The final mode, called fragment-free switching, lies between the extremes of cut-through switching
and store-and-forward switching. With fragment-free switching, the first 64 bytes of the frame are read
and stored. The switch examines the first 64 bytes (which contain all the header information for the
frame), and if all the header data appear correct, the switch presumes that the rest of the frame is error
free and begins transmitting. Fragment-free switching is a compromise between cut-through and store-
and-forward switching because it has higher latency and better error control than cut-through switching,
but lower latency and worse error control than store-and-forward switching. Most switches today use cut-
through or fragment-free switching.
7.3.2 Media Access Control
When several computers share the same collision domain (i.e., multipoint circuit), it is important to
control their access to the media. If two computers on the same circuit transmit at the same time, their
transmissions will become garbled. These collisions must be prevented, or if they do occur, there must be
a way to recover from them. This is called media access control.
Ethernet uses a contention-based media access control technique called Carrier Sense Multiple
Access with Collision Detection (CSMA/CD). CSMA/CD, like all contention-based techniques, is
very simple in concept: wait until the circuit is free and then transmit. Computers wait until no other
devices are transmitting, then transmit their frames. As an analogy, suppose you are talking with a small
group of friends (four or five people). As the discussion progresses, each person tries to grab the floor
when the previous speaker finishes. Usually, the other members of the group yield to the first person who
jumps in right after the previous speaker.
Ethernet’s CSMA/CD protocol can be termed “ordered chaos.” As long as no other computer attempts to


transmit at the same time, everything is fine. However, it is possible that two computers located some
distance from each other can both listen to the circuit, find it empty, and begin simultaneously. This
simultaneous transmission is called a collision. The two frames collide and destroy each other.
The solution to this is to listen while transmitting, better known as collision detection (CD). If the NIC
detects any signal other than its own, it presumes that a collision has occurred and sends a jamming
signal. All computers stop transmitting and wait for the circuit to become free before trying to retransmit.
The problem is that the computers that caused the collision could attempt to retransmit at the same time.
To prevent this, each computer waits a random amount of time after the colliding frame disappears before
attempting to retransmit. Chances are both computers will choose a different random amount of time and
one will begin to transmit before the other, thus preventing a second collision. However, if another
collision occurs, the computers wait a random amount of time before trying again. This does not eliminate


collisions completely, but it reduces them to manageable proportions.
7.3.3 Types of Ethernet
Figure 7-6 summarizes the many different types of Ethernet in use today. The 10Base-T standard
revolutionized Ethernet and made it the most popular type of LANin the world. Today, 100Base-T and
1000Base-T are the most common forms of Ethernet.
Other types of Ethernet include 1000Base-F (which runs at 1 Gbps and is sometimes called 1 GbE), 10
GbE (10 Gbps), 40 GbE (40 Gbps), and 100 GbE (100 Gbps). They can use Ethernet’s traditional half-
duplex approach, but most are configured to use full duplex. Each is also designed to run over fiber-optic
cables, but some may also use traditional twisted-pair cables (e.g., Cat 5e). For example, two common
versions of 1000Base-F are 1000Base-LX and 1000Base-SX, both of which use fiber-optic cable, running
up to 440 and 260 meters, respectively; 1000Base-T, which runs on four pairs of category 5 twisted-pair
cable, but only up to 100 meters; and 1000Base-CX, which runs up to 24 meters on one category 5 cable.
Similar versions of 10 and 40 GbE that use different media are also available.
FIGURE 7-6 Types of Ethernet
MANAGEMENT FOCUS 7-3
Moving to Gigabit Ethernet
Kotak Mahindra Group, one of India’s leading financial services provider, offers comprehensive
financial solutions such as commercial banking, stock brokering, mutual funds, life insurance, and


financial solutions such as commercial banking, stock brokering, mutual funds, life insurance, and
investment banking. They employ 20,000 people at more than 1,300 branches in India and around
the world.
Because of the high network traffic in their main data center location, Kotak installed gigabit


Ethernet switches in their core network. The switches provide 512 ports of 10 GbE, with the ability to
upgrade to 40 and 100 Gbps. The switches have an internal switching capacity of 15 Tbps (15 trillion
bits per second), so there is room for growth.
Source: Adapted from “Kotak Group Builds State-of-the-Art Data Center on Cisco Nexus 7000 Switch,” Cisco Customer Case
Study, Cisco Systems.
Some organizations use 10/100/1000 Ethernet, which is a hybrid that can run at any of these three
speeds; 10/100/1000 NICs and switches detect the signal transmitted by the computer or device on the
other end of the cable and will use 10 Mbps, 100 Mbps, or 1 Gbps, depending on which the other device
uses.
7.4 WIRELESS ETHERNET
Wireless Ethernet (commonly called Wi-Fi) is the commercial name for a set of standards developed
by the IEEE 802.11 standards group. A group of vendors selling 802.11 equipment trademarked the
name Wi-Fi to refer to 802.11 because they believe that consumers are more likely to buy equipment with
a catchier name than 802.11. Wi-Fi is intended to evoke memories of Hi-Fi, as the original stereo music
systems in the 1960s were called.
The 802.11 family of technologies is much like the Ethernet family. They reuse many of the Ethernet 802.3
components and are designed to connect easily into Ethernet LANs. For these reasons, IEEE 802.11 is
often called wireless Ethernet. Just as there are several different types of Ethernet (e.g., 10Base-T,
100Base-T, and 1000Base-T), there are several different types of 802.11.


7.4.1 Topology
The logical and physical topologies of Wi-Fi are the same as those of hub-based Ethernet: a physical star
and a logical bus. There is a central AP to which all computers direct their transmissions (star), and the
radio frequencies are shared (bus) so that all computers must take turns transmitting.
7.4.2 Media Access Control
Media access control in Wi-Fi is Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA), which is similar to the contention-based CSMA/CD approach used by Ethernet. With
CSMA/CA, computers listen before they transmit, and if no one else is transmitting, they proceed with
transmission. Detecting collisions is more difficult in radio transmission than in transmission over wired
networks, so Wi-Fi attempts to avoid collisions to a greater extent than traditional Ethernet. CSMA/CA
has two media access control approaches. However, before a computer can transmit in a WLAN, it must
first establish an association with a specific AP, so that the AP will accept its transmissions.
Associating with an AP
Searching for an available AP is called scanning, and a NIC can engage in either active or passive
scanning. During active scanning, a NIC transmits a special frame called probe frame on all active
channels on its frequency range. When an AP receives a probe frame, it responds with a probe response
that contains all the necessary information for a NIC to associate with it. A NIC can receive several probe
responses from different APs. It is up to the NIC to choose with which AP to associate. This usually
depends on the speed rather than distance from an AP. Once a NIC associates with an AP, they start
exchanging packets over the channel that is specified by the AP.
During passive scanning, the NIC listens on all channels for a special frame called a beacon frame that


During passive scanning, the NIC listens on all channels for a special frame called a beacon frame that
is sent out by an AP. The beacon frame contains all the necessary information for a NIC to associate with
it. Once a NIC detects this beacon frame, it can decide to associate with it and start communication on the
frequency channel set by the AP.
Distributed Coordination Function
The first media access control method is the distributed coordination function (DCF) (also called
physical carrier sense method because it relies on the ability of computers to physically listen before


they transmit). With DCF, each frame in CSMA/CA is sent using stop-and-wait ARQ. After the sender
transmits one frame, it immediately stops and waits for an ACK from the receiver before attempting to
send another frame. When the receiver of a frame detects the end of the frame in a transmission, it waits a
fraction of a second to make sure the sender has really stopped transmitting, and then immediately
transmits an ACK (or a NAK). The original sender can then send another frame, stop and wait for an ACK,
and so on. While the sender and receiver are exchanging frames and ACKs, other computers may also
want to transmit. So when the sender ends its transmission, you might ask, why doesn’t some other
computer begin transmitting before the receiver can transmit an ACK? The answer is that the physical
carrier sense method is designed so that the time the receiver waits after the frame transmission ends
before sending an ACK is significantly less time than the time a computer must listen to determine that no
one else is transmitting before initiating a new transmission. Thus, the time interval between a frame and
the matching ACK is so short that no other computer has the opportunity to begin transmitting.
Point Coordination Function
The second media access control technique is called the point coordination function (PCF) (also
called the virtual carrier sense method). Not all manufacturers have implemented PCF in their APs.
DCF works well in traditional Ethernet because every computer on the shared circuit receives every
transmission on the shared circuit. However, in a wireless environment, this is not always true. A
computer at the extreme edge of the range limit from the AP on one side may not receive transmissions
from a computer on the extreme opposite edge of the AP’s range limit. In Figure 7-1, all computers may be


within the range of the AP, but may not be within the range of each other. In this case, if one computer
transmits, the other computer on the opposite edge may not sense the other transmission and transmit at
the same time causing a collision at the AP. This is called the hidden node problem because the computers
at the opposite edges of the WLAN are hidden from each other.
When the hidden node problem exists, the AP is the only device guaranteed to be able to communicate
with all computers on the WLAN. Therefore, the AP must manage the shared circuit using a controlled-
access technique, not the contention-based approach of traditional Ethernet. With this approach, any
computer wishing to transmit first sends a request to send (RTS) to the AP, which may or may not be
heard by all computers. The RTS requests permission to transmit and to reserve the circuit for the sole use
of the requesting computer for a specified time period. If no other computer is transmitting, the AP
responds with a clear to send (CTS), specifying the amount of time for which the circuit is reserved for
the requesting computer. All computers hear the CTS and remain silent for the specified time period. The
virtual carrier sense method is optional. It can always be used, never used, or used just for frames
exceeding a certain size, as set by the WLAN manager.
Controlled-access methods provide poorer performance in low-traffic networks because computers must
wait for permission before transmitting rather than just waiting for an unused time period. However,
controlled-access techniques work better in high-traffic WLANs, because without controlled access, there
are many collisions. Think of a large class discussion in which the instructor selects who will speak
(controlled access) versus one in which any student can shout out a comment at any time.
7.4.3 Wireless Ethernet Frame Layout


7.4.3 Wireless Ethernet Frame Layout
An 801.11 data frame is illustrated in Figure 7-7. We notice two major differences when we compare the
802.11 frame to the 802.3 frame used in wired Ethernet (see Chapter 4). First, the wireless Ethernet frame
has four address fields rather than two like the wired Ethernet. These four address fields are source
address, transmitter address, receiver address, and destination address. The source and destination
address have the same meaning as in wired Ethernet. However, because every NIC has to communicate
via an AP (it cannot directly communication with another NIC), there is a need to add the address of the
AP and also any other device that might be needed to transmit the frame. To do this, the transmitter and
received address fields are used.
Second, there is new field called sequence control that indicates how a large frame is fragmented—split
into smaller pieces. Recall that in wired networks this is done by the transport layer, not the data link
layer. Moving the segmentation to the data link layer for wireless makes the transmission transparent to
the higher layers. The price, however, is less efficiency because of the size of the frame and thus also a
higher error rate.


