chapter 16 -- wireless networking technologies

(rev. Mar 25, 2017)

Notes On Chapter Sixteen -- Wireless Networking Technologies

16.0 Study Guide
- Know that PANs are implemented with Bluetooth, Infrared, and ZigBee standards and technology.
- Be aware that there are many IEEE 802.11x standards for wireless networks
- Know what spread spectrum technology is, and advantages gained by utilizing it
- Understand how basic service sets (BSSs) interact with access points (APs) in a wireless LAN, and how APs connect up to wired sections of the LAN.
- Know that WiMax is a wireless networking technology that has many potential uses as both an access and interconnect technology.
- Be able to list some of the specific ways that WiMax can be used for access and/or interconnect
- Have basic understanding of cellular architecture - for example, mobile switching centers, wired connections, antennae, cells, and handoffs.
- Some familiarity with the history of cellular technology, including knowledge of capabilities associated with the so-called four generations.
16.1 Introduction
- This chapter is about wireless technologies
- There is a multiplicity of them
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
- Examples of PAN technology uses:
- Categories:
  - Bluetooth - e.g. headset-to-phone, computer-to-keyboard
  - InfraRed - e.g. controller to entertainment system
  - ZigBee - e.g. electric appliance to Smart Grid
  - ISM wireless (frequencies reserved for Industrial Scientific and Medical devices)
16.4 ISM Wireless Bands Used by LANS and PANS
- "not licensed to specific carriers ... broadly available for products"
  - 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 for wireless LANs (802.11)
- IEEE Standards that fall under the "WiFi Alliance" certification organization:
  - 802.11
  - 802.11b
  - 802.11g
  - 802.11n
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.
- Three Key Multiplexing Techniques (The text describes them only very minimally.):
  - DSSS: Direct Sequence Spread Spectrum - similar to CDMA - text says it has good performance
  - FHSS: Frequency Hopping Spread Spectrum - use of a sequence of frequencies - text says it makes transmission more immune to noise
  - OFDM: Orthogonal Frequency Division Multiplexing - band divided into many non-interfering carriers - text says it offers the greatest flexibility
16.7 Other Wireless LAN Standards
- 802.11a: The first variant of 802.11 created to improve speed; no longer popular
- 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, Advanced Encryption Standard; full version is WPA2
- 802.11k: will provide radio resource management, including transmission power
- 802.11p: Dedicated Short-Range Communication (DSRC) among vehicles on a highway and vehicle-to-roadside
- 802.11r: Improved roaming without loss of connectivity
- 802.11s: proposed for a mesh network in which a set of nodes automatically form a network and pass packets
16.8 Wireless LAN Architecture
- Characteristics of an Infrastructure Architecture:
  - See figure 16.7
  - 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
- 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 a wireless host to change the access point to which it is 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 BSS 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 ready-to-send (RTS) message and the access point responds with a short clear-to-send (CTS) message. Then the sender sends its data and the access point responds with an acknowledgement (ACK).
- The 802.11 standard specifies minimum delays between these messages.
- If no ACK arrives the sender employs a back-off strategy.
- Collision detection is not part of the scheme. It is difficult to distinguish collisions 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
- Not too many MAN technologies have caught on.
- 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 among access points.
- 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
- Cellular Communication Systems
  - See figure 16.14
  - 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 Micro Cells
- There may be 'micro cells' that only serve, for example, a subset of the floors of a high-rise apartment building.
- Some providers sell or lease micro cell devices to individuals - a device that connects to the Internet and provides cell service within a residence to a set of recognized devices. (no per-minute charges.)
16.17 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.
16.18 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 (download rates of 400Kbps to 2Mbps) - roaming over N.A. Japan & Europe
- 4G: starting in 2008 - support for real-time media like TV & high-speed video downloads - support for multiple connection technologies, e.g. WiFi and satellite with phone choosing best service dynamically. Designed for the Internet - uses packet switching.
- There is a wide variety of implementation strategies.
- Proposals for 5G call for higher data rates, reduced latency, and more capacity.
16.19 VSAT Satellite Technology
- Antennae with parabolic reflectors cause incoming energy to converge at a focal point.
- 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).
16.20 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.
- Key Features:
  - Accuracy between 20 and 2 meters (military version are even better)
  - 24 total satellites orbit earth
  - Satellites in six orbital planes
  - Provides time synchronization used in some communication networks
16.21 Software Defined Radio and the Future of Wireless
- SDR is a technology that can help deal with the complexity arising from the wide variety of wireless technologies.
- 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.