This document provides information about local area networks (LANs), including their design, topologies, transmission media, and protocols. It discusses common LAN applications and topologies such as bus, star, ring, and mesh. Specific protocols covered include Ethernet, token ring, and spanning tree. Bridges are described as a way to interconnect multiple LANs. Factors in choosing a topology and transmission medium are also summarized.

1. 20CS2007
Computer Communication
Networks
Module 3
Local Area Networks Design Café and residential networks, Setup Wi-Fi networks, Traffic confinement with
VLAN. LANs and Basic Topologies, LAN Protocols, MAC/IP Address Conversion Protocols, Wired LAN, Wireless
LAN, Virtual LAN, IEEE 802.11 Wireless LAN Standard.
Dr.A.Kathirvel, Professor,
DCSE, KITS
kathirvel@karunya.edu

2. LAN and Basic Topologies
9/14/2021 2

3. Local Area Network Overview

4. LAN Applications (1)
• Personal computer LANs
—Low cost
—Limited data rate
• Back end networks
—Interconnecting large systems (mainframes and large
storage devices)
• High data rate
• High speed interface
• Distributed access
• Limited distance
• Limited number of devices

5. LAN Applications (2)
• Storage Area Networks
— Separate network handling storage needs
— Detaches storage tasks from specific servers
— Shared storage facility across high-speed network
— Hard disks, tape libraries, CD arrays
— Improved client-server storage access
— Direct storage to storage communication for backup
• High speed office networks
— Desktop image processing
— High capacity local storage
• Backbone LANs
— Interconnect low speed local LANs
— Reliability
— Capacity
— Cost

6. Storage Area Networks

7. LAN Architecture
• Topologies
• Transmission medium
• Layout
• Medium access control

8. Topologies
• Tree
• Bus
—Special case of tree
• One trunk, no branches
• Ring
• Star

9. LAN Topologies

10. Bus and Tree
• Multipoint medium
• Transmission propagates throughout medium
• Heard by all stations
— Need to identify target station
• Each station has unique address
• Full duplex connection between station and tap
— Allows for transmission and reception
• Need to regulate transmission
— To avoid collisions
— To avoid hogging
• Data in small blocks - frames
• Terminator absorbs frames at end of medium

11. Frame
Transmission
on Bus LAN

12. Ring Topology
• Repeaters joined by point to point links in closed
loop
—Receive data on one link and retransmit on another
—Links unidirectional
—Stations attach to repeaters
• Data in frames
—Circulate past all stations
—Destination recognizes address and copies frame
—Frame circulates back to source where it is removed
• Media access control determines when station
can insert frame

13. Frame
Transmission
Ring LAN

14. Star Topology
• Each station connected directly to central node
—Usually via two point to point links
• Central node can broadcast
—Physical star, logical bus
—Only one station can transmit at a time
• Central node can act as frame switch

15. Choice of Topology
• Reliability
• Expandability
• Performance
• Needs considering in context of:
—Medium
—Wiring layout
—Access control

16. Bus LAN
Transmission Media (1)
• Twisted pair
—Early LANs used voice grade cable
—Didn’t scale for fast LANs
—Not used in bus LANs now
• Baseband coaxial cable
—Uses digital signalling
—Original Ethernet

17. Bus LAN
Transmission Media (2)
• Broadband coaxial cable
— As in cable TV systems
— Analog signals at radio frequencies
— Expensive, hard to install and maintain
— No longer used in LANs
• Optical fiber
— Expensive taps
— Better alternatives available
— Not used in bus LANs
• All hard to work with compared with star topology twisted pair
• Coaxial baseband still used but not often in new
installations

18. Ring and Star Usage
• Ring
—Very high speed links over long distances
—Single link or repeater failure disables network
• Star
—Uses natural layout of wiring in building
—Best for short distances
—High data rates for small number of devices

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Network:
 A network is a set of nodes (devices) connected by communication links.
 A node can be a computer, printer, or any other device capable of sending and/or
receiving data generated by other nodes on the network.
A network is two or more devices connected through links. A link is a
communication
pathway that transfers data from one device to another.
There are two possible types of connections: point-to-point and multipoint
Point-to-Point: A point-to-point connection provides a dedicated link between two
devices. The entire capacity of the link is reserved for transmission between those
two
devices.
Example : connection between the remote control and the television's control
system.

20. 9/14/2021 20
Multipoint :
A multipoint connection is connection in which more than
two specific devices share a single link.
In a multipoint environment, the capacity of the channel is
shared, either spatially
or temporally.
If several devices can use the link simultaneously, it is a
spatially shared
connection. If users must take turns, it is a timeshared
connection.

21. 9/14/2021 21
Station A Station B
Communication Link
Point –to-point Connection
Mainframe
Station 1 Station 2
Station 3
Station 4
Multipoint Connection

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Benefits of Networks:
The following are some of the benefits of networks.
 Provide convenience: Computers on a network can back up their files over
the network.
 Allow sharing: Networked Computers can share resources such as printers
and disks.
 Facilitate communication: Networks facilitate the communication such as
sending and receiving email, transferring files and video conferencing.
 Generate savings: Networked computers can provide more computing
power for less money. Since resources can be shared, not everyone need their
own peripherals which can result in cost savings.
 Provide reliability : If one part of a network is down, useful work may be
still possible using a different network path.
 Simplifying scalability : It is easy to add more computers to an existing
network.

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Network Topologies :
 Network topology refers to the way in which a network is laid out
physically.
 The topology of a network is the geometric representation of the
relationship of all the links and linking devices (nodes) to one another.
There are four basic topologies
 Mesh Topology
 Star Topology
 Bus Topology
 Ring Topology

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1. Mesh Topology :
In a mesh topology, every device has a dedicated point-to-point
link to every other device. The term dedicated means that the link carries
traffic only between the two devices it connects.
Example: connection of telephone regional offices in which each
regional office needs to be connected to every other regional
office.
Advantages :
1. Eliminate the traffic problems that can occur when links shared by
multiple devices.
2. Mesh topology is robust.
3. Privacy and Security
4. Point-to-point links make fault identification and fault isolation easy.
Disadvantages:
1. More amount of cabling and the number of I/O ports required.
2. The hardware required to connect each link (I/O ports and cable) can be
expensive.

