Asynchronous Transfer Mode (ATM) Networks

Thus far, our focus has been on the Internet and its protocols. But many other existing packet-switching technologies can also provide end-to-end networking solutions. Among these alternatives to the Internet, so called Asynchronous Transfer Mode (ATM) networks are perhaps the most well-known. ATM arrived on the scene in the early 1990s. It is useful to discuss ATM for two reasons. First, it provides an interesting contrast to the Internet, and by exploring its differences, we will gain more insight into the Internet. Second, ATM is often used as a link-layer technology in the backbone of the Internet. Since we will refer to ATM throughout this book, we end this chapter with a brief overview of ATM.

The Original Goals of ATM

The standards for ATM were first developed in the mid 1980s. For those too young to remember, at this time there were predominately two types of networks: telephone networks, that were (and still are) primarily used to carry real-time voice; and data networks, that were primarily used to transfer text files, support remote login, and provide email. There were also dedicated private networks available for video conferencing. The Internet existed at this time, but few people were thinking about using it to transport phone calls, and the WWW was as yet unheard of. It was therefore natural to design a networking technology that would be appropriate for transporting real-time audio and video as well as text, email and image files.

ATM achieved this goal. Two standards bodies, the ATM Forum [ATM Forum] and the International Telecommunications Union [ITU] have developed ATM standards for Broadband Integrated Services Digital Networks (BISDNs). The ATM standards call for packet switching with virtual circuits (called virtual channels in ATM jargon); the standards define how applications directly interface with ATM, so that ATM provides complete networking solution for distributed applications. Paralleling the development of the ATM standards, major companies throughout the world made significant investments in ATM research and development. These investments have led to a myriad of high-performing ATM technologies, including ATM switches that can switch terabits per second. In recent years, ATM technology has been deployed very aggressively within both telephone networks and the Internet backbones.

Although ATM has been deployed within networks, it has been unsuccessful in extending itself all the way to desktop PCs and workstations. And it is now questionable whether ATMwill ever have a significant presence at the desktop. Indeed, while ATM was brewing in the standards committees and research labs in the late 1980s and early 1990s, the Internet and its TCP/IP protocols were already operational and making significant headway:

• The TCP/IP protocol suite was integrated into all of the most popular operating systems.
• Companies began to transact commerce (e-commerce) over the Internet.
• Residential Internet access became very cheap.
• Many wonderful desktop applications were developed for TCP/IP networks, including the World Wide Web, Internet phone, and interactive streaming video. Thousands of companies are currently developing new applications and services for the Internet.

Furthermore, throughout the 1990s, several low-cost high-speed LAN technologies were developed, including 100 Mbps Ethernet and more recently Gigabit Ethernet, mitigating the need for ATM use in high-speed LAN applications. Today, we live in a world where almost all networking application products interface directly with TCP/IP. Nevertheless, ATMswitches can switch packets at very high rates, and consequently has been deployed in Internet backbone networks, where the need to transport traffic at high rates is most acute. We will discuss the topic of IP over ATM in Section 5.8.

Principle Characteristics of ATM

We shall discuss ATM in some detail in subsequent chapters. For now we briefly outline its principle characteristics:

• The ATM standard defines a full suite of communication protocols, from the transport layer all the way down through the physical layer.
• It uses packet switching with fixed length packets of 53 bytes. In ATM jargon these packets are called cells. Each cell has 5 bytes of header and 48 bytes of "payload". The fixed length cells and simple headers have facilitated high-speed switching.
• ATM uses virtual circuits (VCs). In ATM jargon, virtual circuits are called virtual channels. The ATM header includes a field for the virtual channel number, which is called the virtual channel identifier (VCI) in ATM jargon. As discussed in Section 1.3, packet switches use the VCI to route cells towards their destinations; ATM switches also perform VCItranslation.
• ATM provides no retransmissions on a link-by-link basis. If a switch detects an error in an ATM cell, it attempts to correct the error using error correcting codes. If it cannot correct the error, it drops the cell and does not ask the preceding switch to retransmit the cell.
• ATM provides congestion control on an end-to-end basis. That is, the transmission of ATM cells is not directly regulated by the switches in times of congestion. However, the network switches themselves do provide feedback to a sending end system to help it regulate its transmission rate when the network becomes congested.
• ATM can run over just about any physical layer. It often runs over fiber optics using the SONET standard at speeds of 155.52 Mbps, 622 Mbps and higher.

Overview of the ATM Layers

As shown in Figure 1.10-1, the ATM protocol stack consists of three layers: the ATM adaptation layer (AAL), the ATM Layer, and the ATM Physical Layer:

The ATM Physical Layer deals with voltages, bit timings, and framing on the physical medium. The ATM Layer is the core of the ATM standard. It defines the structure of the ATM cell. The ATM Adaptation Layer is analogous to the transport layer in the Internet protocol stack. ATM includes many different types of AALs to support many different types of services.

Currently, ATM is often used as a link-layer technology within localized regions of the Internet. A special AAL type, AAL5, has been developed to allow TCP/IP to interface with ATM. At the IP-to-ATM interface, AAL5 prepares IP datagrams for ATM transport; at the ATM-to-IP interface, AAL5 reassembles ATM cells into IP datagrams. Figure 1.10-2 shows the protocol stack for the regions of the Internet that use ATM.

Note that in this configuration, the three ATM layers have been squeezed into the lower two layers of the Internet protocol stack. In particular, the Internet's network layer "sees" ATM as a link-layer protocol.

This concludes our brief introduction to ATM. We will return to ATM from time to time throughout this book.