7.4.4 Types of Wireless Ethernet
Wi-Fi is one of the fastest changing areas in networking. There are many different versions of Wi-Fi; all
but the last two or three versions are obsolete but may still be in use in some companies. All the different
types are backward compatible, which means that laptops and APs that use new versions can
communicate with laptops and APs that use older versions. However, this backward compatibility comes
with a price. These old laptops become confused when other laptops operate at high speeds near them, so
when an AP detects the presence of a laptop using an old version, it prohibits laptops that use the newer
versions from operating at high speeds. Thus, one old laptop will slow down all the other new laptops
around it.
We focus on the three latest versions of 802.11.
FIGURE 7-7 A wireless Ethernet frame
802.11n
IEEE 802.11n is an obsolete version, but many organizations continue to use it because it is cheap. Under
perfect conditions, it provides three channels of 450 Mbps each with a maximum range of 100 meters or
300 feet, although in practice both the speed and range are lower. Older versions of 802.11n provide a
maximum speed of 300 Mbps. You would probably think the three channels are numbered 1, 2, and 3, but
they aren’t. The three channels are numbered 1, 6, and 11, because the underlying technology provides 11
channels, with channels 1, 6, and 11 designed so they do not overlap and cause interference with each
other. It is also possible to configure a dual-band AP so it combines all the channels into one “dual-
band” channel that provides 600 Mbps.
802.11ac


band” channel that provides 600 Mbps.
802.11ac
IEEE 802.11ac is the current version, and it has a number of differences from the earlier version. This
version runs in two different frequency spectrums simultaneously (2.4 and 5 GHz) to provide high speed
data rates. To make things more confusing, there are several different versions of the standard, and
vendors can choose what aspects to implement. Some vendors have introduced products that conform to
the standard but operate only in the 2.4 GHz spectrum. These products offer fewer channels and/or
slower data rates, so we now need to read product labels very carefully before buying equipment. One
important innovation is that the RTS/CTS media access control is sent on a separate frequency range, so
that it does not interfere with data transmission. The default modulation technique is 256-QAM (8 bits
per symbol), increased from 64-QAM (6 bits per symbol) in 802.11n, so the increase in data transmission
speed is greater than the increase in frequency range would suggest (see Chapter 3).
One version of the standard, the one most vendors have implemented as we write this, provides eight
channels each running at 433 Mbps with a maximum effective range of 50 meters (150 feet) under perfect
conditions. The actual throughput after you consider the efficiency of the data link protocol (see Chapter
4) is about 300 Mbps. As you get farther from the AP, the speed drops, so users will only see the
maximum speed within 20–30 meters of the AP, depending on the interference in the environment. At
maximum range, data rates are likely to be about 90 Mbps per channel (60 Mbps throughput).
Another version of the standard enables the user to configure the number of channels and the capacity


Another version of the standard enables the user to configure the number of channels and the capacity
each will have. This is made more complex by enabling each AP to have a different number of antennas,
with each additional antenna enabling faster speeds—but only if the devices attached to the AP also have
more antennas, which is usually not the case; most laptops have only two antennas. It also enables special
antennas that shape the radio beam, so that the signal is focused only in certain directions, to further
improve speed and quality. So technically, 802.11ac could provide one channel of 6.9 Gbps (or a
throughput of 4.9 Gbps), but only under perfect conditions, when talking to a nonstandard laptop.
802.11ax
IEEE 802.11ax (sometimes called Wi-FI 6) is newest version of wireless Ethernet that is being rolled out


in commercial products. This version runs in three different frequency spectrums (2.4, 5, and 6 GHz) to
provide very high speed data rates. It provides 9.6 Gbps across 8 channels or about 1.2 Gbps per channel
under prefect conditions. The entire data stream can be combined into one large channel. The default
modulation technique was improved so it now uses 1024-QAM (10 bits per symbol), which is one reason
for this increase (see Chapter 3). Range is limited to 35 meters, but the effective range is much less, likely
7–10 meters (20–30 feet). This range sounds very short, but is likely to be sufficient in large office
buildings where APs are installed close together because there are many users in the same area. For
example, at many universities, each classroom has its own AP, meaning an effective range of 10 meters is
fine.
One new innovation in 802.11ax is the ability for the AP to combine packets destined for different devices
into the same physical layer transmission. With prior versions of 802.11, each Ethernet packet was sent as
a separate transmission. Now the same physical layer transmission can contain Ethernet packets destined
to different computers. A second new innovation is the ability to “color” packets so an AP can tell that an
incoming packet is actually from another AP whose transmission range overlaps with its range, not one of
the client computers in its WLAN. This means the AP can ignore the incoming packet and thus save a little
bit of processing time. APs will also have the ability to detect overlapping APs and weaken their
transmission signal to shorten the range so there is less overlap.
7.4.5 Security
Security is important to all networks and types of technology, but it is especially important for wireless


Security is important to all networks and types of technology, but it is especially important for wireless
networks. With a WLAN, anyone walking or driving within the range of an AP (even outside the offices)
can begin to use the network.
Finding WLANs is quite simple. You just walk or drive around different office buildings with your WLAN-
equipped client computer and see if it picks up a signal. There are also many special-purpose software
tools available on the Internet that will enable you to learn more about the WLANs you discover, with the
intent of helping you to break into them. This type of wireless reconnaissance is often called wardriving
(see www.wardriving.com).
WEP
One older wireless security technique is Wired Equivalent Privacy (WEP). With WEP, the AP
requires the user to have a key to communicate with it. All data sent to and from the AP are encrypted so
that they can only be understood by computers or devices that have the key (encryption is discussed in
more detail in Chapter 11). If a computer does not have the correct WEP key, it cannot understand any
messages transmitted by the AP, and the AP will not accept any data that are not encrypted with the
correct key.
The WEP keys are produced dynamically, much like the way in which a DHCP server is used to
dynamically produce IP addresses. When an AP first discovers a new client computer, it requires the user
to log in before it will communicate with the client computer. The user ID and password supplied by the
user are transmitted to a login server, and if the server determines that they are valid, the server generates
a WEP key that will be used by the AP and client computer to communicate for this session. Once the
client logs out or leaves the WLAN, the WEP key is discarded, and the client must log in again and receive


a new WEP key.
WEP has a number of serious weaknesses, and most experts agree that a determined hacker can break
into a WLAN that uses only WEP security. A good way to think about WEP is that it is like locking your
doors when you leave: It won’t keep out a professional criminal, but it will protect against a casual thief.
WPA
Wi-Fi Protected Access (WPA) is a newer, more secure type of security. WPA works in ways similar to
WEP: Every frame is encrypted using a key, and the key can be fixed in the AP or can be assigned
dynamically as users login. The difference is that the WPA key is longer than the WEP key and thus is
harder to break. More importantly, the key is changed for every frame that is transmitted to the client.
Each time a frame is transmitted, the key is changed.


802.11i
802.11i (also called WPA2) is the newest, most secure type of WLAN security. The user logs in to a login
server to obtain the master key. Armed with this master key, the user’s computer and the AP negotiate a
new key that will be used for this session until the user leaves the WLAN. 802.11i uses the Advanced
Encryption Standard (AES) discussed in Chapter 11 as its encryption method.
MAC Address Filtering
With MAC address filtering, the AP permits the owner to provide a list of MAC addresses (i.e., layer 2
addresses). The AP only processes frames sent by computers whose MAC address is in the address list; if a
computer with a MAC address not in the list sends a frame, the AP ignores it. Unfortunately, this provides
no security against a determined hacker. There is software available that will change the MAC address on
a wireless NIC, so a determined hacker could use a packet sniffer (e.g., Wireshark) to discover a valid
MAC address and then use the software to change his MAC address to one the AP would accept. MAC
address filtering is like WEP; it will protect against a casual thief, but not against a professional.
MANAGEMENT FOCUS 7-4
Wi-Fi Access Points in Light Bulbs
One product that has the potential to revolutionize SOHO networking is the Wi-Fi light bulb. For
years, we’ve had LED bulbs that can be controlled over Wi-Fi. The new bulbs take this one step
further: each bulb becomes a Wi-Fi AP.
These light bulbs contain a Wi-Fi extender chip. They receive the Wi-Fi signal from your existing AP,
and then retransmit it. The current range is about 100 feet (30 meters). This means you put the first


Wi-Fi bulb in a light socket that is within range of your AP. Then you put the next bulb within 100
feet of the first bulb, and so on, until your entire house, patio, backyard, etc. is covered in Wi-Fi.
Anywhere you can plug in a light bulb, you can have Wi-Fi.
7.5 THE BEST PRACTICE LAN DESIGN
This section focuses on the design of wired and wireless LANs that provide network access to users. The
data center and e-commerce edge also use LANs, so we include sections on the unique needs of these two
network architecture components. The past few years have seen major changes in LAN technologies (e.g.,
gigabit Ethernet and high-speed wireless Ethernet). As technologies have changed and costs have
dropped, so too has our understanding of the best practice design for LANs.
One of the key questions facing network designers is the relationship between Wi-Fi and wired Ethernet.
The data rates for Wi-Fi have increased substantially with the introduction of each new version of 802.11,
so they are similar to the data rates offered by 100Base-T wired Ethernet. The key difference is that
100Base-T wired Ethernet using switches provides 100 Mbps to each user, whereas Wi-Fi shares its
available capacity among every user on the same AP, so as more users connect to the APs, the network
gets slower and slower.
Wi-Fi is considerably cheaper than wired Ethernet because the largest cost of LANs is not the equipment,
but in paying someone to install the cables. The cost to install a cable in an existing building is typically
between $150 and $400 per cable, depending on whether the cable will have to be run through drywall,
brick, ceilings, and so on. Installing cable in a new building during construction is cheaper, typically $50–
$100 per cable.


