Modems
Lets go through some of the basics. Everyone has some type of modem in their house. Whether it be for the internet, telephone line, tv, or any other device that uses a modulated line. A modem is exactly this. A Modulating/De-Modulating mechanism, the correct term for these devices are CSU/DSU. The function of the modem is to encode a signal so that it can be carried over a form of media from point A to point B and be easily decoded on the other end. Mostly anything that is brought to you by data from the internet is directly transferred through a modem at some point. Originally, the modem started out as a 300 bit/ps piece of hardware looking like this:
Credit to:
Jason "Textfiles" Scott for uploading the picture.
This piece of hardware is basically an acoustic coupler, picking up the fluctuations and tones of the signal. What usually would happen would there would be two modems, similar to that of a client and server, however these modems would be called the Originate modem and the other called an Answer modem. However, though you may be thinking that the Originate modem would be the computer, and the the Answer Modem would be the server it is in reality the opposite. The computer was always the answer modem, and the dumb terminal was the Originate modem. Because the modems would transfer two signals at a time, they were considered full-duplex modems. Surprisingly almost all modems operated in a full-duplex mode and there was a rare occurrence if it operated in half-duplex mode. If you think about the time now, Gigabit Ethernet has now only emerged as completely full-duplex. All speeds 10/100 have been functioning as both half- and full- duplex. These modems would connect to that dumb terminal to gain access to the web, however these days, the modem connects to the Internet Service Provider (ISP) and the ISP connects us to the internet. How this operates is that we have a Point-to-Point Protocol. Once the information (packet) has been transferred to the modem, it is then transferred to the ISP's server and to the server. The same steps are required to get the data from the Server back to your computer. There are different types of connections and classifications for modems.
- Cable
- DSL/ADSL/VDSL
- Phone-line
- Fiber-Optic
And future implementations of networking technology will most likely also have modems.
To understand how a cable modem works, you need to understand a bit about electricity. A signal operates over an electrical field. In the instance of cable or co-axial connections, the data being sent to the modem and TV receiver are able to be put into the same electrical range. Electrical Range is commonly referred to as Bandwidth. Bandwidth is measurable in Hertz, since the transfer rate is measured in Hertz. The only ways to increase and boost transmission range is to increase the amount of lines or increase the speed on the lines. Due to the standards of coaxial cables, these cables can carry several hundreds of megahertz on a given line. However, bitrate is not dependent on frequency. Frequency is a basic ceiling of how high one can go. Since most cable modems can do 45 mbps each way, the bandwidth limit is usually imposed at the ISP's center.
This is a Cable Modem:
Surfboard Cable Modem
A Digital Subscriber Line (DSL) modem works on a Plain Old Telephone Service infrastructure. This infrastructure is basically using the telephone lines that are already in place for day to day internet services. How this service works is the reverse of how the Cable Modem infrastructure works. DSL Providers limit the frequency of the line as much as possible, this also helps limit crosstalk on cables. By keeping the frequency range to a smaller speed they can stuff as many cables/wiring as they can in the given area. Since these cables are usually a standard set however, the cables themselves can support anywhere up to higher speeds, ranging from anywhere to 10 Mpbs to 25 Mbps. A downfall to DSL however is that the signal looses its strength the longer it travels from the provider's office to the box, this is one of the many reasons why when you pay for a service you don't receive the entire rated speed. For instance you buy 10 Mbps download and 2 Mbps upload, and you only receive 5 Mbps download and 264 kbps upload.
This is a DSL/ADSL modem:
A phone-line modem is what several used in the 90's, and still use today either because cable, DSL, or fiber is not in the area, or as a backup for businesses when their own internet system goes down. One of the earliest forms of Internet Connectivity. Since recent forms of dial-up modems max out their transfer rate at 56kpbs, ISP's have come up with server side compression. This allows images, text, and other media on the requested media to be compressed to allow a higher rate of transfer. How this system works is similar to the older methods of modems. It uses a Line-to-Line connection to initiate and connect to the ISP's hub.
Picture of a phone-line modem:
Fiber-optic modems are very similar to cable modems. The reason that they are similar is because that the difference in a cable system backbone and Fiber-optic backbone is that once it hits the providers drop-box, the fiber-optic cabling is terminated and clipped, and then changed to a coaxial cable medium.
Edited by GH0 - 8/4/10 at 9:12pm 
Association control service element (ACSE)—Creates associations between two application entities in preparation for application-to-application communication 
Thanks to Enterprise for his picture
