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Wireless System Architecture:
How Wireless Works
• components of a wireless network
• general wireless network architectural
elements
• information flows through a wireless
network
• Wireless networks utilize components similar
to wired networks.
• wireless networks must convert information
signals into a form suitable for transmission
through the air medium.
• Wireless networks directly contribute only to a
portion of the overall network infrastructure,
attention to all network functions is necessary
to counter impairments resulting from the
wireless medium.
Wireless Network System
Components
• A wireless network consists of several
components that support communications
using radio or light waves propagating through
an air medium.
• Some of these elements overlap with those of
wired networks, but special consideration is
necessary for all of these components when
deploying a wireless network.
Users
• A user can be anything that directly utilizes the
wireless network. One of the most common types
of user is a person.
• The user initiates and terminates use of a
wireless network, making the term end-user
appropriate.
• Users of wireless networks tend to be mobile,
constantly moving throughout a facility, campus,
or city.
• Mobility is one of the most prominent benefits of
deploying a wireless network.
Computer Devices
• Many types of computer devices, sometimes
referred to as clients, operate on a wireless
network.
• Some computer devices might be specifically
designed for users, whereas some computer
devices are end systems.
• In generally, any computer device might
communicate with any other computer device
on the same wireless network.
• To support mobile applications, computer devices
are often small, making them practical for people
to carry with them at all times.
• With portable and stationary applications,
however, the computer devices are much larger.
• Computer devices within a wireless network also
include end systems such as servers, databases,
and websites.
• Users can adapt many existing computer devices
to operate on a wireless network.
• A computer device also has an operating system,
such as Windows XP, LINUX, or MAC OS.
NICs
• The network interface card provides the interface
between the computer device and the wireless
network infrastructure.
• The NIC fits inside the computer device, but
external network adaptors are available that plug
in and remain outside the computer device.
• Wireless network standards define how a
wireless NIC operates.
• Wireless NICs also comply with a specific form
factor, which defines the physical and electrical
bus interface that enables the card to
communicate with the computer device.
Air Medium
• Air provides a medium for the propagation of
wireless communications signals, which is the
heart of wireless networking.
• Air is the conduit by which information flows
between computer devices and the wireless
infrastructure.
• Wireless information signals cannot be heard by
humans, so it's possible to amplify the signals to a
higher level without disturbing human ears.
• Wireless information signals cannot be heard
by humans, so it's possible to amplify the
signals to a higher level without disturbing
human ears.
• With wireless networks, the air medium
supports the propagation of radio and light
waves that travel from one point to another.
Wireless Network Infrastructures
• The infrastructure of a wireless network
interconnects wireless users and end systems.
• The infrastructure might consist of base
stations, access controllers, application
connectivity software, and a distribution
system.
• These
components
enhance
wireless
communications
and
fulfill
important
functions necessary for specific applications.
Base Stations
• The base station is a common infrastructure
component that interfaces the wireless
communications signals traveling through the air
medium to a wired network—often referred to as
a distribution system.
• Base station enables users to access a wide range
of network services, such as web browsing, email access, and database applications.
• A base station often contains a wireless NIC that
implements the same technology in operation by
the user's wireless NIC.
• Base stations go by different names, depending
on their purpose.
• An access point, for instance, represents a
generic base station for a wireless LAN. A
collection of access points within a wireless LAN,
for example, supports roaming throughout a
facility.
• Residential gateways and routers are more
advanced forms of base stations that enable
additional network functions.
• The gateway might have functions, such as access
control and application connectivity, that better
serve distributed, public networks.
• A router would enable operation of multiple
computers on a single broadband connection.
• a base station might support point-to-point or
point-to-multipoint communications.
• Point-to-point systems enable communications
signals to flow from one particular base station or
computer device directly to another one.
• point-to-multipoint functionality enables a base
station to communicate with more than one
wireless computer device or base station.
• An access point within a wireless LAN implements
this form of communications.
• The access point represents a single point
whereby many computer devices connect to and
communicate with each other and systems within
the wireless infrastructure.
