The document discusses wireless local area networks (WLANs) and the IEEE 802.11 standards. It describes some of the challenges of wireless transmissions and provides an overview of the IEEE 802.11 standards including 802.11, 802.11a, 802.11b, 802.11e, and 802.11g. It also summarizes the IEEE 802.11 network architecture including the physical layer, data link layer, and different network topologies like IBSS, BSS, and ESS.

1. Wireless LANs
• Rat‟s nest of wires
• Variety of power cords and adapters
• Cables for modems, printers, scanner,
mouse and keyboards
• NEED OF CONNECTING COMPUTERS

2. Challenges
• Radio and Infrared transmissions susceptible to
noise and interference
– Unreliable transmission
• Strength varies in time and space
– Fading
• Finite Radio Spectrum
– Shared with other systems
• Radio spectrum regulated by different bodies

3. Wireless LANs
• IEEE 802.11 Standards
• Non IEEE Standards like
– Bluetooth, HIPERLAN, HomeRF

4. What is IEEE 802.11?
• IEEE:
– Institute of Electrical and Electronics Engineers
• 802.11:
– Family of standards set forth by the IEEE to
define the specifications for wireless LANs
– Defines:
• Medium Access Control (MAC)
• Physical Layer (PHY) Specifications

5. IEEE 802.11 and the ISO stack

6. What is IEEE 802.11?
• Local, high-speed wireless connectivity for
fixed, portable and moving stations
– stations can be moving at pedestrian and
vehicular speeds
• Standard promises interoperability
– vendors products on the same physical layer
should interoperate
• Targetted for use in
– inside buildings, outdoor areas, anywhere!

7. IEEE 802.11
• Uses Direct Sequence spread spectrum
(DSSS) technology
– Frequency-Hopping spread spectrum (FHSS)
can only be used for 1 or 2Mbps in US due to
FCC regulations
• Operates in unlicensed 2.4 GHz ISM band
– ISM: Industrial, Scientific and Medical
– ISM regulatory range:
• 2.4 GHz to 2.4835 GHz for North America

8. IEEE 802.11
• Supported Speeds and Distances
– 1, 2, 5.5, 11 Mbps at distances of 150-2000 feet
without special antenna
– Greater distances can be achieved by using
special antennas
– Distance (or signal strength) greatly depends on
obstructions such as buildings and other objects
– Maximum speed obtained depends on signal
strength

9. IEEE 802.11b
• „b‟ in IEEE 802.11b
– September 1999, 802.11b “High Rate”
amendment was ratified by the IEEE
– 802.11b amendment to 802.11 only affects the
physical layer, basic architecture is the same
• Added two higher speeds
– 5.5 and 11 Mbps
• More robust connectivity
• 802.11b is the current „favorite‟ in 802.11
– also known as Wi-Fi (Wireless Fidelity)

10. IEEE 802.11a
• “Fast Ethernet” standard of wireless LANs
• Speeds of up to 54 Mbps
• 5 GHz (U-NII band) instead of 2.4 GHz
– Unlicensed National Information Infrastructure
• OFDM instead of DSSS for encoding
– Orthogonal Frequency Division Multiplexing

11. IEEE 802.11a
• Advantages
– higher speed
– less RF interference than 2.4 GHz
• 2.4 GHz used by Bluetooth, cordless/cellular phones,
etc.
– some interoperability, vendors currently have
“dual-standard” 802.11a/b equipment
• Disadvantages
– shorter range, need to increase AP density or
power 4X to compensate

12. IEEE 802.11g
• Another high-speed standard
• Viewed as a „step‟ towards 802.11a
• Speeds of up to 54 Mbps
– may be more like 20+ Mbps
• Still works at 2.4 GHz
– not in the 5 GHz range like 802.11a
• Advantages
– compatible with 802.11b
– better range than 802.11a, for now

13. IEEE 802.11e
• Another upcoming standard for WLANs
– adds quality-of-service features to MAC layer of
802.11b compatible networks
• error correction
• better bandwidth management
– significantly improves multimedia performance
• works around RF interference
– handles interference by moving away from it
– i.e., moves to a new frequency when interference from a 2.4
GHz cordless phone is detected

14. IEEE 802.11 and the ISO stack

15. IEEE 802.11 Physical Layer
• 802.11 Physical Layer Specifications
– include FHSS, DSSS, IR
• PLCP: Physical Layer Convergence Protocol
– interface used by the other physical layer specs
– maps data units into a suitable framing format
• PMD system: Physical Medium Dependent
– defines the characteristics/method of Tx/Rx data
through a wireless medium between 2 or more
stations

