Recommendation G.773
Recommendation G.773
PROTOCOL  SUITES  FOR  Q-INTERFACES  FOR  MANAGEMENT
OF  TRANSMISSION  SYSTEMS
1	Introduction
1.1	Scope
	This Recommendation defines the characteristics of protocol suites for Q-interfaces of transmission systems/equipments, as defined in Recommendations M.30 [1] and G.771 [58]. Protocol suites for Q-interfaces of other systems/equipments will be specified in other Recommendations. The interfaces will support bidirectional data transfer for the management of telecommunications systems.
	This Recommendation defines:
—	the layer services;
—	the layer protocols;
—	the application service elements and protocols;
—	the conformance requirements to be met by an implementation of these interfaces.
	This Recommendation does not define:
—	the structure or meaning of the management information that is transmitted by means of the protocol suites;
—	the manner in which management is accomplished as a result of the application protocol exchanges;
—	the interactions which result in the use of the application layer protocols.
1.2	Abbreviations and symbols
1.2.1	Abbreviations
AARE		A-associate response
AARQ		A-associate request
ACSE 		Association control service element
AFI	 	Authority and format identifier
APDU		Application protocol data unit
ASE		Application Service element
ASN.1		Abstract syntax notation one
CD	 	Collision detection
CDO		Connect data overflow
CLNS		Connectionless-mode network service
CMIP		Common management information protocol
CMIS		Common management information service

CMISE		Common management information service element
Conf		Confirm
CONS		Connection oriented-mode network service
CSMA		Carrier sense multiple access
DCE		Data circuit terminating equipment
DCN		Data communication network
DIS	 	Draft international standard
DLC		Data link connection
DLS	 	Data link service
DSP		Domain specific part
DTE		Data terminal equipment
EOC		Embedded operations channel
FU		Functional unit
HDLC		High-level data link control
IDI		Initial domain identifier
IDP		Initial domain part
Ind	 	Indication
ISO	 	International organization for standardization
LCN		Local communication network
LLC		Logical link control
LME		Layer management entity
MAC		Media access control
MD	 	Mediation device
NDM		Normal disconnected mode
NE	 	Network element
NLR		Network layer relay
NM-ASE 	Network management-application service element
NRM		Normal response mode
NRZ		Non return to zero
NRZI		Non return to zero inverted
NS	 	Network service

NSAP		Network service access point
OA	 	Overflow accept
OS	 	Operations system
OSI	 	Open systems interconnection
PDU		Protocol data unit
PhC	 	Physical connection
Ph	 	Physical
PhS	 	Physical service
PICS		Protocol implementation conformance statement
PLS	 	Physical layer service
PPDU		Presentation protocol data unit
PV	 	Parameter value
PVC	 	Permanent virtual circuit
PU	 	Protocol unit
QOS	 	Quality of service
Req	 	Request
Res	 	Response
ROSE		Remote operations service element
SDH		Synchronous digital hierarchy
SP	 	Session protocol
SPDU		Session protocol data unit
SPF		Segmentation permitted flag
SVC	 	Switched virtual circuit 1)
TMN		Telecommunications management network
TPDU		Transport protocol data unit
TSAP		Transport service access point
UNC		Unbalanced operation normal response mode class
1.2.2	Symbols and abbreviations used in tables 2)
M	Mandatory
—	The parameter is not present in the interaction described by the service or primitive concerned.
(=)	The value of the parameter is equal to the value of the parameter in the column to the left.
2	Protocol suites overview
2.1	Introduction
	Recommendation G.771 [58] provides guidance for the selection of protocol suites from Recommendation G.773 and the domain of application of these standard protocol suites.
	The structures of the protocol suites with the present layers are shown in Figure 1/G.773. The defined communication services and protocols are in accordance with the Open System Interconnection (OSI) reference model [2].
	The protocols for the different layers are based on Recommendations and/or ISO standards.
	Two types of protocol suites are defined in this Recommendation:
—	short stack: protocol suites A1 and A2,
—	full 7 layer stack: protocol suites B1, B2 and B3.
	The short stack protocol suites (A1 and A2) will be used mainly for LCN application as specified in Recommendation M.30 [1]. The full 7 layer stack protocol suites (B1, B2 and B3) can be applied to both LCN and DCN applications, as defined by Recommendation M.30 [1].
	Because of the nulling of the transport layer, session layer and presentation layer for short stack protocol suites, mapping functions have been defined.
	The full 7 layer protocol suites satisfy the requirements of complex NEs (e.g. equipments for the SDH). To support already existing networks and to provide maximum flexibility, several possibilities are defined for layers 1, 2 and 3. Each Administration should select depending on its own specific requirements and needs. Layers 5, 6 and 7 are identical for the three protocol suites B1, B2 and B3, whilst almost identical requirements apply to layer 4.
3	Protocol suites A1 and A2
3.1	Physical layer
3.1.1	Physical layer for A1
3.1.1.1	Service
3.1.1.1.1	Definition
	The service definition for the physical layer is in accordance with Recommendation X.211 [3].
	The following classes of physical services shall be supported:
—	type of transmission is synchronous;
—	mode of operation is half-duplex;
—	topology is point-to-multipoint by a bus.

Figure 1/G.773 = 22 cm

3.1.1.1.2	Service provided by the physical layer
	The physical layer provides the physical service primitives and parameters as listed in Table 1/G.773.
TABLE 1/G.773
Provided physical service
Service	Primitive	Parameters	
	PH-ACTIVATE-request		
PhC-Activation	PH-ACTIVATE-indication		
	PH-DATA-request		
Data Transfer	PH-DATA-indication	PhS-User data	
	PH-DEACTIVATE-request		
PhC-Deactivation	PH-DEACTIVATE-indication		

	The services PhC-Activation and PhC-Deactivation will be provided to the Layer Management Entity (LME) of the physical layer.
3.1.1.2	Physical interface
3.1.1.2.1	Physical characteristics
3.1.1.2.1.1	Configuration
	Serial bus operation in accordance with ISO 8482 3) [4], in half-duplex mode.
3.1.1.2.1.2	Transmission pairs
	Two screened balanced pairs, one for each direction of transmission.
3.1.1.2.1.3	Connector
	The Administration shall specify the connector type.
3.1.1.2.2	Electrical characteristics
3.1.1.2.2.1	Static and dynamic characteristics
	The static and dynamic characteristics of each bus connection shall be in accordance with ISO 8482 [4]. When all generators connected to the bus are in the high impedance state, the bus shall be set to logical level “1”.
3.1.1.2.2.2	Bus termination
	Each bus end shall be terminated in accordance with ISO 8482 [4].
3.1.1.2.2.3	Load connection
	Each receiver shall present a maximum of one unit load, as defined in ISO 8482 [4], to the bus. The number of load connections is limited to 32.
3.1.1.2.2.4	Bit rate
	The bit rate shall be 19 200 bit/s or 64 000 bit/s. A bit rate of 128 000 bit/s may be necessary in some applications. The bit rate tolerance shall be ± 0.05%.
3.1.1.2.2.5	Turn-off time
	For bit rates of 19 200 bit/s and 64 000 bit/s a transmitting station shall put its generator in the high impedance state within 0.750 ms from the end of the last bit of the closing flag. For a bit rate of 128 000 bit/s the turn- off time shall be not more than 0.375 ms. This point is not applicable to a primary station (see § 3.2.1.2.1.3).
3.1.1.2.2.6	Switch-on transient
	Following the enabling of the generator an implementation dependent preamble of no more than 4 bit times is allowed. No assumption as to the state of the bus during this preamble is allowed.
3.1.1.2.3	Line code
	The line code shall be NRZI.
3.1.1.2.3.1	Principle
	Each ISO 8482 [4] transition shall represent a ZERO, and no transition shall represent a ONE bit.
3.1.1.2.3.2	Lock in sequence
	Where required for clock extraction, it shall be possible to send a lock in sequence containing at least four transitions immediately prior to the beginning of the starting flag of the frame to be transmitted.
3.1.1.2.4	Extended mode
	An example of extended mode is given in Annex A.
3.1.2	Physical layer for A2
3.1.2.1	Overview
	Protocol suite A2 employs local area network technology for the physical and data link layers. Administrations will select the appropriate physical medium, e.g. coaxial cable, screened pairs, optical fibre according to technological and operational requirements.
3.1.2.2	Service
3.1.2.2.1	Definition
	The service definition for the physical layer shall comply with that specified in clause 6 of ISO 8802-3 [20].