Access Controllers
• Access controller-hardware that resides on the
wired portion of the network between the access
points and the protected side of the network.
• Access controllers provide centralized intelligence
behind the access points to regulate traffic
between the open wireless network and
important resources.
• In some cases, the access point contains the
access control function.
• Access controller regulates access to the
Internet by authenticating and authorizing
users based on a subscription plan.
• Access controller help a hacker sitting in the
company's parking lot from getting entry to
sensitive data and applications.
• The use of an access controller reduces the
need for smart access points.
• Thin access points primarily implement the
basic wireless network standard (such as IEEE
802.11), and not much more.
Benefits when access controllers
deployed with thin access points
• Lower Costs— Access points with limited
functionality cost less, which generally results in
lower overall system costs.
• Open Connectivity— Smart access points offer
enhancements related to security and
performance to the basic wireless connectivity
that wireless network standards offer.
• Centralized Support— An advantage of placing
the smarts of the network in an access controller
is that the system is easier to support, primarily
because fewer touch points are in the network.
Access controllers features
• Authentication— Most access controllers have a built-in
database for authenticating users; however, some offer
external interfaces to authentication servers such as
Remote Authentication Dial-In User Service (RADIUS) and
Lightweight Directory Access Protocol (LDAP).
• Encryption— Some access controllers provide encryption of
data from the client to the server and back, using such
common methods such as IPSec.
• Bandwidth Management— Because users share bandwidth
in a wireless network, it's important to have a mechanism
to ensure specific users don't hog the bandwidth.
Data Communication Terms
• Data - entities that convey meaning, or
information
• Signals - electric or electromagnetic
representations of data
• Transmission - communication of data by the
propagation and processing of signals
TRANSMISSION
FUNDAMENTALS 2-18
Examples of Analog and Digital Data
• Analog
– Video
– Audio
• Digital
– Text
– Integers
TRANSMISSION
FUNDAMENTALS 2-19
Analog Signals
• A continuously varying electromagnetic wave that
may be propagated over a variety of media,
depending on frequency
• Examples of media:
– Copper wire media (twisted pair and coaxial cable)
– Fiber optic cable
– Atmosphere or space propagation
• Analog signals can propagate analog and digital data
TRANSMISSION
FUNDAMENTALS 2-20
Digital Signals
• A sequence of voltage pulses that may be
transmitted over a copper wire medium
• Generally cheaper than analog signaling
• Less susceptible to noise interference
• Suffer more from attenuation
• Digital signals can propagate analog and
digital data
TRANSMISSION
FUNDAMENTALS 2-21
Reasons for Choosing Data and Signal
Combinations
• Digital data, digital signal
– Equipment for encoding is less expensive than digital-toanalog equipment
• Analog data, digital signal
– Conversion permits use of modern digital transmission and
switching equipment
• Digital data, analog signal
– Some transmission media will only propagate analog signals
– Examples include optical fiber and satellite
• Analog data, analog signal
– Analog data easily converted to analog signal
TRANSMISSION
FUNDAMENTALS 2-22
2.8 Analog and Digital Signaling of Analog and Digital Data
TRANSMISSION FUNDAMENTALS 2-23
Analog Transmission
• Transmit analog signals without regard to content
• Attenuation limits length of transmission link
• Cascaded amplifiers boost signal’s energy for longer
distances but cause distortion
– Analog data can tolerate distortion
– Introduces errors in digital data
TRANSMISSION
FUNDAMENTALS 2-24
Digital Transmission
• Concerned with the content of the signal
• Attenuation endangers integrity of data
• Digital Signal
– Repeaters achieve greater distance
– Repeaters recover the signal and retransmit
• Analog signal carrying digital data
– Retransmission device recovers the digital data from
analog signal
– Generates new, clean analog signal
TRANSMISSION
FUNDAMENTALS 2-25
About Channel Capacity
• Impairments, such as noise, limit data rate
that can be achieved
• For digital data, to what extent do
impairments limit data rate?