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Device 1
Device 2 Device 3
Device 4 Device 5
Mesh Topology

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2. Star Topology :
 In a star topology, each device has a dedicated point-to-point link only to a central
controller, called a hub.
 The devices are not directly linked to one another.
 A star topology does not allow direct traffic between devices. The controller acts as
an exchange.
 If one device wants to send data to another, it sends the data to the controller,
which then relays the data to the other connected device.
Example: The star topology is used in local-area networks (LANs). High-speed LANs
often use a star topology with a central hub.
Advantages:
1. A star topology is less expensive than a mesh topology.
2. Robustness
3. easy to install and reconfigure
Disadvantages:
1. Star topology is dependent on hub . If the hub goes down, the whole system is
dead.

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Central Hub
Device 1 Device 2 Device 3 Device 4
Star Topology

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3. Bus Topology :
A bus Topology is multipoint connection. One long cable acts as a backbone to link
the devices in a network.
Device 1 Device 2 Device 3
Tap Tap
Tap
Cable
Drop Line Drop Line
Drop Line

29. 9/14/2021 29
 Nodes are connected to the bus cable by drop lines and taps.
 A drop line is a connection running between the device and the main cable.
 A tap is a connector that either splices into the main cable or punctures the sheathing
of a cable to create a contact with the metallic core.
Advantages:
 Ease of installation
 Bus topology uses less cabling than mesh or star topologies.
Disadvantages:
 Difficult reconnection and fault isolation
 Difficult to add new devices

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4. Ring Topology:
In a ring topology, each device has a dedicated point-to-point connection with
only the two devices on either side of it.
A signal is passed along the ring in one direction, from device to device, until it
reaches its destination.
Each device in the ring incorporates a repeater. When a device receives a signal
intended for another device, its repeater regenerates the messages and passes
them along path.
Example : Token Ring.
Advantages :
Easy to install and reconfigure
Fault isolation is simplified
Disadvantages:
Unidirectional traffic
Break in the ring disable the entire network

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Device 1
Device
2
Device
3
Device
4
Device
5
Device
6
Repeate
r
Repeat
er
Repeater
Repeate
r Repeater
Repeat
er
Ring Topology

32. 32
Network Protocols Stack
Application
Transport
Network
Link
Application protocol
TCP protocol
IP protocol
Data
Link
IP
Network
Access
IP protocol
Data
Link
Application
Transport
Network
Link

33. Types of Addresses in Internet
• Media Access Control (MAC) addresses in the network
access layer
– Associated w/ network interface card (NIC)
– 48 bits or 64 bits
• IP addresses for the network layer
– 32 bits for IPv4, and 128 bits for IPv6
– E.g., 128.3.23.3
• IP addresses + ports for the transport layer
– E.g., 128.3.23.3:80
• Domain names for the application/human layer
– E.g., www.purdue.edu
33

34. Routing and Translation of
Addresses
• Translation between IP addresses and MAC
addresses
– Address Resolution Protocol (ARP) for IPv4
– Neighbor Discovery Protocol (NDP) for IPv6
• Routing with IP addresses
– TCP, UDP, IP for routing packets, connections
– Border Gateway Protocol for routing table updates
• Translation between IP addresses and domain
names
– Domain Name System (DNS)
34

35. Wired LAN
• Constrained by LAN topology
• Capacity
• Reliability
• Types of data supported
• Environmental scope

36. Media Available (1)
• Voice grade unshielded twisted pair (UTP)
—Cat 3
—Cheap
—Well understood
—Use existing telephone wiring in office building
—Low data rates
• Shielded twisted pair and baseband coaxial
—More expensive than UTP but higher data rates
• Broadband cable
—Still more expensive and higher data rate

37. Media Available (2)
• High performance UTP
— Cat 5 and above
— High data rate for small number of devices
— Switched star topology for large installations
• Optical fiber
— Electromagnetic isolation
— High capacity
— Small size
— High cost of components
— High skill needed to install and maintain
• Prices are coming down as demand and product range increases

38. Protocol Architecture
• Lower layers of OSI model
• IEEE 802 reference model
• Physical
• Logical link control (LLC)
• Media access control (MAC)

39. IEEE 802 v OSI

40. 802 Layers -
Physical
• Encoding/decoding
• Preamble generation/removal
• Bit transmission/reception
• Transmission medium and topology

41. 802 Layers -
Logical Link Control
• Interface to higher levels
• Flow and error control

42. Logical Link Control
• Transmission of link level PDUs between two
stations
• Must support multiaccess, shared medium
• Relieved of some link access details by MAC
layer
• Addressing involves specifying source and
destination LLC users
—Referred to as service access points (SAP)
—Typically higher level protocol

43. LLC Services
• Based on HDLC
• Unacknowledged connectionless service
• Connection mode service
• Acknowledged connectionless service

44. LLC Protocol
• Modeled after HDLC
• Asynchronous balanced mode to support
connection mode LLC service (type 2 operation)
• Unnumbered information PDUs to support
Acknowledged connectionless service (type 1)
• Multiplexing using LSAPs

45. Media Access Control
• Assembly of data into frame with address and
error detection fields
• Disassembly of frame
—Address recognition
—Error detection
• Govern access to transmission medium
—Not found in traditional layer 2 data link control
• For the same LLC, several MAC options may be
available

46. LAN Protocols in Context

47. Media Access Control
• Where
— Central
• Greater control
• Simple access logic at station
• Avoids problems of co-ordination
• Single point of failure
• Potential bottleneck
— Distributed
• How
— Synchronous
• Specific capacity dedicated to connection
— Asynchronous
• In response to demand

48. Asynchronous Systems
• Round robin
— Good if many stations have data to transmit over extended
period
• Reservation
— Good for stream traffic
• Contention
— Good for bursty traffic
— All stations contend for time
— Distributed
— Simple to implement
— Efficient under moderate load
— Tend to collapse under heavy load

49. MAC Frame Format
• MAC layer receives data from LLC layer
• MAC control
• Destination MAC address
• Source MAC address
• LLS
• CRC
• MAC layer detects errors and discards frames
• LLC optionally retransmits unsuccessful frames

