Tuesday, December 24, 2013

B.Sc. IT BT0088 (Semester 5, Cryptography and Network Security) Assignment

Fall 2013 Assignment
Bachelor of Science in Information Technology (BSc IT) – Semester 5
BT0088 – Cryptography and Network Security – 4 Credits
(Book ID: B1183)
Assignment Set (60 Marks)


1.      What is the need for security? Explain types of security attacks.
Ans.-    Computer security is required because many organizations will be damaged by hostile software or intruders. There may be several forms of damage which are obviously interrelated. These include:
·         Damage or destruction of computer systems.
·         Damage or destruction of internal data.
·         Loss of sensitive information to hostile parties.
·         Use of sensitive information to steal elements of monitary value.
·         Use of sensitive information against the customers which may result in legal action by customers against the organization and loss of customers.
·         Damage to the reputation of an organization.
·         Monitory damage, due to loss of sensitive information, destruction of data, hostile use of sensitive data, or damage to the reputation of the organization.

Types of Threats (Attacks)
Now you would see the various types of threats which a computing environment would encounter.

·         Interception:
This type of threat occurs when an unauthorized party(outsider) has gained access. The outside party can be a person, a program, or a computing system. Examples of this type of failure are illicit copying of program or data files, or wiretapping to obtain data in a network. Although a loss may be discovered fairly quickly, a silent interceptor may leave no traces by which the interception can be readily detected.

·         Interruption:
This occurs when an asset of the system becomes lost, unavailable, or unusable. An example is the malicious destruction of a hardware device, erasure of a program or data file, or malfunction of an operating system file manager so that it cannot find a particular disk file.

Passive attacks:- Passive attacks are in the nature of eavesdropping on, or monitoring of transmissions. The goal of the opponent is to obtain information that is being transmitted. Two types of passive attacks are release of message contents and traffic analysis.
The release of message content is easily understood. A telephone conversation, an electronic mail message, and a transferred file may contain sensitive or confidential information. We would like to prevent the opponent from learning the contents of these transmissions.
A second type of passive attack is traffic analysis. Suppose a sender is masking the content by using encryption( will be discussed later) an attacker still be able to observe the pattern of these messages. The attacker (Opponent) could determine the location and identify the communicating hosts and could observe the frequency and length of messages being exchanged. This information might be useful in guessing the nature of the communication that has taken place.
Passive attacks are very difficult to detect because they do not involve any alteration of the data.

Active Attacks:- Active attacks involve some modification of the data stream or the creation of a false stream and can be subdivided into four categories: masquerade, replay, modification of messages and denial of service.
A Masquerade takes place when one entity pretends to be a different entity. A masquerade attack usually includes one of the other forms of active attack. Replay involves the passive capture of a data unit and its subsequent retransmission to produce an unauthorized effect.

2.      List substitution techniques. Explain Ceaser’s cipher.
Ans.-    Substitutions are the simple form of encryption in which one letter is exchanged for another. A substitution is an acceptable way of encrypting text. There are four types of Substitutions techniques--

1.      The Caesar Cipher
2.      One-Time Pads
3.      The Vernam Cipher
4.      Book Cipher
The Caesar Cipher:- The Caesar cipher has an important place in history. Julius Caesar is said to have been the first to use this scheme, in which each letter is translated to a letter a fixed number of places after it in the alphabet. Caesar used a shift of 3, so that plaintext letter pi was enciphered as ciphertext letter ci by the rule
A full translation chart of the Caesar cipher is shown here.



Using this encryption, the message
SIKKIM MANIPAL UNIVERSITY
would be encoded as
S I K K I M M A N I P A L U N I V E R S I T Y
v l n n l p p d q l s d o x q l y h u v l w b

Cryptanalysis of the Caesar Cipher
Let us take a closer look at the result of applying Caesar's encryption technique to "SIKKIM MANIPAL UNIVERSITY" If we did not know the plaintext and were trying to guess it, we would have many clues from the ciphertext. For example, the break between the two words is preserved in the ciphertext, and double letters are preserved: The SS is translated to vv. We might also notice that when a letter is repeated, it maps again to the same ciphertext as it did previously. So the letter K always translate to n. These clues make this cipher easy to break.
Suppose you are given the following ciphertext message, and you want to try to determine the original plaintext.

wklv phvvdjh lv qrw wrr kdug wr euhdn

The message has actually been enciphered with a 27-symbol alphabet: A through Z plus the "blank" character or separator between words. As a start, assume that the coder was lazy and has allowed the blank to be translated to itself. If your assumption is true, it is an exceptional piece of information; knowing where the spaces are allows us to see which are the small words. English has relatively few small words, such as am, is, to, be, he, we, and, are, you, she, and so on. Therefore, one way to attack this problem and break the encryption is to substitute known short words at appropriate places in the ciphertext until you have something that seems to be meaningful. Once the small words fall into place, you can try substituting for matching characters at other places in the ciphertext.
Look again at the ciphertext you are decrypting. There is a strong clue in the repeated r of the word wrr. You might use this text to guess at three-letter words that you know. For instance, two very common three-letter words having the pattern xyy are see and too; other less common possibilities are add, odd, and off. (Of course, there are also obscure possibilities like woo or gee, but it makes more sense to try the common cases first.) Moreover, the combination wr appears in the ciphertext, too, so you can determine whether the first two letters of the three-letter word also form a two-letter word.

