Atm

Running head: ATM

The Many Faces of Asynchronous Transfer Mode

Table of Contents

Introduction 3

ATM Objectives 3

Basic Concepts in ATM 4

ATM Standards 7

ATM Switching 11

Switching Networks 12

Performance Issues 12

ATM Applications 14

Consumer Applications 14

Commercial Applications 16

Summary 17

References 19

The Many Faces of Asynchronous Transfer Mode

Introduction

Asynchronous Transfer Mode (ATM) has been accepted universally as the transfer mode of choice for Broadband Integrated Services Digital Networks (BISDN). It can handle any kind of information such as voice, data, image, text and video in an integrated manner. ATM provides a good bandwidth flexibility and can be used efficiently from desktop computers to local area, metropolitan are network and wide area networks (LAN, MAN and WAN). It is a connection- oriented packet switching technique in which all packets are of fixed length.

Although ATM is a technology, most users experience ATM through the use of both ATM equipment and services. This paper will attempt to explain a few of the basic principles and concept of Asynchronous Transfer Mode through objectives, basic concepts, standards, switching and application.

ATM Objectives

One must note that ATM meets the following objectives for BISDN networks. They support all existing services as well as emerging services in the future, utilize network resources very efficiently, minimizes the switching complexity, minimizes the processing time at the intermediate nodes and supports very high transmission speeds, minimizes the number of buffers required at the intermediate nodes to bound the delay and the complexity of buffer management, and guarantees performance requirements of existing and emerging applications.

Basic Concepts in ATM

Before continuing we must first review some basic concepts in ATM design. These terms were copied in their entirely to maintain definition integrity from McGraw-Hill, ATM Theory and Application, 1999.

Information Transfer - ATM is a fast packet oriented transfer mode based on asynchronous time division multiplexing and it uses fixed length (53 bytes) cells. Each ATM cell consists of 48 bytes for information field and 5 bytes for header. The header is used to identify cells belonging to the same virtual channel and thus used in appropriate routing. Cell sequence integrity is preserved per virtual channel. ATM Adaptation layers (AAL) are used to support various services and provide service specific functions. This AAL specific information is contained in the information field of the ATM cell. Basic ATM cell structure can be shown as follows.

Routing -ATM is a connection oriented mode. The header values such as VCI and VPI are assigned during the connection set up phase and translated when switched from one section to other. Signaling information is carried on a separate virtual channel than the user information. In routing, there are two types of connections i.e. Virtual channel connection (VCC) and Virtual path connection (VPC). A VPC is an aggregate of VCCs. Switching on cells is first done on the VPC and then on the VCC.

ATM Resources - ATM is connection-oriented and the establishment of the connections includes the allocation of a VCI i.e. virtual channel identifier and/or VPI i.e. virtual path identifier and also includes the allocation of the required resources on the user access and inside the network. These resources, expressed in terms of throughput and quality of service, can be negotiated between user and network either before the call-set up or during the call.

ATM Cell Identifiers - ATM cell identifiers, i.e. Virtual Path Identifier, Virtual Channel Identifier and Payload Type Identifier (PTI) are used to recognize an ATM cell on a physical transmission medium. VPI and VCI are same for cells belonging to the same virtual connection on a shared transmission medium.

Throughput - Peak Cell Rate (PCR) can be defined as a Throughput parameter which in turn is defined as the inverse of the minimum inter-arrival time T between two consecutive basic events and T is the peak emission interval of the ATM connection. PCR applies to both constant bit rate (CBR) and variable bit rate (VBR) services for ATM connections. It is an upper bound of the cell rate of an ATM connection and there is another parameter sustainable cell rate (SCR) allows the ATM network to allocate resources more efficiently.

Quality Of Service - Quality of Service (QOS) parameters include cell loss, the delay and the delay variation incurred by the cells belonging to the connection in an ATM network. QOS parameters can be either specified explicitly by the user or implicitly associated with specific service requests. A limited number of specific QOS classes will be standardized in practice.

Usage Parameter Control - In ATM, excessive reservation of resources by one user affects traffic for other users. So the throughput must be policed at the user-network interface by a Usage Parameter Control function in the network to ensure that the negotiated connection parameters per VCC or VPC between network and subscriber is maintained by each other user. Traffic parameters describe the desired throughput and QOS in the contract. The traffic parameters are to be monitored in real time at the arrival of each cell. CCITT recommends a check of the peak cell rate (PCR) of the high priority cell flow (CLP = 0) and a check of the aggregate cell flow (CLP = 0+1), per virtual connection.

Flow Control - order to control the flow of traffic on ATM connections from a terminal to the network, a Generic Flow Control (GFC) mechanism is proposed by CCITT at the User to Network Interface (UNI). This function is supported by GFC field in the ATM cell header. Two sets of procedures are associated with the GFC field i.e. Uncontrolled Transmission which is for use in point-to-point configurations and Controlled Transmission which can be used in both point-to-point