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Final

Final. What is a wireless LAN?. WLAN, like a LAN, requires a physical medium to transmit signals. Instead of using UTP, WLANs use: Infrared light (IR) 802.11 does include an IR specification limitations, easily blocked, no real 802.11 products (IrDA) Radio frequencies (RFs)

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Final

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  1. Final

  2. What is a wireless LAN? • WLAN, like a LAN, requires a physical medium to transmit signals. • Instead of using UTP, WLANs use: • Infrared light (IR) • 802.11 does include an IR specification • limitations, easily blocked, no real 802.11 products (IrDA) • Radio frequencies (RFs) • Can penetrate ‘most’ office obstructions http://earlyradiohistory.us/1920au.htm

  3. What is a wireless LAN? • WLANs use the 2.4 GHz and 5-GHz frequency bands. • ISM (Industry, Scientific, Medical) license-free (unlicensed) frequency bands. However, FCC wants more control. • L-Band ISM – 900 MHz • S-Band ISM • 802.11b and 802.11g: 2.4- 2.5 GHz • C-Band ISM • 802.11a: 5.725 – 5.875 GHz More later!

  4. What is a wireless LAN? • WLANs use the 2.4 GHz and 5-GHz frequency bands. • ISM (Industry, Scientific, Medical) license-free (unlicensed) frequency bands. However, FCC wants more control. • L-Band ISM – 900 MHz • S-Band ISM • 802.11b and 802.11g: 2.4- 2.5 GHz • C-Band ISM • 802.11a: 5.725 – 5.875 GHz More later!

  5. 54 Mbps Speed 860 Kbps 1 and 2 Mbps 1 and 2 Mbps 11 Mbps Standards-based Network Proprietary 5 GHz Radio 900 MHz 2.4 GHz 2.4 GHz 1986 1998 2000 2003 1988 1990 1992 1994 1996 802.11 PHY (Physical Layer) Technologies • Infrared light • Three types of radio transmission within the unlicensed 2.4-GHz frequency bands: • Frequency hopping spread spectrum (FHSS) 802.11b • Direct sequence spread spectrum (DSSS) 802.11b • Orthogonal frequency-division multiplexing (OFDM) 802.11g • One type of radio transmission within the unlicensed 5-GHz frequency bands: • Orthogonal frequency-division multiplexing (OFDM) 802.11a 802.11 Ratified 802.11a,b Ratified 802.11g Ratified • IEEE 802.11Begins Drafting More later!

  6. Power Consumption • Power consumption is always an issue with laptops, because the power and the battery have limited lives. • 802.11a uses a higher frequency (5 GHz) than 802.11a/g (2.4 GHz) which requires higher power and more of a drain on batteries.

  7. Overview of WLAN Topologies • Three types of WLAN Topologies: • Independent Basic Service Sets (IBSS) • Basic Service Set (BSS) • Extended Service Set (ESS) • Service Set – A logical grouping of devices. • WLANs provide network access by broadcasting a signal across a wireless radio frequency. • Transmitter prefaces its transmissions with a Service Set Identifier (SSID) • A station may receive transmissions from transmitters with the same or different SSIDs.

  8. DCF Operation • In DCF operation, a station wanting to transmit : • Checks to see if radio link is clear, CS/CCA – Carrier Sense, Clear Channel Assessment (Later in PHY presentation) • Checks its NAV timer (virtual carrier-sense mechanism - coming) to see if someone else is using the medium. • If medium is available DCF uses a random backoff timer to avoid collisions and sends the frame. • Transmitting station only knows the 802.11 frame got there if it receives an ACK. • May also use RTS/CTS to reduce collisions (coming) An example will be coming!

  9. Duration Field An example will be coming! • Duration/ID field – The number of microseconds (millionth of a second) that the medium is expected to remain busy for transmission currently in progress. • Transmitting device sets the Duration time in microseconds. • Includes time to: • Transmit this frame to the AP (or to the client if an AP) • The returning ACK • The time in-between frames, IFS (Interframe Spacing -coming) • All stations monitor this field! • All stations update their NAV (Network Allocation Vector) timer. General 802.11 Frame (more on this later)

  10. NAV Timer An example will be coming! • All stations have a NAV(Network Allocation Vector) timer. • Virtual carrier-sensing function which maintains a prediction of future traffic based on info in the duration field of unicast frames. • Protects the sequence of frames from interruption. • Martha sends a frame to George. • Since wireless medium is a “broadcast-based” (not broadcast frame) shared medium, all stations including Vivian receive the frame. • Vivian updates her NAV timer with the duration value. • Vivian will not attempt to transmit until her NAV is decremented to 0. • Stations will only update their NAV when the duration field value received is greater than their current NAV. General 802.11 Frame (more on this later)

  11. RTS/CTS Solution • The hidden node stations cannot see the RTS. • The AP replies to Vivian with a CTS, which all nodes, including the hidden node can see. • Vivian transmits the frame. • The AP returns an ACK to Vivian. • The AP sends the message to George who returns an ACK to the AP. • Vivian attempts to reserve the medium using an RTS control frame to the AP. • The RTS frame indicates to the AP and all stations within range, that Vivian wants to reserve the medium for a certain duration of time, message, ACK, and SIFS.

