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TM355 > Forgotten items > Flashcards

Forgotten items Flashcards

1
Q

Frequency Division Duplexing (FDD)

A
  • uses different frequency bands for uplink and downlink comms
  • means both uplink and downlink can work simultaneous
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2
Q

Integrity

A
  • The message guaranteed not to have been changed by third party while in transit
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3
Q

Authentication

A
  • Received information guaranteed to have come from designated sender
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4
Q

Confidentiality

A
  • Contents of communications invisible to all except intended recipient
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5
Q

How does hash ensure integrity

A
  • Sender of message applies a hash function to message
  • this produces a message digest which is sent to the recipient with the message
  • any change to the message will result in different hash
  • at recipient end same hash function applied and resulting message digest compared with original
  • if they match integrity assured
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6
Q

Source coding vs channel coding

A
  • source coding about removing redundant data
  • this is data that’s not important
  • channel coding is more about adding extra bits like parity check or crcs
  • helps in correction of corrupt data
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7
Q

Differential pulse code modulation (DPCM)

A
  • variant of PCM
  • achieves low bit rate by sample prediction
  • send first number then send difference from it to next number and so on
  • i.e. 221, 223, 219, 229
  • = 221 +2 -4 +10
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8
Q

Hearing threshold graph

A
  • amplitude on left
  • frequency underneath
  • looks like a U
  • anything under U on graph inaudible
  • anything above audible
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9
Q

Factors that limit error free data rate in band limited channel with AWGN

A
  • band width
  • signal-to-noise ratio
  • lower S/N ratio can be compensated for by increasing bandwidth
  • higher S/N ratio means bandwidth can be decreased
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10
Q

OFDM

A
  • relatively low symbol rate
  • low rate actually provides resilience against propagation in mobile comms
  • OFDM combines high data rate with low symbol rate
  • single carrier approach sends one subchannel at a time
  • OFDM sends parts of all subchannels at once
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11
Q

Multicarrier in OFDM

A
  • rather than one frequency broadcast at one time multiple ones transmitted
  • each symbol spread over long time channel
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12
Q

Subchannel in OFDM

A
  • each one centered on subcarrier frequency

- wide enough to enclose most of power associated with modulated subcarrier

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13
Q

Null subchannels in OFDM

A
  • subchannels not being used
  • act to separate different subcarriers and stop interference between them
  • centre frequency of subcarrier set to null
  • allows receiver to work out where transmission band is
  • to find out useful symbol invert subcarrier separation
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14
Q

ADSL/ADSL2+

A
  • pots first band
  • guard band of 5 subchannels
  • smaller upstream group of subchannels than downstream
  • separated by empty subchannel 32
  • each subchannel checked for S/N
  • different modulation schemes used depending on S/N
  • number of bits loaded onto channel depending on S/N (bit loading)
  • if subchannel has spare bits they can be used by other subchannels (bit swapping)
  • 4000 bits per symbol
  • subchannels with very low S/N ratio can still be utilised as they can use just one bit per symbol
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15
Q

perceptual noise substitution (PNS)

A
  • enables bit rate optimisation
  • works on premise one noise sounds like another
  • actual noise signal structure of minor importance to human perception
  • instead of transmitting all spectral noise components, bitstream flags to the decoder that signal in particular band is noise
  • provides some information regarding corresponding power level in the band
  • decoder uses randomly generated noise signal at requisite level as a substitute
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16
Q

public switched telecommunications network (PTSN)

A
  • core network - fixed, high speed, intensively used comms network; often interconnect with other core networks
  • access network - links end users equipment to core via local exchange or local radio node
  • consumer premises equipment (CPE) - devices used by subscribers for consuming data
17
Q

five main types of tech used in access networks

A
  • copper telephone wires
  • coaxial cable
  • optical fibre
  • mobile radio comms
  • nomadic radio comms; WiFi
18
Q

signalling

A
  • in band signalling uses same channels for signalling as user data
  • out of band uses separate channels
19
Q

DSLAM and MSAN

A
  • DSLAM requires splitter
  • local loop terminates on MSAN; directs voice and data as appropriate
  • Both mark edge of access network
  • BRAS - operated by ISP; terminates broadband connection; allows data to be extracted and forwarded to final destination
20
Q

Aggregation

A
  • combining traffic from large number of lightly loaded access lines onto trunk line
21
Q

Multiprotocol label switching (MPLS)

A
  • quality of service; certain types of traffic prioritised
  • fast forwarding of packets
  • facilitated creation of VPNs
  • traffic engineering; reserving capacity; treat packets different according to types of data
22
Q

