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INTERNATIONAL TELECOMMUNICATION UNION
ITU-T RECOMMENDATION SUMMARY
Rec. No. : I.150
Title : B-ISDN asynchronous transfer mode functional characteristics
Study Group : XVIII - ISDN
Version : New
Date of adoption : 1991
Notes :
Recommendation I.150 addresses specifically the functions of the ATM
Layer. This Layer is common to all services including signalling and
OAM.
ATM is the transfer mode solution for implementing a B-ISDN. It
influences the standardization of digital hierarchies, multiplexing
structures, switching and interfaces for broadband signals.
ATM is used in Recommendation I.150 for addressing a specific
packet-oriented transfer mode which uses asynchronous time division
multiplexing techniques. The multiplexed information flow is organized
into blocks of a fixed size called cells. A cell consists of an
information field and a header. The primary role of the header is to
identify cells belonging to the same virtual channel within the
asynchronous time division multiplex. Transfer capacity is assigned
by negotiation and is based on the source requirements and the
available capacity. Cell sequence integrity on a virtual channel
connection is preserved by the ATM Layer.
ATM is a connection-oriented technique. Connection identifiers are
assigned to each link of a connection when required and released when
no longer needed. In general, signalling and user information are
carried on separate ATM Layer connections.
ATM offers a flexible transfer capability common to all services,
including connectionless services. Additional functionalities on top
of the ATM Layer (e.g. in the ATM Adaptation Layer (AAL)) are provided
to accommodate various services. The boundary between the ATM Layer
and the AAL corresponds to the boundary between functions supported by
the contents of the cell header and functions supported by
AAL-specific information. The AAL-specific information is contained in
the information field of the ATM cell.
The information field is transported transparently by the ATM Layer.
No processing, e.g. error control, is performed on the information
field at the ATM Layer.
The header and information field each consists of a fixed integer
number of octets. The header size (5 octets) and the information field
size (48 octets) remain constant at all reference points, including
the User-network interface (UNI) and the Network-node interface (NNI),
where the ATM technique is applied.
To order the complete text of this Recommendation, please use the
Order Form for ITU-T Recommendations. An electronic version of this
form is available on ITUDOC (Winword 2.0: UPI=ITU-5265; ASCII: UPI=ITU-2488).