Information Systems Theory
Chapter 6: Communications and Networks
This chapter discusses various storage methods. The objectives important
to this chapter are:
- Understanding communications
- Understanding how communications technology is used
- Understanding transmission media
- Understanding line configurations
- Understanding data transmission
- Understanding some commonly used equipment
- Understanding LANs, MANs, and WANs
The chapter begins with a definition of communications that says
it is the transmission of data or information between two
or more computers across a communications channel.
Many kinds of computer applications use this kind of communications.
Several are listed on page 6.2, some of which have been discussed in previous
chapters. Some of the new ones are:
- voice mail - this uses a computer to store voice messages
for delivery to a user later
- telecommuting - this involves working at one site and accessing
data or program files at another site
- videoconferencing - transmission of sound, video and data
between locations for users who can't all meet in one place
- electronic data interchange - sharing of database files or
records between companies and their suppliers, to enhance the order
and supply of parts or products
- global positioning systems - this technology uses communications
between satellites and earth-based systems to determine a user's location
- bulletin board systems - like e-mail systems, except that
all users can usually read all messages placed in the system
- online services - often Internet Service Providers who add
access to their own content as well, like America Online and the Microsoft
- Internet - the Internet itself is given as an example of a
network of networks, which is an example of computers linked for communication
A communications system contains several elements, each of which
may come in several types.
Transmission media are the pathways across which data and information
are passed. Several types are listed:
- Twisted pair cable - several insulated copper wires (usually
8) are grouped into pairs. The first graphic on page 6.9 shows four
twisted pairs of wires. Each wire is covered with an insulator,
and the two wires in each pair complete a circuit. This type
of wire suffers from crosstalk, leakage of signal from one circuit
to another. The twists help cancel out such leaks. This type
of cable comes in several varieties: two pair, three pair and four pair
are common. The graphic shows a UTP cable with eight wires in
it, making four pairs. This kind of cable would be connected to a computer
with an RJ-45 connector. If the cable had only four wires in
it (two pairs), it could use an RJ-11 connector, and it would
probably be a telephone cable, not a data network cable. Twisted pair
cable comes in many grades and two types: unshielded (UTP)
and shielded (STP). The shielded type has an extra insulator
to shield it from interference. The unshielded type does not.
- Coaxial cable - is less flexible, costs more and is harder
to work with than UTP. It can be similar to the cable used in cable
television system, or it can be much thicker and even harder to use.
A coaxial cable has one central conductor, enclosed by insulation,
enclosed by a ground wire (or wire mesh), which is enclosed in
the cable insulation.
- Fiber-optic - can be glass or plastic, and is
meant to conduct light instead of electricity. The conductor
is called a waveguide, and is covered with cladding, a
material to reflect the signal back into the center of the conductor.
- Microwave signals are used in two formats: terrestrial
(earth-based) and satellite systems. Terrestrial systems are
used in line of sight connections where it is not possible to
put a wire, such as across several city blocks.
- Satellite systems are used to connect sites that are widely
separated. Usually, signals are sent to geosynchronous satellites,
orbiting 22,300 miles above the earth. This orbit puts the satellite
in the same part of the sky relative to a ground based observer at all
times. Of course, the orbit should be around the equator of the
earth, not the poles as indicated in the picture in your
book. (Best lines in the Pauly Shore film Bio-Dome:
smart aleck question: "What are you, a rocket scientist?"
deadpan answer: "Yes."
no possible response: "...sorry...")
- Wireless transmission media - actually, microwaves are one
form of wireless media, which just means there is no cable. Other
forms often used are infra-red light and radio waves. A cellular
phone actually is a radio which contacts the nearest radio receiver
of a land based telephone network.
An actual channel may consist of several parts, called links,
which can be of any type. The illustration on page 6.14 is of a computer
on the west coast of the US contacting a server on the east coast. The
signal passes through phone lines, terrestrial microwaves, satellite microwaves
and possibly other media along the way.
Lines in communication channels are available to users in several
- switched lines - when a person makes a telephone call, the
switching network of the phone companies set up a channel
for the conversation. This channel will only exist temporarily,
for the time this conversation is in effect, and it may vary in quality
from the next channel set up for a similar conversation.
- dedicated line - this is a guarantee of a connection of a
certain quality through a provider that charges more for this service
than for a switched line
- point to point - this kind of connection only connects
two devices, such as the connection between a television and its
remote control. No other devices share or use the connection.
- multipoint - also called multidrop, because each place
where a device can be connected to the line is called a drop.
Several devices share the connection medium.
