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Networking > Lecture 1 > Flashcards

Lecture 1 Flashcards

1
Q

What do the following stand for?
TCP
IP
SDN
DNS
OSPF
ISP

A

Transmission Control Protocol
Internet Protocol
Software Defined Network
Domain Name Services
Open Shortest Path First
Internet Service Provider

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2
Q

What is the difference between switch, router and network hub?

A

They are the same.

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3
Q

Differentiate between a link and a path with examples.

A

Link goes from end host (e.g. phone) to switch (local internet) Often visible the route it takes.

Path goes from switch to switch (local internet via service provider to information center/servers) Often invisible the route it takes. Made up of a number of links + switches.

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4
Q

Do we differentiate links by whether they are wired or wireless?

A

Yes.

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5
Q

What are the components of a computer network? (5)

A

Message: information to be communicated.
Sender: Device that sends the message
Receiver: Device that receives the message.
Medium: Physical Path from sender to receiver.
Protocol: Set of rules that govern data communications.

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6
Q

How do incompatibility issues arise? (3)

A

Different types of:
-networks
-os
-applications

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7
Q

What is a protocol?

A

Is a set of rules which governs the data communication between different devices on a network and how they interact. Must be consistent across different networks, os and applications.

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8
Q

What is the header?

A

The header is the metadata put in a packet that facilitates the transfer and routing of the payload (data) through the network.

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9
Q

What is a packet?

A

A packet is a chunk of information, a number of bits / bytes, a block of data. Consists of payload (information content) and header which tells you what the packet is.

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10
Q

What is the ISP of UCD?

A

HEA - Higher Education Authority

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11
Q

What is an autonomous system?

A

Are systems that are controlled by something else. Usually an ISP.

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12
Q

How can a network send messages instantly?

A

Never, there will always be delays.

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13
Q

What does it mean to say “Communication feedback is always dated”?

A

Means communication feedback is always delayed.

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14
Q

Define propagation delay.

A

Propagation delay is the delay due to the speed of light. Time it takes a bit to travel along the link “length” of the link.

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15
Q

Differentiate between how header and payload of a packet interact with a network.

A

Data is meaningful only to endpoints. Header is meaningful to the network and the endpoint.

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16
Q

What is a flow?

A

A stream of packets.

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17
Q

Define bandwidth.

A

Number of bits sent per unit time “width” of the link.

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18
Q

Define bandwidth-delay product (BDP).

A

bits/time * propagation delay is the capacity of the link

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19
Q

Distinguish between the units of bandwidth, propagation delay and bandwidth-delay product.

A

Bandwidth - bits/second
Propagation Delay - seconds
Bandwidth-Delay Product - bits

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20
Q

What is the formula for the packet delay?

A

Packet Delay = (Packet Size / Link Bandwidth) + Propagation Delay

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21
Q

Differentiate between a connection and connectionless approach of sending data from one end host to another.

A

Connection:
- establish a connection
- use for data transfer
- release connection
- modelled on telephone service
- sender pushes objects and receiver takes them out in the same order.

Connectionless:
- Independent messages
- modelled on the postal service
- Each message must include receiver address.
- Message can be received in different order

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22
Q

What are the mesh and star approaches to network implementation?

A

Mesh: Directly connect ever pair of sender-receiver pairs.
Star: Connect all to a central site.

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23
Q

Why can’t the star or mesh methods be implemented? (2)
What is the solution?

A

Technology limitations mean that broadcast solutions don’t scale to large numbers of hosts or large geographical distances.
Too expensive.

Network resources must be shared between the users, while still allowing senders to transmit data to their receivers.

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24
Q

What are the two basic techniques that can be used while sharing network resources.

A

Switching - sharing network resources among multiple transmissions.
Multiplexing - sharing a single link among multiple transmissions.

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25
Q

Explain circuit switching.

A
  • Path is set up between sender and receiver by making the appropriate connections in the switches.
  • Network resources are reserved, before data transfer, blocked if not possible.
  • Reserved resources held for duration of transfer.
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26
Q

What is a switch?

A

A device that can create a temporary connection between an input link and an output link.

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27
Q

What are the disadvantages of circuit switching? What is an advantage?

A

Links monopolised, prevents other devices.
Set-up delay
Inefficient for bursty

For smooth traffic, avoids any queuing delays at each link.

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28
Q

Distinguish between smooth and bursty traffic.

A

Smooth traffic - peak to mean is approx 1.
Bursty traffic - where the rate varies widely around its average value.

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29
Q

What could you do to attempt to solve bursty problem with circuit switching? Why dont we do these?

