Networking Explained, 2nd Edition

Chapter 6: Network Hardware Components (Layers 1 and 2) 219

12. OK. I now understand AUI, but what do AAUI and MAU mean?

AAUI stands for Apple Attachment Unit Interface, which was Apple’s proprietary AUI.

Older model Macintosh computers like the Centris 650 and Power Macintosh 7100 have

built-in AAUI ports, as evidenced by yet another variety of subminiature 15-pin connector

on their rear panels. To connect these machines to an Ethernet/802.3 LAN, you must first

purchase an AAUI-to-AUI adapter to change the proprietary interface to AUI, BNC, or RJ-

45. Why Apple chose to do this is anyone’s guess. Later model Macintoshes have built-in

standard connectors—mainly RJ45. As for MAU, this stands for Media Attachment Unit; it

is another term for a transceiver.

13. Enough about transceivers. Let’s talk about repeaters.

OK. Repeaters, like transceivers, provide both physical and electrical connections.

Their function is to regenerate and propagate a signal.

14. Give me a little more detail. Where are they used? Why are they used?

Repeaters, which are also called concentrators, are used in Ethernet/802.3 LANs to

extend the length of the LAN. You see, depending on the type of medium, the length of an

Ethernet/802.3 LAN segment has specific length restrictions. For example, a 10 Mbps

Ethernet/802.3 LAN that uses UTP cable (10BASE-T), has a maximum length restriction of

100 meters, and a ThinWire (coax) Ethernet/802.3 segment (10BASE2) cannot exceed 185

meters. The reason for these restrictions is signal quality. As a segment exceeds its maxi-

mum length, the signal quality begins to deteriorate. (Recall the concept of attenuation from

Chapter 4.) In many instances, these length restrictions are not always practical, so network

managers have to extend their LANs by interconnecting individual segments. A repeater

makes this possible. It receives signals from one cable segment, regenerates, retimes, and

amplifies them, and then transmits these “revitalized” signals to another cable segment. We

discuss the application of repeaters in Ethernet/802.3 LANs in more detail in Chapter 8.

15. Do repeaters introduce delay in a network?

Yes. Repeaters are a source of propagation delay in a network. Propagation delay is

the amount of time a signal spends getting from one point in a circuit to another. It is

affected by the speed and efficiency of the components between the two points.

16. Are repeaters also called hubs?

It depends on who you talk to. Some people think of a hub as a repeater, whereas oth-

ers view a hub generically as any device that connects two or more network segments. A

hub can also be a device that supports several different media (see Figure 6.8).

17. What about a stackable hub? What’s that?

Stackable repeater hubs consist of individual repeater units “stacked” one on top of

another. Instead of a common shared backplane that is part of a chassis-based repeater,

stackable hubs use a “pseudo-backplane” based on a common connector interface. An

external cable interconnects the individual hubs in a daisy-chained manner. Once intercon-

220 Networking Explained, Second Edition

nected, the entire chain of hubs becomes a single logical unit that functions as a single

repeater. Stackable hubs are less expensive than chassis-based devices. An illustration of a

stackable hub is shown in Figure 6.9.

18. So what you are saying then is that when I hear the term “Ethernet hub” I

shouldn’t assume it is a repeater.

Correct. You need qualification. At one point, several years ago, an Ethernet hub was a

repeater. Today, however, switches have replaced repeaters in most Ethernet/802.3 LANs

Power Supply

Module

8-port AUI

12-port

Fiber

Module

Module

5-port UTP

Module

FIGURE 6.8 This multislot chassis-based repeater hub (also called a concentrator) can accommo-

date several different media types, including UTP, coax, and fiber. Since each interface module shares

the same backplane as the repeater module, the Ethernet ports on each module use the same repeater.

Thus, repeater hubs are capable of supporting many Ethernet connections using only a single repeat-

er. For example, if one of the modules is a 12-port ThinWire board, then this one board can support

12 separate 185-meter ThinWire segments. Since 30 devices can be connected to one ThinWire seg-

ment, this board can support 12 × 30 = 360 nodes.

IGURE 6.9 Stackable hubs are daisy-chained together using an external cable, which enables them

o function as a single hub.