Cisco Systems 8600 Series manual Functional Description, Physical Layer, ATM Layer, Isdn Model

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

Managementplane

Controlplane Userplane

Higher layer functions

AALs	Convergence sublayer
(ATM	(CS)
adaptation
	SAR
layers)
	ATM layer
Physical	TC
layer	PM

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

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.

General Description 2-7