Chapter 8 is about Wireless Networking - Welcome to San Ramon Campus_

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					Hello Networking Class:

Chapter 8 is about Wireless Networking
How nodes exchange wireless signals

The wireless spectrum is a continuum of the electromagnetic waves used for data and
voice communication. On the spectrum, waves are arranged according to their
frequencies, from lowest to highest. The wireless spectrum (as defined by the FCC,
which controls its use) spans frequencies between 9 KHz and 300 GHz.

In the United States, the collection of frequencies available for communication—also
known as ―the airwaves‖—is a natural resource available for public use. The FCC grants
organizations in different locations exclusive rights to use each frequency. It also
determines what frequency ranges can be used for what purposes.

Potential obstacles to successful wireless transmission

Each type of wireless service requires an antenna specifically designed for that service.
The service’s specifications determine the antenna’s power output, frequency, and
radiation pattern. Note that an antenna’s radiation pattern describes the relative strength
over a three-dimensional area of all the electromagnetic energy the antenna sends or
receives.

Reflection in wireless signaling is no different from reflection of other electromagnetic
waves, such as light. The wave encounters an obstacle and reflects—or bounces
back—toward its source.

In diffraction, a wireless signal splits into secondary waves when it encounters an
obstruction. The secondary waves continue to propagate in the direction in which they
were split.

Scattering is the diffusion, or the reflection in multiple different directions, of a signal.
Scattering occurs when a wireless signal encounters an object that has small
dimensions compared to the signal’s wavelength.

No matter what paths wireless signals take, they are bound to run into obstacles. When
they do, the original signal issued by the transmitter will experience fading, or a change
in signal strength as a result of some of the electromagnetic energy being scattered,
reflected, or diffracted after being issued by the transmitter.

For many years WLANs have relied on frequencies in the range of 2.4–2.4835 GHz,
more commonly known as the 2.4-GHz band, to send and receive signals. This band
offers 11 communications channels that are unlicensed in the United States.
In narrowband, a transmitter concentrates the signal energy at a single frequency or in a
very small range of frequencies. In contrast to narrowband, broadband uses a relatively
wide band of the wireless spectrum. Broadband technologies, as a result of their wider
frequency bands, offer higher throughputs than narrowband technologies.

The use of multiple frequencies to transmit a signal is known as spread-spectrum
technology (because the signal is spread out over the wireless spectrum). In other
words, a signal never stays continuously within one frequency range during its
transmission.

Each type of wireless communication falls into one of two categories: fixed or mobile. In
fixed wireless systems, the locations of the transmitter and receiver do not move. The
transmitting antenna focuses its energy directly toward the receiving antenna. In mobile
wireless, the receiver can be located anywhere within the transmitter’s range. This
allows the receiver to roam from one place to another while continuing to pick up its
signal.

WLAN (wireless LAN) architecture

Because they are not bound by cabling paths between nodes and connectivity devices,
wireless networks are not laid out using the same topologies as wired networks. They
have their own, different layouts. Smaller wireless networks, in which a small number of
nodes closely positioned need to exchange data, can be arranged in an ad hoc fashion.

It is common for a WLAN to include several access points (a device that accepts
wireless signals from multiple nodes and retransmits them to the rest of the network).
The number of access points depends on the number of stations a WLAN connects.
The maximum number of stations each access point can serve varies from 10 to 100,
depending on the wireless technology used. Exceeding the recommended maximum
leads to a greater incidence of errors and slower overall transmission.

WLANs support the same protocols (for example, TCP/IP) and operating systems (for
example, UNIX, Linux, or Windows) as wired LANs. This compatibility ensures that
wireless and wired transmission methods can be integrated on the same network.

Characteristics of popular WLAN transmission methods, including 802.11 a/b/g/n

IEEE released its first wireless network standard in 1997. Since then, its WLAN
standards committee, also known as the 802.11 committee, has published several
distinct standards related to wireless networking. Each IEEE wireless network access
standard is named after the 802.11 task group (or subcommittee) that developed it. The
three IEEE 802.11 task groups that have generated notable wireless standards are
802.11b, 802.11a, and 802.11g.
802.11 standards specify the use of CSMA/CA (Carrier Sense Multiple Access with
Collision Avoidance) to access a shared medium. Using CSMA/CA, before a station
begins to send data on an 802.11 network, it checks for existing wireless transmissions.
If the source node detects no transmission activity on the network, it waits a brief,
random amount of time, and then sends its transmission.

One way to ensure that packets are not inhibited by other transmissions is to reserve
the medium for one station’s use. In 802.11, this can be accomplished through the
optional RTS/ CTS (Request to Send/Clear to Send) protocol.

