1. Wireless# Guide to Wireless Communications
Chapter 7
Low-Speed Wireless Local Area Networks

2. Objectives Describe how WLANs are used
List the components and modes of a WLAN
Describe how an RF WLAN works
Explain the differences between IR, IEEE , and IEEE b WLANs
Outline the user mobility features offered by IEEE networks
Wireless# Guide to Wireless Communications

3. WLAN Applications Wireless networks are increasing in popularity
Installing cabling is inconvenient and very expensive
Wireless networks solve this problem
With a wireless network
Multiple users can share a single Internet connection
Wireless residential gateway
Device that combines a router, Ethernet switch, and wireless access point
Also allows Internet and printer sharing
Wireless# Guide to Wireless Communications

4. WLAN Components Minimal hardware needed for a WLAN Computer ISP
Wireless network interface card
Access point (AP)
Wireless# Guide to Wireless Communications

5. Wireless Network Interface Card
Network interface card (NIC)
Allows a computer to be connected to a network
Wireless NIC
Connects a computer to a network without cables
Most often a separate card
Mini PCI
Small card that is functionally equivalent to a standard PCI expansion card
Used with notebook computers
Wireless# Guide to Wireless Communications

6. Wireless Network Interface Card (continued)
Wireless# Guide to Wireless Communications

7. Wireless Network Interface Card (continued)
Smaller devices (ex. PDAs) have two options for wireless NICs
Optional sled
Includes a Type II PC Card slot and battery
Compact flash (CF) card or secure digital (SD) card
Smaller and consume less power
Transmit and receive with lower power levels
Intel Centrino chipset
Integrates all of the functions of a wireless NIC
Mounted directly on the motherboard
Wireless# Guide to Wireless Communications

8. Wireless Network Interface Card (continued)
Wireless# Guide to Wireless Communications

9. Access Points Access point (AP) AP parts AP functions
Provides wireless LAN devices with a point of access into a wired network
AP parts
Radio transceiver
Antenna
RJ-45 wired network interface port
AP functions
Wireless communications base station
Bridge between the wireless and wired networks
Wireless# Guide to Wireless Communications

10. Access Points (continued)
Wireless# Guide to Wireless Communications

11. Access Points (continued)
The range of an AP is approximately 115 meters
Dynamic rate selection
AP will automatically select the highest possible data rate for transmission
Depending on the strength and quality of the signal
An AP can generally support over 100 users
Power over Ethernet (PoE)
DC power is delivered to the AP through the unused wires in a standard UTP Ethernet cable
Wireless# Guide to Wireless Communications

12. WLAN Modes Ad hoc (infrastructure-less) mode Infrastructure mode
Wireless# Guide to Wireless Communications

13. Ad Hoc Mode Also known as peer-to-peer mode
Formal name: Independent Basic Service Set (IBSS) mode
Wireless clients communicate directly among themselves without using an AP
Quick and easy setup of a wireless network
Drawback is that wireless clients can only communicate among themselves
Wireless# Guide to Wireless Communications

14. Ad Hoc Mode (continued)
Wireless# Guide to Wireless Communications

15. Infrastructure Mode Also known as the Basic Service Set (BSS)
Consists of wireless clients and an AP
Extended Service Set (ESS)
Two or more BSS wireless networks installed in same area
Provides users with uninterrupted mobile access to the network
All wireless clients and APs must be part of the same network
For users to be able to roam freely
Wireless# Guide to Wireless Communications

16. Infrastructure Mode (continued)
Wireless# Guide to Wireless Communications

17. Infrastructure Mode (continued)
Can be difficult to manage one large network
Performance and security may also be adversely affected
Subnets
Network units that contain fewer computers
In an ESS divided into subnets
A mobile user might not be able to freely roam between APs
Wireless# Guide to Wireless Communications

18. Wireless LAN Standards and Operation
Most WLANs are based on the same initial IEEE standards
Wireless# Guide to Wireless Communications

19. IEEE 802.11 Standards 802.11 standard
Defines a local area network that provides cable-free data access for clients
That are either mobile or in a fixed location
At a rate of either 1 or 2 Mbps
Using either diffused infrared or RF transmission
Specifies that the features of a WLAN be transparent to the upper layers of the TCP/IP protocol stack or the OSI protocol model
No Modifications are needed at upper layers
Wireless# Guide to Wireless Communications

20. IEEE 802.11 Standards (continued)
Wireless# Guide to Wireless Communications

21. IEEE 802.11 Infrared WLAN Standard
IEEE standards outline the specifications for infrared as well as RF WLANs
Infrared specification is based on diffused transmissions
PHY layer sublayers
Physical Medium Dependent (PMD): Defines the method for transmitting and receiving through the wireless medium (IR, RF)
Physical Layer Convergence Procedure (PLCP): Reformats the data received from the MAC layer, and listens to the medium
Wireless# Guide to Wireless Communications