3.      Explain in brief types of encryption systems.
Ans.-    The two basic kinds of encryption systems are key based and block based. Key based encryption is based on either single key or multiple keys. Block based encryption is based on either stream or block of characters.

Based on Key :- We have two types of encryptions based on keys they are symmetric (also called "secret key") and asymmetric (also called "public key"). Symmetric algorithms use one key, which works for both encryption and decryption. Usually, the decryption algorithm is closely related to the encryption one.
The symmetric system means both encryption and the decryption are performed using the same key. They provide a two-way channel to their users: A and B share a secret key, and they can both encrypt information to send to the other as well as decrypt information from the other. As long as the key remains secret, the system also provides authentication, proof that a message received was not fabricated by someone other than the declared sender. Authenticity is ensured because only the legitimate sender can produce a message that will decrypt properly with the shared key.
Public key systems, on the other hand, excel at key management. By the nature of the public key approach, you can send a public key in an e-mail message or post it in a public directory. Only the corresponding private key, which presumably is kept private, can decrypt what has been encrypted with the public key.
But for both kinds of encryption, a key must be kept well secured. Once the symmetric or private key is known by an outsider, all messages written previously or in the future can be decrypted (and hence read or modified) by the outsider. So, for all encryption algorithms, key management is a major issue. It involves storing, safeguarding, and activating keys.

Based on Block:- Block based encryption system is classified as stream and block encryption system. Stream encryption algorithm convert one symbol of plaintext immediately into a symbol of ciphertext. The transformation depends only on the symbol, the key, and the control information of the encipherment algorithm. A model of stream enciphering is shown in below figure.



Some kinds of errors, such as skipping a character in the key during encryption, affect the encryption of all future characters. However, such errors can sometimes be recognized during decryption because the plaintext will be properly recovered up to a point, and then all following characters will be wrong. If that is the case, the receiver may be able to recover from the error by dropping a character of the key on the receiving end. Once the receiver has successfully recalibrated the key with the ciphertext, there will be no further effects from this error.
To address this problem and make it harder for a cryptanalyst to break the code, we can use block encryption algorithm. A block encryption encrypts a group of plaintext symbols as one block. The columnar transposition and other transpositions are examples of block ciphers. In the columnar transposition, the entire message is translated as one block. The block size need not have any particular relationship to the size of a character. Block ciphers work on blocks of plaintext and produce blocks of ciphertext, as shown in below figure. In this figure, the central box represents an encryption machine: The previous plaintext pair is converted to po, the current one being converted is IH, and the machine is soon to convert ES.




4.      Explain authentication header with necessary diagrams.
Ans.-    Authentication Header (AH) is one of the two core security protocols in IPSec protocol suite. AH provides data integrity, data source authentication, and protection against replay attacks. It does not provide confidentiality. This makes AH header much simpler than ESP. It is merely a header and not a header plus trailer. The below figure shows the AH protected IP packet.


It provides authentication of either all or part of the contents of a datagram through the addition of a header that is calculated based on the values in the datagram. What parts of the datagram are used for the calculation, and the placement of the header, depends on the mode (tunnel or transport) and the version of IP. The below figure shows the AH protocol structure.


The fields comprising the AH header are:
·         Next Header: The next header field identifies the protocol type of the next packet header after the AH packet header.
·         Payload Length: The length field states the length of the AH header information.
·         Reserved field: It is for future extensions of the AH protocol.
·         SPI field: shows to which SA the packet belongs.
·         Sequence number: It is an incrementing value that prevents against replay attacks.
·         The authentication data: contains the information for authenticating the packet.

