Wireless Local Area Networks

Wireless Local Area Networks
Chapter 21-21.8

By
Donavon M. Norwood
CS286 Dr. Moh
SJSU
08/18/2009

Introduction - 21.1

WLANs are a flexible data communication systems that can be used for
applications in which mobility is required. WLANs are designed to
operate in the following bands:

Industrial

Scientific

Medical (ISM)

Currently WLANs provide speeds up to 11 Mbps but in the future
manufacturers are trying to make them have speeds up to 54 Mbps. In
the USA the FCC regulates radio transmission however they do not
require a license for a user to use the ISM or U-NII bands. The IEEE
802.11 are responsible for the WLAN standards which include:

802.11a (Wifi 5)

802.11b (Wifi)

802.11g

802.11n 2

Figure 21.1 IEEE 802.11 WLAN standards

3

Introduction – 21.1 (Continued)


In WLANs a connection between the client and the user is
accomplished by the use of a wireless medium such as a RF or Infrared
(IR) communications instead of a cable. This will allow a remote user to
stay connected to the network while mobile or not physically attached to
the network.

Wireless connections are made through a hand held device terminal or
laptop that has an RF interface inside the terminal or through a PC card
slot of the laptop. The connection from the wired LAN is made through
an access point (AP) which can act as a gateway for wireless users
data to be routed onto the wired network.

An important feature of WLANs is that they can be used independently
of wired networks. The network spectrum for communications is the one
designed as license free and in this band 2.4-2.5 GHz users can
operate in this band without a license as long as they have equipment
designed to use the free band.
4

Figure 21.2 WLAN Applications

5

Introduction – 21.1 (Continued)

The following are advantages of deploying WLANs:

Mobility improves productivity with real time access to
information regardless of worker location.

Cost effective network set up.

Reduced cost of ownership.

The following are some issues with deploying WLANs:

Frequency allocation

Interference and reliability

Security

Power consumptions

Mobility

Throughput 6

WLAN equipment - 21.2

There are three main links that form the basis of a wireless
network:

LAN adapter are made in the same fashion as wired adapters:
PCMCIA, Card bus, PCI and USB. They enable users to access
the network.

Access point (AP) is the wireless equivalent of a LAN hub. It
receives, buffers and transmits data between the WLAN and the
wired network.

Outdoor LAN bridges are used to connect LANs in different
buildings.

7

WLAN Topologies - 21.3

WLANs can be built using the following topologies:

• Peer-to-peer (adhoc) topology in which client devices
in the same cell communicate with each other directly.

• Access point based topology uses access points to
bridge traffic onto a wired (Ethernet/Token ring) or
wireless backbone.

• Point-to-multipoint topologyy in which
wirelessbridges connect LANs in one building to LANs
in another building even if the buildings are miles apart.
8

Figure 21.3 Peer-to-peer topology
(Ad hoc Network)

9

Figure 21.4 Access Point based topology

10

WLAN Technologies - 21.4

WLANs include the following technologies:

• Infrared

• UHF (narrowband)

• Spread spectrum

Each implemantation comes with its own advantages and
disadvantages.

11

Infrared Technology - 21.4.1

Infrared is an invisible band of radiation that exists at the
lower end of the visible electromagnatic spectrum. This
type of transmission is most effective when a clear line
exists between the sender and receiver.

Two type of infared solutions are available:

• Diffused beam uses reflected rays to transmit/receive
a data signal. Data rate for it is lower data rates in the
1-2 Mbps range.

