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Departamento de
Tecnología Electrónica
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Chapter 5.
Wireless Local Area
Network
Computer Networking:
A Top Down Approach
5th edition.
Jim Kurose, Keith Ross
Addison-Wesley, April
2009.
1
Chapter 5: Wireless Local Area
Networks
 5.1 Introduction
 5.2 Standard IEEE 802.11
 5.2.1 Standards
 5.2.2 IEEE 802.11 network components
 5.2.3 Standard IEEE 802.11
 5.2.4 IEEE 802.11 network topologies
Wireless Local Area Networks
2
Chapter 5: Wireless Local Area
Networks
 5.1 Introduction
 5.2 Standard IEEE 802.11
 5.2.1 Standards
 5.2.2 IEEE 802.11 network components
 5.2.3 Standard IEEE 802.11
 5.2.4 IEEE 802.11 network topologies
Wireless Local Area Networks
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Introduction
Introduction
 No cables
 They use the electromagnetic spectrum
 Generally they are integrated within wired LANs
Why?
 Allow mobility
 Allow installation in places where cables can’t be
installed (or are expensive)
Wireless Local Area Networks
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Introduction
Introduction
Wireless Local Area Networks
5
Introduction
 Advantages:
 Allow the same features as wired LANs, but without
cable limitations.
 Mobility



Reduce installation time/cost
Flexibility
May work inside buildings or between buildings
 Disadvantages:
 Need a trasnsmission medium based on radio frequency
(RF) -> Electromagnetic spectrum is limited
 Transmission rates are slower than in wired LANs
 Security problems
Wireless Local Area Networks
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Electromagnetic spectrum
 Use of electromagnetic spectrum







0-200 MHz: Radio, television, remote controls, wireless
phones, etc.
200 MHz- 1GHz: alarms, medical implants, walkie-talkies,
television, mobile phones.
1- 2 GHz: GPS, medical telemetry, phone mobiles
2.4 GHz: free band… satellite radio, satellite phones,
microwave ovens, weather radars, WI-FI, BLUETOOTH.
2.5- 5 GHz: satellite communications (e.g, TV)
5-50 GHz: Wi-fi, police radars
50-300 GHz: short-distance signals.
Wireless Local Area Networks
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Electromagnetic spectrum
 ISM frequency bands:
 900 – 928 MHz
 2,400 – 2,4835 GHz
 5,725 – 5,850 GHz
Wireless Local Area Networks
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Chapter 5: Wireless Local Area
Networks
 5.1 Introduction
 5.2 Standard IEEE 802.11
 5.2.1 Standards
 5.2.2 IEEE 802.11 network components
 5.2.3 Standard IEEE 802.11
 5.2.4 IEEE 802.11 network topologies
Wireless Local Area Networks
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Standards
Standars
 Standardization of WLANs is in charge of
IEEE & WIFI Alliance.

IEEE in standard 802.11 is in charge of:
Define specifications of high performance WLANs
 Ensures Interoperability
Security
Quality of Service.

WIFI Alliance is in charge of:
Certificates that a manufacturer’s product may
interoperate with another manufacturer’s one.
Promote the use of WLANs
Wireless Local Area Networks
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Standards
LAN/MAN Standards
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Chapter 5: Wireless Local Area
Networks
 5.1 Introduction
 5.2 Standard IEEE 802.11
 5.2.1 Standards
 5.2.2 IEEE 802.11 network components
 5.2.3 Standard IEEE 802.11
 5.2.4 IEEE 802.11 network topologies
Wireless Local Area Networks
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802.11 Network components
IEEE 802.11Network components
 Networks that follow standard 802.11 are formed by four
basic elements
Distribution system
Access Point
Station or client
Wireless medium
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802.11 Network components
 Station or client: Device using a NIC that follows standard
IEEE 802.11
PC, laptop, PDA,…
Wireless Network
Adapter
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802.11 Network components
 Station or client

