used for host addresses must be used as part of the subnet address. How many bits do you need to use
from the host space to create 10 subnets?
Solution: If we use 1 bit, we will be able to create 2 subnets (they will have the following binary
representation: 11111111.11111111.11111111.00000000 and 111111111.11111111.11111111.10000000). If we use
2 bits, we will be able to create 22 subnets, which is 4. Using 3 bits will give us 23 subnets, which is 8.
Therefore, we need to use 4 bits (24 = 16), which will give us 16 subnets. This is more than we need, but if
we use 3 bits, it will not meet our needs. The subnet mask for this network will be
1111111.11111111.11111111.11110000, or 255.255.255.240. This also means that we now only have 4 bits to
use for the host address on each network. So this means the maximum number of host addresses on each
subnet is 24 = 16.
Deliverables
Now that you understand how to make decisions regarding subnet masks, work on the following
problems:
1. Given a Class C network and a number of subnets required, complete the table to identify the number
of bits to borrow from the host field to use for the subnet field and the maximum number of host
addresses available per subnet.
Number of
Subnets
Required
Number of Bits to
Borrow for the Subnet
Field
Maximum Number
of Hosts per Subnet
Subnet Mask in Binary
and Decimal
Representation
2
5
12
24
40


and Decimal
Representation
2
5
12
24
40
2. Given a Class B network and a number of subnets required, complete the table to identify the number
of bits to borrow from the host field for the subnet field and the maximum number of host addresses
available per subnet.
Number of
Subnets
Required
Number of Bits to
Borrow for the Subnet
Field
Maximum Number
of Hosts per Subnet
Subnet Mask in Binary
and Decimal
Representation
5
8
35
200
400
3. Given a Class A network and a number of subnets required, complete the table to identify the number
of bits to borrow from the host field for the subnet field and the maximum number of host addresses
available per subnet.
Number of
Subnets
Required
Number of Bits to
Borrow for the Subnet
Field
Maximum Number
of Hosts per Subnet
Subnet Mask in Binary
and Decimal
Representation
10
20
80
400
2000


HANDS-ON ACTIVITY 5E
Subnetting Class C Addresses
To do this activity, you need to do Hands-On Activity 5D. First, we explain how to find the subnet address
for each subnet, the range of host addresses, and the direct broadcast address. Then you will be asked to
do a similar exercise by yourself.
Assume that you have been assigned the 192.168.1.0/24 network. You need to create 6 subnets.
a. How many bits do you need to borrow from the host field for the subnet field?
We need to borrow 3 bits: 1 bit would give us 2 subnets, 2 would give us 4, and 3 would give us 8.
b. What is the maximum number of subnets that can be created with this number of bits?
We can create 23 = 8 subnets.
c. How many bits can be used to create the host space?
Each byte has 8 bits—we are using 3 bits to define the subnets, and this leaves us with 5 bits for the
host space.
d. What is the maximum number of host addresses available per subnet?
25 − 2 = 32 − 2 = 30. We have 5 bits for the host space, and each bit can assume a value of 1 or 0 (25).
However, two addresses on each subnet are reserved—the first one (all zeros) for the subnet address
and last address (all ones) for the broadcast address.
e. What prefix would you use? What is the subnet mask, in binary and decimal format?
Recall that the prefix indicates the number of bits used to identify the network. Given that this is a


Class C address and we use 3 bytes plus 3 additional bits for subnets (8 + 8 + 8 + 3 = 27), our prefix
will be /27. Recall that the subnet mask is a continuous stream of 1s—in our case, 27 of them.
Therefore, the subnet mask in binary is 11111111.11111111.11111111.11100000.
Subnet
Number
Subnet Address (first
address on the subnet)
Range of Host
Addresses
Direct Broadcast Address (last
address on the subnet)
0
192.168.1.0
192.168.1.1–
192.168.1.30
192.168.1.31
1
192.168.1.32
192.168.1.33–
192.168.1.62
192.168.1.63
2
192.168.1.64
192.168.1.65–
192.168.1.94
192.168.1.95
3
192.168.1.96
192.168.1.97–
192.168.1.126
192.168.1.127
4
192.168.1.128
192.168.1.129–
192.168.1.158
192.168.1.159
5
192.168.1.160
192.168.1.161–
192.168.1.190
192.168.1.191
6
192.168.1.192
192.168.1.193–
192.168.1.222
192.168.1.223
7
192.168.1.224
192.168.1.225–
192.168.1.254


192.168.1.225–
192.168.1.254
192.168.1.255
We need to convert this binary number into a decimal to get the subnet mask. Hands-On Activity 5C
might come in handy here. The subnet mask in decimal is 255.255.255.224.
f. What is the increment value?
The increment value is the amount by which the subnet address increases from one subnet to the next


and is given by the placeholder value of the last 1 in the subnet mask. Because the last byte in the
subnet mask has three 1s, the third number 1 represents 32 (see Hands-On Activity 5C). So, the
increment value is 32.
g. Complete the following table; define each of the subnets, the range of host addresses on the subnet,
and the directed broadcast address on the subnet. Explanation of this table:
In part b, we indicated that there were eight subnets. The best way to fill out the table is to identify
the subnet addresses for all subnets. The very first subnet’s IP address is when all the bits in the last
byte are 0, giving us the following decimal value: 192.168.1.0. Recall from part f that the incremental
value is 32, which means that the second subnet’s IP address will have the third placeholder equal to
1, giving us the following address: 192.168.1.32. To find the third subnet’s IP address, we need to
multiply the increment value (32) by 2, resulting in 192.168.1.64. You would continue until the eighth
subnet, in which all the first 3 bits in the last byte equal 1, giving us 192.168.1.224. The direct
broadcast address’s value is one less than the next subnet’s IP address. Also, this address will have all
the host bits in the last byte equal to 1. For simplicity, I will only convert the last byte of several
broadcast addresses to binary to illustrate this:
Broadcast Address Last Byte Converted to Binary (network bits | host bits)
192.168.1.31
0 0 0 | 1 1 1 1 1
192.168.1.63


192.168.1.63
0 0 1 | 1 1 1 1 1
192.168.1.95
0 1 0 | 1 1 1 1 1
192.168.1.127
0 1 1 | 1 1 1 1 1
The addresses between the subnet address and the broadcast address can be assigned to any hosts on
the network.
Deliverables
Assume that you have been assigned 192.168.111.129/28.
1. How many bits are borrowed to create the subnet field? ________________
2. What is the maximum number of subnets that can be created with this number of bits?
________________
3. How many bits can be used to create the host space? ________________
4. What is the maximum number of host addresses available per subnet? ________________
5. What is the subnet mask, in binary and decimal format? ________________
6. Complete the following table and calculate the subnet that this address is on, and define all the other
subnets (the range of host addresses on the subnet and the directed broadcast address on the subnet).
Subnet Number
Subnet Address Range of Host Addresses Direct Broadcast Address
0
1
2
3
4
5
6
7
…
Last subnet number
7. Answer the following:


a. What subnet is 192.168.111.129 on?
b. A junior network administrator is trying to assign 192.168.111.127 as a static IP address for a
computer on the network but is getting an error message. Why?
c. Can 192.168.111.39 be assigned as an IP address?
HANDS-ON ACTIVITY 5F
IPv6 Addresses
The IPv4 address space provides approximately 4.3 billion addresses, but only 3.7 billion addresses are
assignable because the IPv4 addressing system separates the addresses into classes and reserves some
addresses for multicasting, testing, and other specific uses. The IPv4 address space has almost been
exhausted, which is why it is vital to understand the IPv6 protocol.
The 32-bit IPv4 address is a series of four 8-bit fields separated by dots, such as 129.79.126.1. However,
larger 128-bit IPv6 addresses need a different representation because of their size. IPv6 addresses use
colons to separate entries in a series of 16-bit hexadecimal.
A sample IPv6 address is 2031:0000:130F:0000:0000: 09C0:876A:130B. IPv6 does not require
explicit address string notation. Because the IPv6 addresses are much longer than their IPv4 counterparts,
several rules to shorten the addresses were developed:
Leading zeros in a field are optional. For example, the field 09C0 equals 9C0, and the field 0000
equals 0. So 2031:0000:130F:0000:0000:09C0:876A:130B can be written as
2031:0:130F:0:0:9C0:876A:130B.
Preferred Representation


Preferred Representation
Compressed Representation
A0B0:10F0:A110:1001:5000:0000:0000:0001
0000:0000:0000:0000:0000:0000:0000:0001
2001:0000:0000:1234:0000:0000:0000:45FF
3ffe:0000:0010:0000:1010:2a2a:0000:1001
3FFE:0B00:0C18:0001:0000:1234:AB34:0002
FEC0:0000:0000:1000:1000:0000:0000:0009
FF80:0000:0000:0000:0250:FFFF:FFFF:FFFF
Successive fields of zeros can be represented as two colons “::”. However, this shorthand method can
only be used once in an address, for example, 2031:0:130F:0000:0000:9C0:876A:130B can be
written as 2031:0:130F::9C0:876A:130B.
An unspecified address is written as “::” because it contains only zeros.
Deliverables
1. Use the preceding guidelines to compress the following IPv6 addresses into the shortest forms
possible.
2. Research on the Internet which IPv6 addresses are routable on the Internet.