• Direct beam which is more directional therefore it is
more faster than diffused beam.
12

UHF Narrowband Technology - 21.4.2

UHF which has been around since the early 1980s
normally transmit in the 430 to 470 MHz frequency
range with systems who rarely use the 800 MHz range.
The lower portion of this band (430-450 MHz) is called
the unprotected or unlicensed band and the 450-470
MHz is referred to the protected or licensed band. In
the unprotected band RF licenses are not granted for a
specific frequencies and anyone is allowed to use any
frequencies. The term narrowband is used to
described this technology because the RF signal is
sent in a very narrow bandwidth, typically 12.5 kHz or
25 Khz and power levels range from 1-2 watts in
narrowband RF systems.
13

Spread Spectrum Technology - 21.4.3

Many WLANs use Spread Spectrum technology which is a
wideband radio frequency technique that uses the entire allotted
spectrum in a shared fashion. It spreads the transmission power
over the entire usable spectrum. Spread Spectrum technology
makes eavesdropping and jamming inherently difficult. Two
modulation schemes are used in Spread Spectrum technology:

• Direct sequence spread spectrum (DSSS) generates a
redundant bit pattern for each bit to be transmitted and the
pattern is known as a spreading code. The longer the spreading
code the greater the probability the data can be recovered.

• Frequency Hopping spread spectrum which uses a
narrowband carrier that changes frequency in a pattern known to
the sender and receiver. When properly synchronized the net
effect is to maintain a single logical channel. 14

IEEE 802.11 Architecture 21.5.1

The architecture of the IEEE 802.11 WLAN is designed to
support a network where most decision making is
distributed through the mobile stations. The network
architectures are defined for the IEEE 802.11 standard:

• Infrastructure network is a network architecture for
providing communication between wireless clients and and
wired network services. The transition from wireless to
wired is done through a access point (AP).

• Point-to-point (ad hoc) network is an architecture that is
used to support wireless communication between wireless
clients. This type of network does not provide access to a
wired network.
15

(Continued)

IEEE 802.11 supports three type of topologies:

• Independent basic service set (IBSS) is referred to as an
idependent network configuration for an ad hoc network in
which no single node is required to act as a server. It is
normally a short lived network.

• Basic service set relies on an access point (AP) that acts
as a logical server for a single WLAN cell or channel.

• Extended service set consists of multiple basic service set
cells that can be linked together by either a wired or
wireless backbones called distributed systems.
16

Figure 21.5 BSS and ESS configuration
of IEEE 802.11 WLAN

17

802.11 Physical layer (PHY) - 21.5.2
At the physical layer IEEE 802.11 defines three physical standards
for WLANs:

• Diffused infrared (baseband)
• DSSS
• FHSS

All three support a 1-2 Mbps data rate. Both DSSS and FHSS use
the ISM band (2.4-2.4835 GHz). The physical layer provides three
levels of functionality:

• Frame exchange between MAC and PHY under the control of
the physical layer convergence procedure (PLCP)
• Use of signal carrier and spread spectrum (SS) modulation to
transmit data frames over the media under the control of the
physical medium dependent (PMD) sublayer.
• Provide a carrier sense indication back to the MAC to verify 18
activity on the media.

Figure 21.6 OSI model for IEEE 802.11 WLAN

19

DSSS PHY
In the DSSS PHY data transmission over the media is controlled by the PMD
sublayer as directed by the PLCP sublayer. The DSS PMD are scrambled
using a self-synchronizing 7-bit polynomial and uses a 11-bit Baker code (1,
-1, 1, -1, 1, 1, -1, -1, -1) for spreading. PLCP takes the binary information bits
from the PLCP protocol data unit (PPDU) and converts them into a RF
signals by using DSSS and modulation. The PLCP preamble/header are
transmitted at 1 Mbps and the MPDU at 1-2 Mbps. The PPDU frame consists
of the follow fields:

• Start of frame delimiter (SFD) which contains information that marks the
start of the PPDU frame.
• Signal field indicates which modulation scheme to use to receive the
incoming MPDU.
• Service field is reserved for future use.
• Length field indicates the number of microseconds necessary to to transmit
the MPDU and is also used to indicate the end of the PPDU frame.
• CRC field contains the results of a calculated CRC from the sending station
which is a ITU CRC-16 error detection algorithm.
• SYNC field is 128 bits and contains a string of 1s which are scrambeled prior
to transmission. 20

Figure 21.7 DSS Transmit and receive DSS PPDU

21

Figure 21.8 DSS PHY PPDU frame

22

DSSS PHY (Continued)

Each DSS Phy channel occupies 22 MHz of bandwidth and allows for three
non-interfering channels that are spaced 25 MHz in the 2.4 frequency band.
With this channel arrangement a user can configure multiple DSSS networks
to operate simultaneously in the same area.

