The document summarizes a lecture on wireless networks. It outlines the key requirements for wireless LANs including throughput, number of supported nodes, quality of service, energy saving, and roaming support. It also describes various wireless LAN technologies like infrared, UHF narrowband, and spread spectrum. Finally, it provides an overview of the IEEE 802.11 wireless LAN standard including the physical layers, bands, and data rates supported by different versions of the standard.

1. Wireless Networks
Lecture 3
Dr .Ekhlas Kadhum Hamza
Dept. of Electrical Eng.
Baghdad University
Master course
Second semester
25.3.2015

2. 2
Lecture Outline
 Wireless LAN Requirements
 The Physical Layer(The Infrared Physical Layer
 Microwave-based Physical Layer Alternatives
 Spread Spectrum Technology, UHF
Narrowband Technology
 IEEE 802.11 WLAN, IEEE 802.11 Architecture
 802.11 Physical Layer (PHY), IEEE 802.11 Data
Link Layer

3. 3
1 - Wireless LAN Requirements
 A WLAN is expected to meet the same requirements as a traditional
wired LAN, such as high capacity, robustness, broadcast and multicast
capability, etc. However, due to the use of the wireless medium for
data transmission, there are additional requirements to be met. Those
requirements affect the implementation of the physical and MAC
layers and are summarized below:
 Throughput. Although this is a general requirement for every network,
it is an even more crucial aspect for WLANs. The issue of concern in
this case is the system’s operating throughput and not the maximum
throughput it can achieve. In a wired 802.3 network, for example,
although a peak throughput in the area of 8 Mbps is achievable, it is
accompanied by great delay. Operating throughput in this case is
measured to be around 4 Mbps, only 40% of the link’s capacity. Such
a scenario in today’s WLANs with physical layers of a couple of
Mbps, would be undesirable. Thus, MAC sublayers that shift
operating throughput towards the theoretical figure are required.

4. 4
1 - Wireless LAN Requirements
 Number of nodes. WLANs often need to support tens or hundreds of
nodes. Therefore the WLAN design should pose no limit to the
network’s maximum number of nodes.
 Ability to serve multimedia, priority traffic and client server
applications. In order to serve today’s multimedia applications, such as
video conferencing and voice transmission, a WLAN must be able to
provide QoS connections and support priority traffic among its nodes.
Moreover, since many of today’s WLAN applications use the client-
server model, a WLAN is expected to support nonreciprocal traffic.
Consequently, WLAN designs must take into consideration the fact
that flow of traffic from the server to the clients can often be greater
than the opposite

5. 5
1 - Wireless LAN Requirements
 Energy saving. Mobile nodes are powered by batteries having a finite
time of operation. A node consumes battery power for packet
reception and transmission, handshakes with BSs and exchange of
control information. Typically a mobile node may operate either in
normal or sleep mode. In the latter case, however, a procedure that
wakes up a transmission’s destination node needs to be implemented.
Alternatively, buffering can be used at the sender, posing the danger of
buffer overflows and packet losses, however. The above discussion
suggests that schemes resulting in efficient power use should be
adopted.

6. 6
1 - Wireless LAN Requirements
 Collocated network operation. With the increasing popularity of
WLANs, another issue that surfaces is the ability for two or more
WLANs to operate in the same geographical area or in regions
that partly overlap. Collocated networks may cause interference
with each other, which may result in performance degradation.
One example of this case is neighboring CSMA WLANs.
Suppose that two networks, A and B are located in adjacent
buildings and that some of their nodes are able to sense
transmissions originating from the other WLAN. Furthermore,
assume that in a certain time period, no transmissions are in
progress in WLAN A and a transmitting node exists in WLAN B.
Nodes in A may sense B’s traffic and falsely defer transmission,
despite the fact that no transmissions are taking place in their
own network.

