1.10 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 ATM will 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,
ATM switches 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 VCI translation.
-
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:
ATM Adaptation Layer (AAL)
|
ATM Layer
|
ATM Physical Layer
|
Figure 1.10-1: The three ATM layers.
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.
Application Layer (HTTP, FTP, etc.)
|
Transport Layer (TCP or UDP)
|
Network Layer (IP)
|
AAL5
|
ATM Layer
|
ATM Physical Layer
|
Figure 1.10-2: Internet-over-ATM protocol
stack.
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.
References
[ATM Forum] The ATM Forum Web site, http://www.atmforum.com
[ITU] The ITU Web site, http://www.itu.ch
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Copyright Keith W. Ross and Jim
Kurose 1996-2000