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Wireless Principles Flashcards

1
Q

frequency

A
  • how often a signal is seen, expressed as cycles per second
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2
Q

wavelength

A
  • the size of the cycle pattern of an electromagnetic wave
  • often represented by the Greek symbol lambda
  • AM radio uses 400 or 500m wavelength
  • Wi-Fi uses wavelengths a few cm long
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3
Q

alternating current (AC)

A
  • an electrical current in which the direction of the current changes cyclically
  • shape and form is a sine wave
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4
Q

amplitude

A
  • strength of the signal
  • often represented by the Greek symbol gamma (γ)
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5
Q

amplication types

A

1) active - the applied power is increased
2) passive - accomplished by focusing the energy in one direction by using an antenna

the opposite of amplication is called attenuation

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6
Q

amplitude modulation (AM)

A
  • the transmitter can dynamically modify amplitude
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7
Q

frequency modulation (FM)

A
  • changing the frequency of the signal to encode information
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8
Q

free path loss

A

the attentuation of signal strength, unrelated to any obstacles
- the amount of energy does not change, but when the area over which it is distributed increases, the signal strength decreases
- the receiver antenna can’t pick up more than a portion of the original signal, and the rest of the sent energy is lost

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9
Q

Received Signal Strength Indicator (RSSI)

A
  • the signal strength that one device receives from another device
  • usually expressed in decibels (dBs) referenced to 1 milliwatt (dBm)
  • RSSI is a grade value ranging from 0 (no signal or no reference) to a max of 255, but many vendors use a lower max like 100 or 60
  • from the RSSI grade value, an equivalent dBm is displayed
  • values vary from vendor to vendor, so only good for comparing against same vendor or itself
  • for Cisco, good RSSI values are -67 dBm or better
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10
Q

Signal-To-Noise Ratio (SNR)

A
  • the evaluation of signal strength after it has been affected by noise
  • RSSI - Noise = SNR
  • general principle is that any SNR above 20 dB is good
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11
Q

Signal to Interference Plus Noise Ratio (SINR)

A
  • current calculation considers the noise floor and the strength of any interference to the signal
  • SINR = RSSI minus combination of noise and interference
  • an SINR of 25 or better is required for voice over wireless applications
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12
Q

Received Channel Power Indicator (RCPI)

A
  • attempt to unify the grade levels of signal strength across vendors
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13
Q

Decibels

A
  • a logarithmic unit of measurement that expresses the amount of power relative to a reference
  • since the signal that a transmitter emits is an AC current, the power levels are expressed in milliwatts, and comparing these powers uses the dBm symbol
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14
Q

Watt

A
  • first unit of power that is used in power measurement
  • 1W = 1 J per second
  • direct formulat for converting mW to dBm is
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15
Q

Joule

A
  • amount of energy that is generated by a force of 1 newton (N) movign 1m in one direction
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16
Q

Newton

A
  • the force required to accelerate 1kg at a rate of 1m/s^2
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17
Q

Decibel Calculation References

A
  • 0 dB = same power
  • 3 dB = 2x the power
  • -3dB = 1/2 the power
  • 10dB = 10x the power
  • -10dB = 1/10 the power
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18
Q

mW to dBm Calculation Method

A
  • convert the mW into factors of 10 & 2, and then replace with 10 and 3, respectively
  • example, 50mW = 10x10/2, so the conversion is 10+10-3 = 17dBm
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19
Q

Antenna Power

A
  • an antenna doesn’t send an electric current, but instead, sends an electromagnetic field
  • you can compare the power of antennas by measuring the power gain relative to a reference antenna
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20
Q

Isotropic Antenna

A
  • a theoretical reference antenna that is 1 dot large, and radiates equally in all directions
  • the scale used to compare powers that antennas radiate to an isotropic antenna is called dBi
  • dipole antenna comparisons use the dBd unit
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21
Q

Effective Isotropic Radiated Power (EIRP)

A
  • measures how much energy is actually radiated from an antenna toward the main beam
  • EIRP = Tx power (dBm) + antenna gain (dBi) - cable loss (dB)
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22
Q

Antenna Types

A

1) Omnidirectional
2) Directional

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23
Q

Beamwidth

A
  • the angle through which an antenna signal is emitted
  • omnidirectional antennas are 360 degrees
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24
Q

