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55. WIRELESS FUNDAMENTALS

  • Although we will briefly look at other types of WIRELESS NETWORKS, in this section of the course we will be focusing on WIRELESS LANs using WI-FI
  • The STANDARDS we use for WIRELESS LANs are defined in IEEE 802.11
  • The term WI-FI is a trademark of the WI-FI ALLIANCE, not directly connected to the IEEE
  • The WI-FI ALLIANCE tests and certifies equipment for 802.11 standards compliance
  • However, WI-FI has become the common term that people use to refer to 802.11 WIRELESS LANs and that term will be used through the course videos

WIRELESS NETWORKS

  • WIRELESS NETWORKS have some issues that we need to deal with

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  1. ALL DEVICES within range receive ALL FRAMES, like DEVICES connected to an ETHERNET HUB

  • Privacy of DATA within the LAN is a greater concern

  • CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) is used to facilitate HALF-DUPLEX communications

  • CSMA / CD is used in WIRED NETWORKS to detect and recover from COLLISIONS

  • CSMA / CA is used in WIRELESS NETWORKS to avoid COLLISIONS

  • When using CSMA / CA, a DEVICE will wait for other DEVICES to STOP TRANSMITTING before it TRANSMITS DATA itself.

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  1. WIRELESS COMMUNICATIONS are regulated by various INTERNATIONAL and NATIONAL bodies

  2. WIRELESS SIGNAL COVERAGE AREA must be considered

  • Signal Range
  • Signal ABSORPTION, REFLECTION, REFRACTION, DIFFRACTION, and SCATTERING

SIGNAL ABSORPTION

  • ABSOPTION happens when a WIRELESS SIGNAL PASSES THROUGH a material and is converted into HEAT, weakening the SIGNAL

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SIGNAL REFLECTION

  • REFLECTION happens when a SIGNAL BOUNCES off a material (like metal)
    • This is why WI-FI reception is usually POOR in elevators. The SIGNAL bounces off the metal and very little penetrates into the elevator

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SIGNAL REFRACTION

  • REFRACTION happens when a WAVE is BENT when entering a medium where the SIGNAL travels at a different speed
    • For example, glass and water can refract waves

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SIGNAL DIFFRACTION

  • DIFFRACTION happens when a WAVE encounters an OBSTACLE and travels AROUND it
    • This can result in “BLIND SPOTS” behind the obstacle

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SIGNAL SCATTERING

  • SCATTERING happens when a material causes a SIGNAL to SCATTER in all directions
    • Dust, smog, uneven surfaces, etc. can cause scattering

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  1. Other DEVICES using the SAME CHANNELS can cause INTERFERENCE
  • For example, a WIRELESS LAN in your neighbor’s house / apartment

RADIO FREQUENCY (RF)

  • To send WIRELESS SIGNALS, the SENDER applies an ALTERNATING CURRENT to an antenna
    • This creates ELECTROMAGNETIC WAVES which propagate out as WAVES
  • ELECTROMAGENETIC WAVES can be measured in multiple ways - for example AMPLITUDE and FREQUENCY
  • AMPLITUDE is the MAXIMUM STRENGTH of the ELECTRIC and MAGNETIC FIELDS

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  • FREQUENCY measures the number of UP / DOWN CYCLES per a GIVEN UNIT of TIME
  • The most COMMON measurement of FREQUENCE is HERTZ
    • Hz (HERTZ) = cycles per second
    • kHz (KILOHERZ) = 1,000 cycles per second
    • MHz (MEGAHERZ) = 1,000,000 cycles per second
    • GHz (GIGAHERTZ) = 1,000,000,000 cycles per second
    • THz (TERAHERTZ) = 1,000,000,000,000 cycles per second

4 CYCLES per 1 SECOND = 4 HERTZ

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  • Another important term is PERIOD, the amount of TIME of ONE CYCLE
    • If the FREQUENCY is 4 Hz, the PERIOD is 0.25 SECONDS

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  • The VISIBLE FREQUENCY RANGE is ~400 THz to 790 THz
  • The RADIO FREQUENCY RANGE is 30 Hz to 300 GHz and is used for many purposes.

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RADIO FREQUENCY BANDS

  • WI-FI uses TWO MAIN BANDS (FREQUENCY RANGES)

  • 2.4 GHz band

    • Range is 2.400 - 2.4835 GHz

  • 5 GHz band

    • Range is 5.150 - 5.825 GHz
    • Divided into FOUR SMALLER BANDS:
      • 5.150 - 5.250 GHz
      • 5.250 - 5.350 GHz
      • 5.470 - 5.725 GHz
      • 5.725 - 5.825 GHz

  • The 2.4 GHz band typically provides FURTHER REACH in open space and BETTER PENETRATION of obstacles such as walls.

    • HOWEVER, more DEVICES tend to use the 2.4 GHz BAND so INTERFERENCE can be a BIGGER PROBLEM compared to 5GHz

** WI-FI 6 (802.11ax) has EXPANDED the spectrum range to include a band in the 6 GHz RANGE

CHANNELS

  • Each BAND is divided up into MULTIPLE “CHANNELS”
    • DEVICES are configured to TRANSMIT and RECEIVE traffic on one (or more) of these CHANNELS
  • The 2.4 GHz BAND is divided into several CHANNELS, each with a 22 MHz RANGE
  • In a SMALL WIRELESS LAN with only a single ACCESS POINT (AP), you can use ANY channel
  • However, in larger WLANs with multiple APs, it’s important that adjacent APs don’t use OVERLAPPING CHANNELS. This helps avoid INTERFERENCE
  • In the 2.4 GHz BAND, it is recommended to use CHANNELS 1, 6 and 11

