1

Dr. Sanjay Tiwari

PRSU,Raipur

2

Introduction

• Wireless LANs transmit data through the air

using radio frequency transmissions.

• Several standards for WLANs have recently

emerged facilitating market to take off.

• Currently the three principal WLAN

technologies are: 802.11b (low speed), 802.11a

(a higher speed protocol) and Bluetooth.

• An emerging WLAN standard that may prove

more important in the future is 802.11g.

3

WLAN in the workplace

• WLANs are popular because they:

– Eliminate cabling and make network access possible from a variety of locations

– Facilitate computing for mobile workers at different office locations or as those workers move around the office.

– Are increasingly used in hospitals because they enable doctors and nurses access patient records.

– Are becoming popular in airports because they enable business travelers to access the Internet while waiting for their flights to leave.

4

Wireless Ethernet (IEEE 802.11b)

• The IEEE 802.11b, also called wireless

Ethernet, is now the dominant WLAN

standard.

• Two version of IEEE 802.11b exist:

– Frequency-hopping spread-spectrum (FHSS)

with data rates of 1 and 2 Mbps and

– Direct-sequence spread-spectrum (DHSS)

with data rates of 1, 2, 5.5 and 11Mbps, which

dominates the market due to its higher speed.

5

Types of Wireless Ethernet

• Two forms of the IEEE 802.11b standard currently exist:

• Direct Sequence Spread Spectrum (DSSS) uses the entire 2.4 GHz WLAN frequency band to transmit information. DSSS is capable of data rates of up to 11 Mbps with fallback rates of 5.5, 2 and 1 Mbps. Lower rates are used whenever interference or congestion occurs.

• Frequency Hopping Spread Spectrum (FHSS) divides the frequency band into a series of channels and then changes its frequency channel about every half a second, using a pseudorandom sequence. FHSS is more secure, but is only capable of data rates of 1 or 2 Mbps, since the frequency band gets divided up into a number of channels.

• IEEE 802.11a is another Wireless LAN standard that is still being defined. It will operate in the 5 GHz band and be capable of data rates of up to 54 Mbps, but will probably average about 20 Mbps in practice.

6

IEEE 802.11b Wireless LAN Topology

• WLANs use a physical star, logical bus topology (Fig. 8-1).

• Each WLAN computer uses a wireless NIC that transmits radio signals to the AP.

• WLAN network access is through devices called access points (APs), which have a maximum transmission range of about 100-500 feet.

• AP also connect into the wired LAN. The AP acts as a repeater by retransmitting frames from client computers over the wired network.

• Multiple APs are needed to make wireless access possible in most areas of a building.

• IEEE 802.11 also uses 3 separate radio channels, allowing APs with overlapping ranges to be set up without interfering with each other’s signals.

7

Wireless Local Area Networks (WLANs)

WLANs connect “local” computers (100m range)

Breaks data into packets

Channel access is shared (random access)

Backbone Internet provides best-effort service

Poor performance in some apps (e.g. video)

01011011

Internet

Access

Point

0101 1011

8

Wireless LAN Standards

• 802.11b (Current Generation) – Standard for 2.4GHz ISM band (80 MHz) – Frequency hopped spread spectrum – 1.6-10 Mbps, 500 ft range

• 802.11a (Emerging Generation) – Standard for 5GHz NII band (300 MHz) – OFDM with time division – 20-70 Mbps, variable range – Similar to HiperLAN in Europe

• 802.11g (New Standard) – Standard in 2.4 GHz and 5 GHz bands – OFDM – Speeds up to 54 Mbps

In 200?,

all WLAN

cards will

have all 3

standards

9

Wireless Local Area Networks

(continued)

10

Wireless Local Area Networks

(continued)

11

Figure 8-1 A wireless Ethernet access point

connected into an Ethernet Switch.

12

Fixed Broadband Wireless

• Integrated Services Digital Networks (ISDN)

– Transmit at 256 Kbps over regular phone lines

• T1 lines

– Transmit at 1.544 Mbps

• Cable modems and digital subscriber lines

(DSL)

– Generally only available in residential areas

– Maximum transmission speed is only about 8 Mbps

13

Wireless Wide Area Network

(continued)

14

Wireless Wide Area Network

(continued) • Programming languages

– BREW (Binary Run-Time Environment for

Wireless)

– J2ME (Java 2 Micro Edition)

• Wireless Wide Area Network (WWAN)

– Enables employees to access corporate data and

applications from virtually anywhere

15

The Wireless Landscape

16

The Wireless Landscape (continued)

