Wireless and mobile networks Flashcards
(143 cards)
1
Q
Which 3 functions are necessary for a DCF?
A
NAV, RTS/CTS, CSMA/CA.
2
Q
How does the IEEE 802.11 ad hoc mode work?
A
The nodes can communicate directly with each other, meaning that one node can send a packet to another. It is decentralized.
3
Q
How does the IEEE 802.11 infrastructure mode work?
A
In the infrastructure mode, an access point (AP) is necessary. The nodes communicate with each other through the AP. It is centralized. The client sends and receives its packets through the AP.
4
Q
Which IEEE 802.11 work on the 2.4 GHz ISM band?
A
802.11b, 802.11g, 802.11n.
5
Q
Which IEEE 802.11 work on the 5 GHz UNII band?
A
802.11a, 802.11n, 802.11ac.
6
Q
How many channels are there in the ISM band?
A
14, but only 13 used in Europe.
7
Q
How many non-overlapping channels are there in the 5 GHz UNII band? Which ones?
A
24.
UNII-1: 36, 40, 44, 48
UNII-2: 52, 56, 60, 64
UNII-2e: 100-144
UNII-3: 149, 153, 157, 161, 165
8
Q
How many devices can there be in a Bluetooth piconet?
A
Up to 7 devices/slaves and one master.
9
Q
Which channels don’t overlap in the 2.4 GHz ISM band?
A
1, 6, 11.
10
Q
Make a comparison of 2.4 GHz band and the 5 GHz band regarding Wi-Fi and range.
A
2.4 GHz:
- Plauged by non-Wi-Fi interference since Bluetooth,
microwaves etc operate on the same channel.
- Longer range.
- Universal compatibility.
5 GHz:
- Very little non-Wi-Fi interference.
- Shorter range.
- Limited compatibility (802.11a/n/ac).
11
Q
Which devices use the 2.4 GHz ISM band?
A
Microwaves, Bluetooth.
12
Q
What does FHSS stand for and what is the purpose of it?
A
FHSS = frequency hopping spread spectrum. A signal modulation method, introduced in the IEEE 802.11 base version.
The purpose of FHSS is to avoid interference, to prevent eavesdropping and to enable code-division multiple access (CDMA).
13
Q
What does DSSS stand for and how does it work?
What about the bandwidth?
A
DSSS = direct-sequence spread spectrum. A modulation method used to reduce overall signal interference. DSSS makes the transmitted signal wider in bandwidth than the information bandwidth.
14
Q
How does beamforming work? Can you give one example?
A
Beamforming has directional transmission, meaning that the signal is stronger. A flashlight is a type of beamforming since it only sends light in one direction.
15
Q
How does CSMA/CA work?
A
Carrier-sensing multiple access/collision avoidance. The goal is to avoid collisions from occurring, not detecting them.
Nodes attempt to avoid collisions by starting transmission after the channel is sensed to be idle. Important for wireless networks.
16
Q
What is the objective of power management?
A
The objective is to increase the battery life.
17
Q
How many channels does Bluetooth have?
A
79
18
Q
How does a piconet work and where is it mainly used?
A
A piconet is a small ad hoc network, used in Bluetooth. It has one master and up to 7 active slaves. It can have a total of 255 slaves, but not in active status at the same time. The master can put a parked slave in active mode by inactivating another.
19
Q
What is the bandwidth of each channel in the 2.4 GHz ISM band?
A
22 MHz.
20
Q
What does NAV stand for and how does it work? Where is it used?
A
NAV = network allocation vector. It should be thought of as a counter, counting down to zero (0) at a uniform rate. Used in DCF.
NAV is a virtual carrier-sensing mechanism used with Wi-Fi.
21
Q
What does RTS/CTS stand for and how does it work? Where is it used?
A
RTS = request to send. CTS = clear to send. RTS/CTS is used in DCF mode. A frame sends a RTS to see if the medium is idle, if it is then the frame is transmitted, CTS. If the medium is not idle, the medium waits for a random amount of time and then it sends a new RTS.
22
Q
What is spatial multiplexing (SM) and how does it work?
