U-8 – Communications and Networks
Overview
Telecommunications, or telecom, refers to
communications over a distance—over long-distance phone lines, via privately
owned cable, or by satellite, for instance.
Telecommunications has extended the usefulness of
the computer in the workplace and has made the computer an ever more popular
fixture in the home.
The Major Players
There are many players in the telecommunications
industry, including phone companies, cable-TV and satellite companies, service
and content providers, and software and hardware companies.
The entire telecom industry is presently
characterized by volatility—mergers and acquisitions, partnering arrangements,
and shakeouts. On top of all of this
turmoil, computer-and-communications technology itself is undergoing extremely
rapid change.
Government Legislation
Before 1968, AT&T was the only phone company in
the United States. It owned all of the
phone lines and, also, all of the phones on its lines. In 1968, the Federal Communications Commission
(FCC) produced the Carterfone Decision, in which a small company was allowed to
connect its own two-way radios to the phone lines, and the doors swung open for
anyone to buy a non-AT&T phone and hook it up to the AT&T system.
Later federal-government rulings forced AT&T to
divest itself of its regional services—the companies that are today the Baby
Bells—and enabled companies like MCI and Sprint to enter the long-distance
phone business as competitors to AT&T.
Despite the interjection of competition, industry
regulation was still relatively tight at the end of the 1980s. The Telecommunications Act of 1996
essentially deregulated the entire telecommunications industry, enabling
companies to expand well beyond their traditional "turfs."
Today, a wide variety of important business applications involve
communications, and the roster of uses is growing rapidly. Some activities
aside from using the Web and exchanging e-mail
are listed next.
Through a pocket-sized wireless device called a pager, short numeric or
text messages can be sent from one person to another—carrying the pager—who is
on the move. More advanced messaging systems allow two-way paging, as well as
possibly Web and e-mail access.
The most common type of wireless phone is the cellular (cell) phone.
Another possibility is the satellite phone. Smart phones can be used for e-mail
and Web access; some can also be used for messaging.
Cell phone use is dramatically increasing, especially in countries with
poor traditional communications infrastructure. Cell-based Web activity is
especially common in countries where home PC Internet access is less common,
due to expense or space.
A facsimile, or fax, resembles e-mail, but it allows users to send
images of documents from one location to another over ordinary phone lines.
Faxes can be sent or received via a PC or conventional fax machine. Internet
faxing (e-faxing) is also available.
Global Positioning Systems (GPS)
A global positioning system (GPS) uses a GPS receiver in conjunction
with a system of DOD satellites. The receiver can determine its location to
within about 10 feet. GPS receivers are commonly used by hikers, motorists,
farmers, surveyors, etc.
Satellite radio, typically used in cars, broadcasts to an entire
country instead of a small local area. This enables travelers to hear the same
radio station while driving across the country. Satellite radio is available by
subscription, typically up to $15 per month.
Videoconferencing, or teleconferencing, refers to the use of computer
and communications technology to conduct meetings between people in different
locations. It can be done via PCs with microphones and video cameras, or using
a sophisticated videoconferencing room.
Videoconferencing performed over the Web is sometimes called Web
conferencing.
Telecommuting refers to people working from their homes and being
connected to their co-workers through such means as the Internet, fax machines,
personal computers, pagers, and the like.
Telecommuting can save a business office and parking space, while
allowing employees some freedom in their work schedules. It also helps cut down
on commuting traffic and pollution.
A network consists of a collection of computers and other hardware
connected together to share data, hardware, software, and programs, and to
facilitate electronic communication.
Network Topologies
Telecommunications networks can be classified in
terms of their topology, or pattern.
Three common topologies are the star, bus, and ring.
Star Networks: The star network—which represents the
oldest topology as far as computers go—often consists of a large “host”
computer that's hierarchically connected to several “dumb” workstations in a
point-to-point fashion.
Bus Networks: A bus network commonly consists of a
high-speed cable, with inexpensive twisted-pair wires dropped off each
workstation. A bus network contains no
host computer.
Ring Networks: A less common and more expensive
alternative to the star and bus is the ring network, in which a host computer
is absent and a number of computers or other devices are connected by a loop.
Combination Topologies: Network topologies are often
combined when smaller networks are aggregated into larger ones. For instance, two or more star networks can
be daisy chained together with a bus or can be linked together in a ring with a
high-speed cable called a fiber-optic backbone.
