Data Networks

1.6 Building block 4: Hardware 37

Chapter 1

1.6 Building block 4: Hardware

In this section we briefly explore some of the key hardware devices you will

encounter when designing networks. In order to place these devices in con-

text, it is useful to position them using the ISO OSI seven-layer model, as

shown in Figure 1.10.

Since the mid-1980s, we have seen a gradual shift in the presentation of

networked devices from largely discrete units (such as standalone bridges,

standalone repeaters, etc.) to highly integrated devices with many hybrid

functions (such as multimedia hubs with repeater, bridge, and multiproto-

col router interface cards). This is largely the result of functionality becom-

ing a commodity, a general trend toward increased miniaturization, and the

need to improve functionality to remain competitive. As these tools have

matured and improved in performance there has also been a shift in net-

work design from the use of simple bridged and repeated networks to more

sophisticated router-switch networks capable of optimizing traffic flows

with much greater accuracy and granularity. Scalability, convergence, and

traffic optimization are now key driving forces behind today’s large-scale

network designs. We will now examine some of these devices in more detail.

Figure 1.11 is a somewhat simplified network design illustrating where

you would typically expect to find these devices in a real network today. It

shows discrete devices, although it is common to see much of the function-

ality integrated into a single device. The Head Office site uses a firewall to

prevent unauthorized internal access and offers a Demilitarized Zone

(DMZ) for shared hosts at lower security levels. A gateway is used to con-

vert IBM SNA into TCP/IP protocol for wide area transport. Campus 1 has

a number of LANs, segregated either via repeaters (LAN extension) or

bridges and routers. Line drivers (LD) are used to extend the campus to a

remote office in Building 9. Building 5 is a multistory building with Layer 2

and Layer 3 switches to provide Virtual LAN (VLAN) traffic domains.

Figure 1.10

Network devices in

context.

38 1.6 Building block 4: Hardware

Note that the Layer 3 switch includes wide area support for access to the

meshed WAN.

1.6.1 Media Attachment Units (MAUs)

MAUs, or transceivers, provide the means of encoding data (framed bits)

into purely electrical or light signals ready for transmission onto the physi-

cal media, typically a piece of cable. An MAU is also responsible for decod-

ing electrical or light signals and converting them back into data for

receiving stations. Note that MAU should not be confused with the Token

Ring concentrator Multiple Access Unit. All devices attached to a network

will typically have either a built-in transceiver interface (such as an onboard

10 Base T interface provided on a PC network adapter) or will provide a

standard AUI interface, which can be mated to a discrete transceiver via a

Figure 1.11 Simplified network design illustrating the typical locations for key hardware devices.