Page 6.16 brings up the concepts of digital and analog again. This time,
it is explained that the signals on a channel could be either digital
or analog. A digital signal might be a series of pulses,
where a pulse might stand for one kind of bit and the absence of
a pulse might stand for the other kind. An analog signal might
consist of one or more continuous electrical waves. We might send one
kind of wave (one frequency) to represent a one and another kind of wave
(another frequency) to represent a zero.
Digital data lines exist, and they are available in several grades
of service. Some types are:
- T1 lines - can carry up to 1.544 Megabits per second
- T3 lines - can carry 45 Megabits per second
- ISDN lines - can have multiple data channels, each of which
can carry 128 Kilobits per second
Data transmissions can be classified into three types:
- simplex - data flows one way from one device to anther,
but does not flow the other direction. This is like a remote control.
- half-duplex - data can flow both directions between devices,
but only one way at a time. Devices must take turns. This is
like CB radio, ham radio, and walkie-talkies.
- full-duplex - data can flow both directions between
devices, at the same time. This is like a telephone.
Several kinds of communication equipment are described in the chapter:
- modem - stands for modulator/demodulator, often used
with computers connected to phone lines to convert between digital
computer signals and analog phone signals. Available as internal
(installed inside the computer) and external (attached to a port)
- multiplexer - sometimes called a MUX, a multiplexer
combines several signals from different devices, passes them
along a communications channel to another MUX, which separates the
signals and passes them to their intended destination.
- front end processor - this is a computer that does the communications
work for another computer
- network interface card - a NIC (don't call it a "NIC
card", that's redundant) is used to connect a computer, printer, or
other device to a network
- wiring hubs - a hub can act like a telephone switchboard in
a network, passing signals from one device to another
- gateway - the word "gateway" can mean several different things.
In this chapter, a gateway is a device that connects networks that
are different from each other, and acts as a translator between
- bridge - a bridge connects networks that are similar to
each other, so translation is not necessary
- routers - a router provides connections to several
networks. Routers can act as bridges or as gateways.
Three size designations are used for networks:
- LAN - a Local Area Network is often inside one building,
may cover several buildings, but is never as large as a city.
- MAN - a Metropolitan Area Network spans an area the
size of a city. It is larger than a LAN, smaller than a WAN.
- WAN - a Wide Area Network spans an area larger than
a city, often covering many states or countries. The Internet, which
covers the world, is a WAN.
Two major types of networks are:
- Client-Server - clients are machines that request
services on a network, servers are machines that provide
services. In a client-server network, most machines are only one or
- Peer-to-Peer - peers are machines that can both request
and provide services on a network.
A network operating system (NOS) is the set of programs
that make it possible to provide and share resources on a network.
Several software publishers offer NOSs, such as Novell (NetWare),
Microsoft (Windows NT, Windows 2000), and IBM (LAN Server).
Three network configurations are described in the text. Before you consider
them, a word of advice: things do not always work the way they look. Another
phrase that describes this topic is topology, with is the science
of shapes. Physical topology describes how network is actually
shaped, but logical topology describes how it acts. The three physical
topologies described here are:
- star - all the workstations, printers, etc. on a star network
are wired to one or more central point. Imagine the wire running
from the central point to each device, and you can see the star.
- bus - a single continuous line (wire, fiber optic,
etc.) is run from device to device. Each device is said to tap in to
the line (bus). The ends of the bus are not connected to anything.
- ring - like a bus, in that the line used runs from one
device to the next, but different in that the line eventually returns
and connects to the first device, making a circle (ring)
Protocols are sets of rules that describe how networks
function. Many sets of protocols exist. The text summarizes two types
- Ethernet - an Ethernet is a contention based system.
Contention systems work by letting each device try to send a
message on the net as needed, contending or competing
with all the other devices for the bandwidth. An example of a protocols
that supports such a systems is CSMA/CD (Carrier Sense, Multiple
Access, with Collision Detection). A collision occurs when two
signals collide on the medium, causing signal loss. In a CSMA/CD system,
like Ethernet, the collision is detected and the devices each
wait a random number of seconds before sending again. This usually
results in one device going ahead of the other.
- Token Ring - a token ring system uses a protocol that is referred
to as token passing. Token-Passing involves passing a token,
a few bytes, from workstation to workstation. When a station has the
token, it is that station's turn to access the medium. This type of
media access is predictable and consistent, allowing large or
small transmissions. It is not the best for time sensitive data
since waits are built in, but it will support more devices
An Ethernet acts like a logical bus, a Token Ring
acts like a logical ring, but both kinds of network can actually
be wired like a star (and still act the way they are supposed
to act). If that doesn't confuse you, you may have a future in networking.