Notes On Chapter Sixteen -- Wireless Networking Technologies

• 16.1 Introduction
  
  
  • There are multiple wireless technologies
  
  
• 16.2 A Taxonomy of Wireless Networks
  
  
  • Local Area Networks (LANs)
  • Metropolitan Area Networks (MANs)
  • Wide Area Networks (WANs)
  • Personal Area Networks (PANs)
  
  
• 16.3 Personal Area Networks
  
  
  • PAN technology use examples: headset-to-phone, computer-to-keyboard
  • Categories:
    • Bluetooth
    • InfraRed
    • ISM wireless (frequencies reserved for Industrial Scientific and Medical devices)
  
  
• 16.4 ISM Wireless Bands Used by LANS and PANS
  
  
  • 902-928 MHz
  • 2.4-2.84 GHz
  • 5.725-5.850 GHz
  
  
• 16.5 Wireless LAN Technologies and Wi-Fi
  
  
  • IEEE provides most standards (802.11)
  • See figure 16.4 on page 269
  • IEEE Standards:
    • 802.11
    • 802.11a
    • 802.11b
    • 802.11g
  
  
• 16.6 Spread Spectrum Techniques
  
  
  • Advantages: increased performance and noise immunity
  • Spread Spectrum uses multiple frequencies to send data
  • Sender spreads data across multiple frequencies and receiver recombines it.
  • DSSS: Direct Sequence Spread Spectrum - similar to CDMA
  • FHSS: Frequency Hopping Spread Spectrum - use of a sequence of frequencies
  • OFDM: Orthogonal Frequency Division Multiplexing - band divided into many non-interfering carriers.
  
  
• 16.7 Other Wireless LAN Standards
  
  
  • 802.11e: improved quality of service, i.e. low jitter
  • 802.11h: like 802.11a with control of spectrum and power
  • 802.11i: enhanced security, encryption
  • 802.11k: radio resource management
  • 802.11n: over 100 Mbps
  • 802.11p: Dedicated Short-Range Communication among vehicles on Hwy
  • 802.11r: Improved roaming
  • 802.11s: For a mesh network
  
  
• 16.8 Wireless LAN Architecture
  
  
  • Characteristics of an Infrastructure Architecture:
    • See figure 16.7 on page 272
    • wireless hosts: computers that communicate using a wireless network
    • access point: aka base station - typically a wireless host communicates with an access point and the access point relays all packets.
    • switch or router: an interconnect mechanism used to connect access points - typically access points and switches or routers are connected by a twisted-pair Ethernet.
    • Basic Service Set (BSS): the set of computers within range of a given access point.
  • It is possible to have an Ad Hoc Wireless Network in which wireless hosts communicate with each other without relying on an access point. This architecture is not common.
  
  
• 16.9 Overlap, Association, and 802.11 Frame Format
  
  
  • See figure 16.8 on page 273
  • So that there will not be dead zones there must be overlaps - areas where a wireless host can reach more than one access point.
  • According to 802.11, a wireless host must associate with (send frames to) only one access point.
  • In an infrastructure architecture the 802.11 frame carries the MAC address of the target access point, and also the MAC address of the target router connected to the access point.
  
  
• 16.10 Coordination Among Access Points
  
  
  • Some types of access points coordinate to insure smooth handoff as a wireless computer moves from the region reached by one access point to another.
  • Other types of access points just operate independently and rely on the ability of wireless hosts to change the the access point to which they are associated.
  
  
• 16.11 Contention and Contention-Free Access
  
  
  • General Approaches are:
    • Point Coordinated Functions (PCF)
    • Distributed Coordinated Functions (DCF)
  • With PCF the access point controls stations in the BCF to make sure they do not interfere, for example by assigning a separate frequency to each station. This method is seldom if ever used.
  • With DCF, each station in the BSS runs a random access protocol like Carrier Sense Multi-Access with Collision Avoidance (CSMA/CA).
  • In CSMA/CA, a sender begins with a short RTS message and the access point responds with a short CTS message, then the sender sends its data and the access point response with an ACK.
  • The 802.11 standard specifies minimum delays between these messages.
  • If no ACK arrives the sender employs a backoff strategy.
  • Collision detection is not part of the scheme. It is difficult to a collision from weak signals and interference.
  • Conditions vary and some wireless networks must depend heavily on retransmission - others hardly at all.
  