The operation of the AH protocol is simple especially for any protocol that has anything to do with network security. It can be considered analogous to the algorithms used to calculate checksums or perform CRC checks for error detection. In those cases, a standard algorithm is used by the sender to compute a checksum or CRC code based on the contents of a message. This computed result is transmitted along with the original data to the destination, which repeats the calculation and discards the message if any discrepancy is found between its calculation and the one done by the source.
This is the same idea behind AH, except that instead of using a simple algorithm known to everyone, it uses a special hashing algorithm and a specific key known only to the source and the destination. SA between two devices is set up that specifies these particulars so that the source and destination know how to perform the computation, but nobody else can. On the source device, AH performs the computation and puts the result (called the Integrity Check Value or ICV) into a special header with other fields for transmission. The destination device does the same calculation using the key the two devices share, which enables it to see immediately if any of the fields in the original datagram were modified either due to error or malice.
It's important to point here that just as a checksum doesn't change the original data, neither does the ICV calculation change it. The presence of the AH header allows us to verify the integrity of the message, but doesn't encrypt it. Thus, AH provides authentication but not privacy.

5.      Explain the processing of Encrypted E-Mail.
Ans.-    The sender chooses a (random) symmetric algorithm encryption key. Then, the sender encrypts a copy of the entire message to be transmitted, including FROM:, TO:, SUBJECT:, and DATE: headers. Next, the sender prepends plaintext headers. For key management, the sender encrypts the message key under the recipient's public key, and attaches that to the message as well. The process of creating an encrypted e-mail message is shown in Figure A.


Encryption can potentially yield any string as output. Many e-mail handlers expect that message traffic will not contain characters other than the normal printable characters. Network e-mail handlers use unprintable characters as control signals in the traffic stream. To avoid problems in transmission, encrypted e-mail converts the entire ciphertext message to printable characters. An example of an encrypted e-mail message is shown in above Figure A. Notice the three portions: an external (plaintext) header, a section by which the message encryption key can be transferred, and the encrypted message itself. (The encryption is shown with shading.)


The encrypted e-mail standard works most easily as just described, using both symmetric and asymmetric encryption. The standard is also defined for symmetric encryption only: To use symmetric encryption, the sender and receiver must have previously established a shared secret encryption key. The processing type ("Proc-Type") field tells what privacy enhancement services have been applied. In the data exchange key field ("DEK-Info"), the kind of key exchange (symmetric or asymmetric) is shown. The key exchange ("Key-Info") field contains the message encryption key, encrypted under this shared encryption key. The field also identifies the originator (sender) so that the receiver can determine which shared symmetric key was used. If the key exchange technique were to use asymmetric encryption, the key exchange field would contain the message encryption field, encrypted under the recipient's public key. Also included could be the sender's certificate (used for determining authenticity and for generating replies).
To ensure the authenticity of the sender, the encrypted e-mail messages always carry a digital signature along with the message. The integrity is also assured because of a hash function (called a message integrity check, or MIC) in the digital signature. Optionally, encrypted e-mail messages can be encrypted for confidentiality.
Notice in above Figure A. that the header inside the message (in the encrypted portion) differs from that outside. A sender's identity or the actual subject of a message can be concealed within the encrypted portion.
The encrypted e-mail processing can integrate with ordinary e-mail packages, so a person can send both enhanced and nonenhanced messages, as shown in below Figure B. If the sender decides to add enhancements, an extra bit of encrypted e-mail processing is invoked on the sender's end; the receiver must also remove the enhancements. But without enhancements, messages flow through the mail handlers as usual.



6.      Explain characteristics of good security policy.
Ans.-    Characteristics of a good security policy
If a security policy is written poorly, it cannot guide the developers and users in providing appropriate security mechanisms to protect important assets. Certain characteristics make a security policy a good one.

1. Coverage: A security policy must be comprehensive: It must either apply to or explicitly exclude all possible situations. Furthermore, a security policy may not be updated as each new situation arises, so it must be general enough to apply naturally to new cases that occur as the system is used in unusual or unexpected ways.

2.Durability: A security policy must grow and adapt well. In large measure, it will survive the system's growth and expansion without change. If written in a flexible way, the existing policy will be applicable to new situations. However, there are times when the policy must change (such as when government regulations mandate new security constraints), so the policy must be changeable when it needs to be.
An important key to durability is keeping the policy free from ties to specific data or protection mechanisms that almost certainly will change. For example, an initial version of a security policy might require a ten-character password for anyone needing access to data on the Sun workstation in room 110. But when that workstation is replaced or moved, the policy's guidance becomes useless. It is preferable to describe assets needing protection in terms of their function and characteristics, rather than in terms of specific implementation. For example, the policy on Sun workstations could be re-worded to mandate strong authentication for access to sensitive student grades or customers' proprietary data. Better still, we can separate the elements of the policy, having one policy statement for student grades and another for customers' proprietary data. Similarly, we may want to define one policy that applies to preserving the confidentiality of relationships, and another protecting the use of system through strong authentication.

3.Realism: The policy must be realistic. That is, it must be possible to implement the stated security requirements with existing technology. Moreover, the implementation must be beneficial in terms of time, cost, and convenience; the policy should not recommend a control that works but prevents the system or its users from performing their activities and functions. It is important to make economically worthwhile investments in security, just as for any other careful business investment.