Wireless Network Adapters
 Radio units
 Several types of Wi-Fi adapters
 PCI Cards
• With internal antenna
• With external antenna
 USB Adapters
• With internal antenna
• With external antenna
 PCMCIA Adapters
• With internal antenna
• With external antenna
Wireless Local Area Networks
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802.11 Network components
 Station or client

Wireless Network Adapters
PCI Cards
 With internal antenna
• More usual
• Disadvantage: very sensitive to the host
location
 With external antenna
• Allows locating the antenna in the best
place for receiving a more powerful
signal.
• PCI cards in 802.11n have three
antennas.
Wireless Local Area Networks
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802.11 Network components
 Station or client

Wireless Network Adapters
PCI Cards
 Advantages
• Reliable: once they are installed, there
are usually no problems
 Disadvantages
• Need HW installation
• Allow their use in an only host
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802.11 Network components
 Station or client

Wireless Network Adapters
USB Adapters
 With internal antenna
• More usual.
• Less range, cheaper.
 With external antenna
• More gain: more quality of signal
• Also in 802.11n there are three antennas.
Wireless Local Area Networks
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802.11 Network components
 Station or client

Wireless Network Adapters
 USB Adapters
 Advantages
• Mobility: Allows locating them in the best place
for receiving a more powerful signal.
• May be used in any host. Only a USB port is
needed.
• If it is neccessary, they may pass from a host to
another (only driver installation is needed)
 Disadvantages
• Much more unstable than Wi-Fi PCI Cards.
• Units with interior antenna do not usually have
high gain, so that they do not work properly in
places where the quality of the signal is bad.
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802.11 Network components
 Station or client

Wireless Network Adapters
PCMCIA Cards
 With internal antenna
• More useful for laptops but less range than
units with external antenna.
 With external antenna
• Higher range. Antenna is not usually very
large, and it is generally folding
• Also in 802.11n there are three antennas
(internal, for practical reasons).
Wireless Local Area Networks
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802.11 Network components
 Station or client

Wireless Network Adapters
PCMCIA Cards
 Adavantages
• More quality of signal than USB Adapters
and simillar to Wi-Fi PCI cards.
 Disadvantages
• May be used only in hosts with PCMCIA
port.
• They all need driver installation
Wireless Local Area Networks
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802.11 Network components
Access Point (AP):



Device in charge of “Medium Control Access” for the
clients of a WLAN and allows connection to a wired
network (bridge)
An AP is a wireless hub.
Different from a wireless router (very common
nowadays). A wireless router is a combination of an AP
and a router and may execute more complex functions
than a simple AP.
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802.11 Network components
 Access Point (AP):



Bridge: allows interconnecting different
networks, regardless of the protocol they
are using. Works in 1 & 2 OSI layers.
A router also allows interconnecting several
networks, but the network layer protocol
must be common (E.g, IP)
If we want to interconnect two networks
that are using the same protocol, it is
desirable to use a router.
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802.11 Network components
 Station & AP

Antennas
Directional antennas or beam antennas
 Radiate signal in a very determined direction.
Narrow beam with high range.
 Range is determined by the combination of the
gain of the antenna, emitting power in the
transmitter AP & sensitivity in the receiver AP.
No signal is captured out of range.
 Very useful for long-distance point to point
communication
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802.11 Network components
 Station & AP

Antennas
Onmidirectional antennas
 Signal is radiated in all directions, but the
signal’s range is short
 Useful for radiating a signal in an extensive area.
Sectorial antennas
 A combination of directional & omnidirectional
antennas.
 Expensive
 When long distance and extensive area range are
neccesary.
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802.11 Network components
 Wireless Medium: Use of RF
for the transport of
MAC_PDUs.
Frequency
Standard
Wavelength
2.4 Ghz
802.11b/g
12.5 cm
5.x GHz
802.11a
5-6 cm
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802.11 Network components
 Distribution System: LAN or WLAN technology
used to make WLAN range wider.