Figure 21.9 Channel spacing for IEEE 802.11 DSSS networks

23

FHSS PHY

In FHSS PHY data transmission over media is controlled by the FHSS PMD
sublayer directed by the FHSS PLCP sublayer. Channel hopping is
controlled by FHSS PMD. The FHSS PMD takes the binary from the
whitened PSDU and converts them into RF signals by using carrier
modulation and FHSS techniques. The PLCP preamble are transmitted at 1
Mbps. The format of the PHSS PHY PPDU frame consists of the following
fields:

• Sync field contains information marking the start of the PSDU frame.
• PLCP length word (PLW) specifies the length of the PSDU in octets and
used by the MAC layer to indicate the end of the PPDU frame.
• PLCP signaling field (PSF) identifies the data rate of the whitened PSDU
ranging from 1-4.5 Mbps in increments of 0.5 Mbps.
• Header error check field contains information of a calculated frame check
sequence from the sending station which uses a ITU CRC-16 error detection
algorithm.
• Data whitening is used for the PSDU before transmission to minimize DC
bias on the data if long strips of 1s or 0s are contained in the PSDU.
24

Figure 21.11 FHSS PHY PPDU frame

25

Figure 21.10 FHSS PHY transmitter and receiver

26

802.11a Orthogonal Frequency Division Multiplexing
(OFDM)
Orthogonal Frequency Division Multiplexing (OFDM) PHY provides
The capability to transmit PSDU frames at multiple data rates up to
54 Mbps for a WLAN where the transmission of multimedia content
is a consideration. The PLCP preamble/signal fields are always
transmitted at 6 Mbps, BPSK-OFDM modulated using a
coventional encoding rate R = 1/2. The PPDU frame consists of
the followinf fields:
• PLCP preamble is used to acquire the incoming signal and train and
synchronize the receiver.
• Signal field is a 24-bit field that contains data about the rate and length of
the PSDU which is encoded at the rate R = ½, BPSK-OFDM modulated.
• Length field a 12-bit integer used to indicate the number of octets in the
PSDU. Four bits are used to encode the rate, eleven bits to define the length,
one reserved bit, and six 0 tail bits.
• Data field contains the service field, PSDU, tail bits and pad bits. Six tail bits
containing 0s are appended to the PPDU to ensure that the encoder is
brought back to zero.
27

Figure 21.12 OFDM PLCP preamble, header, and PSDU

28

802.11a Orthogonal Frequency Division Multiplexing
(OFDM) - Continued
In OFDM modulation the basic principal of operation is to divide a high speed
binary signal to be transmitted into a number of lower data rate sub-carriers
There are 48 subcarriers and 4 carrier pilot subcarriers for a total of 52 nonzero
subcarriers defined in IEEE 802.11a. Prior to transmission the PPDU is
encoded using a coded rate of R = 1/2, and the bits are recorded and bit
interleaved for the desired rate. Each bit is then mapped into into a complex
number according to modulation type and divided into 48 subcarriers and 4
carrier pilot subcarriers. The subcarriers are combined and using inverse fast
Fourier transform (IFFT) and then transmitted. At the receiver the carrier is
converted back into a multicarrier lower data rate using fast frquency transform
(FFT) and then combined to form a high rate PPDU.
Figure 21.13 IEEE 802.11a transmit/receive OFDM PMD

29

IEEE 802.11 Data Link Layer - 21.5.3

The data link layer in 802.11 consists of two sub-layers:

• Logical Link Control (LLC) is where framing takes
place in which it inserts certain fields in the frame such
source/destination address at the head of the frame
and error handling at the end of the frame.

• Media Access Control (MAC) is similar to the 802.3
standard but it is designed to support multiple access
to the medium for users by having a sender sense the
medium before sending data. 802.3 uses carrier sense
multiple access/collision detection (CSMA/CD), but
collision detection is not possible with 802.11.
30

IEEE 802.11 Medium Access Control (MAC) - 21.5.4

MAC schemes include:

• Random access which include ALOHA, CSMA, CSMA/CD, and CSMA/CA.

