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Chapter 15: Networks
Chapter 15Networks
Page 1
Data communication networks are set up in two basic configurations.
While this approach guarantees a direct connection between any pair of devices, the amount of cabling and I/O hardware required at each device makes it impractical in most situations.The alternative is to establish a broadcast network in which network lines are shared.
Simple configurations associated with the broadcast approach include the ring and bus topologies, at left and right, respectively.
The mesh configuration uses direct, point-to-point connections between each pair of communicating devices.
Local Area Networks
Chapter 15Networks
Page 2
LANs are privately owned networks containing perhaps dozens of devices.
Example: Ethernet
Computer
Bus Cable
ComputerComputerComputerComputer Computer
Ethernet uses a bus configuration and a protocol for accessing the bus called CSMA/CD.
• CS - Carrier Sense: Each machine constantly listens to the traffic that’s passing by on the bus.
• MA - Multiple Access: Every machine has equal access to the communication medium (i.e., the bus).• CD - Collision Detection: Each machine is capable of detecting whether its transmitted message “collided” with that of another machine, thus corrupting the message and forcing a retransmission.
Ethernet Carrier Sense: Incoming Traffic
Chapter 15Networks
Page 3
As messages pass by, each machine examines the first few bits of the message, the “address” of the message, and determines whether or not the message is intended for itself.
Computer
Bus Cable
ComputerComputerComputerComputer Computer
message
When the destination machine sees its address in the
message’s prefix, it copies the passing message as the
original proceeds down the bus.
message
message
Ethernet Carrier Sense: Outgoing Traffic
Chapter 15Networks
Page 4
When a machine wants to transmit a message, it waits a system-dependent amount of time, using its carrier sense to see if any traffic appears on the bus. If not, it transmits its message and “hopes” that no collision occurs.
Computer
Bus Cable
ComputerComputerComputerComputer Computer
When the transmitting machine detects no passing
traffic for a certain time interval, it places its message
on the bus.
message
Ethernet Carrier Sense: Colliding Traffic
Chapter 15Networks
Page 5
Computer
Bus Cable
ComputerComputerComputerComputer Computer
After a machine transmits a message, it continues to use its carrier sense to see if there’s any difference between its transmitted message and the message on the bus. If so, it interprets the problem as a collision.
When each transmitting machine detects that its
message has been corrupted, it waits a random
interval of time, and then retransmits.
A LAN Alternative: Token Ring
Chapter 15Networks
Page 6
Computer
ComputerComputer
Computer Computer
Computer
Ring
Computer
ComputerComputer
Computer
Computer ComputerComputer
Collisions tend to exacerbate traffic problems on congested Ethernet LANs, so an alternative to the CSMA/CD approach is desirable.
Token Ring uses a ring topology, with a specially formatted “token” message perpetually traversing the ring.
TOKEN
message
TOKEN
message
message
When a machine wishes to transmit, it merely waits for the token to arrive, removes it, and transmits its message.When its message returns to the transmitting machine, it’s removed from the ring, and the machine places the token back on the ring.
Wide Area Networks
Chapter 15Networks
Page 7
WANs are large collections of smaller networks, with special interconnection devices known as “routers” to make adjacent sub-networks compatible.
Router
Router
Router
Router
RouterRouter
Switching
Chapter 15Networks
Page 8
Data communications is characterized by two switching technologies.
Technology #1: Circuit SwitchingOnce the circuit is
established, it is maintained until one of the endstations terminates the connection.
Problems with Circuit Switching
Chapter 15Networks
Page 9
Circuit switching has one great advantage: once established, the circuit is dedicated, i.e., a communication line is completely open until formally terminated.
• Many networked applications don’t require a dedicated circuit, so reserving a communication line until an endstation formally terminates it can represent a serious waste of resources.
However, this approach has a number of serious problems:
• The route originally selected for the circuit may be optimal to begin with, but may prove to be suboptimal as the communication continues.• An entire, end-to-end path must be found and reserved before any communication is allowed between the two endstations; this is definitely not conducive to many modern applications (e.g., Web surfing, videoconferencing).• Transmission errors are propagated all the way to the destination, requiring retransmission across the entire network.
