ECEN 1200 - Telecommunications 1

Peter Mathys, Fall 2006, 9/01/06

Homework/Computer Lab 1: Resource Identification, Directories and Files

How to Submit Solutions for this Homework/Computer Lab
Due Date:	Friday 9-08-06
E-mail To:
Subject Line:	HWCL 01
1'st Line:	Your name and student number

This computer lab is written for Windows 9x/2000/NT/XT based PCs, with a Mozilla/Firefox Browser or an Internet Explorer. Those portions of the lab that are not platform-specific can also be run on a Mac.

Goals of this Lab

The goals of this computer lab are:

• Create a user account at CU Boulder if necessary.
• Know where the computer labs on the CU-Boulder campus are.
• Learn about addresses, protocols, and languages.
• Understand how files are organized in directories and subdirectories.
• Understand the structure of URIs and URLs and access URLs with a Web browser.
• Know about special characters that sould not be used in URLs and/or filenames.
• Identify top level domains on the Internet.

Information Technology Services

Computer resources and user accounts on the CU Boulder campus are handled by Information Technology Services (ITS). If you need to create an account, change a password, or find the locations of computing labs on the campus go to ITS.

What Does it Take to Communicate?

In order for two (or more) users to communicate with each other it clearly takes some infrastructure, e.g., in the form of wires or radio transmitters between the users, or in the form of a network to which the users are connected. Examples of such networks are the PSTN (public switched telephone network) and the Internet.

To use a communication infrastructure like the Internet for communication between arbitrary sets of users, the following three ingredients are necessary:

For example, if user A wants to talk to user B over the telephone, user A first dials user B's telephone number (address). User B picks up the phone when it rings and says hello (part of protocol). User A and B then talk to each other, e.g., in english (language). At the end of the call both users hang up (part of protocol).

Addresses

In everyday life you are used to addresses in the form of postal addresses, telephone numbers, social security numbers, etc. Inside computers numerical addresses in binary form are used to locate information in memory and to communicate with peripherals such as the keyboard, the monitor, disk drives, printers, etc.

On the Internet addresses take on the form of a fully qualified domain name (see below) of a host computer, such as yahoo.search.com or, in the form of a numeric IP (Internet Protocol) address, such as 66.218.71.200 (dotted decimal address).

Protocols

A simple example of an everyday protocol is going to the store, selecting groceries, having them scanned and bagged at the checkout stand, paying the amount shown on the cash register, and then returning home to eat. In this case you interact with the grocery store and its clerks and you get food in exchange for money.

On the Internet several different protocols exist according to which computers interact to accomplish specific tasks. Some of the most commonly used (application-layer) protocols on the Internet are:

Languages

Natural languages like english, french, german, etc, are quite complex because of the many irregularities and exceptions that need to be known to use and interpret these languages correctly.

For the purpose of programming computers, or storing, tranmsitting, and rendering text, images, and sound, languages with much more rigorous structure and unambiguous interpretation are needed. On the WWW the most common document language is html (hypertext markup language) and its various variants. The language of resources on the Internet is usually known from the filename extension of the file in which the resource is stored. A file with the name index.html, for instance, contains hypertext markup language. The filename extensions of the most common "languages" used on the Internet are shown in the table below.

Directories, Subdirectories, and Files

Documents, images, sounds, programs, etc, are stored in the form of data files with appropriate filename extensions on computers. All major computer platforms use hierarchically organized file systems with directories, subdirectories, sub-subdirectories (or folders, subfolders, sub-subfolders), etc.

The conventions that determine the syntax and other restrictions for directories and file names are dependent on the operating system under which a particular host computer runs. The main variants of operating systems are Unix/Linux, Windows, and MacOS. For historical reasons Windows is not truly distinguishing between filenames in upper and lower case (even though it lets you use upper and lower case letters in file and directory names). Thus, Windows is usually quite forgiving if you make upper/lower case errors when specifying a file or path name. Unix/Linux, however, which is very often used for WWW servers, is case sensitive when it comes to directory and file names and index.html and Index.html refer to two different files.

