quiz 1 Flashcards
LAN
Local Area Network
examples of wireless networks
LTE
eNB: cellular tower
DSD communications
vehicular communications
explain what a data center network is and how it works
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
what is a computer network?
telecommunications network which allows computers to exchange data
what is the internet/how does it work?
- 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
what are hosts?
PC, servers, laptops, cellphones
need to be connected to internet
examples of communication links
fiber, copper, radio, satellite
what is transmission rate?
bandwidth
how fast you can transmit or receive data
what is the function of a router?
forwards packets (chunks of data) from one location to another
what is the purpose of protocols?
control sending and receiving data
communication infrastructure enables distributed applications like …
web, email, gaming, e-commerce, file sharing, video calls
communication services provide applications with …
reliable data delivery from source to destination, “best effort” data delivery
network edge
applications and hosts
examples of edge-devices are
cell phones, laptops, etc.
access networks, physical media is done by:
wired (ethernet)
wireless communication links (LTE)
network core is
interconnected routers
the function of end systems are
hosts
run application programs
examples of end systems
web and email
describe the client/server model
client host requests, receives service from always-on server
example of client/server model
client: user
server: youtube, web browser
describe the peer-peer model
minimal or no use of dedicated services, no client or server
example of peer-peer model
skype
what is the purpose of access networks and physical media?
connect end systems to edge router
what are the 3 types of access networks?
residential, institutional, mobile
what is important to keep in mind in terms of access networks?
bandwidth
shared or dedicated
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
Dial-Up Modem
used existing telephone infrastructure
shared connection
home directly connected to central office
up to 56 kbps direct access to router
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
describe residential access: cable modems
cable TV infrastructure
hybrid fiber coaxial cables
fiber attaches homes to ISP routers
homes share access to router
what is the bps for hybrid fiber coaxial cables
30 Mbps downstream
2 Mbps upstream
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
what is a WAN?
wireless area network
shared wireless access network connecting end system to router
define physical media - bit
propagates between transmitter/receiver pairs
define physical media - physical link
what lies between transmitter and receiver
define physical media - guided media
signals propagate in solid media - copper, fiber, coaxial
define physical media - unguided media
signals propagate freely (radio)
what is a twisted pair (TP)?
2 insulated copper wires
what is a coaxial cable?
2 concentric copper conductors
bidirectional
what are the 2 types of coaxial cables?
baseband: single channel on cable
broadband: multiple channels on cable
what are fibre optic cables?
glass fibers carrying light pulses, each pulse is a bit
high-speed operation/high speed point to point transmission
how do fiber optic cables have a low error rate?
repeaters spaced far apart, immune to electromagnetic noise
how does radio transmission work?
signal carried in EM spectrum
no physical wires
bidirectional
types of radio links
microwave
LAN
WAN
satellite
describe satellite
Kbps to 45 Mbps channel (or multiple small channels)
270 ms end-end delay
what is circuit switching?
dedicated circuit between switch and destination
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)
circuit switching in terms of time and frequency division
freq: each user takes a piece of bw
time: takes turn using bw
packet switching is known as _____ sharing
dynamic
how does packet switching work?
multiple sessions share 1 link
resources used as needed
which is more efficient: circuit or packet switching?
packet
do not need to dedicate resources to a single user
what is packetization?
message segmented into blocks of data
what is a packet?
group of bits, few hundred to thousand
what are the 3 parts of a packet
header
data
trailer
header contains
addresses of destination and source of packet
sequence number that destination users to verify all packets received or to reorder them
trailer contains
error control bits that nodes use to verify that they received the packet correctly
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
what is the benefit of the store-and-forward method of packet switching?
reduce message delivery time
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
store-and-forward: router needs to check …
header where address is contained of source and destination
trailer to look at error bits
bandwidth shared on demand is
statistical multiplexing
explain resource contention
aggregate resource demand can exceed amount available leading to congestion, packets queueing and waiting for link use
_____ switching allows more users to use networks
packet
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
adv and disadv to circuit switching
dedicated circuit
throughput and delay will not change
not as efficient use of resources and power
internet structure is a _____ of _______
network of networks
tier-1 networks
commercial ISPs, national and international coverage
tier-2 networks
regional ISPs
each tier-1 has many tier-2 customer nets
tier-3 networks
customer of tier-1 or tier-2 network
last hop network
why computer networks?