A
  • set up new circuit for each burst.
  • hold original circuit for duration

Both are wasteful and inefficent.

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30
Q

What is P(f) and A(f)? How is aggregate of peaks calculated? How is peak of aggregate calculated?

A

P: Peak rate of flow.
A: Average rate of flow.
Sum of the peak rate of flows. (Add all the peaks)
Peak of each flow added together. (Add all the data then find the peak)

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31
Q

Which is typically higher? The aggregate of peaks or the peak of aggregate.

A

Aggregate of peaks

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32
Q

What do we hope with large number of flows with the peak and aggregates?

A

That the peak of aggregate is approximately equivalent to the average of the flows added together.

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33
Q

What is statistical multiplexing?

A

Means not provisioning for the worst case (when all flows peak together) but instead sharing resources and hoping they don’t peak at the same time.

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34
Q

Message switching isn’t used very much as messages can be large and is better to break them up. Why?

A

The overall delay is decreased by splitting the message into a lot of smaller packets that are pipelined in transmission.
Smaller messages smaller bit error probability.

35
Q

What is store and forward?

A

Send on the packet before the whole of the packet has been received.

36
Q

Explain datagram packet switching.

A

Each packet is treated individually within the network so successive packets may follow different routes. Each packet contains the receiver’s address and a sequence number as packets aren’t neccesarily received in the correct order. Network notes are routers which have routing tables telling them which output link to use for each possible destination.

37
Q

Explain virtual circuit packet switching.

A

A route is set up in the network between sender and receiver. Resources may or may not be reserved for this route. Each packet contains a VCI. Routing tables are aligned for the virtual circuit path created. The packets at the far end could have variable delays between them.

38
Q

What does VCI stand for?

A

Virtual Circuit Identifier

39
Q

What is jitter?

A

Variable delays between packets.

40
Q

Differentiate between datagram and virtual circuit methods.

A

Virtual:
-Connection set up delay
- resources not shared at same time
- Connection needs to be re-established if node goes down.
- Ideal for long lived bursty traffic
- Less work for intermediate routers.
- Packets generally in the right order.
Datagram:
- No connection set up
- resources not shared at same time
- Flexible routing possible. More Ro bust to network problems.
- Ideal for short-lived bursty traffic
- More work for intermediate routers.
- Packets not neccessarily in the right order.

41
Q

Explain why virtual circuit is a compromise between circuit switching and datagram?

A

Circuit switching: Creates path in network. Links cannot be shared.
Virtual: Creates a route which exists only in software. Links can be shared.
Datagram: Doesn’t have routes. Links can be shared.

42
Q

Explain throughput. If the network is filled with packets what is maximum throughput?

A

How fast data can pass a certain point. Measured in bits/s in packets/s.

1 packet / s

43
Q

Explain efficiency. If half the slots in the network are filled what is the efficiency.

A

Efficiency = throughput / bandwidth * 100

50%

44
Q

How do we characterise smooth compared to bursty?

A

The ratio between the peak to average transmission time. 1:1 is smooth 100:1 is very bursty.

45
Q

Compare how packet switching and circuit switching deal with node or link failure.

A

PS:
Network detects failure
Recalculates route
Endhosts and individual flows do nothing special.

CS:
Network detects failure
Recalculates route
Endhosts must detect failure, teardown old reservations, send a new reservation request.
If lots of slows then lots of reservation requests are being simultaneously re-established leading to further delays.

46
Q

Compare pros for circuit switching vs packet switching.

A

CS:
Better application performance.
More predictable and understandable.
PS:
Better efficiency.
Faster startup to first packet delivered.
Easier recovery from failure.
Simpler implementation.

47
Q

What is overload?

A

When a queue at a node reaches its limit of the storage of the queue and overflows dropping packets.

48
Q

How can packet switching be ensured to have reliable delivery?

A

Congestion control, endhosts decide at what rate they will send packets. How fast the packets are sent will impact if packets are dropped.

49
Q

T or F. Not all packets have to have a header in them.

A

False

50
Q

T or F. When the length of a link is increased the propagation delay is increased.

A

True

51
Q

T or F. When the length of a link is doubled the propagation delay is halved.

A

False

52
Q

Finish this sentence the bandwidth delay product is the ____ of the link.

A

capacity

53
Q

T or F. Increasing the length of the link will increase the packet transmission time.

A

False

54
Q

T or F. The transmission time can be order of magnitudes larger or order of magnitudes smaller than the propagation delay.

A

True

55
Q

T or F. The propagation delay does not depend on the packet size or the bit rate of transmission.