As an example, the following figure shows all subfolders of a folder called "directories", as displayed in Windows Explorer (set to "Detailed View").

The starting point, which is also called the "root" of a directory tree, is the folder or directory called "directories" in the above figure. One level below are the subfolders or subdirectories "Dogs", "e-texts", "images", and "Monkeys". Each of these subdirectories can contain other sub-subdirectories, etc. Often the names of directories and their subdirectories are chosen in a logical fashion. For example, the electronic text versions of the works of Edgar Allan Poe are stored using the directory path directories/e-texts/Poe,EdgarAllan.

The above figure does not show the actual files that are stored in the various directories. A more explicit graph of the directory "directories" and its subdirectories (highlighted in yellow) that shows all files (the entries without highlighting) is shown in the next figure.

The representation in this figure is called a tree representation of the root directory "directories" because the blue lines that lead from directories to their subdirectories resemble the branches of a tree. There are no further branches emanating from the actual stored files, and thus the filenames are called the leafs of the tree. Note that any (sub)directory may contain files as well as further (sub)directories, e.g., "gutindex.all" is a file in the subdirectory "e-texts", whereas "Maugham,W.Somerset" is another subdirectory.

URIs and URLs

URI stands for uniform resource identifier, and URL stands for uniform resource locator. A URI is characterized by the following definitions:

Comment: The concept of a URI is more general than the concept of a URL because a URI can describe any resource (e.g., a book that was written 1000 years ago and subsequently lost in a fire), whereas a URL must also specify a location where the resource can be retrieved from. In practice the terms URI and URL are often used interchangeably.

To ensure that every user in a network executes a given protocol according to the same rules and every address is specified and used in a uniform manner, standards that everyone in the network agrees upon are necessary.

Definition: A standard is a documented agreement containing technical specifications or other precise criteria to be used consistently as rules, guidelines, or definitions of characteristics, to ensure that materials, products, processes and services are fit for their purpose.

The generic syntax of a URI, as defined in RFC 3986, consists of five main components as follows:

Here is an example of a URI (or URL) for the WWW and its interpretation:

Here is another example of a URI (or URL) for the WWW and its interpretation:

More generally, if the URI specifies the location of a retrievable resource on a network, then the five components have the following meanings (the specification in RFC 3986 is actually more complex and general, but this generality is not currently needed in the context of the WWW):

Not every URI (or URL) will consist of all five parts. The <query> part is usually only needed for searching databases or submitting data from forms embedded in a Web page. If the <path> component is omitted, then the host retrieves a default document, e.g., named index.html. If the <scheme> and <authority> components are omitted in a document, then the URL is taken to be relative to the URL of the document.

Upper/Lower Case Letters. The portions of a URL that specify the protocol and the the host computer address are case insensitive, i.e., http, HttP, and HTTP all specify the same protocol and www.colorado.edu, WWW.Colorado.EDU, and WwW.cOlOrAdO.eDu all specify the same host computer. But the <path>, <query>, and <fragment> strings are specific to the host computer and programs running on it, and will in most cases be case sensitive. Thus, whereas http://ece.colorado.edu/~mathys/ecen1200 takes you to the WWW site of this class, http://ece.colorado.edu/~mathys/ECEN1200 will give you an error message.

Special Characters, "Escaped" Characters in URIs

Special Characters. Certain characters cannot be used in URIs or should be avoided. Here's a table that lists all printable ASCII (American Standard Code for Information Interchange) characters and their classification for URIs:

Non-printable characters, such as tabs, carriage return and line feeds are not allowed in URIs. If a reserved, excluded or unwise character needs to be included as part of a URI, then it must be "escaped" according to the table below. The % sign is the escape character, and the 2-digit number after it is the hexadecimal representation of the ASCII code of the character.

Host Names

A host name, such as ece.colorado.edu consists of fields separated by dots. The fields correspond to different levels in the hierarchical domain name system (DNS), with the highest hierarchy level (top level) at the end ("edu") and the lowest hierarchy level ("ece") at the beginning. Another example is shown in the following table.

