This is Section 2.9.1 of the new Telecom 101, 4th edition
print and ebook available January 2016
.
Brought to you by BOOT CAMP
January 25-29 2016 – Silicon Valley
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This diagram provides a very high-level block diagram view of the processes involved in communicating speech in IP packets from one person to another:
Starting on the left, commands from the speaker’s brain cause combinations of lungs, diaphragm, vocal cords, tongue, jaw and lips to form sounds.
A microphone is positioned in front of the mouth and acts as a transducer, creating a fluctuating voltage which is an analog or representation of the sound pressure waves coming out of the speaker’s throat.
This is fed into a codec, which digitizes the voltage analog by taking samples of it 8,000 times per second and coding the value of sample into binary 1s and 0s. Typically, the value of each sample is represented with a byte, meaning 64 kb/s to be transmitted.
Approximately 20 ms worth of coded speech is taken as a segment and placed or encapsulated in an IP packet.
The IP packet begins with a header, which is control information, the most interesting part being the IP address of the source telephone and IP address of the destination telephone.
IP packets are moved from the source to the destination over a sequence of links.
The links are connected with routers, which relay the packets from one link to the next.
Lower level functions such as framing and link addressing are usually performed following the IEEE Ethernet and MAC standards.
At the lowest level, the links are physcially implemented with Category 6 LAN cables, DSL modems, Cable modems, fiber optics and radio systems.
At the destination, the bits are extracted from the IP packet and fed into a codec, which re-creates the analog voltage.
This voltage drives a speaker, which re-creates the sound pressure waves, which travel down the ear canal to the inner ear, causing hairs on the cochlea to vibrate, triggering neural impulses to the brain, making the listener think they are hearing something.
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It’s important to note that the voice packets are communicated directly from one telephone to the other over the IP network.
The packets do not pass through a CO telephone switch, for example.
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