4.Usefulness: An obscure or incomplete security policy can not be implemented properly, if at all. The policy must be written in a language that can be read, understood and followed by anyone who must implement it or is affected by it. For this reason, the policy should be succinct, clear, and direct.

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B.Sc. IT BT0087 (Semester 5, WML and WAP Programming) Assignment

Fall 2013 Assignment
Bachelor of Science in Information Technology (BSc IT) – Semester 5
BT0087 – WML and WAP Programming – 2 Credits
(Book ID: B1182)
Assignment Set (30 Marks)

1.      Write short note on WAP toolkit software.
Ans.-   The WAP Toolkit simulates a generic WAP mobile phone (you can choose from two models), later referred to as user agent. A WAP user agent is similar to a web browser with the distinction that in the WAP environment Uniform Resources Locators (URLs) are requested is sent to a web server via a WAP Gateway. The web server (also called an "origin server" because content originates with it) contains WML and WML Script content.
Having received the request for this content, the web server sends it to the WAP Gateway, which encodes it into binary form (to reduce its size). The WAP Gateway then transmits the encoded content to the requesting mobile phone. The content received by the phone is optimized for small displays.
WML is based on the Extensible Markup Language (XML) and was developed by the Wireless Application Protocol (WAP) Forum for specifying content and the user interface for narrow and devices such as mobile phones.
WML Script can be used to add client side procedural logic to WML cards and decks. The language is based on ECMA Script, but has been modified to better support low band with devices such as mobile phones. You can use WML Script along with WML to provide intelligence to the client, or you can use it as a stand-alone tool.
The WAP toolkit provides tools for creating services on the WAP platform. It includes the following components:
·         The WAP Toolkit application for Windows NT 4.0, Windows 95, and Windows 98, which simulates the behavior of WML and WML script, allows you to create, edit and debug WML and WML Script services.

The simulation client includes the following components:
– WML Script interpreter and WML Script libraries
– WAP phone simulations
– WML and WML Script encoders Compiled
– WML, WML Script, and WBMP editors
– WAP Protocol Stack, HTTP and file access modules
– Debugging views
– WAP Server Simulator for devices that require a connection to a WAP Gateway
– WAP Push Simulator. (Blueprint phone simulation only)

An installation utility and practical WML example applications and source code. You can use these samples as a starting point for creating your own services.
·         XML library that you can use to construct valid WML documents for use by server-side components, or for general use. The library only allows you to create valid content as validated by the standard DTD for WML 1.1.
·         User documentation in Adobe PDF format, this guide, a Developer's Guide, and a Designer's Guide.
The components of the WAP Toolkit software can be divided into three main categories discussed below:

Phone simulations
The phone simulations are on-screen images of phones that have functional displays and keys for navigating and entering data. There are two phone simulations: a simulation of the actual 7110 phone and a Java-based Blueprint phone simulation, which has no real-world counterpart.

Editors
The toolkit provides editors for convenience in developing WAP applications. There are four editors a WML Deck editor, a WML Script editor, a Push Message editor, and a WBMP graphics editor. The editor interface also provides an easy means to compile the textual source code into binary data for display on the phone simulation.

Debugging aids
The Toolkit provides several tab views with in its main window. Some of these views are used for editing, while others provide information for debugging. These latter views include the history view, sessions view, and variables view.

2.      What is WML input field? Explain WML password input field.
Ans.-   WML Input Fields
Input fields are used to obtain alphanumeric data from users. The <input/> tag is used to create input fields. Let's first take a look at the following WML example. Then we will mention about some of the commonly used attributes of the <input> element.

<?xml version="1.0"?>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.3//EN" "http://www.wapforum.org/DTD/wml13.dtd">
<wml>
  <card id="card1" title="WML Input Fields">
    <p>
      Hello, welcome to our WML tutorial.<br/>
      What's your name?
      <input name="myname" maxlength="16"/>
    </p>
  </card>
</wml>

Figure shows the result of the above WML example.


Like a selection list, an input field is associated with a variable, which stores the data entered by the user. The variable name is specified with the name attribute of the <input> element. In the above WML example, a variable myname is associated with the input field. If you enter Tom in the input field, the variable myname will contain the valueTom
The maxlength attribute of the <input> element limits the number of characters that a user can enter in an input field. In the above WML example, a user can enter at most 16 characters in the input field.
In the above WML example, we have not set a default value to the input field. So, as you can see in the screenshots, the input field is empty at the beginning. If you want to set a default value to the input field, you need to specify the value attribute in the <input> element. For example, if we change the following line:

<input name="myname" maxlength="16"/>
to: 
<input name="name" maxlength="16" value="Jack"/>
Then the input field will contain the word "Jack" by default.