50. Generic MAC Frame Format

51. Bridges
• Ability to expand beyond single LAN
• Provide interconnection to other LANs/WANs
• Use Bridge or router
• Bridge is simpler
—Connects similar LANs
—Identical protocols for physical and link layers
—Minimal processing
• Router more general purpose
—Interconnect various LANs and WANs
—see later

52. Why Bridge?
• Reliability
• Performance
• Security
• Geography

53. Functions of a Bridge
• Read all frames transmitted on one LAN and
accept those address to any station on the other
LAN
• Using MAC protocol for second LAN, retransmit
each frame
• Do the same the other way round

54. Bridge Operation

55. Bridge Design Aspects
• No modification to content or format of frame
• No encapsulation
• Exact bitwise copy of frame
• Minimal buffering to meet peak demand
• Contains routing and address intelligence
— Must be able to tell which frames to pass
— May be more than one bridge to cross
• May connect more than two LANs
• Bridging is transparent to stations
— Appears to all stations on multiple LANs as if they are on one
single LAN

56. Bridge Protocol Architecture
• IEEE 802.1D
• MAC level
— Station address is at this level
• Bridge does not need LLC layer
— It is relaying MAC frames
• Can pass frame over external comms system
— e.g. WAN link
— Capture frame
— Encapsulate it
— Forward it across link
— Remove encapsulation and forward over LAN link

57. Connection of Two LANs

58. Fixed Routing
• Complex large LANs need alternative routes
—Load balancing
—Fault tolerance
• Bridge must decide whether to forward frame
• Bridge must decide which LAN to forward frame
on
• Routing selected for each source-destination
pair of LANs
—Done in configuration
—Usually least hop route
—Only changed when topology changes

59. Bridges and
LANs with
Alternative
Routes

60. Spanning Tree
• Bridge automatically develops routing table
• Automatically update in response to changes
• Frame forwarding
• Address learning
• Loop resolution

61. Frame forwarding
• Maintain forwarding database for each port
—List station addresses reached through each port
• For a frame arriving on port X:
—Search forwarding database to see if MAC address is
listed for any port except X
—If address not found, forward to all ports except X
—If address listed for port Y, check port Y for blocking
or forwarding state
• Blocking prevents port from receiving or transmitting
—If not blocked, transmit frame through port Y

62. Address Learning
• Can preload forwarding database
• Can be learned
• When frame arrives at port X, it has come form
the LAN attached to port X
• Use the source address to update forwarding
database for port X to include that address
• Timer on each entry in database
• Each time frame arrives, source address
checked against forwarding database

63. Spanning Tree Algorithm
• Address learning works for tree layout
—i.e. no closed loops
• For any connected graph there is a spanning
tree that maintains connectivity but contains no
closed loops
• Each bridge assigned unique identifier
• Exchange between bridges to establish spanning
tree

64. Loop of Bridges

65. Layer 2 and Layer 3 Switches
• Now many types of devices for interconnecting
LANs
• Beyond bridges and routers
• Layer 2 switches
• Layer 3 switches

66. Hubs
• Active central element of star layout
• Each station connected to hub by two lines
— Transmit and receive
• Hub acts as a repeater
• When single station transmits, hub repeats signal on outgoing line
to each station
• Line consists of two unshielded twisted pairs
• Limited to about 100 m
— High data rate and poor transmission qualities of UTP
• Optical fiber may be used
— Max about 500 m
• Physically star, logically bus
• Transmission from any station received by all other stations
• If two stations transmit at the same time, collision

67. Hub Layouts
• Multiple levels of hubs cascaded
• Each hub may have a mixture of stations and other hubs
attached to from below
• Fits well with building wiring practices
— Wiring closet on each floor
— Hub can be placed in each one
— Each hub services stations on its floor

68. Two Level Star Topology

69. Buses and Hubs
• Bus configuration
—All stations share capacity of bus (e.g. 10Mbps)
—Only one station transmitting at a time
• Hub uses star wiring to attach stations to hub
—Transmission from any station received by hub and
retransmitted on all outgoing lines
—Only one station can transmit at a time
—Total capacity of LAN is 10 Mbps
• Improve performance with layer 2 switch

70. Shared Medium Bus and Hub

71. Shared Medium Hub and
Layer 2 Switch

72. Layer 2 Switches
• Central hub acts as switch
• Incoming frame from particular station switched
to appropriate output line
• Unused lines can switch other traffic
• More than one station transmitting at a time
• Multiplying capacity of LAN

73. Layer 2 Switch Benefits
• No change to attached devices to convert bus LAN or
hub LAN to switched LAN
• For Ethernet LAN, each device uses Ethernet MAC
protocol
• Device has dedicated capacity equal to original LAN
— Assuming switch has sufficient capacity to keep up with all
devices
— For example if switch can sustain throughput of 20 Mbps, each
device appears to have dedicated capacity for either input or
output of 10 Mbps
• Layer 2 switch scales easily
— Additional devices attached to switch by increasing capacity of
layer 2

74. Types of Layer 2 Switch
• Store-and-forward switch
— Accepts frame on input line
— Buffers it briefly,
— Then routes it to appropriate output line
— Delay between sender and receiver
— Boosts integrity of network
• Cut-through switch
— Takes advantage of destination address appearing at beginning
of frame
— Switch begins repeating frame onto output line as soon as it
recognizes destination address
— Highest possible throughput
— Risk of propagating bad frames
• Switch unable to check CRC prior to retransmission

75. Layer 2 Switch v Bridge
• Layer 2 switch can be viewed as full-duplex hub
• Can incorporate logic to function as multiport bridge
• Bridge frame handling done in software
• Switch performs address recognition and frame
forwarding in hardware
• Bridge only analyzes and forwards one frame at a time
• Switch has multiple parallel data paths
— Can handle multiple frames at a time
• Bridge uses store-and-forward operation
• Switch can have cut-through operation
• Bridge suffered commercially
— New installations typically include layer 2 switches with bridge
functionality rather than bridges

76. Problems with Layer 2
Switches (1)
• As number of devices in building grows, layer 2 switches
reveal some inadequacies
• Broadcast overload
• Lack of multiple links
• Set of devices and LANs connected by layer 2 switches
have flat address space
— Allusers share common MAC broadcast address
— If any device issues broadcast frame, that frame is delivered to
all devices attached to network connected by layer 2 switches
and/or bridges
— In large network, broadcast frames can create big overhead
— Malfunctioning device can create broadcast storm
• Numerous broadcast frames clog network

