Making Remote Connections

So far, this chapter has discussed input and output in relation to a single computer. Input and output also can occur between two computers in different locations. Special devices called modems enable computers to connect to one another over the public telephone network. Through the use of modems, digital signals from the computer are sent over regular phone lines or, in some cases, over television cable.

Accessing the Internet through a traditional modem requires several steps. First, there must be a phone connection to the central office (CO) of the telephone company. The physical line between the CO and the commercial location is called the local loop. The local loop is terminated at both ends. At the commercial office end, the phone is plugged into a wall jack. At the telephone company, the cable terminates and is plugged into a telephone switch. Incoming calls to the switch are then forwarded to the appropriate location.

local loop

The two-wire copper telephone cable that runs from a home or office to the central office of the telephone company.

bits per second (bps)

The number of bits, or ones and zeros, transmitted each second.

modulate

To convert digital data into analog signals. Modulation enables digital computer data to be transferred over standard telephone lines.

demodulate

To convert an analog signal back to digital data. This is typically done on the receiving end of a computer transmission using standard phone service.

When data is transmitted between two computers, the transmission speed is measured in terms of bits per second (bps) and not bytes per second. This table shows the difference between the two.

Bits	Transmission Speed
1 bit	1 bit per second (bps)
1,024 bits	1 kilobit per second (Kbps)
1,024,000 bits	1 megabit per second (Mbps)
1,024,000,000 bits	1 gigabit per second (Gbps)

There are several communication technologies that enable computers in different locations to share data. Each uses a different signaling format, medium, or both, which yields varying levels of performance. In the following, you will learn about these technologies.

Analog Modems

Modems function as input and output devices. They enable computers to communicate with one another over great distances. The word modem comes from the merging of the two words modulate anddemodulate. Modems that send data convert digital signals from a computer into analog audible tone signals that can be transmitted over phone lines, and modems that receive data do the reverse.

Digital signals are made up of discrete values. The values represent an on or off state, which are the ones and zeros of computer language. Discrete voltages represent the on or off conditions. An on condition is represented by +5 volts, and an off condition by -5 volts or a zero value. The values do not change over time; they change instantaneously from one state to another.

Analog signals are constantly changing values. The values represent fluctuations in voltage or sound. An analog signal can consist of an infinite number of possible values.

RS-232

An interface standard for use between data communications equipment (DCE) and data terminal equipment (DTE).

baud

A measurement of the number of signals that are transmitted each second.

Digital Subscriber Line (DSL)

A digital signaling method used to transmit data over regular phone lines at speeds up to 6Mbps. DSL uses Asynchronous Transfer Mode to pass data in fixed-size cells.

Modems can communicate at speeds from 300bps to more than 56,000bps. Modems are backward compatible and slow their transmission speed to communicate with slower modems.

Modems come in two types: internal and external. External modems require a connection to an available serial port on the back of a computer using an RS-232 cable. Internal modems plug into an expansion slot inside a computer.

Note 

A term often associated with modems is baud. Baud is the number of frequency changes made per second. This term is often used synonymously with bps. This is accurate at low speeds, such as 300 baud. At higher speeds, modems use a technique that enables multiple bits to be sent on each frequency change. So, for example, a modem operating at 9,600bps may operate at only 2,400 baud.

Digital Subscriber Line (DSL)

Digital Subscriber Line (DSL) is a relatively new and inexpensive technology offered through the local telephone company. DSL offers much promise in the residential/small office Internet access market because of its high bandwidth potential. Unlike modems, which must convert digital signals to analog to transmit over the public telephone network, DSL signals are digital from beginning to end.

bridge

A network device that enables networks using different transmission signals to communicate with one another.

Asynchronous Transfer Mode (ATM)

A network technology that uses fixed-size cells to transfer data. The fixed-size cells enable it to provide better performance.

Internet service provider (ISP)

A company or organization that provides the user with access to the Internet, typically for a fee. Users may gain access by using any one of many remote connection technologies, including modems, DSL, ISDN, cable modems, and others.

Ethernet

A network communication technology developed by Xerox that encloses data with a destination and source address for delivery, which is called a frame. Additional information for Ethernet is also added to the frame.

The completely digital transmission vastly improves performance in several ways. First, DSL does not have the transmission limitation of 56Kbps found with traditional modems. In fact, DSL is capable of transmitting 6Mbps-nearly as fast as a network connection. Second, the DSL modem does not have to convert a signal from analog to digital, and thus slow transmission, as modems do. A DSL modem is not a modem at all; it is actually a bridge. Since most people are comfortable with the term modem, it made more sense to use modem rather than introducing a new term.

DSL modems are able to achieve high transfer rates by using a technology called Asynchronous Transfer Mode (ATM). ATM uses cells to transfer data. The DSL modem is a bridge that converts ATM cells coming from your Internet service provider (ISP) into Ethernet frames. Most computers with the right hardware can communicate via Ethernet network technology.

