midterm Flashcards
AM radio
f ≈ 1 MHz
FM radio
f ≈ 100 MHz
wave properties and which are dependent
A
f
phase
dependent: A and phase
physical layer function
converting data into electrical, radio or optical signals
it is responsible for the transmission and reception of raw data streams over a physical medium
data link layer function
to provide node-to-node transfer-a link between two directly connected nodes
responsible for communications between adjacent network nodes by handling the data moving in and out of a physical link in a network
packet-switched
data packets take various routes to the destination
circuit-switched
create a dedicated path for the entire communication session
CDMA
allows multiple users to share the same bandwidth simultaneously; each user is assigned unique code to differentiate their data from others on the same channel
allows stations to transmit at the same time and over the same frequency
function of CSMA
detecting signals from other devices before transmitting
CSMA
protocol that helps devices sense whether the medium is in use or not -> prevents data collisions
role of MAC
controlling how devices access the shared physical medium
sliding window: high latency ->
decreased efficiency
messages containing ACKs to the previous message take longer to transmit, and the sender can not send another frame until it has received the ACKs for the previous frame
Ethernet Frames use ____ for error detection
CRC
Which collision-free protocol is most efficient in terms of bandwidth utilisation and minimising time for collision resolution?
binary countdown
Big Endian
most significant byte at the lowest address
1 KiB = __ bytes
2^10
kibi
2^10
ASCII
a character encoding standard used to represent text in computers and other devices. It encodes 128 specified characters into seven-bit integers, including letters, numerals, and special symbols
Why choose layered over monolithic architecture?
A layered architecture breaks down a system into manageable parts, making it easier to understand, maintain, and develop, as opposed to a monolithic architecture which is typically a single, indivisible unit
What is the purpose of framing?
Splitting data into manageable units.
What is the OSI model primarily used for?
Designing layered network architectures
OSI model is a conceptual framework used to understand and standardise the functions of a telecommunication or computing system without regard to its underlying internal structure and technology. It is organised into seven layers, each specifying particular network functions
OSI layers
application
presentation
session
transport
network
data link
physical
TCP/IP layers
application
transport
internet
host-to-network
modulation
putting information into a signal
twisted pair
commonly used for networks and wired LANs
bandwidth: 500 MHz
coax cable
commonly used for telephone networks, cable television, wired MANs
bandwidth: GHz
optical fiber
commonly used for: long-distance network, wired MANs, high-performance MANs
bandwidth: 100 GHz
radiowave
advantage: can travel reasonably long distances
microwave
satellite dishes
disadvantage: needs a line of sight
infrared
remote controls
satellite networks
radio waves allow for a high bit rate and have low attenuation
sending signals to artificial satellites and back has significant latency
lower latency requires lower orbits requires more satellites
baseband transmission
idea: send signals that represent one or more bits
Non-Return to Zero
clock problems
Manchester encoding
halves the available bandwidth
passband transmission
idea: move from [0, B] Hz to [S, S+B] Hz - the passband
grey encoding
every adjacent pair of symbols only differs by one bit
simplex channels
only allow data to pass in one direction
duplex channels
allows data to pass through in both directions at the same time
half-duplex channels
allow data in both directions, but not at the same time
FDM
all stations send at the same time, at different frequencies
CDMA
stations send at the same time, at the same frequency
receiver figures out who sent what
TDM
stations take turns on a fixed schedule
How does static multiplexing affect apps?
inefficient resource usage
video streaming no longer works
acknowledgements
let the sender know it does not need to retransmit
ARQ
keeps track of frames using sequence numbers
wait until previous frame has been accepted
adds error control (ACK)
stop-and-wait
1-bit sliding window protocol
half the bandwidth is wasted on retransmissinos
when using stop-and-wait, data rate decrease when:
latency increases
frame size decreases
sender window
specifies how many frames a sender is allowed to send before waiting for an acknowledgement
receiver window
specifies the range of frames that the receiver is allowed to accept
small window reduces performance
CRC vs checksum
can detect all double bit errors
detect all bit errors <= r
not vulnerable to systematic errors
parity reliably detects any __ number of errors
odd
checksum vs parity
improved error detection
detects burst up to N errors, N - num of words
convolutional codes
operate on a stream of bits, keeping internal state
determines most likely input for given output
contented approach
if there is data to send, send it
collisions are a fact of life
keep trying until sending successful
coordinated approach
if there is data to send, let other stations knows
send when it is your turn
ALOHA
in pure ALOHA, users transmit frames whenever they have data; if a collision occurs, users retry after a random delay
slotted ALOHA
reduced frame of collisions, frames overlap completely or not at all
CSMA
senders detect if the channel is in use
protocols that apply it:
1-persistent
nonpersistent
p-persistent
CSMA/CD
idea: when collision is detected, do not finish sending, stop transmission instead
separates contention periods from transmission periods -> saves time and bandwidth
contention period
check if it is safe to send data
transmission period
send data
classic ethernet
uses 1-persistent CSMA/CD
large frames → less time spent in connection periods
uses CRC
ethernet frames
preamble: 8
destination: 6
source: 6
T/L: 2
Data: 0-1500
Pad: 0-46
CRC: 4
ethernet evolution
- classic ethernet
- fast ethernet
- gigabit ethernet
- 10-Gigabit ethernet
properties of wireless channels affect
MAC protocol design
MACA
an approach to solve the hidden and exposed terminal problem
CSMA/CA
starts with back-off regardless of collision
stop and wait
poor utilisation of bandwidth
one frame at a time
sliding window protocol
noiseless channels, no error control
multiple frames at a time
data loss -> sender waits for data an infinite amount of time and receiver waits for ack
controlled access protocols
reservation
polling
token passing
