structures,

unanimously declares

	(1)	that the optional user facilities should be standardized for each of 
the user classes of service indicated in Recommendation X.1 for each of the 
following:

i)	circuit switched data transmission services;

ii)	packet switched data transmission services;

iii)	leased circuit data transmission services.

	Note 1 – Recommendation X.10 defines the various categories of access to 
public data communication services.

	Note 2 – Further study is necessary to establish whether the service and 
optional user facilities provided by ISDNs for circuit switched data trans-
mission services are adequately defined in Recommendation X.2.

	(2)	that the optional user facilities to be made available on an international 
basis are indicated in in the following sections. Some of the optional user 
facilities are available on a per–call basis and others may be assigned for an 
agreed contractual period. In all cases, the user has the option of requesting 
a given optional user facility;

	(3)	that the DTE services and the relative identification methods used 
when the packet switched data transmission service is obtained via a Public 
Switched Telephone Network (PSTN), a Circuit Switched Public Data Net-
work (CSPDN) or an Integrated Services Digital Network (ISDN) should 
also be standardized and are indicated in the following sections.

1		Circuit switched data transmission service

	Table 1/X.2 indicates the optional user facilities which should be 
made available on an international basis in the circuit switched data trans-
mission service and those facilities which may be available in certain data 
networks and may also be available internationally.

	Note – The subject of interworking between the packet switching ser-
vice and the circuit switching service is for further study.

TABLE 1/X.2

Optional user facilities in circuit switched data transmission service





Optional user facility

 All user classes
of service

 1.	Optional user facilities assigned for an agreed 
contractual period

1.1	Direct call			

A

1.2	Closed user group		

E

1.3	Closed user group with outgoing access	

A

1.4	Closed user group with incoming access	

A

1.5	Incoming call barred within a closed user 
group	

A

1.6	Outgoing call barred within a closed user group	

A

1.7	Calling line identification	

A

1.8	Outgoing calls barred	

A

1.9	Bilateral closed user group	

A

1.10	Bilateral closed user group with outgoing 
access	

A

1.11	Incoming calls barred	

A

1.12	Reverse charging acceptance	

A

1.13	Connect when free	

A

1.14	Waiting allowed	

A

1.15	Redirection of calls	

A

1.16	On–line facility parameter registration/cancel-
lation	

A

1.17	DTE inactive registration/cancellation	

A

1.18	Date and time indication	

A

1.19	Hunt group		

A

2.	Optional user facilities requested by the DTE 
on a per–call basis

2.1	Direct call			

A

2.2	Abbreviated address calling	

A

2.3	Multi–address calling (see Note)	

A

2.4	Reverse charging	

A

2.5	RPOA selection		

A

2.6	Charging information	

A

2.7	Called line identification	

A

Note – This optional user facility provides also the user with the capability 
to request the establishment of a point–to– multipoint configuration 
amongst the following: centralized multipoint, decentralized multipoint, 
broadcasting.

2		Packet switched data transmission services

2.1	Direct connection to a packet switched data transmission service

	Table 2/X.2 and Table 3/X.2 indicate the services and the optional 
user facilities, respectively, which should be made available on an interna-
tional basis in the packet switched data transmission service and those 
which may be available on certain data networks and may also be available 
internationally, in case of a direct connection to a packet switched data 
transmission service.

	These services and optional user facilities are described in Recom-
mendation X.25.

	A DTE may make use of one or more of these services and facilities.

	It should be noted that Recommendation X.25 has inherent features 
(e.g. the conveying of an address extension), which can be used end–to–end 
by users for providing an OSI network service (see Recommendations 
X.213 and X.223). Such features are described in Recommendation X.25 
and are so–called “CCITT–defined DTE facilties” (see Recommendation 
X.25, Annex G).

	Note 1 – The subject of interworking between the packet switching 
service and the circuit switching service is for further study.

	Note 2 – The study of “connectionless services” is for further study.

TABLE 2/X.2

Services in packet switched data transmission service (direct connection)





User classes of 
service



 

8–11
and 13

20–23

Virtual call service			

E

 E



Permanent virtual circuit service	

E
(Note)

 FS

Note – This service continues to be allocated an “E” classification. How-
ever, while some Administrations continue to believe that the offering of 
this service is an essential feature of the international service, other Admin-
istrations have expressed the view that they will not offer the service inter-
nationally. The international application depends on bilateral agreement. 
This service may not be available for the maritime mobile services.