Local Area Networks (LANs)
Local area networks (LANs) are local networks that
do not utilize a host computer as such.
Instead, most use either a bus or ring topology, and it is computers
within the network itself that manage workstations as they demand the shared
facilities.
LANs are available principally as client-server LANs
and peer-to-peer LANs.
With client-server LANs, each workstation getting
network service is called a client, while the computers managing the requests
for facilities within the network are called servers.
Small networks often use peer-to-peer LANs. These LANs do not use predesignated clients
and servers per se. Instead, all of the
user workstations and shared peripherals are on the same level, and users have
direct access to each other's workstations and the peripherals.
LANs are used for a variety of applications, the
simplest of which involves just sharing expensive hard disks and laser
printers. One of the newest types of
LANs is the intranet—a private LAN, set up by a company for its employees, that
follows the infrastructure and standards of the Internet and its World Wide
Web.
Wide Area Networks
Wide area networks (WANs) are communications
networks that encompass a relatively wide geographical area. WANs may be publicly accessible, like the
Internet, or be privately owned and operated.
Hubs provide a common connection point for a variety of devices. All
packets coming into the hub are sent to all other network nodes.
Switches are similar to hubs, but send data to only the appropriate
node, not all network nodes. This increases the efficiency of the network.
Routers are used in large WANs—like the Internet—to pass packets along
to their destinations. They decide the most efficient path to send the packets
over the network.
A gateway is a collection of hardware and software resources that
enables devices on one network to communicate with devices on another,
dissimilar network.
When the two networks being linked are based on similar technology—such
as a LAN in one city and a LAN in another—a device called a bridge is used to
connect them.
Repeaters are devices that amplify signals along a network. They are
used because signals often have to travel farther than the wires or cable that
carry them are designed for.
Because communications lines are expensive, it is desirable that
several low-speed devices share the same line. A special device called a
multiplexer—which interleaves the messages of several low-speed devices and
sends them along a single, high-speed path—makes this possible.
A concentrator is a multiplexer that combines and sends multiple
messages in a way that leaves the individual messages simultaneously active,
instead of being sent as one combined message.
An intranet is a private network, usually set up by a company for its
employees, that follows the infrastructure and standards of the Internet and
its World Wide Web. They are used for a variety of purposes and provide a
familiar structure with which to build a company network.
Extranets are intranets that are at least partially accessible to
outsiders, such as selected customers and suppliers.
Storage Area Networks (SANs)
A storage area network (SAN) is a high-speed, dedicated secure network
of shared hard drives or other storage devices. SANs are becoming more common
because of the vast amount of storage required with e-commerce and related
activities. Devices can be added to the SAN as needed, without interrupting the
regular network.
Virtual Private Networks (VPNs)
A virtual private network (VPN) is used to securely transfer private
information over a public network, such as the Internet. Information sent over
the VPN is encrypted and can only be decrypted by the intended recipient. VPNs
are growing in importance because of the amount of e-commerce and other secure
Internet activity performed today.
Metropolitan Area Networks (WANs)
A metropolitan area network (MAN) is a network designed for a town or
city. MANs are generally not owned by a single company or organization and are
commonly used to connect multiple LANs.
Personal Area Networks (PANs)
A personal area network (PAN) is a network of all devices in a person’s
life. PANs can keep portable devices synchronized with a desktop PC and
coordinate Internet access and e-mail. Bluetooth (discussed shortly) and other
wireless technologies are helping to increase the feasibility of PANs.
Telecommunications Media
Any two hardware units that are distant from each
other transfer messages over some type of communications medium. One of the hardware units is designated as
the sender and the other as the receiver.
Communications media fall into one of two classes:
wire and wireless.
Wire Media
Three types of wiring are commonly used:
twisted-pair wires, coaxial cable, and fiber optic cable.
Twisted-pair wire, in which strands of wire are
twisted in groups of two and bound into a cable, is the communications
technology that has been in use the longest. The telephone system still
consists heavily of twisted-pair wires. Twisted-pair wire can be shielded for
better performance, and is available in a variety of categories—Category 3 is
regular telephone wire; Categories 5, 6, and higher are used for networks.
Coaxial cable, the medium pioneered by the cable
television industry, was originally developed to provide high-speed,
interference-free video transmission. Coaxial cable is now also widely used in
other types of communication applications, such as linking computers in office
networks. In general, coaxial cable can carry more data more quickly than
twisted-pair wire.
