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
Scrambling code
- 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
JPEG
- 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
carrier sense multiple access with collision avoidance (CSMA/CA)
- 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
Cyclic prefix
- can be used with OFDM
- adds copy of end of signal to start of signal
- provides guard band
29
Pilot signals in OFDM/ADSL
- used for reference
- used in channel equalisation and scheduling
- important to keep synchronisation between base and receiver as orthogonality can be lost otherwise
30
Problems with local loop
- 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
Vectoring in VDSL
- 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
Passive optical networks (PONs)
- much more fibre shared than in active optical network
- nodes are passive nodes
- splitters used to distribute optical signal
33
Algorithms for secure comms
- Integrity - MD5, SHA
- Authentication - HMAC-MD5, HMAC-SHA-1, RSA or DSA
- Confidentiality - DES, 3DES, AES
34
Integrity
- 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
Authentication
- keys sued for authentication
- at receiver regeneration of hash needs key as well to decode
- as both key and hash needed ensures authentication
36
Confidentiality
- 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
Key management
- 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
IPsec
- transport mode - provides security at layer 4
| - tunnel mode - provides more complete protection at layer 3
39
Linear predictive coding (LPC)
- 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
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