Chapter 14: Asynchronous Transfer Mode (ATM) 441
35. OK. I think I have had my fill of LUNI and FUNI to the point where I’m going
loony now. Let’s wrap things up with a few more “clean-up”–type questions.
First, how does frame relay stack up against ATM?
Recall that ATM is similar to frame relay. In fact, ATM is sometime referred to as cell
relay to distinguish it from frame relay. The key difference is that frame relay uses vari-
able-length frames as its main transmission unit, whereas ATM uses fixed-length packets
of exactly 53 bytes known as cells. Although ATM’s use of smaller, fixed-length cells
results in higher overhead than frame relay incurs, it also provides two critical advantages
over frame relay, namely, speed and traffic type. Since all ATM cells are exactly the same
size, they are much easier (and hence, faster) to process. Second, by using short cells with
predictable transmission delay, ATM can combine cells carrying delay-sensitive traffic like
interactive video and voice along with data cells. This concept, called interleaving, isn’t
possible with frame relay because longer data frames create longer and unpredictable
delays when processing voice and video traffic. Thus, frame relay is less suitable for real-
time videoconferencing, for example.
36. Second, please explain the difference between ATM and SONET.
When discussing ATM and SONET, it is important to realize two things. First, SONET
is nothing more than a transport mechanism (see Chapter 7), and second, ATM does not
require the use of any specific physical layer protocol. As a high-bandwidth carrier ser-
LAN Emulation Device
Running LAN Emulation
Client (LEC) Process
Native ATM Station
Running LAN Emulation
Server (LES) Process
End Node
End Node
LAN Emulation Device
Running LAN Emulation
Client (LEC) Process
FIGURE 14.8 A typical ATM local area network emulation (LANE) configuration in an Ethernet/
802.3 environment. Source: Adapted from King, 1994.
442 Networking Explained, Second Edition
vice, SONET can serve as the transport facility for any network technology or service,
including ATM, FDDI, SMDS, and ISDN. SONET also can support various topologies
including point-to-point, star, and ring. Frequently, though, SONET is used to carry ATM
traffic. The two are kind of linked to each other to the point where they can be considered
as “words that come in pairs” (e.g., hue and cry, or table and chair)—ATM and SONET.
There is a reason for this. Instead of developing a new physical layer, the designers of
ATM borrowed SONET’s link-level technology and used it for ATM switching. Further-
more, the ATM Forum has defined 622-Mbps ATM (OC-12) (and higher) to run over only
SONET. This does not mean though, that OC-12 is ATM. As we discussed in Chapter 7,
OC-12 is simply the label given to denote the concatenation of 12 DS-3 channels, which
provides an aggregate bandwidth of 622.08 Mbps. So, in a nutshell, SONET is a carrier
service that transports bits from a source to destination, and ATM is a technology and pro-
tocol that was designed to use SONET as its carrier service.
This concludes our discussion of ATM. The information contained in this chapter provides
only a working overview of the technology. For detailed technical information, you are
encouraged to visit the ATM Forum Web site (, or consult some
of the references given for this chapter. You might also want to revisit other chapters within
this book that included ATM as part of the discussion, most notably: Chapter 7, which
includes a discussion of SONET; Chapter 8, which examines Gigabit Ethernet (1 GbE and
10 GbE) and its relationship to ATM; and Chapters 12 and 13, which provide a comparison
of frame relay, SMDS, and ATM. The Glossary also provides definitions of additional ATM
terms, and the Web site is a terrific resource
for ATM information. Finally, as is the case with many technologies, ATM continues to
evolve, and there is one evolving technology that has caught the eye of WAN operators.
It is CIF, which was mentioned earlier as a technology for transporting ATM protocols
over Ethernet and token ring LANs. The CIF Alliance is actively modifying CIF to work
over SONET and PPP links. This proposed WAN version of CIF presumably maintains
all of native ATM’s key features but requires less overhead. The ATM Forum is working
with the CIF Alliance to bring the concept of using CIF to carry ATM protocols to fruition.

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