wireless LANs

wireless LANs

1. Introduction

   • OSI Standards
     • single network, require physical & data link layer standards
   • 802.11 = Wi-Fi
     • use radio for physical layer transmission, unlicensed service band
       • 2.4 GHz & 5 GHz
     • the dominant WLAN standards are the 802.11 standards, created by the IEEE 802.11 Working Group
   • Basic Access Point Operation
     • p.183 figure 6-2

2. Radio Signal Propagation

   • Perfidious Radio
     • propagation effects in wired transmission can be well controlled by respecting cord distance limits and taking other installation precautions because wired propagation is predictable
     • radio propagation is very unreliable. propagation effects in wireless networks are complex and difficult to solve
   • Frequencies
     • Radios for data transmission are called transceivers because they both transmit and receive
     • Waves have amplitude and wavelength
     • Optical fiber waves are described wavelength; radio waves are frequency
     • frequency is the number of complete cycles per second. One cycle per second is one hertz (Hz)
   • Antennas
     • A transceiver uses an antenna to transmit its signal
     • 2 types of radio antennas
       • omnidirectional
         • equally strongly/well in all directions, does not need to point in the direction of the receiver, only small partion reaches the receiver, used for short distances
       • dish
         • concentrate signals in a particular direction, receive weaker incoming, longer distances, need to know the direction
   • Wireless Propagation Problems
     • Electromagnetic Interference (EMI)
     • Inverse Square Law Attenuation
       • P1/P2=(r1/r2)^2
     • Absorptive Attenuation
       • signal loss through energy absorption
       • higher-frequency waves suffer more rapidly from absorptive attenuation
     • Dead/Shadow Zone
       • become worse with increasing frequency
     • Multipath Interference
       • Most serious
       • cancel out

3. Service Bands and Bandwidth

   • Service Bands
     • divided the frequency spectrum into contiguous spectrum ranges called service band, are subdivided further into smaller frequency ranges called channels
   • Signal and Channel Bandwidth
     • In reality real singals do not travel at a single frequency. Real signals spread over a range of frequencies. This range is called the signal's bandwidth. Signal bandwidth is measure by subtracting the lowest frequency from the highest frequency
     • channel also has bandwidth. The channel bandwidth must be wide enough for a signal's bandwidth.
     • High bandwidth brings high radio transmission speed. To transmit N times as fast, you need N times as much channel bandwidth
     • Radio channels with large bandwidths are called broadband channels. Broadband refers only to the width
   • Licensed and Unlicensed Service Bands
     • unlicensed: a company can add or drop access pints any time, have as many wireless hosts as it wishes
   • Channel Use and Co-Channel Interference
     • When co-channel interference occurs, it does not stop transmissions, but it does slow them down. To reduce it, set adjacent access points on different channels
   • The 2.4 Ghz and 5 Ghz Unlicensed Service Bands
     • 5 GHz wider than the 2.4 GHz, number of channels in the 5 GHz band is going down as channels become wider to provide higher speed per channel.
     • 2.4 GHz band has no expansion potential because it is bordered by services that cannot be moved

4. Spread Spectrum Transmission

   • Normal v.s. Spread Spectrum Transmission
   • Orthogonal Frequency Division Multiplexing (OFDM) Spread Spectrum Transmission
     • each broadband channel is divided into many smaller subchannels called subcarriers.
     • send many slow signals in many small subcarriers is easier than sending data over a very wide channel
   • Use far wider channel bandwidth than the transmission speed requires to reduce multipath interference problems at Wi-Fi frequencies. channels are much wider than normal channels, but they do not transmit signals faster.
   • is used to reduce propagation problems, not to provide security or higher transmission speed.