PART THREE NETWORK TECHNOLOGIES


CHAPTER 6
NETWORK DESIGN
The chapters in the first section of the book provided a fundamental understanding of how networks work
using the five-layer model. This chapter starts the next section of the book, which focuses on how we
design networks. We usually design networks in seven network architecture components: local area
networks (LANs), building backbone networks, campus backbones that connect buildings, wide area
networks (WANs) that connect campuses, Internet access, e-commerce edge, and data centers. Network
design is an iterative process in which the designer examines users’ needs, develops an initial set of
technology designs, assesses their cost, and then revisits the needs analysis until the final network design
emerges.
OBJECTIVES
Understand the seven network architecture components
Describe the overall process of designing and implementing a network
Describe techniques for developing a logical network design
Describe techniques for developing a physical network design
Understand network design principles
OUTLINE
6.1 Introduction
6.1.1 Network Architecture Components
6.1.2 The Traditional Network Design Process
6.1.3 The Building-Block Network Design Process
6.2 Needs Analysis
6.2.1 Network Architecture Component
6.2.2 Application Systems
6.2.3 Network Users
6.2.4 Categorizing Network Needs
6.2.5 Deliverables
6.3 Technology Design
6.3.1 Designing Clients and Servers
6.3.2 Designing Circuits
6.3.3 Network Design Tools
6.3.4 Deliverables
6.4 Cost Assessment
6.4.1 Request for Proposal
6.4.2 Selling the Proposal to Management
6.4.3 Deliverables
6.5 Implications for Cyber Security


Summary
6.1 INTRODUCTION
All but the smallest organizations have networks, which means that most network design projects are the
design of upgrades or extensions to existing networks, rather than the construction of entirely new
networks. Even the network for an entirely new building is likely to be integrated with the organization’s
existing backbone network or wide area network (WAN), so even new projects can be seen as extensions
of existing networks. Nonetheless, network design is very challenging.
6.1.1 Network Architecture Components
Network designers usually think about networks as seven distinct network architecture components when
they design networks. Figure 6-1 shows a typical network for a large enterprise. This organization has
three enterprise campuses in different cities that are connected by a WAN provided by a common
carrier such as AT&T. Each campus has several buildings that are connected by a backbone network.
The first network architecture component is the local area network (LAN), which enables users to
access the network. Some vendors call this component the access layer because it provides access to the
network. Chapter 7 describes LANs, both wired and wireless, because when we build LANs today, we
usually provide both wired and wireless access. The technologies we use in the LAN are probably familiar
to you because you have used them, and perhaps you have even installed them in your home or
apartment: They are network hubs, switches, and wireless access points.
The second network architecture component is the building backbone network, which some vendors
call the distribution layer, because it distributes network traffic to and from the LANs. The building
backbone typically uses the same basic technology that we use in the LAN (a network switch), but usually
we buy faster switches because the building backbone carries more network traffic than a LAN. Chapter 8
describes building backbones.
The third network architecture component is the campus backbone (sometimes called the core layer),


The third network architecture component is the campus backbone (sometimes called the core layer),
which connects all the buildings on one campus. The campus backbone is usually faster than the
backbones we use inside buildings because it typically carries more traffic than they do. We use routers or
layer 3 switches that do routing when we design the campus backbone. Chapter 8 also describes campus
backbones.
The fourth network architecture component is the data center, which contains the organization’s servers
(e.g., database servers, email servers). The data center is essentially a LAN, but because so much traffic
goes to and from the data center, it is typically designed and managed very differently than the LANs
intended for user access. The data center is usually located centrally on the enterprise campus, with a
very, very-high-speed connection into the campus backbone. There is usually one primary data center for
the organization, typically found on its main headquarters campus. It is common for large organizations
to have several data centers spread around the world. Many enterprise campuses have their own smaller
data centers that store data just for that campus. We briefly discuss data center LAN design in Chapter 7.


FIGURE 6-1 Network architecture components
The last three components of the network architecture make up the enterprise edge, the parts of the
network that are at the edge of an enterprise campus and connect that campus to the rest of the world.
One of these is the wide area network (WAN), which is discussed in Chapter 9. A WAN is a private
network that connects its different campus locations, usually leased from a common carrier such as
AT&T. The WAN is for the private use of the organization and only carries its network traffic from one
campus to another, unlike the Internet, which carries traffic from many different organizations. The
circuits used in the WAN are traditionally very different than the Ethernet we use in the LAN, but this is
changing.
Another network architecture component is the Internet access component, which enables the
organization to connect to the Internet. The Internet and the technologies we use to connect to it are
discussed in Chapter 10. Large organizations use the same technologies to connect to the Internet as they
use in the WAN. Small companies and individuals like us typically use cable modem from your cable
company (e.g., Comcast or Time Warner) or DSL from AT&T.
The final network architecture component is the e-commerce edge. The e-commerce edge is a special
LAN with a group of servers that enables electronic data exchange between the organization and the
external entities with which it does business (such as its customers or suppliers). For example, the
organization’s primary Web server is located in the e-commerce edge. Like the data center, the design of
the LAN for the e-commerce edge is specialized; we briefly discuss it in Chapter 7 and again in Chapter 11
on security, because the e-commerce edge often requires different security.
Network design usually begins at the access layer, not the core layer. The needs of the users drive the


network design (as well as the applications in the data center). This is the reason that we discuss LANs
first (Chapter 7) and then move into the distribution and core layers (Chapter 8), with the enterprise edge
coming last (WANs in Chapter 9 and the Internet in Chapter 10).
Most organizations put the last five components in the same building. The switches and routers that


compose the campus backbone, the data center, and the enterprise edge are usually placed in one central
building on campus so that data move very quickly between the enterprise edge, the campus backbone,
and the data center.
6.1.2 The Traditional Network Design Process
The traditional network design process follows a very structured systems analysis and design
process similar to that used to build application systems. First, the network analyst meets with users to
identify user needs and the application systems planned for the network. Second, the analyst develops a
precise estimate of the amount of data that each user will send and receive and uses this to estimate the
total amount of traffic on each part of the network. Third, the circuits needed to support this traffic plus a
modest increase in traffic are designed, and cost estimates are obtained from vendors. Finally, 1 or 2 years
later, the network is built and implemented.
This traditional process, although expensive and time consuming, works well for static or slowly evolving
networks. Unfortunately, networking today is significantly different from what it was when the traditional
process was developed. Three forces are making the traditional design process less appropriate for many
of today’s networks.
First, the underlying technology of the client and server computers, networking devices, and the circuits
themselves is changing very rapidly. In the early 1990s, mainframes dominated networks, the typical
client computer was an 8-MHz 386 with 1 megabyte (MB) of random access memory (RAM) and 40 MB of
hard disk space, and a typical circuit was a 9,600-bps mainframe connection or a 1-Mbps LAN. Today,
client computers and servers are significantly more powerful, and circuit speeds of 1 Gbps (one billion bits
per second) are common. We now have more processing capability and network capacity than ever before;


per second) are common. We now have more processing capability and network capacity than ever before;
both are no longer scarce commodities that we need to manage carefully.
Second, the growth in network traffic is immense. The challenge is not in estimating today’s user demand
but in estimating its growth rate. In the early 1990s, email and the Web were novelties primarily used by
university professors and scientists. In the past, network demand was essentially driven by predictable
business systems such as order processing. Today, much network demand is driven by less predictable
user behavior, such as email and the Web. Many experts expect the rapid increase in network demand to
continue, especially as video, voice, and multimedia applications become commonplace on networks. At a
10% growth rate, user demand on a given network will increase by one-third in 3 years. At 20%, it will
increase by about 75% in 3 years. At 30%, it will double in less than 3 years. A minor mistake in estimating
the growth rate can lead to major problems. With such rapid growth, it is no longer possible to accurately
predict network needs for most networks. In the past, it was not uncommon for networks to be designed
to last for 5–10 years. Today, most network designers use a 3- to 5-year planning horizon.
Finally, the balance of costs has changed dramatically over the years. In the early 1990s, the most
expensive item in any network was the hardware (circuits, devices, and servers). Today, the most
expensive part of the network is the staff members who design, operate, and maintain it. As the costs have
shifted, the emphasis in network design is no longer on minimizing hardware cost (although it is
important); the emphasis today is on designing networks to reduce the staff time needed to operate them.


important); the emphasis today is on designing networks to reduce the staff time needed to operate them.
The traditional process minimizes the equipment cost by tailoring the equipment to a careful assessment
of needs but often results in a mishmash of different devices with different capabilities. Two resulting
problems are that staff members need to learn to operate and maintain many different devices and that it
often takes longer to perform network management activities because each device may use slightly
different software.
Today, the cost of staff time is far more expensive than the cost of equipment. Thus, the traditional
process can lead to a false economy—save money now in equipment costs but pay much more over the
long term in staff costs.
MANAGEMENT FOCUS 6-1
Average Life-Spans