  
• 16.12 Wireless MAN Technology and WiMax
  
  
  • WiMax is one MAN technology that seems to have a lot of potential
  • WiMax = World-wide Interoperability for Microwave Access = 802.16
  • Fixed WiMax (802.16-2004, 802.16d) does not provide for handoff
  • Mobile WiMax (802.16e-2005, 802.16e) does provide handoff among access points and can be used with mobile computers, phones and other mobile devices.
  • A WiMax cell has a range of 3-10 Km
  • WiMax can provide data rates of 10-70 Mbps
  • Applications include:
    • Last Mile Internet Access - alternative to DSL or cable modem
    • General-purpose interconnection within a city
    • Backhaul - e.g. service provider central tower to remote cell towers (high data-rate LOS frequencies).
    • Unified data and telecommunications access
  
  
• 16.13 PAN Technologies and Standards
  
  
  • There are various wireless PAN standards
  • Bluetooth: high frequency, short distance, master-slave, 721 Kbps
  • Ultra Wideband (UWB): saves power by spreading across many frequencies, goes through walls, up to 500 Mbps
  • Zigbee: for remote control in industry and homes, not for data, three frequency bands and data rates up to 250 Kbps, low power consumption, three levels of security being defined.
  
  
• 16.14 Other Short-Distance Communication Technologies
  
  
  • InfraRED is used for remote control and low-speed data transfer - up to about 16 Mbps
  • Small RFID tags used e.g. for inventory control, sensors, and passports can contain information that a receiver can "pull" - passive RFID's draw power from reader's signal and active RFID's have a battery that can last 10 years.
  
  
• 16.15 Wireless WAN Technologies
  
  
  • WAN Technologies:
    • Cellular communication systems, or
    • Satellite communication systems
    
    
  • 16.15.1 Cellular Communication Systems
    • See figure 16.14 on page 281
    • Cellular systems are being used more and more for data and Internet service, in addition to voice.
    • Each cell has a tower and groups of cells are connected to a Mobile Switching Center that connects in with the Public-Switched Telephone Network and the Internet too.
    • The Centers track users and manage handoff as a user passes from one cell to another.
    • In a handoff from one group to another, two Mobile Switching Centers are involved.
    • Real cell coverage maps do not correspond to perfect honeycomb patterns.
    • Cell sizes differ between rural and metropolitan areas.
  
  
• 16.16 Cell Clusters and Frequency Reuse
  
  
  • There are schemes for assigning frequencies to (e.g. phone) cells so that no two adjacent cells use the same frequency, This lessens problems with interference.
  
  
• 16.17 Generations of Cellular Technologies
  
  
  • 1G: 1970's through '80's - analog voice
  • 2G: early 1990's to present - digital voice
  • 3G: starting in the 2000's - addition of high-speed data services for web-browsing, photo-sharing, and the like - roaming over N.A. Japan & Europe
  • 4G: starting in 2008 - support for real-time media & multiple connection technologies, e.g. WiFi and satellite with phone choosing best service dynamically.
  • There is a wide variety of implementation strategies.
  • Forms of CDMA seem to be converging toward a global standard.
  
  
• 16.18 VSAT Satellite Technology
  
  
  • Early receivers used large antennae (e.g. 3m+ diameter)
  • Very Small Aperture Terminal (VSAT) technology allows diameters more in the 1m range.
  • VSAT is used to link businesses, to download entertainment, and for Internet access.
  • There are three frequency ranges offered with differing footprint, signal strength and sensitivity to atmospheric effects (e.g. rain).
  
  
• 16.19 GPS Satellites
  
  
  • GPS satellites provide accurate time and location information
  • Such information is used increasingly in mobile networking - e.g. location-based services.
  • A receiver can determine its distance to three of the satellites and from that work out its position on the earth.
  
  
• 16.20 Software Radio and the Future of Wireless
  
  
  • A software radio is a device that uses tunable analog filter chips, digital signal processor chips, and multiple antenna management to dynamically control:
    • frequencies used for receiving and transmitting
    • transmitter power
    • channel coding and modulation scheme(s) in use
    • multiplexing scheme(s) in use
    • direction to which various antennae are tuned
    • all aspects of MAC framing and MAC addressing
  • The US military uses software radio
  • The Universal Software Radio Peripheral (USRP) and GNU Radio are available for experimentation
  • Some details have to be worked out before they appear in commercial products - e.g. high cost and protection of other devices from interference that might be caused by software radios.