Cisco 3600 (continued)

Cisco 3600 supports a selection of CODECs:

G.711 A-law 64,000 bits per second (bps)
G.711 µ-law 64,000 bps
G.729 8000 bps

Using Voice Activity Detection (VAD) - you only need to send traffic if there is something to send {Note: telecom operators like this because it enables even higher levels of statistical multiplexing}.

An interesting aspect is that users worry when they hear absolute silence, so to help make them comfortable, it is useful to play noise when there is nothing useful to output. Cisco provides a “comfort-noise command to generate background noise to fill silent gaps during calls if VAD is activated”.

Cisco 3800 supports even more CODECs:

ITU G.726 standard, 32k rate
ITU G.726 standard, 24k rate
ITU G.726 standard, 16k rate
ITU G.728 standard, 16k rate (default)
ITU G.729 standard, 8k rate

Transcript

[slide42] It supported a variety of different CODECs. From G.711 A-law/µ-law coding to G.729 and lots more. But it introduced a very interesting feature called voice activity detection. And voice activity detection is really very simple. If you have nothing to send, you don't send any bits. Right? Makes sense. Why send traffic when I don't have anything to convey? It turns out, however, that while operators love that, because it gives you really great statistical multiplexing, so you're not using any bandwidth when no one's talking. What's the problem? [student answers] No. No timeouts. You are still in the session. The problem is the humans timeout. Because they think it's broken. I don't hear anything. What's wrong? Right? It has to be broken. So the trick was to introduce something called comfort noise. And it turns out that if you play noise in the background that sounds like the noise that you hear when someone's speaking, everyone thinks it's perfectly okay. But what's the advantage? I can generate the comfort noise at the receiver. I don't actually have to send that noise content across the network. Right? So I get the advantages: the users are comfortable and happy. But I got the advantage of not having to send any traffic when I didn't have anything to send. Now, in reality, I actually send some traffic there to make sure that things aren't really broken. But I don't have to send very much traffic to do that.