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  • Depends on device drive of wireless adapter or the software utility you are using. Cisco adapters do active scanning when associating , but use passive scanning for some tests. In either case, beacons are still received and used by the wireless stations for other things besides scanning (coming).
  • Review

    1. 1. Final
    2. 2. What is a wireless LAN? <ul><li>WLAN, like a LAN, requires a physical medium to transmit signals. </li></ul><ul><li>Instead of using UTP, WLANs use: </li></ul><ul><ul><li>Infrared light (IR) </li></ul></ul><ul><ul><ul><li>802.11 does include an IR specification </li></ul></ul></ul><ul><ul><ul><li>limitations, easily blocked, no real 802.11 products (IrDA) </li></ul></ul></ul><ul><ul><li>Radio frequencies (RFs) </li></ul></ul><ul><ul><ul><li>Can penetrate ‘most’ office obstructions </li></ul></ul></ul>http://earlyradiohistory.us/1920au.htm
    3. 3. What is a wireless LAN? <ul><li>WLANs use the 2.4 GHz and 5-GHz frequency bands. </li></ul><ul><li>ISM (Industry, Scientific, Medical) license-free (unlicensed) frequency bands. However, FCC wants more control . </li></ul><ul><li>L-Band ISM – 900 MHz </li></ul><ul><li>S-Band ISM </li></ul><ul><ul><li>802.11b and 802.11g: 2.4- 2.5 GHz </li></ul></ul><ul><li>C-Band ISM </li></ul><ul><ul><li>802.11a: 5.725 – 5.875 GHz </li></ul></ul>More later!
    4. 4. What is a wireless LAN? <ul><li>WLANs use the 2.4 GHz and 5-GHz frequency bands. </li></ul><ul><li>ISM (Industry, Scientific, Medical) license-free (unlicensed) frequency bands. However, FCC wants more control . </li></ul><ul><li>L-Band ISM – 900 MHz </li></ul><ul><li>S-Band ISM </li></ul><ul><ul><li>802.11b and 802.11g: 2.4- 2.5 GHz </li></ul></ul><ul><li>C-Band ISM </li></ul><ul><ul><li>802.11a: 5.725 – 5.875 GHz </li></ul></ul>More later!
    5. 5. 802.11 PHY (Physical Layer) Technologies <ul><li>Infrared light </li></ul><ul><li>Three types of radio transmission within the unlicensed 2.4-GHz frequency bands: </li></ul><ul><ul><li>Frequency hopping spread spectrum (FHSS) 802.11b </li></ul></ul><ul><ul><li>Direct sequence spread spectrum (DSSS) 802.11b </li></ul></ul><ul><ul><li>Orthogonal frequency-division multiplexing (OFDM) 802.11g </li></ul></ul><ul><li>One type of radio transmission within the unlicensed 5-GHz frequency bands: </li></ul><ul><ul><li>Orthogonal frequency-division multiplexing (OFDM) 802.11a </li></ul></ul>802.11 Ratified 802.11a,b Ratified 802.11g Ratified <ul><li>IEEE 802.11Begins Drafting </li></ul>More later! 860 Kbps 900 MHz 1 and 2 Mbps 2.4 GHz Proprietary 1986 1988 1990 1992 1994 1996 1998 2000 2003 1 and 2 Mbps 2.4 GHz 11 Mbps 54 Mbps Standards-based 5 GHz Radio Network Speed
    6. 6. Power Consumption <ul><li>Power consumption is always an issue with laptops, because the power and the battery have limited lives. </li></ul><ul><li>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. </li></ul>
    7. 7. Overview of WLAN Topologies <ul><li>Three types of WLAN Topologies: </li></ul><ul><ul><li>Independent Basic Service Sets (IBSS) </li></ul></ul><ul><ul><li>Basic Service Set (BSS) </li></ul></ul><ul><ul><li>Extended Service Set (ESS) </li></ul></ul><ul><li>Service Set – A logical grouping of devices. </li></ul><ul><li>WLANs provide network access by broadcasting a signal across a wireless radio frequency. </li></ul><ul><li>Transmitter prefaces its transmissions with a Service Set Identifier (SSID) </li></ul><ul><li>A station may receive transmissions from transmitters with the same or different SSIDs. </li></ul>
    8. 8. DCF Operation <ul><li>In DCF operation , a station wanting to transmit : </li></ul><ul><ul><li>Checks to see if radio link is clear , CS/CCA – Carrier Sense, Clear Channel Assessment (Later in PHY presentation) </li></ul></ul><ul><ul><li>Checks its NAV timer (virtual carrier-sense mechanism - coming ) to see if someone else is using the medium. </li></ul></ul><ul><ul><li>If medium is available DCF uses a random backoff timer to avoid collisions and sends the frame. </li></ul></ul><ul><li>Transmitting station only knows the 802.11 frame got there if it receives an ACK . </li></ul><ul><li>May also use RTS/CTS to reduce collisions ( coming ) </li></ul>An example will be coming!