17

Digital Convergence

• Digital convergence

– Refers to the power of digital devices to

combine voice, video, and text processing

capabilities

• As well as to be connected to business and home

networks and to the Internet

18

Wireless Applications • Main areas

– Education

– Home entertainment

– Health Care

– Government and Military

– Office environments

– Event management

– Travel

– Construction and warehouse management

– Environmental research

– Industrial control

19

Advantages of Wireless

Networking • Mobility

– Freedom to move about without being tethered by wires

– Permits many industries to shift toward an increasingly mobile workforce

– Gives team-based workers the ability to access the network resources

• Easier and less expensive installation

– Installing network cabling in older buildings can be a difficult, slow, and costly task

– Makes it easier for any office to be modified with new cubicles or furniture

20

Advantages of Wireless

Networking (continued)

• Increased reliability

– Network cable failures may be the most common source of network problems

• Disaster recovery

– In the event of a disaster, managers can quickly relocate the office

21

Disadvantages of Wireless

Networking

• Radio signal interference

– The potential for two types of signal interference

exists

• Security

– It is possible for an intruder to be lurking outdoors

with a notebook computer and wireless NIC

• With the intent of intercepting the signals from a nearby

wireless network

– Some wireless technologies can provide added levels of security

22

Disadvantages of Wireless

Networking (continued)

• Health risks

– High levels of RF can produce biological damage through heating effects

• Wireless devices emit low levels of RF while being used

23

Summary

• Wireless communications have become

commonplace

• Wireless networks and devices are found in all

circles of life today

• Wireless wide area networks will enable

companies of all sizes to interconnect their

offices

– Without the high cost charged by telephone carriers

for their landline connections

• WLAN applications are found in a wide variety

of industries and organizations

24

Summary (continued)

• Remote sensors

– Capable of communicating using wireless

technologies

– Used in large manufacturing facilities

• To monitor equipment and for scientific research

• Wireless communication advantages

– Mobility

– Easier and less expensive installation

– Increased network reliability

– Support for disaster recovery

25

Summary (continued)

• Wireless communication disadvantages

– Radio signal interference

– Security issues

– Health risks

26

WLAN Media Access Control

• Wireless LANs use CSMA/CA where CA = collision avoidance (CA).

• With CA, a station waits until another station is finished transmitting then continues to wait an additional random period of time before sending anything, thus ensuring that the network is truly not being used.

• Two WLAN MAC techniques are now in use:

– Distributed Coordination Function (DCF)

– Point Coordination Function (PCF).

27

Distributed Coordination Function (DCF)

• With DCF, also known as physical sense carrier method, a node that wants to send first listens to make sure that the transmitting node has finished, then waits a random period of time longer.

• During transmission, each frame is sent using the Stop and Wait ARQ, so by waiting, the listening node can detect that the sending node has finished and can then begin sending its transmission.

• With Wireless LANs, ACK/NAK signals are sent a short time after a frame is received.

• Stations wishing to send a frame wait a somewhat longer time, ensuring that no collision will occur.

28

Point Coordination Function (PCF)

• When a computer on a Wireless LAN is near the

transmission limits of the AP at one end and another

computer is near the transmission limits at the other end of

the AP’s range, both computers may be able to transmit to

the AP, but can not detect each other’s signals.

• This is known as the hidden node problem. When it

occurs, the physical carrier sense method will not work.

• The virtual carrier sense method solves this problem by

having a transmitting station first send a request to send

(RTS) signal to the AP. If the AP responds with a clear to

send (CTS) signal, the computer wishing to send a frame

can then begin transmitting.

29

IEEE 802.11b Message Delineation

• IEEE 802.11b frames (Figure 8-3) have 5 sections:

• preamble, which contains a preamble used to mark the start of the frame.

• physical layer convergence protocol (PLCP) header, enables the NIC and AP to determine the best data rate to use. The length field also gives the length of the payload portion of the frame.

• payload header contains frame control, source and dest. addresses, & sequence ctr’l information.

• LLC PDU is the data field.

• payload trailer, contains the CRC-32 error detection value.

6.30

6-1 SPREAD SPECTRUM

In spread spectrum (SS), we combine signals from

different sources to fit into a larger bandwidth, but our

goals are to prevent eavesdropping and jamming. To

achieve these goals, spread spectrum techniques add

redundancy.

Frequency Hopping Spread Spectrum (FHSS)

Direct Sequence Spread Spectrum (DSSS)

Topics discussed in this section:

6.31

Spread Spectrum

• A signal that occupies a bandwidth of B, is spread

out to occupy a bandwidth of Bss

• All signals are spread to occupy the same

bandwidth Bss

• Signals are spread with different codes so that they

can be separated at the receivers.

• Signals can be spread in the frequency domain or

in the time domain.