A
SM is a multiplexing technique used in MIMO. Simultaneous transmission of streams by different antennas, all operating on the same channel.
23
Q
What is a basic service set (BSS)?
A
A BSS is a group of stations, with only one access point (AP). The AP is considered as the master that controls all wireless devices within the BSS.
BSS is a small range network –> more secure.
24
Q
What is an extended service set (ESS)? Range?
A
An ESS is a group of BSS’s, which means that an ESS had more than one access point (AP). ESS is a long range network –> less secure than BSS.
25
What happens when a user walks from one side of the building to another in terms of changing AP's?
The user will stay connected to the same network, but the user will go from one AP to another and she will also get a new SSID.
26
What is roaming?
Roaming is when a user changes network location, meaning that the user will connect to a new network and therefore a new AP. Roaming is when a phone is used outside the range of its native network and connecting to another available network.
27
What is the hidden terminal problem?
The hidden terminal means that there are stations hidden to one another. This means that the two stations can transmit a package at the same time to a third station in the middle, and collision will occur at the middle station since the outer two stations cannot sense each other and do therefore not know that the other one is already transmitting a package.
28
What is IFS and how does it work?
IFS = interframe spacing. IFS is a period of time where the medium is idle. IFS is the time from the end of one frame to the beginning of the next one.
IFS is part of a greater process which is used to avoid collisions with other frames.
29
How does SIFS work and what does it stand for?
SIFS = short interframe spacing. It is the difference in time between the first symbol of the response frame in the air and the last symbol of the received frame in the air. It has higher priority.
SIFS < DIFS.
30
How does DIFS work and what does it stand for?
DIFS = DCF interframe spacing.
DIFS is the time interval that a station should wait before it sends its request frame. It is the minimum medium idle time.
31
Which access method do the IEEE 802.11 networks use?
802.11b/a/g/n/ac all use CSMA/CA as access method.
32
Which is the maximum data rate of 802.11 base version?
2 Mbps.
33
Which is the maximum data rate of 802.11b?
11 Mbps.
34
Which is the maximum data rate of 802.11a?
54 Mbps.
35
Which is the maximum data rate of 802.11g?
54 Mbps.
36
Which is the maximum data rate of 802.11n?
600 Mbps.
37
Which is the maximum data rate of 802.11ac?
6.9 Gbps.
38
What is the WLAN architecture?
WLAN consists of a distribution system (DS), BSS with their APs and stations(computers), a portal and all of these elements are part of an ESS.
39
Which protocols belong to the physical layer?
PMD = physical medium dependent and
PLCP = physical layer convergence protocol
40
Which protocols belong to the data link control layer?
MAC = medium access control and
LLC = logical link control
41
What is a distribution system (DS)?
DS has a centralized computer system in the middle, other computer systems connected to it and then the DS software where it stores databases.
42
What does the MAC layer do?
Encryption, fragmentation, numbering, addressing.
43
What does the PMD layer do?
Online coding and modulation.
44
What does the PLCP layer do?
PLCP listens to the carrier and unit adaption of the MAC to PMD data.
45
What does the LLC layer do?
It controls the logic parts such as synchronization, multiplexing and flow control.
46
What does a service access point (SAP) do?
SAP requests the service of another protocol layer, for example can the MAC layer request service from the PHY layer.
47
What does MIMO stand for and how does it work?
MIMO = multiple-input, multiple-output.
MIMO is when you multiply the capacity of a radio link using the transmission and receiving antennas to exploit multipath propagation.
Used mainly in IEEE 802.11n and 802.11ac.
MU-MIMO only works in the 5 GHz in 802.11ac. Only affects the download link, not the upload link.
48
What does PCF stand for and how does it work?
PCF = point coordination function.
PCF > DCF.
49
What does DCF stand for and how does it work? In which layer is DCF used?
DCF = distribution coordination function.
DCF is the fundamental MAC technique of WLAN.
A station which only uses DCF might not gain access to the medium due to the fact that PFC > DCF. The solution to this is to use a repetition interval which covers both the contention free PCF and contention based DCF.
beacon frame - PCF - DCF
-------------CFP------------|CP
50
What are the advantages and disadvantages of using wireless communication instead of Ethernet?
