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elec 373 > quiz 1 > Flashcards

quiz 1 Flashcards

1
Q

LAN

A

Local Area Network

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2
Q

examples of wireless networks

A

LTE
eNB: cellular tower
DSD communications
vehicular communications

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3
Q

explain what a data center network is and how it works

A 
servers on racks
cloud-based
red-dots: switches
grey-dots: servers
any server in network can reach any other server in network due to interconnection of all
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4
Q

what is a computer network?

A

telecommunications network which allows computers to exchange data

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5
Q

what is the internet/how does it work?

A
  • a network of networks: permits reliable exchange of info w low cost
  • global system of interconnected computer networks that use the standard internet protocol suite to link several billion devices worldwide and exchange information
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6
Q

what are hosts?

A

PC, servers, laptops, cellphones

need to be connected to internet

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7
Q

examples of communication links

A

fiber, copper, radio, satellite

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8
Q

what is transmission rate?

A

bandwidth

how fast you can transmit or receive data

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9
Q

what is the function of a router?

A

forwards packets (chunks of data) from one location to another

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10
Q

what is the purpose of protocols?

A

control sending and receiving data

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11
Q

communication infrastructure enables distributed applications like …

A

web, email, gaming, e-commerce, file sharing, video calls

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12
Q

communication services provide applications with …

A

reliable data delivery from source to destination, “best effort” data delivery

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13
Q

network edge

A

applications and hosts

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14
Q

examples of edge-devices are

A

cell phones, laptops, etc.

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15
Q

access networks, physical media is done by:

A

wired (ethernet)

wireless communication links (LTE)

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16
Q

network core is

A

interconnected routers

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17
Q

the function of end systems are

A

hosts

run application programs

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18
Q

examples of end systems

A

web and email

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19
Q

describe the client/server model

A

client host requests, receives service from always-on server

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20
Q

example of client/server model

A

client: user
server: youtube, web browser

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21
Q

describe the peer-peer model

A

minimal or no use of dedicated services, no client or server

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22
Q

example of peer-peer model

A

skype

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23
Q

what is the purpose of access networks and physical media?

A

connect end systems to edge router

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24
Q

what are the 3 types of access networks?

A

residential, institutional, mobile

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25
Q

what is important to keep in mind in terms of access networks?

A

bandwidth

shared or dedicated

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26
Q

what is the difference between shared and dedicated access networks

A

dedicated: fiber in house, dedicated connection to internet
shared: all devices shared resources provided by router, Wi-Fi router

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27
Q

Dial-Up Modem

A

used existing telephone infrastructure
shared connection
home directly connected to central office
up to 56 kbps direct access to router

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28
Q

Digital Subscriber Line (DSL)

A

used existing telephone infrastructure
1 Mbps upstream
8 Mbps downstream
phone and internet at same time, dedicated physical line to telephone central office

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29
Q

describe residential access: cable modems

A

cable TV infrastructure
hybrid fiber coaxial cables
fiber attaches homes to ISP routers
homes share access to router

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30
Q

what is the bps for hybrid fiber coaxial cables

A

30 Mbps downstream

2 Mbps upstream

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31
Q

describe internet ethernet access

A

nodes share the medium
up to 1 Gbps
devices connected to ethernet which that is connected to a server
ethernet switch is connected to institutional router

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32
Q

what is a WAN?

A

wireless area network

shared wireless access network connecting end system to router

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33
Q

define physical media - bit

A

propagates between transmitter/receiver pairs

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34
Q

define physical media - physical link

A

what lies between transmitter and receiver

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35
Q

define physical media - guided media

A

signals propagate in solid media - copper, fiber, coaxial

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36
Q

define physical media - unguided media

A

signals propagate freely (radio)

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37
Q

what is a twisted pair (TP)?

A

2 insulated copper wires

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38
Q

what is a coaxial cable?

A

2 concentric copper conductors

bidirectional

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39
Q

what are the 2 types of coaxial cables?

A

baseband: single channel on cable
broadband: multiple channels on cable

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40
Q

what are fibre optic cables?

A

glass fibers carrying light pulses, each pulse is a bit

high-speed operation/high speed point to point transmission

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41
Q

how do fiber optic cables have a low error rate?

A

repeaters spaced far apart, immune to electromagnetic noise

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42
Q

how does radio transmission work?

A

signal carried in EM spectrum
no physical wires
bidirectional

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43
Q

types of radio links

A

microwave
LAN
WAN
satellite

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44
Q

describe satellite

A

Kbps to 45 Mbps channel (or multiple small channels)

270 ms end-end delay

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45
Q

what is circuit switching?