An Alternative to Circuit Switching
Chapter 15NetworksPage 10
Technology #2: Packet SwitchingThe source’s message is broken into manageable “packets”
that are transmitted to the destination individually, not necessarily along the same path.
The Pros and Cons of Packet Switching
Chapter 15NetworksPage 11
Packet switching remedies circuit switching’s principal problems:• Lines aren’t dedicated, so
their utilization is higher.• Messages are “packetized”,
so line-sharing is reasonably fair.
• Routing may be dynamic, i.e., an alternate route may be chosen when traffic patterns change.
• The entire route does not have to be chosen prior to sending any data.
• Errors aren’t propagated end-to-end.
However, packet switching does have its own set of problems:• Switches must be programmed to make sophisticated
routing decisions.• Switches must manage memory for queued packets that
await forwarding.• Packets must be prefixed with control headers, increasing
overhead.• Endstations must deal with missing packets and out-of-
order packets.• Without a dedicated circuit, transmission times become
unpredictable.
Chapter 15NetworksPage 12
Frequency-Division MultiplexingThe spectrum of frequencies transmittable via the physical medium is divided into several channels (e.g., cable TV).
Time-Division MultiplexingEach transmitter is allocated a periodic time interval in which to transmit.
MultiplexingTo more efficiently utilize a physical medium, multiple higher-level connections might “share” the medium simultaneously.
Chapter 15NetworksPage 13
Code Division Multiplexing
Frequency Division
Multiplexing Everyone
gets to talk at the same
time, but only across their
narrow channels.
(Commonly used with
copper cables)
Time Division Multiplexing
Everyone gets to talk using
the entire bandwidth, but
they have to take turns
talking.(Commonly
user with fiber optics)
Code Division MultiplexingEveryone gets to talk simultaneously, using the entire bandwidth! They do this by coding their transmissions in a unique fashion (as if every pair were speaking a different language, and
each other language merely sounds like background noise).
(Commonly used with wireless communications)
Chapter 15NetworksPage 14
Modems
Cables between the workstation and the
modem and between the modem and the telephone jack
Digital data must be modulated into analog signals if it’s going to be transmitted across an analog medium.
After transmission, it must be demodulated back into its original digital form.
Dial-in pool of 96 modems:
Six terminal servers, each connecting 16 modems to a LAN
Network Protocol Layers
Chapter 15NetworksPage 15
Physical Medium
In an effort to simplify networks, they are often organized as layered hierarchies of protocols, with hardware-intensive protocols on the bottom and user applications at the top.
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
SourceHost
Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
DestinationHost
Layer 4
Layer 3
Layer 2
Layer 1
IntermediateRouter
Layer 4
Layer 3
Layer 2
Layer 1
IntermediateRouter
Layer 2
Layer 1
Low-LevelBridge
Communicating Via Layered Protocols
Chapter 15NetworksPage 16
Consecutive network nodes only communicate directly at the lowest (hardware) layer; to communicate at higher layers, networking software inserts certain relevant data as headers and trailers to the message coming from the source. Layer 5
Layer 4
Layer 3
Layer 2
Layer 1
Layer 4
Layer 3
Layer 2
Layer 1
message
msgBhdr4hdr3
msgAhdr4hdr3
msgBhdr4hdr3 trl2hdr2hdr1 msgAhdr4hdr3 trl2hdr2hdr1
msgBhdr4hdr3
msgAhdr4hdr3
trl2
trl2
hdr2
hdr2
msgAhdr4hdr3
msgBhdr4hdr3
trl2
trl2
hdr2
hdr2
messagehdr4
msgAhdr4hdr3
msgBhdr4hdr3
messagehdr4
Layered Protocol Models
Chapter 15NetworksPage 17
Several models have been developed to implement protocol hierarchies for networks.Reference Model #1: Open Systems Interconnection (OSI)• End-user protocols, e.g., e-mail, file
transfer, Web browsing, network management, videoconferencing.
Application• Format data according to negotiations
between source and destination; encrypt and decrypt messages.
Presentation• Establish, maintain, and discontinue
dialogues between a source and a destination; synchronize data transfer.
Session• Detect and handle end-to-end
transmission errors; alter transmission rate when too much congestion is encountered.