77. Problems with Layer 2
Switches (2)
• Current standards for bridge protocols dictate no closed
loops
— Only one path between any two devices
— Impossible in standards-based implementation to provide
multiple paths through multiple switches between devices
• Limits both performance and reliability.
• Solution: break up network into subnetworks connected
by routers
• MAC broadcast frame limited to devices and switches
contained in single subnetwork
• IP-based routers employ sophisticated routing
algorithms
— Allow use of multiple paths between subnetworks going through
different routers

78. Problems with Routers
• Routers do all IP-level processing in software
—High-speed LANs and high-performance layer 2
switches pump millions of packets per second
—Software-based router only able to handle well under
a million packets per second
• Solution: layer 3 switches
—Implementpacket-forwarding logic of router in
hardware
• Two categories
—Packet by packet
—Flow based

79. Packet by Packet or
Flow Based
• Operates insame way as traditional router
• Order of magnitude increase in performance
compared to software-based router
• Flow-based switch tries to enhance performance
by identifying flows of IP packets
—Same source and destination
—Done by observing ongoing traffic or using a special
flow label in packet header (IPv6)
—Once flow is identified, predefined route can be
established

80. Typical Large LAN Organization
• Thousands to tens of thousands of devices
• Desktop systems links 10 Mbps to 100 Mbps
— Into layer 2 switch
• Wireless LAN connectivity available for mobile users
• Layer 3 switches at local network's core
— Form local backbone
— Interconnected at 1 Gbps
— Connect to layer 2 switches at 100 Mbps to 1 Gbps
• Servers connect directly to layer 2 or layer 3 switches at
1 Gbps
• Lower-cost software-based router provides WAN
connection
• Circles in diagram identify separate LAN subnetworks
• MAC broadcast frame limited to own subnetwork

81. Typical
Large
LAN
Organization
Diagram

82. Overview
• A wireless LAN uses wireless transmission
medium
• Used to have high prices, low data rates,
occupational safety concerns, and licensing
requirements
• Problems have been addressed
• Popularity of wireless LANs has grown rapidly

83. Applications - LAN Extension
• Saves installation of LAN cabling
• Eases relocation and other modifications to network
structure
• However, increasing reliance on twisted pair cabling for
LANs
— Most older buildings already wired with Cat 3 cable
— Newer buildings are prewired with Cat 5
• Wireless LAN to replace wired LANs has not happened
• In some environments, role for the wireless LAN
— Buildings with large open areas
• Manufacturing plants, stock exchange trading floors, warehouses
• Historical buildings
• Small offices where wired LANs not economical
• May also have wired LAN
— Servers and stationary workstations

84. Single Cell Wireless LAN
Configuration

85. Multi-Cell Wireless LAN
Configuration

86. Applications –
Cross-Building Interconnect
• Connect LANs in nearby buildings
• Point-to-point wireless link
• Connect bridges or routers
• Not a LAN per se
—Usual to include this application under heading of
wireless LAN
•

87. Applications - Nomadic Access
• Link between LAN hub and mobile data terminal
—Laptop or notepad computer
—Enable employee returning from trip to transfer data
from portable computer to server
• Also useful in extended environment such as
campus or cluster of buildings
—Users move around with portable computers
—May wish access to servers on wired LAN

88. Infrastructure Wireless LAN

89. Applications –
Ad Hoc Networking
• Peer-to-peer network
• Set up temporarily to meet some immediate
need
• E.g. group of employees, each with laptop or
palmtop, in business or classroom meeting
• Network for duration of meeting

90. Add Hoc LAN

91. Wireless LAN Requirements
• Same as any LAN
— High capacity, short distances, full connectivity, broadcast capability
• Throughput: efficient use 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 mobile stations
— Mustn't require nodes to monitor access points or frequent handshakes
• Transmission robustness and security:Interference prone and easily
eavesdropped
• Collocated network operation:Two or more wireless LANs in same
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

92. Technology
• Infrared (IR) LANs: Individual cell of IR LAN
limited to single room
—IR light does not penetrate opaque walls
• Spread spectrum LANs: Mostly operate in ISM
(industrial, scientific, and medical) bands
—No Federal Communications Commission (FCC)
licensing is required in USA
• Narrowband microwave: Microwave frequencies
but not use spread spectrum
—Some require FCC licensing

93. Infrared LANs
Strengths and Weaknesses
• Spectrum virtually unlimited
— Infrared spectrum is unregulated worldwide
— Extremely high data rates
• Infrared shares some properties of visible light
— Diffusely reflected by light-colored objects
• Use ceiling reflection to cover entire room
— Does not penetrate walls or other opaque objects
• More easily secured against eavesdropping than microwave
• Separate installation in every room without interference
• Inexpensive and simple
— Uses intensity modulation, so receivers need to detect only
amplitude
• Background radiation
— Sunlight, indoor lighting
— Noise, requiring higher power and limiting range
— Power limited by concerns of eye safety and power consumption

94. Infrared LANs
Transmission Techniques
• Directed-beam IR
— Point-to-point links
— Range depends on power and focusing
• Can be kilometers
• Used for building interconnect within line of sight
— Indoor use to set up token ring LAN
— IR transceivers positioned so that data circulate in ring
• Omnidirectional
— Single base station within line of sight of all other stations
• Typically, mounted on ceiling
— Acts as a multiport repeater
— Other transceivers use directional beam aimed at ceiling unit
• Diffused configuration
— Transmitters are focused and aimed at diffusely reflecting
ceiling

95. Spread Spectrum LANs
Hub Configuration
• Usually use multiple-cell arrangement
• Adjacent cells use different center frequencies
• Hub is typically mounted on ceiling
— Connected to wired LAN
— Connect to stations attached to wired LAN and in other cells
— May also control access
• IEEE 802.11 point coordination function
— May also act as multiport repeater
• Stations transmit to hub and receive from hub
— Stations may broadcast using an omnidirectional antenna
• Logical bus configuration
• Hub may do automatic handoff
— Weakening signal, hand off

96. Spread Spectrum LANs
Peer-to-Peer Configuration
• No hub
• MAC algorithm such as CSMA used to control
access
• Ad hoc LANs
•