Fiber-optic cable consists of hundreds of clear
glass or plastic fiber strands, each approximately the thickness of a human
hair. Transmission is made possible by the transformation of data into light
beams, which are sent through the cable by a laser device at speeds on the
order of billions or even trillions of bits per second. Fiber-optic cable is
typically used for the backbone of a network. It is typically faster, more
secure, and more reliable than other types of wired media and has a higher
bandwidth. The disadvantages of fiber-optic cable include expense and more
difficult installation, although the cost can be somewhat made up if the cable
is used for multiple purposes.
Wireless technology allows communications in situations where physical
wiring is impractical or where portability is desired.
Broadcast radio requires the use of a transmitter to send data through
the air and a receiver to accept the data at the other end. Fixed wireless is
the term used to describe radio Internet access at a stationary location, such
as a home or office. Short-range radio is commonly used to connect wireless
devices (keyboards, a mouse, etc.) to a PC, as well as for connecting PCs to
LANs.
Microwaves are high-frequency, high-speed radio signals. Text,
graphics, audio, and video data can all be converted to microwave impulses.
Microwave signals can be sent in two ways. Microwave stations communicate with
each other directly (line-of-sight) within about 30 miles or so. Communications
satellites are placed in orbit around the earth to receive and transmit data to
and from microwave stations. Geosynchronous satellites are located 22,300 miles
above the earth and have a slight delay (half-second) during transmission. Low
earth orbit (LEO) satellites are used for telephone systems to avoid that
delay.
Cellular technology is a form of radio used for cellular phones. Cell
phones operate by being in contact with cellular antennae, placed within
calling cells. Cell phones are very popular. The newest devices are third
generation (3G) devices that are optimized to provide enhanced data
communication services within the cellular network (second generation phones
are digital, such as PCS phones).
Infrared technology has become popular in recent years for setting up
wireless links between office PCs. As opposed to microwave and cellular
technologies, which use radio waves, infrared technology relies on sending data
as light rays. Common applications include beaming data between PCs or sending
documents from a portable PC to a printer.
The phone system, established many years ago to handle voice traffic,
carries signals in an analog fashion—that is, by a continuous wave over a
certain frequency range. In contrast, most business computing equipment is
digital, which means it is built to handle data coded in two discrete states:
0- and 1-bits.
The difference between the highest and lowest frequencies available on
an analog medium is known as a medium’s bandwidth. In computer networks, the
greater the bandwidth, the greater the amount of data that can pass through the
media within a given period of time. High-bandwidth or broadband Internet
access is becoming more common to facilitate the growing amount of multimedia
data available over the Internet. The speed at which data travels over both
analog and digital media is often given in bits per second (bps), kbps
(thousands of bits per second), or mbps (millions of bits per second).
In serial transmission, all of the bits sent in a message follow one
another along a single path. If, on the other hand, each set of eight bits
needed to convey each byte in a message is sent out at once in eight separate
paths, parallel transmission is being used. Parallel transmission is usually
limited to short distances, such as computer-to-printer communications.
Three different types of transmission direction are possible.
With simplex transmission, data can be sent only in a single,
prespecified direction (doorbell).
In half-duplex transmission, messages can be carried in either
direction, but only one way at a time (two-way radio/walkie-talkie).
In full-duplex transmission, the flow moves in two directions at the
same time (telephone).
Three ways of timing transmissions are available.
In synchronous transmission, data is sent in blocks of characters and
the blocks are timed so both the sending and receiving devices know when they
will arrive.
In asynchronous transmission, one character at a time is transmitted at
a time. The transmission isn’t synchronized, so a start and stop bit identify
the beginning and end of each byte.
Isochronous (time-dependent) transmission can be used when data must be
delivered within certain time restraints, such as the audio and video of a
video clip, which must be played at the same time on the receiving end.
User PCs are usually connected to a network with either a network
interface card or with a modem.
Network interface cards (NICs) are typically add-in boards that plug
into an expansion slot within the system unit and have ports to connect to the
network using coaxial cable or twisted-pair cable. Some network interfaces may
be built directly into the device; wireless NICs have no external port,
although they may have an external antenna.
A modem is used to connect PCs to the Internet or other network via the
telephone lines. Modems can be internal or external and there are a variety of
types of devices referred to as “modems.”