A

dedicated circuit between switch and destination

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46
Q

explain circuit switching in terms of a telephone network

A

network resources divided into “pieces” (bw)
pieces are allocated to calls
resource piece idle if not used by owning call (no sharing)

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47
Q

circuit switching in terms of time and frequency division

A

freq: each user takes a piece of bw
time: takes turn using bw

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48
Q

packet switching is known as _____ sharing

A

dynamic

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49
Q

how does packet switching work?

A

multiple sessions share 1 link

resources used as needed

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50
Q

which is more efficient: circuit or packet switching?

A

packet

do not need to dedicate resources to a single user

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51
Q

what is packetization?

A

message segmented into blocks of data

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52
Q

what is a packet?

A

group of bits, few hundred to thousand

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53
Q

what are the 3 parts of a packet

A

header
data
trailer

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54
Q

header contains

A

addresses of destination and source of packet

sequence number that destination users to verify all packets received or to reorder them

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55
Q

trailer contains

A

error control bits that nodes use to verify that they received the packet correctly

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56
Q

what happens when a packet is received at a switch?

A

inspected to determine output link
if output link to the next switch is on its way to destination is available it is transmitted
else it is stored and forwarded when it becomes available

57
Q

what is the benefit of the store-and-forward method of packet switching?

A

reduce message delivery time

58
Q

explain the store-and-forward packetization scheme

A

takes L/R seconds to transmit packet of L bits on to link at R bps
entire packet must arrive at router before it can be transmitted on next link

59
Q

store-and-forward: router needs to check …

A

header where address is contained of source and destination

trailer to look at error bits

60
Q

bandwidth shared on demand is

A

statistical multiplexing

61
Q

explain resource contention

A

aggregate resource demand can exceed amount available leading to congestion, packets queueing and waiting for link use

62
Q

_____ switching allows more users to use networks

A

packet

63
Q

adv and disadv to packet switching

A

no need to allocate resources first, transmit at will as long as protocols are followed
scalability due to statistical multiplexing

best effort service, links get congested, messages can arrive out of order

64
Q

adv and disadv to circuit switching

A

dedicated circuit
throughput and delay will not change

not as efficient use of resources and power

65
Q

internet structure is a _____ of _______

A

network of networks

66
Q

tier-1 networks

A

commercial ISPs, national and international coverage

67
Q

tier-2 networks

A

regional ISPs

each tier-1 has many tier-2 customer nets

68
Q

tier-3 networks

A

customer of tier-1 or tier-2 network

last hop network

69
Q

why computer networks?

A 
resource sharing
efficiency
high reliability
access to remote info
person to person communication
interactive entertainment
70
Q

how are network functions organized

A

in a layered structure

71
Q

what is the benefit to the layered structure?

A

modularity
computer on one network can access computers on all networks independently of specific implementations of different networks

72
Q

how does one layer implement a service?

A

via its own internal-layer actions relying on services provided by the layer below

73
Q

protocol

A

a set of rules that governs comms, defines what is communicated, how and when

74
Q

network architecture

A

a set of layers and protocols

75
Q

peer-to-peer protocols

A

protocols which make the layer N of the source and destination conceptual understanding

76
Q

interface

A

defines what info and services a layer must provide for the layer above it

77
Q

how is peer-to-peer protocol achieved?

A

using the service provided by the lower level entities

78
Q

application layer

A

implements common user communication services such as files transfer, directory services, virtual terminal

79
Q

presentation layer

A

takes care of data compression, security and format conversions so that nodes using different representations of information can communicate efficiently and securely

80
Q

session layer

A

uses transmission layer services to set up and supervise connections between end systems

81
Q

transport layer

A

supervises end-to-end transmission of packets, may arrange for retransmission of erroneous packets

82
Q

network layer

A

guides packets from their source to their destination, along a path that may comprise a number of links

83
Q

data link layer

A

reliable transmission between nodes that are attached to the same physical link

84
Q

physical layer

A

transmits raw bit stream over physical channel

85
Q

network support layers

A

1, 2, 3

physical aspects of moving data from one device to another (specs, connections, addressing, timing, reliability)

86
Q

user support layers

A

5, 6, 7

interoperability among unrelated software systems

87
Q

layer 4 links …

A

the two subgroups of layers and ensures that what the lower layers have transmitted is in a form that the upper layers can use

88
Q

what do network protocols define?

A

format
order of messages sent and received among network entities
actions taken on message transmission
receipt

89
Q

perf metrics: how do loss and delay occur?