Transport• Route messages from their source to
their destination; reroute traffic when heavy congestion is encountered.
Network• Detect and handle transmission errors
between consecutive network devices; control access to the shared medium.
Data Link• Transmit bits across physical medium;
determine frequencies to use when transmitting; specify role of connector pins.
Physical
Chapter 15NetworksPage 18
Reference Model #2: Transmission Control Protocol/Internet Protocol (TCP/IP)
•Analogous to OSI’s Application LayerApplication
•Analogous to OSI’s Transport LayerTCP
•Analogous to OSI’s Network LayerIP
•Analogous to OSI’s Physical and Data Link LayersLower Levels
Chapter 15NetworksPage 19
Version HdrLen Service Type
Flags Fragment Offset
Total Length
Identification
Protocol Header ChecksumTime To Live
Source IP Address
Destination IP Address
Options & Padding (if any)
The IP Header
Version: The version of IP used to create the packet, used by nodes to process it correctly.HdrLen: The length of the header in 32-bit words (because the Options field has no fixed size).Service Type: Six bits to represent the relative priority and delay sensitivity of the packet.
Flags: Don’t-Fragment flag and More-Fragments flag.
Time To Live: Length of time (in seconds) the packet may stay in the Internet.
Identification: All fragments of the same packet have the same ID number.
Version HdrLen Service Type
Flags Fragment Offset
Total Length
Identification
Total Length: The length of the entire packet in bytes (16-bit field means a 65,535-byte max).
Fragment Offset: Offset from start of packet (in bytes) of current fragment.
Protocol Header ChecksumTime To Live
Source IP Address
Destination IP Address
Options & Padding (if any)
Header Checksum: Error-checking sum of all of the 16-bit values in the header.
Destination IP Address: 32-bit IP address of the packet’s final destination.
Protocol: Global ID # of the protocol used to create the packet (e.g., TCP).
Source IP Address: 32-bit IP address of the packet’s original source.
Options & Padding: Options include: No-operation-just-align; Military-security-application; Loose-source-routing; Record-route; Strict-source-routing; Record-internet-timestamps.
Chapter 15NetworksPage 20
File Transfer Protocol (FTP)
Operating System
controlprocess
Client System
controlprocess
Server System
TCP/IP Internet
Operating System
One active TCP connection before and
after data transfer, just for control.
Operating System
datatransfer
controlprocess
Client System
controlprocess
datatransfer
Server System
TCP/IP Internet
Operating System
Two active TCP connections during
data transfer, one for control and one for
data.
Chapter 15NetworksPage 21
Firewalls
packetpacket packetpacket
My SecureNetwork
Outgoing packets for bad address/port combinations
are rejected (e.g., no insider can “http” an
external site).
Incoming or outgoing packets are rejected on the basis of size or payload info.
To ensure the security of a private network, “firewall” programs have been developed.
A common approach is to filter incoming and outgoing packets based upon header information, and to use an application gateway to inhibit application-specific traffic.
Incoming packets for bad address/port combinations
are rejected (e.g., no outsider can “finger” an
internal site).
Chapter 16: The World Wide Web
Chapter 16The World Wide WebPage 22
Click on this piece of hypertext, and
you’re taken to...Type in a keyword and click the “Go!”
pushbutton, and you’re taken to...Click on this
pushbutton, and you’re taken to...
The World Wide Web applies point-and-click hypermedia application software to facilitate accessing documents on the Internet via TCP-based protocols.
And with additional downloaded software, elaborate Web sites that include more than text and simple images can be accessed.