DSL is available in two flavors. Asymmetric DSL (ADSL) has a different download speed, usually faster, than the upload speed. This improves download times where file transfers are the largest. Symmetric DSL (SDSL) has equal download and upload speeds. If one of these versions of DSL seems more appealing than the other, you will need to find an ISP that supports your preferred technology. Most ISPs support only one of the technologies.

Note 

DSL is not available everywhere. Two factors stand in the way of ubiquitous access. First, the current limitation for DSL connectivity is 1,750 feet from the CO. Longer distances are not able to support the minimum quality requirements for transmitting digital signals. Second, the telephone company in your area must invest a substantial amount of money toward upgrading their main facilities to support DSL. In metropolitan areas, this is not a problem, but in rural or semi-rural areas, justifying the cost is difficult.

DSL check-off list:

• Does your computer support Ethernet?

• Do you have a network card?

• Are you close enough to the CO in your area?

Cable Modems

Cable modems allow high-speed access to the Internet over cable TV (CATV) lines. The cable modem requires two connections: one to the cable outlet and the other to the computer. Cable modems are more economical than some other technologies. DSL is as cost effective, but performance is more difficult to measure. One difficulty for DSL is that it has not had the rapid growth that cable technology has, and therefore DSL is not available everywhere. Compared to ISDN, described in the next section, cable modems offer much better performance at a fraction of the cost. There is no need to dial; the connection is always active.

10BASE-T

A signal capable of transmitting at 10Mbps over twisted-pair cable by using baseband signaling. It is the IEEE 802.3 standard for unshielded twisted-pair Ethernet.

wide area network (WAN)

A relatively low-speed data connection (typically 1.546 Mbps) that uses the telephone company to connect two locations separated by a large geographical area.

Cable Modem Termination System (CMTS) device

A device used to forward user data to the Internet. Downstream data from the Internet is forwarded to the cable television equipment in that neighborhood, where it is then forwarded to the home user.

The range at which a cable modem operates varies from 200Kbps to 10Mbps. The typical range is more like from 2Mbps to 10Mbps. Data that is down loaded from the Internet is transmitted at a higher rate than data that is uploaded.

In your home, the cable company will attach a splitter to the incoming cable. One connection is used for your television and the other connects to the cable modem. Your computer connects to the cable modem via a 10BASE-T Ethernet connection to a network card installed in your computer.

When you connect your computer to a cable network, you are actually part of a large Ethernet wide area network (WAN). Each house plugs into the network via the cable in the home. The other end of that cable goes into a cable hub called a Cable Modem Termination System (CMTS) device. The CMTS will manage where data is being sent.

CMTS devices can be two-way, meaning that they support upstream (to the Internet) and downstream (to the user) data transmission. In some locations where CMTS devices are not installed or available, a telephone connection is used for upstream data.

ISDN

Integrated Services Digital Network (ISDN) is a digital dial service that transmits digital data at a higher transmission rate than a standard modem. This makes ISDN more expensive than using a modem. ISDN is still cost-effective, considering that the rate for one ISDN line is less than two business lines, and it can support data, voice, and faxing capabilities.

Integrated Services Digital Network (ISDN)

A technology that combines digital and voice transmission onto a single wire.

telecommuter

Someone who remotely connects to his or her office to work from home or a remote location.

B channel

Stands for bearer channel and is a 64Kbps circuit-switched channel. Used to carry voice and data.

D channel

Stands for delta channel and is a 16Kbps circuit-switched channel. Used to manage control signals.

Basic Rate Interface (BRI)

The basic ISDN service offered by telecommunication companies. BRI consists of two B channels and a single D channel.

H0

Another ISDN channel that includes 6 B channels. Other H channel definitions include H-10 and H-11, which are just another way of identifying the 23 B channels of the Primary Rate Interface.

Primary Rate Interface (PRI)

The high-end ISDN service offered by telecommunication companies. PRI provides 23 B channels and one D channel. This is equivalent to the 24 channels of a T1 line.

You can connect ISDN adapters to the computer in two ways:

• Internal ISDN adapters plug into an available expansion slot inside a computer.

• External ISDN adapters connect to an available serial port on the back of a computer via a null-modem cable.

The most typical ISDN user is the telecommuter, who uses the high-speed service to connect with his or her main office from home.

ISDN adapters often link small branch offices that do not transmit large amounts of data to one another. A connection begins when data needs transmission.

You can identify the speed of an ISDN adapter by the number of signal bearer channels, or B channels. An ISDN adapter configured with a single B channel can support 64Kbps, whereas a D channel supports only 16Kbps.

Here is a summary of available ISDN services.

ISDN Service	Channel Type	Speed
Basic Rate Interface (BRI)	2 B + 1 D channel (2 x 64K + 16K)	144Kbps
H0	6 B channels (6 x 64K)	384Kbps
Primary Rate Interface (PRI)	23 B (64K) + 1 D (64K) channel	1.536Mbps