TABLE 3/X.2

Facilities of packet switched data transmission service (direct connections)





User classes of service



Optional user facility 

8–11
and 13

20–23
(Note 1)



 

VC

PVC

VC

PVC



1.	Optional user facilities assigned for an 
agreed contractual period

1.1	Extended frame sequence numbering	

A

 A

 –

 –

1.2	Multilink procedure		

A

 A

 –

 –

1.3	On–line facility registration	

A

 –

 FS

 –

1.4	Extended packet sequence numbering 
(modulo 128)	

A

 A

 –

 –

1.5	D–bit modification		

A

 A

 FS

 –

1.6	Packet retransmission	

A

 A

 –

 –

1.7	Incoming calls barred	

E

 –

 A

 –

1.8	Outgoing calls barred	

E

 –

 A

 –

1.9	One–way logical channel outgoing	

E

 –

 –

 –

1.10	One–way logical channel incoming	

A

 –

 –

 –

1.11	Nonstandard default packet sizes 16, 
32, 64, 256, 512, 1024,

	2048, 4096			

A

 A

 FS

 FS

1.12	Nonstandard default window sizes	

A

 A

 –

 –

1.13	Default throughput classes assign-
ment	

A

 A

 FS

 FS

1.14	Flow control parameter negotiation	

E

 –

 FS

 –

1.15	Throughput class negotiation	

E

 –

 FS

 –

1.16	Closed user group		

E

 –

 E

 –

1.17	Closed user group with outgoing 
access	

A

 –

 A

 –

1.18	Closed user group with incoming 
access	

A

 –

 A

 –

1.19	Incoming call barred within a closed 
user group	

A

 –

 A

 –

1.20	Outgoing call barred within a closed 
user group	

A

 –

 A

 –

1.21	Bilateral closed user group	

A

 –

 A

 –

1.22	Bilateral closed user group with out-
going access	

A

 –

 A

 –

1.23	Fast select acceptance	

E

 –

 FS

 –

1.24	Reverse charging acceptance	

A

 –

 A

 –

1.25	Local charging prevention	

A

 –

 FS

 –

1.26	NUI subscription		

A

 –

 A

 –

1.27	NUI override			

A

 –

 –

 –

1.28	Charging information	

A

 –

 A

 –

1.29	RPOA subscription		

A

 –

 A

 –

1.30	Hunt group			

A

 –

 A

 –

1.31	Call redirection		

A

 –

 FS

 –

1.32	Call deflection subscription	

A

 –

 –

 –

1.33	TOA/NPI address subscription	

FS

 –

 FS

 –

1.34	Direct call	

FS

 –

 A

 –

TABLE 3/X.2 (cont.)



User classes of service



Optional user facility 

8–11
and 13

20–23
(Note 1)



2.	Optional user facilities on a per–call 
basis	

2.1	Flow control parameter negotiation	

E

 –

 –

 –

2.2	Throughput class negotiation	

E

 –

 –

 –

2.3	Closed user group selection	

E

 –

 E

 –

2.4	Closed user group with outgoing 
access selection	

A

 –

 A

 –

2.5	Bilateral closed user group selection	

A

 –

 FS

 –

2.6	Reverse charging		

A

 –

 A

 –

2.7	Fast select			

E

 –

 FS

 –

2.8	NUI selection			

A

 –

 A

 –

2.9	Charging information	

A

 –

 A

 –

2.10	RPOA selection		

A

 –

 A

 –

2.11	Call deflection selection	

A

 –

 –

 –

2.12	Call redirection or call deflection noti-
fication	

A

 –

 FS

 –

2.13	Called line address modified notifica-
tion	

A

 –

 FS

 –

2.14	Transit delay selection and indication 
(see Note 2)	

E

 –

 –

 –

2.15	Abbreviated address calling	

FS

 –

 A

 –

 E	An essential user service or facility to be made available internation-
ally.

A	An additional user service or facility which may be available in certain 
data networks and may also be available internationally.

FS	For further study.

–	Not applicable.

VC	Applicable when the virtual call service is being used.

PVC	Applicable when the permanent virtual circuit service is being used.

Note 1 – The use of a PAD function is assumed for virtual call service (see 
Recommendation X.3). Its applicability for permanent virtual circuit service 
is for further study.

Note 2 – Attention of users is drawn to the fact that the implementation of 
such a facility might happen with different time schedules, depending on 
national conditions.

2.2	Switched connection to a packet switched data transmission service 
(classes of service 20–23)

	The definition of services and optional user facilities is for further 
study.