Chapter 16The World Wide WebPage 23
HyperText Transport Protocol
(HTTP)
<html><head><title>Welcome to IEEExpl</title></head> <body><table width="550" border="0" cellspacing="0" cellpadding="0" align="center"> <tr><td valign="top"> <p><font face="Verdana, Arial, Helvetica, sans-serif" size="3"><b> <font face="Verdana, Arial, Helvetica, sans-serif" size="3"><b> <font size="5" color="#3333CC"><img src="/images/subidinfmast_01.gif" width="550" height="39“ vspace="3" hspace="0" border="0" alt="IEEE xpl Subscription Access Information“ usemap="#Map"></font></b></font> <map name="Map"> <area shape="rect" coords="6,5,182,31" href="http://ieeexpl.ieee.org"> </map> </b></font></p> <br><table width=510 border=1 cellspacing=0 cellpadding=5 bordercolor=#CCCCCC align=center><tr><td> <font face=Verdana, Arial, Helvetica, sans-serif size=2 color=#3333CC> <b>Your institution subscribes to:</b></font></td></tr><tr><td bgcolor=#E8E8E8><strong> <font size=2 face=Verdana> </font><font face=Verdana, Arial, Helvetica, sans-serif size=2 color=#3333CC>•<b></b></font><font size=2 face=Verdana> Search Xplore Database</font></strong></td></tr></table><br><table width=510 border=1 cellspacing=0 cellpadding=5 bordercolor=#CCCCCC align=center><tr><td> <font face=Verdana, Arial, Helvetica, sans-serif size=2 color=#3333CC><b>Your institution subscribes to:</b> </font></td> </tr> <tr> <td bgcolor=#E8E8E8><strong><font size=2 face=Verdana> </font><font face=Verdana, Arial, Helvetica, sans-serif size=2 color=#3333CC>•<b> </b> </font><font size=2 face=Verdana>IEEE All Society Periodicals Package (ASPP) </font></strong></td> </tr> <tr> <td bgcolor=#FFFFFF> <blockquote> <blockquote> <p><font face=Verdana size=2><b><font color=#3333CC>Your online subscription includes access to the abstracts and full-text of IEEE journals, transactions, and magazines published since 1998:</font></b></font></p> <p><font size=2 face=Verdana> <font face=Verdana, Arial, Helvetica, sans-serif size=2 color=#3333CC><b>•</b> </font> <a href=/xpl/ieeejrns.jsp?hr=1>IEEE journals, transactions, and magazines</a> </p> </blockquote> </blockquote> </td> </tr> </table><br><table width="500" border="0" cellspacing="0" cellpadding="0" align="center"><tr> <td> <p><font face="Verdana" size="2">In addtion to the above, you may purchase individual IEEE journal/magazine articles and conference papers that are not included in your institution's online subscription. If you want a copy of an article or paper, first check with your librarian for a copy available locally.</font></p> <p><font face="Verdana" size="2"><font color="#000000">Any questions? <a href="/xpl/techform.jsp">Contact IEEE Customer Service</a>.</font></font></p></td></tr></table><br><table width="500" border="0" cellspacing="0" cellpadding="0" align="center"> <tr> <td valign="top"> <div align="right"> <font face="Verdana, Arial, Helvetica, sans-serif" size="2"><br><A href='javascript:window.close();'> <img src="/images/backbutton.gif" width="165" height="34" vspace="0" hspace="0" border="0" alt="Close this Window"></A> </font></div></td></tr></table></td></tr></table></body></html>
HTTP is the software defining the format of requests relayed from a Web browser to a Web server, as well as the replies relayed from the server to the browser.HTML (HyperText Markup Language) is the standard used to write Web pages; it doesn’t require specific formatting instructions, so two browsers may display a page differently.
Chapter 16The World Wide WebPage 24
Extensible Markup Language (XML)Unlike HTML, XML allows users to include content-specific information in their files, facilitating its being formatted to suit various display devices.A job vacancy at the
department of Tourism is added to the job database.
XML technology stores the data separately
from the presentation, allowing for a variety
of platforms.
The vacancy is displayed on the job
center kiosk...
...can be accessed by mobile devices...
...and is displayed on the Department of
Tourism website.
Chapter 16The World Wide WebPage 25
Uniform Resource Locators (URLs)A URL specifies three pieces of information: 1. Type of file transfer 2. Name of computer 3. Name of file
For example:
http://www.cs.siue.edu/undergrad/BS/files/prerequisites.pdf
1. http:// - type of file transfer 2. cs.siue.edu - name of the computer 3. /undergrad/BS/files/prerequisites.pdf - location of the file on the computer
Different types of file transfer: http:// - hypertext transfer
protocol ftp:// - file transfer protocol telnet:// - opens a telnet
session gopher:// - transfer file from a
gopher server mailto: - open a mail
session
Chapter 16The World Wide WebPage 26
Network Interface
HTTPClient
FTPClient
BrowserController
Client software runningon the workstation
JavaInterpreter
HTMLInterpreter
Dis
pla
y D
river
Issuing requests for external files
Web Browsing
Input frommouse &keyboard
Communicationswith remoteWeb server
Output touser’sdisplay
InterpretingdownloadedWeb pagesand files
Chapter 16The World Wide WebPage 27
Internet Search EnginesSearch engines use “spider” programs to “crawl” through the Web, building a list of words.