97. Spread Spectrum LANs
Transmission Issues
• Licensing regulations differ from one country to another
• USA FCC authorized two unlicensed applications within
the ISM band:
— Spread spectrum - up to 1 watt
— Very low power systems- up to 0.5 watts
— 902 - 928 MHz (915-MHz band)
— 2.4 - 2.4835 GHz (2.4-GHz band)
— 5.725 - 5.825 GHz (5.8-GHz band)
— 2.4 GHz also in Europe and Japan
— Higher frequency means higher potential bandwidth
• Interference
— Devices at around 900 MHz, including cordless telephones,
wireless microphones, and amateur radio
— Fewer devices at 2.4 GHz; microwave oven
— Little competition at 5.8 GHz
• Higher frequency band, more expensive equipment

98. Narrow Band Microwave LANs
• Just wide enough to accommodate signal
• Until recently, all products used licensed band
• At least one vendor has produced LAN product
in ISM band

99. Licensed Narrowband RF
• Microwave frequencies usable for voice, data, and video licensed
within specific geographic areas to avoid interference
— Radium 28 km
— Can contain five licenses
— Each covering two frequencies
— Motorola holds 600 licenses (1200 frequencies) in the 18-GHz range
— Cover all metropolitan areas with populations of 30,000 or more in USA
• Use of cell configuration
• Adjacent cells use nonoverlapping frequency bands
• Motorola controls frequency band
— Can assure nearby independent LANs do not interfere
• All transmissions are encrypted
• Licensed narrowband LAN guarantees interference-free
communication
• License holder has legal right tointerference-free data channel

100. Unlicensed Narrowband RF
• 1995, RadioLAN introduced narrowband wireless LAN
using unlicensed ISM spectrum
— Used for narrowband transmission at low power
• 0.5 watts or less
— Operates at 10 Mbps
— 5.8-GHz band
— 50 m in semiopen office and 100 m in open office
• Peer-to-peer configuration
• Elects one node as dynamic master
— Based on location, interference, and signal strength
• Master can change automatically as conditions change
• Includes dynamic relay function
• Stations can act as repeater to move data between
stations that are out of range of each other

101. IEEE 802.11 - BSS
• MAC protocol and physical medium specification for
wireless LANs
• Smallest building block is basic service set (BSS)
— Number of stations
— Same MAC protocol
— Competing for access to same shared wireless medium
• May be isolated or connect to backbone distribution
system (DS) through access point (AP)
— AP functions as bridge
• MAC protocol may be distributed or controlled by central
coordination function in AP
• BSS generally corresponds to cell
• DS can be switch, wired network, or wireless network

102. BSS Configuration
• Simplest: each station belongs to single BSS
—Within range only of other stations within BSS
• Can have two BSSs overlap
—Station could participate in more than one BSS
• Association between station and BSS dynamic
—Stations may turn off, come within range, and go out
of range

103. Extended Service Set (ESS)
• Two or more BSS interconnected by DS
—Typically, DS is wired backbone but can be any
network
• Appears as single logical LAN to LLC

104. Access Point (AP)
• Logic within station that provides access to DS
—Provides DS services in addition to acting as station
• To integrate IEEE 802.11 architecture with
wired LAN, portal used
• Portal logic implemented in device that is part of
wired LAN and attached to DS
—E.g. Bridge or router

105. IEEE 802.11 Architecture

106. Services
Service Provider Category
Association Distribution system MSDU delivery
Authentication Station LAN access and
security
Deauthentication Station LAN access and
security
Dissassociation Distribution system MSDU delivery
Distribution Distribution system MSDU delivery
Integration Distribution system MSDU delivery
MSDU delivery Station MSDU delivery
Privacy Station LAN access and
security
Reassocation Distribution system MSDU delivery

107. Categorizing Services
• Station services implemented in every 802.11 station
— Including AP stations
• Distribution services provided between BSSs
— May be implemented in AP or special-purpose device
• Three services used to control access and confidentiality
• Six services used to support delivery of MAC service
data units (MSDUs) between stations
— Block of data passed down from MAC user to MAC layer
— Typically LLC PDU
— If MSDU too large for MAC frame, fragment and transmit in
series of frames (see later)

108. Distribution of Messages
Within a DS
• Distribution is primary service used by stations to
exchange MAC frames when frame must traverse DS
— From station in one BSS to station in another BSS
— Transport of message through DS is beyond scope of 802.11
— If stations within same BSS, distribution service logically goes
through single AP of that BSS
• Integration service enables transfer of data between
station on 802.11 LAN and one on an integrated 802.x
LAN
— Integrated refers to wired LAN physically connected to DS
• Stations may be logically connected to 802.11 LAN via integration
service
— Integration service takes care of address translation and media
conversion

109. Association Related Services
• Purpose of MAC layer transfer MSDUs between MAC
entities
• Fulfilled by distribution service (DS)
• DS requires information about stations within ESS
— Provided by association-related services
— Station must be associated before communicating
• Three transition types of based on mobility
— No transition: Stationary or moves within range of single BSS
— BSS transition: From one BSS to another within same ESS
• Requires addressing capability be able to recognize new location
• ESS transition: From BSS in one ESS to BSS in another
ESS
— Only supported in sense that the station can move
— Maintenance of upper-layer connections not guaranteed
— Disruption of service likely

110. Station Location
• DS needs to know where destination station is
— Identity of AP to which message should be delivered
— Station must maintain association with AP within current BSS
• Three services relate to this requirement:
— Association: Establishes initial association between station and
AP
• To make identity and address known
• Station must establish association with AP within particular BSS
• AP then communicates information to other APs within ESS
— Reassociation: Transfer established association to another AP
• Allows station to move from one BSS to another
— Disassociation: From either station or AP that association is
terminated
— Given before station leaves ESS or shuts
• MAC management facility protects itself against stations that
disappear without notification

111. Access and Privacy Services -
Authentication
• On wireless LAN, any station within radio range other devices can
transmit
• Any station within radio range can receive
• Authentication: Used to establish identity of stations to each other
— Wired LANs assume access to physical connection conveys authority to
connect to LAN
— Not valid assumption for wireless LANs
• Connectivity achieved by having properly tuned antenna
— Authentication service used to establish station identity
— 802.11 supports several authentication schemes
• Allows expansion of these schemes
— Does not mandate any particular scheme
— Range from relatively insecure handshaking to public-key encryption
schemes
— 802.11 requires mutually acceptable, successful authentication before
association

