quiz 1 Flashcards
(139 cards)
1
Q
LAN
A
Local Area Network
2
Q
examples of wireless networks
A
LTE
eNB: cellular tower
DSD communications
vehicular communications
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
4
Q
what is a computer network?
A
telecommunications network which allows computers to exchange data
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
6
Q
what are hosts?
A
PC, servers, laptops, cellphones
need to be connected to internet
7
Q
examples of communication links
A
fiber, copper, radio, satellite
8
Q
what is transmission rate?
A
bandwidth
how fast you can transmit or receive data
9
Q
what is the function of a router?
A
forwards packets (chunks of data) from one location to another
10
Q
what is the purpose of protocols?
A
control sending and receiving data
11
Q
communication infrastructure enables distributed applications like …
A
web, email, gaming, e-commerce, file sharing, video calls
12
Q
communication services provide applications with …
A
reliable data delivery from source to destination, “best effort” data delivery
13
Q
network edge
A
applications and hosts
14
Q
examples of edge-devices are
A
cell phones, laptops, etc.
15
Q
access networks, physical media is done by:
A
wired (ethernet)
wireless communication links (LTE)
16
Q
network core is
A
interconnected routers
17
Q
the function of end systems are
A
hosts
run application programs
18
Q
examples of end systems
A
web and email
19
Q
describe the client/server model
A
client host requests, receives service from always-on server
20
Q
example of client/server model
A
client: user
server: youtube, web browser
21
Q
describe the peer-peer model
A
minimal or no use of dedicated services, no client or server
22
Q
example of peer-peer model
A
skype
23
Q
what is the purpose of access networks and physical media?
A
connect end systems to edge router
24
Q
what are the 3 types of access networks?
A
residential, institutional, mobile
25
what is important to keep in mind in terms of access networks?
bandwidth
| shared or dedicated
26
what is the difference between shared and dedicated access networks
dedicated: fiber in house, dedicated connection to internet
shared: all devices shared resources provided by router, Wi-Fi router
27
Dial-Up Modem
used existing telephone infrastructure
shared connection
home directly connected to central office
up to 56 kbps direct access to router
28
Digital Subscriber Line (DSL)
used existing telephone infrastructure
1 Mbps upstream
8 Mbps downstream
phone and internet at same time, dedicated physical line to telephone central office
29
describe residential access: cable modems
cable TV infrastructure
hybrid fiber coaxial cables
fiber attaches homes to ISP routers
homes share access to router
30
what is the bps for hybrid fiber coaxial cables
30 Mbps downstream
| 2 Mbps upstream
31
describe internet ethernet access
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
32
what is a WAN?
wireless area network
| shared wireless access network connecting end system to router
33
define physical media - bit
propagates between transmitter/receiver pairs
34
define physical media - physical link
what lies between transmitter and receiver
35
define physical media - guided media
signals propagate in solid media - copper, fiber, coaxial
36
define physical media - unguided media
signals propagate freely (radio)
37
what is a twisted pair (TP)?
2 insulated copper wires
38
what is a coaxial cable?
2 concentric copper conductors
| bidirectional
39
what are the 2 types of coaxial cables?
baseband: single channel on cable
broadband: multiple channels on cable
40
what are fibre optic cables?
glass fibers carrying light pulses, each pulse is a bit
| high-speed operation/high speed point to point transmission
41
how do fiber optic cables have a low error rate?
repeaters spaced far apart, immune to electromagnetic noise
42
how does radio transmission work?
signal carried in EM spectrum
no physical wires
bidirectional
43
types of radio links
microwave
LAN
WAN
satellite
44
describe satellite
Kbps to 45 Mbps channel (or multiple small channels)
| 270 ms end-end delay
45
what is circuit switching?
dedicated circuit between switch and destination
46
explain circuit switching in terms of a telephone network
network resources divided into "pieces" (bw)
pieces are allocated to calls
resource piece idle if not used by owning call (no sharing)
47
circuit switching in terms of time and frequency division
freq: each user takes a piece of bw
time: takes turn using bw
48
packet switching is known as _____ sharing
dynamic
49
how does packet switching work?
multiple sessions share 1 link
| resources used as needed
50
which is more efficient: circuit or packet switching?
packet
| do not need to dedicate resources to a single user
51
what is packetization?
message segmented into blocks of data
52
what is a packet?
group of bits, few hundred to thousand
53
what are the 3 parts of a packet
header
data
trailer
54
header contains
addresses of destination and source of packet
| sequence number that destination users to verify all packets received or to reorder them
55
trailer contains
error control bits that nodes use to verify that they received the packet correctly
56
what happens when a packet is received at a switch?
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
what is the benefit of the store-and-forward method of packet switching?
reduce message delivery time
58
explain the store-and-forward packetization scheme
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
store-and-forward: router needs to check ...
header where address is contained of source and destination
| trailer to look at error bits
60
bandwidth shared on demand is
statistical multiplexing
61
explain resource contention
aggregate resource demand can exceed amount available leading to congestion, packets queueing and waiting for link use
62
_____ switching allows more users to use networks
packet
63
adv and disadv to packet switching
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
adv and disadv to circuit switching
dedicated circuit
throughput and delay will not change
not as efficient use of resources and power
65
internet structure is a _____ of _______
network of networks
66
tier-1 networks
commercial ISPs, national and international coverage
67
tier-2 networks
regional ISPs
| each tier-1 has many tier-2 customer nets
68
tier-3 networks
customer of tier-1 or tier-2 network
| last hop network
69
why computer networks?
