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Systems and Networks > Packet switching and network technologies > Flashcards

Packet switching and network technologies Flashcards

1
Q

What is circuit switching?

A

A communication mechanism that establishes a distinct path between the sender and receiver before sending data

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2
Q

What does circuit switching guarantee?

A

isolation from paths used by other senders and receivers

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3
Q

What technology is circuit switching usually associated with?

A

Analogue telephone technology

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4
Q

In regard to circuit switching, does each circuit correspond to a physical path?

A

No (virtual circuits or physical paths)

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5
Q

What are the 3 general properties of a circuit switched paradigm?

A
  • Point to point communication
  • separate steps for circuit creation, use and termination
  • performance equivalent to an isolated physical path even though it isn’t
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6
Q

What is the main alternate for circuit switching? What does it form the basis of?

A

Packet switching
- forms the basis of the internet

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7
Q

Generally, what method does packet switching use?

A

Statistical multiplexing (communication from several sources competes for use of shared media)

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8
Q

What does a packet switching system require of the sender?

A

The sender needs to divide each ‘message’ into blocks of data (packets)

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9
Q

In packet switching - is packet size fixed?

A

The size of the packet varies depending on the packet switching tech

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10
Q

What are 3 general properties of packet switching?

A
  • arbitrary, asynchronous communication
  • no setup needed before communication starts
  • performance varies due to statistical multiplexing among packets
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11
Q

What are two advantages of packet switching?

A
  • Helps in detecting transmission errors
  • Gives fair access for a shared connection between many computers
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12
Q

What type of multiplexing does packet switching use?

A

Time division multiplexing

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13
Q

What are pros and cons of shared communication channels?

A
  • low cost
  • not suitable for a wide area as delays inhibit coordination of sharing the network
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14
Q

When were shared LANs invented?

A

1960s

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15
Q

What do LAN, MAN, WAN stand for?

A
  • local area network (single room or building
  • Metropolitan area net.. (major city)
  • Wide area.. (spans sites in multiple cities)
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16
Q

What are some examples of LAN topologies?

A

star (hub in centre)
ring (each connected to 2 others)
mesh (4 connected to all others)
bus (shared cable)

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17
Q

What are the pros and cons of a a star LAN topology?

A

More robust but may be a bottleneck

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18
Q

What are the pros and cons of a ring LAN topology?

A

Enables easy co-ordination but sensitive to a cable being cut

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19
Q

What are the pros and cons of a bus LAN topology?

A

Requires least wiring but is also sensitive to cable being cut

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20
Q

What is the main problem with mesh LAN topologies?

A

Poor scalability, 3 links for 3, 6 links for 4, etc etc

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21
Q

What LAN topology is a non switched ethernet cable an example of?

A

Bus network (single cable to which computers connect)

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22
Q

What is a mobile ad-hoc network an example of?

A

A mesh network

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23
Q

What are the features of a mobile ad-hoc network (MANET)?

A

-self configuring
- each node has to discover the network shape/topology
- announces it’s presence and listens for broadcasts

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24
Q

What distinguishes physical and logical topology?

A

Technology defines logical topology
Wiring scheme defines physical topology

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25
Q

What is the physical and logical topology of an twisted pair ethernet cable?

A

Logically - bus
Physically - star

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26
Q

Who specifies the scheme for hardware adressing?

A

The IEEE

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27
Q

What are the different terms for a hardware address?

A
  • media access control address (MAC address)
  • Ethernet address
  • NOT IP address
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28
Q

How many bits is a hardware address?

A

48

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29
Q

What do different types of hardware have to define for packets?

A

A format/wrapping called a frame

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30
Q

What distinguishes packets and frames?

A
  • Packet is logical
  • Frame is actual
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31
Q

In a packet switched network, each frame corresponds to a…..

A

Packet

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32
Q

What are the two conceptual parts of a frame?

A

Header (metadata e.g. address)
Payload (contains actual data)

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33
Q

What does CRC stand for?

A

cyclic redundancy check

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34
Q

What does an ethernet frame consist of?

A

-fixed length header
- variable length payload
- fixed length crc

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35
Q

What is byte stuffing?