Advantages: more robust to natural disasters since it doesn't require cables, less costly since there is no cables.
Disadvantages: wireless communication is less secure, interference more likely.
51
Differences between Bluetooth and Wi-Fi?
Bluetooth:
- Used to connect different devices to each other.
- Usually no password --> less secure.
- Shorter range than Wi-Fi, usually 10 m.
- Slower data rate than Wi-Fi.
- Bluetooth uses less power --> smaller batteries.
- Less vulnerable to interference, since it uses FHSS.
Wi-Fi:
- Used to connect devices to internet.
- Usually requires a password.
- Longer range than Bluetooth, 30-91 m.
- Faster data rate than Bluetooth.
- Wi-Fi uses more power --> more costly.
52
What is ad hoc VANET mode and when is it used?
VANET = vehicular ad hoc network.
It is used to improve road safety by improving communication between the vehicles and the road.
53
What does WSN stand for? When is it used?
WSN = wireless sensor network.
Applications: surveillance and monitoring for security.
54
What does WMN stand for? When is it used?
WMN = wireless mesh network.
Applications: medical and industrial monitoring.
55
What does OFDM stand for? What is it?
OFDM = orthogonal frequency division multiplexing.
If we have three waves/signals, when one of them is at its peak, the other two will be at null.
It is a type of digital transmission and a method of encoding digital data on multiple carrier frequencies.
56
What are the layers of the OSI model called?
The OSI model contains 7 protocol layers.
1. Physical layer
2. Data link layer
3. Network layer
4. Transport layer
5. Session layer
6. Presentation layer
7. Application layer
57
What is clear channel assessment (CCA)?
CCA is when the station checks if the medium is idle or not, so it knows if it can transmit a package or if it has to wait.
58
When is the contention period (CP) used?
CP is used for "best-effort" traffic.
59
Which is the only mode that DCF and PCF can be used?
Infrastructure mode.
60
When is the contention free period (CFP) used?
CFP is used for traffic with constraints of delays and flow rates.
61
How does an access point (AP) work?
An AP is connected to either a router or a switch. The router/switch enables the AP to have an internet connection which it transmits as a Wi-Fi signal to specific areas.
62
Which devices can use Bluetooth?
Mobiles, speakers, cars...
63
Which radio technology does Bluetooth use?
FHSS = frequency-hopping spread spectrum.
64
What kind of protocol is Bluetooth?
It's a packet-based protocol with a main/follower structure.
65
What are the advantages and disadvantages with MIMO?
Advantages: link robustness.
Disadvantages: additional processing, cost, size.
66
What is the solution to the hidden terminal problem?
The solution is to use RTS/CTS, i.e., nodes send short packets to request permission of the AP to send longer data packets.
67
What is the main purpose of RTS/CTS?
Avoiding frame collisions introduced by the hidden node problem.
68
What is CSMA/CA and where does it operate?
CSMA/CA is a protocol that operates in layer 2 (data link layer).
69
How are BSS's connected in an ESS?
Through a distribution system (DS).
70
In what is a MAC frame encapsulated when being sent?
In a PLCP frame.
71
How can the physical layer be implemented?
Through a PHY chip.
72
What does the data link layer do? Mention a type of a data link protocol.
Ethernet is a type of a data link protocol.
This layer is concerned with local delivery of frames between nodes on the same level of the network.
73
Which OSI layers are responsible for the software?
The three upper ones, i.e., application, presentation and session layer.
74
Which OSI layers are responsible for the hardware?
The three lower ones, i.e., physical, data link and network layers.
75
How can WLAN provide connection to the wider internet?
Through a gateway.
76
Which are the two categories of wireless stations within a WLAN?
1. Wireless access points (WAP).
2. Clients.
77
What are the advantages of a distribution system?
Shorter response time and higher throughput,
better price performance ratio and flexibility,
resource sharing.
78
Give some examples of wireless networks.
WLANs, cell phone networks, satellite communication network, microwave network.
79
How does FHSS work?
FHSS is a method of transmitting radio signals by rapidly changing the carrier frequency. The changes are controlled by a code known to both transmitter and receiver.
