Chapter 6
At what network layer does 802.11 operate?
802.11 / Wi-Fi operates on Layers 1 and 2.
At what Service Band frequencies does 802.11 operate? Are they licensed? If not, what does this imply?
802.11 Wi-Fi WLANs technology today use two service bands: 2.4 GHz band and 5 GHz band. Each 802.11 channel was 20 MHz wide (40MHz introduced in 802.11n). 2.4 GHz: Centered on channels 1, 6, and 11. There are only three possible non-overlapping 20 MHz 802.11 channels. If nearby access points operate in same channel, their signals will interfere with each other unless the access points are far apart (AKA: co-channel interference). 5 GHz: Provides between 11 and 24 non-overlapping 20 MHz channels today, depending on the frequencies allocated to this service band in a particular country. More expensive, but better than 2.4 GHz. Makes it easy to have nearby access points operate on non-overlapping channels. Increasing channel bandwidth in newer standards reduces the number of possible channels. The 2.4 GHz band is unlicensed (and 802.11 Wi-Fi operates in unlicensed radio bands): difficult or impossible to put nearby access points on different channels. Hosts in these service bands do not need to be licensed to be turned on or used. Anybody can operate anywhere, allows access points to be moved freely. No legal recourse against interference from other nearby users, must negotiate. Cannot use illegally high power,
Name the currently defined 802.11 standards, their rated speeds, and at what service band they operate.
802.1g: 54 Mbps (rated speed), 2.4 GHz (unlicensed bands), and no channels in 5GHz band in the USA. 802.11a: 54 Mbps (rated speed), 5 GHz (unlicensed bands), 20-25 channels in 5GHz band in the USA. 802.11n: 100-600Mbps (rated speed), 2.4 GHz and 5 GHz (unlicensed bands), and 20-25 at 20 MHz (8-12 at 40 MHz) channels in 5GHz band in the USA. 802.1ac: 433 Mbps-6.93Gbps (rated speed), 2.4GHz and 5GHz (unlicensed bands), and 1-2 at 160MHz (4-6 at 80 MHz) channels in 5GHz band in the USA.
List and define wireless propagation problems: (1) Inverse Square Law of Attenuation, (2) Absorptive Attenuation, (3) Dead Zones, (4) Multi-Path Interference, and (5) EMI.
Already done
Compare and contrast 2.4 and 5 Ghz service bands.
Already done.
What is co-channel interference? At what service ban is it most problematic?
Already done.
Define the four characteristics of a Wave: Amplitude, Wavelength, Frequency, Phase.
Amplitude: The power of the wave. Wavelength: The physical distance between comparable points on adjacent cycles. (Optical fiber transmission is described in terms of wavelength.) Frequency: The number of cycles per second. (Radio transmission is measured in terms of frequency.) Phase: The position of a point in time (an instant) on a cycle.
Compare a BSSs to an ESSs. Why is a ECCs necessary? What would happen without it?
BSS = Basic Service Set : an access point and its wireless hosts, Service Set ID (SSID) is the name of the network. ESS = Extended Service Set : collection of access points that all have the same SSID. If both APs belong to the same ESS, the host can be handed off to that AP for service (in WLANs, called 'roaming'). If mobile host travels too far from a wireless AP, its signal will become too weak to reach the access point. However, if there is a closer AP, the host can be handed off.
What wave characteristic does Beam Forming manipulate? What is the result on this manipulation?
Beamforming allows an access point to focus its transmission and reception. Means that when the AP transmits to (or receives from) a wireless device, the signal will be stronger. The radio can operate at lower power or send the signal faster. Phase of wave is manipulated to create beamforming. Defined in 802.11ac and there is a single standard, so adoption is more likely. However, initial pre-standard 802.11ac products typically do not offer it.
CSMA/CA + ACK, define the term. Why does a 802.11 AP transmitting on a signal channel require it? Which mechanisms help avoid collisions? Is CSMA/CA + ACK efficient?
CSMA/CA + ACK = Carrier Sense Multiple Access with Collision Avoidance and Acknowledgement. Carrier Sense (CS): to listen to (sense) traffic. Multiple Access (MA): this method uses listening to control how multiple hosts can access the network to transmit. Quite simply, if another device has been transmitting, the wireless host or access does not transmit. Collision Avoidance (CA): the methods attempts to avoid two devices transmitting at the same time. ACK: if the receiver received a message correctly, it immediately sends an acknowledgement to the sender, not waiting at all. This is another reason to require station to delay before sending when a sender stops transmitting. Makes 802.11 Wi-Fi transmission reliable because it provides both error detection and error correction, is the only reliable transmission method (besides TCP). Not efficient because there is a lot of waiting with CSMA/CA + ACK. Transmitting in a single channel: Channel sharing is when the AP and all the hosts it serves transmit in a single channel. If two devices transmit at the same time, their signals will collide, becoming unreadable. MAC methods govern where devices transmit so that only one device transmits at a time.
