INTERNATIONAL  TELECOMMUNICATION  UNION	

CCITT	I.361
THE  INTERNATIONAL
TELEGRAPH  AND  TELEPHONE
CONSULTATIVE  COMMITTEE

INTEGRATED  SERVICES
DIGITAL  NETWORK  (ISDN)
OVERALL  NETWORK  ASPECTS
AND  FUNCTIONS,
ISDN  USER-NETWORK  INTERFACES

B-ISDN  ATM  LAYER
SPECIFICATION

Recommendation  I.361

	Geneva, 1991	

Printed in Switzerland

FOREWORD
	The CCITT (the International Telegraph and Telephone Consultative Committee) is the 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 CCITT Resolution No. 2 (Melbourne, 1988).
	Recommendation I.361 was prepared by Study Group XVIII and was approved under the Resolution No. 2 procedure on the 5 of April 1991.

___________________

CCITT  NOTES
1)	In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication Administration and a recognized private operating agency.
2)	A list of abbreviations used in this Recommendation can be found in Annex A.

ã  ITU  1991
All rights reserved. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU.

	Preamble to B-ISDN Recommendations
	In 1990, CCITT SG XVIII approved a first set of Recommendations on B-ISDN. These are:
I.113 — Vocabulary of terms for broadband aspects of ISDN
I.121 — Broadband aspects of ISDN
I.150 — B-ISDN asynchronous transfer mode functional characteristics
I.211 — B-ISDN service aspects
I.311 — B-ISDN general network aspects
I.321 — B-ISDN Protocol Reference Model and its application
I.327 — B-ISDN functional architecture
I.361 — B-ISDN ATM Layer specification
I.362 — B-ISDN ATM Adaptation Layer (AAL) functional description
I.363 — B-ISDN ATM Adaptation Layer (AAL) specification
I.413 — B-ISDN user-network interface
I.432 — B-ISDN user-network interface — Physical Layer specification
I.610 — Operation and maintenance principles of B-ISDN access
	These Recommendations address general B-ISDN aspects as well as specific service- and network-oriented issues, the fundamental characteristics of the asynchronous transfer mode (ATM), a first set of relevant ATM oriented parameters and their application at the user-network interface as well as impact on operation and maintenance of the B-ISDN access. They are an integral part of the well established I-Series Recommendations. The set of Recommendations are intended to serve as a consolidated basis for ongoing work relative to B-ISDN both within CCITT and in other organizations. They may also be used as a first basis towards the development of network elements.
	CCITT will continue to further develop and complete these Recommendations in areas where there are unresolved issues and develop additional Recommendations on B-ISDN in the I-Series and other series in the future.

PAGE BLANCHE

Recommendation I.361
Recommendation I.361
B-ISDN  ATM  LAYER  SPECIFICATION
1	Introduction
	This Recommendation specifically addresses:
a)	the cell structure and the ATM cell coding;
b)	the ATM protocol procedures.
2	Cell structure coding
	Two different coding schemes are adopted according to the interface being considered, i.e. the user-network interface (UNI) or the network-node interface (NNI). They are described in §§ 2.2 and 2.3.
2.1	Cell structure
	The cell consists of a five octet header and a 48-octet information field as shown in Figure 1/I.361.
FIGURE 1/I.361 = 6,5 cm

	Note — The header will be sent first followed by the information field.
	When a field within the header is contained within a single octet, the lowest bit number of the field represents the lowest order value.
	When a field spans more than one octet, the order of bit values within each octet progressively decreases as the octet number increases; the lowest bit number associated with the field represents the lowest order value.
	This leads to the following conventions:
—	bits within an octet are sent in decreasing order, starting with bit 8;
—	octets are sent in increasing order, starting with octet 1;
—	for all fields, the first bit sent is the most significant bit (MSB).
2.2	Cell header format and encoding at UNI
	The structure of the header is shown in Figure 2/I.361. The fields contained in the header and their encoding are described in the following sections.