$100 per cable.
Most organizations today install wired Ethernet to provide access for desktop users and install Wi-Fi as
overlay networks. They build the usual switched Ethernet networks as the primary LAN, but they
also install Wi-Fi for laptops and mobile devices. Some organizations have begun experimenting with Wi-
Fi by moving groups of users off the wired networks onto Wi-Fi as their primary network to see whether
Wi-Fi is suitable as a primary network.


Today, we still believe the best practice is to use wired Ethernet for the primary LAN, with Wi-Fi as an
overlay network. However, this may change. Stay tuned.
MANAGEMENT FOCUS 7-5
Will Wi-Fi Replace Wired LANS?
As KPMG, one of the largest consulting firms in the world, began to build a new 2,800-person
headquarters near Amsterdam, KPMG’s IT group realized that their traditional wired network
approach would have required 18,000 cable runs, 55 chassis switches, and 260 LAN switches. The
up-front cost was expected to exceed $6 million, and the recurring operating costs would run into
the millions annually as well.
KPMG began to wonder if there was a better way. Could they build an entirely wireless network that
would meet their needs?
After careful analysis, KPMG decided they were not ready to go completely wireless. However, they
decided to shift a substantial portion of their traditionally wired users to wireless. They cut their
wired network by half and installed more than 500 802.11n APs throughout the new facility to
provide complete coverage for data and voice. The new network design cut the initial cost by $2
million and reduced annual operating costs by $750,000 per year.
The new design also delivered substantial green benefits. APs use about 5% of the electricity that 48-
port switches require for power and cooling. By eliminating half the switches, the new design
eliminated more than 350 metric tons of carbon dioxide emissions each year.
7.5.1 Designing User Access with Wired Ethernet
Many organizations today install switched 100Base-T or 1000Base-T over category 5e wiring for their
wired LANs. It is relatively low cost and fast.


wired LANs. It is relatively low cost and fast.
In the early days of LANs, it was common practice to install network cable wherever it was convenient.
Little long-term planning was done. The exact placement of the cables was often not documented, making
future expansion more difficult—you had to find the cable before you could add a new user.
With today’s explosion in LAN use, it is critical to plan for the effective installation and use of LAN
cabling. The cheapest point at which to install network cable is during the construction of the building;
adding cable to an existing building can cost significantly more. Indeed, the costs to install cable (i.e.,
paying those doing the installation and additional construction) are usually substantially more than the
cost of the hubs and switches, making it expensive to reinstall the cable if the cable plan does not meet the
organization’s needs.
Most buildings under construction today have a separate LAN cable plan, as they have plans for
electrical cables. Each floor has a data wiring closet that contains one or more network hubs or switches.
Cables are run from each room on the floor to this wiring closet.
7.5.2 Designing User Access with Wireless Ethernet
Selecting the best practice wireless technology is usually simple. You pick the newest one, cost permitting.
Today, 802.11ax is the newest standard, but in time, there will be a new one.
Designing the physical WLAN is more challenging than designing a wired LAN because the potential for
radio interference means that extra care must be taken in the placement of APs to ensure that their signals
do not overlap. With the design of LANs, there is considerable freedom in the placement of switches,
subject to the maximum limits to the length of network cables. In WLANs, however, the placement of the


APs needs to consider both the placement of other APs and the sources of interference in the building.
The physical WLAN design begins with a site survey. The site survey determines the feasibility of the
desired coverage, the potential sources of interference, the current locations of the wired network into


which the WLAN will connect, and an estimate of the number of APs required to provide coverage.
WLANs work very well when there is a clear line of sight between the AP and the wireless computer. The
more walls there are between the AP and the computer, the weaker the wireless signal becomes. The type
and thickness of the wall also has an impact; traditional drywall construction provides less interference
than does concrete block construction.
An AP with an omnidirectional antenna broadcasts in all directions. Its coverage area is a circle with a
certain radius. Wi-Fi has a long range, but real-world tests of Wi-Fi in typical office environments have
shown that data rates slow down dramatically when the distance from a laptop to the AP exceeds 50 feet.
Therefore, many wireless designers use a radius of 50 feet when planning traditional office environments,
which ensures access high-quality coverage.
One may design wireless LANs using this 50-foot-radius circle, but because most buildings are square, it
is usually easier to design using squares. Figure 7-8 shows that a 50-foot radius translates into a square
that is approximately 70 feet on each edge. For this reason, most designers plan wireless LANs using 50-
to 75-foot squares, depending on the construction of the building: smaller squares in areas where there
are more walls that can cause more interference and larger squares in areas with fewer walls. Each new
generation of wireless LAN technology has reduced the effective range of APs to increase their speed. So
some designers prefer to use 50-foot squares everywhere.
Figure 7-9 shows a sample building that has two parts. The lower-left corner is a 150 foot × 150 foot
square, while the rest of the building is a 150 foot × 450 foot rectangle. Let’s assume that the large


rectangle part is an open office environment, while the smaller part uses drywall. If we put two rows of
APs in the large rectangle part, we could probably space them so that each AP covered a 75-foot square.
This would take a total of 12 APs for this area (see Figure 7-9). This same spacing probably won’t work for
the small area with drywall, so we would probably design using 50-foot squares, meaning we need nine
APs in this area (see Figure 7-9).
When designing a wireless LAN, 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
where their signals overlapped—just like what happens on your car radio when two stations are in the
same frequency. Therefore, each AP is set to transmit on a different channel, very much like the different
channels on your TV. Figure 7-9 shows how we could set the APs to the three commonly used channels (1,
6, and 11) so that there is minimal overlap between APs using the same channel.


FIGURE 7-8 Design parameters for Wi-Fi access point range
FIGURE 7-9 A Wi-Fi design (the numbers indicate the channel numbers)


Suppose you had a conference room or classroom that needed several APs to provide adequate Wi-Fi for
everyone who will use it. You could put several APs in the same room and set them on different channels
so their signals did not interfere with each other. One challenge is managing the number of users on each
AP. Laptops and mobile phones connect to the AP with the strongest signal, which means most will
connect to the same AP. If too many users connect to one AP, it will get very busy and Wi-Fi speeds will be
slow, while the other APs in the room will be only lightly used. This occurs because standard APs are
autonomous and do not talk to each other. Each AP only responds to the devices that request access to it.
Most large companies install managed APs that are different than the SOHO APs we install in our
homes and apartments. Managed APs are wired into a Wi-Fi Controller (rather than a normal hub or
switch). They report what devices are attached to them and how busy they are to the controller, which
balances traffic across the APs it manages. If a laptop connects to a very busy AP when there are less busy
APs nearby, the controller will instruct the AP to deny access to the laptop and the laptop will
automatically try to connect to the next AP it sees. As a result, the number of devices connected to each AP
and the amount of traffic each receives is balanced across the set of APs managed by the controller, and
overall network performance improves.
After the initial design is complete, a site survey is done using a temporary AP and a computer or device
that can actually measure the strength of the wireless signal. The temporary AP is installed in the area as
called for in the initial design, and the computer or device is carried throughout the building measuring


called for in the initial design, and the computer or device is carried throughout the building measuring
the strength of the signal. Actually measuring the strength of the signal in the environment is far more
accurate than relying on estimated ranges.
Design becomes more difficult in a multistory building because the signals from the APs travel up and
down as well as in all horizontal directions. The design must include the usual horizontal mapping but
also an added vertical mapping to ensure that APs on different floors do not interfere with one another
(Figure 7-10). Because floors are usually thicker than walls, signals travel further horizontally than
vertically, making design a bit more difficult. It becomes even more difficult if your set of floors in a large
office tower is surrounded by APs of other companies. You have to design your network not to interfere
with theirs.
Most wireless LAN APs offer the ability to provide two separate wireless networks. The primary network is
secured by a password that is entered when you first connect to the network. This password is
remembered by the device so that you never have to enter the password a second time. This password
secures the access to the network, and all connections use some form of encryption, such as WPA2, so that
no one can read your messages (even if someone accesses the same AP using the same password). This
network is typically used by regular users of the network such as employees of an organization or the
homeowner in a SOHO network.


FIGURE 7-10 A Wi-Fi design in the three dimensions (the numbers indicate the channel numbers)
The second network is a guest network that is secured by a separate password that is entered on a Web
page when you first connect to the network. This network is not secure, meaning that other users with the
right hacking software can read the messages you send and receive. However, because the network will
not allow users on the network without the password, it means that access can be controlled so that only
authorized users have access. This network is typically used by guests who need temporary access. The
guest network is often configured so it provides slower speeds than the primary network, so if the AP gets
busy, it prioritizes traffic for regular users over traffic for guest users.
7.5.3 Designing the Data Center
The data center is where the organization houses its primary servers. In most large organizations, the data
center is huge because it contains the data center as well as the campus backbone switches and the
enterprise edge. Figure 7-11 shows the data center building at Indiana University. This building, which is
built partially underground to withstand an F5 tornado, is 87,000 square feet, of which 33,000 square
feet is used for servers. The servers can store about 50 petabytes of data (about 50 million gigabytes).
Designing the data center requires considerable expertise, because most data on a network flow from or to
the data center. In all large-scale networks today, servers are placed together in server farms or clusters,
which sometimes have hundreds of servers that perform the same task. Yahoo.com, for example, has more
than a thousand Web servers that do nothing but respond to Web search requests. In this case, it is
important to ensure that when a request arrives at the server farm, it is immediately forwarded to a server


that is not busy—or that is the least busy.