WML Password Input Fields
The type attribute is another useful attribute of the <input> element. Its value can be either text (default value) or password. If the value of the type attribute is text, the characters in the input field will be visible; if you change it to password, all characters in the input field will be displayed as asterisks (*). Note that data in password input fields will not be transmitted in a more secure way than that in ordinary input fields. The following WML example demonstrates how to create a password input field:

<?xml version="1.0"?>
<!DOCTYPE wml PUBLIC "-//WAPFORUM//DTD WML 1.3//EN" "http://www.wapforum.org/DTD/wml13.dtd">
<wml>
  <card id="card1" title="Password Input Fields">
    <p>
      Please login before reading our WML tutorial.<br/><br/>
      User Name:
      <input type="text" name="name" maxlength="16" value="Jack"/><br/>
      Password:
      <input type="password" name="passwd" maxlength="10" value="abc"/>
    </p>
  </card>
</wml>

The result of the above WML example in WAP browsers is shown in figure.



3.      Explain Naming convention for WML Scripts and Characteristics of the functions in WML scripts.
Ans.-   1. Naming convention for WMLScripts
In WMLScript, function names must be started with a letter or an underscore character. The rest of the characters can be letters, numbers, or underscores. Special characters other than the underscore are not permitted. You cannot use WMLScript reserved words and keywords as function names. Remember that WMLScript is case-sensitive. The names wmlscript_function and WMLScript_Function refer to two different functions.
Here are some examples of valid function names:
·         wmlscriptFunction
·         _wmlscript_function
·         wmlscript_function1
·         wmlscript_1st_function
·         WMLSCRIPT_FUNCTION

And, here are some examples of invalid function names:
·         1st_wmlscript_function (Reason: you cannot start a function name with a number)
·         11111 (Reason: You cannot start a function name with a number)
·         wmlscript-function (Reason: The - character is not permitted)
·         ~wmlscript_function (Reason: The ~ character is not permitted)

2. Characteristics of the functions
The characteristics of the functions in WML Script are mentioned below
·         Function declarations cannot be nested.
·         Function names must be unique within one compilation unit.
·         All parameters to functions are passed by value.
·         Function calls must pass exactly the same number of arguments to the called function as specified in the function declaration.
·         Function parameters behave like local variables that have been initialized before the function body (block of statements) is executed.
·         A function always returns a value. By default, it is an empty string (" ").
·         However, you can use a return statement to specify other return values.
A function in WMLScript is defined using the following format. The parts enclosed inside brackets [] are optional.

[extern] function function_name ([argument1, argument2...])
{
  WMLScript statements here
  [return (some_value);]
}

The extern keyword
The extern keyword is used to specify that a function can be called from both inside and outside of the WMLScript file, i.e. the function can be called from functions in the same WMLScript file, from functions in a different WMLScript file, or in a WML file. If you define a function without the extern keyword, the function can only be called from functions in the same WMLScript file.

WMLScript function Arguments
Arguments are used to pass values into a function. Unlike programming languages like C++ or Java, WMLScript does not require you to specify the data type of an argument. For example, to pass two numbers into the WMLScript function wmlscript_function(), you will define something like this:
function wmlscript_function (number1, number2)
{
...
}
If a function does not require any arguments, you still need to include the parentheses (), like this:

function wmlscript_function ()
{
...
}

The return statement
The "return (some_value);" statement is used to return a value back to the calling function in WMLScript. For example, the calculateSum() function below returns the sum of two numbers back to the calling function each time it is executed:
function calculateSum(number1, number2)
{
  return (number1 + number2);
}
The wmlscript_function() function below calls calculateSum() with two arguments 1 and 2. The value 3 is returned from calculateSum() and is assigned to the sum variable:
function wmlscript_function()
{
 ... sum = calculateSum(1, 2);
 ...
 }
It is not compulsory to include a return statement in a WMLScript function. If no return statement is included, the default.


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B.Sc. IT BT0086 (Semester 5, Mobile Computing) Assignment

Fall 2013 Assignment
Bachelor of Science in Information Technology (BSc IT) – Semester 5
BT0086 – Mobile Computing – 4 Credits
(Book ID: B1181)
Assignment Set (60 Marks)

1.      Write notes on FDMA and CDMA techniques.
Ans. -  Frequency Division Multiple Access (FDMA):- FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth. To better understand FDMA, think of radio stations. Each station sends its signal at a different frequency within the available band. FDMA is used mainly for analog transmission. While it is certainly capable of carrying digital information, FDMA is not considered to be an efficient method for digital transmission.



