370 Final Flashcards
A codeword is a….
A codeword is a unique sequence of bits that satisfies certain criteria
Error control codes are….
A set of codewords used by an encoder and decoder pair to detect and possibly correct errors
What does the encoder do?
The encoder adds redundant bits to the sender’s bitstream to create the codewords
What does the decoder do?
The decoder uses the redundant bits to detect and/or correct as many bit errors as the particularly used error-control code will allow
What error does single parity bit check?
The parity check detects odd number of errors only, won’t detect even number of errors
What are some examples of errors?
Noise, fading, EM interference, ISI
The redundancy bits are computed using..?
The redundancy bits are computed according to some criteria (usually, a mathematical algorithm)
The parity bit is computed for the transmitted data block at….?
The transmitter side
The parity bit is checked for the transmitted data block at….?
The receiver’s side
The checksum is an error detection technique that ensures that the sum of packet words is….?
always zero
In checksum the receiver accepts the data only if the sum of the checksum and the packet bits is zero, if not it means something was wrong with the packet. True or false?
False, something could also be wrong in the checksum
What does CRC stand for? What does it do?
it is an error detection technique that is based on binary division to ensure that the total packet is divisible by a known divisor
What does FCS stand for?
frame check sequence
In CRC, what does Tx need to find?
FCS (F), such that : [𝐷, 𝐹 ] divisible by 𝐺, where D is the data bits, and G is the generator
The FCS field contains…?
The FCS field contains a checksum value, which is used by the receiving side to prove that the data is not corrupted
CRC checks for errors by..1 & .. 2?
1- Checking that the received codeword T’=T (the transmitted codeword)
2- Dividing T’ by G to verify it’s still divisible
if the Divisor G has 𝑛 bits, the FCS field F must have..?
FCS field F must have 𝑘=𝑛-1 bits
An error correcting code is an algorithm that …?
expresses a sequence of bits such that any introduced errors can be detected and corrected
What does ARQ stand for? what does it do?
Automatic repeat request,
A data block is encoded for error detection and when an error is detected at the receiver it requests a retransmission
What does FEC stand for? what does it do?
Forward error correction, When FEC is employed, a data block or sequence is encoded for error correction (or error control)
What is controlled redundancy?
It’s a type of error-correcting coding where bits or codewords are repeated so the original message can be recovered if it has been corrupted
A code is defined as…?
A code is defined as an n-tuple of q elements where q is any alphabet
What is the naive approach for repetition codes?
In repetition codes, the same message is appended (transmitted) multiple times. Then, decide the value with the highest average
The 𝑅x attempts to…?
The 𝑅x attempts to locally correct errors without going back to the 𝑇x
How is the Hamming Distance calculated? Which is better codes with shorter or larger distances? How is it detected?
the distance is the bit difference between the codes, Codes with larger distance are better, counting the number of 1’s after Detecting using XOR-ing
𝑑min = ?
smallest distance between all codeword pairs
Coding gain is..?
The difference (in dB) in the required signal-to-noise ratio to maintain reliable communications after coding is employed
What does ARQ use? What are the ARQ protocols used for?
ARQ protocols use the concept of 𝑇 and 𝑅 windows, ARQ protocols are also used to control the flow of information between the 𝑇 and the 𝑅
What are Tx windows? What are Rx windows?
𝑇 window: how many packets can the 𝑇 send before expecting a positive acknowledgment (ACK), 𝑅 window: how many packets can 𝑅 accept and send Acks for
Two main types of ARQ protocols…?
– Stop-and-Wait ARQ: only one frame is sent at a time
– Sliding-window ARQ: several frames can be sent at a time – (time permits)
What happens in Stop-and-wait ARQ?
The transmitter sends a block of data (packet) and remains idle until the
reception of a positive acknowledgment (ACK) or a negative acknowledgment
(NACK)
What happens in Stop and wait ARQ if a NACK is received?
when a negative acknowledgment (NACK) is returned, the transmitted data block (packet) that was kept in the buffer is retransmitted
Is the Stop and Wait ARQ efficient?
The stop & wait method is inherently inefficient due to the idle time spent waiting for confirmation
What is the window size of Tx & Rx in S&W? How many bits are needed for sequencing?
1,1
S&W is known as..?
alternating-bit protocol
In S&W, If ACK is not received before the timer expires then…?
𝑇x automatically retransmits the previous packet
what is Go-back-N ARQ?
The idle time is eliminated by continuously sending data blocks (packets) while waiting for confirmation (acknowledgment)
If a data block is Nack’d,….?
this block and the 𝑁 1 subsequent blocks sent
during the round-trip delay are retransmitted
IS GO BACK IN efficient?
o-back-N preserves the ordering of received data blocks at the
receiver, it still suffers from some inefficiency
what’s the differnce between Selective Repeat & GO BACK IN?
data blocks are continuously transmitted, except that
only the block corresponding to the Nack message is retransmitted, Order of transmitted packets is not preserved, Throughput is improved
what’s Hybrid ARQ?