• Channel Capacity – the maximum rate at
which data can be transmitted over a given
communication path, or channel, under given
conditions
TRANSMISSION
FUNDAMENTALS 2-26
2.9 Effect of Noise on Digital Signal
TRANSMISSION FUNDAMENTALS 2-27
Concepts Related to Channel Capacity
• Data rate - rate at which data can be communicated
(bps)
• Bandwidth - the bandwidth of the transmitted signal
as constrained by the transmitter and the nature of
the transmission medium (Hertz)
• Noise - average level of noise over the
communications path
• Error rate - rate at which errors occur
– Error = transmit 1 and receive 0; transmit 0 and receive 1
TRANSMISSION
FUNDAMENTALS 2-28
Classifications of Transmission Media
• Transmission Medium
– Physical path between transmitter and receiver
• Guided Media
– Waves are guided along a solid medium
– E.g., copper twisted pair, copper coaxial cable, optical fiber
• Unguided Media
– Provides means of transmission but does not guide
electromagnetic signals
– Usually referred to as wireless transmission
– E.g., atmosphere, outer space
TRANSMISSION
FUNDAMENTALS 2-29
Unguided Media
• Transmission and reception are achieved by
means of an antenna
• Configurations for wireless transmission
– Directional
– Omnidirectional
TRANSMISSION
FUNDAMENTALS 2-30
2.10 Electromagnetic spectrum of Telecommunications
TRANSMISSION FUNDAMENTALS 2-31
General Frequency Ranges
• Microwave frequency range
–
–
–
–
1 GHz to 40 GHz
Directional beams possible
Suitable for point-to-point transmission
Used for satellite communications
• Radio frequency range
– 30 MHz to 1 GHz
– Suitable for omnidirectional applications
• Infrared frequency range
– Roughly, 3x1011 to 2x1014 Hz
– Useful in local point-to-point multipoint applications within
confined areas
TRANSMISSION
FUNDAMENTALS 2-32
Terrestrial Microwave
• Description of common microwave antenna
–
–
–
–
Parabolic "dish", 3 m in diameter
Fixed rigidly and focuses a narrow beam
Achieves line-of-sight transmission to receiving antenna
Located at substantial heights above ground level
• Applications
– Long haul telecommunications service
– Short point-to-point links between buildings
TRANSMISSION
FUNDAMENTALS 2-33
Satellite Microwave
• Description of communication satellite
– Microwave relay station
– Used to link two or more ground-based microwave
transmitter/receivers
– Receives transmissions on one frequency band (uplink),
amplifies or repeats the signal, and transmits it on another
frequency (downlink)
• Applications
– Television distribution
– Long-distance telephone transmission
– Private business networks
TRANSMISSION
FUNDAMENTALS 2-34
Broadcast Radio
• Description of broadcast radio antennas
– Omnidirectional
– Antennas not required to be dish-shaped
– Antennas need not be rigidly mounted to a precise
alignment
• Applications
– Broadcast radio
• VHF and part of the UHF band; 30 MHZ to 1GHz
• Covers FM radio and UHF and VHF television
TRANSMISSION
FUNDAMENTALS 2-35
Multiplexing
• Capacity of transmission medium usually
exceeds capacity required for transmission of
a single signal
• Multiplexing - carrying multiple signals on a
single medium
– More efficient use of transmission medium
TRANSMISSION
FUNDAMENTALS 2-36
2.11 Multiplexing
TRANSMISSION FUNDAMENTALS 2-37
Reasons for Widespread Use of
Multiplexing
• Cost per kbps of transmission facility declines with an
increase in the data rate
• Cost of transmission and receiving equipment
declines with increased data rate
• Most individual data communicating devices require
relatively modest data rate support
TRANSMISSION
FUNDAMENTALS 2-38
Multiplexing Techniques
• Frequency-division multiplexing (FDM)
– Takes advantage of the fact that the useful
bandwidth of the medium exceeds the required
bandwidth of a given signal
• Time-division multiplexing (TDM)
– Takes advantage of the fact that the achievable bit
rate of the medium exceeds the required data rate
of a digital signal
TRANSMISSION
FUNDAMENTALS 2-39
2.12 FDM and TDM
TRANSMISSION FUNDAMENTALS 2-40
2.13 Synchronous TDM System
TRANSMISSION FUNDAMENTALS 2-41
Key Features of a Protocol
