Chapter 4: Wireless LANs and PANs

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 802.11). Now, 802.11g 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.

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.

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

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

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

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 802.11 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.

IEEE 802.11 Standard IEEE 802.11b 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 802.11 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 802.11 used CSMA/CD for MAC.

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

Personal Area Networks (PANs)

Wireless LANS Devices wireless router wireless network card

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).

Wireless LAN configuration

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

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.

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.

Characteristics 2.4 GHz ISM band, 79 RF channels, 1 MHz carrier spacing Channel 0: 2402 MHz … channel 78: 2480 MHz GFSK modulation, 1-100 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 433.9 kbit/s symmetric or 723.2/57.6 kbit/s asymmetric, packet switched Topology: Overlapping piconets (stars) forming a scatternet

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