3.1.2.2.2	Service provided by the physical layer
	All of the primitives defined and listed in Table 2/G.773 are mandatory.
TABLE 2/G.773
Primitives of the physical layer
Primitive	
PLS-DATA-request	
PLS-DATA-indication	
PLS-CARRIER-indication	
PLS-SIGNAL-indication	

3.1.2.3	Bit rate
	The bit rate will be 1 Mbit/s or higher.
3.2	Data link layer
3.2.1	Data link layer for A1
3.2.1.1	Service
3.2.1.1.1	Definition
	The service definition of the data link layer is in accordance with Recommendation X.212 [5]. The class of data link service that shall be provided by the data link layer is:
—	a connection-mode service.
3.2.1.1.2	Service required from the physical layer
	The data link layer requires the Data Transfer service from the physical layer.
3.2.1.1.3	Service provided by the data link layer
	The data link layer shall provide the Data Link service, primitives and parameters as listed in Tables 3/G.773 to 5/G.773.
3.2.1.1.3.1	DLC-Establishment
TABLE 3/G.773
DLC-establishment service
Parameter name	Req/Ind	Res/Conf	
Called address
Calling address
Responding address
QOS	M
M
—
M	—
—
M
M	

3.2.1.1.3.2	DLC-Release
TABLE 4/G.773
DLC-release service
Parameter name	Request	Indication	
Originator
Reason	—
M	M
M	

3.2.1.1.3.3	Normal Data Transfer
TABLE 5/G.773
Normal data transfer service
Parameter name	Request	Indication	
DLS-User data	M	M	

3.2.1.2	Data link protocol
	The data link protocol is synchronous HDLC type.
3.2.1.2.1	HDLC frame structure
	The HDLC frame structure shall conform to ISO 3309 (frame structure) [6].

3.2.1.2.1.1	Addressing field
	The addressing field shall be one octet.
3.2.1.2.1.2	Information field
	The information field in any HDLC frame shall be an integral number of octets.
	Information field octets shall be sent least significant bit first. The maximum length of the information field shall be 256 octets.
3.2.1.2.1.3	Interframe time fill
	A primary station shall transmit contiguous flags as interframe time fill.
3.2.1.2.2	Addressing
	The secondary station shall be capable of being assigned any address in the range 1 to 254.
3.2.1.2.2.1	All station address
	The address field pattern “11111111” is defined as the all-station address.
3.2.1.2.2.2	No-station address
	The address field pattern “00000000” is defined as the no-station address. The no-station address shall never be assigned to a secondary station.
3.2.1.2.2.3	Group addresses
	Not used.
3.2.1.2.3	HDLC procedure
	The HDLC procedure is defined in ISO 4335 [7].
3.2.1.2.3.1	Commands and response
	The following HDLC commands and responses must be supported:
—	commands
SNRM:	Set normal response mode
DISC:	Disconnect
—	commands or responses
I:		Information
RR:		Receive ready
RNR:	Receive not ready
—	responses
FRMR:	Frame reject
UA:		Unnumbered acknowledgement
DM:		Disconnect mode
3.2.1.2.3.2	Modes
	Two modes are selected:
—	one operational mode: normal response mode (NRM);
—	one non-operational mode: normal disconnected mode (NDM).
3.2.1.2.4	Class of procedure
	The unbalanced operation normal response mode class (UNC) as defined in ISO 7809 [8] shall be implemented.
3.2.1.2.4.1	HDLC optional functions
	The following HDLC optional functions shall be implemented:
—	unnumbered information (option No. 4);
—	data link test (option No. 12).
3.2.1.2.5	Other parameters of data link layer
3.2.1.2.5.1	Window size
	The window size for unacknowledged frames is to be optional between 1 and 7. The default value is 1.
3.2.1.2.5.2	Waiting-time before a repetition
	In the case of no-reply or lost-reply, the primary station shall provide a waiting time function. The waiting-time before a repetition shall be greater than the duration of the longest frame to be sent by the primary station, added to the response-time of the secondary station and the duration of the longest frame to be sent by the secondary station.
3.2.1.2.5.3	Number of repetitions
	Under the conditions described in § 3.2.1.2.5.2, the maximum number of repetition before detecting a no-reply or a lost-reply condition is fixed to 5 (6 requests).
3.2.1.2.5.4	Response time
	The secondary station shall commence the opening flag of its response not later than 5 ms after the end of the closing flag of the frame sent from the primary station.
3.2.2	Data link layer for A2
3.2.2.1	Overview
	The data link layer provides the acknowledged connectionless-mode service. The access method employed is Carrier Sense Multiple Access with Collision Detection (CSMA/CD).
3.2.2.2	Media access control (MAC)
3.2.2.2.1	The services and protocol of the CSMA/CD access method shall comply with those specified in ISO 8802‑3 [20].
3.2.2.2.2	The address length used at the MAC sublayer shall be 48 bits.
3.2.2.3	Logical link control (LLC)
3.2.2.3.1	The definition of the acknowledged connectionless mode LLC service shall comply with that specified in ISO 8802-2/DAD 2 [23]. All of the primitives defined for type 3 operation (Table 6/G.773) are mandatory.
TABLE 6/G.773
Primitives for type 3 operation
 DL-DATA-ACK-request/indication	
DL-DATA-ACK-STATUS-indication	
DL-REPLY-request/indication	
DL-REPLY-STATUS-indication	
DL-REPLY-UPDATE-request 	
DL-REPLY-UPDATE-STATUS-indication	

3.2.2.3.2	The protocol used to provide the acknowledged connectionless-mode LLC service shall be as specified in ISO 8802-2 [22] and ISO 8802/DAD 2 [23]. All of the commands and responses defined for type 3 operation (Table 7/G.773) are mandatory.
TABLE 7/G.773
Commands and responses for type 3 operation
Commands	Responses	
AC0
AC1	AC0
AC1	

3.3	Network layer for A1 and A2
3.3.1	Service
3.3.1.1	Service definition
	The definition of the connectionless-mode network service shall comply with that specified in ISO 8348/AD 1 [9]. Address formats supported shall conform to ISO 8348/AD 2 [10].
3.3.1.2	Service required from the data link layer
	The network layer requires the Normal Data Transfer service from the data link layer.
3.3.1.3	Service provided by the network layer
	The network layer shall provide the N-UNITDATA service as listed in Table 8/G.773.
TABLE 8/G.773
N-UNITDATA service
Parameter name	Request	Indication	
Source address
Destination address
QOS
NS-User data	M
M
M
M	M(=)
M(=)
M
M(=)	

3.3.2	Network protocol
3.3.2.1	General
	The Network protocol is as specified in ISO 8473 [11]. The sub-network dependent convergence function required for protocol suite A1 is specified in ISO 8473/AD 3 [19]. ISO 8473 [11] defines in addition to the full protocol (see § 3.3.2.4), two subsets namely:
—	inactive network layer protocol (see § 3.3.2.2),
—	non-segmenting network layer protocol (see § 3.3.2.3).
	The address part shall have the structure as defined in ISO 8348/AD 2 [10].
	For protocol suite A1 the Authority and Format Identifier (AFI) shall be set to 49, coded by 2 decimal digits as defined in ISO 8348/AD 2 [10], which specifies “local” and binary coding of the Domain Specific Part (DSP).
	For protocol suite A2 the authority and format identifier (AFI) shall be set to 38, 39, 48 or 49, coded by two decimal digits as defined in ISO 8348/AD 2 [10], which means ISO Data Country Code (ISO DCC) and decimal coding of domain specific part (DSP), ISO DCC and binary coding of DSP, “local” and decimal coding of DSP, or “local” and binary coding of DSP, respectively.
	The full protocol and the two subsets permit the use of known sub-network characteristics and are therefore not sub-network independent.
	Depending on the required usage and the sub-network architecture the full protocol, or one or both subsets, shall be supported by protocol suite A. The selection shall be put in the Protocol Implementation Conformance Statement (PICS).
3.3.2.2	Inactive network layer protocol
	The protocol shall be in accordance with the inactive subset of the protocol as defined in ISO 8473 [11].
3.3.2.3	Non-segmenting network layer protocol
	The protocol shall be in accordance with category type 1 functions of the non-segmenting subset of the protocol as defined in ISO 8473 [11].
	From the optional functions (type 3) defined in the non-segmenting subset only the “priority function” shall be supported as defined in ISO 8473 [11].
3.3.2.4	Full network layer protocol
	The full protocol subset of category type 1 functions, as specified in ISO 8473 [11], shall be supported.
	An implementation shall not transmit PDUs encoded using the inactive subset. Received PDUs encoded using the inactive subset will be discarded.
	An implementation shall not generate data PDUs without a segmentation part, i.e. the Segmentation Permitted Flag (SPF) shall be set to 1 and the segmentation part shall be included. However, an implementation shall be capable of receiving and correctly processing PDUs which do not contain the segmentation part.
3.4	Mapping functions for A1 and A2
3.4.1	Introduction
	No transport layer, session layer and presentation layer will be specified for protocol suites A1 and A2.
	To provide the required service to the application layer and using the provided service of the network layer a mapping function is defined.
	No protocol for the mapping function is defined.
3.4.2	Service
3.4.2.1	Service definition
	The service definition of the mapping function, which provides the required presentation service to the application layer, shall be in accordance with Recommendation X.216 [12].
3.4.2.2	Service required from network layer
	The mapping function requires the N-UNITDATA as the connectionless-mode network service.
3.4.2.3	Service provided by the mapping function
	The mapping function shall provide the presentation service as listed in Table 9/G.773.
TABLE 9/G.773
Service provided by the mapping function
Service	Primitive	Parameters	
P-DATA	P-DATA-request	User data 	
 	P-DATA-indication		

	When ACSE is supported in the application layer the mapping function shall also provide the presentation services P-CONNECT, P-RELEASE, P-U-ABORT and P-P-ABORT. Only the parameters defined as mandatory in Recommendation X.216 [12] shall be supported. The value of the Mode parameter of P-CONNECT shall be “normal”.
3.4.3	Procedure
	The mapping function will provide the values for the source address, destination address, QOS and NS-User data as required by the network service parameters. The mapping function will translate the presentation addresses to the Network-Service-Access-Point (NSAP) addresses and vice versa. It will provide the value of the quality of service parameter of N-UNITDATA Request. The NS-User data will be provided by the User data of P-DATA and vice versa.
	Note — This is not a mapping protocol. While the service description of this function is standard, the implementation itself needs not to be standardized.