Forward error correction (FEC) is employed in conjunction with ARQ, applications have a lower bound on the data rate, an upper band on the BER & delay.
ARQ protocol work in ….?
packet switching networks
What does RDT stand for?
reliable data transfer: “No errors” + “No data loss” + “No out-of-order data”
MA protocol stands for?
multiple access
In random access scenarios Stations 𝑇x data packets arrive from upper layer, at……..times
random times
ALOHA transmission protocol
-The transmitter 𝑇 sends data to a receiver 𝑅 via intermediary base station
-The BS relays data to the intended 𝑅 station
-All stations use the same frequency to talk to the BS
ALOHA uses………..used after collision occurs
exponential backoff
pure ALOHA
The 𝑇x node transmits a packet then waits for an ACK from the receiver
– If the ACK message arrives within 2𝑇fr then the transmission is successful
– If no ACK is received within 2𝑇 fr the 𝑇x needs to retransmit
does pure ALOHA produce optimal throughput?
no, there is some associated time waste
Throughput is..?
Throughput is the how efficiently I’m using the shared resource
In the Throughput equation g is…..?
number of transmission attempts per frame time
Slotted ALOHA
-The 𝑇x node transmits a packet then waits for an ACK from the receiver
-The link time is divided into time slots 𝑇s=𝑇fr
difference between pure & slotted ALOHA collisions..?
in pure ALOHA the collisions occur in twice the time frame, in slotted ALOHA collisions occur in one frame time
difference between pure & slotted ALOHA throughput..?
- Pure: Maximum throughput is 𝑅 0.184, which occurs when 𝑔 0.5
- Slotted: The maximum throughput is 𝑅 0.368, which occurs when 𝑔 1
CSMA protocols:
Carrier Sense Multiple Access
In CSMA protocols, nodes follow two rules:
1- “listen before you talk”
2- “stop talking (sending) if someone else begins talking (sending) at the same time”
What dictates the size of LAN networks implementing CSMA?
Propagation delay
CSMA/CD is used in…?
wired Ethernet LANs
(CSMA/CD)
Carrier Sense Multiple Access with collision
detection
In CSMA/CD, when hearing a collision, a station…?
– Uses a persistence strategy
– Aborts transmission
– Sends jamming signal to inform other stations of collision
Each time there’s a collision in CSMA/CD a station increases its..and……?
back-off time to avoid new collisions, Repeats the algorithm until successful transmission or reaching the maximum limit
In CSMA/CD, when does a station stop re-transmitting?
1- successful transmission
2- reaching the maximum time limit
Multiplication by an exponential in one domain (time or frequency) corresponds to a…..?
shift in the other domain
What is a link budget?
The calculation of losses over communication medium while still getting a correct reception
Losses & gains are dimensionless so they use
dB not dBm
What is receiver sensitivity?
The minimum power required by a receiver to properly operate
Bandwidth-limited channels induce ……………., which increases…….?
distortion, BER
How does distortion occur?
Channels are limited and limit the bandwidth which causes expansion in the time domain which causes distortion
What is Thermal noise? Why is it also called white noise?
-It is caused due to electron motion
-The spectrum of thermal noise is flat over a wide range of frequencies, and hence it is said to be white.
The receiver noise………..as we increase the receiver bandwidth
increases
Sampling is……?
Reading analog signal values at discrete moments in time
Sampling converts analog signals….?
from continuous time to discrete time
Quantization is….?
the use of a finite set of amplitude levels and the selection of a level nearest to sample value
What is the quantization noise?
The rounding error introduced by quantization
The minimum required sampling 𝑓 is called the……? It is…….?
Nyquist sampling frequency, 2* fmax (Baseband),
2*(fmax-fmin) (bandpass)
Aliasing occurs if……?
the signal is sampled below its Nyquist rate (under-sampled)
The quantization noise decreases as the number of quantization levels 𝐿 ………….?
increases
(PCM) is an implementation of……….?
Pulse code modulation , A/D conversion
PCM involves…?
1- Sampling
2- Quantization
3- Encoding
A PCM encoder translates the quantized sample into…..?
a codeword in binary sequence
The number of pulses per second is equal to
the…………..rate in units of……….
the symbol rate in units of baud
What are the 5 desirable properties of a line code?
- Adequate timing content
- Good transparency
- Good spectral efficiency for channel bandwidth
- Favorable signal spectrum
- Robustness in presence of channel impairments
What is Adequate timing content in line code?