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  • The 5 GHz BAND consists of NON-OVERLAPPING channels so it’s much EASIER to avoid INTERFERENCE between adjacent APs

  • Using CHANNELS 1, 6, 11, you can place APs in a “HONEYCOMB” pattern to provide COMPLETE coverage of an area without INTERFERENCE between CHANNELS

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WI-FI STANDARDS (802.11)

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SERVICE SETS

  • 802.11 defines different kinds of SERVICE SETS which are groups of WIRELESS NETWORK DEVICES
  • There are THREE MAIN TYPES:
    • INDEPENDENT
    • INFRASTRUCTURE
    • MESH
  • ALL DEVICES in a SERVICE SET share the same SSID (Service Set Identifier)
  • The SSID is a HUMAN-READABLE NAME which identifies the SERVICE SET
  • The SSID does NOT have to be UNQUE

SERVICE SETS : IBSS

  • An IBSS (INDEPENDENT BASIC SERVICE SET) is a WIRELESS NETWORK in which TWO or MORE WIRELESS DEVICES connect directly without using an AP (ACCESS POINT)
  • Also called an AD HOC NETWORK
  • Can be used for FILE TRANSFER (ie: AirDrop)
  • Not scalable beyond a few DEVICES

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SERVICE SETS : BSS

  • A BSS (BASIC SERVICE SET) is a kind of infrastructure SERVICE SET in which CLIENTS connect to each other via an AP (ACCESS POINT) but not DIRECTLY to each other
  • A BSSID (BASIC SERVICE SET ID) is used to uniquely identify the AP
    • Other APs can use the SAME SSID but NOT THE SAME BSSID
    • The BSSID is the MAC ADDRESS of the APs RADIO
  • WIRELESS DEVICES request to associate with the BSS
  • WIRELESS DEVICES that have associated with the BSS are called “CLIENTS” or “STATIONS”
  • The AREA around an AP where its SIGNAL is usable is called a BSA (BASIC SERVICE AREA)

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SERVICE SETS: ESS

  • To create LARGER WIRELESS LANS beyond the range of a SINGLE AP, we use an ESS (EXTENDED SERVICE SET)
  • APs with their own BSSs are connected by a WIRED NETWORK
    • Each BSS uses the SAME SSID
    • Each BSS has a UNIQUE BSSID
    • Each BSS uses a DIFFERENT channel to avoid INTERFERENCE
  • CLIENTS can pass between APs without having to RECONNECT, providing a SEAMLESS WI-FI experience when moving between APs
    • This is called ROAMING
  • The BSAs should overlap about 10-15%

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SERVICE SETS: MBSS

  • An MBSS (MESH BASIC SERVICE SET) can be used in situations where it’s difficult to run an ETHERNET connection to every AP
  • MESH APs use TWO RADIOS:
    • ONE provides BSS to WIRELESS CLIENTS
    • ONE forms a “BACKHAUL NETWORK” which is used to BRIDGE traffic from AP to AP
  • At least ONE AP is connected to the WIRED NETWORK and it is called the RAP (ROOT ACCESS POINT)
  • The OTHER APs are called MAPs (MESH ACCESS POINTS)
  • A PROTOCOL is used to determine the BEST PATH through the MESH (similar to how DYNAMIC ROUTING PROTOCOLS are used to determine the BEST PATH to a DESTINATION)

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DISTRIBUTION SYSTEM

  • Most WIRELESS NETWORKS are not STANDALONE NETWORKS
    • Rather, they are a way for WIRELESS CLIENTS to connect to the WIRED NETWORK INFRASTRUCTURE
  • In 802.11, the UPSTREAM WIRED NETWORK is called the DS (DISTRIBUTION SYSTEM)
  • Each WIRELESS BSS or ESS is mapped to a VLAN in the WIRED NETWORK

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  • It is possible for an AP to provide MULTIPLE WIRELESS LANs, each with a unique SSID
  • Each WLAN is mapped to a separate VLAN and connected to the WIRED NETWORK via a TRUNK
  • Each WLAN uses a UNIQUE BSSID, usually by INCREMENTING the LAST digit of the BBSID by one

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ADDITIONAL AP OPERATIONAL MODES

  • APs can operate in ADDITIONAL MODES beyond the ones we’ve introduced so far

  • An AP in REPEATER MODE can be used to EXTEND the RANGE of a BSS

  • The REPEATER will re-transmit ANY SIGNAL it receives from the AP

    • A REPEATER with a SINGLE RADIO must operate on the SAME CHANNEL as the AP, but this can drastically reduce the overall THROUGHPUT on the CHANNEL
    • A REPEATER with TWO RADIOS can receive on ONE CHANNEL and then retransmit on ANOTHER CHANNEL

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  • A WORKGROUP BRIDGE (WGB) operates as a WIRELESS CLIENT of another AP and can be used to CONNECT WIRED DEVICES to the WIRELESS NETWORK
  • In the example below, PC1 does NOT have WIRELESS CAPABILITIES, and also DOES NOT have ACCESS to WIRED CONNECTIONS to SW1
  • PC1 has a WIRED CONNECTION to the WGB, which has a WIRELESS CONNECTION to the AP

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  • AN OUTDOOR BRIDGE can be used to connect NETWORKS over LONG DISTANCES without a PHYSICAL CABLE connecting them
  • The APs will use SPECIALIZED ANTENNAS that focus most of the SIGNAL POWER in one direction, which allows the WIRELESS CONNECTION to be made over LONGER DISTANCES than normally possible
  • The CONNECTION can be POINT-TO-POINT as in the diagram below, or POINT-TO-MULTIPOINT in which MULTIPLE SITES connect to on CENTRAL SITE

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