Some follow every link on every home page.
Some ignore links that lead to graphics, sound, or animation files, or to newsgroups.
Some concentrate primarily on the most popular Web pages.
Chapter 16The World Wide WebPage 28
Comparing Search Engines
91
60
2816 13 6
Searches Per Day (in millions)
GoogleYahooMSNAOLAskOther
Security IssuesRed = distribute adware, send
a high volume of spam, or make unauthorized changes
to a user's computer Yellow = send a high volume
of "non-spammy" email, display many popup ads, or
prompt a user to change browser settings
Share of Searches
Series1
0 1 2 3 4 5 6
1.9
3.6
2.3
1.4
1.8
1.5
1.8
1.9
1.5
1.7
Percentage of Search Results
Chapter 17: Limitations of Computing
Chapter 17Limitations
of Computing
Page 29
What problems cannot be solved on a computer?
What problems cannot be solved on a computer in a “reasonable” amount of
time?These aren’t just philosophical questions; their answers will determine how practical it is to
pursue computer-based solutions to real-world problems like hurricane prediction, disease
control, and economic forecasting.
Chapter 17Limitations
of Computing
Page 30
ComputabilityTo examine the limits of what it is possible to do with a computer, Alan Turing (1912-1954) developed a simplified mathematical model, called a Turing machine.A Turing machine consists of three
parts:1) A tape of cells from which symbols can be read and into which symbols can be written,
2) A read/write head that moves back and forth across the tape, reading the symbol inside the current cell and/or writing a new symbol into the current cell, and3) A control unit that keeps track of what “state” the machine is in, and uses that state and the current symbol under the read/write head to:a) Determine which symbol to place in the
current cell,b) Determine whether to move the read/write head one cell to the left or right, andc) Determine the next state of the machine.
Tape
Read/Write Head
Control
Unit
Chapter 17Limitations
of Computing
Page 31
State Transition DiagramA state transition diagram may be used to define a Turing machine.Each // transition signifies reading on the tape, replacing it with , and then moving the read/write head in the direction.
The state transition diagram above defines a Turing machine that increments a binary number on the tape by one.
START
ADD
CARRY
NO CARRY
OVERFLOW
HALT
RETURN
*/*/L 1/0/L1/0/L
0/1/L0/1/L
0/0/L,1/1/L
*/1/L
*/*/R*/*/R
*/*/R
*/*/-0/0/R,1/1/R
* 1 0 1 *State:START
* 1 0 1 *State:ADD
* 1 0 0 *State:CARRY
* 1 1 0 *
State:NO
CARRY
* 1 1 0 *
State:NO
CARRY
* 1 1 0 *State:
RETURN
* 1 1 0 *State:RETURN
* 1 1 0 *State:RETURN
* 1 1 0 *State:RETURN
* 1 1 0 *State:HALT
Chapter 17Limitations
of Computing
Page 32
The Church-Turing ThesisComputer scientists commonly accept the Church-Turing Thesis, which states that the set of functions that can be calculated on a computer is exactly the same as the set of functions for which a Turing machine can be devised.There are problems that have been proven to be non-computable (i.e., no Turing machine can be devised to calculate their solutions).
The Halting Problem
Given a program with a set of input values,
does the program halt on that input, or does it get stuck in
an infinite loop?