Narrowband and Spread Spectrum

• Different ways of transmitting over RF

• Narrowband uses little bandwidth, but high power

– 2 Mhz @ 80 Watts

– Easier to block/jam

• Spread Spectrum uses more bandwidth than needed and spreads the signal

– 22 Mhz at 100 mW

– Harder to jam

Pg 195

Transmission issues

• Multipath

– When a reflected signal arrives at receiving antenna

after the primary signal

• Delay between main and reflected signal is the

delay spread

– If delay spread is long enough to interfere with next

part of main signal it is intersymbol interference (ISI)

• Spread Spectrum technologies try to avoid ISI by

spreading

– More tolerant than narrowband

Pg 197

FHSS

• Used in 802.11 prime

• 1 and 2 Mbps in 2.4 Ghz ISM

• Original spec for 79 Mhz between 2.402 and 2.480

– Mostly between 1997 and 1999

• Transmits small amount and then hops

– Dwell time is amount of time on each frequency

• Hopping sequences need to sync between devices

– Original spec of 1 Mhz hop

– 802.11 standard included for hopping sequence information to be sent in the beacon frame to client stations

6.36

Figure 6.27 Spread spectrum

37

6.38

Figure 6.32 DSSS

39

40

6.41

Figure 6.33 DSSS example

6.42

Figure 6.28 Frequency hopping spread spectrum (FHSS)

6.43

Figure 6.29 Frequency selection in FHSS

6.44

Figure 6.30 FHSS cycles

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6.46

Figure 6.31 Bandwidth sharing

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Figure 8-3

Ethernet 802.11b

DSSS frame

layout

48

802.11b DSSS Data Transmission

• 802.11b transmits data using radio waves, a form of analog transmission.

• The direct sequence spread spectrum (DSSS) form of 802.11b divides the bandwidth into three 22 MHz channels, each separated by a 3 MHz guardband at: 2.412, 2.437 and 2.642 GHz.

• In all cases, an 11 Mbps symbol rate is used, but since each symbol the number of bits per symbol is usually higher, the actual data rates are lower.

• For example, 1 Mbps version of 802.11b transmits 11 bits for each data bit.

49

Barker Codes and DPSK (Figure 8-4)

• The 1 Mbps version of DSSS first converts each bit into an 11-bit Barker sequence, used to minimize signal interference.

• A 1011101000 sequence is used for a 0 and its inverse, 0100010111 for a 1.

• The bits in the Barker sequence are sent using differential binary phase shift keying (DBPSK) in which a 0 is sent as a 00 phase shift and a 1 is sent as a 1800 phase shift.

• The 2 Mbps version of DSSS uses differential quadrature phase shift keying (DQPSK) in which four phase shifts are used (00, 900, 1800 and 2700) to encode 2 bits per phase shift.

50

Figure 8-4 How 1 Mbps 802.11b DSSS uses

Barker sequences to transmit data

51

5.5 Mbps 802.11b using CCK

• The 5.5 Mbps version of DSSS uses complementary code keying (CCK)

• With CCK, data is broken up into groups of four bits (see Figure 8-5).

• The second two bits are used to select an 8-bit CCK code word, again designed to minimize signal interference.

• The first two data bits are used to select one of four phase shifts that will be used for transmitting the data using DQPSK.

52

Figure 8-5 How 5.5 Mbps 802.11b DSSS uses

CCK code words to transmit data

53

IEEE 802.11b

54

IEEE 802.11a WLAN Topology

• 802.11a uses the same topology as 802.11b, transmitting data rates of up to 54 Mbps using frequencies in the 5 GHz range, with a total available bandwidth of 300 MHz.

• The signal range for 802.11a is also reduced to only 50m, for 12-24 Mbps and to only 15m for 54 Mbps.

• This means that more APs are needed to cover the same area as for 802.11b (see Figure 8-6).

• 802.11a uses 12 channels instead of the 3 802.11b uses, making AP co-location possible. Higher data rates are then possible by having multiple APs co-located and assigning each to a different frequency.

55

Figure 8-6 It takes more 802.11a APs to provide the

same coverage as one 802.11b access point.

56

IEEE 802.11a: MAC, Error Control

and Message Delineation

• Media access control and error control: same

as with IEEE 802.11b

• IEEE 802.11a frames: very similar structure

to 802.11b frames (Figure 8-7).

57

Fig. 8-7

Ethernet

802.11a

frame

layout

58

Data Transmission using OFDM and BPSK

• 802.11a uses a technique called orthogonal

frequency division multiplexing (OFDM) to transmit

data.

• With OFDM, the 20 MHz channels are broken up

into 48 352 kHz channel (see Figure 8-8).

• Transmissions are made up of 48 bit OFDM

symbols.

• In the case of 6 Mbps 802.11a, each group of 24 bits

of data is converted into a 48-bit OFDM symbol.

• Data is sent through each channel using BPSK.

