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
2
Q
Integrity
A
- The message guaranteed not to have been changed by third party while in transit
3
Q
Authentication
A
- Received information guaranteed to have come from designated sender
4
Q
Confidentiality
A
- Contents of communications invisible to all except intended recipient
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
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
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
8
Q
Hearing threshold graph
A
- amplitude on left
- frequency underneath
- looks like a U
- anything under U on graph inaudible
- anything above audible
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
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
11
Q
Multicarrier in OFDM
A
- rather than one frequency broadcast at one time multiple ones transmitted
- each symbol spread over long time channel
12
Q
Subchannel in OFDM
A
- each one centered on subcarrier frequency
- wide enough to enclose most of power associated with modulated subcarrier
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
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
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
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
