smu assignment of Bsc IT 2nd sem computer networks

                                             [SPRING 2015] ASSIGNMENT

PROGRAM	BSc IT
SEMESTER	SECOND
SUBJECT CODE & NAME	BT0072, COMPUTER NETWORKS

Q. NO. 1. What is OSI model? Explain all its layers with diagram.

Ans. The Open System Interconnection model (OSI Model) is a conceptual model that characterizes and standardizes the communication function of a tele communication or computing system without regard of their underlying internal structure and technology. Its goal is the interoperability of diverse communication system with standard protocols. The original versions of the model defined seven layers.

Layers in the Model:  This section discusses the function of the 7 layers of OSI model.

                                               

                                                         Layers of the OSI Model

1.       Physical Layer: This layer coordinates the function required to carry a bit stream over a physical medium. It deals with the electrical and mechanical specification of the interface and transmission medium. It defines the procedures and function that physical devices and interface have to perform for transmission to occur.

2.       Data Link Layer: This layer transform the physical layer, a raw transmission facility, to a reliable link. It makes the physical layer appear error-free to the upper layer. It is also responsible for other function such as framing, error control, flow control, physical addressing, and access control mechanisms.

3.       Network Layers: This layer is responsibility for the source-to-destination delivery of a packet, possibly across multiple networks. The Data Link Layer overseas the delivery of the packet between two systems on the same network, the network layer ensures that each packets gets from its point of origin to its final destination. Other responsibilities of the network layer include logical addressing, and routing.

4.       Transport Layer: This layer is responsible for process-to-process delivery of the entire message. A process is an application program running on the host. The network layer overseas the source-to- destination delivery of individual packets.

5.       Session Layer: This layer acts as the network dialog controller. It establishes, maintains, and synchronizes the interaction among communicating systems.

6.       Presentation Layer: This layer is concerned with the syntax and semantic of the information exchanged between two systems. The specific responsibility of this layer includes Translation, Encryption, and Compression.

7.       Application Layer: This layer enables the user, whether human or software to access the network. It provides user interface and support for service such as electronic mail, remote file access and transfer, shared database management, and other types of distributed information services.

Q. NO. 2. Write a short note on ALOHA protocol.

Ans. ALOHAnet, also known as the ALOHA System, or simply ALOHA, was a pioneering computer networking system developed at the University of Hawaii. ALOHAnet became operational in June, 1971, providing the first public demonstration of a wireless packet data network.

The ALOHAnet used a new method of medium access and experimental ultra-high frequency (UHF) for its operation, since frequency assignment for communication to and from a computer were not available for commercial application in 1970s. But even before such frequencies were assigned there were two other media available for the application of an ALOHA channel- cables and satellites. In the 1970s ALOHA random access was employed in the widely used Ethernet cable based network and then in Marisat satellite network.

In the early 1980s frequency for mobile network became available, and in 1985 frequency suitable for what became known as Wi-Fi were allocated in the US. These regulatory development made it possible to use the ALOHA random-access technique in both Wi-Fi and in mobile telephone networks.

ALOHA channel were used in a limited way in 1980s, in 1G mobile phones for signaling and control process. In the late 1980s, the European standardization group GSM who worked on the Pan –European Digital mobile communication system GSM greatly expanded the use of ALOHA channel for access to radio channel in mobile telephony.

It has two sub parts:

·         Pure or Un-slotted

·         Slotted or Impure ALOHA

Q. NO. 3. What is the function of data link layer? Explain its services.

Ans. The main function of the data link layer is to provide services to the network layer. The principal service is transferring data from the network layer on the source machine to the network layer on the destination machine. The services of the data link layer can be categorized as:

1.       Unacknowledged Connectionless Service: In this the source machine sends independent frames to the destination machines without having the destination machines acknowledge them. No connection is established before the hand or afterward. This class of service is appropriate when the error rate is very low and recovery is left to the higher layers.

2.       Acknowledge Connectionless Service: in this type of service, there are still no connection used, but each frame sent is individually acknowledge. If the sender does not receive the acknowledgement within a specified time interval, it would be transmitted.

3.       Connection-Oriented Service: It is the most sophisticated service provided by the data link layer to the network layer. The source and the destination machines establish a connection before any data transfer takes place.