  12. RTS/CTS Solution • The RTS/CTS procedure can be enabled/controlled by setting the RTS threshold on the 802.11 client NIC. • RTS/CTS is also used during frame fragmentation (coming). • RTS/CTS consumes a fair amount of capacity and overhead, resulting in additional latency. • Normally used in high capacity environments.

  13. Setting the RTS Threshold on a Cisco Client RTS Threshold • Specifies the data packet size beyond which the low-level RF protocol invokes RTS/CTS flow control. A small value causes RTS packets to be sent more often, which consumes more of the available bandwidth and reduces the throughput of other network packets. However, small values help the system recover from interference or collisions, which can occur in environments with obstructions or metallic surfaces that create complex multipath signals.

  14. Frame Fragmentation • Since we have already discussed RTS/CTS, let’s also discuss frame fragmentation. • Later, we will see that RTS/CTS and fragmentation are typically combined. • Frame fragmentation is a MAC layer function that is designed to increase the reliability of transmitting frames across a wireless medium.

  15. Frame Fragmentation • In a “hostile wireless medium” (interference, noise) larger frames may have more of a problem reaching the receiver without any errors. • By decreasing the size of the frame, the probability of interference during transmission can be reduced. • Breaking up a large frame into smaller frames, allows a larger percentage of frames to arrive undamaged (without errors). • “Easier to poor sand down a hole than boulders.”

  16. Frame Fragmentation • Frame fragmentation can increase the reliability of frame transmissions but there is additional overhead: • Each frame fragment includes the 802.11 MAC protocol header. • Each frame fragment requires a corresponding acknowledgement. • If a frame fragment encounters errors or a collision, only that fragment needs to be retransmitted, not the entire frame. • The frame control field includes information that this is a fragmented frame.

  17. Frame Fragmentation Fragment Threshold: Defines the largest RF packet that the client adapter sends without splitting the packet into two or more smaller fragments. If a single fragment experiences interference during transmission, only that fragment must be resent. Fragmentation generally reduces throughput because the packet overhead for each fragment consumes a higher portion of the RF bandwidth. • The “network administrator” (user) can define the fragment size. • Fragment size – The largest packet that the client adapter sends without fragmenting the packet. • Only unicast packets will be fragmented, not broadcasts or multicasts.

  18. Frame Fragmentation • Frame fragments are sent in a burst, using a single iteration of DCF to access the medium. • In other words the NAV is set in the first fragment and later fragments to reserve the medium for the entire original frame. • FYI – Some of the detail • The first frame sets the NAV to be long enough to include the returning ACK, the next fragment, its ACK, and 3 SIFS. • The following frames set the NAV to include successive ACKs and SIFS. From 802.11 Wireless Networks, by Matthew Gast

  19. 802.11 MAC Addressing - DS X • Distribution System (DS) • “The distribution system is the logical component of 802.11 used to forward frames to their destination. 802.11 does not specify any particular technology for the distribution system.” Matthew Gast • The DS is the exiting network from the AP. (For purposes of this discussion.) • It can be a wired network (Ethernet) or a wireless network (wireless bridge) or something else. • We will assume it is a wired network for these discussions. Y Distribution System (DS) Access Point 1 Access Point 2 C A B D

  20. Station Connectivity Probe process Authentication process Association process • We will look at three processes: • Probe Process (or scanning) • The Authentication Process • The Association Process • Only after a station has both authenticated and associated with the access point can it use the Distribution System (DS) services and communicate with devices beyond the access point. Successful Authentication Successful Association State 1 Unauthenticated Unassociated State 2 Authenticated Unassociated State 3 Authenticated Associated Deauthentication Disassociation

  21. Station Connectivity – Passive Scanning • The Probe Process (Scanning) done by the wireless station • Passive - Beacons • Active – Probe Requests • Passive Scanning • Saves battery power • Station moves to each channel and waits for Beacon frames from the AP. • Records any beacons received. • Beacon frames allow a station to find out every thing it needs to begin communications with the AP including: • SSID • Supported Rates • Kismet/KisMAC uses passive scanning