3.5	Application layer for A1 and A2
3.5.1	Overview
	The network management application layer shall provide the CMISE service to the NM-ASE.
	The required application service elements for this service are Common Management Information Service Element (CMISE) and Remote Operations Service Element (ROSE). Some applications may require the addition of the Association Control Service Element (ACSE).
3.5.2	Syntax and encoding
	The application layer protocol data unit presentation is described by using Abstract Syntax Notation One (ASN.1), as defined in Recommendation X.208 [15] and is encoded in accordance with the basic encoding rules for ASN.1, as defined in Recommendation X.209 [16].
3.5.3	Association control
3.5.3.1	The ACSE service description is detailed in Recommendation X.217 [25]. When the ACSE is used all of the defined ACSE services (Table 10/G.773) are mandatory. The value of mode parameter of A-ASSOCIATE shall be “normal”.
3.5.3.2	The protocol specification for ACSE shall follow Recommendation X.227 [26]. When the ACSE is used all five APDUs (see Table 10/G.773) specified in the standard are mandatory. The value of protocol version field of AARQ and AARE shall be version 1 only.
TABLE 10/G.773
ACSE services and associated APDUs
ACSE service	Associated APDUs	Related P-Service	
A-ASSOCIATE
A-RELEASE
A-ABORT
A-P-ABORT	AARQ, AARE
RLRQ, RLRE
ABRT
(None)	P-CONNECT
P-RELEASE
P-U-ABORT
P-P-ABORT	

3.5.4	Remote operations
3.5.4.1	The remote operations service element (ROSE) shall be a mandatory service element for the protocol suites A1 and A2. The ROSE service description is detailed in Recommendation X.219 [14]. All of the defined ROSE services (Table 11/G.773) are mandatory.
3.5.4.2	The protocol specification for ROSE shall follow Recommendation X.229 [18]. All four APDUs specified in the standard (see Table 11/G.773) are mandatory. In addition, the ability to support correct origination and reception of the linked-ID protocol element is required for protocol suites A1 and A2.
	The requirement specified in Table 11/G.773 implies association class 3 in ROSE.
TABLE 11/G.773
ROSE services and associated APDUs
ROSE service	Associated APDUs	Related underlying service 	
RO-INVOKE	ROIV	P-DATA	
RO-RESULT	RORS	P-DATA	
RO-ERROR	ROER	P-DATA	
RO-REJECT-U	RORJ	P-DATA	
RO-REJECT-P	RORJ	P-DATA	

3.5.5	Common management information
3.5.5.1	The common management information service element (CMISE) shall be a mandatory service element for the protocol suites A1 and A2. The CMISE service description is detailed in ISO 9595 [13], ISO 9595/DAD 1 [27] and ISO 9595/DAD 2 [28]. The CMISE services are listed in Table 12/G.773.
TABLE 12/G.773
CMISE services
Service	Type	
M-EVENT-REPORT	confirmed/non-confirmed	
M-GET	confirmed	
M-SET	confirmed/non-confirmed	
M-ACTION	confirmed/non-confirmed	
M-CREATE	confirmed	
M-DELETE	confirmed	
M-CANCEL-GET 	confirmed	

3.5.5.2	The protocol specification for CMISE shall follow ISO 9596 [17], ISO 9596/DAD 1 [29] and ISO 9596/DAD 2 [30].
3.6	Conformance
	For further study.
4	Protocol suites B1, B2 and B3
4.1	Physical layer
4.1.1	Physical layer for B1 and B2
4.1.1.1	Protocol
	The protocol of the physical layer of protocol suites B1 and B2 shall comply with the following specifications:
—	X.21 interface in accordance with § 1.1 of Recommendation X.25 [39];
—	X.21 bis interface in accordance with § 1.2 of Recommendation X.25 [39];
—	V-Series interface in accordance with § 1.3 of Recommendation X.25 [39].
4.1.1.2	Bit rate
	The supported bit rates are: 1200, 2400, 4800, 9600, 19 200 and 64 000 bit/s. The bit rates 48 000 bit/s and 56 000 bit/s may be used for an interim period (see note a) to Table 19b/G.773).
4.1.1.3	Connector
	Table 13/G.773 lists the connectors to be used in accessing the X.21 [61] and X.21 bis [62] interfaces. Tables 14/G.773, 15/G.773 and 16/G.773 list respectively the pin descriptions of ISO 2110 [37], ISO 2593 [38], ISO 4902 [24] and ISO 4903 [63].
TABLE 13/G.773
X.21/X.21 bis connectors
Data signalling rate	X.21 bis	X.21	
2 400 bit/s	ISO 2110	ISO 4903	
4 800 bit/s	ISO 2110	ISO 4903	
9 600 bit/s	ISO 2110	ISO 4903	
19 200 bit/s	ISO 2110	ISO 4903	
48 000 bit/s	ISO 2593
ISO 4902	ISO 4903	
56 000 bit/s	ISO 2593	ISO 2593	
64 000 bit/s	ISO 4902	ISO 4903	

TABLE 14/G.773
ISO 2110 [37] pin description (Note 6)
Pin	V.24 [32] circuit	Description	Notes	
1
7	101
102	Protective ground (shield)
Signal ground	1
2	
2
3	103
104	Transmitted data
Received data	2
2	
4
5
6
20
22
8	105
106
107
108.2
125
109	Request to send
Clear to send
Data set ready (DCE Ready)
Data terminal ready (DTE Ready)
Ring indicator
Received line signal detector	2
2
2
3
3
2	
24
15	113
114	Transmitter signal element timing (DTE to DCE)
Transmitter signal element timing (DCE to DTE)	4
5	
Note 1 — Equipment: removable strap to frame ground or other equivalent grounding arrangement.
Note 1 — Cable: connected to shield.
Note 2 — Basic interchange circuits, all systems.
Note 3 — Additional interchange circuits required for switched service.
Note 4 — Circuit 113 is not used in OS/MD-NE interfaces.
Note 5 — Additional interchange circuits required for synchronous channel.
Note 6 — Duplex, Interface Type D.
Circuits are grouped by function: ground, data, control and timing.
For further information, see Recommendations V.24 [32] and V.28 [34], and ISO 2110 [37].

TABLE 15/G.773
V.35 [35], ISO 2593 [38] pin description (Note 3)
Pin	Circuit	Description	Notes	
A
B	101
102	Protective ground
Signal ground	1
	
P
S
R
T	103
103
104
104	Transmitted data A-wire
Transmitted data B-wire
Received data A-wire
Received data B-wire	2
2
2
2	
C
D
E
F	105
106
107
109	Request to send
Ready for sending
Data set ready
Data channel receive line signal detector		
Y
AA
V
X	114
114
115
115	Transmitter signal element timing A (DCE to DTE)
Transmitter signal element timing B (DCE to DTE)
Receiver signal element timing A (DCE to DTE)
Receiver signal element timing B (DCE to DTE)	2
2
2
2	
Note 1 — Equipment: removable strap to frame ground or other equivalent grounding arrangement.
Note 1 — Cable: connected to shield.
Note 2 — The electrical characteristics of the interchange circuits 103, 104, 114 and 115 shall be balanced double-current, conforming to Appendix II of Recommendation V.35 [35].
All other circuits shall conform to Recommendation V.28 [34].
Note 3 — The mode is synchronous at 64 000 bit/s.
Some countries may use 56 000 bit/s for an interim period of time.
Circuits are grouped by function: ground, data, control and timing.
For further information, see Recommendations V.35 [35], V.24 [32] and V.28 [34] and ISO 2593 [38].

TABLE 16/G.773
ISO 4903 [63] pin description (Note 2)
Pin	X.21
circuit	Description	Notes	
1
8	—
G	Protective ground
Signal ground or common return	1	
2
9
4
11	T
T
R
R	Transmit A-wire
Transmit B-wire
Receive A-wire
Receive B-wire		
3
10
5
12	C
C
I
I	Control A-wire
Control B-wire
Indication A-wire
Indication B-wire 		
6
13	S
S	Signal element timing A-wire
Signal element timing B-wire		
Note 1 — Equipment: removable strap to frame ground or other equivalent grounding arrange-ment. 
Note 1 — Cable: connected to shield.
Note 2 — Circuits are grouped by functions: ground, data, control and timing.
For further information: see Recommendations V.10 [64],V.11 [36] and X.21 [61] and ISO 4903 [63].

