WIRELESS LANS By: M. Habibullah Pagarkar Mandar Gori Rajesh Jaiswal
Introduction Why wireless? World will become fully mobile Increase in wireless devices usage Wireless will succeed; integrates into many devices and can meet a variety of needs
WLANs What are WLANs? Primarily 802.11 b and HiperLAN Application areas LAN extension Cross building interconnect Nomadic Access Ad-hoc networks
Advantages of WLANs Cheaper Convenient Easy to install Expandable
Disadvantages of WLANs Not TRULY Mobile Device limitations small LCD screens Data rates WML instead of HTML Compatibility issues Cannot use 802.11 and Bluetooth in the same device.yet!
Techniques WLANs Infrared Spread Spectrum Narrowband Microwave Diffused Directed Frequency Hopping Direct Sequence
Comparison Chart Infra-red Spread Spectrum Microwave Diffused I.R. Directed I.R. Frequency Hopping Direct Sequencing Data rate (Mbps) 1-4 1-10 1-3 2-20 10-20 Range (m) 15-60 25 30-100 30-250 10-40 Detectable Negligible Little Some Modulation ASK FSK QPSK QPSK Radiated Power - < 1 W 25 mw Access Methods CSMA Token Ring CSMA CSMA CSMA
Examples of Networks Peer to Peer Transmitter/Receiver
Overview of 802.11 IEEE 802.11 is an extension of 802.3 into wireless communication Allows roaming computers to talk to other devices (peer to peer) or connect to a wired network (transmitter/receiver) Interoperability between multiple vendor products
Comparison With OSI Model
IEEE 802.11 Architecture 802.11 protocol covers MAC & Physical layer Access method CSMA/CA (802.3 uses CSMA/CD)
IEEE 802.11 Architecture contd. Extended Services Set IEEE 802.x LAN Distribution System Portal AP STA 1 STA 2 STA 3 STA 4 Basic Services Set AP STA5 STA 7 STA 6 Basic Service Set
IEEE 802.11 Terminology Access Point (AP) Basic Services Set (BSS) Coordination Function Distribution System (DS) Extended Service Set (ESS) MAC protocol data unit (MPDU) MAC service data unit (MSDU) Station
IEEE 802.11 Services MSDU Distribution of Messages within a DS Distribution Integration Association related services Association Re-association Disassociation
IEEE 802.11 Services contd.. Access & privacy services Authentication De-authentication Privacy
IEEE 802.11 MAC Reliable Data Delivery Access Control Security Distribution Coordination Function (DCF) uses CSMA (diagram on next slide) Point coordination function (PCF) on top of DCF
Medium Access Control Logic Wait for transmit Y Idle? Wait IFS Still Idle? N Y N Transmit frame Wait IFS Wait till transmission ends N Still Idle? Y Exponential backoff Transmit frame
MAC Frame Frame Control Duration Id Sequence Control FCS 2 2 6 6 6 2 6 0-2312 4 Address 1 Address 2 Address 3 Address 4 Frame Body MAC Header
Frame Format 3 types of frames Data Frames data transmission Control Frames control access to medium; assist in reliable delivery Management Frames not forwarded to upper layers Each type is further divided into sub-types
802.11 Physical Layer 2 types of transmission media -Micro-wave Radio(SS/NB) -Infrared 1.DS-SS 2.FHSS 1.Direct Beam 2.Omnidirection 3.Diffused
802.11 Variations IEEE has a veritable alphabet soup 802.11 a 6-54 Mbps 5 GHz band Uses OFDM instead of FHSS or DSSS Interference 802.11 b (also called as Wi-Fi) 5.5-11 Mbps 2.4 Ghz band DSSS only Physical layer combines CCK & PBCC
802.11 Variations contd.. 802.11 g -802.11a + 802.11b -OFDM from a & CCK-PBCC from b -Facilitate development of DUAL-BAND radio 802.11e -Not a protocol but a technology -Can move 2 a new frequency if interference -Used in time sensitive applications (voice & multimedia)
802.11 Variations contd. 802.11f -deals with communication among AP 2 -supports roaming 802.11i -advanced ENCRYPTION standard for better PRIVACY
802.11 Power Saving Battery power is a scarce resource Two power saving modes Continuous aware mode Power save polling mode
802.11 Security Uses Wired Equivalent Privacy (WEP) protocol There are 2 issues -access control (ESSID & mac addr.) -encryption mechanism(rc4prng-rsa) WEP slows WLAN by 20-50%
References Wireless Communication & Networks Stallings IEEE Spectrum September 2002 IEEE Communications Magazine www.wirelesscentral.net www.cwt.vt.edu www.wlana.com