Association is another function of the MAC sublayer described in the 802.11 standard.

As long as a station is on and has its wireless protocols running, it periodically surveys
its surroundings for evidence of an access point, a task known as scanning. A station
can use either active scanning or passive scanning.

For each function, the 802.11 standard specifies a frame type at the MAC sublayer.
These multiple frame types are divided into three groups: control, management, and
data. Management frames are those involved in association and reassociation, such as
the probe and beacon frames. Control frames are those related to medium access and
data delivery, such as the ACK and RTS/CTS frames. Data frames are those that carry
the data sent between stations.

In 1999, the IEEE released its 802.11b standard, which is unique among 802.11
standards in its use of DSSS (direct-sequence spread spectrum) signaling. 802.11b
uses the 2.4–2.4835-GHz frequency range (better known as the 2.4-GHz band) and
separates it into 22-MHz channels. 802.11b provides a theoretical maximum of 11-Mbps
throughput; actual throughput is typically around 5 Mbps.

The 802.11a standard differs from 802.11b and 802.11g in that it uses channels in the
5-GHz band and provides a maximum theoretical throughput of 54 Mbps, though its
effective throughput falls generally between 11 and 18 Mbps.

IEEE’s 802.11g WLAN standard is designed to be just as affordable as 802.11b while
increasing its maximum theoretical throughput from 11 Mbps to 54 Mbps through
different data modulation techniques. The effective throughput of 802.11g ranges
generally from 20 to 25 Mbps.

The primary goal of IEEE’s 802.11n committee was to create a wireless standard that
provided much higher effective throughput than the other 802.11 standards. By all
accounts, they succeeded. 802.11n boasts a maximum throughput of 600 Mbps,
making it a threat to Fast Ethernet and a realistic platform for telephone and video
signals.
Installation and configuration of wireless access points and their clients

Placement of an access point on a WLAN must take into account the typical distances
between the access point and its clients. If your small office spans three floors, for
instance, and clients are evenly distributed among the floors, you might choose to
situate the access point on the second floor.

Larger WLANs warrant a more systematic approach to access point placement. Before
placing access points in every telco room, it’s wise to conduct a site survey. A site
survey assesses client requirements, facility characteristics, and coverage areas to
determine an access point arrangement that will ensure reliable wireless connectivity
within a given area.

When designing an enterprise-wide WLAN, you must consider how the wireless
portions of the LAN will integrate with the wired portions. Access points connect the two.
But an access point may perform other functions as well. It may provide security
features, by, for example, including and excluding certain clients. It may also participate
in VLANs, allowing mobile clients to move from one access point’s range to another
while belonging to the same virtual LAN.

For initial configuration, it’s best to connect to an access point or wireless router directly
using a patch cable. Most wireless routers come with installation software designed to
guide you through the configuration process step-by-step.

If something goes awry during your wireless router configuration, you can force all of the
variables you changed to be reset. Wireless routers feature a reset button on their back
panel.

As with Windows XP, most Linux and UNIX clients provide a graphical interface for
configuring their wireless interfaces. Because each version differs somewhat from the
others, describing the steps required for each graphical interface is beyond the scope of
this book. However, iwconfig, a command-line function for viewing and setting wireless
interface parameters, is common to nearly all versions of Linux and UNIX.

Wireless configuration pitfalls to avoid include: SSID mismatch, incorrect encryption,
incorrect channel or frequency, standard mismatch (802.11 a/b/g/n), incorrect antenna
placement, and interference.

Wireless MAN and WAN technologies, including 802.16 and satellite
communications

Places where wireless Internet access is available to the public are called hot spots.
Some organizations, such as T-Mobile, have established a network of hot spots across
the nation. Other organizations, such as a local coffee shop, might have only one hot
spot. Note that at each hot spot, the access point available for public use is connected
to the Internet using technology other than 802.11.

In 2001, IEEE standardized a new wireless technology under its 802.16 (wireless MAN)
committee. Since that time, IEEE has released several versions of the 802.16 standard.
Collectively, the 802.16 standards are known as WiMAX, which stands for Worldwide
Interoperability for Microwave Access, the name of a group of manufacturers, including
Intel and Nokia, who banded together to promote and develop 802.16 products and
services. The currently favored IEEE 802.16 version is 802.16e, which was approved in
2005. With 802.16e, IEEE improved the mobility and QoS characteristics of WiMAX,
making it better suited to carrying digital voice signals and serving mobile phone users.