2.3	Switched connection to a packet switched data transmission service 
(classes of service 8–13)

2.3.1		DTE services and identification methods

	Table 4/X.2 indicates the DTE services and the relative identification 
methods which should be made available on an international basis with 
classes 8–13 of the packet switched data transmission service when the ser-
vice is accessed via a switched connection, and those which may be avail-
able in certain data networks and may also be available internationally, in 
case of a direct connection to a packet switched data transmission service.

	Permanent virtual circuits are not provided in the scope of § 2.3.

	The detailed definitions of the DTE services and the identification 
methods are contained in Recommendation X.32.

TABLE 4/X.2

DTE services and identification methods






DTE services

User 
classes 
of ser-
vice

DTE identification

DCE identifica-
tion



8–13

NO

PSN

XI
D

Re
g

N
UI

N
O

PS
N

XI
D

Re
g



1.  Nonidentified
(dial–in–by–the–
DTE)

A/E
(Note 
1)

X
(Note
 1)

X

X

X

2.  Nonidentified
(dial–out–by–the–
PSPDN)


A

 X

 X

 X

X

X



Dial–
in–by–
the–
DTE


A/E

X
(Note 
1)

X

X

X

X

X

X



3. Identi-
fied

Dial–
out–by–
the–
PSPDN

(Note 
1) 

X

X



4. Cus-
tomized

Dial–
in–by–
the–
DTE



A

X

X

X

X

X

X

(Note 2)

Dial–
out–by–
the–
PSPDN

X

X

X

X

X

X

X

NO	No identification.

PSN	Identification provided by Public Switched Network.

XID	Identification provided by means of the link layer XID procedure.

Reg	Identification provided by means of the packet layer registration proce-
dure.

NUI	Identification provided by means of the NUI selection facility.

PSPDN	Packet Switched Public Data Network.

E	An essential DTE service to be made available internationally.

A	An additional DTE service which may be available in certain data net-
works and may also be available internationally.

X	DTE or DCE identification method which can be provided by the network 
when it provides the corresponding DTE service.

Note 1 – An Administration providing a switched connection to a packet 
switched data transmission service for classes of service 8–13 should pro-
vide at least the nonidentified DTE service (dial–in–by–the–DTE) with no 
DTE identification method or the identified DTE service (dial–in–by–the–
DTE) with the provided–by–PSN DTE identification method.

Note 2 – The customized DTE service is one DTE service for which dial–
out–by–the–PSPDN may be provided or not provided. However, the possi-
ble identification methods are different for dial–in–by–the–DTE and dial–
out–by–the–PSPDN.

2.3.2	Facilities

	For the nonidentified (dial–in–by–the–DTE), the nonidentified (dial–
out–by–the–PSPDN), and the identified DTE services, the optional user 
facilities provided at the DTE/DCE interface are default values specified in 
Recommendation X.32.

	For the customized DTE service, the optional user facilities which 
should be made available internationally, and those which may be available 
on certain data networks and may also be available internationally are indi-
cated in Table 5/X.2.

TABLE 5/X.2

Facilities of packet switched data transmission service for customized DTE 
service



