FitzGerald J., Dennis A., Durcikova A. Business Data Communications and Networking

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HANDS-ON ACTIVITY 6C 235

(Orcelia), my router (192.168.1.1), a wireless access point

(Aironet) with two connections (into the LAN and out to

the wireless LAN), and the Indiana University VPN server

(because I had my VPN connected; Chapter 11 discusses

VPNs). You can see that all of these devices have little

utilization (under 1 percent), as well as the total number

of packets these devices have sent and received. You can

click through the other tabs in this area to see the packet

distribution.

The panel on the left of the screen shows additional

information about the types of packets, errors, and packet

sizes.

Deliverables

1. How many packets your computer can send and

receive?

2. What is the total data rate on your network?

3. What is your network utilization?

HANDS-ON ACTIVITY 6C

War-Driving and War-Walking

Wireless LANS are often not secure. It is simple to bring

your laptop computer into a public area and listen for

wireless networks. This is called War-Driving (if you

are in a car) or War-Walking (if you’re walking). As

long as you do not attempt to use any networks with-

out authorization, War-Driving and War-Walking are quite

legal. There are many good software tools available for

War-Driving. My favorites are Net Stumbler (available

from www.netstumbler.com) or Wireless NetView (avail-

able from www.nirsoft.com). Both are simple to use, yet

FIGURE 6.14 NetStumbler shows the networks in one downtown building

powerful. Net Stumbler historically has been the most

commonly used tool, but some users have reported prob-

lems using it on Windows 7.

The ﬁrst step is to download and install the Net Stum-

bler software on a laptop computer that has wireless capa-

bility. The software is available at www.netstumbler.com.

Once you have installed the software, simply walk or

drive to a public area and start it up. Figure 6.14 shows an

example of the seven networks I discovered in my home

town of Bloomington, Indiana when I walked through one

236 CHAPTER 6 WIRED AND WIRELESS LOCAL AREA NETWORKS

FIGURE 6.15 Wireless NetView shows the networks in Alan’s ofﬁce at Indiana University

FIGURE 6.16 Changes in signal strength

HANDS-ON ACTIVITY 6C 237

building downtown using NetStumbler. For each network,

Net Stumbler displays the MAC address of the access

point (or physical address if you prefer to use that term).

It shows the SSID, the channel number the AP is con-

ﬁgured to use, the speed of the network, the access point

vendor (which can be disabled by the access point owner

to increase security), and the type of encryption in use

(if any). It also shows the signal strength both by color

coding the network (green is good) and by showing the

signal-to-noise ratio (SNR) and the strength of the signal

and the noise.

Figure 6.15 shows a similar display from Wireless

NetView when it was used in Alan’s ofﬁce at Indiana

University. Wireless NetView presents the same basic

information as Net Stumbler. Notice how many networks

there are that touch this one location, although the signal

strength is too weak to be used for most of them.

In these ﬁgures, you can see a mix of WLANs, both

11 Mbps and 54 Mbps. The channels we usually use for

802.11b and 802.11g are channels 1, 6, and 11. In this

ﬁgure, you’ll see a mix of channels 1 and 6, plus one chan-

nel 8 and one channel 10 WLAN. 802.11b and 802.11g

can be conﬁgured to use four channels (1, 4, 8, and 11),

although the channels overlap to some extent. So if you

run an AP on channel 1 and another on channel 4, there

will be some interference between the two APs. The best

practice recommendation that most companies follow is

to use a three-channel conﬁguration. In Figure 6.14, you

can see that most companies are using the three-channel

conﬁguration, but one is not; it’s using the four-channel

conﬁguration.

If you click on an access point in the left panel, Net

Stumbler shows you a real time graph of the signal and

noise for that network. Figure 6.16 shows how the sig-

nal strength changed for one of the networks as I walked

through the building. The left edge of the graph shows

that the network started with a good signal (the green or

light colored area at the top of the bars) was much higher

than the noise (the red or dark colored area at the bot-

tom of the bars). As I walked around, the signal became

weaker; the signal was barely higher than the noise. As I

walked more, the signal dropped so that it was too weak

for me to detect it from the noise.

Deliverables

1. Capture a snapshot for the screen having all the

information related to the various network connec-

tions that you collected during your warwalking.

2. How many networks kept the original/default

SSID? Why could this be a security problem?

3. How many networks were secure?

4. What is your overall assessment of the WLAN

usage with respect to security?

CHAPTER7

BACKBONE NETWORKS

The Three Faces of Networking

Fundamental Concepts Network Technologies

Backbone

Network Management

ecurit

Application layer

Network layer

Data Link layer

Physical layer

LAN

WLAN

Transport layer

Backbone

WAN

Internet

7.1 INTRODUCTION 239

THIS CHAPTER examines backbone networks (BNs) that are

used to link LANs together and to link BNs to WANs. We begin with

the various types of devices used in BNs and discuss several backbone

architectures. The chapter ends with a discussion of how to improve BN

performance and of the future of BNs.