A recent survey of network managers found that most expect their network hardware to last 3–5
years—not because the equipment wears out, but because rapid changes in capabilities make
otherwise good equipment obsolete. As Joel Snyder, a senior partner at OpusOne (a network
consulting firm), puts it: “You might go buy a firewall for a 1.5 Mbps circuit at a remote office and
then 2 weeks later have your cable provider offer you 50 Mbps.”
Life expectancy for selected network equipment:
Rack-mounted switch 4.5 years Wi-Fi access point 3 years
Chassis switch
4.5 years Desktop PC
3.5 years
Backbone router
5 years
Laptop PC
2.5 years
Branch office router
4 years
Mainframe
8.5 years
6.1.3 The Building-Block Network Design Process
Some organizations still use the traditional process of network design, particularly for those applications
for which hardware or network circuits are unusually expensive (e.g., WANs that cover long distances
through many different countries). However, many other organizations now use a simpler approach to
network design that we call the building-block process. The key concept in the building-block process
is that networks that use a few standard components throughout the network are cheaper in the long run
than networks that use a variety of different components on different parts of the network.
Rather than attempting to accurately predict user traffic on the network and build networks to meet those
demands, the building-block process instead starts with a few standard components and uses them over
and over again, even if they provide more capacity than is needed. The goal is simplicity of design. This
strategy is sometimes called “narrow and deep” because a very narrow range of technologies and devices
is used over and over again (very deeply throughout the organization). The results are a simpler design


is used over and over again (very deeply throughout the organization). The results are a simpler design
process and a more easily managed network built with a smaller range of components.
In this chapter, we focus on the building-block process to network design. The basic design process
involves three steps that are performed repeatedly: needs analysis, technology design, and cost
assessment (Figure 6-2). This process begins with needs analysis, during which the designer attempts
to understand the fundamental current and future network needs of the various users, departments, and
applications. This is likely to be an educated guess at best. Users’ access needs and the needs of
applications drive the network design process from the top into the center of the network. These needs are
classified as typical or high volume. Specific technology needs are identified (e.g., the ability to dial in with
current modem technologies).
The next step, technology design, examines the available technologies and assesses which options will
meet users’ needs. The designer makes some estimates about the network needs of each category of user
and circuit in terms of current technology (e.g., 1 Gbps Ethernet) and matches needs to technologies.
Because the basic network design is general, it can easily be changed as needs and technologies change.
The difficulty, of course, lies in predicting user demand so one can define the technologies needed. Most
organizations solve this by building more capacity than they expect to need and by designing networks
that can easily grow and then closely monitoring growth so they expand the network ahead of the growth
pattern.
In the third step, cost assessment, the relative costs of the technologies are considered. The process
then cycles back to the needs analysis, which is refined using the technology and cost information to
produce a new assessment of users’ needs. This, in turn, triggers changes in the technology design and


cost assessment, and so on. By cycling through these three processes, the final network design is
established (Figure 6-3).
6.2 NEEDS ANALYSIS
The goal of needs analysis is to understand why the network is being built and what users and applications
it will support. In many cases, the network is being designed to improve poor performance or enable new


applications to be used. In other cases, the network is upgraded to replace unreliable or aging equipment
or to standardize equipment so that only one type of equipment, one protocol (e.g., TCP/IP, Ethernet), or
one vendor’s equipment is used everywhere in the network.
Often, the goals in network design are slightly different between LANs and backbone networks (BNs) on
the one hand and WANs on the other. In the LAN and BN environments, the organization owns and
operates the equipment and the circuits. Once they are paid for, there are no additional charges for usage.
However, if major changes must be made, the organization will need to spend additional funds. In this
case, most network designers tend to err on the side of building too big a network—that is, building more
capacity than they expect to need.
FIGURE 6-2 Network design


FIGURE 6-3 The cyclical nature of network design
In contrast, in most WANs, the organization leases circuits from a common carrier and pays for them on a
monthly or per-use basis. Understanding capacity becomes more important in this situation because
additional capacity comes at a noticeable cost. In this case, most network designers tend to err on the side
of building too small a network, because they can lease additional capacity if they need it—but it is much
more difficult to cancel a long-term contract for capacity they are not using.
Much of the needs analysis may already have been done because most network design projects today are
network upgrades rather than the design of entirely new networks. In this case, there is already a fairly
good understanding of the existing traffic in the network and, most important, of the rate of growth of
network traffic. It is important to gain an understanding of the current operations (application systems
and messages). This step provides a baseline against which future design requirements can be gauged. It
should provide a clear picture of the present sequence of operations, processing times, work volumes, the
current communication network (if one exists), existing costs, and user/management needs. Whether the
network is a new network or a network upgrade, the primary objective of this stage is to define (1) the


geographic scope of the network and (2) the users and applications that will use it.
The goal of the needs analysis step is to produce a logical network design, which is a statement of the
network elements needed to meet the needs of the organization. The logical design does not specify
technologies or products to be used (although any specific requirements are noted). Instead, it focuses on
the fundamental functionality needed, such as a high-speed access network, which in the technology
design stage will be translated into specific technologies (e.g., switched 100Base-T).
6.2.1 Network Architecture Component
The first step in needs analysis is to break the network into the seven network architecture
components in Figure 6-1: LANs, building backbones, campus backbones, WANs, Internet access, e-
commerce edge, and data centers. Not all layers are present in all networks. Small networks, for example,
may not have a core backbone because there is only one building. Likewise, the data center is typically
designed and managed separately.
Sometimes, the current network infrastructure imposes constraints. For example, if we are adding a new
building to an existing office complex that used 1 Gbps Ethernet in the LANs, then we will probably
choose to use the same in the new building. All such constraints are noted.
It is easiest to start with the highest level, so most designers begin by drawing a network diagram for any
WANs with enterprise campuses that must be connected. A diagram that shows the logical network going
between the locations is sufficient. Details such as the type of circuit and other considerations will be
added later. Next, the individual enterprise campus diagrams are drawn, usually in a series of separate
diagrams, but for a simple network, one diagram may be sufficient.
MANAGEMENT FOCUS 6-2
A New Hospital Network


MANAGEMENT FOCUS 6-2
A New Hospital Network
Kingston Hospital is part of the National Health Service in the United Kingdom. The hospital is one
of the largest in London, with more than 3,500 employees. As the health-care industry moves into a
digital environment, the network becomes critical. Mobile computing on tablets at the patient
bedside enables doctors, nurses, and other staff to provide care without relying on paper notes,
which can be easily lost or misunderstood because of illegible handwriting.
The access layer is primarily wireless LAN, with 650 wireless access points spread throughout the
hospital. The hospital provides more than 200 tablets to staff to review patient records, update
observations, and order tests and medications. Pharmacists use laptops and workstations on wheels
to manage and deliver prescriptions.
These access points and wired LANs are connected into building backbones that run at 1 Gbps. These
in turn are connected into two large campus backbone switches that provide 10 Gbps.
Adapted from: “Laying Foundations for Digital Hospital Vision,” Cisco Systems Inc.
At this point, the designers gather general information and characteristics of the environment in which
the network must operate. For example, they determine whether there are any legal requirements, such as
local, state/provincial, federal, or international laws, regulations, or building codes, that might affect the
network.
6.2.2 Application Systems
Next, the designers must review the list of applications that will use the network and identify the location
of each. This information should be added to the emerging network documentation. This process is called
baselining. Next, those applications that are expected to use the network in the future are added.
In many cases, the applications will be relatively well defined. Specific internal applications (e.g., payroll)


and external applications (e.g., Web servers) may already be part of the “old” network. However, it is
important to review the organization’s long- and short-range plans concerning changes in company goals,


strategic plans, development plans for new products or services, projections of sales, research and
development projects, major capital expenditures, possible changes in product mix, new offices that must
be served by the communications network, security issues, and future commitments to technology. For
example, a major expansion in the number of offices or a major electronic commerce initiative will have a
significant impact on network requirements.
It is also helpful to identify the hardware and software requirements of each application that will use the
network and, if possible, the protocol each application uses (e.g., HTTP over TCP/IP, Windows file
access). This knowledge helps now and will be particularly useful later when designers develop
technological solutions.
6.2.3 Network Users
In the past, application systems accounted for the majority of network traffic. Today, much network traffic
is produced by the discretionary use of the Internet. Applications such as email and the Web are
generating significant traffic, so the network manager is no longer in total control of the network traffic
generated on his or her networks. This is likely to continue in the future as network-hungry applications
such as desktop videoconferencing become more common. Therefore, in addition to understanding the
applications, you must also assess the number and type of users that will generate and receive network
traffic and identify their location on the emerging network diagram. We usually assume that most users
will want both wired and wireless access to the network, although there are exceptions. Hotels may only
provide wireless access for guests, and some offices may only provide wired access.
6.2.4 Categorizing Network Needs
At this point, the network has been designed in terms of geographic scope, application systems, and users.
The next step is to assess the relative amount of traffic generated in each part of the network. With the
traditional design approach, this involves considerable detailed analysis. With the building-block


traditional design approach, this involves considerable detailed analysis. With the building-block
approach, the goal is to provide some rough assessment of the relative magnitude of network needs. Each
application system is assessed in general terms to determine the amount of network traffic that can be
expected to generate today and in the future, compared with other applications. Likewise, each user is
categorized as either a typical user or a high-traffic user. These assessments will be refined in the next
stage of the design process.
This assessment can be problematic, but the goal is some relative understanding of the network needs.
Some simple rules of thumb can help. For example, applications that require large amounts of multimedia
data or those that load executables over the network are likely to be high-traffic applications. Applications
that are time sensitive or need constant updates (e.g., financial information systems, order processing) are
likely to be high-traffic applications.
Once the network requirements have been identified, they also should be organized into mandatory
requirements, desirable requirements, and wish-list requirements. This information enables
the development of a minimum level of mandatory requirements and a negotiable list of desirable
requirements that are dependent on cost and availability. For example, desktop video-conferencing may
be a wish-list item, but it will be omitted if it increases the cost of the network beyond what is desired.
At this point, the local facility network diagrams are prepared. For a really large network, there may be
several levels. The choice is up to the designer, provided the diagrams and supporting text clearly explain
the network’s needs.