    9. 9. Duration Field <ul><li>Duration/ID field – The number of microseconds (millionth of a second) that the medium is expected to remain busy for transmission currently in progress. </li></ul><ul><ul><li>Transmitting device sets the Duration time in microseconds. </li></ul></ul><ul><ul><li>Includes time to: </li></ul></ul><ul><ul><ul><li>Transmit this frame to the AP (or to the client if an AP) </li></ul></ul></ul><ul><ul><ul><li>The returning ACK </li></ul></ul></ul><ul><ul><ul><li>The time in-between frames, IFS (Interframe Spacing - coming ) </li></ul></ul></ul><ul><li>All stations monitor this field! </li></ul><ul><li>All stations update their NAV (Network Allocation Vector) timer. </li></ul>General 802.11 Frame (more on this later) An example will be coming !
    10. 10. NAV Timer <ul><li>All stations have a NAV (Network Allocation Vector) timer . </li></ul><ul><li>Virtual carrier-sensing function which maintains a prediction of future traffic based on info in the duration field of unicast frames. </li></ul><ul><li>Protects the sequence of frames from interruption. </li></ul><ul><li>Martha sends a frame to George. </li></ul><ul><li>Since wireless medium is a “broadcast-based” ( not broadcast frame) shared medium, all stations including Vivian receive the frame. </li></ul><ul><li>Vivian updates her NAV timer with the duration value. </li></ul><ul><li>Vivian will not attempt to transmit until her NAV is decremented to 0. </li></ul><ul><li>Stations will only update their NAV when the duration field value received is greater than their current NAV. </li></ul>General 802.11 Frame (more on this later) An example will be coming !
    11. 11. RTS/CTS Solution <ul><li>The hidden node stations cannot see the RTS . </li></ul><ul><li>The AP replies to Vivian with a CTS , which all nodes, including the hidden node can see. </li></ul><ul><li>Vivian transmits the frame. </li></ul><ul><li>The AP returns an ACK to Vivian. </li></ul><ul><li>The AP sends the message to George who returns an ACK to the AP. </li></ul><ul><li>Vivian attempts to reserve the medium using an RTS control frame to the AP . </li></ul><ul><li>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. </li></ul>
    12. 12. RTS/CTS Solution <ul><li>The RTS/CTS procedure can be enabled/controlled by setting the RTS threshold on the 802.11 client NIC . </li></ul><ul><li>RTS/CTS is also used during frame fragmentation (coming). </li></ul><ul><li>RTS/CTS consumes a fair amount of capacity and overhead , resulting in additional latency. </li></ul><ul><li>Normally used in high capacity environments. </li></ul>
    13. 13. Setting the RTS Threshold on a Cisco Client <ul><li>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. </li></ul>RTS Threshold
    14. 14. Frame Fragmentation <ul><li>Since we have already discussed RTS/CTS, let’s also discuss frame fragmentation. </li></ul><ul><li>Later, we will see that RTS/CTS and fragmentation are typically combined. </li></ul><ul><li>Frame fragmentation is a MAC layer function that is designed to increase the reliability of transmitting frames across a wireless medium. </li></ul>
    15. 15. Frame Fragmentation <ul><li>In a “hostile wireless medium” (interference, noise) larger frames may have more of a problem reaching the receiver without any errors. </li></ul><ul><li>By decreasing the size of the frame, the probability of interference during transmission can be reduced. </li></ul><ul><li>Breaking up a large frame into smaller frames, allows a larger percentage of frames to arrive undamaged (without errors). </li></ul><ul><li>“ Easier to poor sand down a hole than boulders.” </li></ul>
    16. 16. Frame Fragmentation <ul><li>Frame fragmentation can increase the reliability of frame transmissions but there is additional overhead : </li></ul><ul><ul><li>Each frame fragment includes the 802.11 MAC protocol header. </li></ul></ul><ul><ul><li>Each frame fragment requires a corresponding acknowledgement. </li></ul></ul><ul><li>If a frame fragment encounters errors or a collision, only that fragment needs to be retransmitted, not the entire frame. </li></ul><ul><li>The frame control field includes information that this is a fragmented frame. </li></ul>
    17. 17. Frame Fragmentation <ul><li>The “network administrator” (user) can define the fragment size. </li></ul><ul><li>Fragment size – The largest packet that the client adapter sends without fragmenting the packet. </li></ul><ul><li>Only unicast packets will be fragmented, not broadcasts or multicasts. </li></ul>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.