7. 7
1 - Wireless LAN Requirements
 Handoff – roaming support. As mentioned earlier, in cell structured WLANs a
user may move from one cell to another while maintaining all logical
connections. Moreover, the presence of mobile multimedia applications that
pose time bounds on the wireless traffic makes this issue of even greater imp
ortance. Mobile users using such applications must be able to roam from cell
to cell without perceiving degradation in service quality or connection losses.
Therefore, WLANs must be designed in a way that allows roaming to be
implemented in a fast and reliable way.
 Effect of propagation delay. A typical coverage area for WLANs can be up to
150--300 m In diameter. The effect of propagation delay can be significant,
especially where a WLAN MAC demands precise synchronization among
mobile nodes. For example, in cases where unslotted CSMA is used, increased
propagation delays result in a rising number of collisions, reducing the
WLANs performance. Thus, a WLAN MAC should not be heavily dependent
on propagation delay.
 Dynamic topology. In a WLAN, fully connected topologies cannot be
assumed, due to the presence of the ‘hidden’ and ‘exposed’ terminal problems.
A good WLAN design should take this issue into consideration limiting its
negative effect on network performance.

8. 8
1 - Wireless LAN Requirements
 Effect of propagation delay. A typical coverage area for WLANs can be up to
150--300 m In diameter. The effect of propagation delay can be significant,
especially where a WLAN MAC demands precise synchronization among
mobile nodes. For example, in cases where unslotted CSMA is used, increased
propagation delays result in a rising number of collisions, reducing the
WLANs performance. Thus, a WLAN MAC should not be heavily dependent
on propagation delay.
 Dynamic topology. In a WLAN, fully connected topologies cannot be
assumed, due to the presence of the ‘hidden’ and ‘exposed’ terminal problems.
A good WLAN design should take this issue into consideration limiting its
negative effect on network performance.
 Compliance with standards. As the WLAN market progressively
matures, it is of significant importance to comply with existing
standards. Design and product implementations based on new ideas
are always welcome, provided, however, that they are optional exten-
sions to a given standard. In this way, interoperability is achieved

9. 9
2-WLAN Technologies
 The technologies available for use in a WLAN include infrared, UHF
(narrowband), and spread spectrum implementation. Each implementation
comes with its own set of advantages and limitations.
1. Infrared Technology
Infrared is an invisible band of radiation that exists at the lower end of the visible
electromagnetic spectrum. This type of transmission is most effective when a
clear line-of-sight exists between the transmitter and the receiver
Two types of infrared WLAN solutions are available: diffused-beam and direct-
beam (or line-of-sight). Currently, direct-beam WLANs offer a faster data
rate than the diffused-beam networks. Direct-beam is more directional since
diffused-beam technology uses reflected rays to transmit/receive a data signal

10. 10
2-WLAN Technologies

. It achieves lower data rates in the 1–2 Mbps range. Infrared is a short-range
technology. When used indoors, it can be limited by solid objects such as
doors, walls, merchandise, or racking. In addition, the lighting environment
can affect signal quality. For example, loss of communication may occur
because of the large amount of sunlight or background light in an
environment. Fluorescent lights also may contain large amounts of infrared.
This problem may be solved by using high signal power and an optimal
bandwidth filter, which reduces the infrared signals coming from an outside
source. In an outdoor environment, snow, ice, and fog may affect the
operation of an infrared- based system.

11. 11
WLAN Technologies
Table : Considerations for choosing
infrared technology.

12. 12

13. 13
2-WLAN Technologies
2-UHF Narrowband Technology
UHF wireless data communication systems have been available since the early
1980s. These systems normally transmit in the 430 to 470 MHz frequency range,
with rare systems using segments of the 800 MHz range. The lower portion of this
band — 430–450 MHz — is referred to as the unprotected (unlicensed), and 450–
470 MHz is referred to as the protected (licensed) band. In the unprotected band, RF
licenses are not granted for specifi c frequencies and anyone is allowed to use any
frequencies, giving customers some assurance that they will have complete use of
that frequency.
Modern UHF systems allow APs to be individually confi gured for operation on
one of the several preprogrammed frequencies. Terminals are programmed with a
list of all frequencies used in the installed APs, allowing them to change frequencies
when roaming. To increase throughput, APs may be installed with overlapping
coverage but use different frequencies.

