1: Introduction Flashcards
what does a protocol do?
protocols define format, order of messages sent and received among network entities, and actions taken on message transmission, receipt
analogous to human conversation, “Hello” “Hello” “I have a question” “Go ahead.” “What’s the time?” “The time is 4pm”
what are the network edge and core? what is an access network?
edge: hosts/end systems are clients and servers. most servers are in data centers
access network: the network that connections an end system to the first router/edge router e.g. home network or mobile network
core: the mesh of switches and links that interconnects the internets end systems
describe DSL and Cable
DSL: digital subscriber line uses preexisting telephone lines to a DSLAM (access multiplexer) in the telco’s central office. data is transmitted at different frequencies to the CO
cable: homes connect to a cable modem termination system CMTS, another multiplexer. shared cable, if multiple users are downloading they must share. different channels are trans. at different freq. bands. asymmetric upsteam and downstrema rates
example of a wireless LAN and wireless WAN
LAN - home
WAN - mobile network
how to calculate delay?
delay = time needed to transmit L-bit packet into link = L(bits)/R(bits/sec) where R is the link transmission rate
what is store and forward in packet switching/
the entire packet must arrive at a router before it can be transmitted onto the next link
what causes queueing delay, and loss in queueing delay?
if the arrival rate exceeds the transmission rate packets will be queued up
if the buffer fills up then packets will be dropped
what are routing and forwarding? how are they different
routing is the process of determining the source-destination route raken by packets
forwarding is the router-level process of transmitting packets to an appropriate output based on the packet’s destination
what are FDM and TDM
in FDM the frequencies are divided across the users so each user has a constant access
in TDM each user gets the entire frequency for a peroid of time. advantage is that most applications dont require a constant connection, and so user gets good connection
dif between coaxial and fibre optic?`
coaxial uses conductors
fibre optic sends pulses of light, each is a bit
what is circuit switching? advantages and disadvanages or packet switching?
end to end resources are allocated and reserved for end systems. involves no sharing and means that there may be idle segments if its not used to its full potential
used in telephone networks
good because we dont have to wait for anything, but there might be no circuit available and wasted links with inactive users
packet switching means we dont have to set anything up but there may be delays in packets arring, allows more users to use the network
restrauant analyogy
packet switching better for “bursty data” whereas circuit will allow better eg skype cals
what is a content provider network
a private netowrk that are distrubted around close to end users to provide better services to users,
what are the four sources of packet delay? how do calculate?
at a given node the delay is
processing delay (check bit errors, determine output)
+
queueing delay (time waiting at output for transmission)
+
transmission delay (time for router to push out the pcket. toll booth analogy) = packet length in bits/link bandwidth in bps
+
propagation delay (time to get across physical link) = length of link/propagationspeed
how does traceroute work?
for all i
sends three packets that will reach router i on a parth towards a destination, which are returned. sender times interval between transmission and reply
what is throughput (instantaeous and average)
rate (bits/timeunit) at which bits transfered between s and t
instant - at a given point in time
average - over a period of time
constrained by the bottleneck link - the link on the path that has the smallest throughput