  22. Authentication Process • Authentication • Open-System • Shared-Key (WEP) • Encryption • None • WEP only or

  23. Multipath Reflection Multipath Reflection Interactive Activity 3.7.5 • Advantage: Can use reflection to go around obstruction. • Disadvantage: Multipath reflection – occurs when reflections cause more than one copy of the same transmission to arrive at the receiver at slightly different times. Usually caused by poor signal quality levels or high RF signal strength

  24. Diffraction • Diffraction of a wireless signal occurs when the signal is partially blocked or obstructed by a large object in the signal’s path. • A diffracted signal is usually attenuated so much it is too weak to provide a reliable microwave connection. • Do not plan to use a diffracted signal, and always try to obtain an unobstructed path between microwave antennas. Diffracted Signal

  25. Refraction Sub-Refraction Interactive Activity 3.7.2 Refraction (straight line) • Refraction (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere. • When a ray of light traveling in one medium enters a second medium and is not perpendicular to the surface of this second medium, it bends • The refractivity gradient (k-factor) usually causes microwave signals to curve slightly downward toward the earth, making the radio horizon father away than the visual horizon. • This can increase the microwave path by about 15%, Normal Refraction Earth

  26. Watts • One definition of energy is the ability to do work. • There are many forms of energy, including: • electrical energy • chemical energy • thermal energy • gravitational potential energy • The metric unit for measuring energy is the Joule. • Energy can be thought of as an amount. • 1 Watt = I Joule of energy / one second • If one Joule of energy is transferred in one second, this is one watt (W) of power.

  27. Watts • The U.S. Federal Communications Commission allows a maximum of 4 watts of power to be emitted in point-to-multipoint WLAN transmissions in the unlicensed 2.4-GHz band. • In WLANs, power levels as low as one milliwatt (mW), or one one-thousandth (1/1000th) of a watt, can be used for a small area. • Typical WLAN NICS transmit at 100 mW. • Typical Access Points can transmit between 30 to 100 mW (plus the gain from the Antenna).

  28. Watts • Power levels on a single WLAN segment are rarely higher than 100 mW, enough to communicate for up to three-fourths of a kilometer or one-half of a mile under optimum conditions. • Access points generally have the ability to radiate from 30 to100 mW, depending on the manufacturer. • Outdoor building-to-building applications (bridges) are the only ones that use power levels over 100 mW.

  29. Decibels 10x • The dB is measured on a base 10 logarithmic scale. • The base increases ten-fold for every ten dB measured. The decibel scale allows people to work more easily with large numbers. • A similar scale called the Richter Scale. • The Richter scale is logarithmic, that is an increase of 1 magnitude unit represents a factor of ten times in amplitude. • The seismic waves of a magnitude 6 earthquake are 10 times greater in amplitude than those of a magnitude 5 earthquake. • Each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy. 10x

  30. Decibels - FYI • Calculating dB The formula for calculating dB is as follows: dB = 10 log10 (Pfinal/Pref) • dB = The amount of decibels. • This usually represents: • a loss in power such as when the wave travels or interacts with matter, • can also represent a gain as when traveling through an amplifier. • Pfinal = The final power. This is the delivered power after some process has occurred. • Pref = The reference power. This is the original power.

  31. Logarithms – Just another way of expressing powers (10n) - FYI x = ay logax = y • Example: 100 = 102 • This is equivalent to saying that the base-10 logarithm of 100 is 2; that is: 100 = 102 same as log10100 = 2 • Example 2: 1000 = 103 is the same as: log10 1000 = 3 • Notes: • With base-10 logarithms, the subscript 10 is often omitted; log 100 = 2 same as log 1000 = 3 • When the base-10 logarithm of a quantity increases by 1, the quantity itself increases by a factor of 10, ie. 2 to 3 increases the quantity 100 to 1000. • A 10-to-1 change in the size of a quantity, resulting in a logarithmic increase or decrease of 1, is called an order of magnitude. • Thus, 1000 is one order of magnitude larger than 100.

  32. Decibel references WLANs work in milliwatts or 1/1,000th of a Watt • dB has no particular defined reference • Most common reference when working with WLANs is: • dBm • m = milliwatt or 1/1,000th of a watt • 1,000 mW = 1 W (Watt) • Milliwatt = .001 Watt or 1/1,000th of a watt • Since the dBm has a defined reference, it can also be converted back to watts, if desired. • The power gain or loss in a signal is determined by comparing it to this fixed reference point, the milliwatt.