16. IEEE 802.11 Physical Layer
• Spread Spectrum
– spreads the transmitted signal over a wide range
of spectrum
– avoids concentrating power in a single narrow
frequency band
– noise makes this necessary so that receiver can
accurately decode the transmitted signal
– 2 major approaches to spread spectrum:
• FHSS: Frequency Hopping Spread Spectrum
• DSSS: Direct Sequence Spread Spectrum

17. IEEE 802.11 Data Link Layer
• 2 Sublayers
– Logical Link Control (LLC)
– Media Access Control (MAC)
• 802.11 uses the same 802.2 LLC
– same 48-bit addressing as other 802 LANs
• MAC address is 6 bytes or 48 bits
– allows for simple bridging to wired networks
• MAC sublayer is unique in 802.11

18. IEEE 802.11 MAC Sublayer
• MAC: Regulates access to the medium
• Wired IEEE 802 LANs use CSMA/CD
• 802.11 uses CSMA/CA
• CSMA: carrier sense multiple access
– CD: with collision detection
– CA: with collision avoidance
• Collision detection is not possible in 802.11
– near/far problem: can‟t transmit and “hear” a
collision at the same time

19. IEEE 802.11 MAC Sublayer
• CSMA/CA avoids collisions by explicit packet
acknowledgment (ACK)
– station wishing to transmit first senses the medium
– if no activity detected, station waits an additional,
random amount of time then transmits if the
medium is still free
– ACK packet is sent by receiving station to confirm
the data packet arrived intact
– collision assumed if sending station doesn‟t get
ACK, data is retransmitted after a random time

20. IEEE 802.11 MAC Sublayer
• Other unique features in 802.11
– IFS: Inter Frame Space
• time interval between frames
– Handling hidden stations (hidden-node problem)
• virtual carrier sense
– Power management functions
– Data security (MAC address, WEP)
• WEP: Wired Equivalent Privacy
– Multirate support
– Fragmentation / Defragmentation

21. Coordination Functions of MAC
• Determine when a station in a BSS is allowed
to transmit and when it may be able to receive
PDUs over the wireless medium
• Distributed Coordination Function (DCF)
– Provides support for asynchronous data transfer of
MAC SDUs on a best effort basis
– Contention Mode for all station

22. Coordination Functions of MAC
• Point Coordination Function
– Optional and sits on top of DCF
– May be implemented by an AP
– Connection-oriented time bound transfer of MAC
SDUs
– Contention and contenion-free periods
– Medium usage controlled by AP (synchronization
and timing)

23. DCF
• Basic access method
• Contention services for fair service to all stations
• All stations required to support DCF
• Based on CSMA-CA protocol
– All stations obliged to remain quiet for a certain minimum
period after a transmission has been completed called the
interframe space (IFS)
– High priority frames: SIFS
– PCF Priority access to the medium: PIFS
– DCF Interframe Space: DIFS
• Transmit data and management MPDUs

24. IEEE 802.11: A Closer Look

25. IEEE 802.11 Frame Types
• Three types of frames
– Control
• RTS, CTS, ACK, Contention-Free (CF), PS-Poll
• Used for handshaking and for positive acknowledgements during
the data transfer
– Management
• Probe request/response
• Station Association and Disassociation with the AP
• Timing and Synchronization
• Authentication / deauthentication
• Announcement traffic indication message (ATIM)
– sent after each frame
– Data
• Transmission of data

26. CSMA-CA operation
• A station is allowed to transmit an initial MAC
PDU under DCF if the station detects the medium
idle for a period DIFS or greater. If the station
detects medium busy, then it must calculate a
random backoff time to schedule a reattempt. A
station that has scheduled a reattempt monitors the
medium and decrements a counter each time an
idle contention slot expires. The station is allowed
to transmit when its backoff timer expires during
the contention period.
• Idle period after a DIFS period called contention
window (CW)

27. Handshaking in CSMA-CA
• Required when there is hidden station problem. If a
station A wants to send data frame to station B, station A
first sends a request-to-send (RTS) frame. If station B
receives the RTS frame, then B issues a clear-to-send
(CTS) frame. All stations within range of B receive CTS
frame and are aware that station A has been given
permission to send, so they remain quiet while station A
proceeds with its data frame transmission. If the data
frame arrives without error, station B responds with an
ACK. If two stations send RTS frames at the same time
and they collide at B then the stations must execute a
backoff to schedule a later attempt.