A special device called a load balancer or load balancing switch acts as a router at the front of the
server farm (Figure 7-12). All requests are directed to the load balancer at its IP address. When a request
hits the load balancer, it forwards it to one specific server using its IP address. Sometimes a simple round-
robin formula is used (requests go to each server one after the other in turn); in other cases, more
complex formulas track how busy each server actually is. If a server crashes, the load balancer stops
sending requests to it, and the network continues to operate without the failed server. Load balancing
makes it simple to add servers (or remove servers) without affecting users. You simply add or remove the
server(s) and change the software configuration in the load balancing switch; no one is aware of the
change.
FIGURE 7-11 The data center at Indiana University


FIGURE 7-12 Network with load balancer
Server virtualization is somewhat the opposite of server farms and load balancing. Server
virtualization is the process of creating several logically separate servers (e.g., a Web server, an email
server, and a file server) on the same physical computer. The virtual servers run on the same physical
computer but appear completely separate to the network (and if one crashes, it does not affect the others
running on the same computer).
Over time, many firms have installed new servers to support new projects, only to find that the new server
was not fully used; the server might only be running at 10% of its capacity and sitting idle for the rest of
the time. One underutilized server is not a problem, but imagine if 20–30% of a company’s servers are
underutilized. The company has spent too much money to acquire the servers, and more importantly, it is
continuing to spend money to monitor, manage, and update the underused servers. Even the space and
power used by having many separate computers can noticeably increase operating costs. Server
virtualization enables firms to save money by reducing the number of physical servers they buy and
operate, while still providing all the benefits of having logically separate devices and operating systems.


FIGURE 7-13 The storage area network (SAN) at the Kelley School of Business at Indiana University
Some operating systems enable virtualization natively, which means that it is easy to configure and run
separate virtual servers. In other cases, special-purpose virtualization software (e.g., VMware) is installed
on the server and sits between the hardware and the operating systems; this software means that several
different operating systems can be installed on the same physical computer.


A storage area network (SAN) is a LAN devoted solely to data storage. When the amount of data to be
stored exceeds the practical limits of servers, the SAN plays a critical role. The SAN has a set of high-speed
storage devices and servers that are networked together using a very high speed network. When data are
needed, clients send the request to a server on the LAN, which obtains the information from the devices
on the SAN and then returns it to the client.
The devices on the SAN may be a large set of database servers or a set of network-attached disk arrays. In
other cases, the devices may be network-attached storage (NAS) devices. A NAS is not a general-
purpose computer, such as a server that runs a server operating system (e.g., Windows and Linux);
instead, it has a small processor and a large amount of disk storage and is designed solely to respond to
requests for files and data. NAS can also be attached to LANs, where they function as fast file servers.
Figure 7-13 shows the SAN for the Kelley School of Business at Indiana University. This SAN stores 125
terabytes of data.
7.5.4 Designing the e-Commerce Edge
The e-commerce edge contains the servers that are designed to serve data to customers and suppliers,
such as the corporate Web server. The e-commerce edge is essentially a smaller, specialized version of the
data center. It contains all the same equipment as the data center (e.g., load balancer, SAN, and UPS), but
this equipment supports access by users external to the organization. It is often connected directly to the
Internet access part of the network via a very-high-speed circuit as well as the campus backbone.
The e-commerce edge often has different security requirements than the servers in the data center


The e-commerce edge often has different security requirements than the servers in the data center
intended for use by employees inside the organization because the e-commerce edge is primarily intended
to serve those external to the organization. We discuss the special security needs of the e-commerce edge
in Chapter 11.
7.5.5 Designing the SOHO Environment
Most of what we have discussed so far has focused on network design in large enterprises. What about
LAN design for SOHO environments? SOHO environments can be small versions of enterprise designs, or
can take a very different approach.
Figure 7-14a shows a SOHO LAN designed similar to a small enterprise design that provides both wired
and wireless Ethernet (it’s in Alan’s house). Virtually all of the rooms in the house are wired with
1000Base-T Ethernet over Cat 5e cable, which terminates in a 24-port patch panel. You can see from the
figure that only five of the rooms are actually wired from the patch panel into the 16-port switch; one of
those wires connects the AP mounted in an upstairs hallway (not shown) that provides wireless access
throughout the house and onto the back deck and gazebo. There is a separate router and cable modem.
The AP, switch, and router are all Cisco or Linksys equipment and are the original 2001 equipment, and
still work well. The cable modem is an off-brand provided by the ISP and has broken and been replaced
every 3 years.
Figure 7-14b shows a more modern—and probably more common—SOHO LAN that provides only
wireless access (it’s in Alexandra’s house). This has a cable modem that connects into a wireless router;
the wireless router is a wireless AP, a router, and a switch for wired Ethernet all in one box. This network


is simpler and cheaper because it contains fewer devices and is used only for wireless access. Alexandra
doesn’t have a desktop computer at home, but she could easily connect one if she wanted by adding a
wireless NIC into a desktop; the 802.11n WLAN provides ample capacity for a small SOHO network.


FIGURE 7-14 SOHO LAN designs


FIGURE 7-15 Powerline adapter
Installing cables for wired Ethernet is expensive, so most SOHO designs use wireless Ethernet. Sometimes
a house is big enough that one WAP won’t cover the entire building and the outdoor area. Powerline
networking is an old technology that is making a comeback for exactly this situation. Powerline
networking provides Ethernet over the existing electrical power wires in your house at rates up to 1 Gbps.
The powerline adapters convert the traditional wired Ethernet signal that runs over Cat 5e cables into a


signal that can travel over the electrical powerwires.
You buy a kit that has two powerline adapters. You plug one adapter into a power outlet in one room and
the other into an outlet in a different room. Then you connect an Ethernet cable into each adapter and you
can begin transmitting between the two powerline adapters. Figure 7-15 shows the powerline adapter that
one of our friends has at his house in Hawaii. He has one WAP near his back door to provide wireless
coverage for the rear of his house and the backyard, which is plugged into this powerline adapter. He has a
second WAP connected to the second powerline adapter upstairs near the front of his house to provide
coverage for the rest of his house and his front yard.
7.6 IMPROVING LAN PERFORMANCE
When LANs had only a few users, performance was usually very good. Today, however, when most
computers in an organization are on LANs, performance can be a problem. Performance is usually
expressed in terms of throughput (the total amount of user data transmitted in a given time period) or in
response time (how long it takes to get a response from the destination). In this section, we discuss how to
improve throughput. We focus on dedicated-server networks because they are the most commonly used
type of LANs, but many of these concepts also apply to peer-to-peer networks.
TECHNICAL FOCUS 7-2
Error Control in Wired Ethernet
Ethernet provides a strong error control method using stop-and-wait ARQ with a CRC-32 error
detection field (see Chapter 4). However, the normal way of installing wired Ethernet doesn’t use
stop-and-wait ARQ.
In the early days of Ethernet, LAN environments were not very reliable, so error control was


In the early days of Ethernet, LAN environments were not very reliable, so error control was
important. However, today’s wired Ethernet LANs are very reliable; errors seldom occur. Stop-and-
wait ARQ uses considerable network capacity because every time a frame is transmitted, the sender
must stop and wait for the receiver to send an ACK. By eliminating the need to stop and wait and the
need to send acknowledgments, Ethernet can significantly improve network performance—almost
doubling the number of messages that can be transmitted in the same time period. Ethernet does
still add the CRC and does still check it for errors, but any frame with an error is simply discarded.
If Ethernet doesn’t provide error control, then higher layers in the network model must. In general,
TCP is configured to provide error control by using continuous ARQ (see Chapter 5) to ensure that
all frames that have been sent are actually received at the final destination. If a frame with an error is
discarded by Ethernet, TCP will recognize that a frame has been lost and ask the sender to
retransmit. This moves responsibility for error control to the edges of the network (i.e., the sender
and receiver) rather than making every computer along the way responsible for ensuring reliable
message delivery.
To improve performance, you must locate the bottleneck, the part of the network that is restricting the
data flow. Generally speaking, the bottleneck will lie in one of two places. The first is the network server.
In this case, the client computers have no difficulty sending requests to the network server, but the server
lacks sufficient capacity to process all the requests it receives in a timely manner. The second location is a
network circuit, either the access LAN, the building backbone, the campus backbone, or the circuit into


the data center. In this case, the server (or more likely, a server farm) can easily process all the client
requests it receives, but a circuit lacks enough capacity to transmit all the requests to the server.
The first step in improving performance, therefore, is to identify whether the bottleneck lies in a circuit or
the server. To do so, you simply watch the utilization of the server during periods of poor performance. If
the server utilization is high (e.g., 80–100%), then the bottleneck is the server; it cannot process all the
requests it receives in a timely manner. If the server utilization is low during periods of poor performance,
then the problem lies with a network circuit; some circuits cannot transmit messages as quickly as