    18. 18. Frame Fragmentation <ul><li>Frame fragments are sent in a burst , using a single iteration of DCF to access the medium. </li></ul><ul><li>In other words the NAV is set in the first fragment and later fragments to reserve the medium for the entire original frame. </li></ul><ul><li>FYI – Some of the detail </li></ul><ul><ul><li>The first frame sets the NAV to be long enough to include the returning ACK, the next fragment, its ACK, and 3 SIFS. </li></ul></ul><ul><ul><li>The following frames set the NAV to include successive ACKs and SIFS. </li></ul></ul>From 802.11 Wireless Networks, by Matthew Gast
    19. 19. 802.11 MAC Addressing - DS <ul><li>Distribution System (DS) </li></ul><ul><ul><li>“ 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 </li></ul></ul><ul><ul><li>The DS is the exiting network from the AP. (For purposes of this discussion.) </li></ul></ul><ul><ul><li>It can be a wired network (Ethernet) or a wireless network (wireless bridge) or something else. </li></ul></ul><ul><ul><li>We will assume it is a wired network for these discussions. </li></ul></ul>Distribution System (DS) A B C D Access Point 1 Access Point 2 X Y
    20. 20. Station Connectivity <ul><li>We will look at three processes: </li></ul><ul><ul><li>Probe Process (or scanning) </li></ul></ul><ul><ul><li>The Authentication Process </li></ul></ul><ul><ul><li>The Association Process </li></ul></ul><ul><li>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. </li></ul>State 1 Unauthenticated Unassociated State 2 Authenticated Unassociated State 3 Authenticated Associated Successful Authentication Successful Association Deauthentication Disassociation Probe process Authentication process Association process
    21. 21. Station Connectivity – Passive Scanning <ul><li>The Probe Process (Scanning) done by the wireless station </li></ul><ul><ul><li>Passive - Beacons </li></ul></ul><ul><ul><li>Active – Probe Requests </li></ul></ul><ul><li>Passive Scanning </li></ul><ul><ul><li>Saves battery power </li></ul></ul><ul><ul><li>Station moves to each channel and waits for Beacon frames from the AP. </li></ul></ul><ul><ul><li>Records any beacons received. </li></ul></ul><ul><li>Beacon frames allow a station to find out every thing it needs to begin communications with the AP including: </li></ul><ul><ul><li>SSID </li></ul></ul><ul><ul><li>Supported Rates </li></ul></ul><ul><li>Kismet/KisMAC uses passive scanning </li></ul>
    22. 22. Authentication Process <ul><li>Authentication </li></ul><ul><ul><li>Open-System </li></ul></ul><ul><ul><li>Shared-Key (WEP) </li></ul></ul><ul><li>Encryption </li></ul><ul><ul><li>None </li></ul></ul><ul><ul><li>WEP </li></ul></ul>or only
    23. 23. Multipath Reflection <ul><li>Advantage : Can use reflection to go around obstruction. </li></ul><ul><li>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 </li></ul>Multipath Reflection Interactive Activity 3.7.5
    24. 24. Diffraction <ul><li>Diffraction of a wireless signal occurs when the signal is partially blocked or obstructed by a large object in the signal’s path. </li></ul><ul><li>A diffracted signal is usually attenuated so much it is too weak to provide a reliable microwave connection. </li></ul><ul><li>Do not plan to use a diffracted signal, and always try to obtain an unobstructed path between microwave antennas. </li></ul>Diffracted Signal
