Download Communication and Networks

12 Nov 2003
Communication and Networks
Communication and Networks
Communication requires four (4) things: a sender, receiver, message, and medium (or media).
Sender 
speaker
radio/TV station
Message
Medium
conversation
air
sound / pictures
air, vacuum
 Receiver
listener
radios/TVs
phone
conversation, text
copper wire, optic fiber,
satellite, radio,
microwave, infrared
phone
writer
letter, book, magazine
USPS, truck, airplane
reader
sender
files / documents
communication network
receiver
Data Communication is the process of exchanging data over communications facilities.
A communication network consists of the equipment
and connections to provide reliable communication
between selected points.
Switching equipment may be a large computer or a
dedicated switching microprocessor.
Connections between switching points are
sometimes called links. They may be twisted
pair, coax cable, optic fiber, microwave, radio,
or satellite links.
The communications network provides
communication services only. Nodes that connect
into the network provide the data to be sent.
ARPANet, the Advanced Research Project Agency
Network, was a military network started in 1969 joining military bases and university research
centers. Because of security problems, it was eventually split into a strictly defense network,
DARPA, and an academic one (CSNet, 1981; NSFNET, 1987). Some universities started
BITNET, "Because It's Time Network," 1981, as an alternative to the ARPANet.
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12 Nov 2003
Communication and Networks
RIC had a BITNET connection to Brown University
until July 1993, but now has an Internet connection via
optic cable to downtown Providence.
BITNET was a store-and-forward network.
Sender, S, --> A --> B --> C --> Receiver, R
S sends a message to R. A receives the message and sends it on to B, but retains a copy of it until
B acknowledges receipt of the sent message. B likewise passes the messages onto C, keeping a
copy again. If C is broken or unavailable, the BITNET node, B, will keep trying to deliver the
message for up to 72 hours before returning the original message to the sender.
The BITNET service was somewhat like US Postal mail. Communication went in one direction
from S to R, and the receiver could send a reply if needed, R back to S.
Unlike the mail, the path between S and R was fixed. If any link in the path failed, the message
waited unless a network manager decided to define a new path around the failure. (BITNET
history)
The Internet provides a virtual circuit like the phone system. S and R can send messages back
and forth in lock step fashion, or at the same time: S-->R-->S-->R-->S-->R, or S<==>R.
The path between S and R is not fixed beforehand, but is established when S initiates steps to
create a connection to R. The same path is used for the duration of the connection. If another,
new connection, is established between S and R, a different path may be used. This is like a long
distance call from Providence to L.A. via Chicago or Saint Louis or Dallas. If a connection
cannot be made along one link in a possible path, another, alternate connection will be found.
Thus, the Internet is more robust than BITNET which relied on predetermined paths.
Some Communications History
Yahoo has collected general information about what’s on the [Web] including a section of
Internet history. The Smithsonian Institute maintains a computer history collection and an
exhibit. "Information Age: People, Information & Society" [which] chronicles the birth and
growth of the electronic information age from Samuel Morse's invention of the telegraph in the
1830s through the development of the telephone, radio, television, and computer.
Some Cyberspace Maps
An Atlas of Cyberspaces has a "range of the historical maps of ARPANET, the Internet, Usenet,