112. Access and Privacy Services -
Deauthentication and Privacy
• Deauthentication: Invoked whenever an existing
authentication is to be terminated
• Privacy: Used to prevent messages being read
by others
• 802.11 provides for optional use of encryption

113. Medium Access Control
• MAC layer covers three functional areas
• Reliable data delivery
• Access control
• Security
—Beyond our scope

114. Reliable Data Delivery
• 802.11 physical and MAC layers subject to unreliability
• Noise, interference, and other propagation effects result
in loss of frames
• Even with error-correction codes, frames may not
successfully be received
• Can be dealt with at a higher layer, such as TCP
— However, retransmission timers at higher layers typically order
of seconds
— More efficient to deal with errors at the MAC level
• 802.11 includes frame exchange protocol
— Station receiving frame returns acknowledgment (ACK) frame
— Exchange treated as atomic unit
• Not interrupted by any other station
— If noACK within short period of time, retransmit

115. Four Frame Exchange
• Basic data transfer involves exchange of two frames
• To further enhance reliability, four-frame exchange may
be used
— Source issues a Request to Send (RTS) frame to destination
— Destination responds with Clear to Send (CTS)
— After receiving CTS, source transmits data
— Destination responds with ACK
• RTS alerts all stations within range of source that
exchange is under way
• CTS alerts all stations within range of destination
• Stations refrain from transmission to avoid collision
• RTS/CTS exchange is required function of MAC but may
be disabled

116. Media Access Control
• Distributed wireless foundation MAC (DWFMAC)
—Distributed access control mechanism
—Optional centralized control on top
• Lower sublayer is distributed coordination
function (DCF)
—Contention algorithm to provide access to all traffic
—Asynchronous traffic
• Point coordination function (PCF)
—Centralized MAC algorithm
—Contention free
—Built on top of DCF

117. IEEE 802.11 Protocol
Architecture

118. Distributed Coordination
Function
• DCF sublayer uses CSMA
• If station has frame to transmit, it listens to medium
• If medium idle, station may transmit
• Otherwise must wait until current transmission complete
• No collision detection
— Not practical on wireless network
— Dynamic range of signals very large
— Transmitting station cannot distinguish incoming weak signals
from noise and effects of own transmission
• DCF includes delays
— Amounts to priority scheme
• Interframe space

119. Interframe Space
• Single delay known as interframe space (IFS)
• Using IFS, rules for CSMA:
1. Station with frame senses medium
• If idle, wait to see if remains idle for one IFS. If so, may
transmit immediately
2. If busy (either initially or becomes busy during IFS)
station defers transmission
• Continue to monitor until current transmission is over
3. Once current transmission over, delay another IFS
• If remains idle, back off random time and again sense
• If medium still idle, station may transmit
• During backoff time, if becomes busy, backoff timer is halted
and resumes when medium becomes idle
• To ensure stability, binary exponential backoff used

120. IEEE 802.11
Medium
Access
Control
Logic

121. Priority
• Use three values for IFS
• SIFS (short IFS):
— Shortest IFS
— For all immediate response actions (see later)
• PIFS (point coordination function IFS):
— Midlength IFS
— Used by the centralized controller in PCF scheme when issuing
polls
• DIFS (distributed coordination function IFS):
— Longest IFS
— Used as minimum delay for asynchronous frames contending for
access

122. SIFS Use - ACK
• Station using SIFS to determine transmission
opportunity has highest priority
— In preference to station waiting PIFS or DIFS time
• SIFS used in following circumstances:
• Acknowledgment (ACK): Station responds with ACK after
waiting SIFS gap
— No collision detection so likelihood of collisions greater than
CSMA/CD
• MAC-level ACK gives efficient collision recovery
— SIFS provide efficient delivery of multiple frame LLC PDU
• Station with multiframe LLC PDU to transmit sends out MAC frames
one at a time
• Each frame acknowledged after SIFS by recipient
• When source receives ACK, immediately (after SIFS) sends next
frame in sequence
• Once station has contended for channel, it maintains control of all
fragments sent

123. SIFS Use – CTS
• Clear to Send (CTS): Station can ensure data
frame will get through by issuing RTS
—Destination station should immediately respond with
CTS if ready to receive
—All other stations hear RTS and defer
• Poll response: See Point coordination Function
(PCF)

124. PIFS and DIFS
• PIFS used by centralized controller
—Issuing polls
—Takes precedence over normal contention traffic
—Frames using SIFS have precedence over PCF poll
• DIFS used for all ordinary asynchronous traffic

125. IEEE 802.11 MAC Timing
Basic Access Method

126. Point Coordination Function
(PCF)
• Alternative access method implemented on top of DCF
• Polling by centralized polling master (point coordinator)
• Uses PIFS when issuing polls
— PIFS smaller than DIFS
— Can seize medium and lock out all asynchronous traffic while it issues
polls and receives responses
• E.g. wireless network configured so number of stations with time-
sensitive traffic controlled by point coordinator
— Remaining traffic contends for access using CSMA
• Point coordinator polls in round-robin to stations configured for
polling
• When poll issued, polled station may respond using SIFS
• If point coordinator receives response, it issues another poll using
PIFS
• If no response during expected turnaround time, coordinator issues
poll

127. Superframe
• Point coordinator would lock out asynchronous traffic by issuing
polls
• Superframe interval defined
— During first part of superframe interval, point coordinator polls round-
robin to all stations configured for polling
— Point coordinator then idles for remainder of superframe
— Allowing contention period for asynchronous access
• At beginning of superframe, point coordinator may seize control
and issue polls for given period
— Time varies because of variable frame size issued by responding
stations
• Rest of superframe available for contention-based access
• At end of superframe interval, point coordinator contends for access
using PIFS
• If idle, point coordinator gains immediate access
— Full superframe period follows
— If busy, point coordinator must wait for idle to gain access
— Results in foreshortened superframe period for next cycle

