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2.2.1 Pre-assigned values of the cell header
2.3.2 Routing field (VPI/VCI)
_______________
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 Inter-
national 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 telecommunica-
tion Administration and a recognized private operating agency.
2) A list of abbreviations used in this Recommendation can be found in Annex A.
aITU1991
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 û Vocabulary of terms for broadband aspects of ISDN
I.121 û Broadband aspects of ISDN
I.150 û B-ISDN asynchronous transfer mode functional characteristics
I.211 û B-ISDN service aspects
I.311 û B-ISDN general network aspects
I.321 û B-ISDN Protocol Reference Model and its application
I.327 û B-ISDN functional architecture
I.361 û B-ISDN ATM Layer specification
I.362 û B-ISDN ATM Adaptation Layer (AAL) functional description
I.363 û B-ISDN ATM Adaptation Layer (AAL) specification
I.413 û B-ISDN user-network interface
I.432 û B-ISDN user-network interface û Physical Layer specification
I.610 û Operation and maintenance principles of B-ISDN access
These Recommendations address general B-ISDN aspects as well as specific service- and network-ori-
ented 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 Recommen-
dations. 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 devel-
opment 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-net-
work 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 û 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:
û bits within an octet are sent in decreasing order, starting with bit 8;
û octets are sent in increasing order, starting with octet 1;
û 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.
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.
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:
û the allocated bits of the VPI field will be contiguous;
û the allocated bits of the VPI field will be the least significant bits of the VPI field (beginning at bit 5 of
octet 2);
û the allocated bits of the VCI field will be contiguous;
û 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 val-
ues 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.
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.
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