– Line codes should allow extracting the timing information from the received signal
– The 𝑅 timing circuit job is easier if the pulse train has transitions
What is Good transparency in line code?
should work fine regardless of the transmitted bit pattern
What is Good spectral efficiency in line code?
𝑇 data per second should be as high as possible
𝑇 power should be as low as possible
What is a Favorable signal spectrum in line code?
Pulse train shouldn’t have any DC component if the transmission line uses AC coupling
What are some benefits of AMI?
1- synchronous (bipolar)
2- no dc component
3- Occupies less bandwidth than unipolar and polar NRZ schemes
What is spectral efficiency?
is the number of bits transmitted per second per Hz, Maximizing the ratio of 𝑅b/𝐵 in bps/Hz is the goal
Most of the power of a bipolar signal is contained in [range]?
0 to Rb (bit rate can also be considered frequency)
Square pulses have………….bandwidth since its Fourier transform is a sinc function
infinite
What can prevent ISI?
Pulse shaping with raised-cosine and duo-binary filters can prevent ISI
modulation is an important property of the
Fourier transform
Why do we need modulation (list 5)?
- The carrier frequency is better suited for transmission
- Modulation techniques can enhance the robustness of a signal
- Modulated signals can travel longer distances
- Allows multiplexing
- Allows for smaller antenna size
Digital & analog modulation systems are in general baseband or bandpass systems?
bandpass systems
Demodulation involves one of two detection types, what are they?
- Coherent detection: the received 𝑅𝑋 signal is mixed with LO signal
- Direct detection: other electronics are used; no mixing
What is another name for BASK?
OOK (On Off Keying)
What is BASK?
The carrier amplitude is modulated by the binary data
What is BPSK?
The carrier frequency is modulated by the message binary data as follows.
What is BFSK?
He carrier frequency is modulated by the message binary data
Binary modulation uses how many distinct message symbols?
2
𝑀 different symbols allow sending how many bits/symbol?
log2 𝑀
QPSK is a common M-ary modulation scheme, QPSK uses how many signal phases to transmit 2 bits/symbol?
4
M-ary Quadrature Amplitude (M-QAM) Modulation, what is modulated?
In M-QAM, the carrier amplitude & phase are modulated
The spectrum of a modulated signal can be thought of as……..?
a lower sideband & upper sideband
the bandwidth of a baseband signal is…?
Bm
the bandwidth of a bandpass signal is….?
2Bm
Bandwidth of the AM signal..?
modulated=2*unmodulated bandwidth
In AM, the modulation index describes….?
the amount by which the envelope of the modulated carrier varies about its static level 𝐴c
The modulation depth is…?
the modulation index expressed as a percentage
The FM-modulated carrier has a bandwidth…?
2 (1+𝛽) 𝐵m
FM has………noise performance than AM
better
The PM-modulated carrier has bandwidth…?
2 (1+𝛽) 𝐵m, 𝛽 is the PM modulation index
The PM-modulated carrier consumes……..bandwidth as FM
the same
FM SNR is…………….than the baseband SNR
higher
FM has …….. noise performance than AM
better
What is the figure of merit in digital modulation?
the SNR per bit not SNR
For M-ary digital the BER is……than in binary systems
higher
Compare STP with UTP
STP is less noise & less flexible, more expensive & more heavy compared to UTP
Coaxial cable……….BW and………. loss than UTP
higher BW, higher loss
Back-reflection occurs when Zo……?
Zo != ZL
Maximum power coupling occurs when Zo…?
Zo = ZL (no back reflection)
optical fiber offers……….. loss and………BW
low-loss, high
The injection angle must be…………acceptance angle
less than
Modal dispersion is…?
is distortion that occurs in multimode fibers in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes.
Chromatic dispersion….?
is when different frequency components travel at different speeds in silica
SMF travels ………… distances & has…… bandwidth which means it has…?
longer distances, higher BW, (less attenuation & Lower ISI)
What is an antenna?
is a device that converts the electrical\physical signal into an EM wave
Antenna size typically around…?
𝜆/2
The attenuation coefficient describes….?
the attenuation of an EM wave propagating through a medium per unit distance from the source
As the frequency increases, the attenuation coefficient 𝛼 (dB/km)……?
increases
BNC connectors are used with………….cables
coaxial
What specifies the impedance 𝑍0 of coaxial cables?
Radio-Government
What is the power reflection coefficient?
It is the ratio of the power reflected
from the load to the power sent into
the transmission line
Optical fiber uses the principle of ………….. to capture the light transmitted in an optical fiber and confine the light to the core of the fiber
“total internal reflection” (TIR)
……………is a low-loss, high-bandwidth dielectric medium
Fiber
The V number (parameter) is a dimensionless parameter used to describe…..?
the number of modes (light paths), which is used in the context of step-index fibers
A step-index fiber is…..?
the kind of fiber for which the refractive index is constant within the fiber core and is higher than in the cladding.
