Wireless LAN & Bluetooth

8/7/2019 Wireless LAN & Bluetooth

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Wireless LANs and Bluetooth Lecture 5

A wireless LAN uses wireless transmission medium.

Used to have high prices, low data rates, occupationalsafety concerns, and licensing requirements.

Problems have been addressed.

Popularity of wireless LANs has grown rapidly.



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Applications of Wireless LANs

Wired LAN Extension

Saves installation of LAN cabling Eases relocation and other modifications to network structure However, increasing reliance on twisted pair cabling for LANs

² Most older buildings already wired with Cat 3 cable² Newer buildings are prewired with Cat 5

Wireless LAN to replace wired LANs has not happened In some environments, role for the wireless LAN

² Buildings with large open areas

Manufacturing plants, stock exchange trading floors,warehouses

Historical buildings Small offices where wired LANs not economical



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Single Cell Wireless LAN



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Multi-Cell Wireless LAN



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Applications of Wireless LANsAd Hoc Networking

Peer-to-peer network,

No centralised server,

No infrastructure,Temporary nature.



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Wireless LAN Requirements Same as any LAN

± High capacity, short distances, full connectivity, broadcast capability Throughput: efficient use of wireless medium Number of nodes:Hundreds of nodes across multiple cells Connection to backbone LAN: Use control modules to connect to

both types of LANs

Service area: 100 to 300 m Low power consumption:Need long battery life on mobilestations ± Mustn¶t require nodes to monitor access points or frequent handshakes

Transmission robustness and security:Interference prone andeasily eavesdropped

Collocated network operation:Two or more wireless LANs insame area

License-free operation Handoff/roaming: Move from one cell to another Dynamic configuration: Addition, deletion, and relocation of end

systems without disruption to users



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IEEE 802.11 Wireless LANs

One of the major problems in wireless LANs today is theabundance of different wireless technologies and the lack

of compatibility among them.

IEEE 802.11 (also known as Wi-Fi) defines a standard for

the physical and the data link layers of wireless LANs.The standard is defined for the license-free Industrial,

Scientific, Medical (ISM) bands.

ISM Bands



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IEEE 802.11 Architecture



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IEEE 802.11 Protocol Stack

Physical

Layer

(PCF)

OFDM



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IEEE 802.11 MAC Sublayer

CSMA/CD cannot be used in wireless LANs.

(a) The hidden station problem. (b) The exposed station problem

Because not all stations are within the radio range of each other.It is also not possible to detect collision while transmittingbecause most stations are half-duplex.



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IEEE 802.11 MAC algorithm is called Distributed Foundation

Wireless MAC (DFWMAC).

DFWMAC defines two sublayers. Distributed Coordination Function

(DCF) and Point Coordination Function (PCF).

DCF is distributed with an optional centralised access control thatworks on top of that (i.e., PCF).

DCF is based on CSMA/CA (CSMA with Collision Avoidance) or

MACAW.

All frame transmissions are acknowledged with ACK packets. This is

the way collision is avoided.



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A B

CD

Distributed Coordination Function (DCF)



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IEEE 802.11 MAC Sublayer Point Coordination Function (PCF)

The access point periodically broadcasts beacon frames thatcontains system parameters and invites stations to requestbandwidth.

The access point can provide guaranteed bandwidth to stations

that are working in PCF mode.

PCF allows the transport of real-time traffic over the wirelessLAN.

PCF is not very well defined in the standard, and not commonly

implemented in most commercial access points. However, it existsin the standard specification.



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IEEE 802.11

MAC Sublayer

More on Distributed

Coordination Function (DCF)

IFS: InterFrame Space



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SIFS (Short IFS): Shortest IFS. Defined for ACKs, CTSs, and

poll responses

PIFS (PCF IFS): Middle-length IFS. Used by the access pointwhen issuing polls.

DIFS (DCF IFS): Longest IFS. Any station may attempt to

acquire channel to send a new frame.



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Bluetooth

A piconet comprised by up to seven active slaves and a master.

IEEE 802.15 (Personal Area Networks) is based on Bluetooth.

Bluetooth is an end-to-end specification, whereas IEEE 802.15

only covers physical and data link layers.



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Bluetooth

Bluetooth physical layer uses FHSS at 1600 hops/sec.

2.4 GHz ISM Band is used with 79 channels of 1 MHz each.

Gross transmission rate is 1 Mbps.

Dwell time (time spent in one carrier) is 625 sec.

It takes 250-260 sec to settle in one carrier frequency.

The radio designated as the master makes the determination of

the channel (frequency hopping sequence) and phase (timing

offset, i.e., when to transmit) that shall be used by all devices on

this piconet.

A slave may only communicate with the master and may onlycommunicate when granted permission by the master.



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Bluetooth

Time is divided in 625 bits, 3x625 bits, or 5x625 bits slots.

Regardless of the slot type there is one settling time (250-260 sec).

Master uses even numbered slots.

Slaves use odd numbered slots that are designated by the

master.



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Bluetooth



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Bluetooth

Two types of logical links are defined between master and slave.

Asynchronous ConnectionLess (ACL) links are used bursty data

traffic with no QoS guarantee. Frames can be lost and

retransmitted on ACL links.

Synchronous Connection Oriented (SCO) links are used for real-

time traffic (typically telephony). This type of link is allocated a

fixed slot in each direction. There are no retransmissions.

Instead there is forward error correction on SCO.



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Bluetooth

Access code defines the master node (in case there are several

masters within the radio range).

Address defines destination node.

Type defines link type (e.g., SCO, ACL)F: Flow control bit

A: Acknowledgement bit

S: Sequence number



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Bluetooth

SCO Links always operate on single-slot (i.e., 240 bits data

field) configuration. ACL links may operate on three or five slot

configuration (i.e., 2744 bits data field).

The most reliable configuration is single-slot with 80 bits of

data repeated three times in a 240-bit data field. 64000 bps

can be achieved this way.



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William Stallings, ´Data and ComputerCommunicationsµ, chapter 13, 14.

A. S. Tanenbaum, ´Computer Networksµ, chapter 4. http://iamwww.unibe.ch/~rvs/lectures/SS98/cn/applets/Ethernet/et

hernet.htm (CSMA/CD applet)

http://www.wi-fiplanet.com/

http://www.vicomsoft.com/knowledge/reference/wir

eless1.html



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Tutorial

1- Explain the ³Hidden Node´ and ³Exposed Node´ problems in wirelessLANs?

2- What is the solution for medium access control that is proposed byIEEE 802.11 standard?

3- How are PCF and DCF different in DFWMAC?

4-Explain the relationship between master and slave stations in Bluetooth.

5-What kind of logical links are supported by Bluetooth? What kind of alink would be required by an email application?

6-Explain why CSMA/CD is not used in wireless LANs?

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Wireless LAN & Bluetooth