4.1.2	Physical layer for B3
4.1.2.1	Overview
	Protocol suite B3 employs local area network technology for the physical and data link layers. Administrations will select the appropriate physical medium, e.g. coaxial cable, screened pairs, optical fibre according to technological and operational requirements.
4.1.2.2	Service
	The service definition for the physical layer shall comply with that specified in clause 6 of ISO 8802-3 [20].
	All of the primitives defined and listed in Table 17/G.773 are mandatory.
TABLE 17/G.773
Primitives of the physical layer
Primitive	
PLS-DATA-request	
PLS-DATA-indication	
PLS-CARRIER-indication	
PLS-SIGNAL-indication	

4.1.2.3	Bit rate
	The possible bit rate will be 1 Mbit/s, 10 Mbit/s or higher.
4.2	Data link layer
4.2.1	Data link layer for B1 and B2
	It is mandatory that the data link layer conforms to LAPB as defined in Recommendation X.25 [39]. In addition, provision shall be made for connection between data terminal equipments without an intervening packet switched network. The interface shall conform to ISO 7776 [40]. Further detail is provided in § 4.2.1.1.
	The following link layer specification applies to all cases.
4.2.1.1	Equipment type during link set-up and reset
	When a packet switched network is used to connect systems, they are each designated Data Terminal Equipment (DTE) and the network acts as a Data Circuit-Terminating Equipment (DCE). When a dedicated or dial-up link is provided, other means must be used to supply the DCE role.
	At the physical layer, the modems will provide the DCE interface, supplying bit synchronization.
	At the link level, the procedures specified in ISO 7776 [40] shall be followed. A system must be able to start the set-up or reset of the link (a DCE function in Recommendation X.25 [39]). In addition, provision must be made for assignments of the A/B addresses. This mandatory option is to be field-settable and stored in non-volatile memory. Equipment which meets this requirement is compatible with connection to either a DCE or remote DTE.
4.2.1.2	Window
	Modulo 8 operation shall be used. Support of modulo 128 is optional. The window for unacknowledged frames is to be optional between 1 and 7 frames and 1 to 127 with modulo 128. The standard default is 7.
4.2.1.3	User information
	The user information is to be arranged in an integral number of octets.
	The maximum length of the user information shall be user settable, consistent with the range of values for the N1 parameter as shown in Table 18/G.773. Maximum information field lengths that shall be supported are 131 and 259 octets with optionally 515, 1027, 2051 or 4099 octets. These values provide for three packet header octets and maximum length of packet data units of 128, 256, 512, 1024, 2048 and 4096 octets, respectively.
4.2.1.4	Other frame parameters
	Certain other frame parameters shall be set by the user to be consistent with the bit rate, frame size and characteristics of the connecting network. A system design should be sufficiently flexible to accommodate parameter sets for diverse networks, both as order options and later reconfigurations. The range of parameters is shown in Table 18/G.773. These options, like those of the physical layer, are to be set at installation, changeable by the user, and non-volatile.
TABLE 18/G.773
LAPB data link layer attributes
LAPB protocol
octet aligned 
Single link procedure (SLP) 				
Parameter	Function	Range	Default	
K	I-Frames window	1 to 7 (with modulo 8)
1 to 127 (with optional modulo 128)	7
7	
T1	Waiting acknowledgement
(retry) timer a)
for up to 9600 bit/s
for 56  000 bit/s	

2 to 20 seconds
0.2 to 20 seconds	

3
3	
T2	Response delay parameter a)	Not greater than 0.3 seconds		
T3	Disconnect timer	b)		
T4	No activity timer	4 to 120 seconds	20	
N1	Bits per I-frame, excluding flags and zero bit insertion for transparency c)	1080, 2104 (with modulo 8)
Optional d) (with modulo 8): 4152, 8248, 16 440, 32 824
1096, 2120 (with modulo 128)
Optional d) (with modulo 128): 4168, 8264, 16 456, 32 840	2104

2120	
N2	Retransmission count	2 to 16	7	
A/B	Address assignment	Selectable by the user		
a)	Further guidelines on the use of T1 and T2 can be found in Recommendation X.25 [39] and ISO 7776 [40]. The transport layer T1 timer should always be greater than the link layer T1 timer.
b)	The value of timer T3, the disconnect timer, is not critical for successful interworking of OSs and NEs. Therefore no value is specified.
c)	In some cases, users may need to choose a maximum information field length of 259 octets (N1 = 2104 for modulo 8 or N1 = 2120 for modulo 128) with a 128 octet packet data unit in order to accommodate call request packets containing 128 octet user data fields in addition to the packet header and facility fields. These values are based on modulo 8 or modulo 128 operation at both link and packet layer.
d)	Optional
	The default values shall be part of a vendor's offering. That is, unless otherwise specified by the user, the default parameters shall be the initial values supplied. They can be subsequently changed by the user within the specified range.

4.2.2	Data link layer for B3
4.2.2.1	Overview
	The data link layer provides the unacknowledged connectionless-mode service. The access method employed is carrier sense multiple access with collision detection (CSMA/CD).
4.2.2.2	Media access control (MAC)
	The services and protocol of the CSMA/CD access method shall comply with those specified in ISO 8802‑3 [20].
	The address length used at the MAC sublayer shall be 48 bits.
4.2.2.3	Logical link control (LLC)
	The definition of the unacknowledged connectionless-mode LLC service shall comply with that specified in ISO 8802-2 [22]. All of the primitives defined for type 1 operation shall be supported.
	The protocol used to provide the unacknowledged connectionless-mode LLC service shall be as specified in ISO 8802-2 [22]. All of the commands and responses defined for type 1 operation shall be supported.
4.3	Network layer
4.3.1	Network layer for B1
	It is mandatory that the packet layer conforms to Recommendation X.25 [39]. In addition, the packet layer must provide for connection of data terminal equipment without an intervening packet network; the required interface for this purpose conforms to ISO 8208 [41]. In addition, the provisions of Recommendation X.223 [42] shall apply.
	The attributes which must be supported are summarized in Tables 19a/G.773 and 19b/G.773. Note in particular that these tables show the different attributes needed to support PVCs (the X.25/PVC [39] procedures) and SVCs (the X.25/SVC [39] procedures).
4.3.1.1	Equipment type during restart
	When the packet level X.25 [39] interface is used, automatic selection of the DCE/DTE role during restart is required, as specified in ISO 8208 [41].
4.3.1.2	Other features and parameters
	The packet layer attributes are summarized in Tables 19a/G.773 and 19b/G.773.
TABLE 19a/G.773
X.25 [39] packet layer attributes for permanent virtual circuits
	Range	Default	
Extended packet sequence numbering	Modulo 128 optional		
Packet size (octets)	128, 256
512, 1024, 2048, 4096 optional	128	
Window size
Extended sequence number option	1-7 (with modulo 8)
1-127 (with optional modulo 128)	122
122	
Interrupt packets	Optional		
Note 1 — The default values shall be part of a vendor's offering. That is, unless otherwise specified by the user, the default parameters shall be the initial values supplied. They can be subsequently changed by the user within the specified range.
Note  2 — The attributes which are not marked optional are mandatory.
Note 3 — The ranges specified for negotiated parameters in no way affect the normal negotiation rules specified in the international standards.

TABLE 19b/G.773
X.25 [39] packet layer attributes for switched virtual circuits
	Range	Default	
Flow control parameters			
Packet size (octets)	128, 256	2128	
	(512 optional)		
			
Window size	1-7 (with modulo 8)	2122	
			
Extended sequence number option	1-127 (with optional modulo 128)	2122	
			
Throughput class a) bit rate (bit/s)	1200, 2400, 4800, 9600, 19 200 and 64 000	2400	
			
Expedited data negotiation closed user			
group			
Closed user group selection basic format	2 decimal digits		
			
Fast select			
Fast select acceptance	128 octets		
			
Hunt group	Optional		
Transit delay selection and indication			
Calling address extension			
Called address extension  			
Minimum throughput class  negotiation			
End-to-end transit delay negotiation			
a)	Some countries may use 56 000 bit/s for an interim period of time. In addition to the codes specified in the table in § 7.2.2.2 of Recommendation X.25 [39], 56  000 bit/s shall be encoded as binary “1100”. 48  000 bit/s is encoded as binary “1100” in that table, but when 56 000 bit/s is supported, the code shall stand for 56 000 bit/s.
Note 1 — The default values shall be part of a vendor's offering. That is, unless otherwise specified by the user, the default parameters shall be the initial values supplied. They can be subsequently changed by the user within the specified range.
Note 2 — The attributes which are not marked optional are mandatory.
Note 3 — The ranges specified for negotiated parameters in no way affect the normal A negotiation rules specified in the international standards.