14. 14
2-WLAN Technologies
Table :Considerations for choosing UHF
technology.

15. 15
2-WLAN Technologies
3-Spread Spectrum Technology
Most WLANs use spread spectrum technology, a wideband
radio frequency technique that uses the entire allotted
spectrum in a shared fashion as opposed to dividing it into
discrete private pieces (as with narrowband)
By operating across a broad range of radio frequencies, a
spread spectrum device could communicate clearly despite
interference from other devices using the same spectrum in
the same physical location. In addition to its relative immunity
to interference, spread spectrum makes eavesdropping and
jamming inherently diffi cult.

16. 16
2-WLAN Technologies
 In commercial applications, spread spectrum techniques
currently offer data rates up to 2 Mbps. Because the FCC
does not require site licensing for the bands used by
spread spectrum systems, this technology has become
the standard for high-speed RF data transmission. Two
modulation schemes are commonly used to encode
spread spectrum signals: direct sequence spread
spectrum (DSSS) and frequency hopping spread
spectrum (FHSS)

17. 17

18. 18
 In 1997 the IEEE developed an international standard for WLANs: IEEE 802.11-
1997. This standard was revised in 1999. Like other IEEE 802 standards, the
802.11 standard focuses on the bottom two layers of the OSI model, the physical
layer (PHY) and data link layer (DLL). Because of the common interface
provided to upper layers, any LAN application, network operating system, or
protocol including TCP/IP and Novell Netware will run on an 802.11- compliant
WLAN.
 Because of the common interface provided to upper layers, any LAN application,
network operating system, or protocol including TCP/IP and Novell Netware will
run on an 802.11- compliant WLAN. The objective of the IEEE 802.11 standard
was to defi ne an medium access control (MAC) sublayer, MAC management
protocols and services, and three PHYs for wireless connectivity of fi xed,
portable, and moving devices within a local area
3-IEEE 802.11 WLAN

19. 19
IEEE 802.11 WLAN
 The three physical layers are an IR baseband PHY, an FHSS radio in the 2.4 GHz
band, and a DSSS radio in the 2.4 GHz. All three physical layers support both 1 and 2
Mbps operations. WLANs support asynchronous data transfers that refer to the traffi c
that is relatively insensitive to time delays such as electronic mail and fi le transfers.
Optionally WLANs can also support the traffic, which is bounded by the specifi ed
time delay, to achieve an acceptable quality of service (QoS), such as packetized voice
and video.

20. 20
3-802.11 WLANs - Outline
 801.11 bands and layers
 Link layer
 Media access layer
 frames and headers
 CSMA/CD
 Physical layer
 frames
 modulation

Frequency hopping

Direct sequence

Infrared
 Security
 Implementation
Based on: Jim Geier: Wireless LANs, SAMS publishing and IEEE 802 - standards

21. 21
802.11 WLAN technologies
 IEEE 802.11 standards and rates
 IEEE 802.11 (1997) 1 Mbps and 2 Mbps (2.4 GHz band )
 IEEE 802.11b (1999) 11 Mbps (2.4 GHz band) = Wi-Fi
 IEEE 802.11a (1999) 6, 9, 12, 18, 24, 36, 48, 54 Mbps (5 GHz
band)
 IEEE 802.11g (2001 ... 2003) up to 54 Mbps (2.4 GHz)
backward compatible to 802.11b
 IEEE 802.11 networks work on license free industrial, science,
medicine (ISM) bands:
902 928 2400 2484 5150 5350 5470 5725 f/MHz
26 MHz 83.5 MHz 200 MHz
100 mW
Equipment technical requirements for radio frequency usage defined in ETS 300 328
255 MHz
200 mW
indoors only
1 WEIRP power
in Finland
EIRP: Effective Isotropically Radiated Power - radiated power measured immediately after antenna