• Syntax
– Concerns the format of the data blocks
• Semantics
– Includes control information for coordination and
error handling
• Timing
– Includes speed matching and sequencing
Protocols and the TCP/IP Suite
4-42
Agents Involved in Communication
• Applications
– Exchange data between computers (e.g.,
electronic mail)
• Computers
– Connected to networks
• Networks
– Transfers data from one computer to another
Protocols and the TCP/IP Suite
4-43
4.1 TCP/IP Concepts
Protocols and the TCP/IP Suite 4-44
TCP/IP Layers
•
•
•
•
•
Physical layer
Network access layer
Internet layer
Host-to-host, or transport layer
Application layer
Protocols and the TCP/IP Suite
4-45
4.2 Protocol Data Units (PDUs) in the TCP/IP
Architecture
Protocols and the TCP/IP Suite 4-46
TCP/IP Physical Layer
• Covers the physical interface between a data
transmission device and a
transmission medium or network
• Physical layer specifies:
– Characteristics of the transmission medium
– The nature of the signals
– The data rate
– Other related matters
Protocols and the TCP/IP Suite
4-47
TCP/IP Network Access Layer
• Concerned with the exchange of data between an
end system and the network to which it's attached
• Software used depends on type of network
–
–
–
–
Circuit switching
Packet switching (e.g., X.25)
LANs (e.g., Ethernet)
Others
Protocols and the TCP/IP Suite
4-48
T:TCP/IP Internet Layer
• Uses internet protocol (IP)
• Provides routing functions to allow data to
traverse multiple interconnected networks
• Implemented in end systems and routers
Protocols and the TCP/IP Suite
4-49
TCP/IP Host-to-Host, or Transport
Layer
• Commonly uses transmission control protocol
(tcp)
• Provides reliability during data exchange
– Completeness
– Order
Protocols and the TCP/IP Suite
4-50
TCP/IP Application Layer
• Logic supports user applications
• Uses separate modules that are peculiar to
each different type of application
Protocols and the TCP/IP Suite 4-51
Common TCP/IP Applications
• Simple mail transfer protocol (SMTP)
– Provides a basic electronic mail facility
• File Transfer Protocol (FTP)
– Allows files to be sent from one system to another
• Hypertext Transfer Protocol (HTTP)
– Transfers information for the World Wide Web
Protocols and the TCP/IP Suite
4-52
Network functionality for OSI layers
• Layer 7—Application layer: Establishes
communications among users and provides basic
communications services such as file transfer and
e-mail.
• Layer 6—Presentation layer: Negotiates data
transfer syntax for the application layer and
performs translations between different data
formats, if necessary.
• Layer 5—Session layer: Establishes, manages, and
terminates sessions between applications.
• Layer 4—Transport layer: Provides mechanisms
for the establishment, maintenance, and orderly
termination of virtual circuits, while shielding the
higher layers from the network implementation
details.
• Layer 3—Network layer: Provides the routing of
packets though a network from source to
destination.
• Layer 2—Data link layer: Ensures medium access,
as well as synchronization and error control
between two entities.
• Layer 1—Physical layer: Provides the actual
transmission of information through the medium.
Physical layers include radio waves and infrared
light.
• Wireless networks directly implement only the
lower layers of the model.
• wireless NIC, for example, implements the
data link layer and physical layer functions.
• wireless middleware offer functions that the
session layer implements.
• Each layer of the OSI model supports the
layers above it.
• protocols at each layer communicate across
the network to the respective peer layer. The
actual transmission of data, however, occurs
at the physical layer.
• The architecture allows for a layering process
where a particular layer embeds its protocol
information into frames that are placed within
frames at lower layers.
• The frame that is sent by the physical layer
actually contains frames from all higher layers.
• At the destination, each layer passes
applicable frames to higher layers to facilitate
the protocol between peer layers.