Code Division Multiple Access (CDMA):- CDMA takes an entirely different approach from TDMA. CDMA, after digitizing data, spreads it out over the entire bandwidth it has available. Multiple calls are overlaid on the channel, with each assigned a unique sequence code. CDMA is a form of spread spectrum, which simply means that data is sent in small pieces over a number of the discrete frequencies available for use at any time in the specified range.


All the users transmit in the same wide-band chunk of spectrum. Each user's signal is spread over the entire bandwidth by a unique spreading code. At the receiver, that same unique code is used to recover the signal. Because CDMA systems need to put an accurate time stamp on each piece of a signal, it refers the GPS system for this information. Between eight and 10, separate calls can be carried in the same channel space as one analog AMPS call. CDMA technology is the basis for Interim Standard 95 (IS-95) and operates in both the 800 MHz and 1900 MHz frequency bands. Ideally, TDMA and CDMA are transparent to each other. In practice, high power CDMA signals will raise the noise floor for TDMA receivers, and high power TDMA signals can cause overloading and jamming of CDMA receivers.

2.      Explain the three basic communication modes.
Ans. -  Three Modes of Communication
1. Interpersonal Mode: - The Interpersonal Mode is characterized by active negotiation of meaning among individuals. Participants observe and monitor one another to see how their meaning and intentions are being communicated. Adjustments and clarifications can be made accordingly. As a result, there is a higher probability of ultimately achieving the goal of successful communication in this mode than in the other two modes. The Interpersonal Mode is most obvious in conversation, but both the interpersonal and negotiated dimensions can be realized through reading and writing, such as the exchange of personal letters or electronic mail messages.

Interpersonal examples:-
1.      Exchange information via letters, e-mail/video mail, notes, conversations or interviews on familiar topics (e.g., school events, weekend activities, memorable experiences, family life).
2.      Express and compare opinions and preferences about information gathered regarding events, experiences and other school subjects.
3.      Clarify meaning (e.g., paraphrasing, questioning).
4.      Give and follow directions, instructions and requests (e.g., installing software, dance steps).
5.      Demonstrate the ability to acquire goods, services or information (e.g., using public transportation, making a hotel reservation, buying food).

2. Interpretive Mode: - The Interpretive Mode is focused on the appropriate cultural interpretation of meanings that occur in written and spoken form where there is no recourse to the active negotiation of meaning with the writer or the speaker. Such instances of “one-way” reading or listening include the cultural interpretation of texts, oral or written, must be distinguished from the notion of reading and listening “comprehension,” where the term could refer to understanding a text with an American mindset. Put another way, interpretation differs from comprehension in that the former implies the ability to “read (or listen) between the lines.”
Since the Interpretive Mode does not allow for active negotiation between the reader and the writer or the listener and the speaker, it requires a much more profound knowledge of culture from the outset. The more one knows about the other language and culture, the greater the chances of creating the appropriate cultural interpretations of a written or spoken text. It must be noted, however, that cultural literacy and the ability to read or listen between the lines are developed over time and through exposure to the language and culture.

Interpretive examples:-
1.      Follow directions, instructions and requests (e.g., recipes, travel directions, prompts on ATMs).
2.      Use listening and reading strategies (e.g., skimming and scanning techniques) to determine main ideas and purpose.

3. Presentational Mode: - The Presentational Mode refers to the creation of messages in a manner that facilities interpretation by members of the other culture where no direct opportunity for active negotiation of meaning between members of the two cultures exists. Examples of the “one-way” writing and speaking require a substantial knowledge of language and culture from the outset, since the goal is to make sure that members of the other culture, the audience, will be successful in reading and listening between the lines.

Presentational examples:-
1.      Summarize information from authentic language materials and artifacts (e.g., TV programs, articles from youth magazines, Internet, videos, currency) and give personal reactions.
2.      Use information acquired from target language sources to solve everyday problems and situations (e.g., using a newspaper to make plans to see a movie, perusing a catalog to shop for a birthday gift, watching a weather forecast to help plan an activity).
3.      Create and present a narrative (e.g., current events, personal experiences, school happenings).
4.      Present differences in products and practices (e.g., sports, celebrations, school life) found in the target culture.
5.      Prepare and deliver a summary of characters and plot in selected pieces of literature.

Apply age-appropriate writing process strategies (prewriting, drafting, and revising, editing, publishing).


1.      Differentiate circuit switching and packet switching techniques.
Ans. -  Packet Switching: - Packet-switched networks move data in separate, small blocks -- packets -- based on the destination address in each packet. When received, packets are reassembled in the proper sequence to make up the message. Circuit-switched networks require dedicated point-to-point connections during calls.
In packet-based networks, however, the message gets broken into small data packets that seek out the most efficient route as circuits become available. Each packet may go a different route; its header address tells it where to go and describes the sequence for reassembly at the destination computer.

