Statistical Multiplexing

This is a discussion of statistical multiplexing, and the difference between the old paradigm of time division multiplexing and the new paradigm of statistical multiplexing. It is what the Internet and increasingly, all of our networks are based upon.

The entire Internet is a statistically multiplexed place, unlike the Telephone network, which is a "deterministically" multiplexed place . They are, historically two fundamentally different transport paradigms. That is why you had "data communications," and then "telecommunications" industries. They are colliding now, as "data communications" technologies are becoming more capable of providing the "telecommunications" services.

The division of course, was between "data" and "voice" which have entirely different requirements in transport. Voice (or video to a lesser degree), requires almost no delay, and almost no "jitter" in the underlying transport in order to be intelligible. Data, such as your latest MP3 download, could really care less if there is some random delay in the transport, and if it is very bursty (jittery).

That fundamental distinction enables data communications to built with much cheaper technology, both in transport and computing.

Consider the following. An ADSL line has typically a 1 MBit/s download speed. As many as 500 ADSL customers will share a DS3 pipe which feeds all of their downloads. But a DS3 has only a speed of 40 Mbit/s. So it is only capable of feeding 40 people continually at the 1 Mbit/s download speed. But because of the usage and traffic patterns of the ADSL customers, they do not use 1 Mbit/s at all times. In fact they only average maybe 10 Kbit/s. So we can "statistically share that 40 Mbit/s downstream pipe over all 500 customers, and they will be served just fine.

Telecommunications circuits dedicate bandwidth at both upstream and downstream. Your voice channel thru the telephone network is typically 64Kbit/s. And this bandwidth is reserved throughout the entire telephone network for the entire duration of the telephone call to guarantee that there will be no delay or jitter in the delivery of voice.

Now in the modern world, it is much more complicated than this simple explanation, but this is the foundation for the differences. An ADSL service is typically 1.5 Mb/s down, and 256 Kbit/s upstream. But the effective "dedicated" bandwidth on the upstream pipe may be 60 Kbits and 15 Kbit/s [just for example]. A DS1 service, which is a telecommunications service has a bandwidth of 1.4 Mbit/s down and 1.4 Mbit/s upstream, and this bandwidth is not statistically multiplexed with any other traffic. It is there at all times. The "effective" dedicated bandwidth is the same as the actual service bandwidth. So a DS1 service costs typically ten times what an ADSL service costs, say $500 instead of $50. So it makes perfect sense for the pricing difference.

So there you have a discussion of "data communications" and "telecommunications." Increasingly in this day and age, everything is being provided over data communications transport, which means "statistically multiplexed transport," because of the traffic and usage characteristics of the underlying data, and the ability to build the underlying transport with tremendous excess bandwidth. The underlying technology is so fast that it can smooth out the delay and jitter in the underlying data delivery to make it appear like a "telecommunications" circuit. Thus you have "voice over IP," "Packet over SONET," "webcasting," "Internet telephones," "video conferencing over the Internet using Netmeeting or similar technologies.

TCP/IP STUFF

WIRELESS STUFF

PC STUFF

ADSL/CABLE MODEM STUFF