Spectrum considerations
• Controlled by regulatory bodies
– Carrier frequency
– Signal Power
– Multiple Access Scheme
• Divide into time slots –Time Division Multiple Access
(TDMA)
• Divide into frequency bands – Frequency Division
Multiple Access (FDMA)
• Different signal encodings – Code Division Multiple
Access (CDMA)
Overview of Wireless 5-57
Spectrum considerations
• Federal Communications Commission (FCC) in the
United States regulates spectrum
–
–
–
–
–
–
Military
Broadcasting
Public Safety
Mobile
Amateur
Government exclusive, non-government exclusive, or
both
– Many other categories
Overview of Wireless 5-58
Spectrum considerations
• Industrial, Scientific, and Medical (ISM) bands
– Can be used without a license
– As long as power and spread spectrum regulations
are followed
• ISM bands are used for
– WLANs
– Wireless Personal Area networks
– Internet of Things
Overview of Wireless 5-59
Propagation Modes
• Ground-wave propagation
• Sky-wave propagation
• Line-of-sight propagation
Overview of Wireless 5-60
5.1 Wireless Propagation Modes
Overview of Wireless 5-61
Ground Wave Propagation
•
•
•
•
Follows contour of the earth
Can propagate considerable distances
Frequencies up to 2 MHz
Example
– AM radio
Overview of Wireless 5-62
Sky Wave Propagation
• Signal reflected from ionized layer of atmosphere
back down to earth
• Signal can travel a number of hops, back and forth
between ionosphere and earth’s surface
• Reflection effect caused by refraction
• Examples
– Amateur radio
– CB radio
Overview of Wireless 5-63
Line-of-Sight Propagation
• Transmitting and receiving antennas must be within
line of sight
– Satellite communication – signal above 30 MHz not
reflected by ionosphere
– Ground communication – antennas within effective line of
site due to refraction
• Refraction – bending of microwaves by the
atmosphere
– Velocity of electromagnetic wave is a function of the
density of the medium
– When wave changes medium, speed changes
– Wave bends at the boundary between mediums
Overview of Wireless 5-64
Five basic propagation mechanisms
1. Free-space propagation
2. Transmission
– Through a medium
– Refraction occurs at boundaries
3. Reflections
– Waves impinge upon surfaces that are large compared to
the signal wavelength
4. Diffraction
– Secondary waves behind objects with sharp edges
5. Scattering
– Interactions between small objects or rough surfaces
Overview of Wireless 5-65
antennas
• An antenna is an electrical conductor or
system of conductors
– Transmission - radiates electromagnetic energy
into space
– Reception - collects electromagnetic energy from
space
• In two-way communication, the same antenna
can be used for transmission and reception
Overview of Wireless 5-66
Radiation Patterns
• Radiation pattern
– Graphical representation of radiation properties of an antenna
– Depicted as two-dimensional cross section
• Beam width (or half-power beam width)
– Measure of directivity of antenna
• Reception pattern
– Receiving antenna’s equivalent to radiation pattern
• Sidelobes
– Extra energy in directions outside the mainlobe
• Nulls
– Very low energy in between mainlobe and sidelobes
Overview of Wireless 5-67
5.2 Antenna Radiation Patterns
Overview of Wireless 5-68
Attenuation
• Strength of signal falls off with distance over
transmission medium
• Attenuation factors for unguided media:
– Received signal must have sufficient strength so that
circuitry in the receiver can interpret the signal
– Signal must maintain a level sufficiently higher than noise
to be received without error
– Attenuation is greater at higher frequencies, causing
distortion
Overview of Wireless 5-69
Error Control
• wireless NICs implement error control
mechanisms that detect and correct bit errors.
• Error control techniques highly reduce the
number of transmission errors.
• The two primary types of error control are
automatic repeat-request (ARQ) and forward
error correction (FEC).
• With ARQ, which operates at the data link layer, the
receiving wireless NIC detects errors and uses a
feedback path to the sending wireless NIC for
requesting the retransmission of frames having bit
errors.
