lesson 1

Question 1

pots

Answer 1

• plain old telephone service
• voice grade telephone service
• analog signal transmission
• copper loops

Question 2

opening of first manual telephone exchange

Answer 2

-1878

Question 3

first commercial automatic telephone exchange was patented in

Answer 3

• 1891
• strowger switch designed by almon brown strowger
• DC current at -48v
• telephones used on-off current pulses to communicate digits 0-9

Question 4

dtmf

Answer 4

• dual tone multi frequency
• developed in the bell system
• known as touch tone
• quickest time between digits = 45 ms
• longest time between digits = 3 s

Question 5

pstn

Answer 5

• public switched telephone network
• the worlds circuit switched telephone networks
• reserved and consistent path for analog signals is allocated upon call establishment

Question 6

isdn

Answer 6

• integrated services digital network
• copper wire pair
• tdm
• virtual out of band control channel
• virtual data channels
• basic rate and primary rate types

Question 7

isdn tdm

Answer 7

-time division multiplexing

Question 8

isdn virtual out of band control channel

Answer 8

-delta (D) channel

Question 9

isdn virtual data channels

Answer 9

-bearer (B) channels

Question 10

isdn varieties

Answer 10

• basic rate (bri)

- primary rate (pri)

Question 11

business telephone systems

Answer 11

• multiline access
• key telephone system (kts)
• private branch exchange (pbx)

Question 12

pbx

Answer 12

• private branch exchange
• operates similar to automatic telephone exchange
• route calls within a business
• can connect to outside world over the pstn using pots or isdn

Question 13

kts

Answer 13

• key telephone system
• hard wired
• shared business lines
• small business

Question 14

pbx cards

Answer 14

• line cards (lan)
• trunk cards (wan)
• control cards (routers/switches)

Question 15

voip

Answer 15

• voice over ip
• allows voice communication to be encapsulated into binary packets for an ip network
• developed in the 80s
• broadband and high speed internet allowed it to be implemented in the 90s

Question 16

uc

Answer 16

• unified communications
• set of products
• the ability to unify multiple modes of communication under a single network

Question 17

analog to digital signal encoding steps

Answer 17

1. sample
2. quantize
3. encode
4. compress

Question 18

nyquist theorem

Answer 18

• determines minimum sampling rate

- sampling rate has to be at least twice the maximum frequency

Question 19

quantization

Answer 19

• assigning amplitude step values
• 256 step scale is used
• rounding introduces quantization noise

Question 20

quantization techniques

Answer 20

• linear quantization

- logarithmic quantization

Question 21

linear quantization

Answer 21

• lower snr on small signals (worse voice quality)

- higher snr on large signals (better voice quality)

Question 22

logarithmic quantization

Answer 22

• uniform snr for all signals
• higher granularity for lower signals
• corresponds with the logarithmic behaviour of the human ear

Question 23

quantization methods

Answer 23

• u-law (Mu-law)

- a-law

Question 24

u-law

Answer 24

• canada, us, japan
• logarithmic scale
• eight positive and negative segments
• 16 steps per segment

Question 25

a-law

Answer 25

- other countries - logarithmic scale - eight positive and negative segments - 16 steps per segment - quantization method of international connections

Question 26

digital voice encoding sample bits

Answer 26

- 8 bits - 1 polarity - 3 segment - 4 step

Question 27

bandwidth required for one call

Answer 27

- 64kbps - 8000 samples per second - 8 bit samples - tdm is used to combine multiple 64kbps channels into one physical line

Question 28

compression codecs

Answer 28

- g.711 - g.722 - g.729 - g.729b

Question 29

g.711

Answer 29

- pulse code modulation (pcm) - digital representation of analog signal - sampled regularly at uniform levels - pcm samples voice 8000 times per second

Question 30

g.722

Answer 30

- adaptive differential pulse code modulation (adpcm) | - only the difference between one sample and the next is transmitted

Question 31

g.729

Answer 31

- conjugate structure algebraic code excited linear prediction (cs-acelp) - commonly used is annex b (g.729b) - current revision is g.729.1 (annex j)

Question 32

mos

Answer 32

- mean opinion score - used in qos - number 1-5 - based on rating of, speech quality and level of distortion

Question 33

mos rating 1

Answer 33

- speech quality: unsatisfactory | - level of distortion: very annoying and objectionable

Question 34

mos rating 2

Answer 34

- speech quality: poor | - level of distortion: annoying but not objectionable

Question 35

mos rating 3

Answer 35

- speech quality: fair | - level of distortion: perceptible and slightly annoying

Question 36

mos rating 4

Answer 36

- speech quality: good | - level of distortion: just perceptible but not annoying

Question 37

mas rating 5

Answer 37

- speech quality: excellent | - level of distortion: imperceptible

Question 38

pesq

Answer 38

- perceptual evaluation of speech quality | - worldwide industry standard

Question 39

dsp

Answer 39

- digital signal processor - application specific integrated circuit (asic) used for telephony - used at endpoints and audio conferencing - converts analog to digital and back again - translates codecs as well (transcoding) - installed on a isr as a pvdm (packet voice digital-signal-processor module)

Question 40

features of dsp

Answer 40

- echo cancellation - vad - cng - jitter removal