General Description 2-7
Functional Description
Functional Description
ATM
ATM transmits broadband information using fixed length, relatively small, 53-byte cells which are
suitable for carrying both constant rate data (e.g., voice and video) as well as bursty data.
ATM evolved from the Broadband Integrated Services Digital Network (B-ISDN) standard, which
in turn is an extension of ISDN. ISDN defines service and interfaces for public telecommunications
networks. B-ISDN utilizes a 7-layer reference model similar to the Open Systems Interconnection
(OSI) 7-layer architecture. ATM redefines the lower three levels as shown in Figure 2-6. These are
the Physical Layer, the ATM layer, and the ATM Adaptation Layer (AAL).
Figure 2-6 B-ISDN Model
Physical Layer
The physical layer is divided into two parts, the Transmission Convergence sub-layer and the
Physical Medium sub-layer.
The Physical Medium sub-layer (PMD) handles processing specific to a particular physical layer,
such as transmission rate, clock extractions, etc.
The Transmission Convergence sub-layer (TC) extracts the information content from the physical
layer data format. This includes HEC generation and checking, extraction of cells from the data
stream, processing of idle cells, etc.
ATM Layer
The ATM layer processes ATM cells. The ATM cell consists of a 5-byte header and a 48-byte
payload. The header contains the ATM cell address and other management information Figure 2-7.
Higher layer functions
Convergence sublayer
(CS)
Physical
layer
SAR
ATM layer
TC
PM
AALs
(ATM
adaptation
layers)
Management plane
Control plane User plane
Service specific, e.g., FR-SSCS
Common part convergence
sublayer CPCS
Segmentation and reassembly
Cell header insert/extract
Cell multiplexing/demultiplexing
VPI/VCI addressing and translation
Generic flow control
Transmission convergence
Physical medium
H8021
