Networking Explained, 2nd Edition

346 Networking Explained, Second Edition

10. What are some of the advantages and disadvantages of token ring networks?

Some of token ring’s advantages include its ability to run on many different media types,

its efficient use of bandwidth (e.g., if packet size is 1,000 bits, efficiency is 99 percent), its

stable behavior during high load times, its deterministic nature (it has a known upper bound

on channel access), and its priority scheme that enables lobes with high-priority data to

reserve the network for data transmission. Disadvantages include the need for special recov-

ery procedures when the network fails, difficulty in configuring new hosts to an established

LAN, and in the case of priority scheduling, the susceptibility of low priority lobes to

increased delays in accessing the network.

11. I realize that token ring is not as popular as Ethernet. However, is there a “second

generation” token ring as there is with Ethernet?

Yes. Token ring has benefited from some of the technological advances made for

Ethernet/802.3. These include switched token ring, dedicated token ring, full-duplex token

ring, and 100-Mbps token ring. A gigabit token ring specification is also in development.

12. Let’s start with switched token ring. I can understand how switches benefit tradi-

tional Ethernet/802.3 LANs. But how do token ring LANs benefit from switching

technology? They don’t have collisions.

You are right. Ethernet/802.3’s MAC layer protocol (CSMA/CD) is contention-based

and highly susceptible to performance problems during periods of high activity. Token

ring’s token-passing protocol, however, is inherently deterministic and quite stable during

peak load times. Token ring networks do not experience the same type of performance

problems as those found in Ethernet/802.3 networks. A token ring network’s performance,

for example, does not begin to degrade until its utilization rate approaches 90 percent, and

in many cases can achieve a bandwidth utilization rate as high as 95 percent. So, yes,

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FIGURE 9.6 A token bus network physically resembles a bus topology; logically, however, it is a

ring. A token is transmitted from lobe to lobe using network addresses and occurs in descending or-

der. The lobe that possesses the token is permitted to transmit data. The lobe order in the figure is 70-

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