22. 22
Other WLAN technologies
 High performance LAN or HiperLAN (ETSI-BRAN EN 300
652) in the 5 GHz ISM
 version 1 up to 24 Mbps
 version 2 up to 54 Mbps
 HiperLAN provides also QoS for data, video, voice and
images
 Bluetooth
 range up to 100 meters only (cable replacement tech.)
 Bluetooth Special Interest Group (SIG)
 Operates at max of 740 kbps at 2.4 GHz ISM band
 Applies fast frequency hopping 1600 hops/second
 Can have serious interference with 802.11 2.4 GHz
range network

23. 23
802.11a
 Operates at 5 GHz band
 Supports multi-rate 6 Mbps, 9 Mbps,… up to 54 Mbps
 Use Orthogonal Frequency Division Multiplexing (OFDM) with 52
subcarriers, 4 us symbols (0.8 us guard interval)
 Use inverse discrete Fourier transform (IFFT) to combine multi-carrier
signals to single time domain symbol
902 928 2400 2484 5150 5350 5470 5725 f/MHz
26 MHz 83.5 MHz 200 MHz 255 MHz

24. 24
IEEE 802.11a rates and modulation
formats
Data Rate
(Mbps)
Modulation Coding Rate
Coded bits per
sub-carrier
Code bits per
OFDM symbol
Data bits per
OFDM symbol
6 BPSK 1 / 2 1 48 24
9 BPSK 3 / 4 1 48 36
12 QPSK 1 / 2 2 96 48
18 QPSK 3 / 4 2 96 72
24 16QAM 1 / 2 4 192 96
36 16QAM 3 / 4 4 192 144
48 64QAM 2 / 3 6 288 192
54 64QAM 3 / 4 6 288 216

25. 25
IEEE 802-series of LAN standards
 802 standards free to
download from
http://standards.ieee.org
/getieee802/portfolio.html
hubhub
hubhub
hubhub
hubhub
routerrouter
serverserver
stationsstations
stationsstations
stationsstations
DQDB: Distributed queue dual buss, see PSTN lecture 2
Demand priority: A round-robin (see token rings-later) arbitration
method to provide LAN access based on message priority level

26. 26
The IEEE 802.11 and
supporting LAN Standards
 See also IEEE LAN/MAN Standards Committee Web site
www.manta.ieee.org/groups/802/
IEEE 802.3
Carrier
Sense
IEEE 802.4
Token
Bus
IEEE 802.5
Token
Ring
IEEE 802.11
Wireless
IEEE 802.2
Logical Link Control (LLC)
MAC
PHY
OSI Layer 2
(data link)
OSI Layer 1
(physical)
bus star ring
a b g

27. 27
4-IEEE 802.11 Architecture
 The architecture of the IEEE 802.11 WLAN is designed to support a network
where most decision making is distributed to mobile stations. This type of
architecture has several advantages. It is tolerant of faults in all of the WLAN
equipment and eliminates possible bottlenecks a centralized architecture
would introduce. The architecture is flexible and can easily support both
small, transient networks and large, semipermanent or permanent networks.
In addition, the architecture and protocols offer signifi cant power saving and
prolong the battery life of mobile equipment without losing network
connectivity
 Two network architectures are defined in the IEEE 802.11 standard:
Infrastructure network: An infrastructure network is the network architecture
for providing communication between wireless clients and wired network
resources. The transition of data from the wireless to wired medium occurs
via an AP. An AP and its associated wireless clients define the coverage area.
Together all the devices form a basic service set (see Figure 1).