```
resource sharing
efficiency
high reliability
access to remote info
person to person communication
interactive entertainment
```
70
how are network functions organized
in a layered structure
71
what is the benefit to the layered structure?
modularity
computer on one network can access computers on all networks independently of specific implementations of different networks
72
how does one layer implement a service?
via its own internal-layer actions relying on services provided by the layer below
73
protocol
a set of rules that governs comms, defines what is communicated, how and when
74
network architecture
a set of layers and protocols
75
peer-to-peer protocols
protocols which make the layer N of the source and destination conceptual understanding
76
interface
defines what info and services a layer must provide for the layer above it
77
how is peer-to-peer protocol achieved?
using the service provided by the lower level entities
78
application layer
implements common user communication services such as files transfer, directory services, virtual terminal
79
presentation layer
takes care of data compression, security and format conversions so that nodes using different representations of information can communicate efficiently and securely
80
session layer
uses transmission layer services to set up and supervise connections between end systems
81
transport layer
supervises end-to-end transmission of packets, may arrange for retransmission of erroneous packets
82
network layer
guides packets from their source to their destination, along a path that may comprise a number of links
83
data link layer
reliable transmission between nodes that are attached to the same physical link
84
physical layer
transmits raw bit stream over physical channel
85
network support layers
1, 2, 3
| physical aspects of moving data from one device to another (specs, connections, addressing, timing, reliability)
86
user support layers
5, 6, 7
| interoperability among unrelated software systems
87
layer 4 links ...
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
what do network protocols define?
format
order of messages sent and received among network entities
actions taken on message transmission
receipt
89
perf metrics: how do loss and delay occur?
packets queue in router buffers
packet arrival rate to link exceeds output link capacity
90
perf metrics: list the 4 sources of packet delay
nodal processing
queueing delay
transmission delay
propogation delay
91
dproc: nodal processing
check bit errors
| determine output link
92
dproc: queueing delay
time waiting at output link for transmission
| depends on congestion level of router
93
dproc: transmission delay
packet length (bits)/link bw (bps)
94
dprop: propagation delay
d: length of physical link
s: prop speed in medium
= d/s
95
traffic intensity formula and variables
R: link bw (bps)
a: avg packet arrival rate (# packets/s)
L: packet length (bits)
96
perf metrics: packet loss
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
perf metrics: throughput
rate (bits/time) at which bits transferred btw sender and receiver
98
instantaneous throughput
rate at given pt in time
99
avg throughput
rate over longer period of time
100
bottleneck link
link on end-end path that constrains end-end throughput
101
what is the use for mathematical modelling?
provide useful approximations
102
what can models be used for in terms of networks?
evaluate system performance: queue length, wait time, loss probability
improve system performance: service rate, packet loss probability
103
where is randomness present communication?
data generation at source
data transmission at network
104
what is the purpose of buffers?
used to absorb the randomness of the network
105
queueing system: incoming traffic
packet arrivals
106
queueing system: outgoing traffic
packet departures
107
queueing systems: buffers
storing packets waiting for service
108
queueing systems: server
process each packet before it departs
109
queueing systems: service discipline
FIFO, processor sharing
110
what does λ represent?
time between the arrival of 2 packets
111
λ is ________ distributed
geometrically
112
what is service time?
each packet takes a geometrically distributed amount of time to process
113
what does µ represent?
service time
the probably with which a packet departs the system
114
µ is ________ distributed
geometrically
115
are the inter-arrival times and service times of packets dependent or independent?
independent
116
λ represents the probability with which...
a packet arrives in a time slot
a packet arrives w probability λ in each time slot
117
the number of packets arriving in a time slot is a ...
Bernoulli random variable
118
mean service time of a packet
1/µ time slots
119
mean service rate of the server
µ packets/time slot
120
q(k)
queue length
| number of packets in queue at the beginning of time slot k
121
a(k)
variable which takes on a value 1 if there was an arrival in time slot k and 0 ow
122
d(k)
indicator variable indicating if there was a departure in time slot k or not
123
d(k) has to be 0 if
there is no packet in the queue at the beginning of the time slot and there was no arrival
124
P_(i, i+1) is the conditional probability that
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
P_(i+1, i) is the conditional probability that
the queue length decreases by 1
126
P_(i, i+1) =
P_(i, i+1) = λ(1-µ)
127
P_(i+1, i) =
P_(i+1, i) = (1-λ)µ
128
how do we compute the performance measure of such a system?
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
waiting time of a packet
amount of time that a packet stays in the system
130
if a packet arrives in time slot t and departs in time slot t+n then its waiting time is
n
131
what is a Markov chain
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
p_i(∞) =
π_i
133
π_i:
steady-state or stationary probability of being in state i
134
when is π_i a good approximation to the probability that the queue length at current time i
when the system has been in operation for a long time
135
once the system reaches steady-state then the probability distribution over the queue lengths ...
will not change
136
when the arrival rate is greater or equal to the service rate than the queue...
will not be stable and the queue lengths will blow up to infinity
137
as ρ → 1 or equivalently λ → µ, the expected steady state length →
infinity
138
Little's Law is a relationship between what?
expected queue length L and expected waiting time W
139
Little's Law: L =
λW