FIGURE 6-4 Sample needs assessment logical network design for a single building. LAN = local area
network
6.2.5 Deliverables
The key deliverable for the needs assessments stage is a set of logical network diagrams, showing the
applications, circuits, clients, and servers in the proposed network, each categorized as either typical or
high traffic. The logical diagram is the conceptual plan for the network and does not consider the specific
physical elements (e.g., routers, switches, circuits) that will be used to implement the network.
Figure 6-4 shows the results of a needs assessment for a building that includes the access layer (LANs),
the distribution layer (building backbone), and the core layer (campus backbone). This figure shows the
distribution and access components in the building with the series of six access LANs connected by one
building backbone, which is in turn connected to a campus core back-bone. One of the six LANs is
highlighted as a high-traffic LAN, whereas the others are typical ones. We normally would assume that the
LANs need both wired and wireless access unless the requirements stated differently. Three mandatory
applications are identified that will be used by all network users: email, Web, and file sharing. One wish-
list requirement (desktop videoconferencing) is also identified for a portion of the network.


6.3 TECHNOLOGY DESIGN
Once the needs have been defined in the logical network design, the next step is to develop a physical
network design (or a set of possible designs). The physical network design starts with the client and
server computers needed to support the users and applications. If the network is a new network, new
computers will need to be purchased. If the network is an existing network, the servers may need to be
upgraded to the newest technology. Once these are designed, then the circuits and devices connecting
them are designed.
6.3.1 Designing Clients and Servers
The idea behind the building-block approach is to specify needs in terms of some standard units. Typical
users are allocated the base-level client computers, as are servers supporting typical applications. Users
and servers for applications needing more powerful computers are assigned some advanced computer. As
the specifications for computers rapidly improve and costs drop (usually every 6 months), today’s typical
user may receive the type of computer originally intended for the advanced user when the network is
actually implemented, and the advanced users may end up with a computer not available when the
network was designed.
6.3.2 Designing Circuits
The same is true for network circuits and devices (e.g., hubs, routers, switches). There are two interrelated
decisions in designing network circuits and devices: the fundamental technology and protocols (e.g.,
Ethernet) and the capacity of each circuit (e.g., 100 Mbps, 1,000 Mbps). These are interrelated because
each technology offers different circuit capacities.
Designing the circuit capacity means capacity planning, estimating the size and type of the standard
and advanced network circuits for each type of network (LAN, backbone, WAN). As you will learn in
Chapter 7 on LANs, wired and wireless circuits come in standard sizes. Most users with a desktop or


laptop computer don’t need to send files that are over a gigabyte in size at a time (i.e., 1,000 Meg). And if
they do, they understand there may be a slight delay. Therefore, circuits for wired LANs are typically 100
Mbps or 1 Gbps. Wireless circuits are a little different, so we’ll avoid them until Chapter 7.
Designing circuit capacities for backbone networks is more challenging because backbones move traffic
from many computers at one time and there are more choices in standard sizes. This requires some
assessment of the current and future circuit loading (the amount of data transmitted on a circuit). This
analysis can focus on either the average circuit traffic or the peak circuit traffic. For example, in an online
banking network, traffic volume peaks usually are in the midmorning (bank opening) and just prior to
closing. Airline and rental car reservations network designers look for peak volumes before and during
holidays or other vacation periods, whereas telephone companies normally have their highest peak
volumes on Mother’s Day. Designing for peak circuit traffic is the ideal.
The designer usually starts with the total characters transmitted per day on each circuit or, if possible, the
maximum number of characters transmitted per 2-second interval if peaks must be met. You can calculate
message volumes by counting messages in a current network and applying some estimated growth rate. If
an existing network is in place, network monitors/analyzers (see Chapter 12) may be able to provide an
actual circuit character count of the volume transmitted per minute or per day.
A good rule of thumb is that 80% of this circuit loading information is easy to gather. The last 20% needed
for very precise estimates is extremely difficult and expensive to find. However, precision usually is not a
major concern because of the stairstep nature of communication circuits and the need to project future


major concern because of the stairstep nature of communication circuits and the need to project future
needs. For example, the difference between 100 Mbps and 1 Gbps is quite large, and assessing which level
is needed for typical traffic does not require a lot of precision. Forecasts are inherently less precise than
understanding current network traffic. The turnpike effect is an expression that means that traffic
increases much faster than originally forecast. It comes from the traffic forecasting that was done for the
construction of the early interstate highways. When a new, faster highway (or network) is built, people are
more likely to use it than the old slow one because it is available, is very efficient, and provides new
capabilities. The annual growth factor for network use may vary from 5% to 50% and, in some cases, may
exceed 100% for high-growth organizations.


Although no organization wants to overbuild its network and pay for more capacity than it needs, in most
cases, upgrading a network costs 50% to 80% more than building it right the first time. Few organizations
complain about having too much network capacity, but being under capacity can cause significant
problems. Given the rapid growth in network demand and the difficulty in accurately predicting it, most
organizations intentionally overbuild (build more capacity into their network than they plan to use), and
most end up using this supposedly unneeded capacity within 3 years.
In any network, there may be a bottleneck, a circuit that is filled almost to its capacity and thus is the
critical point that determines whether users get good or bad response times. When users complain about a
slow network, it is usually because there is a bottleneck circuit somewhere in the network. Of course, the
bottleneck could also be a slow Web server that is simply receiving more traffic than it can handle, but
usually the problem is a circuit.
Take another look at Figure 6-4. Suppose we specified 1 Gbps circuits as the standard for the LANs. If
each LAN has 20 computers, then this is in theory a total capacity of 120 Gbps in the building (6 LANs ×
20 computers each × 1 Gbps = 120 Gbps). Not all the computers will be sending or receiving at the same
time, so this is artificially high, but it is a theoretical maximum.
If this is the case, what speed should we specify for the building backbone? We have a few standard
speeds, as you will learn in Chapter 8: 1 Gbps, 10 Gbps, 40 Gbps, 100 Gbps. A 1-Gbps backbone is
probably too slow and would end up being a bottleneck. Is 10 Gbps enough? It’s hard to say without


knowing the circuit loading. Without the circuit loading, most network designers would set the building
backbone speed at one level above the standard LAN speed, which in this case would be 10 Gbps.


FIGURE 6-5 Physical network design for a single building
This problem continues at the next architecture component—the campus core backbone. If each building
has a 10-Gbps backbone, what speed should the campus backbone that connects all the buildings be?
Without a circuit loading, it’s hard to say. Once again, most network designers would set the building
backbone speed at one level above the building backbone speed, which in this case would be 40 Gbps. And
this is where reality sets in. Today, the technology for 40 Gbps is very expensive—so expensive, in fact,
that most organizations don’t buy it unless they really need it. Chances are, the campus backbone would
be designed at 10 Gbps, which means it might be the bottleneck—at least for traffic on campus.
Figure 6-5 shows the physical design for the network in Figure 6-4. Take a moment to look at it and
compare Figures 6-4 and 6-5.
As we move beyond the campus to the enterprise edge, network design becomes a bit more difficult. As
you will learn in Chapter 9, on WANs, and Chapter 10, on the Internet, the technologies we use for WANs
and Internet access are quite different from what we use for LANs and backbones. Their speeds are much,
much slower and much more expensive. A typical WAN circuit speed is between 1 Mbps and 50 Mbps.
Yes, that was Mbps; in other words, more than 100 times slower than the speed of our backbone
networks. Thus, the bottleneck in most enterprise networks is the WAN and the Internet, not the
enterprise campus network.


This is also true for the network in your house or apartment. Most wireless LAN access points you buy
today provide speeds of 100–400 Mbps, yet your Internet connection is usually less than 25 Mbps. This
means the response times you experience when you’re on the Internet will be the same whether you buy a
really fast, state-of-the-art wireless access point or an old one that provides only 50 Mbps, because the
bottleneck is the Internet access, not the wireless LAN. Unless you’re spending a lot of money on a really
fast Internet connection, don’t waste your money on a really fast wireless access point.
6.3.3 Network Design Tools
Network modeling and design tools can perform a number of functions to help in the technology design
process. With most tools, the first step is to enter a diagram or model of the existing network or proposed
network design. Some modeling tools require the user to create the network diagram from scratch. That
is, the user must enter all of the network components by hand, placing each server, client computer, and
circuit on the diagram and defining what each is.
Other tools can “discover” the existing network; that is, once installed on the network, they will explore
the network to draw a network diagram. In this case, the user provides some starting point, and the
modeling software explores the network and automatically draws the diagram itself. Once the diagram is
complete, the user can then change it to reflect the new network design. Obviously, a tool that can perform
network discovery by itself is most helpful when the network being designed is an upgrade to an existing
network and when the network is very complex.
Once the diagram is complete, the next step is to add information about the expected network traffic and
see if the network can support the level of traffic that is expected. Simulation, a mathematical technique


see if the network can support the level of traffic that is expected. Simulation, a mathematical technique
in which the network comes to life and behaves as it would under real conditions, is used to model the
behavior of the communication network. Applications and users generate and respond to messages while
the simulator tracks the number of packets in the network and the delays encountered at each point in the
network.
Simulation models may be tailored to the users’ needs by entering parameter values specific to the
network at hand (e.g., this computer will send an average of three 100-byte packets per minute and
receive one hundred 1,500-byte packets per minute). Alternatively, the user may prefer to rely primarily
on the set of average values provided by the network.
Once the simulation is complete, the user can examine the results to see the estimated response times
throughout. It is important to note that these network design tools provide only estimates, which may
vary from the actual results. At this point, the user can change the network design in an attempt to
eliminate bottlenecks and rerun the simulation. Good modeling tools not only produce simulation results
but also highlight potential trouble spots (e.g., servers, circuits, or devices that experienced long response
times). The very best tools offer suggestions on how to overcome the problems that the simulation
identified.
6.3.4 Deliverables
The key deliverable is a set of one or more physical network designs like that in Figure 6-5, which is the
design for a single building. Most designers like to prepare several physical designs so they can trade-off
technical benefits (e.g., performance) against cost. In most cases, the critical part is the design of the
network circuits and devices. In the case of a new network designed from scratch, it is also important to
define the client computers with care because these will form a large portion of the total cost of the


define the client computers with care because these will form a large portion of the total cost of the
network. Usually, however, the network will replace an existing network and only a few of the client
computers in the existing network will be upgraded.
6.4 COST ASSESSMENT
The purpose of this step is to assess the costs of various physical network design alternatives produced in
the previous step. The main items are the costs of software, hardware, and circuits. These three factors are
all interconnected and must be considered along with the performance and reliability required. All factors
are interrelated with regard to cost.