4.3.1.3	Expedited data negotiation
	The initiator shall be capable of proposing the non-use of the Expedited Data service. Responders shall be capable of receiving requests for the Expedited Data service, but shall be capable of responding with non-use of the service. The Expedited Data service is neither required nor precluded by this Recommendation.
4.3.1.4	Receipt confirmation negotiation
	The initiator shall be capable of setting bit 7 of the general format identifier to 0. Responders shall be capable of receiving bit 7 set to 1, but shall be capable of responding with bit 7 set to 0. The Receipt Confirmation service is neither required nor precluded by this Recommendation.

4.3.1.5	Throughput class
	When the end system requires only one network layer connection on a physical access port, support of throughput classes up to the access line transmission rate is required. When multiple network layer connections are required, support of the throughput class equal to the access line transmission rate is optional. Further study of throughput class range and default values at various access line rates is needed.
4.3.1.6	Packet size negotiation
	Interoperability is achieved by having the initiator propose a packet size from the set specified in Tables 19a/G.773 and 19b/G.773, and by the responder selecting the most appropriate packet size between 128 and the proposed packet size. The rules for negotiation of the size of the packet to be used in a given instance of communication are specified in ISO 8208 [41].
	The choice of packet size is a local issue which can depend on, for example, the quality of service requested or needed by the user or application and the sub-network characteristics.
4.3.1.7	User data field
	When layers above X.25 [39] are used, the initial octets of a DATA primitive and the corresponding data transfer packet are used for peer-to-peer protocol data for those layers.
	In following the procedures of Recommendation X.244 [43], ISO DTR 9577 [44], Annex B of Recommendation X.224 [49] and ISO 8073/AD 1 [45], the initial octets of the user data field of the call request packet may only be used for protocol identification. For those cases in which the fast select feature is used, the call request packet may contain a call user data field of up to 128 octets.
4.3.1.8	Numbering plans
	To support communications over public networks, public numbering plans may be used on the packet-switched network between OSs/MDs and NEs. The Recommendations E.164 [21] and X.121 [46] specify public numbering plans. Equipment may be assigned numbers in accordance with either of these international Recommendations. The escape code values of “0” and “9” shall be supported as specified in Table 2/X.121 of Recommendation X.121 [46]. Where a public numbering plan is not necessary, a private numbering plan may be used.
4.3.1.9	Addressing
	Network layer addressing as specified in Recommendation X.213, Annex A [47] and ISO 8348/AD 2 [10] shall be supported.
4.3.2	Network layer for B2
4.3.2.1	Protocol
	The protocols for the network layer shall be identical to the network layer protocol of protocol suite B1 (see § 4.3.1) with the inclusion of ISO 8473 [11] as specified in ISO 8880/3 [59] § 3, to provide the connectionless-mode network service over the connection-mode network service.
	For those instances of communication requiring interworking between a Connection Oriented Service (CONS) and a Connectionless-mode Network Service (CLNS), ISO DTR 10172 [60] provides an ISO compatible interworking capability. This capability is known as a Network Layer Relay (NLR) and utilizes the ISO 8473 [11] protocol to provide this service.
4.3.2.2	Network layer attributes
	Characteristics of the connectionless-mode network layer service, and the connectionless-mode network layer protocol shall be as shown in Table 20/G.773.

4.3.3	Network layer for B3
4.3.3.1	Service
	The definition of the connectionless-mode network service shall comply with that specified in ISO 8348/AD 1 [9]. Address formats supported shall conform to ISO 8348/AD 2 [10].
	The network layer shall provide the N-UNITDATA service as specified in ISO 8348/AD 1 [9].
4.3.3.2	Protocol
	The protocol shall be in accordance with the full protocol subset of category type 1 functions, as specified in ISO 8473 [11].
4.3.3.3	Network layer attributes
	Characteristics of the connectionless-mode network layer service and the connectionless-mode network layer protocol shall be as shown in Table 20/G.773.
TABLE 20/G.773
Network layer service/protocol parameters
a)	Destination and source addresses used by this protocol shall be network service access points (NSAP) addresses, as specified in ISO 8348/AD 2 [10] or Rec. X.213 [47] (Annex A).
	The destination and source addresses are of variable length. The destination and source address fields shall be as network protocol address information using the preferred binary encoding specified in ISO 8348/AD 2 [10].	
b)	The setting of the error reporting flag (E/R) shall be a local matter.
	Note — The use of error reporting and setting the E/R flag to 1 may lead to excessive network traffic.	
c)	Partial source routing shall NOT be supported. A defect exists with this option which can cause PDUs to loop in the network until their lifetime expires.	
d)	Inactive subset — Implementations shall not transmit PDUs encoded using the ISO 8473 [11] inactive subset. Received PDUs encoded with the inactive subset shall be discarded.	
e)	Segmentation — The non-segmentation subset shall NOT be used. However, implementations shall be capable of receiving and correctly processing PDUs which do not contain the segmentation part. 	
f)	Segmentation permitted flag — Implementations shall NOT generate data PDUs without a segmentation part, i.e. the segmentation permitted flag (SPF) shall be set to 1 and segmentation part shall be included.	
g)	Lifetime control — The lifetime parameter shall be used as specified in clause 6.4 of ISO 8473 [11]. This parameter shall have an initial value of at least three times the network span (number of network entities) or three times the maximum transmission delay (in units of 500 milliseconds), whichever is greater. 	

4.4	Transport layer
4.4.1	Transport layer for B1
	It is mandatory that for the connection-oriented network service, the transport layer shall conform to Recommendations X.214 [48] and X.224 [49] and to those provisions of ISO 8072 [50] and 8073 [51] that apply to the use of the Connection-Oriented Network Service (CONS).

4.4.1.1	Class of service
	Classes 4, 2 and 0 shall be supported as shown in Table 21/G.773 in countries requiring the features of transport layer class 4. The conformance rules of Recommendation X.224 [49] require that classes 0 and 2 be supported as well when class 4 is specified.
TABLE 21/G.773
Transport layer attributes for connection-oriented network service
	Range	Default	
Maximum TPDU (octets)	128, 256, 512, 1024 	128	
	(2048, 4096, 8192 optional)		
			
TSAP-ID a)	Up to 32 octets		
			
Class of service	4, 2, 0		
Preferred class	4, 2, 0	124	
Alternative class	0, none	None	
Expedited data	non-use		
			
Options for class 4			
Data TPDU numbering b)	Normal, extended	Normal	
			
Checksum c)	Use, non-use	Non use	
			
Options for class 2			
Data TPDU numbering b)	Normal, extended	Normal	
Flow control	Explicit		
			
Parameters for class 4			
T1	Retransmission time	0.25-64 seconds d)	128	
N	Retransmissions	2 (other values for further study)		
L	Bound on reference	1-256 seconds	132	
I	Inactivity time	2-512 seconds 	164	
a)	Some systems may require TSAP-IDs. However, all systems shall be capable of generating called TSAP-IDs in CR TPDUs and capable of receiving calling and called TSAP-IDs in received CR and CC TPDUs, respectively.
b)	Extended format option shall be implemented. Non-use of this option shall be negotiable. The responder shall honour the initiator's request whenever possible. Negotiation to other than what has been requested shall only occur under abnormal conditions: for example, severe congestion, as determined by the implementor. Initiators shall be prepared to operate in the mode confirmed by the responder.
c)	Use of checksum is required for the CR TPDU. An additional requirement is that all implementations shall support the negotiated “non-use” of the checksum. Initiators shall request and responders shall agree to “non- use” of the checksum.
d)	The transport layer T1 timer should always be greater than the link layer T1 timer.
Note — The default values shall be part of a vendor's offering. That is, unless otherwise specified by the user, the default parameters shall be the initial values supplied. They can be subsequently changed by the user within the specified range.

	In addition to the requirements specified in Recommendation X.224 [49], equipment shall meet the following requirement: if a responder receives an alternate class of “none”, it shall respond with the preferred class. Rules for responders are specified in Table 22a/G.773. Acceptance rules for initiators are specified in Table 22b/G.773.

	User options shall be provided to designate the preferred and alternate classes (see Table 3 of Recommendation X.224 [49]). When all of the classes are supported, the preferred class for connection is class 4.
TABLE 22a/G.773
Transport class selection rules: Response rules for responder
	Alternative class			
Preferred class	0	2	None	
0	Not valid	Not valid	Class 0	
2	Classes 0, 2	Class 2	Class 2	
4	Classes 0, 2, 4	Class 2 or 4	Class 4	
Note 1 — When all of the classes are supported, the preferred class, when initiating a CR-TPDU, shall be class 4.
Note 2 — If a responder receives an alternative class of “none”, it shall respond with the preferred class.