necessary.
Most organizations focus on ways to improve the server and the circuits to remove bottle-necks. These
actions address only the supply side of the equation—that is, increasing the capacity of the LAN as a
whole. The other way to reduce performance problems is to attack the demand side: reduce the amount of
network use by the clients, which we also discuss. Figure 7-16 provides a performance checklist.
7.6.1 Improving Server Performance
Improving server performance can be approached from two directions simultaneously: software and
hardware.
Software
The NOS is the primary software-based approach to improving network performance. Some NOSs are
faster than others, so replacing the NOS with a faster one will improve performance.
Each NOS provides a number of software settings to fine-tune network performance. Depending on the
number, size, and type of messages and requests in your LAN, different settings can have a significant
effect on performance. The specific settings differ by NOS but often include things such as the amount of
memory used for disk caches, the number of simultaneously open files, and the amount of buffer space.
Performance Checklist
Increase Server Performance
Software
Fine-tune the network operating system settings
Hardware
Add more servers and spread the network applications across the servers to balance
the load
Upgrade to a faster computer
Increase the server’s memory
Increase the number and speed of the server’s hard disk(s)
Increase Circuit Capacity
Upgrade to a faster circuit
Increase the number of circuits
Reduce Network Demand
Move files from the server to the client computers
Increase the use of disk caching on client computers
Change user behavior
FIGURE 7-16 Improving local area network performance
Hardware
One obvious solution if your network server is overloaded is to buy a second server (or more). Each server


is then dedicated to supporting one set of application software (e.g., one handles email, another handles
the financial database, and another stores customer records). The bottleneck can be broken by carefully
identifying the demands each major application software package places on the server and allocating them
to different servers.
Sometimes, however, most of the demand on the server is produced by one application that cannot be
split across several servers. In this case, the server itself must be upgraded. The first place to start is with
the server’s CPU. Faster CPUs mean better performance. If you are still using an old computer as a LAN


server, this may be the answer; you probably need to upgrade to the latest and greatest. Clock speed also
matters: the faster, the better. Most computers today also come with CPU-cache (a very fast memory
module directly connected to the CPU). Increasing the cache will increase CPU performance.
A second bottleneck is the amount of memory in the server. Increasing the amount of memory increases
the probability that disk caching will work, thus increasing performance.
A third bottleneck is the number and speed of the hard disks in the server. The primary function of the
LAN server is to process requests for information on its disks. Slow hard disks give slow network
performance. The obvious solution is to buy the fastest disk drive possible. Even more important,
however, is the number of hard disks. Each computer hard disk has only one read/write head, meaning
that all requests must go through this one device. By using several smaller disks rather than one larger
disk (e.g., five 200 gigabyte disks rather than one 1 terabyte disk), you now have more read/write heads,
each of which can be used simultaneously, dramatically improving throughput. A special type of disk drive
called RAID (redundant array of inexpensive disks) builds on this concept and is typically used in
applications requiring very fast processing of large volumes of data, such as multimedia. Of course, RAID
is more expensive than traditional disk drives, but costs have been shrinking. RAID can also provide fault
tolerance, which is discussed in Chapter 11.
Several vendors sell special-purpose network servers that are optimized to provide extremely fast
performance. Many of these provide RAID and use symmetric multiprocessing (SMP) that enables
one server to use up to 16 CPUs. Such servers provide excellent performance but cost more (often
$5,000–$15,000).
7.6.2 Improving Circuit Capacity


7.6.2 Improving Circuit Capacity
Improving the capacity of a circuit means increasing the volume of simultaneous messages the circuit can
transmit from network clients to the server(s). One obvious approach is simply to buy a bigger circuit. For
example, if you are now using a 100Base-T LAN, upgrading to 1000Base-T LAN will improve capacity. Or
if you have 802.11n, then upgrade to 802.11ac or 802.11ax. You can also add more circuits so that there
are two or even three separate high-speed circuits between busy parts of the network, such as the core
backbone and the data center. Most Ethernet circuits can be configured to use full duplex (see Chapter 4),
which is often done for backbones and servers.
Another approach is to segment the network. If there is more traffic on a LAN than it can handle, you can
divide the LAN into several smaller segments. Breaking a network into smaller parts is called network
segmentation. In a wired LAN, this means adding one of more new switches and spreading the
computers across these new switches. In a wireless LAN, this means adding more APs that operate on
different channels. If wireless performance is significantly worse than expected, then it is important to
check for sources of interference near the AP and the computers such as Bluetooth devices and cordless
phones.
7.6.3 Reducing Network Demand
One way to reduce network demand is to move files to client computers. Heavily used software packages
that continually access and load modules from the network can place unusually heavy demands on the
network. Although user data and messages are often only a few kilobytes in size, today’s software packages
can be many megabytes in size. Placing even one or two such applications on client computers can greatly


improve network performance (although this can create other problems, such as increasing the difficulty
in upgrading to new versions of the software).
Most organizations now provide both wired and wireless networks, so another way to reduce demand is to
shift it from wired networks to wireless networks, or vice versa, depending on which has the problem. For
example, you can encourage wired users to go wireless or install wired Ethernet jacks in places where
wireless users often sit.
Because the demand on most LANs is uneven, network performance can be improved by attempting to
move user demands from peak times to off-peak times. For example, early morning and after lunch are
often busy times when people check their email. Telling network users about the peak times and
encouraging them to change their habits may help; however, in practice, it is often difficult to get users to


change. Nonetheless, finding one application that places a large demand on the network and moving it
can have a significant impact (e.g., printing several thousand customer records after midnight).
7.7 IMPLICATIONS FOR CYBER SECURITY
Most attacks from external hackers come over the Internet. However, it is still important to secure the
LAN from unauthorized users. For wired LANs, this means securing network devices, such as switches
and APs. All devices should be placed in locked closets, with access tightly controlled. For WLANs, this
means using encryption and secured access that requires users to login before they can use the Wi-Fi. This
is particularly important for Wi-Fi APs whose signal extends outside of buildings onto public streets, or to
other floors in an office building that are occupied by other firms.
A very useful way to protect both the LAN and WLAN is to do MAC address filtering. Your users would
register all devices that could send and receive packets on a network. Then, when a device wants to
communicate (e.g., is plugged into a port in the wall or tries to associate with an AP), its MAC address is
checked against the list of registered devices. All devices that are not registered are denied access to the
network.
Finally, organizations protect their internal network through network segmentation. On the WLAN, this
can be done through different SSIDs (SSID stands for Service Set Identifier, but nobody ever says this).
SSID is simply the name of the network that you connect to. We are sure you faced this situation when you
went to a coffee shop and wanted to connect to the Internet and your laptop offered you multiple different
networks to connect to. In business networks, one SSID would be used for the registered devices that can
access all services offered by the network, and another guest SSID would only offer Internet access and at
lower capacity (to deter people from staying on it too long).


lower capacity (to deter people from staying on it too long).
SUMMARY
LAN Components The NIC enables the computer to be physically connected to the network and
provides the physical layer connection among the computers. Wired LANs use UTP wires, STP wires,
and/or fiber-optic cable. Network hubs and switches provide an easy way to connect network cables
and act as repeaters. Wireless NICs provide radio connections to APs that link wireless computers
into the wired network. The NOS is the software that performs the functions associated with the data
link and the network layers and interacts with the application software and the computer’s own
operating system. Every NOS provides two sets of software: one that runs on the network server(s)
and one that runs on the network client(s). A network profile specifies what resources on each server
are available for network use by other computers and which devices or people are allowed what
access to the network.
Ethernet (IEEE 802.3) Ethernet, the most commonly used LAN protocol in the world, uses a
contention-based media access technique called CSMA/CD. There are many different types of
Ethernet that use different network cabling (e.g., 10Base-T, 100Base-T, 1000Base-T, and 10 GbE).
Switches are preferred to hubs because they are significantly faster.
Wireless Ethernet Wireless Ethernet (often called Wi-Fi) is the most common type of wireless
LAN. It uses physical star/logical bus topology with both controlled and contention-based media
access control. 802.11ac, the newest version, provides 433Mbps over three channels or faster speeds
over fewer channels.
Best Practice LAN Design Most organizations install 100Base-T or 10/100/1000 Ethernet as their
primary LAN and also provide wireless LANs as an overlay network. For SOHO networks, the best


LAN choice may be wireless. Designing the data center and e-commerce edge often uses specialized
equipment such as server farms, load balancers, virtual servers, SANs, and UPS.
Improving LAN Performance Every LAN has a bottleneck, a narrow point in the network that
limits the number of messages that can be processed. Generally speaking, the bottleneck will lie in
either the network server or a network circuit. Server performance can be improved with a faster NOS
that provides better disk caching, by buying more servers and spreading applications among them or
by upgrading the server’s CPU, memory, NIC, and the speed and number of its hard disks. Circuit


capacity can be improved by using faster technologies (e.g., 1000Base-T), by adding more circuits,
and by segmenting the network into several separate LANs by adding more switches or APs. Overall
LAN performance also can be improved by reducing the demand for the LAN by moving files off the
LAN, moving users from wired Ethernet to wireless or vice versa, and by shifting users’ routines.
KEY TERMS
access point (AP)
Active Directory Service (ADS)
association
beacon frame
bottleneck
bus topology
cable plan
cabling
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
channel
clear to send (CTS)
collision
collision detection (CD)
collision domain
cut-through switching
directional antenna
distributed coordination function (DCF)
domain controller
dual-band AP
Ethernet
fiber-optic cable
forwarding table
fragment-free switching
frame
hub
IEEE 802.3
IEEE 802.11
latency
layer 2 switch
lightweight directory access protocol (LDAP)
load balancer
load balancing switch
logical topology


MAC address filtering
managed APs
network-attached storage (NAS)
network interface card (NIC)
network operating system (NOS)
network profile
network segmentation
network server
omnidirectional antenna
overlay network
physical carrier sense method
physical topology
point coordination function (PCF)
port
power over Ethernet (POE)
powerline networking
probe frame
redundant array of inexpensive disks (RAID)
request to send (RTS)
server virtualization
shielded twisted-pair (STP)
site survey
small-office, home-office (SOHO)
storage area network (SAN)
store-and-forward switching
switch
switched Ethernet
symmetric multiprocessing (SMP)
topology
twisted-pair cable
unshielded twisted-pair (UTP) cable
virtual carrier sense
wardriving
Wireless Ethernet (Wi-Fi)
Wi-Fi Controller
Wi-Fi Protected Access (WPA)
Wired Equivalent Privacy (WEP)
Wireless LAN (WLAN)
10Base-T