8	7	6	5	4	3	2	1	Bit
Octet	
GFC				VPI				1	
VPI				VCI				2	
VCI								3	
VCI				PT		RES	CLP	4	
HEC								5	
CLP	Cell loss priority
GFC	Generic flow control
PT		Payload type
RES	Reserved
HEC	Header error control
VPI	Virtual path identifier
VCI	Virtual channel identifier
FIGURE 2/I.361
Header structure at UNI

2.2.1	Pre-assigned values of the cell header
	Pre-assigned values of the cell header (to differentiate cells for the use of the ATM Layer from cells for the use of the Physical Layer) are given in Table 1/I.361. All other values are for the use of the ATM Layer.
2.2.2	Generic flow control (GFC) field
	The GFC field contains 4 bits. When the GFC function is not used, the value of this field is 0000. When the GFC mechanism is standardized, all values of this field are available for coding. This coding is for further study.
2.2.3	Routing field (VPI/VCI)
	24 bits are available for routing: 8 bits for virtual path identifier (VPI) and 16 bits for virtual channel identifier (VCI). Pre-assigned combinations of VPI and VCI values are shown in Table 2/I.361. Other pre-assigned values of VPI and VCI are for further study. The VCI value of zero is not available for user virtual channel identification.
TABLE 1/I.361
Pre-assigned cell header values at the UNI
(excluding the HEC field)
	Octet 1	Octet 2	Octet 3	Octet 4	
Reserved for use of the physical layer (Notes 1, 2 and 3)	PPPP0000	00000000	00000000	0000PPP1	
Unassigned cell identification (Note 1)	AAAA0000	00000000	00000000	0000AAA0	
A	Indicates the bit is available for use by the ATM Layer.
P	Indicates the bit is available for use by the Physical Layer.
		Values assigned to these bits have no meaning with respect to the fields occupying the corresponding bit positions at the ATM Layer.
Note 1 — In the case of Physical Layer cells or ATM Layer unassigned cells, the bit in the location of the cell loss priority (CLP) indication is not used for the CLP mechanism as specified in § 3.4.2.3.2 of Recommendation I.150.
Note 2 — Cells having header values which are identified as reserved for use of the Physical Layer are not passed to the ATM Layer from the Physical Layer.
Note 3 — Specific pre-assigned Physical Layer cell header values are given in Recommenda‑
tion I.432.

TABLE 2/I.361
Combinations of pre-assigned VPI and VCI values at the UNI
	VPI	VCI	
Meta-signalling virtual channel identification (Note)	00000000	00000000  00000001	
General broadcast signalling virtual channel identification (Note)	00000000	00000000  00000010	
Note — See Recommendation I.311 for use of other VPI/VCI combinations for meta-signalling.

	The number of bits of the VPI and VCI fields used for routing are established by negotiation between the user and the network as described in § 3.1.2.3 of Recommendation I.150. The bits within the VPI and VCI fields used for routing are allocated using the following rules:
—	the allocated bits of the VPI field will be contiguous;
—	the allocated bits of the VPI field will be the least significant bits of the VPI field (beginning at bit 5 of octet 2);
—	the allocated bits of the VCI field will be contiguous;
—	the allocated bits of the VCI field will be the least significant bits of the VCI field (beginning at bit 5 of octet 4).
	In addition, unallocated bits, i.e. bits not utilized by the user or the network, within the 24-bit routing field will be set to zero.

2.2.4	Payload type (PT) field
	2 bits are available for PT identification. The default value for user information is 00. Use of other PT values for user information and for network information is for further study.
2.2.5	Cell loss priority (CLP) field
	If the CLP is set (CLP value is 1), the cell is subject to discard, depending on network conditions. If the CLP is not set (CLP value is 0), the cell has higher priority (see § 3.4.2.3 of Recommendation I.150).
2.2.6	Header error control (HEC) field 
	The HEC field consists of 8 bits. Use of this field is described in § 4.3 of Recommendation I.432.
2.2.7	Reserved (RES) field
	The reserved field (1 bit) is for further enhancement of existing cell header functions or for standardized functions not yet specified. The default value of this field is zero.
2.3	Cell header format and encoding at NNI 
	The structure of the header is shown in Figure 3/I.361. The fields contained in the header and their encoding are described in the following sections.