For an optical fiber cable, the power attenuation coefficient is dependent on…?
wavelength
Pulses spread in time as they propagate inside….. because of ………
fiber, dispersion
List the 6 Antenna Parameters
radiation pattern
gain
bandwidth,
beamwidth
polarization
impedance
The beamwidth is…..?
the angle width of main lobe, 𝜃3dB measured at half of max gain 𝐺𝑚/2
Antenna gain
Maximum radiated power relative to a reference isotropic antenna Measured in dBi
Antenna with……..gain has necessarily narrow beamwidth
high
What are 4 channel impairments of free path signals
- Radio Frequency signals are absorbed by atmosphere
- EM interference has serious effects
- Multi-path causes signal copies to arrive at different times (delay spread resulting in ISI and higher BER)
- Multi-path leads to signal amplitude fluctuations (causes distortion & higher BER)
Received power must be…………….than the 𝑅𝑋 sensitivity to achieve desired BER
larger
A …………….. is a varying function that conveys some information about a physical
quantity
signal
If a signal is classified as an energy signal, it’s average power is….?
zero
If a signal is classified as a power signal, it’s energy is…?
infinite
Any arbitrary signal can be written as a sum of sinusoidal functions, This sum is discrete in the case of……………..signals
periodic signals
Any arbitrary signal can be written as a sum of sinusoidal functions, This sum is continuous in the case of……………..signals
aperiodic signals
A signal is said to be of……….symmetry if:
x(t)=x(-t), for all t
even
A signal is said to be of……….symmetry if:
x(t) =- x(-t), for all t
odd
What’s 3db Bandwidth?
BW where the amplitude drops to ½ of its max
What’s null to null BW?
BW where the 1st spectrum null occurs at
A simple communication model may comprise
Data+Tx+Rx+Protocols+Signals+Channel
A……….is a network that connects a relatively small number of computers in a relatively close by geographical area.
LAN (Local Area Network)
All nodes are directly interconnected with one another for direct communication….?
Mesh topology
All nodes are connected in a closed loop on which
all messages are transmitted in one direction.
Ring topology
All nodes communicate with one another through a
central node.
Star topology
Nodes are connected to a single communication line
that carries messages in both directions
Bus topology
Which topology can manage the highest amounts of traffic since multiple pairs of devices can simultaneously communicate?
mesh topology
Which topology has the least packet collision
ring topology
Which topology is a slower network
topology compared to other topologies?
bus topology
In simplex…?
One end system talks, the other ones listen
In Half duplex…?
One talks at a time, not simultaneously
In Full duplex….?
Information can flow in either direction simultaneously
What is PSTN? Which switching Paradigm is used in PSTN?
public switched telephone networks, circuit switching
which networks are connection-oriented
Circuit-switched
which networks have dedicated links?
Circuit-switched
How do Tx and Rx exchange data in circuit switching?
as bit streams
which networks are connectionless, undedicated models?
Packet-switched
What is multiplexing?
multiplexing is a technique for combining multiple signals into one signal over a shared channel
What is FDM?
Frequency Division Multiplexing, they can use a certain frequency for all the time
What is TDM?
Time Division Multiplexing, they can use any frequency for some time
What is a protocol?
A protocol is a set of rules governing the communication process between network devices
What are the 3 key elements of a protocol?
Syntax - structure or format
Semantics - Interprets the meaning of transmitted bits
Timing - when should it be sent, and how fast (speed)
What does PDU stand for? What is it?
Protocol data units, Internet messages exchanged as packets
What does a packet consist of?
1- The header (Tx & Rx IP addresses, the packet number, the total number of packets, protocols used)
2- The payload, which is the actual message itself
What is TCP/IP? What are its model layers?
Transmission Control Protocol / Internet Protocol
– Application layer
– Transport layer
– Network layer
– Link layer
– Physical layer
What is OSI? What are its model layers?
Open Systems Interconnection
– Application layer
– Presentation layer
– Session layer
– Transport layer
– Network layer
– Link layer
– Physical layer
Which layer Specifies how signals represent data bits, bit duration, transmission media,
power levels, etc?
Physical layer
Which layer Specifies how data is moved between adjacent nodes over a physical link, shared access, etc.?
Data-link layer
Which layer specifies how data moved between nodes in different networks, packet scheduling, buffer management, etc.
Network layer
Which layer Specifies how data can be moved reliably (error-free & flow-controlled ) between end systems
Transport layer
Which layer Specifies how a session is established and terminated between end systems, synchronization, e.g., between audio/video streams?
Session layer
Which layer Specifies how a data is encoded, encrypted, compressed between end
systems, etc?
Presentation layer
Which layer Specifies which protocols are followed between the applications running on
end systems?
Application layer
