CS2363-Computer Networks

Define computer network.

Computer network is a connection of autonomous computers and network devices for:

Resource sharing (data/devices) in an efficient manner Communication amongst them.

Compare simplex and duplex communication with example.

In simplex mode, the communication is unidirectional (Eg: keyboard, monitor).

In half-duplex mode, each station can both transmit and receive, but not simultaneously (Eg. walkie-talkie).

In full-duplex (also called duplex), both stations can transmit and receive simultaneously (Eg. telephone network). 

List the criteria based on which a network can be assessed.

Performance is based on its throughput (no. of packets delivered) and delay. Reliability is how much the network is fault tolerant.

Security includes preventing unauthorized access and recovery from breaches.

What are the two types of line configuration?

A point-to-point connection provides a dedicated link between two nodes.

In a multipoint connection, more than two nodes share a single link, i.e., bandwidth is shared amongst the nodes..

Classify LAN, WAN, MAN and Internetwork.

LAN is privately owned and links the devices in a single office, building, or campus. The LAN size is limited to a few kilometers. A LAN will use only one type of transmission medium. The speed of LAN is in the range 10–1000 Mbps.

WAN provides long-distance transmission of data, image, audio, and video information over large geographic areas that may comprise a country, a continent, or the whole world.

MAN is a network with a size between a LAN and a WAN. It normally covers the area inside a town or a city. It is designed for customers who need a high-speed connectivity to the Internet, and have endpoints spread over a city or part of city.

When two or more independent networks are connected, it becomes an internetwork or internet.

List the advantages of layering.

It decomposes the problem of building a network into more manageable components.

It provides a more modular design. To add a new service, then it is only needed to modify the functionality at one layer, reusing the functions at all the other layers.

Uses abstraction to hide complexity of network from application.

Define protocol.

The abstract objects that make up the layers of a network system are called protocols. Each protocol defines two different interfaces.

o  Service interface that specifies the set of operations

o    Peer-to-peer interface for messages to be exchanged amongst peers Protocol is a set of rules that govern communications between devices.

IT1451 -WEB TECHNOLOGY

9. Define protocol tunneling.

Protocol tunneling is the process of encapsulating one protocol within another

protocol that operates on the same layer.

10. Define URI, URL, URN.

1. URI (Uniform Resource Identifier): It identifies an object on the Internet.

2. URL (Uniform Resource Locator): It is a specification for identifying an such as a file, newsgroup, CGI program or e-mail address by indicating the exact location on the internet.

3. URN (Uniform Resource Name): It is a method for referencing an object   without declaring the full path to the object.

11. What are the components of HTTP URL?

The components are host, an optional port, path, filename, and section and query string

12. Define URL encoding.

URL encoding involves replacing all unsafe and nonprintable characters with a

 Percent sign (%) followed by two hexadecimal digits corresponding to the character’s ASCII value.

13. Explain the various Specifics of SMTP Protocol?

 Operational Scenario.

 Command Semantics & Procedures

 Forwaring of Messages.

 Example of Forwarding

14. What is meant by Scripting Language?Explain any one of the Scripting Languages?

    Definition-To create Dynamic Web content.

        Eg:VB Script & Java Script

        Introduction about VB Script

        Functions of VB Script

       VB Script Execution Environment

        Embeding VB Script in to HTML.

        Objects & Events.

       Manipulating VB Script Variables with Operators.

        Functions & Procedures.

15. Explain the various methodologies by which you can retrieve data from Internet?

     Internet Search Engine

     Protocols used to retrieve the Data

     i)FTP –Concepts

     Commands &Semantics

     Anonymous FTP

     ii)TELNET

     Network Virtual Treminal

     Commands *Control Functions

     Negotiable Options

     iii)HTTP

     iv)TFTP (16

IT1451 -WEB TECHNOLOGY

1. What is the difference between node and host?

A node is any addressable device connected to a network whereas the host is a more specific descriptor that refers to a networked general-purpose computer rather than a single purpose device (such as a printer).

2. What is the purpose of routers?

Router operates like electronic postal workers that evaluate and forward packets

between networks.

3. Define protocol.

A protocol is a formal set of rules that must be followed in order to communicate.

4. Why are the protocols layered?

Layering protocols simplifies the task of communicating over the network and it

allows for reuse of layers that are not specific to a particular application.

5. Define encapsulation.

Placing the data inside a package of headers is known as encapsulation.

6. Define port.

A port is a logical channel to an application running on a host. ie., The applications running on the host machines are uniquely identified by port numbers.

7. What do you mean by well-known ports?

Port numbers can range from 1 to 65535; however ports 1 to 1023 are reserved.

These reserved ports are referred to as we1l-known ports because the Internet Assigned Numbers Authority publicly documents the applications that use them.

8. What is meant by Name Resolution?

Name Resolution is the process of mapping a hostname to its corresponding IP

Address.One way to translate a hostname to an IP address is to look it up in a simple text file.The second way is the domain name service, which is a distributed database containing all registered hostnames on the Internet and their IP addresses.

Error Detection and Correction

Data can be corrupted during transmission. For reliable communication, errors must be detected and corrected.

Types of Errors

Single-bit error

The term Single-bit error means that only one bit of a given data unit (such as byte, character, data unit or packet) is changed from 1 to 0 or from 0 to 1.

Burst Error

The term Burst Error means that two or more bits in the data unit have changed from 1 to 0 or from 0 to 1.