WiMAX offers two distinct advantages over Wi-Fi. First, it can provide much greater
throughput than 802.11a, b, or g—up to 70 Mbps. Second, its range extends much
farther than any of the 802.11 standards, topping out at 50 kilometers (or approximately
30 miles).

Each type of wireless service requires an antenna specifically designed for that service.
The service’s specifications determine the antenna’s power output, frequency, and
radiation pattern. Note that an antenna’s radiation pattern describes the relative strength
over a three-dimensional area of all the electromagnetic energy the antenna sends or
receives.

Reflection in wireless signaling is no different from reflection of other electromagnetic
waves, such as light. The wave encounters an obstacle and reflects—or bounces
back—toward its source.

In diffraction, a wireless signal splits into secondary waves when it encounters an
obstruction. The secondary waves continue to propagate in the direction in which they
were split.

Scattering is the diffusion, or the reflection in multiple different directions, of a signal.
Scattering occurs when a wireless signal encounters an object that has small
dimensions compared to the signal’s wavelength.

No matter what paths wireless signals take, they are bound to run into obstacles. When
they do, the original signal issued by the transmitter will experience fading, or a change
in signal strength as a result of some of the electromagnetic energy being scattered,
reflected, or diffracted after being issued by the transmitter.

For many years WLANs have relied on frequencies in the range of 2.4–2.4835 GHz,
more commonly known as the 2.4-GHz band, to send and receive signals. This band
offers 11 communications channels that are unlicensed in the United States.
In narrowband, a transmitter concentrates the signal energy at a single frequency or in a
very small range of frequencies. In contrast to narrowband, broadband uses a relatively
wide band of the wireless spectrum. Broadband technologies, as a result of their wider
frequency bands, offer higher throughputs than narrowband technologies.

The use of multiple frequencies to transmit a signal is known as spread-spectrum
technology (because the signal is spread out over the wireless spectrum). In other
words, a signal never stays continuously within one frequency range during its
transmission.

Each type of wireless communication falls into one of two categories: fixed or mobile. In
fixed wireless systems, the locations of the transmitter and receiver do not move. The
transmitting antenna focuses its energy directly toward the receiving antenna. In mobile
wireless, the receiver can be located anywhere within the transmitter’s range. This
allows the receiver to roam from one place to another while continuing to pick up its
signal.

Most satellites circle the Earth 22,300 miles above the equator in a geosynchronous
orbit. Geosynchronous orbit (also called geostationary orbit, or GEO) means that
satellites orbit the Earth at the same rate as the Earth turns. Consequently, at every
point in their orbit, the satellites maintain a constant distance from a specific point on the
Earth’s equator.

An alternative to geosynchronous satellites are LEO (low Earth orbiting) satellites. LEO
satellites orbit the Earth with an altitude as low as 100 miles and up to 1240 miles, not
above the equator but closer to the Earth’s poles.

In between the altitudes of LEO and GEO satellites lie MEO (medium Earth orbiting)
satellites. MEO satellites orbit the Earth between 6000 and 12,000 miles above its
surface.

Satellites transmit and receive signals in any of following five frequency bands, which
are roughly defined as: L-band—1.5–2.7 GHz; S-band—2.7–3.5 GHz; C-band—3.4–6.7
GHz; Ku-band—12–18 GHz; and Ka-band—18–40 GHz.

A handful of companies offer high-bandwidth Internet access via GEO satellite links.
Each subscriber uses a small satellite dish antenna and receiver to exchange signals
with the service provider’s satellite network. Subscribers can choose one of two types of
satellite Internet access service: dial return or satellite return.

That’s it for Chapter 8.

Have a good week
Bill

Class Web Site: http://www.srvc.net/hammond/cnt105

Hint of the week:

Here are a few tips to help secure your wireless network.

Make sure that you also change the default password(s) on the router
Change the SSID to something other than what the manufacturer provided
Turn off SSID broadcast
Use MAC address filtering
Turn on 128-bit WEP, better yet WPA, best WPA2
Keep the router at or below ground level to discourage "war driving"
Limit the number of DHCP licenses to only what you need
Change the default frequency (channel) to one that is not in use by your neighbors

Chapter 8 Lab:

The Netstat program is a useful tool for checking network and Internet connections.
Some useful applications for the average PC user include checking for malware
connections. To run the program, drop to the command prompt and type netstat.

You will see your current connections identified by IP address and port number.

Additional commands commands are available. Type netstat /? to see some of the
things you can do with this program.

For your lab, capture the output of your netstat screen into a document and identify all
of your connection points. Try to explain why you have these connections. Email your
report to me with a subject line Lab 8-Netstat.

				
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