  33. Decibel references • Example: • 1 mW = .001 Watts • Using 1 mW as our reference we start at: 0 dB • Using the dB formula: • Doubling the milliwatts to 2 mW or .002 Watts we get +3 dBm • +10 dBm is 10 times the original 1 mW value or 10 mW • +20 dBm is 100 times the original 1 mW value or 100 mW

  34. Ref. • dB milliWatt (dBm) - This is the unit of measurement for signal strength or power level. (milliwatt = 1,000th of a watt or 1/1,000 watt) • If the original signal was 1 mW and a device receives a signal at 1 mW, this is a loss of 0 dBm. • However, if that same device receives a signal that is 0.001 milliwatt, then a loss of 30 dBm occurs, or -30 dBm. • -n dBm is not a negative number, but a value between 0 and 1. • To reduce interference with others, the 802.11b WLAN power levels are limited to the following: • 36 dBm EIRP by the FCC(4 Watts) • 20 dBm EIRP by ETSI

  35. Interactive Activity – Calculating decibelsCurriculum 3.2.3 End • This activity allows the student to enter values for Power final and Power reference, then calculates for decibels. Adding an antenna or other type of amplification. Start Change +10 dBm

  36. Interactive Activity – Calculating decibels • This activity allows the student to enter values for Power final and Power reference, then calculates for decibels. Adding an antenna or other type of amplification. End +20 dBm Start Change

  37. Interactive Activity – Calculating decibels • This activity allows the student to enter values for Power final and Power reference, then calculates for decibels. Adding an antenna or other type of amplification. +3dBm End Start Change

  38. Interactive Activity – Using decibels Change Start End +10 dBm • This activity allows the student to enter a value for the decibels and a value for the reference power resulting in the final power. Adding an antenna or other type of amplification.

  39. Interactive Activity – Using decibels Change Start +3 dBm End • This activity allows the student to enter a value for the decibels and a value for the reference power resulting in the final power. Adding an antenna or other type of amplification.

  40. RF Receivers -90 dBm End • Radio receivers are very sensitive to and may be able to pick up signals as small as 0.000000001 mW or –90 dBm, or a 1 billionth of a milliwatt or 0.000000000001 W. Start Change

  41. Other decibel references besides mW More on this when we discuss antennas. • dB dipole (dBd) - This refers to the gain an antenna has, as compared to a dipole antenna at the same frequency. A dipole antenna is the smallest, least gain practical antenna that can be made. • dB isotropic (dBi) - This refers to the gain a given antenna has, as compared to a theoretical isotropic, or point source, antenna. Unfortunately, an isotropic antenna cannot exist in the real world, but it is useful for calculating theoretical coverage and fade areas. • A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. For example, a simple dipole antenna has a gain of 2.14 dBi or 0 dBd. • Effective Isotropic Radiated Power (EIRP) - EIRP is defined as the effective power found in the main lobe of a transmitter antenna. It is equal to the sum of the antenna gain, in dBi, plus the power level, in dBm, into that antenna. • Gain - This refers to the amount of increase in energy that an antenna appears to add to an RF signal.

  42. 802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS) • In 1999 802.11 introduced 802.11b standard (HR/DSSS) • Data rates of 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps • Backwards compatible with 802.11 • Uses 2.4 GHz ISM band

  43. 802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS) • HR/DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range. • This allows for three non-overlapping channels (three channels that can coexist or overlap without causing interference), channels 1, 6 and 11 (coming).

  44. 802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS) (Once again) • HR/DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range. • This allows for three non-overlapping channels (three channels that can coexist or overlap without causing interference), channels 1, 6 and 11 (coming).

  45. AP Antennas • Cisco Aironet AP 2.4 GHz antennas are compatible with all Cisco RP-TNC equipped APs. • The antennas are available with different gain and range capabilities, beam widths, and form factors. • Coupling the right antenna with the right AP allows for efficient coverage in any facility, as well as better reliability at higher data rates. • A detailed coverage of antennas will be provided later in the course.

  46. Bridge Antennas • Cisco Aironet bridge 2.4 GHz antennas provide transmission between two or more buildings. • Antennas operate at Layer 1 of the OSI Model. • Remember that the physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. • Characteristics such as voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar, attributes are defined by physical layer specifications.

  47. Root modes • Cisco Aironet access points and bridgeshave two different root modes, in which to operate the following: • Root = ON — • The bridge or AP is a root. • If it is a bridge, then it is called the master bridge. • Root = OFF — • The bridge or AP is not a root, non-root.

  48. Root modes

  49. Root modes on on off off off off

  50. Beamwidth 15 dBi • Beamwidth is a measurement used to describe directional antennas. • Beamwidth is sometimes calledhalf-power beamwidth. • Half-power beamwidth is the total width in degrees of the main radiation lobe, at the angle where the radiated power has fallen below that on the centerline of the lobe, by 3 dB (half-power). 3 dBi 12 dBi 15 dBi

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