    25. 25. Refraction <ul><li>Refraction (or bending) of signals is due to temperature, pressure, and water vapor content in the atmosphere. </li></ul><ul><li>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 </li></ul><ul><li>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. </li></ul><ul><li>This can increase the microwave path by about 15%, </li></ul>Normal Refraction Refraction (straight line) Sub-Refraction Earth Interactive Activity 3.7.2
    26. 26. Watts <ul><li>One definition of energy is the ability to do work . </li></ul><ul><li>There are many forms of energy, including: </li></ul><ul><ul><li>electrical energy </li></ul></ul><ul><ul><li>chemical energy </li></ul></ul><ul><ul><li>thermal energy </li></ul></ul><ul><ul><li>gravitational potential energy </li></ul></ul><ul><li>The metric unit for measuring energy is the Joule . </li></ul><ul><li>Energy can be thought of as an amount . </li></ul><ul><li>1 Watt = I Joule of energy / one second </li></ul><ul><ul><li>If one Joule of energy is transferred in one second, this is one watt (W) of power. </li></ul></ul>
    27. 27. Watts <ul><li>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 . </li></ul><ul><li>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. </li></ul><ul><li>Typical WLAN NICS transmit at 100 mW. </li></ul><ul><li>Typical Access Points can transmit between 30 to 100 mW (plus the gain from the Antenna). </li></ul>
    28. 28. Watts <ul><li>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 . </li></ul><ul><li>Access points generally have the ability to radiate from 30 to100 mW , depending on the manufacturer. </li></ul><ul><li>Outdoor building-to-building applications (bridges) are the only ones that use power levels over 100 mW . </li></ul>
    29. 29. Decibels <ul><li>The dB is measured on a base 10 logarithmic scale . </li></ul><ul><li>The base increases ten-fold for every ten dB measured. The decibel scale allows people to work more easily with large numbers. </li></ul><ul><li>A similar scale called the Richter Scale. </li></ul><ul><ul><li>The Richter scale is logarithmic, that is an increase of 1 magnitude unit represents a factor of ten times in amplitude . </li></ul></ul><ul><ul><li>The seismic waves of a magnitude 6 earthquake are 10 times greater in amplitude than those of a magnitude 5 earthquake. </li></ul></ul><ul><ul><li>Each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy. </li></ul></ul>10x 10x
    30. 30. Decibels - FYI <ul><li>Calculating dB The formula for calculating dB is as follows : </li></ul><ul><li>dB = 10 log 10 (Pfinal/Pref) </li></ul><ul><ul><li>dB = The amount of decibels. </li></ul></ul><ul><ul><ul><li>This usually represents: </li></ul></ul></ul><ul><ul><ul><ul><li>a loss in power such as when the wave travels or interacts with matter , </li></ul></ul></ul></ul><ul><ul><ul><ul><li>can also represent a gain as when traveling through an amplifier . </li></ul></ul></ul></ul><ul><ul><li>Pfinal = The final power. This is the delivered power after some process has occurred. </li></ul></ul><ul><ul><li>Pref = The reference power. This is the original power. </li></ul></ul>
    31. 31. Logarithms – Just another way of expressing powers (10 n ) - FYI <ul><li>x = a y </li></ul><ul><li>log a x = y </li></ul><ul><li>Example: 100 = 10 2 </li></ul><ul><li>This is equivalent to saying that the base-10 logarithm of 100 is 2; that is: </li></ul><ul><ul><ul><li>100 = 10 2 same as log 10 100 = 2 </li></ul></ul></ul><ul><li>Example 2: 1000 = 10 3 is the same as: log 10 1000 = 3 </li></ul><ul><li>Notes : </li></ul><ul><ul><li>With base-10 logarithms, the subscript 10 is often omitted; </li></ul></ul><ul><ul><li>log 100 = 2 same as log 1000 = 3 </li></ul></ul><ul><ul><li>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. </li></ul></ul><ul><ul><li>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 . </li></ul></ul><ul><ul><li>Thus, 1000 is one order of magnitude larger than 100. </li></ul></ul>