Optional user facility

User classes of 
service
8–13

1.	Optional user facilities assigned for an agreed con-
tractual period

1.1	Extended frame sequence numbering	

A

1.2	Multilink procedure		

FS

1.3	On–line facility registration	

A

1.4	Extended packet sequence numbering (modulo 
128)	

A

1.5	D–bit modification		

A

1.6	Packet retransmission	

A

1.7	Incoming calls barred	

E

1.8	Outgoing calls barred	

E

1.9	One–way logical channel outgoing	

E

1.10	One–way logical channel incoming	

A

1.11	Nonstandard default packet sizes 16, 32, 64, 256, 
512, 1024, 2048, 4096	

A

1.12	Nonstandard default window sizes	

A

1.13	Default throughput classes assignment	

A

1.14	Flow control parameter negotiation	

E

1.15	Throughput class negotiation	

E

1.16	Closed user group		

E

1.17	Closed user group with outgoing access	

A

1.18	Closed user group with incoming access	

A

1.19	Incoming call barred within a closed user group	

A

1.20	Outgoing call barred within a closed user group	

A

1.21	Bilateral closed user group	

A

1.22	Bilateral closed user group with outgoing access	

A

1.23	Fast select acceptance	

E

1.24	Reverse charging acceptance	

A

1.25	Local charging prevention	

A

1.26	NUI subscription		

A

1.27	NUI override			

A

1.28	Charging information	

A

1.29	RPOA subscription		

A

1.30	Hunt group			

A

1.31	Call redirection		

A

1.32	Call deflection subscription	

A

1.33	TOA/NPI address subscriprion	

FS

1.34	Direct call			

FS

2.	Optional user facilities on a per–call basis

2.1	Flow control parameter negotiation	

E

2.2	Throughput class negotiation	

E

2.3	Closed user group selection	

E

2.4	Closed user group with outgoing access selection	

A

2.5	Bilateral closed user group selection	

A

2.6	Reverse charging		

A

2.7	Fast select			

E

2.8	NUI selection			

A

2.9	Charging information		

A

2.10	RPOA selection		

A

2.11	Call deflection selection	

A

2.12	Call redirection or call deflection notification	

A

2.13	Called line address modified notification	

A

2.14	Transit delay selection and indication (see Note)	

E

2.15	Abbreviated address calling	

FS

TABLE 5/X.2 (cont.)





Optional user facility

 User classes of 
service
8–13

3.	X.32 optional user facilities

3.1	Secure dial–back		

A

3.2	Temporary location		

A

E	An essential user service or facility to be made available internation-
ally.

A	An additional user service or facility which may be available in certain 
data networks and may also be available internationally.

FS	For further study.

Note – Attention of users is drawn to the fact that the implementation of 
such a facility might happen with different time schedules, depending on 
national conditions.

3		Leased circuit data transmission services

	Table 6/X.2 indicates the optional user facilities which should be 
made available on an international basis with the leased circuit data trans-
mission services and those which may be available in certain data networks 
and may also be available internationally.

TABLE 6/X.2

Facilities of leased circuit data transmission service





Optional user facility

User classes of 
service



1–2

3–7



1.	Point to point			

E

E

2.	Multipoint			

2.1	Centralized multipoint	

A

A

2.2	Decentralized multipoint	

A

A

2.3	Broadcasting			

A

A

E	An essential user service or facility to be made available internation-
ally.

A	An additional user service or facility which may be made available in cer-
tain data networks and may also be made available internationally.

APPENDIX I

(to Recommendation X.2)

	Recommendation X.25 uses facility codes in the facility field of call 
set–up and clearing packets, and registration codes in the registration 
field of registration packets. Recommendation X.32 uses X.32 facility 
codes and identification protocol elements in the user data field of the 
XID frames or the registration field of the registration packets. Rec-
ommendation X.75 uses utility codes in the utility field of call set–up 
and clearing packets.

	The principles for the encoding of these codes (i.e., class A, B, C or 
D, depending on the length of the parameter following the code) are 
described in Recommendations X.25 and X.75.

	As far as possible, the same code is used in several contexts only when it 
has an equivalent semantic. However, due to historical reasons, it is not 
always the case.

	Table I–1/X.2 gives the list of the various codes used in these Recommenda-
tions.

TABLE I–1/X.2

Coding of the facility, registration, protocol element and utility codes



Code
Bits
8 7 6 5 4 3 2 1

    
X.2
5 
fac


X.2
5 
dte


X.2
5 
reg


X.3
2


X.7
5



CLASS A



0 0 0 0 0 0 0 0

 X

 X

 X

 X

 X

Marker

0 0 0 0 0 0 0 1

 X


X

Fast select and/or reverse 
charging
Fast select and/or reverse 
charging indication

0 0 0 0 0 0 1 0

X


X



X

Throughput class negotia-
tion
Default throughput classes 
assignment
Throughput class indica-
tion

0 0 0 0 0 0 1 1

X


X

Closed user group selec-
tion (basic format)
Traffic class indication

0 0 0 0 0 1 0 0

X

Charging information 
(requesting service)

0 0 0 0 0 1 0 1

X

Facilities that may be 
negotiated only when all 
logical channels used for 
virtual calls are in state p1

0 0 0 0 0 1 1 0

X

Non–negotiable facilities 
values

0 0 0 0 0 1 1 1

X


X

Diagnostic element
Tariffs

0 0 0 0 1 0 0 0

X 

X

Called line address modi-
fied notification

0 0 0 0 1 0 0 1

X

Closed user group with 
outgoing access selection 
(basic format)

 0 0 0 0 1 0 1 0

X

Quality of service negotia-
tion: minimum throughput 
class

 0 0 0 0 1 0 1 1

X

Expedited data negotiation

 0 0 0 0 1 1 0 0
to
0 0 1 1 1 1 1 1


Unused



TABLE I–1/X.2 (cont.)