28. DIFS Contention
Window
PIFS
SIFS
DIFS
Busy Medium Next Frame
Wait for
Time
Defer Access
reattempt time
Basic CSMA-CA operation

29. IEEE 802.11 Topologies
• Three basic topologies for WLANs
– IBSS: Independent Basic Service Set
– BSS: Basic Service Set
– ESS: Extended Service Set
• Independent of type of PHY chosen

30. IEEE 802.11 IBSS
• IBSS: Independent Basic Service Set
– Peer-to-peer or ad-hoc network
– Wireless stations communicate directly with one
another
– Generally are not connected to a larger network
– No Access Point (AP)

31. IEEE 802.11 BSS
• BSS: Basic Service Set
– Infrastructure mode
– An AP connects clients to a wired network

32. BSS
• Defined as group of stations that coordinate
their access to the medium under a given
instance of the medium access control
• Area covered by BSS called Basic Service
Area (BSA)
– Analogous to a cell in cellular network
– Upto a diameter of tens of meters

33. BSS and Adhoc Wireless
Network
• Adhoc Network consists of group of
stations within range of each other
• Typically temporal in nature
• Can be formed spontaneously anywhere
• Disbanded after a limited period of time

34. IEEE 802.11 ESS
• ESS: Extended Service Set
– A set of BSSs interconnected by a distribution
system
– Consists of overlapping BSSs (each with an AP)
• DS connects APs together, almost always Ethernet
• ESS allows clients to seamlessly roam between APs

35. Access Point
• Similar in functionality to base station in a cellular
system
• ESS can also provide gateway access for wireless
users into a wired network such as Internet
– Such access accomplished via a device called portal
• Infrastructure network refers to combination of
BSSs, a distribution system and portals

36. Access Points (APs)
• To join an infrastructure BSS, a station
must select an AP and establish an
association with it
• This establishes mapping between station
and the AP
• Station can then send and receive data
messages via the AP
• Reassociation and Dissociation services

37. Access Points (APs)
• Usually connects wireless and wired
networks
– if not wired
• acts as an extension point
• Creation of ESS by overlapping AP coverage
– allows roaming operation
– APs should be on different channels

38. Access Points (APs)
• Capacity and Bandwidth
– Possible to keep these higher by using these
techniques
• Reducing size of coverage areas
• Reducing client-to-AP ratio
• Using bandwidth aggregation
– AP-to-client ratio
– load balancing

39. Access Points (APs)
• Roaming
– More than 1 AP provides signals to a single
client
– Client is responsible for choosing the best AP
• first, signal strength. second, network utilization.
– When signal in use degrades, client tries to find
another AP
• if found, tries to authenticate and associate

40. Access Points (APs)
• Configuration
– Management usually done via
• HTTP, Telnet, SNMP, serial interface
– Configuring Security Settings
• SSID: Service Set Identifier
• WEP: Wired Equivalent Privacy
• EAP: Extensible Authentication Protocol
– Configuring Network Settings
• DHCP: Dynamic Host Configuration Protocol
• NAT: Network Address Translation

41. Access Points (APs)
• How to setup a secure access point
– Enable WEP or EAP
– Change SSID and disable broadcast
– Change the management password of your AP
• some have 2: read-only as well as read-write
– Use MAC address filtering
– Consider not using DHCP
• instead use fixed IP addresses for wireless NICs
– Consider other mechanisms for privacy
• PPTP, VPN, SSL, SSH

42. IEEE 802.11 Security
• Authentication
– Open system
– Shared key
• Authorization
– MAC address
• Privacy
– WEP: Wired Equivalent Privacy

43. Overview of the Bluetooth
technology

44. Bluetooth
• Bluetooth must be able to:
• Recognize any other Bluetooth device in radio range
• Permit easy connection of these devices
• Be aware of the device types
• Support service discovery
• Support connectivity aware applications
• Examples of Bluetooth uses:
• Briefcase email: access email while the PC is still in the
briefcase; when PC receives an email, you are notified thru
the mobile phone. Use the mobile phone to browse the
email.
• Cordless desktop: connect your desktop/laptop cordlessly
to printers, scanner, keyboard, mouse, etc.

45. Bluetooth
• Bluetooth radio modules operate in the unlicensed ISM
band centered at at 2.45GHz. RF channels:2420+k MHZ,
k=0..78.
• Bluetooth devices within 10m of each other can share up
to 720kbps of capacity
• Projected cost for a Bluetooth chip is ~$5. Plus its low
power consumption, means you could literally place one
anywhere.
• Can operate on both circuit and packet switching modes,
providing both synchronous and asynchronous data
services
• It is intended to support an open-ended list of applications,
including data, audio, graphics and even video.

46. Bluetooth Architecture
• Up to 8 devices can communicate in a small network, called piconet.
• 10 piconets can coexist in the same coverage range of the Bluetooth
radio.
• Each piconet has 1 MASTER and the rest serve as SLAVES. SLAVES
within a piconet only have links to the MASTER.
• Multi-hop communication is obtained thru the scatternet.

