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Computer Networks > Wireless Networks > Flashcards

Wireless Networks Flashcards

1
Q

What are the wireless hosts in Wireless Network

A

laptop, smartphone etc.

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2
Q

What is the base station in Wireless Network

A

The base station is typically connected in a network, responsible for sending packets between wired network and wireless host in its area

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3
Q

What is the wireless link in Wireless Network

A

typically used to connect mobile(s) to base station. multiple access protocol coordinates link access

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4
Q

Is wireless link used as a backbone link?

A

Yes

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5
Q

What is the infrastructure mode in Wireless Network

A

base station connects mobiles into wired network
▪ handoff: mobile changes base station providing connection into wired network

base stations are connected to the infrastructure mode

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6
Q

What is the ad hoc mode Wireless Network

A

no base stations, nodes can only transmit to other nodes within link coverage. nodes organize themselves into a network: route among themselves

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7
Q

What are the factors that make wireless link more difficult?

A
  1. decreased signal strength
  2. interference from other sources
  3. multipath propagation
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8
Q

What is SNR? if SNR is large, what does it mean?

A

SNR: signal-to-noise ratio

• larger SNR – easier to extract signal from noise (a “good thing”)

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9
Q

Describe SNR vs BER tradeoffs

A

given physical layer: increase power -> increase SNR- >decrease BER (Bit Error Rate)
• given SNR: choose physical layer that meets BER requirement, giving highest throughput
• SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate)

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10
Q

Describe Hidden Terminal Problem

A

▪ B, A hear each other
▪ B, C hear each other
▪ A, C can not hear each other means A, C unaware of their interference at B

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11
Q

Describe Signal Attenuation

A

▪ B, A hear each other
▪ B, C hear each other
▪ A, C can not hear each other interfering at B

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12
Q

Tell encoded signal formula in CDMA

A

encoded signal = (original data) X (chipping

sequence)

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13
Q

Direct Sequence Spread Spectrum is in which protocol?

A

802.11b

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14
Q

Which protocol has 2.4-5Ghz, and up to 11Mbps?

A

802.11b

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15
Q

Which protocol has 2.4-5Ghz, and up to 200Mbps?

A

802.11n

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16
Q

Which protocol has 5-6Ghz, and up to 54Mbps?

A

802.11a

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17
Q

Which protocol has 2.4-5Ghz, and up to 54Mbps?

A

802.11g

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18
Q

All protocols use ___ for multiple access

A

CSMA/CA

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19
Q

Base station is present in what protocol?

How about ad-hock networks?

A

All

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20
Q

Is base station an access point in IEE 802.11 protocols?

A

Yes

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21
Q

What is included in BSS(Basic Service Set)

A

Wireless hosts, access point, ad hoc mode: hosts only

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22
Q

802.11b is divided into how much channels?

A

11

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23
Q

What is the name for access point identifiers?

A

SSID

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24
Q

Apart from SSID, what else should access points have according to the protocol

A

MAC Address

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25
Q

How does CSMA/CA avoid collisions?

A

Prepared RTS(ready to send) message, that includes destination address and duration of message. Tells everyone to backoff. After that Destination sends CTS(clear to send) message. Other stations set their Network Allocation vector and wait for that duration

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26
Q

Can CSMA/CA detect collisions?

A

No

27
Q

What are the two ways to scan the channels?

A

Passive/acting scanning.

28
Q

Explain passive scanning

A
  1. beacon frames sent from AP
  2. request frame from H1 to AP sent
  3. response from AP to H1 sent
29
Q

Explain active scanning

A
  1. probe request frame broadcast from h1
  2. probe response sent from APs
  3. associated request frame from h1 to selected AP
  4. associated response from selected AP to h1
30
Q

Why there is no collision detection in IEEE 802.11?

A

Because of the weak signals from the wireless links

31
Q

List DCF components

A

Carrier sense, interframe spaces, backoff timer, frame transmission

32
Q

What kind of Carrier Sense types exist? Describe each of them

A

Physical and Virtual
Physical Carrier Sense uses Clear Channel Assessments stations check wireless medium to detect use. Two types: Carrier Sense(CS): detects signal strength
Energy Detection(ED): measures RF energy in the channel
Virtual Carrier Sense has NAV timer to calculate transmission opportunity time

33
Q

What kind of Backoff Timer types exist? Describe each of them

A

Contention Window, Random Backoff
Contention Window includes a range of numbers from which stations randomly select backoff value
Random Backoff: number randomly chosen from Contention Window.