Essence of MPLS

A
  • encapsulation of packet from other protocol as payload
  • routing packet through MPLS network using label (number)
  • everytime packet processed by label switching router label changed
23
Q

Multiple access method

A
  • 2G - FDMA/TDMA; channel width 200 kHz
  • 3G - WCDMA; channel width 5 MHz
  • 4G - OFDMA; channel width up to 20 MHz
24
Q

Channelisation codes

A
  • codes same length as chips; 16 chips means 16 codes etc.
  • more chips, more codes
  • long codes have fewer bit rates; shorter higher bit rates
  • short codes less resistant to noise
  • each code represent one bit; if channelization code 4 chips this equals 1 bit
  • inverse channelization code to get zero value
  • codes spread the bandwidth; 16 chip code results in 16 fold spread of bandwidth; more resilient to noise
  • processing gain equal to number of chips used per bit
25
Q

Scrambling code

A
  • chips scrambled to appear random
  • chip by chip multiplication with scrambling code used
  • correlation used for descrambling
  • scrambling done by each base station using own scrambling code.
  • wanted signal in base station descrambled and despread
  • all other signal not needed turned into noise
26
Q

JPEG

A
  • splits image into 8 x 8 macroblocks; transforms image
  • known as discrete cosine transformation (DCT)
  • top left block is d.c coefficient; this is average greyscale value of macroblock; weighted the most
  • all other blocks called a.c. coefficients
  • each block represents cosine wave; combined with other cosine waves to reproduce image
  • all blocks given numbers to weight them; more weight means more of the block added to image
  • at this point everything lossless
  • quantisation then used; uses quantisation table to divide each component
  • quantisation table reflects how our eyes work
  • takes advantage of perceptual redundancy
  • as high frequency lost most ac coefficients set to 0
  • this then read in zigzag pattern
  • results in long line of 0s which can be compressed with RLE then Huffman coding
  • quantisation is lossy
  • process reversed at other end using same quantisation table
27
Q

carrier sense multiple access with collision avoidance (CSMA/CA)

A
  • used on wifi channels
  • device wanting to send data checks channel is free (waits length of time; check period)
  • if channel free starts counting down from random number (contention window)
  • sends data if channel free
  • no interference with other routers using same frequency
28
Q

Cyclic prefix

A
  • can be used with OFDM
  • adds copy of end of signal to start of signal
  • provides guard band
29
Q

Pilot signals in OFDM/ADSL

A
  • used for reference
  • used in channel equalisation and scheduling
  • important to keep synchronisation between base and receiver as orthogonality can be lost otherwise
30
Q

Problems with local loop

A
  • bundling of loops causes crosstalk
  • mechanical connections can deteriorate
  • long lengths of unshielded conductors can act as antennas for radio frequencies
  • signal attenuation can be a problem in long loops
31
Q

Vectoring in VDSL

A
  • lines that are close together can send information to each other to cancel out crosstalk
  • before sending signal one line sends another line what it is going to send and the second line can use this to cancel the induced signal off its line
32
Q

Passive optical networks (PONs)

A
  • much more fibre shared than in active optical network
  • nodes are passive nodes
  • splitters used to distribute optical signal
33
Q

Algorithms for secure comms

A
  • Integrity - MD5, SHA
  • Authentication - HMAC-MD5, HMAC-SHA-1, RSA or DSA
  • Confidentiality - DES, 3DES, AES
34
Q

Integrity

A
  • hash, also called message digest made by applying function to message
  • any change to message produces different hash
  • hash sent to receiver together with message
  • hash function applied at receiver
  • if message digest match, integrity ensured
35
Q

Authentication

A
  • keys sued for authentication
  • at receiver regeneration of hash needs key as well to decode
  • as both key and hash needed ensures authentication
36
Q

Confidentiality

A
  • symmetric keys - same key used for encryption and decryption
  • keys must be pre-shared
  • both keys must be known by sender and receiver
  • keys must be kept secret
  • asymmetric keys - use different keys to encrypt and decrypt data
  • key used for encryption need not be secret, different key used for decryption
37
Q

Key management

A
  • generation - developing new algorithms
  • verification - done by attempting to break new keys
  • storage - key storage must be secure
  • exchange - key exchanges must only go to intended users
38
Q

IPsec

A
  • transport mode - provides security at layer 4

- tunnel mode - provides more complete protection at layer 3

39
Q

Linear predictive coding (LPC)

A
  • estimates key speech in human acoustics
  • uses impulse generator for voiced sound
  • uses white noise generator for unvoiced
  • U/V switch decides which generator to use
  • gain added to sound before added to N quantised model coefficients ready to be transported

TM355 (8 decks)

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