TABLE 22b/G.773
Transport class selection rules: Acceptance rules for initiator
	Alternative class			
Preferred class	0	2	None	
0	Not valid	Not valid	Class 0	
2	Classes 0, 2	Class 2	Class 2	
4	Classes 0, 2, 4	Class 2 or 4	Class 2 or 4	
Note 1 — When all of the classes are supported, the preferred class, when initiating a CR-TPDU, shall be class 4.
Note 2 — If class 4 is proposed, then class 2 is a valid response.
Note 3 — For existing equipment and in countries not requiring class 4, support of classes 0 and 2 is mandatory.

4.4.1.2	Protocol identification
	For the purpose of transport layer protocol identification, the procedures specified in Recommendation X.224 [49] Annex B and ISO 8073/AD 1 [45] shall be used. The conventions for protocol identification given in ISO DTR 10172 [60] should be followed. Selection of codes not specified in the referenced standards is for further study. The absence of call user data in a call request or call accept packet of Recommendation X.25 [39] and ISO 8208 [41] indicates the operation of the transport layer procedures of ISO 8073 [51] and Recommendation X.224 [49].
4.4.1.3	Attributes
	Attributes of the transport layer for use with CONS are summarized in Table 21/G.773. The selection of values within required and optional ranges depends on characteristics of the messages.
	Note — The need to support high priority messages that require low transit delay on a given transport connection must be reflected in the quality of service parameters requested when the transport connection is established. A properly implemented transport entity should not multiplex high priority messages that require low transit delay if it cannot provide the requested quality of service. 
4.4.1.4	User data in connection request and connection confirm TPDUs
	User data in the connection request and connection confirm TPDUs are optional in Recommendation X.224 [49]. No transport service user shall send it: all protocol implementations shall be prepared to receive it and all implementations may ignore it, i.e. it shall not cause a disconnect.
4.4.1.5	Splitting
	Responders may refuse network connections which could impose an unnecessary restriction on the ability to establish outgoing network connections. To prevent repeated ineffective attempts during splitting, initiators shall refrain from immediately requesting additional network connections for a transport connection after a network connection has been refused. The time delay before requesting additional network connections is for further study.
4.4.1.6	Quality of service negotiation
	Quality of service negotiation is outside the scope of this Recommendation. If quality of service negotiation is not supported, receipt of the parameters “throughput”, “residual error rate”, “priority” and “transit delay” in the CR and CC TPDUs shall be ignored.
4.4.1.7	TPDU size negotiation
	Interoperability is achieved by having the initiator propose a TPDU size from the set specified in Table 21/G.773 and by the responder selecting the most appropriate TPDU size between 128 octets and the proposed TPDU size. The rules for negotiation of the size of the TPDU to be used in a given instance of communication are specified in ISO 8073 [51].
	The choice of the TPDU size is a local implementation issue.
4.4.1.8	Class 0 error TPDU
	When transport class 0 has been negotiated, the error transport protocol data unit (ER-TPDU) may be used at any time and upon receipt requires that the recipient disconnect the network connection and, by extension, the transport connection.
4.4.1.9	Negotiation of protection
	Negotiation of protection is outside the scope of this Recommendation. If negotiation of protection is not supported, receipt of the protection parameters in any CR TPDU and any CC TPDU shall be ignored.
4.4.1.10	Unknown CR TPDU parameters
	An unknown parameter in any received CR TPDU shall be ignored.
4.4.1.11	Invalid values of known CR TPDU parameters
	Known parameters with valid lengths but with invalid values in a CR TPDU shall be handled as depicted in Table 23/G.773.
TABLE 23/G.773
TPDU parameters
Parameter	Action 	
TSAP ID	Send DR TPDU	
TPDU size	Ignore parameter, use default	
Version	Ignore parameter, use default	
Checksum	Discard CR TPDU	
Alternate protocol classes	Protocol error	

4.4.1.12	Additional options parameter
	Unrecognized or not applicable bits of the Additional Options shall be ignored.
4.4.1.13	 Code misalignment
	For further study.
	A misalignment between Recommendation X.224 [49] and ISO 8073 [51] code values for subsequence number and flow control confirmation has been identified. As a short-term solution, ISO 8073 [51] shall apply.
Subsequence number			1000	1010
Flow control confirmation		1000	1100
	It is intended that when an ISO/CCITT solution to this defect is available, this Recommendation will be modified to align with the solution.
4.4.2	Transport layer for B2 and B3
4.4.2.1	Protocol
	Operation of the transport protocol over the connectionless-mode network layer service (CLNS), as described in ISO 8348/AD 1 [9], shall use the elements of ISO 8073/AD 2 [52], class 4 operation over the CLNS.
4.4.2.2	Class of service
	Support of class 4 operation of ISO 8073/AD 2 [52] is mandatory.
4.4.2.3	Transport layer attributes
	Transport layer attributes for class 4 operation over the connectionless-mode network layer service shall be as shown in Table 24/G.773.
TABLE 24/G.773
Transport layer attributes for connectionless network service
	Range	Default	
Maximum TPDU (octets)	128, 256, 512, 1024 	128	
	(2048, 4096, 8192 optional)		
			
TSAP-ID a)	Up to 32 octets		
			
Class of service	4		
Preferred class	4		
Alternative class	None		
Expedited data	Non-use		
			
Options for class 4			
Security parameters	Optional		
Data TPDU numbering b)	Normal, extended	Normal	
Checksum c)	Use, non-use	Non use	
			
Parameters 			
T1	Retransmission time	0.25-64 seconds d)	28	
N	Retransmissions	2-15	22	
L	Bound on reference	1-256 seconds	32	
I	Inactivity time	2-512 seconds 	64	
a)	Some systems may require TSAP-IDs. However, all systems shall be capable of generating called TSAP-IDs in CR TPDUs and capable of receiving calling and called TSAP-IDs in received CR and CC TPDUs, respectively.
b)	Extended format option shall be implemented. Non-use of this option shall be negotiable. The responder shall honour the initiator's request whenever possible. Negotiation to other than what has been requested shall only occur under abnormal conditions: for example, severe congestion, as determined by the implementor. Initiators shall be prepared to operate in the mode confirmed by the responder.
c)	Use of checksum is required for the CR TPDU. An additional requirement is that all implementations shall support the negotiated “non-use” of the checksum. Initiators shall request and responders shall agree to “non-use” of the checksum.
d)	The transport layer T1 timer should always be greater than the link layer T1 timer.
Note 1 — The default values shall be part of a vendor's offering. That is, unless otherwise specified by the user, the default parameters shall be the initial values supplied. They can be subsequently changed by the user within the specified range.
Note 2 — A conflict in the code values for subsequence number and flow control confirmation exists between CCITT and ISO. The conflict is expected to be resolved as specified in ISO 8073 [51].

4.5	Session layer for B1, B2 and B3
	The session layer conforms to the service definition and protocol specification in Recommendations X.215 [53] and X.225 [54] respectively. Support of version 2 of the session protocol is mandatory. Two session layer functional units (FU) are required in this Recommendation:
1)	Kernel
2)	Duplex
	Restrictions applied to parameters and their values are specified in the following sections.

4.5.1	Session protocol data units
	The following Session Protocol Data Units (SPDUs) associated with the Kernel and Duplex functional units shall be supported as detailed in Table 25/G.733.
TABLE 25/G.773
Session PDUs
Connect	(CN SPDU)	
Accept	(AC SPDU)	
Refuse	(RF SPDU) 	
Finish	(FN SPDU) 	
Disconnect	(DN SPDU) 	
Abort	(AB SPDU)	
Abort Accepted	(AA SPDU)	
Data Transfer	(DT SPDU)	

4.5.2	Transport expedited service
	The use of the Transport Expedited service is as stated in Recommendation X.225 [54]: if available, it must be used. When the Transport Expedited service is available, the Prepare (PR) SPDU shall be supported as in Recommendation X.225 [54]. The Prepare Type parameter value in the PR SPDU, to indicate the arrival of an Abort (AB) SPDU, is ABORT.
4.5.3	Parameters
	All mandatory parameters defined in Recommendation X.225 [54] for the SPDUs required by the Kernel and Duplex FUs are mandatory parameters for this Recommendation.
4.5.4	User data
	The maximum length of the session user data shall be 10240 octets. This restriction implies that the Overflow Accept (OA) and Connect Data Overflow (CDO) SPDUs are not required to be supported. Session-selector (s-selector) parameter values shall have a maximum length of 16 octets.
4.5.5	Reuse
	Reuse of the transport connection is not required. The Transport Disconnect parameter value (PV) field may be absent or set to “transport connection is released” in appropriate SPDUs. Furthermore, on receipt of a transport disconnect PV field indicating “transport connection is kept”, the transport connection can be released.
4.5.6	Segmentation
	The segmentation feature in the session layer is not required. Support for extended concatenation of SPDUs is not required.
4.5.7	Invalid SPDUs
	Upon receipt of an invalid SPDU, the session protocol machine shall take any action specified in § A.4.3.2 of Recommendation X.225 [54] with the exception of action “d” (take no action).
4.6	Presentation layer for B1, B2 and B3
	It is mandatory that the presentation layer conform to the services and protocols specified in Recommendations X.216 [12] and X.226 [55] respectively. One presentation layer Functional Unit (FU) is required in this Recommendation: Kernel
	The presentation protocol shall be used in the normal mode. Restrictions applied to parameters and their values are specified in the following sections.
4.6.1	Presentation protocol units
	The following presentation protocol data units (PPDU) associated with the Kernel functional unit shall be supported as detailed in Table 26/G.733.
TABLE 26/G.773
Presentation PDUs
Connect Presentation	(CP PPDU)	
Connect Presentation Accept	(CPA PPDU) 	
Connect Presentation Reject	(CPR PPDU)	
Abnormal Release Provider	(ARP PPDU)	
Abnormal Release User	(ARU PPDU)	
Presentation Data	(TD PPDU)	