    32. 32. Decibel references <ul><li>dB has no particular defined reference </li></ul><ul><li>Most common reference when working with WLANs is: </li></ul><ul><ul><li>dB m </li></ul></ul><ul><ul><li>m = milliwatt or 1/1,000 th of a watt </li></ul></ul><ul><ul><li>1,000 mW = 1 W (Watt) </li></ul></ul><ul><li>Milliwatt = . 001 Watt or 1/1,000 th of a watt </li></ul><ul><li>Since the dBm has a defined reference, it can also be converted back to watts, if desired. </li></ul><ul><li>The power gain or loss in a signal is determined by comparing it to this fixed reference point, the milliwatt . </li></ul>WLANs work in milliwatts or 1/1,000 th of a Watt
    33. 33. Decibel references <ul><li>Example: </li></ul><ul><ul><li>1 mW = .001 Watts </li></ul></ul><ul><ul><li>Using 1 mW as our reference we start at: 0 dB </li></ul></ul><ul><ul><li>Using the dB formula: </li></ul></ul><ul><ul><ul><li>Doubling the milliwatts to 2 mW or .002 Watts we get +3 dBm </li></ul></ul></ul><ul><ul><ul><li>+10 dBm is 10 times the original 1 mW value or 10 mW </li></ul></ul></ul><ul><ul><ul><li>+20 dBm is 100 times the original 1 mW value or 100 mW </li></ul></ul></ul>
    34. 34. <ul><li>dB milliWatt (dBm) - This is the unit of measurement for signal strength or power level . (milliwatt = 1,000 th of a watt or 1/1,000 watt) </li></ul><ul><li>If the original signal was 1 mW and a device receives a signal at 1 mW , this is a loss of 0 dBm. </li></ul><ul><li>However, if that same device receives a signal that is 0.001 milliwatt, then a loss of 30 dBm occurs, or -30 dBm . </li></ul><ul><li>-n dBm is not a negative number, but a value between 0 and 1. </li></ul><ul><li>To reduce interference with others, the 802.11b WLAN power levels are limited to the following: </li></ul><ul><ul><li>36 dBm EIRP by the FCC (4 Watts) </li></ul></ul><ul><ul><li>20 dBm EIRP by ETSI </li></ul></ul>Ref.
    35. 35. Interactive Activity – Calculating decibels Curriculum 3.2.3 <ul><li>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. </li></ul>+10 dBm Change End Start
    36. 36. Interactive Activity – Calculating decibels <ul><li>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. </li></ul>+20 dBm Change End Start
    37. 37. Interactive Activity – Calculating decibels <ul><li>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. </li></ul>+3dBm Change End Start
    38. 38. Interactive Activity – Using decibels <ul><li>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. </li></ul>+10 dBm Change End Start
    39. 39. Interactive Activity – Using decibels <ul><li>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. </li></ul>+3 dBm Change End Start
    40. 40. RF Receivers <ul><li>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. </li></ul>-90 dBm Change End Start
    41. 41. Other decibel references besides mW <ul><li>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. </li></ul><ul><li>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. </li></ul><ul><li>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. </li></ul><ul><li>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. </li></ul><ul><li>Gain - This refers to the amount of increase in energy that an antenna appears to add to an RF signal. </li></ul>More on this when we discuss antennas.