Wireless case:
Several AP.
 WDS AP: Master AP.
 The other APs are slaves and act as relay stations: WDS
Stations.
All in the same channel
SSID may be the same one or different
Not standard. Not supported by all hosts and there
may be incompatibilities.
Incompatible with some security mechanisms
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Chapter 5: Wireless Local Area
Networks
 5.1 Introduction
 5.2 Standard IEEE 802.11
 5.2.1 Standards
 5.2.2 IEEE 802.11 network components
 5.2.3 Standard IEEE 802.11
 5.2.4 IEEE 802.11 network topologies
Wireless Local Area Networks
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Standard IEEE 802.11
Standard IEEE 802.11
LLC (802.2)
MAC
PHY
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Standard IEEE 802.11. Physical
Layer
 Physical Layer (PHY)

Cell topology.
Half-duplex
Frequency bands: 2,4 GHz & 5 GHz
 A license is not neccessary.
 Every band has a set of channels
Wireless Local Area Networks
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Standard IEEE 802.11. Physical
Layer
 Physical Layer (PHY)

Cell topology.
 Physical layer options
Standard
Band
Tx Rate
802.11a
5 GHz
54 Mbps
802.11b
2,4 GHz
11 Mbps
802.11g
2,4 GHz
54 Mbps
802.11n
Both
200 Mbps
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
 MAC Sublevel
 Medium Access Technique: CSMA/CA
1) Before transmitting info, a station must
determine the medium state (free or busy)
2) If the channel is not busy, there is an additional
wait: InterFrame Space (IFS)
3) If the channel is busy or it gets busy while IFS,
the tx must wait until current transaction ends.
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
 MAC Sublevel
 Medium Access Technique: CSMA/CA
4) After current transaction ends: Backoff algorithm
is executed
 Additional and random wait, chosen in an interval
called contention window (CW)
 Measured in units of slot time (slots)
5) If during this wait, medium is not free for a time
equal or higher than IFS, wait is suspended until
the condition is fulfilled.
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
Data to tx arrival
Wait
 MAC Sublevel
Medium Access Technique: CSMA/CA
IFS
IFS
IFS
Data
CW

Data
IFS
Station
A
CW
B
Data
Backoff
C
Data
Backoff
CW
D
Backoff
Data
CW
E
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
 MAC Sublevel
 Medium Access Technique: CSMA/CA
Problems in WLAN:
 Hidden nodes. Channel is busy by a station that is not
heard by the correspondent node
 Exposed nodes. A station thinks that the channel is busy
though it is, in fact free, as other nodes does not
intereferes in the communication.
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
 MAC Sublevel
 Medium Access Technique: MACA
Contention that allows reservations to avoid collisions
(CSMA/CA, CA = Collision Avoidance)
RTS (Request to Send) / CTS (Clear to Send)
Not employed (overload) if:
 There are a few stations
 Very dense network: all the stations are in range of every station.
 Small frames.
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Standard IEEE 802.11. Data
Link Layer
 MAC Sublevel