Estimating the cost of a network is quite complex because many factors are not immediately obvious.
Some of the costs that must be considered are as follows:
Circuit costs, including costs of circuits provided by common carriers or the cost of purchasing and
installing your own cable
Network devices such as switches and routers
Hardware costs, including servers, printers, uninterruptible power supplies, and backup tape drives
Software costs for network operating system, application software, and middleware
Network management costs, including special hardware, software, and training needed to develop a
network management system for ongoing redesign, monitoring, and diagnosis of problems
Test and maintenance costs for special monitoring equipment and software, plus the cost of onsite
spare parts
Costs for WAN and Internet circuits leased from common carriers
6.4.1 Request for Proposal
Although some network components can be purchased off the shelf, most organizations develop a
request for proposal (RFP) before making large network purchases. RFPs specify what equipment,
software, and services are desired and ask vendors to provide their best prices. Some RFPs are very
specific about what items are to be provided in what time frame. In other cases, items are defined as
mandatory, important, or desirable, or several scenarios are provided and the vendor is asked to propose
the best solution. In a few cases, RFPs specify generally what is required and the vendors are asked to
propose their own network designs. Figure 6-6 provides a summary of the key parts of an RFP.
Once the vendors have submitted their proposals, the organization evaluates them against specified
criteria and selects the winner(s). Depending on the scope and complexity of the network, it is sometimes
necessary to redesign the network on the basis of the information in the vendors’ proposals.
One of the key decisions in the RFP process is the scope of the RFP. Will you use one vendor or several


vendors for all hardware, software, and services? Multivendor environments tend to provide better
performance because it is unlikely that one vendor provides the best hardware, software, and services in
all categories. Multivendor networks also tend to be less expensive because it is unlikely that one vendor
will always have the cheapest hardware, software, and services in all product categories.
Multivendor environments can be more difficult to manage, however. If equipment is not working
properly and it is provided by two different vendors, each can blame the other for the problem.
6.4.2 Selling the Proposal to Management
One of the main problems in network design is obtaining the support of senior management. To
management, the network is simply a cost center, something on which the organization is spending a lot
of money with little apparent change. The network keeps on running just as it did the year before.
The key to gaining the acceptance of senior management lies in speaking management’s language. It is
pointless to talk about upgrades from 100 Mbps to 1 Gbps on the backbone because this terminology is
meaningless from a business perspective. A more compelling argument is to discuss the growth in
network use. For example, a simple graph that shows network usage growing at 25% per year, compared
with the network budget growing at 10% per year, presents a powerful illustration that the network costs
are well managed, not out of control.
Likewise, a focus on network reliability is an easily understandable issue. For example, if the network
supports a mission-critical system such as order processing or moving point-of-sale data from retail stores
to corporate offices, it is clear from a business perspective that the network must be available and
performing properly, or the organization will lose revenue.
Information in a Typical Request for Proposal
Background information


Organizational profile
Overview of current network
Overview of new network
Goals of new network
Network requirements
Choice sets of possible network designs (hardware, software, circuits)
Mandatory, desirable, and wish-list items
Security and control requirements
Response-time requirements
Guidelines for proposing new network designs
Service requirements
Implementation time plan
Training courses and materials
Support services (e.g., spare parts on site)
Reliability and performance guarantees
Bidding process
Time schedule for the bidding process
Ground rules
Bid evaluation criteria
Availability of additional information
Information required from vendor
Vendor corporate profile
Experience with similar networks
Hardware and software benchmarks
Reference list
FIGURE 6-6 Request for proposal
6.4.3 Deliverables
There are three key deliverables for this step. The first is an RFP that goes to potential vendors. The
second deliverable, after the vendor has been selected, is the revised physical network diagram (e.g.,
Figure 6-5) with the technology design complete. Exact products and costs are specified at this point (e.g.,
a 24-port 1000Base-T Cisco Ethernet switch). The third deliverable is the business case that provides
support for the network design, expressed in business objectives.
6.5 IMPLICATIONS FOR CYBER SECURITY
Thinking of networks as seven separate architectural components also helps with network security. Each
component is a distinct module that can be managed separately in terms of security. The components with
the tightest security are usually the core layer and the data center. Servers in the data center are usually
heavily protected so that no traffic from outside the network can reach them. Data on these servers are
usually encrypted, so that if an attacker manages to break-in and steal data, the encryption must be
broken before the data are useful.


broken before the data are useful.
Security for the enterprise edge is typically managed differently than security for the rest of the network.
Since many external attacks are launched over the Internet, security for the Internet Access component


has to be managed carefully—and constantly. Interestingly, the e-commerce edge often has the lightest
security because it needs to permit access to anyone in the world. After all, if your customers can’t reach
your Web server or your email server, what use are they? Nonetheless, the e-commerce edge is carefully
monitored because it is a prime target.
Each building has its own access and distribution layers, and although there may be some similarities
among buildings, each building can be managed separately. Think about your university. Would you have
similar security for a typical classroom building versus the university administration building?
SUMMARY
Network Architecture Components Network designers usually think about networks as seven
network architecture components. LANs (wired and wireless) provide users access to the network
(access layer). Building backbones (distribution layer) connect the LANs inside one building. Campus
backbones (core layer) connect the different buildings. The data center houses the organization’s
main servers. At the enterprise edge, we have WAN access that connects to other campuses operated
by the organization, Internet access, and the e-commerce edge, which enables a business to support
its customers and/or suppliers.
Traditional Network Design The traditional network design approach follows a very structured
systems analysis and design process similar to that used to build application systems. It attempts to
develop precise estimates of network traffic for each network user and network segment. Although
this is expensive and time consuming, it works well for static or slowly evolving networks.
Unfortunately, computer and networking technology is changing very rapidly, the growth in network
traffic is immense, and hardware and circuit costs are relatively less expensive than they used to be.
Therefore, use of the traditional network design approach is decreasing.
Building-Block Approach to Network Design The building-block approach attempts to build
the network using a series of simple predefined building components, resulting in a simpler design


the network using a series of simple predefined building components, resulting in a simpler design
process and a more easily managed network built with a smaller range of components. The basic
process involves three steps that are performed repeatedly. Needs analysis involves developing a
logical network design that includes the geographic scope of the network and a categorization of
current and future network needs of the various network segments, users, and applications as either
typical or high traffic. The next step, technology design, results in a set of one or more physical
network designs. Network design and simulation tools can play an important role in selecting the
technology that typical and high-volume users, applications, and network segments will use. The final
step, cost assessment, gathers cost information for the network, usually through an RFP that specifies
what equipment, software, and services are desired and asks vendors to provide their best prices. One
of the keys to gaining acceptance by senior management of the network design lies in speaking
management’s language (cost, network growth, and reliability), not the language of the technology
(Ethernet, ATM, and DSL).
KEY TERMS
access layer
baseline
bottleneck
building backbone network
building-block process
campus backbone
capacity planning
circuit loading
common carrier


core layer
cost assessment
data center
distribution layer
e-commerce edge
enterprise campuses
enterprise edge
geographic scope
Internet access
logical network design
local area network (LAN)
mandatory requirements
needs analysis
network architecture component
physical network design
request for proposal (RFP)
simulation
technology design
traditional network design process
turnpike effect
wide area network (WAN)
wish-list requirements
QUESTIONS
1. What are the keys to designing a successful data communications network?
2. How does the traditional approach to network design differ from the building-block approach?
3. Describe the three major steps in the current network design.
4. What is the most important principle in designing networks?
5. Why is it important to analyze needs in terms of both application systems and users?
6. Describe the key parts of the technology design step.
7. How can a network design tool help in network design?
8. On what should the design plan be based?
9. What is an RFP, and why do companies use them?
10. What are the key parts of an RFP?
11. What are some major problems that can cause network designs to fail?
12. What is a network baseline, and when is it established?
13. What issues are important to consider in explaining a network design to senior management?
14. What is the turnpike effect, and why is it important in network design?
15. What are the seven network architecture components?


