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The basics of data and voice transportation
By Red Squirrel
Basic communication
Any type of communication, whether it be voice, or data, needs to be able to receive and send. However, it also depends on the type of circuit. For example, your TV only needs to receive data from your cable TV, and not send, so this type of circuit only sends data to customers, and does not need to receive. But a phone or internet circuit for example, needs to be able to send and receive – at the same time. When you are talking on the phone, you can also hear what the person at the other end is saying, if he or she interrupts you. Type of channels are classified in 3 categories: simplex, half duplex and full duplex.
A channel that can only send in one direction is called a half duplex channel. It can only send data one way and not the other. Cable TV would be a good example, as mentioned above. Half duplex can send and receive, but only one way at a time, for example, a “walkie talkie”. A full duplex channel can send and receive at the same time, for example, a telephone, or the internet.
T1, DS1, DE-4
To make transportation easier, each customer of say, a telecommunication company offering dedicated circuits for various equipment such as cash machines (datapac circuits), the data is transported on a single line with multiple channels. Each channel is 64Kbit/sec. This does not sound fast, but it’s usually enough speed for dedicated circuits, which are usually used by certain companies that have offices at various regions. A T1 is 24 chanels, which means 64Bbits times 24, which makes a T1 roughly 1.5Mbit/sec. When a customer has part of a T1, say, 2 channels, this is called a fractional T1. Suprisingly, video conference usually uses about 256k which is even quite fast for video conference. When you isp advertises 1MB/sec (800Mbits/sec), it is rare that it will get there, as it is not your own dedicated line, you may get as low as 200KB/sec, which is what I get, and I am very close to the central office (where it connects to switches and goes off to "the internet").
In this article, we are mostly talking of dedicated circuits, as in, circuits that a business will purchase and have installed in their store. These are usually 64Kbit circuits which are used simply for radio and other low end communication, and not the internet. However, a school board might own an entire T1 circuit to themselves for the schools’ lans, which is why it is always fast when you connect to other schools with Netbios.
Usually, when a telecommunication company transports all this data from city to city, they won’t send it out as separate wire pairs in 24 channel "bunches". This would be way too many wires to send out! Usually, they will put it into more circuits. A T3 (same as DS3, and T1 is the same as DS1) for example, can carry 28 T1s, so that’s 64kbits/sec times 24 times 28, which makes a T3 about 43Mbit per second, usually referred to as 45Mbit/sec just to round it up. However, a T3 does not have as much bandwidth as fiber optic, and fiber optics are more efficient as they are less "resistant" as they use light and not electricity to communicate. Most of the time, transportation from city to city is send out on FOTS (Fiber Optic Transport System). Usually several fibers are put underground and sent out to various cities and communities. They however need repeater points at each 3 miles are so, as the light starts fading away, as glass, like anything else, is not a vacuum. The smaller the fiber, the farther it can go, as the laser is not bouncing as much inside from "wall" to "wall". When a fiber arrives at it’s destination, it is then read and broken down into various line types such as T1’s, and separated into it’s channels at the central office, and sent out to customers. There are 2 types of fibers. Single mode and multi mode. Single mode is way cheaper, but less efficient as the loss is higher (roughtly 3db per KM) because they are thicker. Multimode are more expensive, but are very small (8 microns thick!) which means there is less loss from data "bouncing around". The loss is about .3db per KM!
How channels work
To send data/voice on a single line, it is separated into channels. If you listened to a T1, you would actually just hear noise, as it is 24 circuits in one and they would mix in. However, to listen to a single channel, the equipment at the other end must know what channel number it wants, and much listen at each 8000th of a second (usually this is the case, depends on the equipment, but 8000 is the standard) and put this data together. Look a it as two wheels turning very fast simultaneously and have 24 pie pieces in them:
Because this "turning of the wheel" is very fast, taking each instant of communication on channel 6 and putting it together will create a fairly clear conversation. Telephones use this exact method and usually these same type of circuits. When you are talking on the phone, your conversation is actually cut 8000 times a second in order for the other conversations to go on the same circuit, but because of this speed, you don’t notice. The faster this is done, the better. 8000 is by far the best and has been standardized in the telecommunication industry.
There is way more information that can be put into this subject, but the article ends here. I hope that it is informative to you and that you enjoyed it! I’m fairly new myself in the telecommunication industry, if you think something is irreverent, please DO let me know, but I am pretty positive on everything.
Red Squirrel,
IceTeks Administrator and Webmaster
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