and other computer networks, tracing how these pioneering networks grew and developed."
[Viewed 8 April 2002, 10:18]
In addition, there are samples “of some of the best network topology maps created by Internet
Service Providers (ISPs) and Internet backbone operators. The maps are often created for
promotional purposes to demonstrate the large bandwidth and good connections available."
[Viewed 8 April 2002, 10:10]
You can also buy wall maps ($85-225) of various communication systems from TeleGeography
and there are some free resources consisting of maps, schematics, statistics and charts.
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Communication and Networks
Transmission of Data
Data
Transmission
Analog
Voice
Analog
Digital
ASCII characters
Digital
Data \ Transmission
Analog
Digital
Use amplifier
Boost weak signal
Use codec
Sample & convert
Use Modem
Smooth for square
Use repeater
New, clean signal
Analog
Digital
Smooth things pass through round "pipes" easily, but square things do not go through easily.
To modulate is to change. AM radio works by changing the amplitude (amplitude modulation),
and FM radio works by modulating the frequency.
A modem, modulator-demodulator, uses analog (continuous) signals, AM, FM, phase
modulation, or a combination, to represent digital (discrete) signals.
1
ASCII A
01000001
0
10
11
ASCII A
01000001
ASCII Z
01011010
01
00
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Modems that deliver more bits per "peak" are more expensive, but deliver information faster.
Modem speed is given in bits per second, 56,000 bps or 56Kbs, but too often modem speed is
given, incorrectly, in baud or the number of symbol changes per second, e.g., 56,000 baud.
A measure of two baud represents two signal symbol changes per second. If each change
represents 1 bit, then a 2-baud signal can send 2 bits per second. This is the only time baud rate
equals the data rate. If each symbol change represents 2 bits, then a 2 baud signal can send 4 bits
per second. An 8000 baud signal that carries 7 bits per symbol can send at a data rate of 56,000
bits per second. How many signal levels are needed for one symbol to represent 7 bits?
Modems work in pairs. Every sending modem needs a receiving modem that can understand the
representation and symbol rate used to send the original digital data. Some modems, called
adaptive modems, can accept a variety of speeds and representations, and, thus, are more
flexible and expensive.
When all transmission facilities become digital, then we can take modems out of our computers.
However, we would need a telephone that contains a codec, an analog-to-digital coder and a
digital-to-analog decoder, to code our analog voice signals into digital signals. These digital
signals can then be transmitted on the all-digital system to the receiving phone where the digital
signals can be decoded into voice for the listener. Phones that record incoming messages in
digital form already have a codec to store the voice message in binary form which can replayed to
sound like the person’s voice.
Important Terms and Acronyms
Analog, continuous
Digital, discrete
Modulation, modulate, AM, amplitude, FM, frequency
Networks: LAN, MAN, WAN, VAN
Topologies: Star, ring, bus, hybrids
EFT, FAX, Telecommuting
E-mail
Node, Link, topology: bus, ring, star, and hybrid
Communication subnet, Internet, connected computers, ISP, Modem, codec, Protocol, TCP/IP
WWW, World Wide Web, linked documents, browser, URL, HTML
Text reference: Snyder, 2ed, Ch.3: Making the Connection
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Computer Communications, the Internet and the Web
Uniform Resource Locator
URL ("earl")
Domains
Old
.com
.edu
.gov
.org
Protocol
http:// https://
Mailto:
Ftp://
News://
Telnet:
Computer