16. What is the difference between a building backbone and a campus backbone, and what are the
implications for the design of each?
17. What are typical speeds for the LAN, building back-bone, and campus backbone? Why?
18. What is a bottleneck, and why do network managers care about them?
19. Is it important to have the fastest wireless LAN technology in your apartment? What about in the
library of your school? Explain.
20. Why do you think some organizations were slow to adopt a building-block approach to network
design?
21. For what types of networks are network design tools most important? Why?
EXERCISES
A. What factors might cause peak loads in a network? How can a network designer determine if they are
important, and how are they taken into account when designing a data communications network?
B. Collect information about two network design tools and compare and contrast what they can and
cannot do.
MINICASES
I. Computer Dynamics Computer Dynamics is a microcomputer software development company
that has a 300-computer network. The company is located in three adjacent five-story buildings in an
office park, with about 100 computers in each building. The LANs in each building are similar, but
one building has the data center on the second floor. There are no other office locations. The current
network is poorly designed for its current needs and must be completely replaced. Develop a logical
design for this enterprise campus that considers the seven network architecture components. There
are no other campuses, so you can omit WAN access. You will need to make some assumptions, so be
sure to document your assumptions and explain why you have designed the network in this way.
II. Drop and Forge Drop and Forge is a manufacturing firm with a 60-computer network on its


Toledo, Ohio, campus. The company has one very large manufacturing plant with an adjacent office
building. The office building houses 50 computers, with an additional 10 computers in the plant. The
current network is old and needs to be completely replaced. Develop a logical design for this
enterprise campus that considers the seven network architecture components. There are no other
campuses, so you can omit WAN access. You will need to make some assumptions, so be sure to
document your assumptions and explain why you have designed the network in this way.
III. AdviceNet AdviceNet is a consulting firm with offices in Toronto, New York, Los Angeles, Dallas,
and Atlanta. The firm currently uses the Internet to transmit data, but its needs are growing and it is
concerned over the security of the Internet. The New York office is the primary headquarters with
200 computers spread across four floors and has the enterprise data center. Develop a logical design
for the New York enterprise campus that considers the seven network architecture components.
Describe the assumptions you have made.
IV. Accurate Accounting Accurate Accounting is a regional accounting firm that has 15 local offices
throughout Georgia, Florida, and the Carolinas. The company is constructing a new office building for
use as its main headquarters. The building will have two floors with a total of 40 offices, each with a
desktop computer. Develop a logical design for the Atlanta headquarters enterprise campus that
considers the seven network architecture components. You will need to make some assumptions, so
be sure to document your assumptions and explain why you have designed the network in this way.
V. Donald’s Distributing Donald’s Distributing is a regional trucking firm that is constructing a new
office building (its only office). The network has 80 desktop computers and 2 servers. Develop a
logical design for the enterprise campus that considers the seven network architecture components.


logical design for the enterprise campus that considers the seven network architecture components.
You will need to make some assumptions, so be sure to document your assumptions and explain why
you have designed the network in this way.


TECH UPDATES
In every chapter, we will offer a couple of ideas to investigate.
Topic A: Securing Code
Securing coding is the practice of developing computer software in a way that guards against the
accidental introduction of security vulnerabilities. Defects, bugs, and logic flaws are consistently the
primary cause of commonly exploited software vulnerabilities. Several vulnerabilities that must be
prevented through secure coding are as follows: buffer overflow, format string attack prevention, and
integer overflow prevention. Investigate these vulnerabilities (and others) and provide examples of how to
protect against them.
Topic B: Emergency of Python as a must Know Language for Cybersecurity
Programming has become essential to cybersecurity. IT security professionals must efficiently write
applications and scripts; often on short notice. The Python language provides unmatched ease, flexibility,
and functionality for both new and experienced coders. It has emerged as a top choice for cybersecurity
professionals because it lessens development effort and the coder’s learning curve. What are some
“basics” that a cybersecurity professional must know when it comes to python. Provide examples.
Deliverables
Your job will be to prepare a presentation/video (7–10 minutes long) that addresses the topic and
provides information on the following items:
1. Title Slide
2. Short description of the topic
3. Main players—these could be people, processes, software, hardware, etc.
4. How it works—use a lot of pictures and be as technical as possible; create this part as a tutorial so that
your audience can follow along
5. How does it relate to material covered in class so far (and in the future)
6. Additional material/books/links where to learn more about this topic
7. Credits
8. List of References
9. Memo addressed to your professor describing all of the above information
HANDS-ON ACTIVITY 6A


HANDS-ON ACTIVITY 6A
Network Design Software
There are many different network design software tools. Some are simple drawing tools; others offer
powerful network simulation modeling capabilities. One powerful tool that provides a free demo version
that can be downloaded is SmartDraw.


FIGURE 6-7 SmartDraw software
The first step is to download and install the SmartDraw software. The software is available at
www.smartdraw.com.
SmartDraw comes with a variety of network icons and templates that can be used to quickly build network
diagrams. Figure 6-7 shows the main drawing screen in SmartDraw and a network diagram.
Deliverable
Select a network and draw it.


CHAPTER 7
WIRED AND WIRELESS LOCAL AREA NETWORKS
This chapter examines the three major network architecture components that use local area networks
(LANs): the LANs that provide network access to users, the data center, and the e-commerce edge. We
focus on the LANs that provide network access to users as these are more common. This chapter draws
together the concepts from the first section of the book on fundamental concepts to describe how wired
and wireless LANs work. We first summarize the major components of LANs and then describe the two
most commonly used LAN technologies: wired and wireless Ethernet. The chapter ends with a discussion
of how to design LANs and how to improve LAN performance.
OBJECTIVES
Understand the major components of LANs
Understand the best practice recommendations for LAN design
Be able to design wired Ethernet LANs
Be able to design wireless Ethernet LANs
Be able to improve LAN performance
OUTLINE
7.1 Introduction
7.2 LAN Components
7.2.1 Network Interface Cards
7.2.2 Network Circuits
7.2.3 Network Hubs, Switches, and Access Points
7.2.4 Network Operating Systems
7.3 Wired Ethernet
7.3.1 Topology
7.3.2 Media Access Control
7.3.3 Types of Ethernet
7.4 Wireless Ethernet
7.4.1 Topology
7.4.2 Media Access Control
7.4.3 Wireless Ethernet Frame Layout
7.4.4 Types of Wireless Ethernet
7.4.5 Security
7.5 The Best Practice LAN Design
7.5.1 Designing User Access with Wired Ethernet
7.5.2 Designing User Access with Wireless Ethernet
7.5.3 Designing the Data Center
7.5.4 Designing the e-Commerce Edge


7.5.5 Designing the SOHO Environment
7.6 Improving LAN Performance
7.6.1 Improving Server Performance
7.6.2 Improving Circuit Capacity
7.6.3 Reducing Network Demand
7.7 Implications for Cyber Security
Summary
7.1 INTRODUCTION
This chapter focuses on the first major network architecture component: the local area networks (LANs)
that provide users access to the network. Most large organizations have numerous wired and wireless
LANs connected by backbone networks. In this chapter, we discuss the fundamental components of a
LAN, along with two technologies commonly used in LANs—traditional wired Ethernet (IEEE 802.3),
which is commonly used to connect desktop computers, and wireless Ethernet (IEEE 802.11, commonly
called Wi-Fi), which often is used to connect laptop computers and mobile devices. There used to be many
different types of LAN technologies, but gradually the world has changed so that Ethernet dominates. The
majority of LAN design is done for the LANs that enable users to access the network, whether wired or
wireless, because there are more of these LANs than any other type. Therefore, this chapter focuses on the
design of these access LANs. However, the data center and e-commerce edge also use LANs, so we include
sections on the unique design needs of these two network architecture components.
A Day in the Life: LAN Administrator
Most days start the same way. The LAN administrator arrives early in the morning before most
people who use the LAN. The first hour is spent checking for problems. All the network hardware
and servers in the server room receive routine diagnostics. All the logs for the previous day are
examined to find problems. If problems are found (e.g., a crashed hard disk), the next few hours are


spent fixing them. Next, the daily backups are done. This usually takes only a few minutes, but
sometimes a problem occurs and it takes an hour.
The next step is to see if there are any other activities that need to be performed to maintain the
network. This involves checking email for security alerts (e.g., Windows updates and antivirus
updates). If critical updates are needed, they are done immediately. There are usually emails from
several users that need to be contacted, concerning either problems with the LAN or requests for
new hardware or software to be installed. These new activities are prioritized into the work queue.
And then the real work begins. Work activities include tasks such as planning for the next roll out of
software upgrades. This involves investigating the new software offerings, identifying what hardware
platforms are required to run them, and determining which users should receive the upgrades. It
also means planning for and installing new servers or network hardware such as firewalls.
Of course, some days can be more exciting than others. When a new virus hits, everyone is involved
in cleaning up the compromised computers and installing security patches on the other computers.
Sometimes, virus attacks can be fun when you see that your security settings work and beat the virus.
Source: With thanks to Steve Bushert.
7.2 LAN COMPONENTS
There are several components in a traditional LAN (Figure 7-1). The first two are the client computer and
the server. Clients and servers have been discussed in Chapter 2, so they are not discussed further here.
The other components are network interface cards (NICs), network circuits, hubs/switches/access points,


and the network operating system.
7.2.1 Network Interface Cards
The network interface card (NIC) is used to connect the computer to the network cable in a wired
network and is one part of the physical layer connection among the computers in the network. In a
wireless network, the NIC is a radio transmitter that sends and receives messages on a specific radio
frequency. All desktop computers have a wired NIC built in, while virtually all laptops have both a wired
NIC and a wireless NIC. You can purchase a wireless NIC for a desktop computer (often as a USB device).
FIGURE 7-1 Local area network components
7.2.2 Network Circuits
Each computer must be physically connected by network circuits to the other computers in the network.
Wired LANs
Most LANs are built with unshielded twisted-pair (UTP) cable, shielded twisted-pair (STP)
cable, or fiber-optic cable. (Common cable standards are discussed in the following box. We should
add that these cable standards specify the minimum quality cable required; it is possible, for example, to
use category 5e UTP cable that is rated for 1,000 Mbps in a LAN that runs at 100 Mbps.)
Many LANs use UTP cable. Its low cost makes it very useful. STP is only used in special areas that produce
electrical interference, such as factories near heavy machinery or hospitals near MRI scanners.
Fiber-optic cable is even thinner than UTP wire and therefore takes far less space when cabled throughout
a building. It is also much lighter, weighing less than 10 pounds per 1,000 feet. Because of its high
capacity, fiber-optic cabling is perfect for BNs, although it is beginning to be used in LANs.
Wireless LANs