A

Special characters at the start and end of frames are reserved for control characters, the transmitter scans data and replaces control characters with specific characters, receiver inverse maps this

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36
Q

IEEE divides layer 2 (network interface) into 2 sub layers, what are they?

A

LLC (logical link control) and MAC (media access control)

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37
Q

What are the 3 subdivisions of Multi-Access protocols? (protocols that control access to a shared medium)

A
  • Controlled Access protocols
  • Random Access protocols
  • Channelisation protocols
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38
Q

What are the 3 types of Controlled Access protocols covered in this module?

A

Polling, Reservation, Token passing

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39
Q

Briefly describe Controlled Access protocol Polling.

A
  • A centralised network controller cycles through all stations on the network and gives an opportunity to transmit a packet (option to give stations higher priority)
40
Q

Briefly describe Controlled Access protocol Reservation.

A

-Uses a centralised controller
- Each potential sender says whether they need to send a packet during the next round and controller sends a list of transmitting stations
- this list is used by stations to know when to transmit

41
Q

Briefly describe Controlled Access protocol token passing.

A

A token is passed around continuously until a station needs to send a packet, it holds onto the token while doing this then sends it on again once finished.

42
Q

What are the two Random access protocols covered in this module.

A

CSMA/CD and CSMA/CA
- Carrier Sense Multi-Access with Collision Detection (CD) or Collision Avoidance (CA)

43
Q

When are CSMA/CD and CSMA/CA used?

A

CD - most widely used and basis of ethernet
CA - basis for wireless networks

44
Q

What is a collision? What do computers detect?

A

When multiple computers transmit at once, detected by other computers as a garbled tranmission

45
Q

What do computers normally do to recover after transmission collision?

A

Wait for a random delay period up to a specified max
- Doubling the max delay for each subsequent collision

46
Q

How does offered traffic affect throughput?

A

More offered traffic means more collisions means more backing off (delay after collision) means more congestion means less overall throughput

47
Q

What is the difference between bandwidth and throughput?

A

Bandwidth refers to capacity, while throughput details how much data actually transmits

48
Q

Wireless LANs use _____ frequency and CSMA/C__ and are an example of passive ____ technology?

A
  • Radio frequency
  • CA (collision avoidance)
  • passive bus technology
49
Q

Why doesn’t CD (collision detection) work on wireless LAN?

A

Collision detection doesn’t work because a transmission by one computer may only be received by its immediate neighbours.

50
Q

Wireless LANs can’t use CSMA/CD, what do they use instead?

A

CSMA/CA

51
Q

How does CA (collision avoidance) work?

A

sender sends a small request to a receiver who responds with a clear to send message received by all adjacent computers.

52
Q

In CSMA/CA how are collisions handled?

A
  • control messages collide rather than real messages and no clear to send is sent
  • a random back off is then applied and control messages are resent.
53
Q

What are the 3 main channelisation protocols?

A

FDMA, TDMA, CDMA
Frequency, Time, Code (Division Multi Access)

54
Q

What do FDMA and TDMA extend?

A

frequency and time division multiplexing respectively.

55
Q

How does the channelisation protocol FDMA work?

A

Independent stations choose carrier frequencies that don’t conflict with other station’s carriers.
- sometimes a central controller tells stations which unused frequencies to use.

56
Q

How does the channelisation protocol TDMA work?

A

Stations get a sequence number and transmit in numerical order

57
Q

LAN access protocols are the….

A

mechanisms that allow computers to access a network for sending.

58
Q

Why are there distance limitations in LANs?

A
  • MAC protocols require time proportional to cable length
  • Electrical signals weaken with distance
59
Q

How does a fibre optic LAN extension work?

A

two fibre modems with a fibre optic cable between is used to extend normal cables that go between a computer and an ethernet hub

60
Q

How do repeaters work (as LAN extenders)?

A

They join sections of maximum size ethernet cable together
- amplify the signal strength
- strengthens the signal but doesn’t help delay
- also amplifies noise.

61
Q

Under the ethernet standard, how many repeaters can be used between 2 computers?

A

No more than four

62
Q

How does a bridge work for LANs?

A

It connects two segments (two hubs) at frame level
- Receives all frames from computers
- Doesn’t forward erroneous frames e.g collisions and noise

63
Q

What are the benefits of learning bridges/ frame filtering?