Redundancy

             One method is to send every data twice, so that receiver checks every bit of two copies and detect error.

Drawbacks

Ø  Sends n-redundant bits for n-bit message.

Ø  Many errors are undetected if both the copies are corrupted.

Instead of adding entire data, some bits are appended to each unit.

             This is called redundant bit because the bits added will not give any new information. These bits are called error detecting codes.

The three error detecting techniques are:

Ø  Parity check

Ø  Check sum algorithm

Ø  Cyclic Redundancy Check

Parity Check

Simple parity check

             Only one redundant bit, called parity bit is added to every data unit so that the total number of 1’s in unit become even (or odd)

Two Dimensional Parity

Ø  It is based on simple parity.

Ø  It performs calculation for each bit position across each byte in the frame.

Ø 

This adds extra parity byte for entire frame, in addition to a parity bit for each byte.

            For example frame containing 6 bytes of data. In this third bit of the parity byte is 1 since there are an odd number of 1’s is in the third bit across the 6 bytes in the frame.

            In this case, 14 bits of redundant information are added with original information.

Check sum algorithm

Ø  In the sender side all the words are added and then transmit the result of sum called checksum with the data.

Ø  The receiver performs the same calculation on the received data and compares the result with the received checksum.

Ø  If any transmitted data, including the checksum itself, is corrupted, then the results will not match, so the receiver knows that an error occurred.

 Ø  Instead of sending the checksum as such, one’s complement of that sum will be send to the receiver. If the receiver gets the result as zero then it will be the correct one.

Ø  In this, we can represent unsigned number from 0 to 2ⁿ using n bits.

Ø  If the number has more than n bits, the extra leftmost bits need to be added to the n rightmost bits.

Ø  Data can be divided in to 16 bit word and the Checksum is initialized to zero.

Issues in the data link layer

Framing

          To transmit frames over the node it is necessary to mention start and end of each frame. There are three techniques to solve this frame

Ø  Byte-Oriented Protocols (BISYNC, PPP, DDCMP)

Ø  Bit-Oriented Protocols (HDLC)

Ø  Clock-Based Framing (SONET)

Byte Oriented protocols

              In this, view each frame as a collection of bytes (characters) rather than a collection of bits. Such a byte-oriented approach is exemplified by the BISYNC (Binary Synchronous Communication) protocol and the DDCMP (Digital Data Communication Message Protocol)

Sentinel Approach

          The BISYNC protocol illustrates the sentinel approach to framing; its frame format is 

Ø  The beginning of a frame is denoted by sending a special SYN (synchronization) character.

Ø  The data portion of the frame is then contained between special sentinel characters: STX (start of text) and ETX (end of text).

Ø  The SOH (start of header) field serves much the same purpose as the STX field.

Ø  The frame format also includes a field labeled CRC (cyclic redundancy check) that is used to detect transmission errors.

          The problem with the sentinel approach is that the ETX character might appear in the data portion of the frame. BISYNC overcomes this problem by “escaping” the ETX character by preceding it with a DLE (data-link-escape) character whenever it appears in the body of a frame; the DLE character is also escaped (by preceding it with an extra DLE) in the frame body. This approach is called character stuffing.

Point-to-Point Protocol (PPP)

                  The more recent Point-to-Point Protocol (PPP). The format of PPP frame is

Ø  The Flag field has 01111110 as starting sequence.

Ø  The Address and Control fields usually contain default values

Ø  The Protocol field is used for demultiplexing.

Ø  The frame payload size can he negotiated, but it is 1500 bytes by default.

Ø  The PPP frame format is unusual in that several of the field sizes are negotiated rather than fixed.

Ø  Negotiation is conducted by a protocol called LCP (Link Control Protocol).

Ø  LCP sends control messages encapsulated in PPP frames—such messages are denoted by an LCP identifier in the PPP Protocol.

Byte-Counting Approach

The number of bytes contained in a frame can he included as a field in the frame header. DDCMP protocol is used for this approach. The frame format is 

Ø  COUNT Field specifies how many bytes are contained in the frame’s body.

Ø  Sometime count field will be corrupted during transmission, so the receiver will accumulate as many bytes as the COUNT field indicates. This is sometimes called a framing error.

Ø  The receiver will then wait until it sees the next SYN character.

Bit-Oriented Protocols (HDLC)

 In this, frames are viewed as collection of bits. High level data link protocol is used. The format is 

Ø  HDLC denotes both the beginning and the end of a frame with the distinguished bit sequence 01111110.

Ø  This sequence might appear anywhere in the body of the frame, it can be avoided by bit stuffing.

Ø  On the sending side, any time five consecutive 1’s have been transmitted from the body of the message (i.e., excluding when the sender is trying to transmit the distinguished 01111110 sequence), the sender inserts a 0 before transmitting the next bit.

Ø  On the receiving side, five consecutive 1’s arrived, the receiver makes its decision based on the next bit it sees (i.e., the bit following the five is).

Ø  If the next bit is a 0, it must have been stuffed, and so the receiver removes it. If the next bit is a 1, then one of two things is true, either this is the end-of-frame marker or an error has been introduced into the bit stream.

Ø  By looking at the next bit, the receiver can distinguish between these two cases:

7     If it sees a 0 (i.e., the last eight bits it has looked at are 01111110), then it is the end-of- frame marker.

8     If it sees a 1 (i.e., the last eight bits it has looked at are 01111111), then there must have been an error and the whole frame is discarded.