One classical example:
Chapter 17Limitations
of Computing
Page 33
ComplexityThe time complexity of an algorithm is a measure of how many steps are executed when the associated program is run.void printA(){ cout << 0 << endl; cout << 0 << endl; cout << 0 << endl; cout << 0 << endl;}
void printB(){ int i; for (i = 1; i <= 100; i++) cout << 0 << endl;}
void printC(int n){ int i; for (i = 1; i <= n; i++) cout << 0 << endl;}
void printD(int n){ int i,j; for (i = 1; i <= n; i++) for (j = 1; j <= n; j++) cout << 0 << endl;}
Number of Output Statements Executed: 4
Time Complexity: O(1)
Number of Output Statements Executed: 100
Time Complexity: O(1)
Number of Output Statements Executed: n
Time Complexity: O(n)
Number of Output Statements Executed: n2
Time Complexity: O(n2)
The “big-O” notation provides information
regarding the program’s “order of complexity”.O(1) indicates that the
execution time doesn’t relate to the size of the number n.
O(n) indicates that the execution time increases linearly as n increases.O(n2) indicates that the
execution time increases quadratically as n increases.
Chapter 17Limitations
of Computing
Page 34
Logarithmic/Polynomial/ExponentialAn algorithm is said to have logarithmic time complexity if the number of steps in its execution is bounded by some logarithmic function:
k log2(n)
Essentially, this means that doubling the size of the problem (n) would only increase the execution time by a constant amount (k).An algorithm is said to have polynomial time complexity if the number of steps in its execution is bounded by some polynomial function:
aknk+ak-1nk-1+…+a2n2+a1n+a0An algorithm is said to have exponential time complexity if the number of steps in its execution is bounded by some exponential function:
k(2n)Essentially, this means that increasing the size of the problem (n) by one would double the execution time.log2(n) n n2 n3 2n
2 5 25 125 31
3 10 100 1000 1024
4 20 400 8000 1048576
Chapter 17Limitations
of Computing
Page 35
Big-OAn algorithm’s time complexity is dominated by its most significant term.
For example, an algorithm that executes in time n2+10n is considered to be O(n2) because, as n increases, the n2 term ultimately dominates the n term.
Additional examples:
5n + n2 + 0.125n3 is O(n3)
log2(n) + n2 + 2n is O(2n)
100n + max(n2, 1000 - n3) is O(n2)
n3 - ⅞(n3 - 80n2 - 800n) is O(n3)
1000000 + 0.000001log2(n) is O(log2(n))
Chapter 17Limitations
of Computing
Page 36
P and NP ProblemsA problem is said to be a P problem if it can be solved with a deterministic, polynomial-time algorithm. (Deterministic algorithms have each step clearly specified.)A problem is said to be an NP problem if it can be solved with a nondeterministic, polynomial-time algorithm.In essence, at a critical point in the NP problem’s algorithm, a decision must be made, and it is assumed that some magical “choice” function (also called an oracle) always chooses correctly.For example, take the Satisfiability Problem:Given a set of n boolean variables b1, b2, … bn, and a boolean function f (b1, b2, …, bn), are there any values that can be assigned to the variables so the function value will evaluate to TRUE?To try every combination of boolean values would take exponential time, but the nondeterministic solution at right has polynomial time complexity.
for (i = 1; i <= n; i++) bi = choice(true, false);if (f(b1, b2,…, bn) == true) satisfiable = true;else satisfiable = false;
Chapter 17Limitations
of Computing
Page 37
The Knapsack ProblemThe Knapsack Problem involves taking n valuable jewels J1, J2,…,Jn, with respective weights w1, w2,…, wn, and prices p1, p2,…, pn, and placing some of them in a knapsack that is capable of supporting a combined weight of M.
The problem is to pack the maximum worth of gems without exceeding the capacity of the knapsack.
(It’s not as easy as it sounds; three lightweight $1000 gems might be preferable to one heavy $2500 gem, and one 20-pound gem worth a lot of money might be preferable to twelve 1-pound gems that are practically worthless.)
A Nondeterministic Polynomial Solution:
TotalWorth = 0;TotalWeight = 0;for (i = 1; i <= n; i++){ bi = choice(true, false); if (b1 == true) { TotalWorth+= pi; TotalWeight += wi; }}if (TotalWeight <= M) cout << “Woo-hoo!” << endl;else cout << “Doh!” << endl;
Supplement: Computer Ethics
SupplementComputer
EthicsPage 38
How dependable is computer technology?
In 1994, Intel’s new Pentium processor was determined to have a flaw…
4195835–(4195835*3145727)/3145727 = 256
The Problem: Five values were left out of a table of 1,066 on the chip. Those five values were looked up when certain divisions were performed and, since they weren’t there, the values were interpreted as zeros.