4.6.2	Parameters
	All mandatory parameters defined in Recommendation X.226 [55] for the above PPDUs are mandatory for this Recommendation. The “presentation context identifier” value shall be encoded in no more than 2 octets. Also, the value(s) in the parameter presentation context definition list shall be consistent with the value(s) defined in the application-specific standards. Presentation-selector (p-selector) parameter values shall have a maximum length of 4 octets.
4.6.3	Encoding rules for transfer syntax
	The encoding rules defined in Recommendation X.209 [16] shall be applied to derive the transfer syntax for the Application Protocol Data Units (APDUs). The ASN.1 OBJECT IDENTIFIER {joint‑iso‑ccitt asn1 (1) basic‑encoding (1)} shall be used as the value for the transfer syntax name. The maximum value of an ASN.1 basic encoding tag that needs to be handled for conformance to this Recommendation is 16383. This is the largest unsigned integer that can be represented in 14 bits. Hence the identifier octets shall consist of an initial octet and up to two more octets, thus occupying a maximum of 3 octets. Also, the largest number of octets in the “contents octets” component of an ASN.1 data value encoding that needs to be handled for conformance to this Recommendation is 4294967295. This is the largest unsigned integer that can be represented in 32 bits. Hence in the “long form” encoding, the length octets shall consist of an initial octet and up to four more octets, thus occupying a maximum of 5 octets. (Note that this restriction does not apply to “indefinite length” encodings.)
4.7	Application layer for B1, B2 and B3
	It is mandatory that the application layer conforms to the architecture for the application layer outlined in ISO 9545 [56]. Abstract Syntax Notation One (ASN.1) shall be used as the abstract syntax for specifying application protocols.
4.7.1	Supporting ASE
	It is mandatory that the association control service elements (ACSE) conform to the services and protocols specified in Recommendations X.217 [25] and X.227 [26]. The ACSE shall establish, release and abort the associations required. The ACSE service shall operate in the “normal mode”.
	Network management applications shall use the common management information service element (CMISE). Services defined by CMISE that are applicable include:
1)	the reporting of an event to an OS/MD;
2)	the transfer of information between OSs/MDs and NEs;
3)	the transer of action requests and results between OSs/MDs and NEs.
4.7.2	Application protocol data units
	The following application protocol data units shall be supported as detailed in Table 27/G.773.
TABLE 27/G.773
Application PDUs
A-Associate-Request	(AARQ APDU)	
A-Associate-Response	(AARE APDU)	
A-Release-Request	(RLRQ APDU)	
A-Release-Response	(RLRE APDU)	
A-Abort	(ABRT APDU)	

	All mandatory parameters defined in Recommendation X.227 [26] for the above APDUs are mandatory for this Recommendation.
4.7.3	Abstract syntax name
	The ACSE abstract syntax name has the ASN.1 type OBJECT IDENTIFIER. The following value shall be used to identify the ACSE abstract-syntax-definition:
	{
	joint-iso-ccitt association-control (2)
	abstract-syntax (1) apdus (0) version (1)
	}
4.7.4	Common management information service (CMIS)
	The common management information service element (CMISE) shall be a mandatory service element for the Protocol Suite B1, B2 and B3. The CMISE service description is detailed in ISO 9595 [13], ISO 9595/DAD 1 [27] and ISO 9595/DAD 2 [28].
	Multiple object selection filter and Multiple reply functional units as defined in ISO 9595 [13] are optional. Their use is application dependent. The negotiation during association establishment to use or not use the Functional Units shall be supported.
	Support of the extended service functional unit defined in ISO 9595 [13] is not required for conformance to this Recommendation and negotiation shall be supported, at association establishment, for its non-use.
4.7.5	Common management information protocol
	Implementations shall support those operations defined in ISO 9596 [17], ISO 9596/DAD 1 [29] and ISO 9596/DAD 2 [30] that are required by specific applications. All mandatory parameters defined in ISO 9596 [17], ISO 9596/DAD 1 [29] and ISO 9596/DAD 2 [30] for the required operations are mandatory parameters for this Recommendation.
4.7.6	Remote operations service element (ROSE)
	Network Management Transaction-oriented applications shall use the following underlying service defined in Recommendation X.219 [14]:
—	Remote operations service element (ROSE). The protocol is specified in Recommendation X.229 [18].
	The requirement specified above implies association class 3 in ROSE.
4.8	Conformance
	For further study.
ANNEX A
(to Recommendation G.773)
Example of extended mode for A1 protocol suite
A.1	Extended mode
	For those cases where it is required to extend beyond the range of the bus, one or several different capabilities may be used. For the case using modems the requirements of §§ 3.1.1.2.1 to 3.1.1.2.3 apply with the following exceptions:
A.1.1	Configuration — Full duplex
	The connector shall conform to IEEE 488 [31]. Appropriate signal lines are to be provided for modem control in accordance with Recommendation V.24 [32]. See Table A-1/G.773.
A.1.2	Electrical requirements
	Data set control leads shall conform to Recommendation V.24 [32].
A.1.3	Line code
	NRZ line code shall be employed. A separate clock distribution shall be provided.
A.1.4	Speed
	The bit rate shall be 9600 bit/s or 64 000 bit/s. Lower speeds, e.g. 1200, 2400 and 4800 bit/s, may be necessary in some applications.
TABLE A-1/G.773
Pin description of 24-pin IEEE STD 488 [31] connector
	Circuit					
Pin	RS 232-C	RS 449	V.24	Description	Notes	
1	AA	Shield	101	Protective ground	1	
13	AB	SG	102	Signal ground		
2	BA	SD	103	Send data A-wire		
14		SD	103	Send data B-wire		
11	BB	RD	104	Receive data A-wire		
23		RD	104	Receive data B-wire		
3	CA	RS	105	Request to send A-wire	2	
15		RS	105	Request to send B-wire	2	
7	CB	CS	106	Clear to send A-wire	2	
19		CS	106	Clear to send B-wire	2	
8	CC	DM	107	Data mode A-wire a)	2	
20		DM	107	Data mode B-wire a)	2	
9	CF	RR	109	Receiver ready A-wire	2	
21		RR	109	Receiver ready B-wire	2	
6	DB	ST	114	Send timing A-wire (DCE to DTE)		
18		ST	114	Send timing B-wire (DCE to DTE)		
10	DD	RT	115	Receive timing A-wire (DCE to DTE)		
22		RT	115	Receive timing B-wire (DCE to DTE)		
Note 1 — Equipment removable strap to frame ground cable connected to shield.
Note 2 — These circuits are optional for connection to an EOC or modem and are not  used for connections to a multipoint bus.
Note 3 — Circuits are grouped by function: ground, data, control and timing.
Provision should be made at each interface point on a multipoint bus for the continuation of the interface to the next network element.
Provision shall be made for the termination of the lines in their characteristics impedance (typically, 120 ohms, resistive), should the equipment be at one end of a multipoint bus.
For further information, see EIA RS 485 [57] and EIA RS 449 [33], Recommendation V.24 [32] and IEEE STD 488 [31].

References
[1]	CCITT Recommendation Principles for a telecommunications management network (TMN), Vol. IV, Rec. M.30.
[2]	CCITT Recommendation Reference model of open system interconnection for CCITT applications, Vol. VIII, Rec X.200 (ISO 7498, 1984).
[3]	CCITT Recommendation Physical layer service definition of open system interconnection (OSI) for CCITT applications, Vol. VIII, Rec. X.211 (ISO 10022, 1989).
[4]	ISO 8482 Information processing systems — Data communication — Twisted pair multipoint interconnections, 1987.
[5]	CCITT Recommendation Data link service definition for open system interconnection for CCITT applications, Vol. VIII, Rec. X.212 (ISO 8886, 1988).
[6]	ISO 3309 Information processing systems — Data communication — High-level data link control procedures — Frame structure, 1984.
[7]	ISO 4335 Information processing systems — Data communication — Consolidation of elements of procedures, 1987.

