(notes after cioara)
Voice
Ground is tip, positive, one wire; battery is ring,
negative, other wire
On hook is open circuit; off hook is closed completing 48vdc
circuit
Glare happens when phone is picked up as a call comes in
Digitizing voice means changing analog signals into numbers
TDM (time division multiplexing) allows for networks to
carry multiple conversations over a single 4 wire pair (numbers sent are
divided by time)
T1 24 separate 64kbps
channels known as a digital signal 0 (DS0)
Signaling – 2 types
CAS and CSS
Channel associated signaling – transmitted using the same bw
as voice
Common channel signaling – transmitted using separate,
dedicated channel
Q.931 is the most popular signaling protocol, which is used
by ISDN
Uh oh, isdn never goes
away…
Just as in life, those who do not pay heed to history are
doomed to repeat it; pstn and pbx should be looked at…
Benefits of VOIP
Cisco loves
comparisons
reduced communications cost
reduced cabling cost
seamless
reduced cost of moving the phone from a to b
unification – email, voicemail, fax
open standards (wait, did I just say open standard and cisco
in the same breath)
quantization is how an analog signal is converted to a
number 0-255 or +127 and -127
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