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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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Applications of Wireless LANs
Single Cell Wireless LAN
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Applications of Wireless LANs
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 Sublayer
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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IEEE 802.11 MAC Sublayer
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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IEEE 802.11 MAC Sublayer
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?