100Base-T
1000Base-T
10/100/1000 Ethernet
1 GbE
10 GbE
40 GbE
100 GbE
802.11ac
802.11ax
802.11n
QUESTIONS
1. Define local area network.
2. Describe at least three types of servers.
3. Describe the basic components of a wired LAN.
4. Describe the basic components of a wireless LAN.
5. What types of cables are commonly used in wired LANs?
6. Compare and contrast category 5 UTP, category 5e UTP, and category 5 STP.
7. What is a cable plan and why would you want one?
8. What does a NOS do? What are the major software parts of a NOS?
9. How does wired Ethernet work?
10. How does a logical topology differ from a physical topology?
11. Briefly describe how CSMA/CD works.
12. Explain the terms 100Base-T, 100Base-F, 1000Base-T, 10 GbE, and 10/100/1000 Ethernet.
13. How do Ethernet switches know where to send the frames they receive? Describe how switches gather
and use this knowledge.
14. Compare and contrast cut-through, store-and-forward, and fragment-free switching.
15. Compare and contrast the two types of antennas.
16. How does Wi-Fi perform media access control?
17. How does Wi-Fi differ from shared Ethernet in terms of topology, media access control, and error
control, Ethernet frame?
18. Explain how CSMA/CA DCF works.
19. Explain how CSMA/CA PCF works.
20. Explain how association works in WLAN.
21. What are the best practice recommendations for wired LAN design?


21. What are the best practice recommendations for wired LAN design?
22. What are the best practice recommendations for WLAN design?
23. What is a site survey, and why is it important?
24. How do you decide how many APs are needed and where they should be placed for best performance?
25. How does the design of the data center differ from the design of the LANs intended to provide user
access to the network?


26. What are three special-purpose devices you might find in a data center and what do they do?
27. What is a bottleneck and how can you locate one?
28. Describe three ways to improve network performance on the server.
29. Describe three ways to improve network performance on circuits.
30. Many of the wired and wireless LANs share the same or similar components (e.g., error control).
Why?
31. As WLANs become more powerful, what are the implications for networks of the future? Will wired
LANS still be common or will we eliminate wired offices?
EXERCISES
A. Survey the LANs used in your organization. Are they wireless or wired? Why?
B. Document one LAN (or LAN segment) in detail. What devices are attached, what cabling is used, and
what is the topology? What does the cable plan look like?
C. You have been hired by a small company to install a simple LAN for its 18 Windows computers.
Develop a simple LAN and determine the total cost; that is, select the cables, hubs/switches, and so
on, and price them. Assume that the company has no network today and that the office is small
enough that you don’t need to worry about cable length.
MINICASES
I. Designing a New Ethernet One important issue in designing Ethernet lies in making sure that if a
computer transmits a frame, any other computer that attempts to transmit at the same time will be
able to hear the incoming frame before it stops transmitting, or else a collision might go unnoticed.
For example, assume that we are on the earth and send an Ethernet frame over a very long piece of
category 5 wire to the moon. If a computer on the moon starts transmitting at the same time as we do


on the earth and finishes transmitting before our frame arrives at the moon, there will be a collision,
but neither computer will detect it; the frame will be garbled, but no one will know why. So, in
designing Ethernet, we must make sure that the length of cable in the LAN is shorter than the length
of the shortest possible frame that can be sent. Otherwise, a collision could go undetected.
a. Let’s assume that the smallest possible message is 64 bytes (including the 33-byte overhead). If
we use 100Base-T, how long (in meters) is a 64-byte message? While electricity in the cable
travels a bit slower than the speed of light, once you include delays in the electrical equipment in
transmitting and receiving the signal, the effective speed is only about 40 million meters per
second. (Hint: First calculate the number of seconds it would take to transmit the frame then
calculate the number of meters the signal would travel in that time, and you have the total length
of the frame.)
b. If we use 10 GbE, how long (in meters) is a 64-byte frame?
c. The answer in part b is the maximum distance any single cable could run from a switch to a
computer in an Ethernet LAN. How would you overcome the problem implied by this?
II. Pat’s Petunias You have been called in as a network consultant by your cousin Pat, who operates a
successful mail-order flower business. She is moving to a new office and wants to install a network for
her telephone operators, who take phone calls and enter orders into the system. The number of
operators working varies depending on the time of day and day of the week. On slow shifts, there are
usually only 10 operators, whereas at peak times, there are 50. She has bids from different companies


to install (1) Wi-Fi or (2) a switched Ethernet 100Base-T network. She wants you to give her some
sense of the relative performance of the alternatives so she can compare that with their different
costs. What would you recommend?
III. Eureka! Eureka! is a telephone- and Internet-based concierge service that specializes in obtaining
things that are hard to find (e.g., Super Bowl tickets, first-edition books from the 1500s, and Fabergé


eggs). It currently employs staff members who work 24 hours per day (over three shifts), with usually
five to seven staff members working at any given time. Staff members answer the phone and respond
to requests entered on the Eureka! website. Much of their work is spent on the phone and on
computers searching on the Internet. They have just leased a new office and are about to wire it. They
have bids from different companies to install (a) a 100Base-T network or (b) a Wi-Fi network. What
would you recommend? Why?
IV. Tom’s Home Automation Your cousin Tom runs a small construction company that builds
custom houses. He has just started a new specialty service that he is offering to other builders on a
subcontracting basis: home automation. He provides a complete service of installing cable in all the
rooms in which the homeowner wants data access and installs the necessary networking devices to
provide a LAN that will connect all the computers in the house to the Internet. Most homeowners
choose to install a DSL or cable modem Internet connection that provides a 12–25 Mbps from the
house to the Internet. Tom has come to you for advice about whether he should continue to offer
wiring services (which often cost $50 per room) or whether wireless is a better direction. What type
of LAN would you recommend?
V. Sally’s Shoes Sally Smith runs a shoe store in the mall that is about 30 feet by 50 feet in size,
including a small office and a storage area in the rear. The store has one inventory computer in the
storage area and one computer in the office. She is replacing the two cash registers with computers
that will act as cash registers but will also be able to communicate with the inventory computer. Sally
wants to network the computers with a LAN. What sort of LAN design would you recommend? Draw
a picture.


a picture.
VI. South West State University South West State University installed a series of four Wi-Fi
omnidirectional APs spread across the ceiling of the main floor of its library. The main floor has
several large, open areas plus two dozen or so small offices spread around the outside walls. The
WLAN worked well for one semester, but now more students are using the network, and performance
has deteriorated significantly. What would you recommend that they do? Be sure to support your
recommendations.
VII. Household Wireless Your sister is building a new two-story house (which measures 50 feet long
by 30 feet wide) and wants to make sure that it is capable of networking her family’s three computers
together. She and her husband are both consultants and work out of their home in the evenings and a
few days a month (each has a separate office with a computer, plus a laptop from the office that is
occasionally used). The kids also have a computer in their playroom. They have several options for
networking their home:
a. Wire the two offices and playroom with Ethernet Cat 5e cable and put in a 1000Base-T switch for
$40.
b. Install one Wi-Fi AP ($85) and put Wi-Fi cards in the three computers for $50 each (their
laptops already have Wi-Fi).
c. Any combination of these options.What would you recommend? Justify your recommendation.
VIII. Ubiquitous Offices Ubiquitous Offices provides temporary office space in cities around the
country. They have a standard office layout that is a single floor with outside dimensions of 150 feet
wide by 150 feet long. The interior is drywall offices. They have 1000Base-T but want to add wireless


access as well. How many APs would you buy, and where would you put them? Draw the office and
show where the APs would go.
IX. ABC Warehouse ABC Warehouse is a single-floor facility with outside dimensions of 100 feet wide
by 350 feet long. The interior is open, but there are large metal shelving units throughout the building
to hold all the goods in the warehouse. How many APs would you buy, and where would you put
them? Draw the warehouse and show where the APs would go.
X. Metro Motel Metro Motel is a four-story motel on the outskirts of town. The outside dimensions of
the motel are 60 feet wide by 200 feet long, and each story is about 10 feet high. Each floor (except
the ground floor) has 20 rooms (drywall construction). There is a central corridor with rooms on both
sides. How many APs would you buy, and where would you put them? Draw the motel and show
where the APs would go.