28. 28
4-IEEE 802.11 Architecture
 Point-to-point (ad hoc) network: An ad hoc network is the architecture that is
used to support mutual communication between wireless clients. Typically,
an ad hoc network is created spontaneously and does not support access to
wired networks. An ad hoc network does not require an AP. IEEE 802.11
supports three basic topologies for WLANs: the independent basic service set
(IBSS), the basic service set, and the extended service set (ESS). The MAC
layer supports implementations of IBSS, basic service set, and ESS
configurations.
 The IBSS configuration is referred to as an independent configuration or an ad
hoc network. An IBSS configuration is analogous to a peer-to-peer office
network in which no single node is required to act as a server. IBSS WLANs
include a number of nodes or wireless stations that communicate directly with
one another on an ad hoc, peer-to-peer basis. Generally, IBSS imple-
mentations cover a limited area and are not connected to any large network.
An IBSS is typically a short-lived network, with a small number of stations,
that is created for a particular purpose. The basic service set configuration
relies on an AP that acts as the logical server for a single WLAN cell or
channel. Communications between station 1 and

29. 29
4-IEEE 802.11 Architecture
 station 4 actually flow from station 1 to AP1 and then from AP1 to AP2
and then from AP2 to AP4 and fi nally AP4 to station 4 (refer to Figure
21.4). An AP performs a bridging function and connects multiple WLAN
cells or chan- nels, and connects WLAN cells to a wired enterprise LAN
 The ESS confi guration consists of multiple basic service set cells that
can be linked by either wired or wireless backbones called a distributed
system. IEEE 802.11 supports ESS confi gurations in which multiple
cells use the same channel, and confi gurations in which multiple cells
use different chan- nels to boost aggregate throughput. To network the
equipment outside of the ESS, the ESS and all of its mobile stations
appear to be a single MAC- layer network where all stations are
physically stationary. Thus, the ESS hides the mobility of the mobile
stations from everything outside the ESS (see Figure 21.5).

30. 30
IEEE 802.11 Architecture4-

31. 31
4-IEEE 802.11 Architecture

32. 32
PHY
IEEE 802.11 Architecture
 IEEE 802.11 defines the physical (PHY), logical link (LLC) and
media access control (MAC) layers for a wireless local area
network
 802.11 networks can work as
 basic service set (BSS)
 extended service set (ESS)
 BSS can also be used in ad-hoc
networking
LLC: Logical Link Control Layer
MAC: Medium Access Control Layer
PHY: Physical Layer
FHSS: Frequency hopping SS
DSSS: Direct sequence SS
SS: Spread spectrum
IR: Infrared light
BSS: Basic Service Set
ESS: Extended Service Set
AP: Access Point
DS: Distribution System
DS,
ESS
ad-hoc network
LLC
MAC
FHSS DSSS IR
Network
802.11

33. 33
5-Physical Layer (PHY)
 At the physical layer, IEEE 802.11 defines three physical characteristics for
WLANs: diffused infrared (baseband), DSSS, and FHSS. All three support
a 1 to 2 Mbps data rate. Both DSSS and FHSS use the 2.4 GHz ISM band
(2.4–2.4835 GHz). The physical layer provides three levels of functionality.
These include:
 (1) frame exchange between the MAC and PHY under the control of the
physical layer convergence
 procedure (PLCP) sublayer; (2) 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; and
 (3) providing a carrier sense indication back to the MAC to verify activity on
the media (see Figure 21.6).
 Each of the physical layers is unique in terms of the modulation type,
designed to coexist with each other and operate with the MAC. The specifi
cations for IEEE 802.11 meet the RF emissions guidelines of FCC, ETSI, and
the Ministry of Telecommunications

34. 34
5-Physical Layer (PHY)

35. 35
Assignment
 Example 21.1
 Example 21.2
 Example 21.3
 Example 21.4
1- What is the frequency range of the IEEE 802.11a
standard?
2- What is the maximum distance running the lowest data
rate for 802.11b?
3- What is the maximum distance with maximum data rate
for 802.11a?
4- What is the frequency range of the IEEE 802.11b
standard?

36. 36
Assignment
 5- You have a Cisco mesh network. What protocol allows
multiple APs to connect with many redundant connections
between nodes?
 6- What is the maximum data rate for the 802.11g
standard?
 7- What is the maximum data rate for the 802.11a
standard?
 8- What is the maximum data rate for the 802.11b
standard
9- What is the frequency range of the IEEE 802.11g
standard?