Termination schemes? What the hell is that?
Ok… Delving into something that you use everyday (I HOPE!) and complain most of the time about it. The internet.
To be precise, it’s the connection rather than the internet itself. To most users that have started using the internet not long ago, I bet you guys have not even heard about dial-up. Dial-up was somewhat similar to the fast ‘cable’ connection that many users are using right now, albeit much slower. The fastest connection back then was using a 56k modem.
So… How far have we leaped in terms of connectivity? Internet pages that used to load so very slowly in the yesteryears now materialise almost instantaneously once called. In Australia, the fastest connection that can be afforded by average users would be the ADSL2 line, sitting comfortably at 24Mbps (Mega-bits per second). This means, on average, the speed should hit about 2.4 Megabytes per second. If you’re downloading a song which is about 6Mb (depending on encoding and quality) it takes about 3-5 seconds to download that song! Fast huh? A cheaper alternative (and the one I’m using cause damn Carlton is limited by the cable type to hit ADSL2) would be ADSL1, which hits a maximum speed of 1.5Mbps. This translate to about 150kb per second, which is a far cry from the ADSL2 line, but still good enough for the things I do.
How do these all work though? How are we able to move from a 56kbps to a 1.5Mbps line without ripping all the cables out of the ground and replacing them? (The same does not apply to 24Mbps since we NEED to rip the cables out of the ground and replace them) The answer lies in termination schemes. Now the question is… What are termination schemes? To put it in short, they are means whereby a transmission line is terminated. Then you’re thinking again… Why must they be terminated? This is a semi-complicated issue, but I’ll try to simplify it as much as possible.
Imagine yourself walking from where you are sitting right now, to say, the toilet. This is assuming you’re sitting comfortably at home, at which the toilet should be at a comfortable walking distance. Now, take that distance, and imagine an ant walking from point A to point B. Far huh? Well, this is what happens in transmission lines. Data bits are transmitted at a rate so fast, that even short wires seem long. This actually translate to the delay and the limitation on how fast data can be transmitted. With these kind of fast transmitted signals, wires which we always assumed to have almost negligable resistances, inductance and even capacitance, now have these properties ‘magnified’. These properties must now be taken into account when transmitting data.
This magnification of the impedances (I will now lump all of them into this term) will cause power loss, and at a worst case, ringing. No, this isn’t the telephone kind of ringing! Ringing occurs when the modelled capacitor and the inductor resonate with one another. Think of an opera singer hitting a note that breaks glass. The principle is the same since the fast changing voltages across those capacitors and inductors (due to the fast changing data transmitted) cause them to resonate. This causes unwanted signal propagating to and fro the transmission line. By limiting the rate at which the data is transferred, we can then avoid corruption of the data transmitted. This would be why we need to rip cables and replace them if we were to transit from ADSL1 to ADSL2. The properties of the cables would inherently be different.
How about signals ‘bouncing’ off the receiver end? Hmm… This actually comes down to refraction. Remember physics in high school? Optics? Well, the same principle applies. Light can be thought of as an extremely fast oscillating wave. By passing it through 2 objects of different medium, (a good example will be from air to water) the light beam will actually ‘bend’ and ‘reflect’. This depends on the angle at which the beam enters the 2nd medium from the first. If the angle is 90 degrees, the reflection will be superimposed onto the transmitted beam, with the ‘bending’ effect unable to be seen. The same actually occurs in transmitted signals. Due to the receiver end having different impedances compared to the transmission line, the signal actually ‘bounces’ back to the transmitter. This obviously corrupts the next signal that is to be transmitted.
The scheme I’ve described is called ’shunt termination’ or ‘end termination’. There are other methods like ’series termination’, ’split termination’ and ‘AC termination’. Each of them have their own pros and cons, and should be applied only where appropriate. Actual analysis of these terminations actually involves a lot of electrical terminology, and an average layman would not be able to follow anything I’ve typed out.
By having a good knowledge of the transmission line, current methods are able to obtain an optimum speed at which data can be transmitted. Ripping the cables out and replacing them will have to happen sooner or later though, since technology is always improving and that seems to be the only way to advance along with it.
KW
P/S: If you feel like I should just delve right into the mechanics of things, feel free to say so. My next posts will then concentrate more towards the basics of things, then moving on to the more advanced sections of electrical engineering.
