Tuesday, March 22, 2005

Slicing and Dicing Digital Carriers

Digital carriers, such as T1 & T3 copper lines and OC3 through OC48 fiber optic lines, are something like roads. Smaller country roads merge into superhighways with enormous traffic capacity and then dissolve into multiple smaller roads again. When this is done with electrical and optical signals it goes by names like multiplexing, demultiplexing and inverse multiplexing.

The first telephones were not multiplexed. It was one conversation on one pair of wires. If you've seen those old time pictures of New York City crammed with telephone poles and a virtual net of phone wires overhead, you can see how this method gets out of hand quickly. The first improvement was to multiplex many telephone calls onto one set of wires. Multiplexing simply means merging a bunch of low capacity circuits into one big one. They did this using carrier signals at widely separated frequencies to create channels for the individual phone calls. It works similar to analog cable TV where the stations come to your set on one wire but are kept separate because each station is on its own channel. This is called frequency division multiplexing.

We think of frequency division multiplexing (FDM) as old hat, but its modern equivalent is wavelength division multiplexing (WDM) used in fiber optic carriers. Each wavelength, called a color or lambda, is also a separate frequency that is digitally modulated by the information it carries. All the colors travel on the same fiber strand without interfering with each other, just like all the TV channels travel on the same cable.

A purely digital form of multiplexing is TDM or time division multiplexing. Groups of bits are loaded onto a wire one after the other. The first 8 data bits may be channel 1, the next 8 can be channel 2, and so on. A T1 line builds 24 of these channels of 8 bits each. It then adds a framing bit at the beginning of this 192 bit stream so it can keep track of where the channels are. A multiplexer takes individual phone calls or modem channels and assigns them to particular channels. At the other end, a demultiplexer takes the bitstream apart and recreates the original 24 channels. Equipment that does this for telephone calls is known as a channel bank.

Now consider a bigger wireline such as a T3, or an optical carrier like an OC3. Instead of multiplexing individual small channels, these lines multiplex entire T1 or larger lines. An OC3 can multiplex 84 T1 lines or 3 T3 lines. Since all these lines are based on TDM, the multiplexing and demultiplexing is fairly straightforward. It's all based on channels having their fixed place in the bitstream.

Like a superhighway, a high speed digital carrier has interchanges known as add/drop multiplexers. These are the on and off ramps. The DS3 service for your building may well be "dropped off" from an OC3 fiber optic line using an optical add/drop multiplexer. Unlike the terminal multiplexers and demultiplexers at each end of the cable, the idea of an add/drop multiplexer is only to access certain channels while letting the others pass through unaffected. For wavelength division multiplexing, a newer device called a ROADM or Reconfigurable Optical Add Drop Multiplexer, can be remotely controlled to insert and drop particular wavelengths at a particular node in the network.

So what is inverse multiplexing? That's when you carry large amounts of traffic using lots of small carriers. A common example is bonding 2 or more T1 lines to make a larger capacity carrier. Bonding up to 6 T1 lines to get as much as 9 Mbps bandwidth from 1.5 Mbps individual T1 lines is often less expensive than buying one 45 Mbps T3 line that may be far more capacity than you really need.

T1 Rex will help you get the best prices for all your bandwidth needs, from fractional T1 lines up to the largest optical carriers. Get a quick complementary digital carrier quote and consultation now.

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