A

A frame is only forwarded when the destination is in the other segment or broadcast or multicast address is used
- The bridge learns which segment a computer is in when that computer sends a frame

64
Q

What happens when a frame arrives at a bridge?

A
  • Source address is extracted and knowledge is updated (where computers are)
  • destination data is inspected to see if it needs forwarding to the other segment
65
Q

Can communication occur simultaneously in both segments of a bridged network?

A

Yes

66
Q

When planning a bridged network, where should two frequently communicating devices go?

A

In the same segment

67
Q

What extension technique might be paired with a bridge to connect LANs in two separate buildings?

A

a fibre optic cable with two fibre modems between buildings and the bridge in one of the two

68
Q

What extension technique might be paired with a bridge to connect LANs very far away from each other?

A

Two bridges each connected to emitters/receivers utilising a satellite

69
Q

What is a switch?

A

A single electronic device that transfers frames between computers

70
Q

A hub between computers simulates a shared medium, what does a switch simulate?

A

A bridged LAN where each computer has one segment

71
Q

Describe the differences between a hub and a switch?

A

hub - operates as an analouge device forwarding signals between computers
switch - digital device forwarding frames between computers

72
Q

What is the advantage of a switch?

A

Greater data transfer as both computers can transfer in parallel like a bridged LAN

73
Q

Why can a switch potentially support multiple transmissions at a time?

A

It handles packets not signals and uses a fabric to provide parallel internal paths within the device.

74
Q

Wireless networks can also be divided into LAN, WAN and MAN like wired networks, what extra category do they have?

A

PAN - personal area network (1-50m)

75
Q

What are the three key parts of a wireless LAN?

A
  • Access points
  • Switch or router
  • Set of wireless hosts
76
Q

What are the two modes of a WIFI network?

A
  • Infrastructure
  • Ad hoc
77
Q

What’s the difference between infrastructure and ad hoc modes of wifi?

A

infrastructure: the host only communicates with an access point (base station) and the access point relays all packets
ad hoc: the hosts communicate among themselves with no base station/access point

78
Q

How does a wireless LAN (e.g. wifi) cover a larger area?

A

with several overlapping access points that clients are able to join (but can only associate with one)

79
Q

How do wireless LAN access points co-ordinate hand offs?

A

Some will co-ordinate among themselves to hand over the client whereas some lower cost access points require the client to change their association manually

80
Q

What are the two key multiplexing techniques used in Wi-Fi networks?

A
  • Frequency division multiplexing
  • Spread spectrum multiplexing
81
Q

Describe spread spectrum multiplexing (for a Wi-Fi network).

A
  • Sender spreads data across multiple frequencies and the receiver combines the information to rebuild the original data
82
Q

What are some potential benefits / reasons for using different standards of wireless LAN?

A

Enhanced security, specific data rates, improved reliability when swapping access points

83
Q

What are four examples of wireless PANs?

A

Bluetooth, InfraRed, ZigBee, ISM Wireless

84
Q

What method of multiplexing does bluetooth use?

A

Spread spectrum frequency hopping

85
Q

What is the network architecture of a bluetooth PAN?

A

Piconet where one master device connects to up to 7 active slaves

86
Q

What type of network is a cellular communications system?

A

A wireless WAN

87
Q

How does a cellular communications system work?

A

System consists of cells each containing a transmitter which are placed side by side to cover wide areas

88
Q

What are all the Base stations of a cellular network connected to?

A

Mobile switching centre

89
Q

How do the frequencies vary across a cellular WAN system of cells?

A

Adjacent cells use different frequencies to prevent interference

90
Q

Give brief details of GPS satellites and GPS.

A

24 satellites orbiting 6 orbital planes
- use triangulation and is accurate to 20-2m ish

91
Q

What are the problems associated with low earth orbit?

A

limited availability and complex to track

92
Q

What does VSAT stand for (satellites)?

A

Very Small Aperture Terminal

93
Q

What is VSAT satellite technology used for?

A
  • entertainment and internet access
  • business use to link stores
94
Q

Why are satellites necessary to extend cellular networks?

A

Microwaves can’t bend round the earth so satellites are needed for long distance communication.

95
Q
A

Systems and Networks (4 decks)

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