128. IEEE 802.11 MAC Timing
PCF Superframe Construction

129. IEEE 802.11 MAC Frame Format

130. MAC Frame Fields (1)
• Frame Control:
— Type of frame
— Control, management, or data
— Provides control information
• Includes whether frame is to or from DS, fragmentation
information, and privacy information
• Duration/Connection ID:
— If used as duration field, indicates time (in s) channel will be
allocated for successful transmission of MAC frame
— In some control frames, contains association or connection
identifier
• Addresses:
— Number and meaning of address fields depend on context
— Types include source, destination, transmitting station, and
receiving station

131. MAC Frame Fields (2)
• Sequence Control:
—4-bit fragment number subfield
• For fragmentation and reassembly
—12-bit sequence number
—Number frames between given transmitter and
receiver
• Frame Body:
—MSDU (or a fragment of)
• LLC PDU or MAC control information
• Frame Check Sequence:
—32-bit cyclic redundancy check

132. Control Frames
• Assist in reliable data delivery
• Power Save-Poll (PS-Poll)
— Sent by any station to station that includes AP
— Request AP transmit frame buffered for this station while station in
power-saving mode
• Request to Send (RTS)
— First frame in four-way frame exchange
• Clear to Send (CTS)
— Second frame in four-way exchange
• Acknowledgment (ACK)
• Contention-Free (CF)-end
— Announces end of contention-free period part of PCF
• CF-End + CF-Ack:
— Acknowledges CF-end
— Ends contention-free period and releases stations from associated
restrictions

133. Data Frames – Data Carrying
• Eight data frame subtypes, in two groups
• First four carry upper-level data from source station to
destination station
• Data
— Simplest data frame
— May be used in contention or contention-free period
• Data + CF-Ack
— Only sent during contention-free period
— Carries data and acknowledges previously received data
• Data + CF-Poll
— Used by point coordinator to deliver data
— Also to request station send data frame it may have buffered
• Data + CF-Ack + CF-Poll
— Combines Data + CF-Ack and Data + CF-Poll

134. Data Frames –
Not Data Carrying
• Remaining four data frames do not carry user
data
• Null Function
—Carries no data, polls, or acknowledgments
—Carries power management bit in frame control field
to AP
—Indicates station is changing to low-power state
• Other three frames (CF-Ack, CF-Poll, CF-Ack +
CF-Poll) same as corresponding frame in
preceding list (Data + CF-Ack, Data + CF-Poll,
Data + CF-Ack + CF-Poll) but without data

135. Management Frames
• Used to manage communications between
stations and Aps
• E.g. management of associations
—Requests, response, reassociation, dissociation, and
authentication

136. 802.11 Physical Layer
• Issued in four stages
• First part in 1997
— IEEE 802.11
— Includes MAC layer and three physical layer specifications
— Two in 2.4-GHz band and one infrared
— All operating at 1 and 2 Mbps
• Two additional parts in 1999
— IEEE 802.11a
• 5-GHz band up to 54 Mbps
— IEEE 802.11b
• 2.4-GHz band at 5.5 and 11 Mbps
• Most recent in 2002
— IEEE 802.g extends IEEE 802.11b to higher data rates

137. Original 802.11 Physical Layer -
DSSS
• Three physical media
• Direct-sequence spread spectrum
—2.4 GHz ISM band at 1 Mbps and 2 Mbps
—Up to seven channels, each 1 Mbps or 2 Mbps, can
be used
—Depends on bandwidth allocated by various national
regulations
• 13 in most European countries
• One in Japan
—Each channel bandwidth 5 MHz
—Encoding scheme DBPSK for 1-Mbps and DQPSK for
2-Mbps

138. Original 802.11 Physical Layer -
FHSS
• Frequency-hopping spread spectrum
— 2.4 GHz ISM band at 1 Mbps and 2 Mbps
— Uses multiple channels
— Signal hopping from one channel to another based on a pseudonoise
sequence
— 1-MHz channels are used
— 23 channels in Japan
— 70 in USA
• Hopping scheme adjustable
— E.g. Minimum hop rate forUSA is 2.5 hops per second
— Minimum hop distance 6 MHz in North America and most of Europe and
5 MHz in Japan
• Two-level Gaussian FSK modulation for 1-Mbps
— Bits encoded as deviations from current carrier frequency
• For 2 Mbps, four-level GFSK used
— Four different deviations from center frequency define four 2-bit
combinations

139. Original 802.11 Physical Layer –
Infrared
• Omnidirectional
• Range up to 20 m
• 1 Mbps used 16-PPM (pulse position modulation)
— Each group of 4 data bits mapped into one of 16-PPM symbols
— Each symbol a string of 16 bits
— Each 16-bit string consists of fifteen 0s and one binary 1
• For 2-Mbps, each group of 2 data bits is mapped into
one of four 4-bit sequences
— Each sequence consists of three 0s and one binary 1
— Intensity modulation
• Presence of signal corresponds to 1

140. 802.11a
• 5-GHz band
• Uses orthogonal frequency division multiplexing (OFDM)
— Not spread spectrum
• Also called multicarrier modulation
• Multiple carrier signals at different frequencies
• Some bits on each channel
— Similar to FDM but all subchannels dedicated to single source
• Data rates 6, 9, 12, 18, 24, 36, 48, and 54 Mbps
• Up to 52 subcarriers modulated using BPSK, QPSK, 16-
QAM, or 64-QAM
— Depending on rate
— Subcarrier frequency spacing 0.3125 MHz
— Convolutional code at rate of 1/2, 2/3, or 3/4 provides forward
error correction

141. 802.11b
• Extension of 802.11 DS-SS scheme
• 5.5 and 11 Mbps
• Chipping rate 11 MHz
— Same as original DS-SS scheme
— Same occupied bandwidth
— Complementary code keying (CCK) modulation to achieve higher
data rate in same bandwidth at same chipping rate
— CCK modulation complex
• Overview on next slide
— Input data treated in blocks of 8 bits at 1.375 MHz
• 8 bits/symbol  1.375 MHz = 11 Mbps
• Six of these bits mapped into one of 64 code sequences
• Output of mapping, plus two additional bits, forms input to QPSK
modulator

142. 11-Mbps CCK Modulation
Scheme

143. 802.11g
• Higher-speed extension to 802.11b
• Combines physical layer encoding techniques
used in 802.11a and 802.11b to provide service
at a variety of data rates

144. Required Reading
• Stallings chapter 17
• Web sites on 802.11

145. Virtual LANs

146. VLAN introduction
VLANs logically segment switched networks
based on the functions, project teams, or
applications of the organization regardless of
the physical location or connections to the
network.
All workstations and servers used by a particular
workgroup share the same VLAN, regardless of
the physical connection or location.