The Response: Initially denying that there was any error at all, Intel ultimately offered to replace customers’ chips with corrected versions. The Reaction: Intel’s competitors condemned Intel, but all of the press Intel received, while negative, turned it into a household name.
Unsolicited Commercial E-Mail
SupplementComputer
EthicsPage 39
BlacklistsOne very effective technique for blocking spam is to maintain a blacklist of spam sources, and to refuse all e-mail from those sources.
The problem: Aggressive blacklist policies might list sources that send legitimate e-mail as well as spam.
Various techniques have been developed for dealing with the “spam” that plagues people’s e-mail accounts.
WhitelistsInstead of trying to block spam while allowing everything else, whitelist software blocks everything except messages from already known, accepted senders, thus changing e-mail from an open system to a closed one.
Whitelists typically allow e-mail from everyone in a user's existing address book. Other, unknown senders receive an automated reply, asking them to take further action, such as explain who they are. Or senders may be asked to identify a partially obscured image of a word. A person can make out the word, but automated spammer software can't.
Identity Theft
SupplementComputer
EthicsPage 40
Rank
Victim State
Victims per
100,000 Populati
on
Number of
Victims
1 Arizona 147.8 9,113
2 Nevada 120.0 2,994
3 California 113.5 41,396
4 Texas 110.6 26,006
5 Florida 98.3 17,780
6 Colorado 92.5 4,395
7 Georgia 86.3 8,084
8 New York 85.2 16,452
9 Washington 83.4 5,336
10 New Mexico 82.9 1,621
11 Maryland 82.9 4,656
12 Illinois 78.6 10.080
13 Oregon 76.1 2,815
14 New Jersey 73.3 6,394
15 Virginia 67.2 5,137
16 Michigan 67.2 6,784
17 Delaware 66.7 569
18 Connecticut 65.8 2,305
19 Pennsylvania 64.9 8,080
20 North Carolina 64.9 5,748
21 Missouri 64.2 3,753
22 Massachusetts 63.7 4,102
23 Oklahoma 63.0 2,254
24 Indiana 62.2 3,928
25 Utah 61.8 1,577
Rank
Victim State
Victims per
100,000 Populati
on
Number of
Victims
26 Tennessee 61.3 3,700
27 Alabama 60.3 2.774
28 Ohio 59.9 6,878
29 Kansas 58.8 1,626
30 Rhode Island 57.6 615
31 Alaska 57.3 384
32 South Carolina 55.7 2,408
33 Minnesota 55.6 2,872
34 Arkansas 54.7 1,537
35 Louisiana 52.6 2,256
36 Mississippi 51.3 1,494
37 Nebraska 49.1 868
38 Idaho 49.0 718
39 Hawaii 47.8 615
40 New Hampshire 46.1 606
41 Montana 45.9 434
42 Wisconsin 45.6 2,536
43 Wyoming 42.3 218
44 Kentucky 42.0 1,766
45 Maine 39.7 525
46 West Virginia 39.3 715
47 Iowa 34.9 1,041
48 South Dakota 30.2 236
49 North Dakota 29.7 189
50 Vermont 28.5 178
Technological advances in recent years have facilitated the ability of criminals to obtain personal information about people and to unlawfully gain access to their financial accounts.
* 2006 FTC Statistics
Electronic Monitoring
SupplementComputer
EthicsPage 41
To combat employee “cyberslacking”, employers are increasing making use of monitoring software that records how employees are using their office communication capabilities.
Legally, the courts have consistently defended the right of employers to monitor telephone conversations, voice-mail messages, e-mail, and Web browsing of employees using employer-provided equipment.
E-Waste
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E-waste represents 2% of America's trash in landfills, but it equals 70% of overall toxic waste. The extreme amount of lead in electronics alone causes damage in the central and peripheral nervous systems, the blood and the kidneys.
Electronic items that are considered to be hazardous include, but are not limited to:
•Televisions and computer monitors that contain cathode ray tubes •LCD desktop monitors •Laptop computers with LCD displays •LCD televisions •Plasma televisions •Portable DVD players with LCD screens.