TECH UPDATES
Pick one of these topics to investigate.
Topic A: Port Scanning
Port scanning is the process of sending a series of messages to a target computer in the hopes of finding a
way to break in. Port scanning software generates standard query messages that target “well-known” port
numbers (e.g., port 25 for email; see Chapter 5) to see if the computer responds, thus giving the assailant
an idea where to probe for weaknesses. What devices are out there that one can use for port scanning?
Why is this activity considered to be so serious? How can a network be protected against port scanning?
Topic B: Common Security Violations
You may try to secure your network as much as you can, but when your users don’t follow your security
policy guidance, all is lost. Investigate what are the five most common cyber security policy violations and
discuss how they open the doors for hackers. For example, you could do a scavenger hunt (and provide
photos) that show that people can be very reckless and leave their laptops open while unattended or post
or have sticky notes with passwords attached to monitors. Have fun! But don’t break the law.
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 topic
3. Main players—these could be people, processes, software, hardware, etc.
4. How it works—use 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


7. Credits
8. List of References
9. Memo addressed to your professor describing all of the above information
HANDS-ON ACTIVITY 7A
Tracing Ethernet
TracePlus Ethernet is a network monitoring tool that enables you to see how much network capacity you
are using. If you’re working from home with a broadband Internet connection, you’ll be surprised how
little of the Ethernet capacity you’re actually using. Your LAN connection is probably 1,000 Mbps (or 300
Mbps if you’re using wireless), while the broadband connection into your home or apartment is only 20–
30 Mbps. The bottleneck is the broadband connection, so you use only a small percentage of your LAN
capacity.
1. Download and install TracePlus. A free trial version of TracePlus is available at Cnet
(download.cnet.com/TracePlus-Ethernet/3000-2085_4-29031.html). The URL might move, so if
this link doesn’t work, search on the Internet. Just be careful what you download and where you get
it. We like Cnet as safe download sites, but you can also check Norton SafeWeb for their ratings of
sites (safeweb.norton.com).
2. Start TracePlus and monitor your network. Leave it open in one part of your screen as you surf the


Internet, check email, or watch a video.
Figure 7-17 shows a sample TracePlus screen while I was surfing the Internet and checking email with
Microsoft Outlook. The dashboard at the bottom of the screen shows the real-time usage. You can see that
when I took this screen shot, my computer was sending and receiving about 100 packets per second (or if
you prefer, 100 frames per second), for a total of just under 1 Mbps of data. This is less than 1% of the total
Ethernet bandwidth (i.e., network capacity), because I have switched to 100Base-T on my computer. The
dashboard also shows that I’m sending and receiving almost no broadcast or multicast data.
Immediately above the dashboard is the summary for my computer (192.1681.104 (Alan 2)). In the 2
minutes and 30 seconds of monitoring, my computer received 1,875 inbound packets with a total of 2.236
megabytes of data for a utilization of 0.118%. The average bits per second was about 118 Kbps. During the
same time, my computer sent slightly fewer outbound packets (1,232), but the average packet was about
10 times smaller because the total amount of data sent was only 218,569 bytes. Most packets were 128–
511 bytes in length, but some were smaller and some were larger.
FIGURE 7-17 TracePlus
The Nodes tab in the upper right of the screen shows the nodes on my network that TracePlus can
monitor. These include my computer (Alan 2), a second computer (Orcelia), my router (192.168.1.1), a
wireless AP (Aironet) with two connections (into the LAN and out to the wireless LAN), and the Indiana


University VPN server (because I had my VPN connected; Chapter 11 discusses VPNs). You can see that all
of these devices have little utilization (under 1%), as well as the total number of packets these devices have
sent and received. You can click through the other tabs in this area to see the packet distribution.
The panel on the left of the screen shows additional information about the types of packets, errors, and
packet sizes.


Deliverables
1. How many packets can your computer send and receive?
2. What is the total data rate on your network?
3. What is your network utilization?
HANDS-ON ACTIVITY 7B
Wardriving
Wireless LANS are often not secure. It is simple to bring your laptop computer into a public area and
listen for wireless networks. This is called wardriving (because it is often done from a car). As long as you
do not attempt to use any networks without authorization, wardriving is quite legal. There are many good
software tools available for wardriving. Our favorites are Net Surveyor (available from
nutsaboutnets.com/netsurveyor-wifi-scanner) or Wireless NetView (available from download.cnet.com).
Both are simple to use, yet powerful.
The first step is to download and install the software on a laptop computer that has wireless capability.
Just be careful what you download as these sites sometimes have other software on the same page. Once
you have installed the software, simply walk or drive to a public area and start it up. Figure 7-18 shows an
example of the 13 networks I discovered in my home town of Bloomington, Indiana, when I parked my car
in a neighborhood near the university that has a lot of rental houses and turned on Wireless Netview. I
rearranged the order of the columns in Netview, so your screen might look a little different than mine
when you first start up Netview.
NetView displays information about each wireless LAN it discovers. The first column shows the name of
the WLAN (the SSID). The second column shows the last signal strength it detected, whereas the third
column shows the average signal strength. I used NetView from my parked car on the street, so the signal


strengths are not strong, and because I wasn’t moving, the average signal strength and the last signal
strength are the same.
You can examine the “PHY Types” column and see that most APs are 802.11n, although there are three
older 802.11g APs. Values in the “Maximum Speed” column are quite variable. There are some newer
8011.n APs that are running at the top speed of 450 Mbps. Some 802.11n APs provide 144 Mbps, which
suggests that these WLANs are likely to be older APs that are not capable of the higher speeds of newer
802.11n APs. You can also see that there are three 802.11g APs that provide only 54 Mbps. The “Channel”
column shows a fairly even distribution of channels 1, 6, and 11, indicating that most users have
configured them to use the three standard channels. However, the owner of the FatJesse WLAN has
configured it to run on channel 2.
All the APs in this neighborhood were secure. They had implemented encryption. However, the very first
AP (2WIRE935) was using WEP, which is a very old standard. It’s better than nothing, but its owner
should switch to WPA or WPA2.
Figure 7-19 shows a similar screen capture in the Kelley School of Business at Indiana University. If you
look closely, you’ll see that this only shows a small subset of the APs that were visible to NetView. There
were more than 50 APs in total. In this case, you’ll see a more standard configuration, with virtually all the
APs being 802.11n running at 216 Mbps in channels 1, 6, and 12 (although you can’t see the ones in


channel 12). All the APs on the IU Secure or eduaroam are secured, whereas attwifi and IU Guest are not
secured. You can also see two rogue APs (both have names starting with “PD”) that are 802.11g, WEP-
secured, running at 54 Mbps.


FIGURE 7-18 WLANs in a neighborhood in Bloomington, Indiana
FIGURE 7-19 WLANs at Indiana University
Deliverables
1. Capture a snapshot for the screen having all the information related to the various network
connections that you collected during your warwalking.
2. What different versions of 802.11 did you see, what were their maximum speeds, and what channels
were used?
3. How many networks were secure?
4. What is your overall assessment of the WLAN usage with respect to security?


HANDS-ON ACTIVITY 7C
Apollo Residence LAN Design
Apollo is a luxury residence hall that will serve honor students at your university. The residence will be
eight floors, with a total of 162 two-bedroom, one-bathroom apartments. The building is steel-frame
construction with concrete on the outside and drywall on the inside that measures 240 feet by 150 feet.
The first floor has an open lobby with a seating area and separate office area, whereas the second floor has
meeting rooms. Floors 3–8 each contain apartments and a large open lounge with a seating area (see
Figure 7-20). Visio files for the residence are available on this book’s website.
Your team was hired to design a network for this residence hall. To improve its quality of service, the
university has decided to install wired network connections in each apartment so that every room can
have one IP phone as well as network access. For security reasons, the university wants two separate
networks: a LAN that will provide secure wired and wireless access to all registered students and a public
wireless LAN that will provide Internet access to visitors.
This activity focuses only on the design of the LAN that will be provided on each floor of six floors with
apartments (floors 3–8). Do not consider floors 1 and 2 at this point; we will add those in the Hands-On
Activity at the end of the next chapter. We have not yet discussed how to design a building backbone or
campus backbone, so just assume that the backbone will connect into a LAN switch using one 100Base-T
or 1000Base-T.
Deliverables
1. Design the network for this residence hall and draw where the network equipment would be placed
(use the floor plans provided).
2. Specify the products in your design and provide their cost and the total cost of the network. There are


two options for specifying product. Option 1 is to use the generic LAN equipment list in Figure 7-21.
Option 2 is to use CDW (www.cdw.com) to find LAN equipment. If you use CDW, you must use only
Cisco devices (to ensure quality).
FIGURE 7-20 Plans for Floors 3–8 of Apollo Residence


FIGURE 7-21 LAN equipment price list


CHAPTER 8
BACKBONE NETWORKS
This chapter examines backbone networks (BNs) that are used in the distribution layer (within building
backbones) and the core layer (campus backbones). We discuss the three primary backbone architectures
and the recommended best practice design guidelines on when to use them. The chapter ends with a
discussion of how to improve BN performance and of the future of BNs.
OBJECTIVES
Understand the Internetworking devices used in BNs
Understand the switched backbone architecture
Understand the routed backbone architecture
Understand virtual LAN architecture
Understand the best practice recommendations for backbone design
Be aware of ways to improve BN performance
OUTLINE
8.1 Introduction
8.2 Switched Backbones
8.3 Routed Backbones
8.4 Virtual LANs
8.4.1 Benefits of VLANs
8.4.2 How VLANs Work
8.5 The Best Practice Backbone Design
8.6 Improving Backbone Performance
8.6.1 Improving Device Performance
8.6.2 Improving Circuit Capacity
8.6.3 Reducing Network Demand
8.7 Implications for Cyber Security
Summary
8.1 INTRODUCTION
Chapter 6 outlined the seven major components in a network (see Figure 6-1). Chapter 7, on LANs,
described how to design the LANs that provide user access to the network as well as the LANs in the data
center and e-commerce edge. This chapter focuses on the next two major network architecture
components: the BNs that connect the access LANs with a building (called the distribution layer) and the
backbone networks that connect the different buildings on one enterprise campus (called the core layer).
Backbones used to be built with special technologies, but today most BNs use high-speed Ethernet. There


are two basic components to a BN: the network cable and the hardware devices that connect other
networks to the BN. The cable is essentially the same as that used in LANs, except that it is often fiber
optic to provide higher data rates. Fiber optic is also used when the distances between the buildings on an