Wireless LANs
Wireless LANs (WLANs) use radio transmissions to send data between the NIC and the access point
(AP). Most countries (but not all) permit WLANs to operate in two frequency ranges: the 2.4 and 5 GHz
range. These same frequency ranges can be used by cordless phones and baby monitors, which means that
your WLAN and your cordless phone may interfere with each other. Under ideal conditions, the radio
transmitters in the NICs and APs can transmit 100–150 meters (300–450 feet). In practice, the range is
much shorter as walls absorb the radio waves. The other problem is that as the distance from the AP
increases, the maximum speed drops, often very dramatically.
When we design a WLAN, it is important to ensure that the APs don’t interfere with each other. If all APs
transmitted on the same frequency, the transmissions of one AP would interfere with another AP.
Therefore, each AP is set to transmit on a different channel, very much like the different channels on
your TV. Each channel uses a different part of the 2.4 or 5 GHz frequency range so that there is no


interference among the different channels. When a computer first starts using the WLAN, its NIC
searches all available channels within the appropriate frequency range and then picks the channel that has
the strongest signal.
TECHNICAL FOCUS 7-1
Commonly Used Network Cable Standards
Name
Type Maximum Data Rate Often Used By
Cost1 ($/foot)
Category 12
UTP
1 Mbps
Telephone
0.04
Category 33
UTP
10 Mbps
10Base-T Ethernet
0.06
Category 5
UTP
100 Mbps
100Base-T Ethernet
0.07
Category 5
STP
100 Mbps
100Base-T Ethernet
0.18
Category 5e4
UTP
1 Gbps
1000Base-T Ethernet
0.10
Category 65
UTP
10 Gbps
10GBase-T
0.20
Category 76
STP
100 Gbps
100GBase-T
0.35
OM1 (62.5/125) Fiber
10 Gbps
10 GbE Ethernet
0.45
OM5 (50/125)
Fiber
100 Gbps
100 GbE Ethernet
0.65
Notes
1These costs are approximate costs for cable only (no connectors). They often change but will give you a sense of the relative
differences in costs among the different options.
2Category 1 is standard voice-grade twisted-pair wires, but it can also be used to support low-speed analog data transmission.
3Categories 2 and 4 cables are old standards and no longer in use today.
4Category 5e is an improved version of category 5 that has better insulation and a center plastic pipe inside the cable to keep the


individual wires in place and reduce noise from cross-talk, so that it is better suited to 1000Base-T.
5Category 6 cable has less ranges than the other types of cable (maximum 55 meters) but they are still long enough for many LAN
installations.
6Category 7 cables have a maximum range of 15 meters, which means they are typically used between networking devices that are near
each other (e.g., in a rack or data center).
7.2.3 Network Hubs, Switches, and Access Points
Network hubs and switches serve two purposes. First, they provide an easy way to connect network
cables. A hub or a switch can be thought of as a junction box, permitting new computers to be connected
to the network as easily as plugging a power cord into an electrical socket. Each connection point where a
cable can be plugged in is called a port. Each port has a unique number. Switches can be designed for use
in small-office, home-office (SOHO) environments (see Figure 7-2a) or for large enterprise
environments (see Figure 7-2b).
Simple hubs and switches are commonly available in 4-, 8-, 16-, and 24-port sizes, meaning that they
provide anywhere between 4 and 24 ports into which network cables can be plugged. When no cables are
plugged in, the signal bypasses the unused port. When a cable is plugged into a port, the signal travels
down the cable as though it were directly connected to the hub or switch. Some switches also enable
different types of cables to be connected and perform the necessary conversions (e.g., twisted-pair cable to
coaxial cable and twisted-pair cable to fiber-optic cable).


FIGURE 7-2 LAN switches
MANAGEMENT FOCUS 7-1
Cable Problems at the University of Georgia
Like many organizations, the Terry College of Business at the University of Georgia is headquartered
in a building built before the computer age. When local area network cabling was first installed in the
early 1980s, no one foresaw the rapid expansion that was to come. Cables and hubs were installed
piecemeal to support the needs of the handful of early users.
The network eventually grew far beyond the number of users it was designed to support. The
network cabling gradually became a complex, confusing, and inefficient mess. There was no logical
pattern for the cables, and there was no network cable plan. Worse still, no one knew where all the
cables and hubs were physically located. Before a new user was added, a network technician had to
open up a ceiling and crawl around to find a hub. Hopefully, the hub had an unused port to connect
the new user, or else the technician would have to find another hub with an empty port.
To complicate matters even more, asbestos was discovered. Now network technicians could not open
the ceiling and work on the cable unless asbestos precautions were taken. This meant calling in the
university’s asbestos team and sealing off nearby offices. Installing a new user to the network (or
fixing a network cable problem) now took 2 days and cost $2,000.
The solution was obvious. The university spent $400,000 to install new category 5 twisted-pair cable
in every office and to install a new high-speed fiber-optic backbone network between network
segments.
Second, hubs and switches act as repeaters. Signals can travel only so far in a network cable before they
attenuate and can no longer be recognized. (Attenuation was discussed in Chapter 4.) All LAN cables are


rated for the maximum distance they can be used (typically 100 meters for twisted-pair cable and 400
meters to several kilometers for fiber-optic cable).
A wireless access point (AP) is a radio transceiver that plays the same role as a hub or switch in wired
Ethernet LANs. It enables the computers near it to communicate with each other, and it also connects
them into wired LANs, typically using 100Base-T or 1000Base-T. All NICs in the WLAN transmit their


frames to the AP, and then the AP retransmits the frames over the wireless network or over the wired
network to their destination. Therefore, if a frame has to be transmitted from one wireless computer to
another, it is transmitted twice, once from the sender to the AP and then from the AP to the destination.
At first glance, this may seem a bit strange because it doubles the number of transmissions in the WLAN.
However, very few frames are ever sent from client computer to client computer in a WLAN. Most frames
are exchanged between client computers and a server of some kind. Therefore, a server should never be
placed on a WLAN because client computers cannot reach it directly but have to communicate with it via
the AP. Even if they are intended to serve clients on a WLAN, they should always be placed on the wired
portion of the LAN.
Figure 7-3a shows an AP for use in SOHO environments. This AP is wired into the regular Ethernet LAN
and has a separate power supply that is plugged into a normal electrical outlet. Figure 7-3b shows an AP
for use in large enterprises. It is also wired into the regular Ethernet LAN, but it uses power over
Ethernet (POE) so it needs no external power; the power is provided from a POE switch over the
unused wires in a category 5/5e cable. POE APs are more expensive, but can be located anywhere you can
run Cat 5/5e cable, even if there are no power outlets nearby.
Most WLANs are installed using APs that have omnidirectional antennas, which means that the
antenna transmits in all directions simultaneously. Some antennas are built into the AP itself, while
others stick up above it. One common omnidirectional antenna is the dipole antenna shown in Figure 7-


3a; others are built into the AP box, as is Figure 7-3b.
The other type of antenna that can be used on APs is the directional antenna, which, as the name
suggests, projects a signal only in one direction. Because the signal is concentrated in a narrower, focused
area, the signal is stronger and therefore will carry farther than the signal from an AP using an
omnidirectional antenna. Directional antennas are most often used on the inside of an exterior wall of a
building, pointing to the inside of the building. This keeps the signal inside the building (to reduce
security issues) and also has the benefit of increasing the range of the AP.
Many wireless routers are sold for use in SOHO environments. The wireless routers are both a wireless AP
and a router, and many also contain a 1000Base-T switch. It is important not to use the term wireless
router when you mean a wireless AP.
FIGURE 7-3 Wireless access points
MANAGEMENT FOCUS 7-2
Managing Network Cabling
You must consider a number of items when installing cables or when performing cable maintenance.
You should do the following:
Perform a physical inventory of any existing cabling systems and document those findings in the
network cable plan.


Properly maintain the network cable plan. Always update cable documentation immediately on
installing or removing a cable or hub. Insist that any cabling contractor provide “as-built” plans
that document where the cabling was actually placed, in case of minor differences from the
construction plan.
Establish a long-term plan for the evolution of the current cabling system to whatever cabling
system will be in place in the future.
Obtain a copy of the local city fire codes and follow them. For example, cables used in airways
without conduit need to be plenum-certified (i.e., covered with a fire-retardant jacket).
Conceal all cables as much as possible to protect them from damage and for security reasons.
Properly number and mark both ends of all cable installations as you install them. If a
contractor installs cabling, always make a complete inspection to ensure that all cables are
labeled.
7.2.4 Network Operating Systems
The network operating system (NOS) is the software that controls the network. Every NOS provides
two sets of software: one that runs on the network server(s) and one that runs on the network client(s).
The server version of the NOS provides the software that performs the functions associated with the data
link, network, and application layers and usually the computer’s own operating system. The client version
of the NOS provides the software that performs the functions associated with the data link and the
network layers and must interact with the application software and the computer’s own operating system.
Most NOSs provide different versions of their client software that run on different types of computers, so
that Windows computers, for example, can function on the same network as Apple computers. In most
cases (e.g., Windows and Linux), the client NOS software is included with the operating system itself.
NOS Server Software


