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.



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