    42. 42. 802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS) <ul><li>In 1999 802.11 introduced 802.11b standard (HR/DSSS) </li></ul><ul><li>Data rates of 1 Mbps, 2 Mbps, 5.5 Mbps and 11 Mbps </li></ul><ul><li>Backwards compatible with 802.11 </li></ul><ul><li>Uses 2.4 GHz ISM band </li></ul>
    43. 43. 802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS) <ul><li>HR/DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range. </li></ul><ul><li>This allows for three non-overlapping channels (three channels that can coexist or overlap without causing interference), channels 1, 6 and 11 (coming). </li></ul>
    44. 44. <ul><li>(Once again) </li></ul><ul><li>HR/DSSS uses 22 MHz channels in the 2.4 to 2.483 GHz range. </li></ul><ul><li>This allows for three non-overlapping channels (three channels that can coexist or overlap without causing interference), channels 1, 6 and 11 (coming). </li></ul>802.11b - High-Rate Direct-sequence spread-spectrum (HR/DSSS)
    45. 45. <ul><li>Cisco Aironet AP 2.4 GHz antennas are compatible with all Cisco RP-TNC equipped APs. </li></ul><ul><li>The antennas are available with different gain and range capabilities, beam widths, and form factors. </li></ul><ul><li>Coupling the right antenna with the right AP allows for efficient coverage in any facility, as well as better reliability at higher data rates. </li></ul><ul><li>A detailed coverage of antennas will be provided later in the course . </li></ul>AP Antennas
    46. 46. <ul><li>Cisco Aironet bridge 2.4 GHz antennas provide transmission between two or more buildings. </li></ul><ul><li>Antennas operate at Layer 1 of the OSI Model. </li></ul><ul><li>Remember that the physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems. </li></ul><ul><li>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. </li></ul>Bridge Antennas
    47. 47. Root modes <ul><li>Cisco Aironet access points and bridges have two different root modes , in which to operate the following: </li></ul><ul><ul><li>Root = ON — </li></ul></ul><ul><ul><ul><li>The bridge or AP is a root . </li></ul></ul></ul><ul><ul><ul><li>If it is a bridge, then it is called the master bridge. </li></ul></ul></ul><ul><ul><li>Root = OFF — </li></ul></ul><ul><ul><ul><li>The bridge or AP is not a root, non-root . </li></ul></ul></ul>
    48. 48. Root modes
    49. 49. Root modes on on off off off off
    50. 50. Beamwidth <ul><li>Beamwidth is a measurement used to describe directional antennas. </li></ul><ul><li>Beamwidth is sometimes called half-power beamwidth . </li></ul><ul><li>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). </li></ul>15 dBi 12 dBi 3 dBi 15 dBi
    51. 51. Gain – It’s all relative! <ul><li>The gain of any antenna is essentially a measurement of how well that antenna focuses radiated RF energy, in a particular direction . </li></ul><ul><li>There are different methods for measuring this. </li></ul><ul><li>Cisco is standardizing on dBi to specify gain measurements. </li></ul><ul><li>This method of measuring gain uses a theoretical isotropic antenna as a reference point. </li></ul><ul><li>Some antennas are rated in dBd , which uses a half -wave dipole type antenna . </li></ul><ul><li>To convert any number from dBd to dBi, simply add 2.14 to the dBd number. </li></ul>dBi = dBd + 2.14 theoretical isotropic antenna Half-wave dipole antenna
    52. 52. Decibel references (Review) <ul><li>Example: </li></ul><ul><ul><li>1 mW = .001 Watts </li></ul></ul><ul><ul><li>Using 1 mW as our reference we start at: 0 dB </li></ul></ul><ul><ul><li>Using the dB formula, doubling the milliwatts to 2 mW or .002 Watts we get +3 dBm </li></ul></ul><ul><ul><li>+10 dBm is 10 times the original 1 mW value or 10 mW </li></ul></ul><ul><ul><li>+20 dBm is 100 times the original 1 mW value or 100 mW </li></ul></ul>
    53. 53. Path-loss (Review) <ul><li>Every time the distance from the transmitter to the receiver is doubled , the signal level is lowered (or increased) by 6 dB 1/4 th or 4 times) . </li></ul><ul><ul><li>6 dBm = 4 times or ¼ </li></ul></ul><ul><ul><li>3 dB + 3dB = 2 times + 2 times = 4 times </li></ul></ul><ul><ul><li>-3dB + -3dB = ½ + ½ = ¼ </li></ul></ul><ul><li>This is also know as the inverse square law. </li></ul><ul><li>“ Signal strength does not fade in a linear manner, but inversely as the square of the distance. This means that if you are a particular distance from an access point and you measure the signal level, and then move twice as far away, the signal level will decrease by a factor of four. You move 2x and the signal decreases by 1/4x; hence the inverse square law. (Move 4x, signal decreases by 1/16x.) In any case, the fact that exponential measurements are involved in signal strength measurement is one reason why the use of logarithmic scale of measurement was developed as an alternative way of representing RF power.” WildPackets White Paper </li></ul>