NOS Server Software
The NOS server software enables the file server, print server, or database server to operate. In addition to
handling all the required network functions, it acts as the application software by executing the requests
sent to it by the clients (e.g., copying a file from its hard disk and transferring it to the client, printing a file
on the printer, executing a database request, and sending the result to the client). NOS server software
replaces the normal operating system on the server. By replacing the existing operating system, it provides
better performance and faster response time because a NOS is optimized for its limited range of
operations. The most commonly used NOS are Windows Server and Linux.
NOS Client Software
The NOS software running at the client computers provides the data link layer and network layer. Most
operating systems today are designed with networking in mind. For example, Windows provides built-in
software that will enable it to act as a client computer with a Windows Server.
One of the most important functions of a NOS is a directory service. Directory services provide
information about resources on the network that are available to the users, such as shared printers,
shared file servers, and application software. A common example of directory services is Microsoft’s
Active Directory Service (ADS).
Active Directory Service works in much the same manner as TCP/IP’s DNS service, and in fact ADS
servers, called domain controllers, can also act as DNS servers. Network resources are typically
organized into a hierarchical tree. Each branch on the tree contains a domain, a group of related
resources. For example, at a university, one domain might be the resources available within the business
school, and another domain might be the resources in the computer science school, while another might
be in the medical school. Domains can contain other domains, and in fact the hierarchical tree of domains


within one organization can be linked to trees in other organizations to create a forest of shared network
resources.
Within each domain, there is a server (the domain controller) that is responsible for resolving address
information (much like a DNS server resolves address information on the Internet). The domain


controller is also responsible for managing authorization information (e.g., who is permitted to use each
resource) and making sure that resources are available only to authorized users. Domain controllers in the
same tree (or forest) can share information among themselves, so that a domain controller in one part of
the tree (or forest) can be configured to permit access to resources to any user that has been approved by
another domain controller in a different part of the tree (or forest).
If you login to a Microsoft server or domain controller that provides ADS, you can see all network
resources that you are authorized to use. When a client computer wishes to view available resources or
access them, it sends a message using an industry standard directory protocol called lightweight
directory access protocol (LDAP) to the ADS domain controller. The ADS domain controller resolves
the textual name in the LDAP request to a network address and—if the user is authorized to access the
resource—provides contact information for the resource.
Network Profiles
A network profile specifies what resources on each server are available on the network for use by other
computers and which devices or people are allowed what access to the network. The network profile is
normally configured when the network is established and remains in place until someone makes a change.
In a LAN, the server hard disk may have various resources that can or cannot be accessed by a specific
network user (e.g., data files and printers). Furthermore, a password may be required to grant network
access to the resources.
If a device such as a hard disk on one of the network’s computers is not included on the network profile, it
cannot be used by another computer on the network. For example, if you have a hard disk (C) on your
computer and your computer is connected to this LAN but the hard disk is not included on the network
profile assignment list, then no other computer can access that hard disk.


profile assignment list, then no other computer can access that hard disk.
In addition to profiling disks and printers, there must be a user profile for each person who uses the LAN,
to add some security. Each device and each user is assigned various access codes, and only those users
who log in with the correct code can use a specific device. Most LANs keep audit files to track who uses
which resource. Security is discussed in Chapter 9.
7.3 WIRED ETHERNET
Almost all LANs installed today use some form of Ethernet. Ethernet was originally developed by DEC,
Xerox, and Intel but has since become a standard formalized by the IEEE as IEEE 802.3. The IEEE
802.3 version of Ethernet is slightly different from the original version but the differences are minor.
Likewise, another version of Ethernet has also been developed that differs slightly from the 802.3
standard.
Ethernet is a layer 2 protocol, which means it operates at the data link layer. Every Ethernet LAN needs
hardware at layer 1, the physical layer, that matches the requirements of the Ethernet software at layer 2.
7.3.1 Topology
Topology is the basic geometric layout of the network—the way in which the computers on the network
are interconnected. It is important to distinguish between a logical topology and a physical topology. A
logical topology is how the network works conceptually, much like a logical data flow diagram (DFD) or
logical entity relation diagram (ERD) in systems analysis and design or database design. A physical
topology is how the network is physically installed, much like a physical DFD or physical ERD.
Hub-Based Ethernet
Hub-based Ethernet is seldom used by companies today, but cheap consumer-grade hubs are still
available. Knowledge about hub-based Ethernet will be useful when we discuss wireless Ethernet. When


available. Knowledge about hub-based Ethernet will be useful when we discuss wireless Ethernet. When
we use hubs, Ethernet’s logical topology is a bus topology. All computers are connected to one half-
duplex circuit running the length of the network that is called the bus. The top part of Figure 7-4 shows
Ethernet’s logical topology. All frames from any computer flow onto the central cable (or bus) and
through it to all computers on the LAN. Every computer on the bus receives all frames sent on the bus,
even those intended for other computers. Before processing incoming frames, the Ethernet software on


each computer checks the data link layer address and processes only those frames addressed to that
computer.
The bottom part of Figure 7-4 shows the physical topology of an Ethernet LAN when a hub is used. From
the outside, an Ethernet LAN appears to be a star topology, because all cables connect to the central hub.
Nonetheless, it is logically a bus.
With hubs, all computers share the same multipoint circuit and must take turns using it. This shared
multipoint circuit is often called a collision domain, because if two computers ever did accidentally
transmit at the same time, there would be a collision. When one computer transmits, all the other
computers must wait, which is very inefficient. Because all frames are sent to all computers in the same
collision domain, security is a problem because any frame can be read by any computer. Most companies
don’t use hub-based Ethernet today, but products are still available and are very cheap. Wireless Ethernet,
which we discuss in a later section, works much the same as hub-based Ethernet.
FIGURE 7-4 Ethernet topology using hubs
Switch-Based Ethernet
Almost all corporate installations of Ethernet today use switches, not hubs. When we use switches,
Ethernet’s topology is a logical star and a physical star (Figure 7-5). From the outside, the switch looks
almost identical to a hub, but inside, it is very different. A switch is an intelligent device with a small
computer built in that is designed to manage a set of separate point-to-point circuits. That means that
each circuit connected to a switch is not shared with any other devices; only the switch and the attached
computer use it. The physical topology looks essentially the same as Ethernet’s physical topology: a star.
On the inside, the logical topology is a set of separate point-to-point circuits, also a star. Many switches


support full duplex circuits, meaning that each circuit can simultaneously send and receive.
When a switch receives a frame from a computer, it looks at the address on the frame and retransmits the
frame only on the circuit connected to that computer, not to all circuits as a hub would. Therefore, no
computer needs to wait because another computer is transmitting; every computer can transmit at the
same time, resulting in much faster performance. As a result, each port on the switch is in a separate


collision domain, and there are only two devices on it: the switch and the computer/device on the other
end of the cable. Today, no one buys a hub unless she or he can’t afford a switch.
So how does a switch know which circuit is connected to what computer? The switch uses a forwarding
table that is very similar to the routing tables discussed in Chapter 5. The table lists the Ethernet address
of the computer connected to each port on the switch. When the switch receives a frame, it compares the
destination address on the frame to the addresses in its forwarding table to find the port number on which
it needs to transmit the frame. Because the switch uses the Ethernet address to decide which port to use
and because Ethernet is a data link layer or layer 2 protocol, this type of switch is called a layer 2 switch.
FIGURE 7-5 Ethernet topology using switches
When switches are first turned on, their forwarding tables are empty; they do not know what Ethernet
address is attached to what port. Switches learn addresses to build the forwarding table. When a switch
receives a frame, it reads the frame’s data link layer source address and compares this address to its
forwarding table. If the address is not in the forwarding table, the switch adds it, along with the port on
which the frame was received.
If a switch receives a frame with a destination address that is not in the forwarding table, the switch must
still send the frame to the correct destination. In this case, it must retransmit the frame to all ports, except
the one on which the frame was received. The attached computers, being Ethernet and assuming they are
attached to a hub, will simply ignore all frames not addressed to them. The one computer for whom the
frame is addressed will recognize its address and will process the frame, which includes sending an


frame is addressed will recognize its address and will process the frame, which includes sending an
acknowledgement (ACK) or a negative acknowledgement (NAK) back to the sender. When the switch
receives the ACK or NAK, it will add this computer’s address and the port number on which the ACK or
NAK was received to its forwarding table and then send the ACK or NAK on its way.
So, for the first few minutes until the forwarding table is complete, the switch acts like a hub. But as its
forwarding table becomes more complete, it begins to act more and more like a switch. In a busy network,
it takes only a few minutes for the switch to learn most addresses and match them to port numbers. To


make a switch work faster, the most active connections are placed on the top of the forwarding table. If a
computer is not communicating for more than 300 seconds, its entry is usually removed from the
forwarding table.
There are three modes in which switches can operate. The first is cut-through switching. With cut-
through switching, the switch begins to transmit the incoming packet on the proper outgoing circuit as
soon as it has read the destination address in the frame. In other words, the switch begins transmitting
before it has received the entire frame. The advantage of this is low latency (the time it takes a device
from receiving a frame to transmitting it) and results in a very fast network. The disadvantage is that the
switch begins transmitting before it has read and processed the frame check sequence at the end of the
frame; the frame may contain an error, but the switch will not notice until after almost all of the frame has
been transmitted. Cut-through switching can only be used when the incoming data circuit has the same
data rate as the outgoing circuit.
With the second switching mode, called store-and-forward switching, the switch does not begin
transmitting the outgoing frame until it has received the entire incoming frame and has checked to make
sure it contains no errors. Only after the switch is sure there are no errors does the switch begin
transmitting the frame on the outgoing circuit. If errors are found, the switch simply discards the frame.
This mode prevents an invalid frame from consuming network capacity, but provides higher latency and
thus results in a slower network (unless many frames contain errors). Store-and-forward switching can be
used regardless of whether the incoming data circuit has the same data rate as the outgoing circuit
because the entire frame must be stored in the switch before it is forwarded on its way.
The final mode, called fragment-free switching, lies between the extremes of cut-through switching