IBM SG24-4817-00 manual Introduction to ATM Networks, ATM Fundamentals, ATM Cells

This soft copy for use by IBM employees only.

Chapter 1. Introduction to ATM Networks

This book is designed to help you to get the most effective use of the IBM 8285 Nways ATM Workgroup Switch as you implement an ATM network. Before going into further details about the 8285, however, it might be useful to review the basics of ATM networking, addressing, and data flows.

1.1 ATM Fundamentals

Asynchronous Transfer Mode (ATM) is a high-performance network technology that is rapidly becoming the standard for high-speed LAN and WAN networks, both public and private. It combines the flexibility and resiliency of connection-less protocols, such as TCP/IP, with the efficiency and manageability of session-oriented protocols, such as SNA. This is because ATM uses small, fixed-size packets called cells which are transported across the network

hop-by-hop along a pre-determined virtual path that can be quickly changed to avoid congestion or failures. Both of these concepts are discussed below.

1.1.1 ATM Cells

ATM uses the concept of cells as its basic delivery vehicle. These cells are similar to the packets (or frames) used in traditional networks, except for two distinguishing features:

1.Fixed Cell Size

All ATM cells are 53-bytes long, of which 48 bytes are payload, and 5 bytes are header information. This payload-size provides the best combination of efficiency (favoring large payloads for data) and latency (favoring small payloads for time-sensitive applications such as voice and video).

The header contains all the information necessary for the cell to enter the network, to be carried to its next (intermediate) destination, and to identify simple errors (single-bit) that might occur.

The most important thing about the fixed cell size, however, is that it enables cells to be switched simply and efficiently, in hardware, without costly (in time and money) large buffers.

2.Minimal Routing Information

ATM cells are connection-oriented, which means that they are not responsible for identifying a destination or determining the best route. In fact, the only routing information necessary is the current hop information (which the next switch uses in its forwarding decision). And, since all cells for a given session follow the same path, no provision is necessary for out-of-sequence arrival. Thus, unlike traditional LAN packets, sequencing numbers are not required, and addressing at the MAC and network layers is eliminated (for native ATM applications). The result is more data, less overhead, and simpler hardware-based switching

 Copyright IBM Corp. 1996 1