147. VLAN introduction
A workstation in a VLAN group is restricted to
communicating with file servers in the same VLAN
group.

148. VLAN introduction
VLANs function by logically segmenting the
network into different broadcast domains so
that packets are only switched between ports
that are designated for the same VLAN.
Routers in VLAN
topologies provide
broadcast filtering,
security, and
traffic flow
management.

149. VLAN introduction
VLANs address scalability, security, and
network management.
Switches may not bridge any traffic between
VLANs, as this would violate the integrity of the
VLAN broadcast domain.
Traffic should only be routed between VLANs.

150. Broadcast domains with VLANs and routers
A VLAN is a broadcast domain created by
one or more switches.

151. Broadcast domains with VLANs and routers
Layer 3 routing allows the router to send
packets to the three different broadcast
domains.

152. Broadcast domains with VLANs and routers
Implementing VLANs on a switch causes the
following to occur:
 The switch maintains a separate bridging table for
each VLAN.
 If the frame comes in on a port in VLAN 1, the
switch searches the bridging table for VLAN 1.
 When the frame is received, the switch adds the
source address to the bridging table if it is
currently unknown.
 The destination is checked so a forwarding
decision can be made.
 For learning and forwarding the search is made
against the address table for that VLAN only.

153. VLAN operation
Each switch port could be assigned to a different VLAN.
Ports assigned to the same VLAN share broadcasts.
Ports that do not belong to that VLAN do not share these broadcasts.

154. VLAN operation
Users attached to the same shared segment, share
the bandwidth of that segment.
Each additional user attached to the shared medium
means less bandwidth and deterioration of network
performance.
VLANs offer more bandwidth to users than a shared
network.
The default VLAN for every port in the switch is the
management VLAN.
The management VLAN is always VLAN 1 and may
not be deleted. All other ports on the switch may be
reassigned to alternate VLANs.

155. VLAN operation
Dynamic VLANs allow for membership based on the MAC
address of the device connected to the switch port.
As a device enters the network, it queries a database within
the switch for a VLAN membership.

156. VLAN operation
In port-based or port-centric VLAN membership, the port is
assigned to a specific VLAN membership independent of the
user or system attached to the port.
All users of the
same port must
be in the same
VLAN.

157. VLAN operation
Network administrators are responsible for
configuring VLANs both manually and statically.

158. Benefits of VLANs
The key benefit of VLANs is that they permit the
network administrator to organize the LAN
logically instead of physically.

159. VLAN types
There are three basic VLAN memberships for
determining and controlling how a packet
gets assigned: -
 Port-based VLANs
 MAC address based
 Protocol based VLANs
The frame headers are encapsulated or
modified to reflect a VLAN ID before the
frame is sent over the link between switches.
Before forwarding to the destination device,
the frame header is changed back to the
original format.

160. VLAN types
Port-based VLANs
MAC address based VLANs
Protocol based VLANs

161. Membership by Port

162. Membership by MAC-
Addresses

163. VLAN types
The number of VLANs in a switch vary
depending on several factors:
 Traffic patterns
 Types of applications
 Network management needs
 Group commonality

164. VLAN types
An important consideration in defining the
size of the switch and the number of VLANs
is the IP addressing scheme.
Because a one-to-one correspondence
between VLANs and IP subnets is strongly
recommended, there can be no more than
254 devices in any one VLAN.
It is further recommended that VLANs should
not extend outside of the Layer 2 domain of
the distribution switch.

165. VLAN types
There are two major methods of frame tagging,
Inter-Switch Link (ISL) and 802.1Q.
ISL used to be the most common, but is now
being replaced by 802.1Q frame tagging.

166. Bluetooth Technology

167. Introduction
• What is Bluetooth?
• Why is it useful?
• Governing Standard – Large Consortium

168. History
• 1998 - Bluetooth technology is officially introduced and the
BLUETOOTH SIG is formed. Bluetooth technology's intended
basic purpose is to be a wire replacement technology in order
to rapidly transfer voice and data.
• 1999 - Bluetooth 1.0 Specification is introduced.
• 2003 - The BLUETOOTH SIG overhauls the Bluetooth Core
Specification with the announcement of Version 2.1.
• 2004 - Bluetooth Version 2.0 + EDR (Enhanced Data Rate) is
introduced.
• 2005 - Devices using Version 2.0 + EDR begin to hit the
market in late 2005.
• 2007 - Bluetooth Core Specification Version 2.1 + EDR is
adopted by the BLUETOOTH SIG.
• 2009 - Bluetooth Core Specification Version 3.0 + HS (High
Speed) is adopted by the BLUETOOTH SIG.

169. How it works?
• Short range wireless connectivity.
• Low power consumption
• Automatic recognition.

170. Privacy/Security
• Very Important.
• Don’t want data to be shared.
• Security not great.
—Pairing security.

171. Reliability
• What it really means?
• Which device are you connecting to?
• How large is the file?
• Which version are you using?
• Have the two devices been connected before?

172. Support of Community

173. Devices
• Laptops
• Gaming Consoles
• Headsets
• Cell Phones
• Printers

174. Ease of Use
• Very easy to use.
• Connection is fast and simple.
• More devices are Bluetooth capable.
• Auto recognition.

175. Future
• Diverse applications with Bluetooth.
• Faster transfer rate.
• Stronger connection.
• Longer distance.

176. Conclusion
• Few flaws
—Security
• Good for short transfers
—Limitations on file size
—Limitations on distance
• Convenient
—Faster
—More simple than flash drive

177. References
• 1.http://www.bluetooth.com/English/Experience
/Pages/On_The_Cutting_Edge.aspx
• 2.
http://electronics.howstuffworks.com/bluetooth
2.htm
• 3.http://www.bluetomorrow.com/about-
bluetooth-technology/history-of-
bluetooth/bluetooth-history.html
• 4.http://www.bluetooth.com/English/SIG/Pages
/default.aspx
• 5. http://en.wikipedia.org/wiki/Bluetooth
• 6.
http://www.allbusiness.com/technology/67826
6-1.html

178. Questions?