lesson 1 Flashcards
pots
- plain old telephone service
- voice grade telephone service
- analog signal transmission
- copper loops
opening of first manual telephone exchange
-1878
first commercial automatic telephone exchange was patented in
- 1891
- strowger switch designed by almon brown strowger
- DC current at -48v
- telephones used on-off current pulses to communicate digits 0-9
dtmf
- 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
pstn
- public switched telephone network
- the worlds circuit switched telephone networks
- reserved and consistent path for analog signals is allocated upon call establishment
isdn
- integrated services digital network
- copper wire pair
- tdm
- virtual out of band control channel
- virtual data channels
- basic rate and primary rate types
isdn tdm
-time division multiplexing
isdn virtual out of band control channel
-delta (D) channel
isdn virtual data channels
-bearer (B) channels
isdn varieties
- basic rate (bri)
- primary rate (pri)
business telephone systems
- multiline access
- key telephone system (kts)
- private branch exchange (pbx)
pbx
- 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
kts
- key telephone system
- hard wired
- shared business lines
- small business
pbx cards
- line cards (lan)
- trunk cards (wan)
- control cards (routers/switches)
voip
- 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
uc
- unified communications
- set of products
- the ability to unify multiple modes of communication under a single network
analog to digital signal encoding steps
- sample
- quantize
- encode
- compress
nyquist theorem
- determines minimum sampling rate
- sampling rate has to be at least twice the maximum frequency
quantization
- assigning amplitude step values
- 256 step scale is used
- rounding introduces quantization noise
quantization techniques
- linear quantization
- logarithmic quantization
linear quantization
- lower snr on small signals (worse voice quality)
- higher snr on large signals (better voice quality)
logarithmic quantization
- uniform snr for all signals
- higher granularity for lower signals
- corresponds with the logarithmic behaviour of the human ear
quantization methods
- u-law (Mu-law)
- a-law
u-law
- canada, us, japan
- logarithmic scale
- eight positive and negative segments
- 16 steps per segment
a-law
- other countries
- logarithmic scale
- eight positive and negative segments
- 16 steps per segment
- quantization method of international connections
digital voice encoding sample bits
- 8 bits
- 1 polarity
- 3 segment
- 4 step
bandwidth required for one call
- 64kbps
- 8000 samples per second
- 8 bit samples
- tdm is used to combine multiple 64kbps channels into one physical line
compression codecs
- g.711
- g.722
- g.729
- g.729b
g.711
- pulse code modulation (pcm)
- digital representation of analog signal
- sampled regularly at uniform levels
- pcm samples voice 8000 times per second
g.722
- adaptive differential pulse code modulation (adpcm)
- only the difference between one sample and the next is transmitted
g.729
- conjugate structure algebraic code excited linear prediction (cs-acelp)
- commonly used is annex b (g.729b)
- current revision is g.729.1 (annex j)
mos
- mean opinion score
- used in qos
- number 1-5
- based on rating of, speech quality and level of distortion
mos rating 1
- speech quality: unsatisfactory
- level of distortion: very annoying and objectionable
mos rating 2
- speech quality: poor
- level of distortion: annoying but not objectionable
mos rating 3
- speech quality: fair
- level of distortion: perceptible and slightly annoying
mos rating 4
- speech quality: good
- level of distortion: just perceptible but not annoying
mas rating 5
- speech quality: excellent
- level of distortion: imperceptible
pesq
- perceptual evaluation of speech quality
- worldwide industry standard
dsp
- 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)
features of dsp
- echo cancellation
- vad
- cng
- jitter removal