Cable modem
Modem
NIC
Browser
Graphical User Interface (GUI)
Netscape, Internet Explorer, AOL
Search engines
Directories
Bookmarks, favorites
Cable System
Phone System
Network
The Web
of Linked Files
file@URL
file@UR
L
Some Common File Types
.htm, .html
.jpg, .jpeg, .gif, .tiff
.doc, .txt
.exe
Added/ New
.aero
.biz
.coop
.info
.museum
.name
.pro
.jobs
.travel
.mobi
Node
Node
Node
Node
Node
Communication Subnet
Links & Nodes
Node
Node
Node
ISP
Gateway
LAN
MAN
WAN
increasing size
additional owners
decreasing speed
Topologies
Bus
Star
Ring
Hybrids
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Viewed 25 Oct 2004, 21:25
Source: http://libweb.uoregon.edu/it/webpub/url-anatomy.html
Web Publishing Curriculum Resources, Anatomy of a URL
URL n. acronym for Uniform Resource Locator. A web resource's unique address.
web server (host name)
named anchor
|
|
http://libweb.uoregon.edu/it/index.html#handouts
|
|
protocol
path & file name
Protocol
A protocol is a means of communication between computers. The most common protocol on the web is the hypertext transfer protocol
(http). Most web addresses begin with this one. Another common protocol is mailto:, e.g., mailto:[email protected], which is
pretty self-explanatory.
Web Server
This next part identifies the web server on which the page exists. The server address holds several clues. If we read the one used in
the example above from right to left, we can tell that it's located at an educational institution (.edu), namely the University of Oregon
(.uoregon), and probably located in the library (libweb).
Path & File Name
This part tells the web browser where to look on the web server to find the specified file.
Named Anchor
This part, which is used only occasionally, tells the browser to jump to a specific (named) spot within a web page. These named spots
within a web page are created using the anchor tag with a "name" attribute instead of the "href" attribute, e.g. <a
name="handouts"></a>. Newer browsers do not require you to place text inside the anchor "nest," but older browsers may be a little
fussier.
More about URLs
http://libweb.uoregon.edu/it/webpub/ Maintained by
Colleen Bell, [email protected]
Last revision: 20 April 2003
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This <H1>Heading-1</H1> is often used by the author as the page
<Title>title</Title> in the blue title bar at the top of the browser
window
[Look up at the top of your browser window, and you should see "This TITLE will
appear at the top of the browser window."]
The following Hypertext Markup Language, " HTML," outlines an unordered list.
o
The unordered list begins with the tag <UL> and contains nine (9) list items.
o
Each list item begins with the tag "<LI>," ends with tag "</LI>," and
appears in a browser preceded by a bullet, 
o
The unordered list ends with tag </UL>.
A list item can link a user of your web page to another web source if you fill-in four
parts:
①
A Preamble that should enhance the Anchor text.
② A URL, an "earl," is a Uniform Resource Locator, the "address" of the web
source to which you want to link.
③
The Anchor text must be present so the user will have something to "click"
on when they want to follow the link.
④
A Postamble that should add additional information about the web source.
Parts ② and ③ are required, and ① and ④ are very important to create a
context of which the link is a part. The "clickable text" is often blue and
underlined and draws the viewer's eye to it, but the pre-and postamble material
should suggest to the viewer why they would want to visit this particular web page
and what they can expect to find and/or get from the visit.
For example, the list item
<li>RIC <a href="http://www.ric.edu/nus/welcome/policy.html">Policy</a> for Responsible Computing</li>
①
②
③
④
would appear in a browser window as:

RIC Policy for Responsible Computing
When the word "Policy" is clicked, the user should be led to a Rhode Island College
policy explaining what it means to be a responsible user of computing resources.
After a link has been clicked, it may change color.
1. The Preamble is "RIC " preceded by <li> which starts the list
item
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2. The URL, Uniform Resource Locator or "earl," is
"http://www.ric.edu/nus/welcome/policy.html" enclosed by <a
href=" and ">. The http indicates that the Hypertext Transport
Protocol should be used to access this particular resource.
3. The Anchor text is "Policy" followed by </a>
4. The Postamble is " for Responsible Computing" followed by </li>
which ends the list item
Here is a skeleton, unordered list, <UL>, containing nine (9) list items that can be
modified to quickly create a web page with nine links to material related to some
particular topic, e.g., Modern Slave Trade.

Preamble Anchor text Postamble

Preamble Anchor text Postamble

Preamble Anchor text Postamble

Preamble Anchor text Postamble

Preamble Anchor text Postamble

Preamble Anchor text Postamble

Preamble Anchor text Postamble

Preamble Anchor text Postamble

Preamble Anchor text Postamble
If you are not presently looking at a printed copy of this material, then you have
already linked to this page from
http://www.ric.edu/schaefer/cs101/InternetDocs/SkeletonPage.htm You can save
a copy of this material on your floppy, or to your H:\ drive, by using File, Save As
YourContentPageName.htm
Send comments to YourEmailAddress. Created 12 October 1887 [This is the date YOU first started creating YOUR page].
Last updated 1 Nov 1887at 10:57. Maintained by YourName
The address tag is used for such information as (an email) address, signature and (claim of)
authorship (and/or copyright). It is often at the beginning or end of the body of a document. Most
browsers render the address element in italics and some will also indent. Copyright ®1995-97
ExperTelligence, Inc.
If you examine the html for the email address, you will find a subject field has also
been selected that will appear in the Subject field of email sent to you about this
particular page!
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Computer Units of Measurement
binary digit
[crumb
[nibble
Byte
Half-word
Word
b
B
2B
4B
8B
bit
2 bits]
[Use Italian Computer Crumbs to stuff Internet turkeys.]
4 bits]
["Nibbles & Bits," a food for robot dogs.]
8 bits
1 ASCII or EBCIDIC character 28 =
256 symbols
16
16-bits
1 Unicode character
2 = 65,536 symbols
32-bits
Memory used to store an integer, or 4 ASCII characters.
64-bits The standard word length for some computers & Java double arithmetic.
Kilobits
Megabits
Gigabits
Terabits
Kb
Mb
Gb
Tb
1,024 bits
10242 bits
10243 bits
10244 bits
1,048,576 bits
1,024 Mb
= 1,073,741,824 bits
1,024 Gb = 1,099,511,627,776 bits
Kilobyte
Megabyte
Gigabyte
Terabyte
KB
MB
GB
TB
1024 Bytes
10242 Bytes
10243 Bytes
10244 Bytes
1,024 KB
1,024 MB
1,024 GB
= 131,072 Bytes
= 134,217,728 Bytes
= 137,438,953,472 Bytes
= 1,048,576 Bytes
= 1,073,741,824 Bytes
= 1,099,511,627,776 Byte
Note: Many sellers of computers use the science meanings of M, G and T, i.e., strict powers of 10.
Telecommunication Terms & Units
Hertz ............... Hz = One cycle per second
Ǻngström...................................... 1x10-10 meters
3  108 m

Wave length = (light speed in vacuum) / frequency
f
Hertz (Hz): 1. The SI unit of frequency, equal to one cycle per second. Note: A periodic phenomenon
that has a period of one second has a frequency of one hertz. 2. A unit of frequency which is equivalent
to one cycle per second. [NTIA] Source: http://www.atis.org/tg2k/ Viewed: 17 Oct 2004.
Kilohertz
Megahertz
Gigahertz
Terahertz
KHz
MHz
GHz
THz
1,000 Hz
1,0002 Hz
1,0003 Hz
1,0004 Hz
1,000 KHz
1,000 MHz
1,000 GHz
1,000,000 Hz
1,000,000,000 Hz
1,000,000,000,000 Hz
Common U.S. Communications Frequency Spectrum Allocations
AM Radio........................ 535 - 1600 KHz
Source: http://www.ntia.doc.gov/osmhome/allochrt.html
TV Ch.2-4 ....................... 54.0 - 72.0 MHz
US Frequency Allocation Chart, 2003
TV Ch.5-6 ....................... 76.0 - 88.0 MHz
|----Ch.5: 76 – 82 MHz----|----Ch.6: 82 – 88M Hz----|
FM Radio ..................... 88.0 – 108.0 MHz
0.20 MHz per channel (station)
TV Ch.7-13 ................. 174.0 - 216.0 MHz
6 MHz per channel
TV Ch.14-20 ............... 470.0 - 512.0 MHz (Shared with Land Mobile)
TV Ch.21-36 ............... 512.0 - 608.0 MHz
TV Broadcasting ........ 614.0 - 806.9 MHz
[HDTV 18- 40 MHz depending on compression]
Personal Frequencies
Soprano operatic voice ....... 3500-5000Hz1
Male speech ...........................85 – 155 Hz
Female speech ........................ 165-255 Hz
Human hearing ...................20 – 20,000Hz
Human eye ............ 4.6x1014 – 7.5x1014Hz
1
http://brainimaging.waisman.wisc.edu/~t
johnstone/lucia.htm
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Revised 10 Apr 2008 / 3 December 2001
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