Networking Explained, 2nd Edition

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

Ethernet/802.3

End Node

Ethernet/802.3

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.

END-OF-CHAPTER COMMENTARY

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 (http://www.atmforum.com), 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 http://www.techfest.com/networking/atm.htm 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.