What is bandwidth? What does more equal faster rated speed mean?
Channel bandwidth is the highest frequency in a channel minus the lowest frequency. Bandwidth is the range of frequencies within a given band, in particular that are used for transmitting a signal. An 88.0 MHz to 88.2 MHz channel has bandwidth of 0.2 MHz (200 kHz). Higher-speed signals need wider channel bandwidths.
In what units is frequency measured?
Hertz
What is MIMO? How does multi-path or signal diversity help MIMO?
Multiple Input / Multiple Output = a technique to double, triple, or even quadruple transmission speed (or even increase it more) without increasing channel bandwidth. Hosts or access points send two or more spatial streams (radio signals) in the same channel between two or more different antennas on access points and wireless hosts. Two signals in the same channel should interfere with each other, but the two spatial streams sent by different antennas will arrive at the two receiving antennas at slightly different times. Using detection and separation methods based on differences in arrival times for the two spatial streams in the same channel and read them individually. (NOT limited to two spatial streams.)
Name and define the use of the two general types of antenna.
Omnidirectional Antenna: signal spreads in all directions, rapid signal attenuation, no need to point at receiver. (best for short distances.) Dish Antenna: focuses signals in a narrow range, signals can be sent over longer distances, must point at receiver. Ex: not as easy to use.
Name the spread spectrum technique used by 802.11.
Orthogonal frequency division multiplexing (OFDM) - each broadband channel is divided into many smaller sub-channels (subcarriers), transmits part of a frame in each subcarrier, sends data redundantly across the subcarriers.
Define the range (lowest to highest) of the Frequency Spectrum.
Range of all possible frequencies is from 0 Hz to infinity.
CSMA/CA + ACK, what makes it reliable? Why does 802.11 need reliability? Why would we still need TCP when using a 802.11 network connection?
Reliable because sending acknowledgements and retransmissions makes 802.11 Wi-Fi transmission reliable because it provides both error detection and correction. Is the only reliable method besides TCP (Transmission Control Protocol). Most early DLL protocols were reliable because transmission then was unreliable, even in wired networks. Under these conditions, error correction is simply not worth the effort when transmission errors were rare.
What happens to the 802.11 packet at the AP? Does it travel to the eventual destination?
Sender puts a packet for the destination host into an 802.11 frame, then sends the frame wirelessly to the access point. The 802.11 frame has the wrong frame format to travel over an 802.3 Ethernet network. The switches and destination host would not know what to do with it. The access point removes the packet from the 802.11 frame and discards the frame. The AP encapsulates the packet in an 802.3 frame and sends this frame on to this destination host via Ethernet switches.
Why does 802.11 use Spread Spectrum transmission?
Spread Spectrum Transmission is transmission that uses far wider channels than transmission speed requires. In WLANs, Spread Spectrum transmission is used to reduce propagation problems, not to provide security.
What is the governing body for 802.11 standards?
Standardized by the 802.11 Working Group of the IEEE 802 Committee.
What are the reasons a hybrid wired / wireless network is generally required for a good wireless network design?
The LAN connection is needed to give clients access to servers and Internet access routers on the wired LAN. The wireless access point connects the wireless client to the wired Ethernet LAN. UTP -(UTP)- AP-(radio)-Client Server^^Router ^Internet
Compare wired and wireless signal propagation. How does the path differ? How does a wireless signal react to obstacles? What happens attenuation-wise as frequency rises?
Wireless transmission has many propagation problems. Electromagnetic Interference (EMI) is unwanted power at the same time frequency from other devices. Shadow Zones are places the signal cannot penetrate because of obstacles in its path. AKA: dead spots (grow worse as frequency increases). Inverse Square Law of Attenuation: after a certain distance, signal starts to die out. Absorptive Attenuation: partially absorbed by air molecules, plants, and other things it passes through. Multi-Path Interference: bounce off ceilings, walls, etc, so receives two signals - direct and reflected. As frequency rises, attenuation grows worse. Most serious propagation problem at WLAN frequencies: Multi-path Interference (direct and reflected signals may interfere and if, two waves are out of phase, they may negate each other, giving no signal.)