Code
Bits
8 7 6 5 4 3 2 1


X.2
5 
fac


X.2
5 
dte


X.2
5 
reg


X.3
2


X.7
5



CLASS B



 0 1 0 0 0 0 0 0

Unused

 0 1 0 0 0 0 0 1

X


X

Bilateral closed user 
group selection
Transit network identifica-
tion

 0 1 0 0 0 0 1 0

 X


X




X

Flow control parameter 
negotiation (packet size)
Non–standard default 
packet sizes
Packet size indication

0 1 0 0 0 0 1 1

 X


X




X

Flow control parameter 
negotiation (window size)
Non–standard default win-
dow sizes
Window size indication

0 1 0 0 0 1 0 0

X


X

RPOA selection (basic for-
mat)
RPOA selection

0 1 0 0 0 1 0 1

X

Facilities that may be 
negotiated at any time

0 1 0 0 0 1 1 0

X

Availability of facilities

0 1 0 0 0 1 1 1

X

Closed user group selec-
tion (extended format)

0 1 0 0 1 0 0 0

X

Closed user group with 
outgoing access selection 
(extended format)

0 1 0 0 1 0 0 1

X


X

Transit delay selection 
and indication
Transit delay indication

0 1 0 0 1 0 1 0

X

Clearing network identifi-
cation code

0 1 0 0 1 0 1 1

X

Transit delay selection

0 1 0 0 1 1 0 0
to
0 1 0 1 1 1 1 1


Unused

 0 1 1 0 0 0 0 0

Reference number (see 
Note)

 0 1 1 0 0 0 0 1
to
0 1 1 1 1 1 1 1


Unused



 CLASS C



 1 0 0 0 0 0 0 0

Unused

 1 0 0 0 0 0 0 1

X

Call identifier

 1 0 0 0 0 0 1 0 
to
1 0 1 1 1 1 1 1


Unused



TABLE I–1/X.2 (cont.)



Code
Bits
8 7 6 5 4 3 2 1

 
X.2
5 
fac


X.2
5 
dte


X.2
5 
reg


X.3
2


X.7
5



CLASS D



1 1 0 0 0 0 0 0

Unused

1 1 0 0 0 0 0 1

 X

Charging information 
(call duration)

1 1 0 0 0 0 1 0

 X

Charging information 
(segment count)

1 1 0 0 0 0 1 1

 X


X

Call redirection or call 
deflection notification
Closed user group indica-
tion

1 1 0 0 0 1 0 0

 X

RPOA selection (extended 
format)

1 1 0 0 0 1 0 1

 X

Charging information 
(monetary unit)

1 1 0 0 0 1 1 0

 X


X

NUI selection
NUI

1 1 0 0 0 1 1 1

X

Closed user group with 
outgoing access indication

1 1 0 0 1 0 0 0

X

Logical channel types 
ranges

1 1 0 0 1 0 0 1

X

Called address extension

1 1 0 0 1 0 1 0

X

Quality of service negotia-
tion: end–to–end transit 
delay

1 1 0 0 1 0 1 1

X

Calling address extension

1 1 0 0 1 1 0 0

X

Identity element

1 1 0 0 1 1 0 1

X

Signature element

1 1 0 0 1 1 1 0

X

Random number element

1 1 0 0 1 1 1 1

X

Signed response element

1 1 0 1 0 0 0 0

X

Temporary location

1 1 0 1 0 0 0 1

X

Call deflection selection

1 1 0 1 0 0 1 0

X

Quality of service negotia-
tion: priority

1 1 0 1 0 0 1 1

X

Quality of service negotia-
tion: protection

1 1 0 1 0 1 0 0 
to
1 1 1 1 1 1 1 0


Unused

1 1 1 1 1 1 1 1

 X

 X

 X

 X

 X

Reserved for extension

Notes to Table I–1/X.2:

X.25 fac:	code used in the facility field of X.25 call set–up and/or clearing 
packets for X.25 facilities.

X.25 dte:	code used in the facility field of X.25 call set–up and/or clearing 
packets for CCITT–defined DTE facilities.

X.25 reg:	code used in the registration field of registration packets.

X.32:	code defined in Recommendation X.32 and used in the user data field 
of the XID frames or the registration field of the registration packets.

X.75:	code defined in Recommendation X.75 and used in the utility field of 
the call set–up and/or clearing packets.

Note – This value is reserved for ISO/8208.