Circuit Switching: - Circuit-switched networks and packet-switched networks have traditionally occupied different spaces within corporations. Circuit-switched networks were used for phone calls and packet-switched networks handled data. But because of the reach of phone lines and the efficiency and low cost of data networks, the two technologies have shared chores for years. 
The primary hardware for a circuit-switched network is the private branch exchange (PBX) system. Computer servers power packet-switched networks.
In modern circuit-switched networks, electronic signals pass through several switches before a connection is established. And during a call, no other network traffic can use those switches.

A basic technical distinction between mobile data networks is whether they are circuit-switched or packet-switched. As a rule of thumb, all analogue and early 2G digital PCS networks provide circuit-switched data services. Newer technologies, such as 2.5G and 3G networks will also offer packet-switched service.

2.      Explain any two tables driven routing protocol.
Ans. -  Fisheye state routing: - Fisheye State Routing (FSR) is an improvement of GSR. The large size of update messages in GSR wastes a considerable amount of network bandwidth. In FSR, each update message does not contain information about all nodes. Instead, it exchanges information about closer nodes more frequently than it does about farther nodes, thus reducing the update message size. Each node receives accurate information about neighbours, and the detail and accuracy of information decreases as the distance from the node increases. The scope is defined in terms of the nodes that can be reached in a certain number of hops. The center node has the most accurate information about the other nodes. Even though a node does not have accurate information about distant nodes, the packets are routed correctly because the route information becomes more and more accurate as the packet moves closer to the destination. FSR scales well to large networks as the overhead is controlled in this scheme.

Hierarchical state routing: - The characteristic feature of Hierarchical State Routing (HSR) is multilevel clustering and logical partitioning of mobile nodes. The network is partitioned into clusters and a cluster head is elected as in a cluster-based algorithm. In HSR, the cluster heads again organize themselves into clusters and so on. The nodes of a physical cluster broadcast their link information to each other. The cluster head summarizes its cluster’s information and sends it to the neighbouring cluster heads via the gateway. These cluster heads are members of the cluster on a level higher and they exchange their link information as well as the summarized lower-level information among themselves and so on. A node at each level floods to its lower level the information that it obtains after the algorithm has run at that level. The lower level has hierarchical topology information. Each node has a hierarchical address. One way to assign hierarchical address is to use the cluster numbers on the way from the root to the node. A gateway can be reached from the root via more than one path. Thus, a gateway can have more than one hierarchical address. A hierarchical address is enough to ensure delivery from anywhere in the network to the host.
In addition, nodes are also partitioned into logical subnetworks and each node is assigned a logical address <subnet, host>. Each subnetwork has a Location Management Server (LMS). All the nodes of that subnet register their logical address with the LMS. The LMS advertise their hierarchical address to the top levels and the information is sent down to all LMS too. The transport layer sends a packet to the network layer with the logical address of the destination. The network layer finds the destination’s LMS from its LMS and then sends the packet to it. The destination’s LMS forwards the packet to the destination. Once the source and destination know each other’s hierarchical addresses, they can bypass the LMS and communicate directly. Since logical address/hierarchical address is used for routing, it is adaptable to network changes.


3.      Explain BCH, CCCH, DCCH control channels.
Ans. -  Broadcast Control Channel: - The Broadcast Control Channel is transmitted by the BTS at all times. The RF carrier used to transmit the BCCH is referred to as the BCCH carrier. The MS monitors the information carried on the BCCH periodically (at least every 30 secs), when it is switched on and not in a call.
The BCCH Consists of:
A.    Broadcast Control Channel (BCCH): Carries the following information:
·         Location Area Identity (LAI).
·         List of neighboring cells that should be monitored by the MS.
·         List of frequencies used in the cell.
·         Cell identity.
·         Power control indicator.
·         DTX permitted.
·         Access control (i.e., emergency calls, call barring ... etc.).
·         CBCH description.
The BCCH is transmitted at constant power at all times, and all MS that may seek to use it to measure its signal strength. “Dummy” bursts are transmitted to ensure continuity when there is no BCCH carrier traffic.
B.     Frequency Correction Channel (FCCH): This is transmitted frequently on the BCCH timeslot and allows the mobile to synchronize its own frequency to that of the transmitting base site. The FCCH may only be sent during timeslot 0 on the BCCH carrier frequency and therefore it acts as a flag to the mobile to identify Timeslot 0.
C.     Synchronization Channel (SCH) The SCH carries the information to enable the MS to synchronize to the TDMA frame structure and know the timing of the individual timeslots. The following parameters are sent:
·         Frame number.
·         Base Site Identity Code (BSIC).
The MS will monitor BCCH information from surrounding cells and store the information from the best six cells. The SCH information on these cells is also stored so that the MS may quickly resynchronize when it enters a new cell.