80
What are the applications of OFDM?
Digital television, audio broadcasting and 4G/5G mobile communications.
81
What are the advantages of OFDM?
It can handle severe channel conditions (such as attenuation of HF in a long copper wire) and
robust against intersymbol interference (ISI).
82
Give an example of a piconet.
A cell phone connected to a computer.
83
Why is the 2.4 GHz ISM band crowded?
Since it is also used by baby monitors, Bluetooth, microwave oven and so on.
84
What is the gap between adjacent channels in the 2.4 GHz ISM band?
5 MHz.
85
What is an advantage with the 2.4 GHz ISM band over the 5 GHz UNII band?
The 2.4 GHz ISM band has lower attenuation given that the energy is the same. Lower attenuation means lower frequency signal --> would travel a longer distance.
86
How does the 802.11 protocol family employ CSMA/CA?
It employs CSMA/CA where equipment listens to a channel for other users before transmitting each frame.
87
Are the 5 GHz UNII band and the 2.4 GHz ISM band license free or license based?
Both are license free frequency bands which means that everyone can operate on them.
88
Where are the UNII-1 and UNII-2 bands used?
Indoor.
89
Where is the UNII-2e used?
Outdoor.
90
Where is the UNII-3 used?
It is prohibited.
91
Where does MANET stand for and where is it used? Why is it autonomous?
MANET = mobile ad hoc network.
Application: urgent intervention operation.
MANET is autonomous because each node acts as host and router.
92
How does a mesh network work?
A mesh network is a group of devices, such as Wi-Fi routers that act as a single network.
93
What is fading?
Fading is a variation of the attenuation of a signal with various variables such as time, position and radio frequency.
94
What is multipath?
Multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths.
95
What is jamming?
Jamming in wireless networks is defined as the disruption of existing wireless communications, by decreasing the signal-to-noise ratio.
Denial of service (DoS) can easily be realized by jamming.
96
Which layers does the WLAN architecture cover?
The physical and the data link layers.
97
How does WLAN work?
- Shared bandwidth.
- Half-duplex.
- Prevent collisions by listening to other nodes in the same medium.
98
Describe the two types of PLCP.
PLCP has one short frame and one long one. The synchronization of the long one has 128 bits while the synchronization of the short one has 56 bits. Used in the 802.11 standard.
99
What is the MAC management responsible for?
Clock synchronization, handover, MIB.
100
What is the PHY management responsible for?
Selection of transmission channel, MIB.
101
What controls CFP and CP?
PCF and HCF.
102
What is a contention window (CW)?
CWmin = 15 and CWmax = 1023 historically. These values are configurable but the standard defines the values by default.
CW is a protocol order parameter fixed by the standard.
103
Which are the 3 types of 802.11 frames? What are they used for?
Management: manages the BSS.
Control: controls the access to the medium.
Data: contain payloads that are layer 3-7 information.
104
Which are the fields in the 802.11 frame? How many bytes do they each have?
1. Frame control - 2
2. Duration/ID - 2
3. Address 1 - 6
4. Address 2 - 6
5. Address 3 - 6
6. Sequence control - 2
7. Address 4 - 6
8. Data - 0-2312
9. CRC - 4
105
What is scanning in an 802.11 network?
Determine available APs. There is passive scanning in which the station passes the channel and stores the beacon frame and active channel in which the station sends a probe frame in the channels.
106
What is selection in an 802.11 network?
Choose which ESS and AP to use.
107
What is authentication in an 802.11 network?
The AP approves access the station to access the wireless network.
108
What is association in an 802.11 network?
See what mutual possibilites a node has.
109
What is handover management used for?
Handover management is used to maintain connection with mobile terminals, it's a three stage process.
First stage: identify the need for a handover.
Second stage: new resources for the handover must be found.
Third stage: data flow control, where the delivery of data from the old connection to the new connection is maintained.
110
What are the two modes of power management?
The normal (awake) mode and the energy saving (sleep) mode.
The stations in energy saving mode pass in the awake mode periodically and listen for the beacon frame before it transmits a frame.
111
What does EDCA stand for and what is it? Where is it used?
EDCA = enhanced distributed channel access.