enterprise campus are farther apart than the 100 meters that standard twisted-pair cable can reach. The
hardware devices can be computers or special-purpose devices that just transfer messages from one
network to another. These include switches, routers, and VLAN switches.
Switches operate at the data link layer. These are the same layer 2 switches discussed in Chapter 7 in
that they use the data link layer address to forward packets between network segments. They learn
addresses by reading the source and destination addresses.
Routers operate at the network layer. They connect two different TCP/IP subnets. Routers are the
“TCP/IP gateways” that we first introduced in Chapter 5. Routers strip off the data link layer packet,
process the network layer packet, and forward only those messages that need to go to other networks on
the basis of their network layer address. Routers may be special-purpose devices or special network
modules in other devices (e.g., wireless access points for home use often include a built-in router). In
general, they perform more processing on each message than switches and therefore operate more slowly.
VLAN switches are a special combination of layer 2 switches and routers. They are complex devices
intended for use in large networks that have special requirements. We discuss these in Section 8.4.
In the sections that follow, we describe the three basic BN architectures and discuss at which layer they
are often used. We assume that you are comfortable with the material on TCP/IP in Chapter 5; if you are
not, you may want to go back and review Section 5.6 “TCP/IP Example,” before you continue reading. We
then explain the best practice design guidelines for the distribution layer and the core layer and discuss
how to improve performance.
8.2 SWITCHED BACKBONES


how to improve performance.
8.2 SWITCHED BACKBONES
Switched backbones are probably the most common type of BN used in the distribution layer (i.e.,
within a building); most new building BNs designed today use switched backbones.
Switched BNs use a star topology with one switch at its center. Figure 8-1 shows a switched backbone
connecting a series of LANs. There is a switch serving each LAN (access layer) that is connected to the
backbone switch at the bottom of the figure (distribution layer). Most organizations now use switched
backbones in which all network devices for one part of the building are physically located in the same
room, often in a rack of equipment. This has the advantage of placing all network equipment in one place
for easy maintenance and upgrade, but it does require more cable. In most cases, the cost of the cable is
only a small part of the overall cost to install the network, so the cost is greatly outweighed by the
simplicity of maintenance and the flexibility it provides for future upgrades.
The room containing the rack of equipment is sometimes called the main distribution facility (MDF)
or central distribution facility (CDF). Figure 8-2 shows a photo of an MDF room at Indiana University.
Figure 8-3 shows the equipment diagram of this same room. The cables from all computers and devices in
the area served by the MDF (often hundreds of cables) are run into the MDF room. Once in the room, they
are connected into the various devices. The devices in the rack are connected among themselves using
very short cables called patch cables.
With rack-mounted equipment, it becomes simple to move computers from one LAN to another. Usually,
all the computers in the same general physical location are connected to the same switch and thus share


all the computers in the same general physical location are connected to the same switch and thus share
the capacity of the switch. Although this often works well, it can cause problems if many of the computers
on the switch are high-traffic computers. For example, if all the busy computers on the network are
located in the upper-left area of the figure, the switch in this area may become a bottleneck.
With an MDF, all cables run into the MDF. If one switch becomes overloaded, it is straight-forward to
unplug the cables from several high-demand computers from the overloaded switch and plug them into
one or more less-busy switches. This effectively spreads the traffic around the network more efficiently
and means that network capacity is no longer tied to the physical location of the computers; computers in
the same physical area can be connected into different network segments.
Sometimes a chassis switch is used instead of a rack. A chassis switch enables users to plug modules
directly into the switch. Each module is a certain type of network device. One module might be a 16-port
100Base-T switch, another might be a router, whereas another might be a 4-port 1000Base-F switch, and
so on. The switch is designed to hold a certain number of modules and has a certain internal capacity, so


that all the modules can be active at one time. For example, a switch with four 1000Base-T switches (with
24 ports each) and one 1000Base-F port would have to have an internal switching capacity of at least 97
Gbps ((4 × 24 × 1 Gbps) + (1 × 1 Gbps)).
FIGURE 8-1 Rack-mounted switched backbone network architecture
The key advantage of chassis switches is their flexibility. It becomes simple to add new modules with
additional ports as the LAN grows and to upgrade the switch to use new technologies. For example, if you
want to add gigabit Ethernet, you simply lay the cable and insert the appropriate module into the chassis
switch.


FIGURE 8-2 An MDF with rack-mounted equipment. A layer 2 chassis switch with five 100Base-T
modules (center of photo) connects to four 24-port 100Base-T switches. The chassis switch is connected
to the campus backbone using 1000Base-F over fiber-optic cable. The cables from each room are wired
into the rear of the patch panel (shown at the top of the photo), with the ports on the front of the patch
panel labeled to show which room is which. Patch cables connect the patch panel ports to the ports on the
switches. MDF = main distribution facility


Source: Photo courtesy of the author, Alan Dennis.
FIGURE 8-3 MDF network diagram. MDF = main distribution facility
MANAGEMENT FOCUS 8-1
Switched Backbones at Indiana University
At Indiana University we commonly use switched backbones in our buildings. Figure 8-4 shows a
typical design. Each floor in the building has a set of switches and access points that serve the LANs
on that floor. Each of these LANs and WLANs are connected into a switch for that floor, thus
forming a switched backbone on each floor. Typically, we use switched 100Base-T within each floor.
The switch forming the switched backbone on each floor is then connected into another switch in the
basement, which provides a switched backbone for the entire building. The building backbone is
usually a higher-speed network running over fiber-optic cable (e.g., 100Base-F or 1 GbE). This
switch, in turn, is connected into a high-speed router that leads to the campus backbone (a routed
backbone design).


FIGURE 8-4 Switched backbones at Indiana University
8.3 ROUTED BACKBONES
Routed backbones move packets along the backbone on the basis of their network layer address (i.e.,
layer 3 address). Routed backbones are sometimes called subnetted backbones or hierarchical backbones
and are most commonly used to connect different buildings on the same enterprise campus backbone
network (i.e., at the core layer).
Figure 8-5 illustrates a routed backbone used at the core layer. A routed backbone is the basic backbone
architecture we used to illustrate how TCP/IP worked in Chapter 5. There are a series of LANs (access
layer) connected to a switched backbone (distribution layer). Each backbone switch is connected to a
router. Each router is connected to a core router (core layer). These routers break the network into
separate subnets. The LANs in one building are a separate subnet from the LANs in a different building.
Message traffic stays within each subnet unless it specifically needs to leave the subnet to travel elsewhere
on the network, in which case the network layer address (e.g., TCP/IP) is used to move the packet. For
example, in a switched backbone, a broadcast message (such as an ARP) would be sent to every single
computer in the network. A routed backbone ensures that broadcast messages stay in the one network
segment (i.e., subnet) where they belong and are not sent to all computers. This leads to a more efficient
network.


FIGURE 8-5 Routed backbone architecture
Each set of LANs is usually a separate entity, relatively isolated from the rest of the network. There is no
requirement that all LANs share the same technologies. Each set of LANs can contain its own server
designed to support the users on that LAN, but users can still easily access servers on other LANs over the
backbone, as needed.
A Day in the Life: Network Operations Manager
The job of the network operations manager is to ensure that the network operates effectively. The
operations manager typically has several network administrators and network managers that report
to him or her and is responsible for both day-to-day operations and long-term planning for the
network. The challenge is to balance daily firefighting with longer-term planning; they're always


looking for a better way to do things. Network operations managers also meet with users to ensure
their needs are met. While network technicians deal primarily with networking technology, a
network operations manager deals extensively with both technology and the users.
Atypical day starts with administrative work that includes checks on all servers and backup
processes to ensure that they are working properly and that there are no security issues. Then it's on
to planning. One typical planning item includes planning for the acquisition of new desktop or
laptop computers, including meeting with vendors to discuss pricing, testing new hardware and
software, and validating new standard configurations for computers. Other planning is done around
network upgrades, such as tracking historical data to monitor network usage, projecting future user
needs, surveying user requirements, testing new hardware and software, and actually planning the
implementation of new network resources.
One recent example of long-term planning was the migration from a Novell file server to Microsoft
ADS file services. The first step was problem definition; what were the goals and the alternatives?
The key driving force behind the decision to migrate was to make it simpler for the users (e.g., now
the users do not need to have different accounts with different passwords) and to make it simpler for
the network staff to provide technical support (e.g., now there is one less type of network software to
support). The next step was to determine the migration strategy: a Big Bang (i.e., the entire network
at once) or a phased implementation (several groups of users at a time). The migration required a
technician to access each individual user's computer, so it was impossible to do a Big Bang. The next
step was to design a migration procedure and schedule whereby groups of users could be moved at a
time (e.g., department by department). A detailed set of procedures and a checklist for network


time (e.g., department by department). A detailed set of procedures and a checklist for network
technicians were developed and extensively tested. Then each department was migrated on a 1-week
schedule. One key issue was revising the procedures and checklist to account for unexpected
occurrences during the migration to ensure that no data were lost. Another key issue was managing
user relationships and dealing with user resistance.
Source: With thanks to Mark Ross.
The primary advantage of the routed backbone is that it clearly segments each part of the network
connected to the backbone. Each segment (usually a set of LANs or switched backbone) has its own
subnet addresses that can be managed by a different network manager. Broadcast messages stay within
each subnet and do not move to other parts of the network.
There are two primary disadvantages to routed backbones. First, the routers in the network impose time
delays. Routing takes more time than switching, so routed networks can sometimes be slower. Second,
routers are more expensive and require more management than switches.
Figure 8-5 shows one core router. Many organizations actually use two core routers to provide better
security, as we discuss in Chapter 11.
8.4 VIRTUAL LANS
For many years, the design of LANs remained relatively constant. However, in recent years, the
introduction of high-speed switches has begun to change the way we think about LANs. Switches offer the
opportunity to design radically new types of LANs. Most large organizations today have implemented the
virtual LAN (VLAN), a new type of LAN–BN architecture made possible by intelligent, high-speed
switches.
Virtual LANs are networks in which computers are assigned to LAN segments by software rather than by
hardware. In the first section, we described how in rack-mounted collapsed BNs a computer could be


moved from one hub to another by unplugging its cable and plugging it into a different hub. VLANs
provide the same capability via software so that the network manager does not have to unplug and replug
physical cables to move computers from one segment to another.
Often, VLANs are faster and provide greater opportunities to manage the flow of traffic on the LAN and
BN than do the traditional LAN and routed BN architectures. However, VLANs are significantly more
complex, so they usually are used only for large networks.