Any host on the Internet has a unique numerical address associated with its name. Under IPv4 (Internet Protocol version 4), these numerical addresses have a fixed length of 32 bits and are usually expressed in dotted decimal form, by converting 8 bits at a time to a decimal number in the range 0..255. For example, the dotted decimal address of the host galileo.jpl.nasa.gov is 137.78.160.55 and the URLs http://www.tonic.to/faq.htm and http://206.14.214.154/faq.htm refer to the same resource (assuming that tonic.to does not change the assignment between the numeric IP address and the domain name).

How about host addresses like 1.2.3.com or 1.2.3.4? Are these numeric IP addresses or domain names? To answer this question you look at the last field of the host address. If it is a number, then the address is a numeric IP address, otherwise it is a domain name.

Under IPv4, a valid numerical IP address has exactly 4 fields with decimal numbers in the range 0..255 in each. Valid domain names can in principle contain any number of fields (but more than 4 or 5 is not very practical), and the last field must be an official top level domain (TLD, see below).

Top Level Domains (TLDs)

In any kind of network, the address of each user must be distinct from the addresses of all other users. In a small network it is possible to just maintain a single table or database which keeps track of all addresses that are used. In a large network it is better to use a more distributed approach and to subdivide the whole address space into non-overlapping sets, which can then be either further subdivided, or administered by individual decentralized databases. The Internet uses a hierarchical decentralized system, called the domain name system (DNS), to ensure unique address allocation on a world-wide basis. At the top of the hierarchy are the top level domains (TLD), which can be either generic (or global) TLDs (gTLD) such as edu or country code TLDs (ccTLD) such as us, as shown in the following tables.

Your Task

To obtain credit for this homework/computer lab you need to answer the questions stated below. Send your solution to , making sure they conform to the following rules.

Questions:

1. URL Syntax. Use the pieces given below to make URLs and access the corresponding Web sites. State the complete URLs that you used and briefly describe the Web documents to which you were taken. State also the (computer as opposed to natural) "language" in which each was written and, if possible (based on the 2-letter country codes), the physical location of the host from which the documents came.
   
   

   URL #	<scheme>	<authority>	<path>	<query>	<fragment>
   1	http	www.pocketmovies.net	pop.html
   2	ftp	128.138.189.30	pub/ecen1200/mm/bike.mpg
   3	http	www.aliceadsl.fr	meteo	ville=07156
   4	http	www.w3.org	TR/html4/types.html		h-6.5

   
   
   
2. RFCs. Use the search feature at http://www.rfc-editor.org to find the newest RFC that describes the generic syntax of URIs (the title of the document is "Uniform Resource Identifier (URI): Generic Syntax"). What is the number of this RFC and when was it issued?
   
   
3. URL Rules. For each of the following URLs, determine whether it is "legal" or not in terms of the rules that govern the construction of URLs (note that the actual Web sites may not exist, even if the URL is "legal")

   hTtP://as.seen.on.tv/miracle_gro.asp?cash-in
   http://show.biz/"oscar"awards.html
   http://just.do.it/sweatshops.cfm?locations
   http://flowers.4.u/order.htm#name

   If a URL is illegal, point out why it is illegal.
   
   
4. Directories and Files. The test directory "directories" and all its subdirectories and files that were used as an example in the section Directories, Subdirectories, and Files have been installed on the server for this class and can be accessed at http://ece.colorado.edu/~mathys/ecen1200/directories. By modifying the <path> directly in the address window of the browser, open several files until you become familiar with the structure of "directories".
   1. The different sentences that can be made up by using the <path>
      
      ~mathys/ecen1200/directories/{Dogs or Monkeys}/{hate or like}/{bananas or bones}.html
      
      are contained in the bananas.html and bones.html files which constitute the leafs of the Dogs and Monkeys directory subtrees. Two of the files got misplaced in the process of uploading to the server. Give the complete <paths> of these two files.
   2. What is the title of the Edgar Allan Poe work in the file story2.txt?
   3. About which people is the image in the file df950523.jpg?
   
   
   
5. Standards.
   1. Give three examples of products for which international standards exist.
   2. Give three examples of products for which international standards do not exist.

©1996-2006, P. Mathys. Last revised: 9-07-06, PM.