48. Bluetooth Limitations
• Does not address routing, most network functions are
pushed into the link layer
• Does not support multi-hop multicasting
• Does not address how to cope with mobility !
• The MASTER node is the bottleneck
• No. of nodes in piconet is limited
• Does not address power-saving methods done at upper
layers, above the link-layer

49. NS : Network Simulator
Any kind of network simulation
Including mobile and wireless network simulation

50. Outlines
 Use NS to simulate wireless network
Extend NS to support mobile and wireless
application: Internal implementation

51. ns-2 overview
•Collection of various protocols at multiple layers
TCP(reno, tahoe, vegas, sack)
MAC(802.11, 802.3, TDMA)
Ad-hoc Routing (DSDV, DSR, AODV, TORA)
Sensor Network (diffusion, gaf)
Multicast protocols, Satellite protocols, and many others
•Codes are contributed from multiple research
communities
Good: Large set of simulation modules
Bad: Level of support and documentation varies
•The source code and documentation is currently
maintained by VINT project at ISI

52. Introduction
•ns-2 is an discrete event driven simulation
Physical activities are translated to events
Events are queued and processed in the order of their
scheduled occurrences
Time progresses as the events are processed
Time: 1.5 sec Time: 1.7 sec
1 2
Time: 2.0 sec Time: 1.8 sec

53. Event Driven Simulation
RX Ack Event
TX Pkt Event
@ 1.5sec
2.0sec Node 1
Module
Event Queue
RX Ack Event
TX Pkt Event
TX
@ 1.5sec
2.0sec
1.8sec
1.7sec
Simulation Finished!
Node 2
Module
TX Ack Event
RX Pkt
@ 1.7sec
1.8sec

55. Network Components inside a
mobile node
 Link Layer
 ARP
 Interface Queue
 Mac Layer: IEEE 802.11
 Network Interface
 Radio Propagation Model
2
Friss-space attenuation(1/ r ) at near distance
2
 Two ray Ground (1/ r ) at far distance

56. Mobile Node Modules
•Agent
Responsible for packet generations and receptions
Can think of it as an Application layer
CBR(Constant Bit Rate), TCP, Sink, FTP, etc.
•RTagent(DSDV, TORA, AODV) or DSR
Ad-hoc network routing protocols
Configure multi hop routes for packets
•LL (Link Layer)
Runs data link protocols
Fragmentation and reassembly of packet
Runs Address Resolution Protocol(ARP) to resolve IP
address to MAC address conversions

57. Mobile Node Modules (Continued)
•IFq (Interface Queue)
PriQueue is implemented to give priority to
routing protocol packets
Supports filter to remove packets destined to
specific address
•Mac Layer
IEEE 802.11 protocol is implemented
Uses RTS/CTS/DATA/ACK pattern for all unicast
pkts and DATA for broadcast pkts

58. Mobile Node Modules (Continued)
•NetIF (Network Interfaces)
Hardware interface used by mobilenode to access the
channel
Simulates signal integrity, collision, tx error
Mark each transmitted packet with transmission power,
wavelength etc.
•Radio Propagation Model
Uses Friss-space attenuation(1/r2) at near distance and
Two ray ground (1/r4) at far distance
Decides whether the packet can be received by the
mobilenode with given distance, transmit power and
wavelength
Implements Omni Directional Antenna module which has
unity gain for all direction

59. Wireless Simulation in ns-2 (Mobile Node
Diagram - DSDV)
Agent
(Src/Sink)
Demux
Port
Demux
Addr
RTagent
(DSDV)
LL ARP
IFq
MAC
Radio
Propagation NetIF
Model
Channel

60. Wireless Simulation in ns-2 (Mobile Node
Diagram - DSR)
Agent
(Src/Sink)
Demux
Port
DSR
LL ARP
IFq
MAC
Radio
Propagation NetIF
Model
Channel

61. Abstract the real mobile world into
your simulation
 Node
 Packets
 Wireless channel and channel access
 Forwarding and routing
 Radio propagation model
 Trace/Visualization
 Event scheduler to make everything running

62. Implementing mobile node by
Extending “standard” NS node
Node Classifier:Forwarding
Agent: Protocol Entity
Routing Node Entry
LL ARP LL LL:Link layer object
IFQ:Interface queue
MAC Radio
Propagation
PHY Model
MAC MAC:Mac object
MobileNode PHY PHY:Net interface
CHANNEL

63. References
Anand Trivedi‟s IEEE 802.11 Page
– http://alpha.fdu.edu/~anandt/introduction.html
• IEEE 802.11 Working Group Page
– http://www.ieee802.org/11/
– Can download the 802 standards here for FREE
– Has links to all the latest 802.11 developments
• O‟Reilly
– http://oreilly.wirelessdevnet.com/
• http://wireless.telerama.com

64. THAT‟S ALL