OBJECTIVES

▲

 Understand the internetworking devices used in BNs

 Understand several common backbone architectures

 Be familiar with gigabit Ethernet

 Understand the best practice recommendations for backbone design

 Be aware of ways to improve BN performance

CHAPTER OUTLINE

▲

7.1 INTRODUCTION

7.2 BACKBONE NETWORK COMPONENTS

7.2.1 Switches

7.2.2 Routers

7.2.3 Gateways

7.2.4 A Caution

7.3 BACKBONE NETWORK ARCHITECTURES

7.3.1 Backbone Architecture Layers

7.3.2 Switched Backbones

7.3.3 Routed Backbones

7.3.4 Virtual LANs

7.4 THE BEST PRACTICE BACKBONE DESIGN

7.5 IMPROVING BACKBONE PERFORMANCE

7.5.1 Improving Computer and Device

Performance

7.5.2 Improving Circuit Capacity

7.5.3 Reducing Network Demand

7.6 IMPLICATIONS FOR MANAGEMENT

7.1 INTRODUCTION

Most business organizations realize that information must be stored, retrieved, analyzed,

acted on, and shared with others at a moment’s notice. Without an enterprisewide network

or an Internet connection, moving information from one department LAN to another or

to customers is difﬁcult.

Interconnecting the organization’s diverse networks is critical. A backbone

network (BN) is a high-speed network that connects many networks. BNs typically use

higher-speed circuits to interconnect a series of LANs and provide connections to other

BNs, MANs, WANs, and the Internet. A backbone that connects many BNs spanning

several nearby buildings for a single organization is often called a campus network.

240 CHAPTER 7 BACKBONE NETWORKS

A BN also may be called an enterprise network if it connects all networks within a

company, regardless of whether it crosses state, national, or international boundaries.

We begin this chapter by describing several commonly used devices in the BN

and then showing how those can be used to create different backbone architectures with

different performance capabilities. Backbones used to be built with special technologies,

but today most BNs use high-speed Ethernet.

7.2 BACKBONE NETWORK COMPONENTS

There are two basic components to a BN: the network cable and the hardware devices

that connect other networks to the BN. The cable is essentially the same as that used

in LANs, except that it is often ﬁber optic to provide higher data rates. The hardware

devices can be computers or special-purpose devices that just transfer messages from

one network to another. These include switches, routers, and gateways (Figure 7.1).

7.2.1 Switches

Most switches operate at the data link layer. They connect two or more network segments

that use the same data link and network protocol. They understand only data link layer

protocols and addresses. They may connect the same or different types of cable. These

are the same layer-2 switches discussed in Chapter 6 in that they use the data link layer

address to forward packets between network segments (Figure 7.2). They learn addresses

by reading the source and destination addresses.

7.2.2 Routers

Routers operate at the network layer. They connect two or more network segments that

use the same or different data link protocols but the same network protocol. They may

connect the same or different types of cable. Routers are the “TCP/IP gateways” that

we ﬁrst introduced in Chapter 5. Routers strip off the data link layer packet, process the

network layer packet, and forward only those messages that need to go to other networks

on the basis of their network layer address (Figure 7.3).

Device

Switch

Router

Gateway

Data link layer

Network layer

Filtered using data

link layer addresses

Routed using network

layer addresses

Routed using network

layer addresses

Same or

different

Same or

different

Same or

different

Same

Same or

different

Same or

different

Same

Same or

different

Operates At Packets

Physical

Layer

Data Link

Layer

Network

Layer

FIGURE 7.1 Backbone network devices

7.2 BACKBONE NETWORK COMPONENTS 241

Switch

Server

Computer

Switch

Computer

FIGURE 7.2 Use of switches to connect local area network segments

Routers may be special purpose devices or special network modules in other devices

(e.g., wireless access points for home use often include a built-in router). In general, they

perform more processing on each message than switches and therefore operate more

slowly.

One major feature of a router is that it can choose the “best” route between networks

when there are several possible routes between them. Because a router knows its own

location, as well as the packet’s ﬁnal destination, it looks in a routing table to identify

thebestrouteorpath.

One other important difference between a router and a switch is that a router

processes only those messages that are speciﬁcally addressed to it. Switches process all

Switch

Router

Server

Computer

Switch

Computer

FIGURE 7.3 Use of routers to connect local area networks

242 CHAPTER 7 BACKBONE NETWORKS

messages that appear on the network and forward them to the appropriate network on the

basis of their data link layer address. Switches simply forward the message unchanged to

the other network. In contrast, because routers operate at the network layer, the router’s

data link layer must ﬁrst recognize that the incoming message is speciﬁcally addressed

to the router at the data link layer level before the message is passed to the network

layer for processing. The router will then process the message by building an entirely

new data link layer packet, then transmit it to the other network.