8	7	6	5	4	3	2	1	Bit
Octet	
VPI								1	
VPI				VCI				2	
VCI								3	
VCI				PT		RES	CLP	4	
HEC								5	
FIGURE 3/I.361
Header structure at NNI

2.3.1	Pre-assigned values of the cell header
	Pre-assigned values of the cell header (to differentiate cells for the use of the ATM Layer from cells for the use of the Physical Layer) are given in Table 3/I.361. All other values are for use of the ATM Layer.
TABLE 3/I.361
Pre-assigned cell header values at the NNI
(excluding the HEC field)
	Octet 1	Octet 2	Octet 3	Octet 4	
Idle cell identification 
(Notes 1 and 2)	00000000	00000000	00000000	00000001	
Physical Layer OAM cell identification (Note 2)	00000000	00000000	00000000	00001001	
Reserved for use of the Physical Layer (Notes 1 and 2)	00000000	00000000	00000000	0000PPP1	
Unassigned cell identification (Note 1)	00000000	00000000	00000000	0000AAA0	
A	Indicates the bit is available for use by the ATM Layer.
P	Indicates the bit is available for use by the Physical Layer.
	Values assigned to these bits have no meaning with respect to the fields occupying the corresponding bit positions at the ATM Layer.
Note 1 — In the case of Physical Layer cells or ATM Layer unassigned cells, the bit in the location of the CLP indication is not used for the CLP mechanism as specified in § 3.4.2.3.2 of Recommendation I.150.
Note 2 — Cells having header values which are identified as idle, Physical Layer OAM, and reserved for use by the Physical Layer, are not passed to the ATM Layer from the Physical Layer.

2.3.2	Routing field (VPI/VCI)
	Twenty-eight bits are available for routing: 12 bits for VPI and 16 bits for VCI. Pre-assigned values of VPI and VCI are for further study. The VCI value of zero is not available for user virtual channel identification.
2.3.3	Payload type (PT) field
	Two bits are available for PT identification. The default value for user information is 00. Use of other PT values for user information and for network information is for further study.
2.3.4	Cell loss priority (CLP) field
	If the CLP is set (CLP value is 1), the cell is subject to discard, depending on network conditions. If the CLP is not set (CLP value is 0), the cell has higher priority (see § 3.4.2.3 of Recommendation I.150). 

2.3.5	Header error control (HEC) field
	The HEC field consists of 8 bits. The HEC mechanism of the NNI is identical to that at the UNI and is described in § 4.3 of Recommendation I.432.
2.3.6	Reserved (RES) field
	The reserved field (1 bit) is for further enhancement of existing cell header functions or for standardized functions not yet specified. The default value of this field is zero.
2.4	Cell information field
2.4.1	Pre-assigned values 
	The pre-assigned values of the information field of all unassigned cells are for further study.
3	ATM protocol procedures
	This section will contain procedures that describe the operation of the ATM protocol (including the peer-to-peer and inter-layer information flows).
3.1	GFC protocol
	For further study.
ANNEX  A
(to Recommendation I.361)
Alpahbetical list of abbreviations used
in this Recommendation
	ATM		Asynchronous transfer mode
	CLP			Cell loss priority
	GFC			Generic flow control
	HEC		Header error control
	MSB		Most significant bit
	NNI			Network-node interface
	OAM		Operation and maintenance
	PT			Payload type
	RES			Reserved
	UNI			User-network interface
	VCI			Virtual channel identifier
	VPI			Virtual path identifier