As already mentioned, the POTS offers more than one alternative for connecting LANs at different locations. Two of these options relate to dial-up connections using modems and connections that used dedicated lines.
In both of these connection strategies, you will run into a device called a modem . Two types of modems exist: asynchronous modems and synchronous modems. Let's take a closer look at how modems are involved in dial-up and dedicated connections over the POTS.
Asynchronous Modem Connections
The simplest and least expensive type of dial-up connection uses a modem to connect two computers. Because computers work with digital information, the digital datathat is, the ones (1) and zeros (0)must be converted so that it can be carried by the analog phone line. This conversion from digital information to analog information is called modulation . When analog information is received on the modem connected to the destination computer (such as a server that is configured with a modem to allow dial-in connections), the data must be converted from analog back to digital. This process is called demodulation . You can see that the name for these devices, modem , is taken from mod ulation and dem odulation.
An asynchronous modem is designed to move data over a regular analog voice-grade telephone line, and it is asynchronous modems that most of us are familiar with. They are used to dial into a network server for telecommuters and are also used by millions of people to connect to the Internet.
An asynchronous modem uses start and stop bits as the way to let the receiving modem know when a particular packet of data starts and stops. Beyond the start and stop bits, many modems incorporate some type of error checking to ensure that the data is sent correctly across the telephone line. The simplest method of error checking involves including a parity bit when transmitting each byte. Essentially, parity checking works like this: The sending modem counts how many 1s are in the data stream. If the number is odd, the parity bit is set to 1. On the other end, the receiving modem also counts the number of 1s and determines whether the number is odd or even. The receiving modem then compares its results with the parity bit. If the results match, the data is probably okay and is sent on for processing. If the parity bits don't match, the data is rejected and the destination computer requests a resending of the data.
Modems are generally available for either internal or external connections to a computer. An internal modem plugs into an expansion slot on the motherboard of your computer and provides an RJ-11 jack (a four-wire telephone jack) from which you can connect a standard telephone cable between the modem and your wall telephone jack.
An external modem is a separate box that is connected to your computer by way of a serial cable (known as an RS-232 cable ). A number of external modems are also available that can be connected to a computer via a USB port. External modems have an RJ-11 jack for your connection to your telephone and normally have lights to indicate various status conditions. Figure 13.2 shows a U.S. Robotics 56K external modem. External modems have a slight advantage over internal modems in that you can reset an external modem ( turn it off, and then on) without having to reboot the computer.
Figure 13.2. External modems can easily be connected to a computer's COM or USB port.
Modem speed is measured in terms of the number of bits per second (bps) that can travel across the phone line. As shown in Table 13.1, the International Telecommunications Union has developed the so-called V series of standards for modem speed. Note that some standard designations contain the words bis or terbo . These designations have nothing to do with speed and are merely the French words for second and third, respectively, indicating that these are revisions of earlier standards. V.90 was declared as the standard for modems in 1988 by the International Telecommunication Union (ITU). V.90-compliant modems can access nearly a 56K connection over ordinary phone lines.
Table 13.1. ITU Standards for Modems
There is some confusion between the bps rate and the baud rate of a modem. Baud is an older term that refers to the oscillation of a sound wave on which one bit of data is carried, whereas bps refers to the actual number of bits transmitted each second. In early modems, the baud rate and the bps rate were equivalent. For instance, an early 300bps modem did, in fact, have 300 oscillations of sound waves each second. However, as technology evolved, engineers developed methods to compress information and manipulate analog signals to send multiple bits with each sound wave oscillation. These improvements rendered the baud rate obsolete as a measurement of speed. For instance, a modem transmitting at 28,800bps might in fact be transmitting at only 9,600 baud.
A synchronous modem doesn't use start and stop bits; instead, the sending and receiving modems actually use synchronization bits to set up the timing for the transmission. Because synchronous modems do not need the start and stop bits used by asynchronous modems, they can achieve much higher data transfer rates than asynchronous modems. However, because synchronous modems are not designed for communication over regular phone lines, they are generally only found in use with dedicated leased lines.
Dedicated leased lines provide a full-time connection between two networks through the POTS or another service provider. Lease lines are typically digital lines and provide much more bandwidth than analog lines; they are also less susceptible to the line noise and interference found on voice-grade connections.
Digital lines commonly used for data transfer include digital data service (DDS) lines and the T-Carrier system, which provides a range of line types for different data rates (T-Carrier lines are discussed in the next section). A DDS line can provide bandwidth of up to 56Kbps and supply your network with a permanent, full-duplex connection (data can be sent and received at the same time).
DDS line connections are actually becoming a thing of the past and are fast being replaced by other WAN technologies. Cheaper T-Carrier rates and other connection possibilities, such as DSL connections (discussed later in "Digital Subscriber Line") and even broadband cable connections, are also making the use of DDS lines a thing of the past.