A

packets queue in router buffers

packet arrival rate to link exceeds output link capacity

90
Q

perf metrics: list the 4 sources of packet delay

A

nodal processing
queueing delay
transmission delay
propogation delay

91
Q

dproc: nodal processing

A

check bit errors

determine output link

92
Q

dproc: queueing delay

A

time waiting at output link for transmission

depends on congestion level of router

93
Q

dproc: transmission delay

A

packet length (bits)/link bw (bps)

94
Q

dprop: propagation delay

A

d: length of physical link
s: prop speed in medium
= d/s

95
Q

traffic intensity formula and variables

A

R: link bw (bps)
a: avg packet arrival rate (# packets/s)
L: packet length (bits)

96
Q

perf metrics: packet loss

A

queue (buffer) preceding link has finite capacity

the packet arrives to a full queue is fully dropped (lost)

lost packet may be retransmitted by previous node, or not at all

97
Q

perf metrics: throughput

A

rate (bits/time) at which bits transferred btw sender and receiver

98
Q

instantaneous throughput

A

rate at given pt in time

99
Q

avg throughput

A

rate over longer period of time

100
Q

bottleneck link

A

link on end-end path that constrains end-end throughput

101
Q

what is the use for mathematical modelling?

A

provide useful approximations

102
Q

what can models be used for in terms of networks?

A

evaluate system performance: queue length, wait time, loss probability

improve system performance: service rate, packet loss probability

103
Q

where is randomness present communication?

A

data generation at source

data transmission at network

104
Q

what is the purpose of buffers?

A

used to absorb the randomness of the network

105
Q

queueing system: incoming traffic

A

packet arrivals

106
Q

queueing system: outgoing traffic

A

packet departures

107
Q

queueing systems: buffers

A

storing packets waiting for service

108
Q

queueing systems: server

A

process each packet before it departs

109
Q

queueing systems: service discipline

A

FIFO, processor sharing

110
Q

what does λ represent?

A

time between the arrival of 2 packets

111
Q

λ is ________ distributed

A

geometrically

112
Q

what is service time?

A

each packet takes a geometrically distributed amount of time to process

113
Q

what does µ represent?

A

service time

the probably with which a packet departs the system

114
Q

µ is ________ distributed

A

geometrically

115
Q

are the inter-arrival times and service times of packets dependent or independent?

A

independent

116
Q

λ represents the probability with which…

A

a packet arrives in a time slot

a packet arrives w probability λ in each time slot

117
Q

the number of packets arriving in a time slot is a …

A

Bernoulli random variable

118
Q

mean service time of a packet

A

1/µ time slots

119
Q

mean service rate of the server

A

µ packets/time slot

120
Q

q(k)

A

queue length

number of packets in queue at the beginning of time slot k

121
Q

a(k)

A

variable which takes on a value 1 if there was an arrival in time slot k and 0 ow

122
Q

d(k)

A

indicator variable indicating if there was a departure in time slot k or not

123
Q

d(k) has to be 0 if

A

there is no packet in the queue at the beginning of the time slot and there was no arrival

124
Q

P_(i, i+1) is the conditional probability that

A

the queue length increases from i to i+1 in one time slot

probability that there is one arrival and no departures in a time slot

125
Q

P_(i+1, i) is the conditional probability that

A

the queue length decreases by 1

126
Q

P_(i, i+1) =

A

P_(i, i+1) = λ(1-µ)

127
Q

P_(i+1, i) =

A

P_(i+1, i) = (1-λ)µ

128
Q

how do we compute the performance measure of such a system?

A
  1. compute expected number of packets in the system at any time instant
  2. compute the mean waiting time of a packet entering the system
129
Q

waiting time of a packet

A

amount of time that a packet stays in the system

130
Q

if a packet arrives in time slot t and departs in time slot t+n then its waiting time is

A

n

131
Q

what is a Markov chain

A

stochastic system where the probabilistic description of the system in time slot k+1 can be written in terms of the probabilistic description in the previous time slot k

132
Q

p_i(∞) =

A

π_i

133
Q

π_i:

A

steady-state or stationary probability of being in state i

134
Q

when is π_i a good approximation to the probability that the queue length at current time i

A

when the system has been in operation for a long time

135
Q

once the system reaches steady-state then the probability distribution over the queue lengths …

A

will not change

136
Q

when the arrival rate is greater or equal to the service rate than the queue…

A

will not be stable and the queue lengths will blow up to infinity

137
Q

as ρ → 1 or equivalently λ → µ, the expected steady state length →

A

infinity

138
Q

Little’s Law is a relationship between what?

A

expected queue length L and expected waiting time W

139
Q

Little’s Law: L =

A

λW

elec 373 (2 decks)

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