1. Chapter 15 Wireless LANs

2. IEEE 802 Standards Working Groups
WiMAX, an acronym that stands for Worldwide Interoperability for Microwave Access(802.16)
WiFi (Wireless Fidelity) :
TAG(Technical Advisory Grooup)

3. Architecture Physical Layer MAC Layer Addressing Mechanism
Introduction
Architecture
Physical Layer
MAC Layer
Addressing Mechanism

4. 15.1 IEEE 802.11 - Architecture Basic Service Set (BSS)
Extended Service Set (ESS)

5. Basic Service Set
made up of stationary or mobile wireless stations, and possibly a central base station (access point)
BSS without an AP is stand-alone, cannot send data to other BSSs

6. ESS – Extended Service Set
Made up of two or more BSSs with APs – connected through a distribution system (usually a wired LAN)

7. Station Types
No-transition mobility – either stationary or moving only inside a BSS
BSS-transition mobility – can move from one BSS to another, confined to one ESS
ESS-transition mobility – can move from one ESS to another

8. Physical layer specifications
defines the specs for the conversion of bits to a signal

9. FHSS – Frequency hopping spread spectrum
sender sends on one carrier frequency for a short amount of time, hops to another frequency for the same time, then another, and so on, eventually the cycle is repeated
makes it difficult for unauthorized person to make sense of data
2.4 GHz industrial scientific, and medical (ISM) band (2.4 ~ 2.48Ghz) – 79 subbands of 1MHz
modulation is FSK (1 or 2 bits/buad, two-level or four-level) : date rate of 1 or 2 Mbps

10. Frequency-Hopping example

11. DSSS – direct sequence spread spectrum
Each bit sent is replaced by a chip code (similar to CDMA, but this is at the physical layer, not a multiple-access data link layer method)
2.4 GHz ISM band
Modulation is PSK (BPSK or QPSK)

12. 802.11 802.11 Infrared Two capacities 1 Mbps or 2 Mbps.
Range is 10 to 20 meters and cannot penetrate walls.
Does not work outdoors.
FHSS (Frequency Hopping Spread Spectrum)
The main issue is multipath fading.
79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band.
Same pseudo-random number generator used by all stations.
Dwell time: min. time on channel before hopping (400msec).

13. 801.11a,b,g 802.11a OFDM – orthogonal frequency division multiplexing
5-GHz ISM band
48 subbands for sending 48 groups of bits at a time, 4 subbands for control information
PSK for 18Mbps and QAM for 54 Mbps
802.11b HR-DSSS – high-rate direct sequence spread spectrum
2.4 GHz band
similar to DSSS except for encoding, complementary code keying (CCK), encodes 4 or 8 bits to one CCK symbol
modulation BPSK and QPSK
4 data rates (1, 2, 5.5, and 11 Mbps) : DSSS for 1, 2 Mbps, BPSK for 5.5, QPSK for 11 Mbps.
802.11b and 11a are incompatible!!

14. 801.11a,b,g (cont’d) 802.11g OFDM 2.4 GHz band 54-Mbps data rate
Is backward compatible with b.

15. MAC layers in IEEE 802.11 standard
PCF : Point Coordination Function
DCF : Distributed Coordination Function

16. CSMA/CA Wireless LAN cannot implement CSMA/CD:
collision detection implies the ability to send and receive at the same time, which implies costly stations and increased bandwidth requirements
collision may not be detected because of a hidden terminal
distance could cause signal fading which could prevent a station from hearing a collision

17. Hidden and Exposed Terminals
(a) The hidden station problem (b) The exposed station problem

18. Another Example for Exposed Terminal Problem

19. CSMA/CA flowchart

20. CSMA/CA and NAV
DIFS – distributed interframe space NAV – network allocation vector
SIFS – short interframe space collision only occurs during
RTS – request to send handshake period (RTS, CTS)
CTS – clear to send

21. Frame format
FC : frame control

22. Subfields in FC (frame control) field
Explanation
Version
The current version is 0.
Type
Type of information: management (00), control (01), or data (10).
Subtype
Defines the subtype of each type (see ).
To DS
Defined later.
From DS
More flag
When set to 1, means more fragments.
Retry
When set to 1, means retransmitted frame.
Pwr mgt
When set to 1, means station is in power management mode.
More data
When set to 1, means station has more data to send.
WEP
Wired equivalent privacy. When set to 1, means encryption implemented.
Rsvd
Reserved.

23. Control frames

24. Values of subfields in control frames
type field : 01
Subtype
Meaning
1011
Request to send (RTS)
1100
Clear to send (CTS)
1101
Acknowledgment (ACK)

25. Subfields in FC field To DS From DS Address 1 Address 2 Address 3
Destination station
Source station
BSS ID
N/A 1
Sending AP
Receiving AP

26. Addressing mechanism: case 1
To DS = 0 and From DS = 0
Frame is not going to a distribution system
Frame is going from one station in a BSS to another
ACK sent to original sender

27. Addressing mechanism: case 2
To DS = 0 and From DS = 1
Frame is coming from a distribution system
Frame is coming from an AP and going to a station
ACK sent to AP

28. Addressing mechanism: case 3
To DS = 1 and From DS = 0
Frame is going to a distribution system
Frame is coming from a station to an AP
ACK sent to original station
address 3 contains final destination of frame

29. Addressing mechanism: case 4
To DS = 1 and From DS = 1
Distribution system is wireless
Frame is coming from an AP going to another AP

30. Architecture Radio Layer Baseband Layer L2CAP Layer Other Upper Layers
Bluetooth
Architecture
Radio Layer
Baseband Layer
L2CAP Layer
Other Upper Layers

31. Architecture - Two Types of Networks
Piconets
Scatternet

32. Piconet up to eight stations, one is the master, rest are slaves
slaves synchronize their clocks and hopping sequence with master
only one master
communication between master and slave can be 1-1 or 1-many
can have up to eight additional slaves in parked state
slave in parked state is synchronized, but cannot communicate

33. Scatternet scatternet – combined piconets
slave in one piconet can be master in another

34. Bluetooth layers

35. Radio Layers
roughly equivalent to the physical layer of the Internet model
2.4-GHz ISM band
FHSS – frequency hopping spread spectrum
modulation – sophisticated version of FSK, called GFSK (FSK w/ Gaussian bandwidth filtering)

36. Baseband Layer roughly equivalent to MAC sublayer in LANs
access method is TDMA
TDD-TDMA (time division duplexing TDMA) – a kind of half duplex communication
single slave communication – master uses even number slots, slave uses odd numbered
multiple slave communication – master uses even number slots, slave sends in the next odd-numbered slot if the packet in the previous slot was addressed to it

37. Single-slave Communication

38. Multiple-slave communication

39. Physical Links
SCO – synchronous connection oriented link – avoiding latency is more important than integrity – damaged packet is never retransmitted
ACL – asynchronous connectionless link – data integrity is more important than avoiding latency – if packet is damaged, it is retransmitted

40. Frame Format Types

41. L2CAP data packet format
Logical link control and adaptation protocol – roughly equivalent to the LLC sublayer in LANs
used for data exchange on an ACL link
specific duties: multiplexing, segmentation and reassembly, quality of service, and group management

42. Questions !