Internetworking

Internetworking is

based on the interconnection (concatenation) of multiple networks
accommodates multiple underlying hardware technologies by providing a way to interconnect heterogeneous networks and makes them inter-operate.

Public Switched Telephony System (PSTN) uses a fixed sampling rate, typically 8 kHz and encoded to 8 bits, this results in 64 kbps voice coding; however, VoIP is not limited to using this coding and could have higher or lower data rates depending on the CODEC(s) used, the available bandwidth between the end points, and the user’s preference(s).

One of the interesting possibilities which VoIP offers is quality which is:

better that “toll grade” telephony or
worse than “toll grade” telephony (but perhaps still acceptable)

This is unlike the fixed quality of traditional phone systems.

Transcript

[slide23] So, we all remember what internetworking is. The idea of internetworking was we could interconnect these different kinds of networks because we agreed upon a common protocol to use. That means we can accommodate heterogeneous networks. So we don't have to care what the underlying media is, as long as it will accommodate the minimum requirements for the service that we want to have. But public switch telephony systems used a fixed sampling rate. So they said, 8 kHz, that's good enough, because it'll more than cover the typical voice frequency range of users. And we'll sample it at 8 bits per sample, that gives us 64 kbps, that's good enough. And PSTN was stuck at that for decades and decades and decades. Everything ran at that bitrate, except if you were in North America and Japan, where it ran at 56 kbps, because they stole some of those bits for signaling. But the problem was, it was one quality. If we sample it at the same rate, now we can choose, of course, do we do mu-law coding or A-law coding, and maybe we can have a little bit of difference in the quality from that. But basically it doesn't change. But suddenly with VoIP, we can do something very cool. We can have better than toll quality. Toll quality was considered the basic quality of audio going between two public switches. Because now I can have surround sound, 7.1 surround sound in really high fidelity, delivered to my device. But we can also have worse than toll grade, but perhaps still acceptable. So if you had an emergency situation, you could think about downgrading, where you would decrease the voice quality, but still have everyone be able to communicate, by sharing the bandwidth. In traditional systems, we couldn't do that, because we had fixed quality. The best we could do with statistical multiplexing is saying, oh, no one's using their 64 kilobit per second channel, so we can put someone else in it. So why do you think VoIP would be interesting to traditional telephoning providers? [student answers] They can provide better quality of service, but there's a much more fundamental reason they're interested in it. [student answers] Yes, that's all wonderful, but if you look at a business, what's the most important thing? [student answers] That's right. It goes back to the economics. And while the most important is the typical voice activity between two people, how active do you think the channel is? [student answers] Yes. [student answers] Yes. So in the traditional model, we reserved that 64 kilobit per second channel for that pair of users, and it was a 64 kilobit per second channel in both directions, but yet in most cases, half of the time, only one person was speaking. Which means we were wasting enormous amounts of resources. Right? So for an operator, boom! Of course I immediately go to this, because I basically double my capacity for the same investment in bandwidth. That's a really big financial impact.