DSL and ISDN

[jbcohen]

Here is what I think I know about how DSL works:

There four coper cables coming into every home but only two are used for voice, one coming in and one going out. DSL remedies this situation by pairing the four coper cables in two groups of two. One set is set aside for voice and the other for data transmissions, which is how it can get people talking and computing at the same time. This understanding does not explain how DSL can manage to achive its broadband bandwith with the remaining two data coper cables. Since DSL runs over coper cables how can the technology manage to achive its bandwidth on the same coper cables that dial up does? What do I not understand about DSL?

ISDN - Integrated Digital Services Network - is an older residential technology that required the pulling of new cables into the homes. It fell out of favor with home owners because it required new cables in the home.

Obviously my knowledge is incomplete about both technologies, which is my reason for posting here. What am I missing?

 

[Trotter]

DSL uses a higher frequency than voice communications. This allows the same wires to carry both. Think of the wires as a four lane highway with POTS (plain old telephone service) using the slow lane only and DSL communications using the fast lane as well as the slow lane.

Here's a decent article explaining DSL:
HowStuffWorks "How DSL Works"

ISDN is a bit different than DSL, even though they operate on the same premise. ISDN uses the phone line as a data conduit, but can use either one or both of the wires. If it uses one, it gets the speed of 64Kb and you use the other for the telephone. If you use both wires, you get 128Kb but can't use the phone.
How ISDN works

 

[jbcohen]

The articles explained a few things. In terms of the highway analogy ISDN takes the data and sends it down all of the faster moving lanes in the highway. DSL uses the same principal by taking the data and using faster moving vehicles to move the data down the same lanes of the highway thus obtaining faster speeds then ISDN could because its using faster vehicles. And Fiber is installing a whole new faster highway.

 

[Nickless]

One of the major benefits of dsl is that the DSL is sent over the ATM network, using dense wave multiplexers. This is how the data cells are encapsulated, saving space on the line, while using a narrower frequency. The only (minor) downside is that there is a small overhead cost on such connections. In our network (AS286) the cells are 53 bytes, using 5 bytes for header information. Larger cells would lower that cost. On our network we take into account about 10% of the bandwith for overhead costs.


Hope that helps.

 

[jbcohen]

Excellent explination Nickless also put in a languege that I understand best. Now on to Fiber, I think that I know that fiber is also a broadband technology but is sent over different cables rather than coper cables it uses fiber cables which carry the data over higher speeds. Its sort of like taking the highway riping it out and replacing it with a whole new higher speed highway.

 

[Trotter]

Forget highway... think bullet train that runs on a laser.

 

[jbcohen]

I have DSL at home and sometimes it runs slowly and some times quickly. Any idea why that is? Based on the explinations given that should not be possible for data to arrive at multiple speeds it should all be either fast or all slow. Perhapse thedifference in the speeds is due to the fact that DSL is a shared technology and needs to fit its data onto a comunal pipe where as fiber is a dedicated technology.

 

[Trotter]

DSL is not throttled as bad by heavy traffic as cable is. Mainly when DSL slows down it is like that for everyone on the internet.

DSL is pretty much a direct connection. Cable is a shared pipeline that is only as fast as the data can move... more people on slows it down as they are all trying to use the same pipe.

 

[majistic]

roger, I have cable and I can always tell when my neighbors are using the pipeline it slows way down. I prefer DSL to cable although I have cable.

 

[oldskool]

I have DSL at home and sometimes it runs slowly and some times quickly. Any idea why that is? Based on the explinations given that should not be possible for data to arrive at multiple speeds it should all be either fast or all slow. Perhapse thedifference in the speeds is due to the fact that DSL is a shared technology and needs to fit its data onto a comunal pipe where as fiber is a dedicated technology.

DSL is digital electronic pulses, but fiber is light photons - small particles carrying made up of light energy. No contest. Light travelling through fiber optic cable approaches the speed of light in a vacuum , but not quite, since the medium is pure silica glass. Not perfect, but compared to a vacuum (like space), light through fiber is in a ratio of about 1.1 : 1 - meaning that it is just a tad bit slower than light in space - which is the fastest known energy in the universe ! Woot !

DSL like Trotter said, is digital pulses over copper , and the reason it speeds up and slows down is due to several factors, network balancing, increased number of users at any given time, etc. Packets traveling through Ethernet arriving at the other end (the target computer) accept the packets, even if the data is out of order, it gets synched up later as it travels through the OSI layers...

It will send that data with its encoded data up through the seven OSI layers, and it is up to the TCP/IP layer to interpret the data order. Your NIC (network interface card) couldn't care less what order it receives the packets, but it matters to the TCP/IP layers (third and fourth layers of the OSI model) which is the different layers from hardware to software that data travels through to get to the application on the other end of the transmission.

The reason I went through all that was to demonstrate that DSL sends packets (blocks of data) using electronic pulses over copper, and that can never compare to the speed of fiber. CAT 6 and now CAT 7 ethernet wire can scream along, but in similar measure lengths - like an imaginary race track - fiber will outperform DSL in a race.

In real world applications, though, fiber hits bottlenecks in ethernet networks and can still be slowed down at the bottleneck. Not the fault of the fiber, but the bottleneck itself.

Thats why it is only feasible in terms of pure speed for fiber to be on fiber-exclusive links (otherwise it gets somewhat limited) , and that is why some businesses and major colleges use fiber links for their backbones - dedicated, reliable speed and huge bandwidth, without the bottlenecks (relatively speaking, anyway).