Dial-up
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.
ISDN
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.
DSL
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.
Cable
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.
Fiber
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.
Satellite
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.
References
-
Wikipedia
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