A

True

56
Q

Name the two types of packet switching.

A

Datagram and virtual circuit

57
Q

T or F. The peak of the aggregate flows is always smaller or equal to the aggregate of peak flows.

A

T

58
Q

T or F. The probability of bit error is 1 in a million and the packet size is 1500B, the probability of error of the packet is therefore 1% rounded.

A

True

59
Q

T or F. An increase in the transmission bit rate will decrease the transmission delay for a packet.

A

True

60
Q

T or F and explain. A smaller packet that follows a larger packet through an intermediate node where the bit rate is the same on the input and the output link will experience queuing delay at that intermediate node.

A

True.

The two packets are right behind each other because the bit rates are the same, the larger packet will take longer at the node forcing the smaller one to wait.

61
Q

T or F and why. A smaller packet that follows a larger Packet through an intermediate node will experience queuing delay at that intermediate node.

A

False.

The bit rate is not the same so there may be a gap between the packets and queuing should be avoided.

62
Q

T or F. The peak of aggregates can be smaller than the average of the flows.

A

False

63
Q

T or F and why. There can be no queuing for a packet if there is only one input to an intermediate node and only one output.

A

False.

If the first packet is bigger than the second than the second will have to queue if they are following right behind each other on the one input.

64
Q

T or F. The bigger the packet size the larger the packet error probability is.

A

True.

65
Q

T or F. The Internet connect 10’s of thousands of networks together.

A

True

66
Q

T or F. When the bit error rate increases the probability of error of a packet also increases.

A

True

67
Q

T or F. We only have to provide the sum of the average flow rates when we aggregrate flows.

A

False

68
Q

A or B.
A: A packet of 1200 Bytes has a bit error rate of 1 in a 100,000. The probability of packet error in percentage rounded is 10%.
B: A packet of 200 Bytes has a bit error rate of 1 in a 100,000. The probability of packet error in percentage rounded is 1%.

A

A

69
Q

T or F. In virtual circuit switching the packets have to be re-ordered at the receiver as they can arrive out of order.

A

False

70
Q

T or F. When a larger number of flows are aggregrated the intention is to bring the Peak of the Aggregrates as close to the sum of the average flows.

A

True

71
Q

T or F. Packets going through a single intermediate node with the same bit rate going in and out will never have to queue.

A

False.

72
Q

If 2 virtual circuits pass through a network node N, and
packets are travelling on both virtual circuits at the same
time so that node N sees an interleaved stream of packets,
how does node N know which packets belong to the first
virtual circuit and which packets belong to the second ?

A

Both virtual circuits give every packet a unique VCI such that the router can tell them apart.

73
Q

What is a normal efficiency level in a network?

A

10%

74
Q

What is the benefit of aggregating a lot of trafifc?

A

If we aggregate a lot of traffic we can get the ratio down to 1:1 to improve efficiency.

75
Q

Why does circuit switching struggle more with errors than packet switching?

A

Circuit switching has to propagate back node failure. Packet switching has per packet switching.

76
Q

What is the term for temporary congestion of packets coming in at the same time?

A

Transient Congestion

77
Q

What is overload?

A

Queue builds up and reaches capacity and will overflow causing failure.

78
Q

Give a formula which demos how long setup delay needs to be for packet switching to be quicker than circuit switching.

A

(R - 1) * p / B
Where R is number of nodes in total (including first node)
Where p is size in bits and B is transmission speed at one node.

79
Q

Formula for throughput. How is it measured?

A

Throughput = min(window size / RTT , bandwidth)

where RTT is delay both ways in s
where window size is number of bits sent in bits

in bps.

80
Q

Formula for circuit switching delay.

A

(setup time) + (time to
transmit x bits at the sender) + (propagation delay
of last bit along the path)

s + (x/b) + kd

where s=setup delay
x=number of bits total
b=transmission speed at 1st node
k=number of nodes
d=propagation delay per node

81
Q

Formula for packet switching delay.

A

(time to transmit x bits
at the sender) + (propagation delays of last bit of
last packet along the path) + (packet retransmission
delays at the intermediate nodes)

(x/b) + kd + (k-1)(p/b)

where x=number of bits total
b=transmission speed at 1st node
k=number of nodes
d=propagation delay per node
p=packet size in bits

82
Q

T or F. Consider a frame error probability that is very small. If the frame size is doubled and the BER is halved, the probability of frame error remains the same.

A

True.

83
Q

T or F. To be able to correct B bit errors there needs to be a Hamming Distance of 2B between the code words.

A

False.

84
Q
A

Networking (5 decks)

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