59

Fig. 8-8 How 6 Mbps 802.11a uses

OFDM and BPSK to transmit data

60

Data Transmission using OFDM at other

data rates

• Data sent at high data rates works in a similar way to

the previous example, but the details may vary

according to:

– How much data is used to create the 48 bit OFDM

symbols

– How many OFDM symbols are created

– The modulation technique (BPSK, QPSK, QAM)

• For example, to send data at 24 Mbps, 96 bits are

converted into 4 OFDM symbols. Data is sent using

16-QAM, i.e., four bits per channel (see Figure 8-9).

61

Fig. 8-9 How 24 Mbps 802.11a uses

OFDM and QAM to transmit data

62

IEEE 802.11g

63

IEEE 802.11g

• 802.11g is a new WLAN standard being developed but not yet finalized.

• The initial proposal provides a 22 Mbps data rate. 54 Mbps is also possible over shorter distances.

• Like 802.11b, 802.11g will transmit using the 2.4 GHz frequency band (like 802.11b) operate over distances of up 100-150m.

• 802.11g will also use the OFDM transmission approach similar to 802.11a.

• Some experts are critical of the new protocol since they believe that it may end up undermining the markets for both 802.11b and 802.11a.

64

Bluetooth

65

Bluetooth

• Bluetooth, standardized as IEEE 802.15, provides networking for small personal networks.

• Bluetooth’s basic data rate is 1 Mbps.

• Devices are small and cheap and have been designed to eliminate cabling between keyboards, mice, telephone handsets and PDAs.

• Bluetooth is not compatible with the other IEEE 802.11 WLAN standards.

66

Bluetooth Media Access Control

• Bluetooth network is called a piconet.

• All communications is between the master devices and the slave devices. Slaves do not communicate directly.

• Bluetooth uses a controlled MAC technique and frequency-hopping spread spectrum (FHSS) using 79 channels.

• During communications the signal makes about 1,600 channel changes per second (called hops).

• Data is encoded using 2-level frequency modulation, with one frequency encoding a binary 0 and another for binary 1.

67

Bluetooth Message Delineation

• A typical Bluetooth frame (see Figure 8-10 for details) has five sections:

• access code, which contains a preamble and special sync bytes based on the sender’s address.

• header, which contains the frame address and error control information (i.e., ACK/NAK, sequence number).

• payload header which contains the channel number and payload length.

• payload which is the data field.

• payload trailer, contains the CRC-16 error detection value.

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Fig. 8-10

Bluetooth

frame

layout

69

The Best Practice WLAN Design

70

Media Access Control Protocol Efficiency

• Unlike CSMA/CD, Wireless Ethernet’s PCF controlled-access technique imposes time delays, even when traffic is low (see Figure 8-11).

• Response time delays increase only slowly with increased traffic up to about 85-90 percent of nominal capacity.

• At traffic levels of about 85-90 percent of nominal capacity performance begins to fall dramatically, though it remains better than with a comparable wired network.

71

Figure 8-11 Performance of wireless

versus wired Ethernet LANs

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Effective Data Rates for WLANs

• Figure 8-12 presents effective data rates of 802.11b and 802.11a protocols under a range of conditions.

• At close range, 802.11a clearly provides superior performance to 802.11b.

• If range is a factor, however, 802.11a performs only modestly better than 802.11b.

• To achieve higher performance, many companies are now installing overlay networks; i.e., combined networks where the wireless portions extend the reach of the wired network into areas not normally wired.

73

802.11b perfect

conditions (11 Mbps)

4.8 Mbps

1.9 Mbps

960 kbps

802.11b normal

conditions (5.5 Mbps)

2.4 Mbps

1 Mbps

480 kbps

802.11a perfect

conditions (54 Mbps)

17.2

Mbps

6.9 Mbps

3.4 Mbps

802.11a long range

(12 Mbps)

3.8 Mbps

1.5 Mbps

760 kbps

802.11b perfect

conditions w/ 4 APs (54

Mbps)

34.4

Mbps

27.5 Mbps

13.7

Mbps

Technology Low Medium High

Network Traffic Conditions

Figure 8-12 Effective data rate estimates

for Wireless Ethernet

Assumes: 1500 byte frames, no transmission errors

(No. of active users: Low traffic = 2, moderate = 5, high = 10)

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Recommendations

• For new construction, WLANs are only modestly more expensive than wired LANs.

• WLANs have the advantage of mobility, linking indoor to outdoor areas as well as areas without wired access.

• Given its lower price, longer track record and ability to operate over greater distances 802.11b is the more attractive of the two WLAN protocols,

• If high capacity is critical, then 802.11a becomes more attractive.

• Over time, as 802.11a technology should drop in price. As experience with the technology increases, its popularity should increase as well.

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End of Chapter 8