22. IEEE 802.11 Infrared WLAN Standard (continued)
Wireless# Guide to Wireless Communications

23. IEEE 802.11 Infrared WLAN Standard (continued)
Diffused infrared PHY layer convergence procedure standards
Reformats the data received from the MAC layer
Into a frame that the PMD sublayer can transmit
Frame’s size is not measured in bits but in time slots
Start frame delimiter for infrared is always 1001
Data rate value determines transmission speed
Direct current level adjustment
Contains a pattern of infrared pulses
Allows receiving device to determine the signal level
Wireless# Guide to Wireless Communications

24. IEEE 802.11 Infrared WLAN Standard (continued)
Wireless# Guide to Wireless Communications

25. IEEE 802.11 Infrared WLAN Standard (continued)
Wireless# Guide to Wireless Communications

26. IEEE 802.11 Infrared WLAN Standard (continued)
Diffused infrared physical medium dependent standards
PMD translates the binary 1s and 0s of the frame into light pulses that are used for transmission
PMD transmits data using a series of light impulses
16-pulse position modulation (16-PPM) 1 Mbps
Translates four data bits into 16 light impulses
4-pulse position modulation (4-PPM)
Used for transmissions at 2 Mbps
Each time slot is 250 ns (nanoseconds)
Wireless# Guide to Wireless Communications

27. IEEE 802.11 Infrared WLAN Standard (continued)
Wireless# Guide to Wireless Communications

28. IEEE 802.11 Infrared WLAN Standard (continued)
Wireless# Guide to Wireless Communications

29. IEEE 802.11b Standard Added two higher speeds, 5.5 Mbps and 11 Mbps
Specified RF and direct sequence spread spectrum (DSSS) as the only transmission technology
Also known as Wi-Fi
Physical layer
Divided into two parts
Physical Medium Dependent (PMD)
Physical Layer Convergence Procedure (PLCP)
Wireless# Guide to Wireless Communications

30. IEEE 802.11b Standard (continued)
Physical layer convergence procedure standards
Based on direct sequence spread spectrum (DSSS)
Must reformat the data received from the MAC layer into a frame that the PMD sublayer can transmit
PLCP frame is made up of three parts: the preamble, the header, and the data
Frame preamble and header are always transmitted at 1 Mbps
Allows communication between slower and faster devices
Wireless# Guide to Wireless Communications

31. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

32. IEEE 802.11b Standard (continued)
Physical medium dependent (PMD) standards
Translate the binary 1s and 0s of the frame into radio signals that can be used for transmission
802.11b standard uses the Industrial, Scientific, and Medical (ISM) band
Specifies 14 available frequencies, beginning at GHz and incrementing by .005 GHz
PMD can transmit the data at 11,5.5, 2,or 1 Mbps
For transmissions at 1 Mbps, two-level Differential Binary Phase Shift Keying (DBPSK) is used (0 bit is shifted 0 degrees, 1 bit is shifted 180 degrees)
For transmissions at 2, 5.5, 11 Mbps, Differential Quadrature Phase Shift Keying (DQPSK) is used (variations in phase/amplitude for 00, 01, 10, 11)
Wireless# Guide to Wireless Communications

33. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

34. IEEE 802.11b Standard (continued)
Physical medium dependent standards (continued)
Transmissions at 2, 5.5, and 11 Mbps use differential quadrature phase shift keying
Media access control (MAC)layer
802.11b Data Link layer consists of two sublayers
Logical Link Control (LLC)
Media Access Control (MAC)
Wireless# Guide to Wireless Communications

35. IEEE 802.11b Standard (continued)
Media access control layer (continued)
Changes for b WLANs are confined to the MAC layer
On a WLAN, all devices share the same RF spectrum
Distributed coordination function
Channel access methods can prevent collisions
Carrier sense multiple access with collision avoidance (CSMA/CA)
Based on CSMA/CD
Wireless# Guide to Wireless Communications

36. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

37. IEEE 802.11b Standard (continued)
Distributed coordination function (DCF) (continued)
CSMA/CD is designed to handle collisions
CSMA/CA attempts to avoid collisions altogether
With contention-based channel access methods
Most collisions occur after a device completes its transmission
CSMA/CA makes all devices wait a random amount of time
CSMA/CA also reduces collisions by using explicit packet acknowledgment (ACK)
Wireless# Guide to Wireless Communications

38. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

39. IEEE 802.11b Standard (continued)
Distributed coordination function (continued)
Additional mechanisms to reduce collisions
Request-to-Send/Clear-to-Send (RTS/CTS) protocol
Fragmentation: reduces the amount of time the medium is being used
Polling: Point coordination function (PCF)
Another type of channel access method
Each computer is asked if it wants to transmit
Polling effectively eliminates collisions
AP serves as the polling device referred to as Point coordinator
Wireless# Guide to Wireless Communications

40. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

41. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

42. IEEE 802.11b Standard (continued)
Association and reassociation
Allow a client to join a WLAN and stay connected
Association begins with the client scanning the airwaves
Types of scanning
Passive scanning
Client listening to each available channel for a set period of time
Client listens for a beacon frame transmitted from all available APs
Frame includes the AP’s SSID
Wireless# Guide to Wireless Communications

43. IEEE 802.11b Standard (continued)
Association and reassociation (continued)
Types of scanning (continued)
Active scanning
Client sends a probe frame to each channel
Client then waits for the probe response frame
Associate request frame
Includes the client’s capabilities and supported rates
Associate response frame
Sent by the AP
Contains a status code and client ID number for that client
Wireless# Guide to Wireless Communications

44. IEEE 802.11b Standard (continued)
Association and reassociation (continued)
Reassociation
Client may drop the connection with one AP and reestablish the connection with another
Necessary when mobile clients roam beyond the coverage area of one AP
Client sends a reassociation request frame to new AP
AP sends back a reassociation response frame
New AP sends a disassociation frame to old AP
Wireless# Guide to Wireless Communications

45. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

46. IEEE 802.11b Standard (continued)
Power management
Most clients in a WLAN are portable devices
To conserve battery power, they can go into sleep mode
When a client is part of a WLAN, it must remain fully powered
Power management allows the mobile client’s NIC to be off as much as possible
But still not miss out on data transmissions
The key to power management is synchronization
Wireless# Guide to Wireless Communications

47. IEEE 802.11b Standard (continued)
Power management (continued)
When a mobile b client goes into sleep mode, the AP is informed of the change
AP temporarily stores the synchronized frames (this function is called buffering)
At predetermined times, the AP sends out a beacon frame to all clients
Containing the traffic indication map (TIM)
Traffic indication map (TIM)
List of clients with buffered frames waiting at the AP
Wireless# Guide to Wireless Communications

48. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

49. IEEE 802.11b Standard (continued)
MAC frame formats
Three types of MAC frame formats
Management frames
Used to set up the initial communications
Control frames
Provide assistance in delivering the frames that contain the data (ex. RTS)
Data frames
Carry the information to be transmitted to the destination client
Wireless# Guide to Wireless Communications

50. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

51. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

52. IEEE 802.11b Standard (continued)
Wireless# Guide to Wireless Communications

53. Distributed Coordination Function Rules
standard defines a number of interframe spaces (IFS)
To handle the contention for the medium
Interframe space types
Short interframe space (SIFS): for ACK frames (10 Microseconds)
Distributed coordination function interframe space (DIFS): between data frames: (50 Microseconds)
Basic rules of communication in an network
Device that wants to transmit begins listening for an RF signal
Wireless# Guide to Wireless Communications

54. Distributed Coordination Function Rules (continued)
Basic rules of communication in an network (continued)
Size of a frame includes both the length of time necessary to send the data plus an SIFS
After receiving an ACK, the transmitting client begins to wait a random backoff interval
If the transmitting device does not receive an ACK within the SIFS
It is allowed to maintain control of the medium
If the frame was acknowledged correctly
Transmitting device listens to the medium again
Wireless# Guide to Wireless Communications

55. Distributed Coordination Function Rules (continued)
Wireless# Guide to Wireless Communications

56. Distributed Coordination Function Rules (continued)
In the case of traffic in the medium
Client A is using DSSS with a backoff interval of 3
While Client B has a backoff interval of only 2
Client A transmits its first frame
Client B detects traffic and waits
Both devices begin carrier sensing during second DIFS
At the end, clients A and B begin their backoff interval
Client B will finish its backoff interval before Client A
Then begins transmitting its first packet
Next time, client A will wait only for 1 interval
Wireless# Guide to Wireless Communications

57. Distributed Coordination Function Rules (continued)
Wireless# Guide to Wireless Communications

58. Summary The wireless technology that attracts the most attention today
Radio frequency (RF) wireless local area networks (WLANs)
Wireless NIC performs same functions as wired NIC
Access points parts
Antenna
Radio transceiver
RJ-45 wired network interface
Wireless# Guide to Wireless Communications

59. Summary (continued)
Data can be sent and received in an RF WLAN either in ad hoc or infrastructure mode
IEEE standard defines a local area network that provides cable-free data access
standard uses an access method known as the distributed coordination function (DCF)
Specifies that carrier sense multiple access with collision avoidance (CSMA/CA) be used
standard provides for an optional polling function known as point coordination function (PCF)
Wireless# Guide to Wireless Communications

60. Summary (continued)
Mobile WLAN devices often depend on batteries as their primary power source
Sleep mode conserves battery power
standard specifies three different types of MAC frame formats
Management frames
Control frames
Data frames
Wireless# Guide to Wireless Communications