    54. 54. Other decibel references besides mW <ul><li>dB dipole (dBd) - This refers to the gain an antenna has, as compared to a dipole antenna at the same frequency . </li></ul><ul><ul><li>A dipole antenna is the smallest, least gain practical antenna that can be made. </li></ul></ul><ul><li>dB isotropic (dBi) - This refers to the gain a given antenna has, as compared to a theoretical isotropic , or point source, antenna. </li></ul><ul><ul><li>Unfortunately, an isotropic antenna cannot exist in the real world, but it is useful for calculating theoretical coverage and fade areas. </li></ul></ul><ul><ul><li>A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. </li></ul></ul><ul><ul><li>For example, a simple dipole antenna has a gain of 2.14 dBi or 0 dBd. </li></ul></ul>From Ch. 3
    55. 55. Gain <ul><li>Like a flashlight: There is always a tradeoff between gain, which is comparable to brightness in a particular direction, and beamwidth, which is comparable to the narrowness of the beam. </li></ul>
    56. 56. Gain <ul><li>Antennas have gain in particular directions </li></ul><ul><li>Direction other than the main intended radiation pattern, are typically related to the main lobe gain </li></ul>
    57. 57. Site survey and path profiling
    58. 58. Alignment and interference <ul><li>When aligning antennas, be sure that the two antennas for the link are not cross-polarized. </li></ul><ul><li>Next, ensure that each antenna is pointed or aligned to maximize the received signal level. </li></ul><ul><li>A signal strength tool is provided, which gives a reading of the received signal level. </li></ul><ul><li>At one end of the link at a time, the antenna pointing direction is carefully adjusted to maximize or peak the reading on the signal-indicator tool. </li></ul><ul><li>After this is done for both ends, it is very important to obtain the actual received signal level, in dBm. </li></ul>
    59. 59. WLAN threats <ul><li>There are four primary classes of threats to wireless security: </li></ul><ul><ul><li>Unstructured threats - individuals using easily available hacking tools </li></ul></ul><ul><ul><li>Structured threats - Hackers who are more highly motivated and technically competent . These people know wireless system vulnerabilities, and they can understand and develop exploit-code, scripts, and programs. </li></ul></ul><ul><ul><li>External threats - They work their way into a network mainly from outside the building such as parking lots, adjacent buildings or common areas. </li></ul></ul><ul><ul><li>Internal threats - internal access and misuse account for 60 to 80 percent of reported incidents. </li></ul></ul>
    60. 60. Wired equivalent privacy (WEP) <ul><li>The IEEE 802.11 standard includes WEP to protect authorized users of a WLAN from casual eavesdropping. </li></ul><ul><li>The IEEE 802.11 WEP standard specified a 40-bit key, so that WEP could be exported and used worldwide. </li></ul><ul><li>Most vendors have extended WEP to 128 bits or more . </li></ul><ul><li>When using WEP , both the wireless client and the access point must have a matching WEP key. </li></ul><ul><li>WEP is based upon an existing and familiar encryption type, Rivest Cipher 4 (RC4). </li></ul>128 bit WEP is sometimes referred to, and more accurately, as 104 bit WEP. Also, be sure Transmit Key numbers match, I.e. Key 1 on the both AP and ACU. AP ACU
    61. 61. Site survey considerations <ul><li>Data rates are inverse proportional to data bit rates </li></ul><ul><li>Range increases in proportion to antenna height and gain. </li></ul><ul><li>Avoid locating the computing device and antenna in a location where there is a barrier between the sending and receiving antennas. </li></ul><ul><li>Drywall construction allows greater range than concrete block </li></ul>
    62. 62. Load and coverage <ul><li>The load on an access point or the total number of potential clients should be considered in any design. </li></ul><ul><li>One problem with WLANs is that the number of potential clients can be unknown, since the freedom of wireless allows any number of people to converge within an area. </li></ul><ul><li>In many situations, AP coverage will be the driving factor over bandwidth and autorate negotiation of bandwidth can be used </li></ul>
    63. 63. Bandwidth and throughput <ul><li>2 Mbps units transmit at 2 Mbps, which takes five times as long to transmit the same data as an 11 Mbps product would. </li></ul><ul><li>You may want to use more APs which are closer together to make higher data rates more available. </li></ul><ul><li>If all devices operate at the same data rate, they will all take the same amount of time to send the same size packets. </li></ul><ul><li>If a single unit is less than the maximum, the overall rate will be somewhat less than the maximum. This is because the base or central unit has to service the slower remote. </li></ul>

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