[8]	ISO 7809 Information processing systems — Data communication — High-level data link control procedures — Consolidation of classes of procedures, 1984.
[9]	ISO 8348/AD 1 Information processing systems — Data communications — Network service definition; Addendum 1: Connectionless-mode transmission, 1987.
[10]	ISO 8348/AD 2 Information processing systems — Data communications — Network service definition; Addendum 2: Network layer addressing, 1988.
[11]	ISO 8473 Information processing systems — Data communications — Protocol for providing the connectionless-mode network service, 1988.
[12]	CCITT Recommendation Presentation service definition for open system interconnection for CCITT applications, Vol. VIII, Rec. X.216 (ISO 8822, 1987).
[13]	ISO 9595 Information processing systems — Open systems interconnection — Common management information service definition (CMIS), 1990.
[14]	CCITT Recommendation Remote operations: model, notation and service definition, Vol. VIII, Rec. X.219, (ISO 9072-1, 1988).
[15]	CCITT Recommendation Specification of abstract syntax notation one (ASN.1), Vol. VIII, Rec. X.208 (ISO 8824, 1987).
[16]	CCITT Recommendation Specification of basic encoding rules for abstract syntax notation one (ASN.1), Vol. VIII, Rec. X.209 (ISO 8825, 1987).
[17]	ISO 9596 Information processing systems — Open systems interconnection — Common management information protocol specification (CMIP), 1990.
[18]	CCITT Recommendation Remote operations: protocols specification, Vol. VIII, Rec. X.229.
[19]	ISO 8473/AD 3 Information processing systems — Data communications — Protocol for providing the connectionless-mode network service; Addendum 3: Provision of the underlying service assumed by ISO 8473 over sub-networks which provide the OSI data link service, 1988.
[20]	ISO 8802-3 Information processing systems — Local area networks — Part 3: Carrier sense multiple access with collision detection — Access method and physical layer specifications, 1989.
[21]	CCITT Recommendation Numbering plan for the ISDN area, Vol. II, Rec. E.164.
[22]	ISO 8802-2 Information processing systems — Local area networks — Part 2: Logical link control, Rec. E.164, 1988.
[23]	ISO 8802-2/DAD 2 Logical Link Control; Addendum 2: Acknowledged connectionless-mode service and protocol, Type 3 operation, 1988.
[24]	ISO 4902 Data communication — 37 pin and 9 pin DTE/DCE interface connectors and pin assignments, 1989.
[25]	CCITT Recommendation Association control service definition for open system interconnections for CCITT applications, Vol. VIII, Rec. X.217 (ISO 8649, 1988).
[26]	CCITT Recommendation Association control protocol specification for open system interconnections for CCITT applications, Rec. X.227 (ISO 8650, 1988).
[27]	ISO 9595/DAD 1 Information processing systems — Open system interconnection — Common management information service element definition, CANCEL-GET.
[28]	ISO 9595/DAD 2 Information processing systems — Open system interconnection — Common management information service element definition, REMOVE.
[29]	ISO 9596/DAD 1 Information processing systems — Open system interconnection — Common management information protocol specification, CANCEL-GET.
[30]	ISO 9596/DAD 2 Information processing systems — Open system interconnection — Common management information protocol specification, REMOVE.
[31]	IEEE STD 488 Standard digital interface for programmable instrumentation, 1978.
[32]	CCITT Recommendation List of definitions for interchange circuits between data terminal equipment (DTE) and data circuit- terminating equipment (DCE), Vol. VIII, Rec. V.24.
[33]	EIA RS 449 General purpose 37-position and 9-position interface for data terminal equipment and data circuit-terminating equipment employing serial binary data interchange, 1977.
[34]	CCITT Recommendation Electrical characteristics for unbalanced double-current interchange circuits, Vol. VIII, Rec. V.28.
[35]	CCITT Recommendation Data transmission at 48 kbits per second using 60-108 kHz group band circuits, Vol. VIII, Rec. V.35.
[36]	CCITT Recommendation Electrical characteristics for balanced double-current interchange circuits for general use with integrated circuit equipment in the field of data communications, Vol. VIII, Recs. V.11 and X.27.
[37]	ISO 2110 Data communication — 25-pin DTE/DCE interface connector and pin assignments, 1989.
[38]	ISO 2593 Data communications — 34-pin DTE/DCE interface connector and pin assignments, 1984.
[39]	CCITT Recommendation Interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) for terminals operating in the packet mode and connected to public data networks by dedicated circuit, Vol. VIII, Rec. X.25 (ISO 8208, 1984 and ISO 7776, 1986).
[40]	ISO 7776 Information processing systems — Data communications — High-level data link control proce-
dures — Description of the X.25 — LAPB-compatible DTE data link procedures, 1986.
[41]	ISO 8208 Information processing systems data communications — X.25 packet level protocol for data terminal equipment, 1987.
[42]	CCITT Recommendation Use of X.25 to provide the OSI connection-mode network service for CCITT applications, Vol. VIII, Rec. X.223 (ISO 8878, 1987).
[43]	CCITT Recommendation Procedure for exchange of protocol identification during virtual call establishment on packet switched public data networks, Vol. VIII, Rec. X.244 (ISO TR 9577, 1990).
[44]	ISO TR 9577 Information technology — Telecommunications and information exchange between systems — Protocol identification in the OSI network layer, 1990.
[45]	ISO IEC 8073/AD 1 Information processing systems — Open systems interconnection — Connection oriented transport protocol specification — Addendum 1: Network connection management subprotocol, 1988 .
[46]	CCITT Recommendation International numbering plan for public data networks, Vol. VIII, Rec. X.121.
[47]	CCITT Recommendation Network service definition for open systems interconnection for CCITT applications, Rec. X.213, 1988 (ISO 8348, 1987; 8348/AD 2, 1989; 8348/AD 3, 1988).
[48]	CCITT Recommendation Transport service definition for open systems interconnection for CCITT applications, Vol. VIII, Rec. X.214 (ISO 8072, 1986).
[49]	CCITT Recommendation Transport protocol specification for open systems interconnection for CCITT applications, Vol. VIII, Rec. X.224 (ISO 8073, 1988).
[50]	ISO 8072 Information processing systems — Open systems interconnection — Transport service definition, 1986.
[51]	ISO 8073 Information processing systems — Open systems interconnection — Connection-oriented transport protocol specification, 1988. 

[52]	ISO 8073/AD 2 Information processing systems — Open systems interconnection — Connection-oriented transport protocol specification, 1988 — Addendum 2: Class 4 operation over connectionless network service, 1989.
[53]	CCITT Recommendation Session service definition for open system interconnection for CCITT applications, Vol. VIII, Rec. X.215 (ISO 8326, 1987 — 8326/AD 1, 1988 — 8326/AD 3, 1989).
[54]	CCITT Recommendation Session protocol specification for open system interconnection (OSI) for CCITT applications, Vol. VIII, Rec. X.225 (ISO 8327 — 8327/AD 1 — 8327/AD 3, 1988).
[55]	CCITT Recommendation Presentation protocol specification for open system interconnection for CCITT applications, Vol. VIII, Rec. X.226 (ISO 8823, 1988).
[56]	ISO 9545 Open systems interconnection application layer structure, 1989.
[57]	EIA RS 485 Standard for electrical characteristics of generators and receivers for use in balanced digital multipoint systems, 1983.
[58]	CCITT Recommendation Q-interfaces protocol selection process for transmission equipment, Vol. III, Rec. G.771.
[59]	ISO 8880/3 Specification of protocols to provide and support the OSI network service — Part 3: Provision and support of connectionless-mode network service, 1988.
[60]	ISO DTR 10172 Informations processing systems — Data communications network/transport protocol interworking specification.
[61]	CCITT Recommendation Interface between data terminal equipment (DTE) and data circuit-terminating equipment (DCE) for synchronous operation on public data networks, Vol. VIII, Rec. X.21.
[62]	CCITT Recommendation Use on public data networks of data terminal equipment (DTE) which is designed for interfacing to synchronous V-Series modems, Vol. VIII, Rec. X.21bis.
[63]	ISO 4903 Data communication — 15-pin DTE/DCE interface connector and pin assignments, 1980.
[64]	CCITT Recommendation Electrical characteristics for unbalanced double-current interchange circuits for general use with field of data communications, Vol. VIII, Recs. V.10 and X.26.

	
INTERNATIONAL  TELECOMMUNICATION  UNION	

CCITT	G.773
THE  INTERNATIONAL
TELEGRAPH  AND  TELEPHONE
CONSULTATIVE  COMMITTEE

GENERAL  ASPECTS  OF  DIGITAL
TRANSMISSION  SYSTEMS;
TERMINAL  EQUIPMENTS

PROTOCOL  SUITES  FOR  Q-INTERFACES
FOR  MANAGEMENT  OF
TRANSMISSION  SYSTEMS

Recommendation  G.773

	Geneva, 1990	

FOREWORD
	The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the International Telecommunication Union (ITU). CCITT is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
	The Plenary Assembly of CCITT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study Groups. The approval of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in Resolution No. 2 (Melbourne, 1988).
	Recommendation G.773 was prepared by Study Group XV and was approved under the Resolution No. 2 procedure on the 14th of December 1990.

___________________

CCITT  NOTE
	In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication Administration and a recognized private operating agency.

ã  ITU  1990
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