Common Control Channels: - The Common Control Channel (CCCH) is responsible for transferring control information between all mobiles and the BTS. This is necessary for the implementation of “call origination” and “call paging” functions. It consists of the following:
A.    Random Access Channel (RACH) Used by the mobile when it requires gaining access to the system. This occurs when the mobile initiates a call or responds to a page.
B.     Paging Channel (PCH) Used by the BTS to page MS, (paging can be performed by an IMSI, TMSI or IMEI).
C.     Access Grant Control Channel (AGCH) Used by the BTS to assign a dedicated control channel to a MS in response to an access message received on the Random Access Channel. The MS will move to the dedicated channel in order to proceed with either a call setup, response to a paging message, Location Area Update or Short Message Service.
D.    Cell Broadcast Channel (CBCH) This channel is used to transmit messages to be broadcast to all MS’s within a cell. The CBCH uses a dedicated control channel to send its messages, however it is considered a common channel because all mobiles in the cell can receive the messages.
Active MS’s must frequently monitor both BCCH and CCCH. The CCCH will be transmitted on the RF carrier with the BCCH.

Dedicated Control Channels: - The DCCH is a single timeslot on an RF carrier that is used to convey eight Stand-alone Dedicated Control Channels (SDCCH). A single MS for call setup, authentication, location updating and SMS point to point use a SDCCH. As we will see later, SDCCH can also be found on a BCCH/CCCH timeslot, this configuration only allows four SDCCHs.
A.    Slow Associated Control Channel (SACCH) Conveys power control and timing information in the downlink direction (towards the MS) and Receive Signal Strength Indicator (RSSI), and link quality reports in the uplink direction.
B.     Fast Associated Control Channel (FACCH) The FACCH is transmitted instead of a TCH. The FACCH ‘‘steals” the TCH burst and inserts its own information. The FACCH is used to carry out user authentication, handovers and immediate assignment.
All of the control channels are required for system operation, however, in the same way that we allow different users to share the radio channel by using different timeslots to carry the conversation data, the control channels share timeslots on the radio channel at different times. This allows efficient passing of control information without wasting capacity that could be used for call traffic. To do this we must organize the timeslots between those, which will be used for traffic, and those, which will carry control signaling.

4.      Write short notes on:
a.      Wireless Routing Protocol
Ans. - Wireless Routing Protocol: - The Wireless Routing Protocol (WRP) is a table-based distance-vector routing protocol. Each node in the network maintains a Distance table, an RT, a Link-cost table, and a Message Retransmission List (MRL). The Distance table of a node x contains the distance of each destination node y via each neighbour z of x. It also contains the downstream neighbour of z through which this path is realized. The RT of node x contains the of each destination node y from node x, the predecessor and the successor of node x on this path. It also contains a tag to identify if the entry is a simple path, a loop, or invalid. Storing predecessor and successor in the table is beneficial in detecting loops and avoiding counting-to-infinity problems. The Link-cost table contains cost of link to each neighbour of the node and the number of timeouts since an error-free message was received from that neighbour. The MRL contains information to let a node know which of its neighbours has not acknowledged its update message and to retransmit the update message to that neighbour.

Nodes exchange RTs with their neighbours using update messages periodically as well as on link changes. The nodes present on the response list of update message (formed using MRL) are required to acknowledge the receipt of the update message. If there is no change in RT since the last update, the node is required to send an idle Hello message to ensure connectivity. On receiving an update message, the node modifies its distance table and looks for better paths using new information. Any new path so found is relayed back to the original nodes so that they can update their tables. The node also updates its RT if the new path is better than the existing path. On receiving an Acknowledgement ACK, the node updates its MRL. A unique feature of this algorithm is that it checks the consistency of all its neighbours every time it detects a change in link of any of its neighbours. A consistency check in this manner helps eliminate looping situations in a better way and it also has fast convergence.

b.      Global State Routing
Ans. -  Global State Routing: - Global State Routing (GSR) is similar to DSDV. It takes the idea of link state routing but improves it by avoiding flooding of routing messages. In this algorithm, each node maintains a Neighbour list, a Topology table, a Next Hop table, and a Distance table. Neighbour list of a node contains the list of its neighbours (here all nodes that can be heard by a node are assumed to be its neighbours). For each destination node, the Topology table contains the link state information as reported by the destination and the timestamp of the information. For each destination, the Next Hop table contains the next hop to which the packets for this destination must be forwarded. The Distance table contains the shortest distance to each destination node.

The routing messages are generated on a link change as in link state protocols. On receiving a routing message, the node updates its Topology table if the sequence number of the message is newer than the sequence number stored in the table. After this, the node reconstructs its RT and broadcasts the information to its neighbours.

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