Fundamentos Atm

C H A P TER

1

ATM Technology Fundamentals
This chapter provides a brief overview of ATM technology. It covers basic principles of ATM, along with the common terminology, and introduces key concepts you need to be familiar with when configuring ATM network equipment. If you already possess this basic knowledge, you can skip this chapter and go on to Chapter 2, “ATM Signaling and Addressing.”Note This chapter provides only generic ATM information. Subsequent chapters in this guide

include implementation-specific information for the Catalyst 8540 MSR, Catalyst 8510 MSR, and LightStream 1010 ATM switch router.

This chapter includes the following sections:

• • • •

What is ATM? on page 1-1 ATM Basics on page 1-2 Traffic Contracts and Service Categories on page 1-12 CommonPhysical Interface Types on page 1-14

What is ATM?
Asynchronous Transfer Mode (ATM) is a technology designed for the high-speed transfer of voice, video, and data through public and private networks using cell relay technology. ATM is an International Telecommunication Union Telecommunication Standardization Sector (ITU-T) standard. Ongoing work on ATM standards is being done primarily by the ATMForum, which was jointly founded by Cisco Systems, NET/ADAPTIVE, Northern Telecom, and Sprint in 1991. A cell switching and multiplexing technology, ATM combines the benefits of circuit switching (constant transmission delay, guaranteed capacity) with those of packet switching (flexibility, efficiency for intermittent traffic). To achieve these benefits, ATM uses the following features:

• • •Fixed-size cells, permitting more efficient switching in hardware than is possible with variable-length packets Connection-oriented service, permitting routing of cells through the ATM network over virtual connections using simple connection identifiers Asynchronous multiplexing, permitting efficient use of bandwidth and interleaving of data of varying priority and size

ATM Technology Fundamentals1-1

ATM Basics

The combination of these features allows ATM to provide different categories of service for different data requirements and to establish a service contract at the time a connection is set up. This means that a virtual connection of a given service category can be guaranteed a certain bandwidth, as well as other traffic parameters, for the life of the connection.

ATM BasicsTo understand how ATM can be used, it is important to have a knowledge of how ATM packages and transfers information. The following sections provide brief descriptions of the format of ATM information transfer and the mechanisms on which ATM networking is based.

ATM Cell Basic Format
The basic unit of information used by ATM is a fixed-size cell consisting of 53 octets, or bytes. The first 5bytes contain header information, such as the connection identifier, while the remaining 48 bytes contain the data, or payload (see Figure 1-1). Because the ATM switch does not have to detect the size of a unit of data, switching can be performed efficiently. The small size of the cell also makes it well suited for the transfer of real-time data, such as voice and video. Such traffic is intolerant ofdelays resulting from having to wait for large data packets to be loaded and forwarded.
Figure 1-1 ATM Cell Basic Format 48 bytes
18332

5 bytes Header

Payload

ATM Device Types
An ATM network is made up of one or more ATM switches and ATM endpoints. An ATM endpoint (or end system) contains an ATM network interface adapter. Workstations, routers, data service units (DSUs), LAN switches, andvideo coder-decoders (CODECs) are examples of ATM end systems that can have an ATM interface. Figure 1-2 illustrates several types of ATM end systems—router, LAN switch, workstation, and DSU/CSU, all with ATM network interfaces—connected to an ATM switch through an ATM network to another ATM switch on the other side.
Note In this document the term ATM switch is used to refer generically to the...