The service primitive used by the data link layer are as follows:

1.       Request: used by the network layer to ask the data link layer to do something.

2.       Indication: used to indicate to the network layer that an event has happened, for example, establishment or release of a connection.

3.       Response: used on the receiving side by the network layer to reply to a previous indication.

4.       Confirm: These primitive provide a way for the data link layer on the requesting side to learn whether the request was successfully carried out and if not, why.

Q. NO. 4. Explain Shortest Path Routing Algorithm.

Ans. The technique is widely used because of its simplicity and easy to understand. It is a static algorithm. A ‘length’ is associated with each edge, which represent the cost of using the link for transmission. Lower the cost, more suitable is the link. The cost is determined depending upon the criteria to be optimized ways of determining the cost are:

·         Minimum number of hopes: if each link is given a unit cost, the shortest path is the one with minimum number of hopes. Such a route is easily obtained by a breadth first search method. This is easy to implement but ignores load, link capacity etc.

·         Transmission and Propagation Delays: if the cost is fixed as a function of transmission and propagation delays, it will reflect the link capacities and the geographical distances. However, these cost are essentially static and do not consider the varying load conditions.

·         Queuing Delays: if the cost of a link is determined through its queuing delays, it takes care of the varying load condition, but not of the propagation delays.

Ideally, the cost parameter should consider all the above mentioned factors, and it should be updated periodically to reflect the changes in the loading condition. However, if the routes are changed according to the load, the load change again. This feedback effect between routing and load can lead to undesirable oscillation and sudden swings.

As mentioned above, the shortest paths are calculated using suitable algorithms on the graph representation of the network. Let the network be represented by graph G (V, E) and let the number of nodes be ‘N’. For all the algorithm discussed below, the costs associated with the links are assumed to be positive. A node has zero cost w.r.t. itself. Further, all the links are assumed to be symmetric, i.e.

If Dij = cost of the link from node I to node j, then Dij = Dj,i. The graph is assumed to be complete. If there exists no edge between two nodes, then a link of infinite cost is assumed. The algorithms given below find costs of the path from all nodes to a particular node; the problem is equivalent to finding the cost of paths from a source to all destination.

Q. NO. 5. Discuss any two design issue of Session Layer.

Ans. The following are the design issue of the session layer:

·         Dialogue Management

·         Synchronization

·         Activity Management

Dialogue Management: Consider a DBMS that can be accessed from remote terminal (e.g., airline reservation or home banking). The most natural mode of operation is for the user to send a query to the DBMS and then wait for a reply. Allowing users to send a second and third query before the first one has been answered needlessly complicates the system. Logically, it is desirable to operate the system in half-duplex mode: either it is the user or system turn to transmit the session. Keeping track of whose turn it is so talk is called Dialog management, and is one of the services that can be provided by the session layer when requested.

The way that the dialog management is implemented is by the use of the data token. When a session is established, half-duplex operation is one of the option that can be selected. If half-duplex operation is chosen, the initial negotiation also determines which side gets the token first.

Activity Management: a key features of the session layer closely related to synchronization. The idea is to let the user split the message stream up into logical units called activities. Each activity is completely independent of any other activities that may have come before it or will come after it.

It is up to the user to determine what an activity is. For example: file transfer is one activity.

Q. NO. 6. Describe the structure of the DNS namespace on the internet.

Ans. Domains defines different levels of authority in a hierarchical structure. The top of the hierarchy is called the root domain. The DNS namespace on the Internet, as in fig, has the following structure:

v  Root domain

v  Top-level domains

v  Second-level domains

                                                      The DNS namespace

Root domain: uses a null label, which you write as a single period (.). In the US, The Internet Assigned Names Authority (IANA) manages several root domain name servers.

The next level in the hierarchy is divided into a series of nodes called the top- level domains. The top level domains are assigned by organization type and country/region. Some of the common top level domains are the following:

com – commercial organization in US (for example, Microsoft.com for the Microsoft Corporation)

edu – Educational organization in the US.

gov – US government organizations.

net – Networking organization.

Org – Noncommercial organization.

Top-level: domains has name servers that IANA administers. Top-level domains can contain second-level domains and hosts.

Second-level: domains contains the domains and names for organization and countries/regions. The names in second-level domain are administrated by the organization or country/region either directly or by using an ISP, who manages the names for an organization or country/region on its customer’s behalf.