Medium Access Technique: MACA
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Standard IEEE 802.11. Data
Link Layer
 MAC Sublevel
 Every MAC_PDU containing data is ACKed by the rcvr
 Encryptation and authenticaction algorithms are
implemented.
 A MAC_PDU may contain up to 4 MAC addresses (source,
destination, transmitter & receiver).
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
 MAC Sublevel
 Three types of MAC_PDUs:
Data
Control
Management
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
 MAC level
 Data MAC_PDUs:
Superior layer info (MAC_SDU) is carried
MAC_PCI is 34 bytes long.
MTU is 2312.
 In case of the existance of many interferences MAC_SDU
fragmentation is enabled
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
 MAC sublevel
 Control MAC_PDUs:
 Used for the “reservation” of the medium and for ACKs
 ACK: sent by MAC sublevel to ACK a Data MAC_PDU.
• Does not indicate that MAC_PDU destination has received
it.
 RTS (Request to Send) / CTS (Clear to Send) for the reservation
of the medium.
• RTS is sent by MAC sublevel to request the use of the
medium and indicate the duration of the reservation (time
needed).
• CTS is sent by MAC sublevel as a response to RTS.
Indicates that the sender is able to transmit and the duration
of the reservation (time left)
Wireless Local Area Networks
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Standard IEEE 802.11. Data
Link Layer
 MAC sublevel
 Management MAC_PDUs:
 To manage wireless links.
 Beacon. Sent periodically by MAC sublevel to inform about the
existance of a wireless network
• Interval: configurable parameter.
 Probe request. Enable MAC sublevel to search for wireless
networks within an area.
• Informs about transmission rates.
 Probe response. Response to Probe Request.
 Association request. MAC sublevel requests the connection to a
wireless network.
 Association response. Connection confirmation.
 Others.
Wireless Local Area Networks
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Chapter 5: Wireless Local Area
Networks
 5.1 Introduction
 5.2 Standard IEEE 802.11
 5.2.1 Standards
 5.2.2 IEEE 802.11 network components
 5.2.3 Standard IEEE 802.11
 5.2.4 IEEE 802.11 network topologies
Wireless Local Area Networks
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802.11 Network topologies
 The basic communication set in a 802.11 network is the BSS
(Basic Service Set) or cell.


Every BSS has a coverage area, so that all the stations
belonging to the BSS can communicate to the others
A name known as SSID (Service Set Identifier) is assigned to
them.
 According to the number of BSSs and the kind of devices
within a network, there are three types of 802.11 networks:

Ad hoc networks or Independent BSS (IBSS).
 There are only clients.

Infrastructure BSS.
 There are clients and an AP.

EBSS (Extended BSS)
 Multiple BSS to allow bigger coverage area.
Wireless Local Area Networks
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802.11 Network topologies
 Ad hoc 802.11 Network
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802.11 Network topologies
 Infrastructure BSS
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802.11 Network topologies
 EBSS 802.11 Network
BSS
BSS
EBSS
Wireless Local Area Networks
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802.11 Network topologies
Functioning in Infrastructure Mode / EBSS
 Every AP has a BSSID (MAC of its wireless
interface) and a SSID (configured by the network
administrator).

In EBSS, every cell has the same SSID, but is
distinguished by its AP’s BSSID.
 802.11 does not limit the number of clients per AP.
 To connect to a wireless network, a client must
know BSSID & SSID in a cell.


APs send Beacon periodically, with BSSID and, optionally
with SSID
The client sends Probe request with SSID, waiting a
Probe response, from the AP, containing its BSSID.
Wireless Local Area Networks
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802.11 Network topologies
Functioning in Infrastructure Mode / EBSS
 A client who know BSSID & SSID of a cell request
the association (connection) with an AP by means
of the Association Request

If the AP accepts the client, it sends an Association
Response with an Association Id.
AP registers client’s MAC in its Addressing Table.
 An AP controls the communication in all its
associated clients

Clients never communicate with each other directly
They only proccess MAC_PDUs coming from their
associated AP.
Wireless Local Area Networks
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802.11 Network topologies
Functioning in Infrastructure Mode / EBSS
 APs keep Addressing tables, like bridges do.
 They learn from the traffic that goes through it.
 They forward info based on the dest MAC addr
 An AP that is connected to a Distribution System
acts as a bridge, but


Injects traffic to the wireless interface if the dest is
one of its clients or if it is broadcast/multicast
Injects traffic in the Distribution System (like a bridge)
 AP adapts logical addressing if it’s neccessary.
 E.g: Distribution System based on 802.3
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802.11 Network topologies
Association
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802.11 Network topologies
Sending of data MAC-PDU
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