With EDCA, high-priority traffic has a higher chance of being sent than low-priority traffic. A station with high-priority traffic waits a little less before it sends its packet than a station with low-priority traffic.
Used in the 802.11e MAC protocol.
112
What is an advantage with EDCA?
EDCA improves the DCF by changing IFS, CW, CFP.
113
What does HCCA stand for and what is it? Where is it used?
HCCA = hybrid controlled channel access.
Works a lot like PCF, but in contrast to PCF, in which the interval between two beacon frames is divided into two periods of CFP and CP, HCCA allows for CFPs being initiated at almost any time during a CP.
Used in the 802.11e MAC protocol.
114
Why is 802.11a not compatible with 802.11b/g?
Because they don't use the same frequency range, 802.11a uses the 5 GHz UNII band while the other two use the 2.4 GHz ISM band.
115
What data transfer rate does the IEEE 802.11g usually reach?
Between 24-31 Mbps.
115
What data transfer rate does the IEEE 802.11g usually reach?
Between 24-31 Mbps.
116
Mention an advantage with spatial multiplexing.
It increases the data rate.
117
What is space-time block coding and where is it used?
It is used in MIMO. It is simultaneous transmission and by redundancy of multiple antennas of the same flux by using two techniques of different coding.
118
Where is beamforming used?
Used for radio and sound waves.
119
What are the advantages of DSSS?
Resistance to jamming, determination of relative timing between transmitter and receiver.
120
What are the advantages and disadvantages with the 2.4 GHz ISM band?
Advantages: farther range, can penetrate solid objects.
Disadvantages: vulnerable to interference, slower speed.
121
What are the advantages and disadvantages with the 5 GHz UNII band?
Advantages: higher transfer speed, less vulnerable to interference.
Disadvantages: harder time penetrating solid objects, shorter range.
122
What is the bandwidth of the 5 GHz UNII band?
20 MHz, but two can be combined and then the bandwidth will be 40 MHz and so on.
123
There is no overlapping in the 24 channels in the 5 GHz UNII band, what are the advantages with that?
Less congestion which will most likely give more stable connections with higher data transfer rate.
124
What is the purpose of adding two 5 GHz UNII bands together?
To increase the bandwidth.
125
Between which two types of a network does an AP connect?
Between a wired network and a wireless network.
126
What does an AP handle?
It handles frames and MAC addresses.
127
What is the BSSID?
It is the AP's physical/MAC address. BSSID is a 48-bit long hexadecimal number.
128
Why is Bluetooth less vulnerable to interference than Wi-Fi?
Since it changes channel 1600 times per second.
129
What are the advantages and disadvantages with WLAN?
Advantages: devices connect wirelessly, supports many devices, device flexibility, easy to install.
Disadvantages: less secure, slower speed, difficult to set up, limited range, interference from physical barriers.
130
What does the length of the IFS depend on?
Previous frame type, following frame type etc.
131
How many non-overlapping channels does 802.11a have?
8 indoor.
132
Can there be several AP's on one channel? If yes, are there any disadvantages?
Yes, there can be several AP's on one channel but that can affect the network performance and cause dropped connections or lost packets while using the internet.
133
Which four speeds can 802.11b operate at?
1, 2, 5.5, 11 Mbps.
134
What is dual band Wi-Fi?
It's when a router uses both the 2.4GHz band and 5 GHz band.
135
Which channel does a Wi-Fi router usually use as default?
Channel 6.
136
Which 802.11 networks are in the physical (PHY) layer?
802.11a, 802.11b, 802.11g.
137
What is the IEEE 802.11 system?
The world's most popular WLAN system.
138
Which modulation technology does 802.11b use?
DSSS.
139
Which modulation technology does 802.11a use?
OFDM.
140
Is it hardware or software that is implemented in the PHY layer and the MAC layer respectively?
PHY: only hardware.
MAC: a combination of hardware and software.
141
What is throughput in terms of networks?
Throughput is a measure of how many units of information a system can process in a given amount of time.
142
What is performance anomaly of 802.11?
The throughput of all hosts transmitting at the higher rate is degraded below the level of the low rate.