• Deterministic access which includes FDMA, TDMA and CDMA.

• Mixed access which includes CSMA/TDMA.

Since wireless networks are not able to detect collisions like Ethernet, 802.11
uses CSMA/CA together with a positive ACK. The MAC layer of the
transmitting station senses the medium and if the medium is free for a
specified amount of time called distributed inter-frame space (DIFS) then
the station is able to send the packet. If the medium becomes busy during
the DIFS interval the station uses the exponential backoff which is commonly
used to resolve contention problems. In 802.11b a slot has a 20 s duration
and the random number must be greater than 0 and smaller than the value of

the contention window (CW).
31

Figure 21.4 CSMA/CA in IEEE 802.11b

32

Hidden and Exposed Node problem

• Another major problem in the 802.11 MAC layer is the
hidden node issue, in which two stations on the opposite
side of an AP can hear the activity of the AP but not from
each other usually due to distance or an obstruction.

• 802.11 solves this problem by using the optional request
to send/clear to send (RTS/CTS) at the MAC layer. A
sending station will send a RTS to the AP and waits for the
AP to reply with a CTS. Since all stations can hear the AP,
the CTS causes them to delay any transmissions and
allowing the sending to transmit and receive a packet ACK
without the chance of a collision.

• The RTS protects the transmitter area from collisions
during a ACK. 33

Figure 21.15(a & b) Hidden and exposed node problem

34

IEEE 802.11 MAC sublayer - 21.5.5

In 802.11 the MAC layer is responsible for synchronous data service, security
service (confidentiality, authentication, access control) and MSDU ordering.
The MAC frame contains the following fields:

• Transmitter address is the address of the MAC that transmitted the frame
onto the wireless medium.
• Receiver address (RA) is the address of the MAC in which the frame is sent
over the wireless meduim.
• Source address (SA) is the address of the MAC that originated the frame.
• Destination address (DA) is the address of the final address to which the
frame is sent.
• Sequence control field is a 16-bit field that contains two subfields which are
a 4-bit fragment number and a 12-bit sequence number.
• Frame body field contains the information specific to a particular data or
management frames.
• Frame check sequence (FCS) 32 bits in length that contains the result of
applying a C-32 polynomial to the MAC header and frame body.
35

Figure 21.16 IEEE 802.11 MAC frame format

36

Joining an existing Basic Service Set - 21.6

The 802.11 MAC layer is responsible for how a station
associates with an AP. When a 802.11 station enters the range
of one or more Aps it chooses the AP to associate with based on
signal strength and observed packet error rates. One accepted
by the AP the station tunes to the channel to which the AP is set.
When a station wishes to access an existing BSS, it needs
synchronization information from the AP in one of two ways:

• Passive scanning in which the station waits to receive a
beacon frame which contains synchronization information from
the AP.

• Active scanning in which the stations tries to contact the AP by
transmitting a probe request frame and then waiting to receive
a probe response from AP.
37

Security of IEEE 802.11 Systems - 21.7

The IEEE 802.11 provides MAC access control and
encryption mechanisms:

• Wireless equivalent privacy (WEP) algorithm used to
encrypt messages and uses Rivest Cipher 4 (RC4)
with 40 and 128 bit keys.

• ESSID is used for access control and programmed into
each AP and is required knowledge in order for a
wireless client to associate with an AP.

• MAC access control list are used to restrict access to
stations to the AP whose MAC address is not listed on
the access control list. 38

Power Management - 21.8

Power management is necessary to minimize power
requirements for battery powered portable mobile
units. The standard supports the two following modes:

• Continuous aware mode in which the radio is always
on and draws power.

• Power save polling mode the radio is dozing with the
AP and is queing any data for it.

39

THANK YOU!

References: Wireless and Data Communications and Networking, Vijay K. Garg

40

Wireless Local Area Networks

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Wireless Local Area Networks