Radiation Pattern Views

A

1) H-plane or azimuth chart
2) Elevation plane (E-plane)

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25
Q

H-Plane, or Azimuth Chart

A
  • represents the radiation pattern as seen from the top
  • shows how the signal spreads ahead, behind, left, and right, but not how the signal spreads up or down
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26
Q

E-plane, or Elevation Chart

A
  • represents the radiation pattern as seen from the side of the antenna
  • shows how the signal spreads ahead, behind, up and down, but not left or right
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27
Q

Patch Antenna

A
  • ideal for indoor b/c it’s flat and discrete
  • radiates slightly toward the back, which is useful for positioning over doors
28
Q

Yagi Antenna

A
  • with its comb shape, is the same type that is used for TV reception, but usually encased in a protective cylinder
  • gain is high at 13.5 dBi
  • well adapted to covering long corridors or large warehouses (if several Yagis are installed next to each other)
29
Q

802.11 b/g

A
  • b was ratified in 1999, has rates of 1-11Mbps, and operates in 2.4-GHz
  • g was ratified in 2003, is backwards compatible with b, supports additional rates of 6, 9, 12, 18, 24, 36, 48, and 54Mbps
30
Q

802.11a

A
  • ratified in 1999, deliver max rate of 54Mbps
  • uses orthogonal frequency-division multplexing (OFDM), which is a multicarrier system
31
Q

Orthogonal Frequency-Division Multiplexing (OFDM)

A
  • multicarrier system
  • allows subchannels to overlap, providing high spectral efficiency
  • more efficient than spread spectrum techniques used with 802.11b
  • immune to interference from devices in the 2.4GHz band bc it operates in an unlicensed portion of the 5GHz band
32
Q

802.11n

A
  • ratified in 2009, backwards compatible with 802.11a & b/g
  • features include channel bonding for up to 40-MHz channels, packet aggregation, and block acknowledgement
  • improved signals from multiple-input and multiple-output (MIMO)-enabled clients can connect with faster data rates
  • extends data rates into the hundreds of megabits per second in the 2.4 & 5-GHz bands
33
Q

802.11ac

A
  • ratified in 2013
  • operates in 5-GHz spectrum as 802.11a
  • initial deployment was “Wave 1” and uses channel bonding for up to 80 MHz channels, 256-QAM coding, and 1-3 spatial streams with data rates up to 1.3 Gbps
  • “Wave 2” uses up to 160 MHz channel bonding, 1-8 spatial streams, and MU-MIMO with data rates up to 6.93 Gbps
  • supports all mandatory modes of 802.11a, and 802.11n
34
Q

802.11ax (Wi-Fi 6)

A
  • ratified in 2021
  • first wave of APs support eight spatial streams, and with 80 MHz channels, they deliver up to 4800 Mbps at the physical layer
  • unlike 802.11ac, ax is dual-band 2.4- and 5-GHz, so legacy 2.4-GHz-only clients can take advantage of its benefits
  • ax also supports downlink MU-MIMO, where a device can transmit concurrently to multiple receivers
  • ax also supports uplink MU-MIMO
35
Q

SISO

A
  • has only one radio that switches between antennas, whichever is best, but only one at a time
36
Q

Multipath Reception

A
  • degraded performance when confronted by reflected copies of the signal
37
Q

MIMO

A
  • makes use of multiple antennas and radios, combined with advanced signal-processing methods
  • incorporates 3 main technologies: maximal ratio combining (MRC), beamforming, and spatial multiplexing
38
Q

Maximal Ratio Combining (MRC)

A
  • 802.11n/802.11ac MIMO
  • allows a receiver to combine energies from multiple receive chains
  • counterpart of Tx beamforming, and takes place on the receiver side, usually on the AP, regardless of whether the send is 802.11n-compatible
39
Q

Multipath Issues

A
  • occurs when one antenna receives reflected signals out of phase, which is destructive to the signal quality
40
Q

Beamforming

A
  • 802.11n/802.11ac MIMO
  • a technique used when there is more than one Tx antenna
  • the signal that is sent from each antenna can be coordinated so that the signal at the receiver is improved, even if the antenna is far from the sender
  • generally used when the client only has one antenna, and when the reflection sources are stable in space (a receiver not moving face indoors)
41
Q