The router attempts to make no changes to the network layer packet and user data

it receives. (As noted previously, it creates a new data link layer packet.) Sometimes,

however, changes are needed, such as when the maximum data link layer packet size on

one network is different from another, which forces the router to split a message into

several smaller messages for transmission.

7.2.3 Gateways

Gateways operate at the network layer and use network layer addresses in processing

messages. Gateways are more complex than switches or routers because they are the

interface between two or more dissimilar networks. Gateways connect two or more

networks that use the same or different (usually different) data link and network protocols.

They may connect the same or different types of cable. Some gateways operate at the

application layer as well. Gateways process only those messages explicitly addressed to

them (i.e., using their data link layer address) and route those messages that need to go

to other networks (Figure 7.4).

Gateways translate one network layer protocol into another, translate data link

layer protocols, and open sessions between application programs, thus overcoming both

Server

Switch

Computer

Switch

Server

Mainframe

Gateway

Computer

FIGURE 7.4 Use of gateways to connect local area networks and a mainframe

7.3 BACKBONE NETWORK ARCHITECTURES 243

hardware and software incompatibilities. A gateway may be a stand-alone computer with

several NICs and special software or a front-end processor connected to a mainframe

computer.

Gateways used to be common, but as TCP/IP has become the dominant network

protocol, they are quickly fading from use.

7.2.4 A Caution

One warning is in order. The terminology used in the marketplace may differ substantially

from that in the preceding discussion. Many new types of switches, routers, and gateways

are being developed, so that one vendor’s “switch” may actually provide the functions

of a “router.” Layer-3 switches function in the same way as layer-2 switches discussed

previously, but they switch messages on the basis of their network layer address (usually

IP address). These switches provide the best of both switches and routers. They can be

used in place of routers but provide the beneﬁts of traditional layer-2 switches: much

faster transmission and more simultaneously active ports than routers.

7.3 BACKBONE NETWORK ARCHITECTURES

The backbone architecture refers to the way in which the backbone interconnects the

networks attached to it and how it manages the way in which packets from one network

move through the backbone to other networks.

While there are an inﬁnite number of ways in which network designers can build

backbone networks, there are really only three fundamental architectures that can be

combined in different ways. These architectures are routed backbones (routers that move

packets on the basis of network layer addresses), switched backbones (switches that

move packets based on data link layer addresses), and virtual LANs (switches that move

packets through LANs that are built virtually, not using physical location).

These architectures are mixed and matched to build sets of BNs. Before we discuss

these architectures, we ﬁrst must discuss the way in which network designers think about

backbone designs and how to combine them; that is, the different layers of backbones

that exist in most organizations today.

7.3.1 Backbone Architecture Layers

Network designers often think about three distinct technology layers

when they design

BNs. The layer closest to the users is the access layer, the technology used in the

LANs attached to the BN as described in the previous chapter (e.g., 100Base-T, wireless

Ethernet) (Figure 7.5). Although the access layer is not part of the BN, the technologies

used in the LANs (or access layer) can have major impacts on the design of the backbone.

Try not to confuse the ﬁve basic layers in the network model (application layer, transport layer, and so on)

with the layers of backbone technology we are describing here. They are different. We would have preferred

to use a different word than layer to describe these, but unfortunately, that is the term used in the industry.

244 CHAPTER 7 BACKBONE NETWORKS

Core Layer Distribution

Layer

Access Layer

LAN

FIGURE 7.5 Backbone network design layers. LAN = local area network

The distribution layer is the part of the backbone that connects the LANs

together. This is the part of the backbone that contains the “TCP/IP gateways” described

in Chapter 5. It usually runs throughout one building.

The core layer is the part of the backbone that connects the different BNs together,

often from building to building. The core layer is technologies used in the campus network

or the enterprise network. Some small organizations are not large enough to have a core

layer; their backbone spans only the distribution layer. Other organizations are large

enough that they have a core network at several locations that are in turn connected by

WANs.

In the sections that follow, we describe the three basic BN architectures and discuss

at which layer they are often used. We focus on TCP/IP networks when comparing

these architectures. We assume that you are comfortable with the material on TCP/IP in

Chapter 5; if you are not, you may want to go back and review the last section of the

chapter, entitled TCP/IP Example, before you continue reading.

7.3.2 Switched Backbones

Switched backbones are probably the most common type of BN used in the distribution

layer (i.e., within a building); most new building BNs designed today use switched

backbones. They also are making their way into the core layer as the campus backbone,

but routed backbones still remain common.