• There are two main events that must occur to correct
errors with ARQ.
-First, a received frame must be checked at the receiver
for possible errors,
-and then the sender must be notified to retransmit the
frames received in error. In some protocols, such as
802.11, the receiver sends an acknowledgement to
the sender if the received frame has no errors. The
absence of an acknowledgement indicates to the
sender to retransmit the frame.
• Two approaches for retransmitting
unsatisfactory blocks of data exist:
• Stop-and-wait ARQ
• Continuous ARQ
Stop-and-Wait ARQ
• In the stop-and-wait method of transmission, the
sending NIC transmits a block of data, then stops and
waits for an acknowledgment from the receiving NIC
on whether a particular frame was acceptable or not.
If the sending side receives a negative
acknowledgment, the previous frame will be sent
again. The sending NIC will send the next frame after
it receives a positive acknowledgment from the
receiving NIC. The IEEE 802.11 standard specifies this
form of error control.
Continuous ARQ
• the transmitter sends data blocks continuously until
the receiving NIC detects an error. The sending NIC is
usually capable of transmitting a certain number of
frames and keeps a log of which frames have been
sent. Once the receiving side detects a bad block, it
will send a signal back to the sending NIC requesting
that the bad frame be sent over again. When the
receiver gets the signal to retransmit a certain frame,
several subsequent frames might have already been
sent because of propagation delays between the
sender and receiver.
Go-back-n technique
• The go-back-n technique is useful in applications where the
receiver has little memory space because all that is needed is
a receiver window size of one (ability to store one frame),
assuming frames do not need to be delivered in order.
• When the receiving NIC rejects an erred frame —sends a
negative acknowledgment—it does not need to store any
subsequent frames for possible reordering while it is waiting
for the retransmission.
• It need not wait because all subsequent frames will also be
resent.
Selective Repeat Approach
• Selective repeat is obviously better than continuous
go-back-n in terms of throughput because the
sending NIC only transmits the erred data block;
• The receiver must be capable of storing a number of
data frames if they are to be processed in order.
• The receiver needs to buffer data that have been
received after an erred frame was requested for
retransmission as only the damaged frame will be
resent.
FEC
• As an alternative to ARQ, FEC automatically corrects
as many errors as it can within the physical layer at
the receiving NIC without referring to the sending
NIC.
• This is possible because the sending NIC includes
enough redundant bits in case some are lost because
of errors.
• This makes FEC well suited for simplex
communications links, and cases where a return path
to the sending NIC is not feasible.
Connecting with the Wireless
Network Infrastructure
• The base station, such as an access point, includes
both a wireless and wired NIC, as well as software
that interfaces the two networks.
• When a wireless user communicates with another
wireless user, the base station might simply resend
the data frame received from one user so that the
other user is able to receive it.
• In this case, the base station is acting as a repeater.
• Alternatively, the base station might forward the
data to the wired side of the base station if the
destination is located somewhere on the wired side
of the network.
• Upon receiving a data frame, the wireless NIC within
the base station converts the analog radio wave or
light signal into a digital signal and performs error
detection to ensure that the resulting data frame
does not have any bit errors.
• The error control mechanism will cause the sending
wireless NIC to retransmit the data frame if errors
are present.
• After taking care of erred frames, the wireless NIC
within the base station will either resend the frame
or forward the frame to the wired side of the base
station.
Summary
• Wireless networks include components that make
mobile and portable application possible.
• Users are end points of the wireless network and
utilize computer devices designed for a particular
application.
• Wireless NICs and base stations are key components
that communicate over the air medium.
• To provide roaming throughout a facility or city, a
distribution system such as Ethernet interconnects
base stations and interfaces users to servers and
applications located on the wired network.
• The seven-layer OSI reference model depicts
functions necessary for a network, but wireless
networks implement only functions defined by the
bottom two layers—the physical and data link layer.
• These functions include medium access, error
control, and formation of radio and light signals for
propagation through the medium.
• When deploying wireless networks, however, it's
important to ensure that protocols operating at
higher layers have features that counter impairments
found in wireless networks.