Explicit Beamforming

A
  • requires the same capabilities in the AP and client
  • AP will dynamically gather information from the client for determining best path
42
Q

Implicit Beamforming

A
  • uses some information from the client at the initial association
  • improves the signal for older devices
43
Q

Cisco ClientLink

A
  • helps solve the problems of mixed-client networks by making sure that older 802.11a/n clients operate at the best possible rates, especially when near cellular boundaries
  • most 802.11ac APs only improve uplink performance, but ClientLink 3.0 improves both up and down
  • based on signal processing enhancements to the AP chipset and doesn’t require changes to the network
44
Q

Spatial Multiplexing

A
  • requires both transmitter and receiver are 802.11n/802.11ac MIMO capable
  • requires a miniumum of 2 receivers and a single transmitter per band, while supporting up to 4 transmitters and 4 receivers per band
  • this makes use of the reflected signals to improve reliability, whereas this used to be detrimental to legacy protocols
  • a signal stream is broken into multiple individual streams, each of which is transmitted from a different antenna, using its own transmitter
  • allows devices to send redundant info for better reliability or greater volume for improved throughput, or both
  • when a transmitter can emit over 3 antennas, is it described as having 3 data streams
  • when a transmitter can receive and combine signals from 3 antennas, it is described as having 3 receive chains
  • this combination is commonly denoted as 3x3 (read 3 by 3)
  • An 802.11ac environment allows more data by increasing the spatial streams up to eight. Therefore, an 80-MHz channel with one stream provides a throughput of 300 Mbps, while eight streams provide a throughput of 2400 Mbps. Using a 160-MHz channel would allow throughputs of 867 Mbps (one stream) to 6900 Mbps (eight streams).
45
Q

Spatial Diversity

A
  • each transmitting antenna sends a signal that follows a different path to the receiver
46
Q

802.11acMU-MIMO

A
  • multiple antennas transmit multiple frames to different clients
  • 802.11ac provides for a feature called MU-MIMO, where an AP can use its antenna resources to Tx multiple frames to up to four different clients all at the same time and over the same frequency spectrum.
47
Q

SU-MIMO

A
  • with 802.11n, a device can transmit multiple spatial streams at once, but only directed to a single address
  • for individually addressed frames, it means that only a single device (or user0 receives data at a time
48
Q

null-steering

A
  • in MU-MIMO, the AP will form a strong beam toward user1, but minimize the energy toward users 2 & 3
  • then user2 will have a strong beam with minimized energy toward users1 & 3, etc
  • in this way, each user receives a strong copy of the desired data, that is only slightly degraded by interference from data for the other users
49
Q

Stations

A

all wireless-capable devices in a network

50
Q

Access Point (AP)

A
  • close in concept to an Ethernet hub in that only one device can speak at a given time, on a given channel with the AP
  • functionas as a translational bridge between the 802.11 wireless media and 802.3 wired media
51
Q

802.11 Connections States

A

1) Not authenticated or associated
2) Authenticated, but not yet associated
3) Authenticated and associated

52
Q

Authentication + Association Message Flow

A

1) Mobile station sends probe request that advertises its own supported data rates and 802.11 capabilities to the destination L2 address and BSSID of ff:ff:ff:ff:ff:ff, all APs that receive it will response
2) APs receiving the probe request check to see if the mobile station has at least one commonly supported data rate. If so, a probe response is sent advertising the SSID, supported data rates, encryption types (if required) and other 802.11 capabilities of the AP
3) A mobile station chooses compatible networks from the probe responses that it receives
4) A mobile station sends a low-level 802.11 authentication frame to a compatible AP, setting the authentication to open and the sequence to 0x0001
5) The AP receives the authentication frame and responds to the mobile station with the authentication frame set to open, indicating a sequence of 0x0002
6) If an AP receives a frame other than an authentication or probe request from a mobile station that is not authenticated, it will respond with a deauthentication frame, placing the mobile into an unauthenticated and unassociated state. A mobile station can be 802.11 authenticated to multiple APs, but it can only be actively associated and transferring data through a single AP at a time.
7) Once a mobile station determines which AP it would like to associate with, it will send an association request to that AP, which contains chosen encryption types (if required), and other compatible 802.11 capabilities
8) If an AP receives a frame from a mobile station that is authenticated, but not yet associated, it will respond with a disassociation frame, placing the mobile inot an authenticated, but unassociated state.
9) If the elements in the association request match the capability of the AP, the AP will create an Association ID for the mobile station, and respond with an association response, a Success message granting network access to the mobile station
10) Now the mobile station is successfully associated to the AP, and data transfer can begin

