Overview of Wireless LANs Use wireless transmission medium Issues of high prices, low data rates, occupational safety concerns, & licensing requirements now addressed key application areas: LAN extension cross-building interconnect ad hoc networking

Wireless LAN Requirements Throughput - efficient use wireless medium No of nodes - hundreds of nodes across multiple cells Connection to backbone LAN - using control modules Service area to 300 m low power consumption - for long battery life on mobiles Transmission robustness and security license-free operation handoff/roaming Dynamic configuration - addition, deletion, and relocation of end systems without disruption to users

Wireless LAN at a glance Wireless LAN Medium access control FDMA CSMA/CA CDMA Transmission media Infrared Radio Topologies Ad hoc Infrastructure Applications Standards IEEEETSI (Hipper LAN) CDMA: Code Division Multiple Access ETSI: European Telecom. Standards Institute CSMA/CD: CSMA with Collision Detection FDMA: Frequency Division Multiple Access CSMA/CA: CSMA with Collision Avoidance TDMA: Time Division Multiple Access CDMA: Code Division Multiple Access ETSI: European Telecom. Standards Institute CSMA/CD: CSMA with Collision Detection FDMA: Frequency Division Multiple Access CSMA/CA: CSMA with Collision Avoidance TDMA: Time Division Multiple Access CSMA/CD TDMA Pulse-position modulation Direct modulatio n Multi- subcarrier modulation Single-carrier modulation Transmission schemes Carrier modulation Spread spectrum On-off keying Direct Sequence Frequency hopping Airports Warehouses Retail stores Old buildings Hospitals

Wireless LANs  IEEE  Basic service set (cell)  Set of stations using same MAC protocol  Competing to access shared medium  May be isolated  May connect to backbone via access point (bridge)  Extended service set  Two or more BSS connected by distributed system  Appears as single logic LAN to LLC level

modes Infrastructure mode All packets go through a base station Cards associate with a BSS (basic service set) Multiple BSSs can be linked into an Extended Service Set (ESS) Handoff to new BSS in ESS is pretty quick Moving to new ESS is slower, may require re- addressing Ad Hoc mode Cards communicate directly. Perform some, but not all, of the AP functions

A BSS without an AP is called an ad hoc network; a BSS with an AP is called an infrastructure network. Note

Basic service sets (BSSs)

Wireless connections: Access Point as a Bridge

Extended service sets (ESSs)

Types of station  No transition  Stationary or moves within direct communication range of single BSS  BSS transition  Moves between BSS within single ESS  ESS transition  From a BSS in one ESS to a BSS in another ESS

MAC layers in IEEE standard

Media Access Control  Distributed coordination function (DCF):  CSMA/CA  No collision detection  After finding channel idle, the station waits for a time period called distributed interframe space (DIFS).  Then sends RTS (Request to send) & waits for time period short interframe space (SIFS)  Destination sends clear to send (CTS) to source station  After time period of SIFS source sends data  After successful transmission of data source waits for acknowledgement  Point coordination function (PCF): It can be implemented in Infrastructure network. AP performs polling for all the stations.

CSMA/CA flowchart

CSMA/CA NAV (Network allocation vector)

Frame format Duration of transmission Sequence control

Subfields in FC field

Control frames

Addresses

Addressing mechanisms

Hidden station problem

The CTS frame in CSMA/CA handshake can prevent collision from a hidden station. Note

Use of handshaking to prevent hidden station problem

Exposed station problem

Use of handshaking in exposed station problem

WIRELESS LAN : Physical layers

Industrial, scientific, and medical (ISM) band

Physical layer of IEEE FHSS

Physical layer of IEEE DSSS

Physical layer of IEEE infrared

BLUETOOTH Bluetooth is a wireless LAN technology designed to connect devices of different functions such as telephones, notebooks, computers, cameras, printers, coffee makers, and so on. A Bluetooth LAN is an ad hoc network, which means that the network is formed spontaneously.

Bluetooth basics Short-range, high-data-rate wireless link for personal devices Originally intended to replace cables in a range of applications e.g., Phone headsets synchronization, remote controls Operates in 2.4 GHz ISM band Same as Frequency Hopping Spread Spectrum across ~ 80 channels

Protocol Architecture

Piconet Architecture One master and up to 7 slave devices in each Piconet: Master controls transmission schedule of all devices in the Piconet Time Division Multiple Access (TDMA): Only one device transmits at a time Frequency hopping used to avoid collisions with other Piconets 79 physical channels of 1 MHz each, hop between channels 1600 times a sec

Scatternets Combine multiple Piconets into a larger Scatternet Device may act as master in one Piconet and slave in another Each Piconet using different FH schedule to avoid interference Can extend the range of Bluetooth, can route across Piconets

Frame format types

L2CAP data packet format