resource sharing efficiency high reliability access to remote info person to person communication interactive entertainment
how are network functions organized
in a layered structure
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
how does one layer implement a service?
via its own internal-layer actions relying on services provided by the layer below
protocol
a set of rules that governs comms, defines what is communicated, how and when
network architecture
a set of layers and protocols
peer-to-peer protocols
protocols which make the layer N of the source and destination conceptual understanding
interface
defines what info and services a layer must provide for the layer above it
how is peer-to-peer protocol achieved?
using the service provided by the lower level entities
application layer
implements common user communication services such as files transfer, directory services, virtual terminal
presentation layer
takes care of data compression, security and format conversions so that nodes using different representations of information can communicate efficiently and securely
session layer
uses transmission layer services to set up and supervise connections between end systems
transport layer
supervises end-to-end transmission of packets, may arrange for retransmission of erroneous packets
network layer
guides packets from their source to their destination, along a path that may comprise a number of links
data link layer
reliable transmission between nodes that are attached to the same physical link
physical layer
transmits raw bit stream over physical channel
network support layers
1, 2, 3
physical aspects of moving data from one device to another (specs, connections, addressing, timing, reliability)
user support layers
5, 6, 7
interoperability among unrelated software systems
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
what do network protocols define?
format
order of messages sent and received among network entities
actions taken on message transmission
receipt
perf metrics: how do loss and delay occur?
packets queue in router buffers
packet arrival rate to link exceeds output link capacity
perf metrics: list the 4 sources of packet delay
nodal processing
queueing delay
transmission delay
propogation delay
dproc: nodal processing
check bit errors
determine output link
dproc: queueing delay
time waiting at output link for transmission
depends on congestion level of router
dproc: transmission delay
packet length (bits)/link bw (bps)
dprop: propagation delay
d: length of physical link
s: prop speed in medium
= d/s
traffic intensity formula and variables
R: link bw (bps)
a: avg packet arrival rate (# packets/s)
L: packet length (bits)
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
perf metrics: throughput
rate (bits/time) at which bits transferred btw sender and receiver
instantaneous throughput
rate at given pt in time
avg throughput
rate over longer period of time
bottleneck link
link on end-end path that constrains end-end throughput
what is the use for mathematical modelling?
provide useful approximations
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
where is randomness present communication?
data generation at source
data transmission at network
what is the purpose of buffers?
used to absorb the randomness of the network
queueing system: incoming traffic
packet arrivals
queueing system: outgoing traffic
packet departures
queueing systems: buffers
storing packets waiting for service
queueing systems: server
process each packet before it departs
queueing systems: service discipline
FIFO, processor sharing
what does λ represent?
time between the arrival of 2 packets
λ is ________ distributed
geometrically
what is service time?
each packet takes a geometrically distributed amount of time to process
what does µ represent?
service time
the probably with which a packet departs the system
µ is ________ distributed
geometrically
are the inter-arrival times and service times of packets dependent or independent?
independent
λ represents the probability with which…
a packet arrives in a time slot
a packet arrives w probability λ in each time slot
the number of packets arriving in a time slot is a …
Bernoulli random variable
mean service time of a packet
1/µ time slots
mean service rate of the server
µ packets/time slot
q(k)
queue length
number of packets in queue at the beginning of time slot k
a(k)
variable which takes on a value 1 if there was an arrival in time slot k and 0 ow
d(k)
indicator variable indicating if there was a departure in time slot k or not
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
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
P_(i+1, i) is the conditional probability that
the queue length decreases by 1
P_(i, i+1) =
P_(i, i+1) = λ(1-µ)
P_(i+1, i) =
P_(i+1, i) = (1-λ)µ
how do we compute the performance measure of such a system?
- compute expected number of packets in the system at any time instant
- compute the mean waiting time of a packet entering the system
waiting time of a packet
amount of time that a packet stays in the system
if a packet arrives in time slot t and departs in time slot t+n then its waiting time is
n
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
p_i(∞) =
π_i
π_i:
steady-state or stationary probability of being in state i
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
once the system reaches steady-state then the probability distribution over the queue lengths …
will not change
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
as ρ → 1 or equivalently λ → µ, the expected steady state length →
infinity
Little’s Law is a relationship between what?
expected queue length L and expected waiting time W
Little’s Law: L =
λW
