Monday, July 3, 2017

Connecting to the Internet


Telephone lines can be used to transmit computer data. This was how people connected to the internet in the early days. However, telephone wires are designed to transmit analog information, and computers can only consume digital information. A dial-up modem converts analog signals to digital and vice versa.

To connect to the internet with a dial-up modem, you enter a phone number for your modem to call, which is provided to you by your ISP. You also have to provide a username and password. The connection process is noisy and takes several seconds to complete. ISPs would often charge by the minute, so you never wanted to leave your connection open when you weren't using it (you also couldn't make phone calls while connected). Dial-up connections use a protocol called Point-to-Point Protocol (PPP), which is specifically designed for transmitting data over dial-up.

The unit of measurement that is used to measure the speed of data across a telephone line is a baud. The maximum speed a telephone line can achieve is 2,400 baud. As modems improved over time, they could pack more and more bits into each baud. For example, a 33.6 Kbps modem can pack 14 bits into each baud (2400 x 14 = 33,600). The highest speed that can be achieved through dial-up is 56 Kbps. Dial-up connections also have fairly high latency compared to other internet connection approaches.

To try to break the 56K barrier, some ISPs experimented with server-side compression. This involved compressing certain kinds of data before sending it over the wire to the client, resulting in higher download speeds. This approach was hugely successful for certain kinds of data that can be easily compressed, such as HTML pages and plain text. But many data formats are already compressed, such as ZIP files and streaming video, so no speed improvements could be gained from them. Image file formats like JPEG and PNG already use compression, but ISPs would compress them even more, resulting faster speeds, but, as a consequence, a loss of image quality.


As dial-up modems began approaching the 56K limit, telephone companies began converting all their analog telephone lines to digital. The process of sending digital signals across digital telephone lines is called ISDN, and it allows speeds of up to 64 Kbps (wow!).

An ISDN line contains two types of channels. Bearer (B) channels are used for voice and digital signals and run at 64 Kbps. Delta (D) channels are used for setup and configuration data and run at 16 Kbps. A common setup would be to install two B channels and one D channel, giving you speeds of up to 128 Kbps. This setup was referred to as basic rate interface (BRI). A more powerful, but less common, setup involved twenty-three B channels (providing 1.544 Mbps) and one 64 Kbps D channel. This was called primary rate interface (PRI) or a T1 line. The main downside to ISDN connections was that you had to be within 18,000 feet of the central ISP building for it to work.


Digital subscriber line connections use your telephone line like dial-up, but the connection is always-on and is much faster. They also allow you to make phone calls while the connection is active. Speeds can vary anywhere from 3 Mbps to hundreds of Mbps. The most common type of DSL connection is asynchronous DSL. ADSL has upload speeds that are slower than download speeds. On the other hand, synchronous DSL (SDSL) gives you identical upload and download speeds, but is more expensive. Just like with ISDN, you must be within a certain distance of the main ISP office. The distance can vary from a few hundred feet to 18,000 feet.


A cable connection piggy-backs off of your cable television connection. It provides upload speeds between up to 20 Mbps and download speeds of over 100 Mbps.


There are two kinds of fiber connections. In fiber-to-the-node (FTTN), the ISP installs a central box somewhere in your neighborhood, which is connected to the actual fiber line. Then, the individual houses connect to the box using standard Ethernet or coaxial cabling. In fiber-to-the-premises (FTTP) your house is directly connected with the central office via fiber. Fiber varies in speed, but can be as fast as 1 Gbps (which is what Google Fiber provides). In some cases the download speed matches the upload speed. I have an FTTP fiber connection that gives me 100 Mbps upload and download speeds.


The main benefit to a satellite connection is that it works anywhere in the world. No infrastructure is required (telephone lines, cable lines, etc). A satellite dish must be professionally setup so that it has line-of-sight communication with the satellite up in space. The main downsides are: higher than average latency and signal degradation in cloudy weather.


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