1. Chapter 4: Wireless LANs and PANs

2. Characteristics of wireless LANs
Advantages
Flexibility: very flexible within the reception area
Planning: Ad-hoc networks without previous planning possible
Design: (almost) no wiring difficulties (e.g. historic buildings, firewalls)
Robustness: more robust against disasters like, e.g., earthquakes, fire or users pulling a plug
Cost: Adding additional users to a wireless network will not increase the cost.
Disadvantages
Quality of service: typically very low bandwidth compared to wired networks (1-10 Mbit/s)
Proprietary solutions: many proprietary solutions, especially for higher bit-rates, standards take time (e.g. IEEE ). Now, g is a popular solution.
Restrictions: products have to follow many national restrictions if working wireless, it takes long time to establish global solutions like, e.g., IMT-2000
Safety and security: Precautions have to be taken to prevent safety hazards. Secrecy and integrity must be assured.

3. Fundamentals of WLANs
Differences between wireless and wired transmission
Address is not equivalent to physical location
Dynamic topology and restricted connectivity
Medium boundaries are not well-defined
Error-prone medium
Use of WLANs
Users can access the Internet on the move.
WLANs are handy in areas affected by earthquakes or other disasters.
WLANs are good solutions in places where wiring may not be permitted.

4. Design goals for wireless LANs
Operational simplicity
Power-efficient operations
License-free operation: no special permissions or licenses needed to use the LAN
Tolerance to interference
Global usability
Security: security (no one should be able to read my data), privacy (no one should be able to collect user profiles),
Safety requirement (low radiation)
Quality of service requirements
Compatibility with other technologies and applications

5. Comparison: Infrastructure vs. Ad-hoc networks
WLANs can be classified into two types:
Infrastructure networks contains access points (APs) and mobile station (STAs).
Ad hoc LANs do not need any fixed infrastructure.
Infrastructure networks
Provide access to other networks
Include forwarding functions
Medium access control
Ad-hoc networks is a group of computers each with wireless adapters, connected as an independent wireless LAN.
Each node can communicate with other nodes

6. Comparison: infrastructure vs. ad-hoc networks
Infrastructure Network
AP: Access Point AP AP
Wired network AP
Ad-hoc network

7. 802.11 Services Distribution Services (for APs)
Association – mobile stations connect themselves to base stations
Reassociation – a station may change its preferred base station
Disassociation – the station or base station breaks the association Distribution – determines how to route frames sent to the base station
Integration – handles the translation from the format to the format of the destination network
Intracell Services (for STAs and APs)
Authentication – a station must authenticate itself before permitted to send data.
Deauthentication – a authenticated station wanting to leave the network is deauthenticated.
Privacy – manages the encryption and decryption. The algorithm specified is RC4 by Ronald Rivest of MIT.
Data Delivery – not reliable.

8. IEEE Standard
IEEE b is known as Wi-Fi (wireless Fidelity).
Mobile Stations (MTs) can operate two modes:
Infrastructure mode, in which MTs can communicate with one or more APs which are connected to a WLAN.
Ad hoc mode, in which MTs can communicate directly with each other without using an AP.
IEEE supports two medium in the physical layer:
Infrared
Radio wave
The physical layer is subdivided into physical medium dependent (PMD) sublayer and physical layer convergence protocol (PLCP).
IEEE used CSMA/CD for MAC.

9. Personal Area Networks (PANs)
A PAN technology provides communication over a short distance
It is intended for use with devices that are owned and operated by a single user. For example
between a wireless headset and a cell phone
between a computer and a nearby wireless mouse or keyboard
PAN technologies can be grouped into three categories
Later sections explain PAN communication in more detail
and list PAN standards

10. Personal Area Networks (PANs)

11. Wireless LANS Devices
wireless router
wireless network card

12. Medium Access Control in Wireless LANs
Because there is higher error rate and signal strength is not uniform throughout the space in which wireless LANs operate, carrier detection may fail in the following ways:
Hidden nodes:
Hidden stations: Carrier sensing may fail to detect another station. For example, A and D.
Fading: The strength of radio signals diminished rapidly with the distance from the transmitter. For example, A and C.
Exposed nodes:
Exposed stations: B is sending to A. C can detect it. C might want to send to E but conclude it cannot transmit because C hears B.
Collision masking: The local signal might drown out the remote transmission.
An early protocol designed for wireless LANs is MACA (Multiple Access with Collision Avoidance).

13. Wireless LAN configuration

14. Comparison: infrared vs. radio transmission
uses IR (Infra-Red) diodes, diffuse light, multiple reflections (walls, furniture etc.)
Advantages
simple, cheap, available in many mobile devices
no licenses needed
simple shielding possible
Disadvantages
interference by sunlight, heat sources etc.
many things shield or absorb IR light
low bandwidth
Example
IrDA (Infrared Data Association) interface available everywhere
Radio
typically using the license free ISM (Industrial, Scientific, Medical) band at 2.4 GHz
Advantages
experience from wireless WAN and mobile phones can be used
coverage of larger areas possible (radio can penetrate walls, furniture etc.)
Disadvantages
limited license free frequency bands
shielding more difficult, interference with other electrical devices
Example
WaveLAN (Lucent), HIPERLAN, Bluetooth

15. RFID – Radio Frequency Identification
RFID (radio frequency identification) is a technology that incorporates the use of electromagnetic or electrostatic coupling in the radio frequency (RF) portion of the electromagnetic spectrum to uniquely identify an object, animal, or person.
RFID is coming into increasing use in industry as an alternative to the bar code.
The advantage of RFID is that it does not require direct contact or line-of-sight scanning.
An RFID system consists of three components: an antenna and transceiver (often combined into one reader) and a transponder (the tag).
The antenna uses radio frequency waves to transmit a signal that activates the transponder. When activated, the tag transmits data back to the antenna.

16. History and Hi-Tech…
1999:
Ericsson mobile communications AB reste denna sten till minne av Harald Blåtand, som fick ge sitt namn åt en ny teknologi för trådlös, mobil kommunikation.

17. Characteristics
2.4 GHz ISM band, 79 RF channels, 1 MHz carrier spacing
Channel 0: 2402 MHz … channel 78: 2480 MHz
GFSK modulation, mW transmit power
FHSS and TDD
Frequency hopping with 1600 hops/s
Hopping sequence in a pseudo random fashion, determined by a master
Time division duplex for send/receive separation
Two type of links:
Voice link – SCO (Synchronous Connection Oriented)
FEC (forward error correction), no retransmission, 64 kbit/s duplex, point-to-point, circuit switched
Data link – ACL (Asynchronous Connectionless)
Asynchronous, fast acknowledge, point-to-multipoint, up to kbit/s symmetric or 723.2/57.6 kbit/s asymmetric, packet switched
Topology: Overlapping piconets (stars) forming a scatternet

18. Forming a Bluetooth Network: Piconet
Collection of devices connected in an ad hoc fashion
One unit acts as master and the others as slaves for the lifetime of the piconet
Master determines hopping pattern, slaves have to synchronize
Each piconet has a unique hopping pattern
Participation in a piconet = synchronization to hopping sequence
Each piconet has one master and up to 7 simultaneous slaves (> 200 could be parked) P S S M P SB S P SB
M=Master
S=Slave
P=Parked
SB=Standby