5. 802.11 (reliable protocol)

   • 802.11 WLAN Operation
     • From 802.11 to 802.3
       • the 802.11 frame travels only between the wireless host and the wireless access point.
       • the 802.3 frame travels only between the wireless access point and the server.
       • p.197 figure 6-16
     • Wireless Networks with Multiple Access Points
       • Access points have limited signal range.
       • user must know its service set ID (SSID)
       • roaming: 802.11 standards. How do access points know that they are on the same network? they all have the same SSID
       • Distribution system
     • Media Access Control (MAC)
       • The access point and all of the wireless hosts it serves transmit and receive in a single channel
       • collision
       • CSMA/CA+ACK. p.201. figure 6-19
       • RTS/CTS - slower than CSMA/CA+ACK
   • 802.11 Transmission Standards: 802.11n, 802.11ac
     • Channel Bandwidth and Service Band Bandwidth
       • 802.11n: 40Hz
       • 802.11ac: 80MHz, 160Hz
       • p.203, figure 6-21
       • As channel bandwidth increases, the number of channels in a service band decreases proportionately.
     • Speed and Market Status
       • Speed: ac (new) > n
     • Your Service Speed Will Vary. A Lot.
     • Multiple Input/Output (MIMO)
       • To increase transmission speed without increasing channel bandwidth
     • Beamforming and Mutiuser MIMO
   • 802.11 Wi-Fi Wireless LAN Management
     • Access Point Placement
       • reduce overlap as much as possible
       • minimize co-channel interference - assign channels to access point positions
       • initial site survery - require analysis software
     • Centralized Management
       • SNMP trap command
         • disassociate messages
           • DoS attack
       • SNMP set commands

6. 802.11i

   • 802.11i WLAN Security
     • 802.11i(WPA2)
       • only extends between the wireless access point and the wireless client host
       • use AES for confidentiality
       • WEP -> WPA -> 802.11i
     • 802.11i Stages
       • initial authentication
         • 802.1X Initial authentication mode was created for corporations with many access points - for enterprise
         • Pre-Shared Key (64 bits)- for personal
           • Unshared pairwise session keys
     • Pre-Shared Key (PSK) Initial Authentication Mode in 802.11i
     • 802.1X Initial Authentication Mode Operation
       • no single verifier
         • 802.1X authenticator
         • 802.1X authentication server (do heavy work)
       • authentication process
         • 1. Requirements for credentials
         • 2. Provide credentials
         • 3. Credential check
         • 4. Authorization message to the Authenticator
         • 5. Authorization message to the client
     • p.224 figure 7-1 drive by hacker
   • Beyond 802.11i Security: 2 attacks
     • Rogue Access Points
       • a single rogue access point destroy the security that the firm has created
       • the least secure access point determines the strength of the entire network
       • a rogue access point is an unauthorized access point set up within a firm by an employee or department
     • Evil Twin Access Points and Virtual Private Networks
       • man-in-the-middle attack using an evil twin access point to intercept traffic passing between a wireless host and a legitimate access point
       • end-to-end VPN key client-server

7. IoT

   • Peer-to-Peer Protocols for the IoT
     • "slow and close"
     • p.242 figure 7-18
   • Bluetooth
     • 2 forms of bluetooth: Classic Bluetooth and Bluetooth Low Engergy (LE)
       • they are incompatible, but both use 2.4 GHz unlicensed radio band, same radio and anthennas
       • Classic bluetooth
         • 2 data rates: an extended data rate speed of 3 Mbps, a high speed rate of 24 Mbps
       • Bluetooth LE
         • for devices with coin batteries
     • One-to-One, Master-Slave Operation
       • bluetooth always uses point-to-point communication between a pair of devices
       • bluetooth always use master-slave control
         • piconet
         • a bluetooth device to be a master and a slave simultaneously
     • Bluetooth Profiles
       • profiles provide application-level functionality
       • govern how devices share information and specify control messages for various uses
     • Bluetooth Low Energy
       • Advertisements and Connections
       • Beacons - coupons
       • Profiles
     • a short-range radio technology designed for personal area networks (PANs), it replaces cable with radio waves
     • not standardized by IEEE, is standardized by the Bluetooth Special Interest Group
   • Other Promising IoT Transmission Standards
     • communication between pairs of devices
       • Near Field Communication (NFC)
       • Wi-Fi Direct
     • network IoT devices in a mesh
       • Zigbee and Z-Wave
   • Security in the IoT