34
Q

What interframe space Highest Priority Frames use?
What interframe space Medium Priority Frames use?
What interframe space Asynchronous Data Frames use?

A
  1. short IFS, 2. Point Coordination Function IFS, 3. Distributed Coordination Function IFS
35
Q

Arbitration Interframe Spaces are used by?

A

Used by Quality of Service stations

36
Q

List and describe components of cellular network architecture?

A

cell: covers geographical region; base station (BS) analogous to 802.11 AP. mobile users attach to network through BS. air-interface: physical and link layer protocol between mobile and BS

37
Q

Mobile Switching Center responsibility is?

A

❖ to connect cells to wired tel. net.
❖ to manage call setup
❖ to handle mobility

38
Q

What are the two ways for sharing mobile radio spectrum?

A
  1. FDMA/TDMA

2. CDMA

39
Q

What is 1G?

A

First generation. Supports voice calls only. Uses FDMA. analog modulation

40
Q

What is 2G?

A

Second. Supports voice and data services. Uses TDMA and Digital Modulation

41
Q

Mobile station components?

A

Mobile equipment and SIM

42
Q

Base stations subsystem components?

A
  1. Base Tranceiver Station:
    * sends & receives signals from MS
    * uses multiplexing, encryption, modulation
  2. Base Station Controller
    * controls BTS
    * allocates radio channels
    * handovers from one BTS to another
43
Q

Network switching subsystem components?

A

MSC(Mobile switching Center):

  • heart of GSM
  • manages mobile services
44
Q

Key insight of 3G?

A

New cellular data network operates in parallel with existing cellular voice network.
voice network unchanged, data network operates in parallel

45
Q

What is the difference between LTE and 3G?

A
  1. IP packets tunneled (through core IP network) from base station to gateway
  2. no separation between voice and data – all traffic carried over IP core to gateway
46
Q

What are the 5 5G Technologies?

A

Millimeter Waves, Small Cells, Massive MIMO, Beamforming, Full duplex

47
Q

Explain Millimeter Waves

A

Expanding the frequency range.

Has problem of being absorbed by nature or not being able to travel the obstacles: solved by small cells

48
Q

Explain small cells

A

Put behind the obstacles so that the signal does not dissapear

49
Q

Explain massive MIMO

A

MIMO: Massive Input Massive Output. 7 times more ports increase the capacity of networks. Problem: massive interference. Solved by beamforming

50
Q

Explain beamforming

A

Serves as a traffic light for signals. Solved the interference

51
Q

Explain full duplex

A

Allows the signals to be sent in both ways at once.

52
Q

ZigBee is?

A

Is a wireless technology standard that finds a set of communication protocols for short range communications

53
Q

Zigbee is a standard that addresses the need of _____ implementation of ___ with ___ data rate and ___ range wireless communication

A

very low cost; low-power devices; low; short

54
Q

List ZigBee applications?

A
  1. Home automation
  2. Medical data collection
  3. Industrial Control Systems
55
Q

Purpose of ZigBee?

A
  1. Collect Information

2. Perform Control Tasks inside a building

56
Q

Give example where ZigBee can be applied and Bluetooth or Wifi cannot?

A

Remote Monitoring System can be applied by Zigbee.
Wi-Fi too powerful
Bluetooth has max 7 devices

57
Q

List three frequency bands assigned to ZigBee

A

Europe: 868Mhz
US and Australia: 915 Mhz, Channels 1-10
Across the world: 2.6Ghz, Channels 11-26

58
Q

List Zigbee devices

A

Coordinators, Routers, End Devices

59
Q

Explain Coordinator in Zigbee

A

Coordinator: most capable device, root of the network, only one
Can perform: channel selection, assign ID to network, allocates unique address to device, initiates and transfers messages

60
Q

Explain Router in Zigbee

A

Routers: intermediate node between coordinator and the end devices. Route traffic between nodes. Receives and store messages intended for their messages, allow other routers and end devices to join the network

61
Q

Explain End devices in Zigbee

A

End Devices: contains enough info to talk to parent, they may sleep(suitable for battery devices), all traffic to end devices is routed to its parent. Responsible for requesting any pending messages from parent

62
Q

Which topologies are used in Zigbee

A

Mesh, star

63
Q

Describe channel accessing in Zigbee

A

Two types: contention-free and contention based methods. In first, coordinator dedicates specific timeslot to each devices(guaranteed time slot).
In second: no synchronization, CSMA/CA.
Step by step: go to receive mode, check for signals, transmit if channel is clear. else backoff

Computer Networks (6 decks)

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