Conventional dial-up modems are used to connect over regular telephone
lines at speeds up to 56 Kbps.
ISDN modems connect over regular phone lines, but use multiplexing and
typically two lines so data can be transferred faster—up to 128 Kbps.
DSL modems allow even faster transmission over phone lines and don’t
tie up your telephone like conventional dial-ups and ISDNs. Most common is
ADSL, with faster download than upload speeds (typically 8 Mbps and 640 Kbps,
respectively).
Cable modems are used for cable Internet access and work similar to a
cable TV box. Cable is fast (roughly 1.5 Mbps or higher), but degrades as more
users in the same immediate area get on the Internet.
Satellite modems are used for satellite Internet access. Satellite is
reasonably fast (500 Kbps to 1 Mbps) and available in rural areas that don’t
offer DSL or cable. Upload speeds are generally slower than download speeds.
Wireless modems are commonly used with handheld PCs and other mobile
devices to connect them to a wireless network. Wireless modems can also be used
for fixed wireless Internet service.
Cellular modems can be used to connect a portable PC to a cell phone.
A communications protocol is a collection of procedures used to
establish, maintain, and terminate transmission between devices. It helps
determine how devices will physically connect to a network, how data will be
packaged for transmission, how receiver devices will acknowledge sender devices
(a process called handshaking), how errors will be handled, and so on.
Ethernet refers to a collection of protocols that specify a standard
way of setting up a LAN in a bus or star Ethernet network. Data communications
and problem checking are done through a set of procedures collectively called
CSMA/CD, which describes how workstations listen for messages and what takes
place if two messages are sent at exactly the same time. Traditional (10BaseT)
Ethernet networks run at 10 Mbps; newer 100 Mbps and 1 Gbps versions are now available.
Token ring is usually used with a ring network topology and employs a
token-passing protocol. Token ring works with a small packet called a
token—which has room for messages and addresses—that is sent around the ring.
The protocol specifies how the ring is seized by sending PCs and how messages
are delivered to receiving PCs.
TCP/IP (transmission control protocol/Internet protocol) specifies how
the hundreds of different types of computers that hook up to the Internet
package and send messages. TCP/IP relies on a procedure known as packet
switching to do this. TCP/IP is also increasingly being used with LANs.
Wireless Application Protocol (WAP) is a standard for delivering
content to mobile devices, such as mobile phones and pagers, using a wireless
network. When Internet content is used, a WAP-enabled microbrowser is needed to
display WAP-enabled Web pages.
Wireless Ethernet (also known as Wi-Fi) allows the Ethernet standard to
be used with a wireless network. Currently, wireless Ethernet is slow (11
Mbps), but is expected to increase soon.
Bluetooth is a standard for low-cost, short-range, wireless connections
between Bluetooth-enabled devices, such as allowing communications between a
headset and wireless phone, a PDA and a printer, a PDA and a PC, etc. When the
devices come within 10 meters (35 feet), they automatically network with one
another.
Other wireless protocols include Home Phoneline Networking Association
(Home PNA) (networking using standard home phone wiring); powerline networks
(networking over standard electrical wiring); and Shared Wireless Access
Protocol (SWAP) (using cordless telephone and wireless LAN technologies to
share voice and data between PCs, printers, and other peripheral devices).
A computer virus is a program designed to cause damage or some other
malicious act. Some lay dominant for a specified period; others act as soon as
the PC is infected. Viruses can be obtained via infected disks, or when
infected files are received over the Internet or other network. Antivirus
programs, discussed in Chapter 15, can help you safeguard your system from
viruses.
Unauthorized access is the use of a computer, network, or network
resource without permission. To prevent it, an identification procedure—such as
using passwords or physical access objects—can be used.
Passwords can be used to restrict access to an entire system, or just
to selected folders, files, or devices. Good password strategies should be used
to avoid having the password be easily guessed.
Common physical access objects include magnetic cards, smart cards, and
encoded badges. Using one of these objects in conjunction with a password helps
with security so that the object can’t be used if it is lost or stolen.
Biometric devices recognize a unique physiological characteristic of a
person, such as a fingerprint, hand geometry, voice, or iris. They are used
today and have great potential for secure access for the future.
A firewall is a collection of hardware and/or software intended to
protect a computer or network from attack over a network. Firewalls are
becoming increasingly important for personal use as more and more individuals
have an always-on Internet connection.
See figure 8-27 on page 235