53
Q

Basic Service Set Identifier (BSSID)

A
  • a 48-bit label that defines a group of devices that share wireless characteristics
54
Q

Service Set Identifier (SSID)

A
  • wireless network name
55
Q

Wired Equivalent Privacy (WEP)

A
  • 802.11 authentication frames that have been proven insecure, and therefore has become deprecated
56
Q

Split MAC

A
  • controller-based architecture model where 802.11 functions are split between the AP, which processes real-time portions of the protocol, and Cisco WLC, which manages items that aren’t time-sensitive
57
Q

Real-time Requirements of 802.11

A

1) Frame-exchange handshaking between client and AP when transferring a frame
2) Transmitting of beacon frames
3) Buffering and transmitting of frames for clients in a power-save operation
4) Responding to probe request frames from clients, forward notifications of received probe requests to the Cisco WLC
5) Providing real-time signal quality information to Cisco WLC with every received frame
6) Monitoring all radio channels for noise, interference, and other WLANs, and monitoring for the presence of other APs
7) Providing wireless encryption and decryption of 802.11 frames

58
Q

Cisco WLC Functions

A

1) Authentication (PSK, EAP, etc)
2) RF Management: control of RF space for APs and clients
3) Client IP Addressing
4) Seamless roaming: L2 or L3 client roaming
5) QoS for voice, video, and data
6) AP configuration management (VLANs, IPs, etc)
7) AP image management (current and consistent software levels across the enterprise)

59
Q

Cisco WLC Form-Factors

A

1) Cloud-base controllers
2) Dedicated hardware appliances running Cisco IOS XE Software
3) Dedicated virtual machines running Cisco AireOS
4) Modules (Cisco WiSMs) in switches running Cisco AireOS
5) Integrated in switches running Cisco IOS XE Software

60
Q

Control and Provisioning of Wireless Access Points (CAPWAP)

A
  • open protocol that enables a WLC to manage a collection of wireless APs
  • CAPWAP control msgs are exchanged between the WLC and AP across an encrypted tunnel
  • includes WLC discovery and joins process, AP configuration and firmware push from the WLC, and statistics gathering and wireless security enforcement
  • CAPWAP-capable APs are authenticated before being able to download any configuration from WLC
  • control plane messages are encapsulated in a DTLS tunnel
  • data plane is not encrypted by default, but encryption can be added using DTLS
  • client data is encapsulated with a CAPWAP header that contains info about the client RSSI and SNR, and then is sent to the WLC, which forwards the data as needed
  • between the AP and the controller, the src IP is the AP, the dst IP is the WLC AP Manager IP, and the dst port is UDP 5247
  • details of the client VLAN and WLAN are hidden inside the encapsulated payload
  • ## CAPWAP control destination UDP port 5246
61
Q

Datagram Transport Layer Security (DTLS)

A
  • the AP and WLC build the DTLS tunnel for the control plane messages to support wireless station access, authentication, and mobility
62
Q

Mobility Agent (MA)

A
  • MA and MC can run on the same Cisco WLC
  • has the responsibility to terminate CAPWAP tunnels
  • maintains the client database
  • configures and enforces security and QoS policies for wireless clients
63
Q

Mobility Controller (MC)

A
  • MA and MC can run on the same Cisco WLC
  • provides mobility management tasks, including roaming, Radio Resource Management (RRM), wireless intrusion prevention, and guest access
  • MA reports local and roamed client states to MC
  • builds database of client stations across all the MA
64
Q

Point of Presence (POP)

A
  • where the wireless user is seen to be within the wired portion of the network
  • ARP would show a wireless user coming from the WLC
  • POP anchors the client IP address, regardless of where the user roams
  • used for security policy application
65
Q

Point of Attachment (PoA)

A
  • is the WLC where the client AP CAPWAP is terminated
  • after roaming, the PoA may move to a different Cisco WLC, while the POP stays fixed
  • used for user mobility and QoS policy application

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