COMPUTER NETWORKS BSIT-34 Bsc-IT 3rd Semester Chapter Solution

Bsc-IT 3rd Semester Chapter Solution

Comments:

1.Briefly Explain what is meant by protocol and protocol architecture?

Ans:- A protocol is used for communication between entities in different systems. It is software that resides either in a computer’s memory or in the memory of a transmission device, like a network interface card. When data is ready for transmission, this software is executed. The software prepares data for transmission and sets the transmission in motion. At the receiving end, the software takes the data off the wire and prepares it for the computer by taking off all the information added by the transmitting end. Having introduced the concept of a protocol, we can now introduce the concept of protocol architecture.

2.Brief explain the OSI architecture?

Ans:-The OSI (Open System Interconnection) model includees a set of protocols that attempt to define and standardize the data communication process. The OSI protocol were defined by the International Standards Organization (ISO) which is a multinational body dedicated to worldwide agreement on international standards.

The OSI model is not a single defination of how data communications actually takes place in the real world. Numerous protocol may exist at layer. The OSI model states how the process should be divided and what protocols should be used at each layer. If a network vendor implements one of the protocols at each layer, its network component should work with other vendors’ offerings.

The OSI model is layered frame work for The design of network system that allows communication between all types of computer systems.It consists of seven separate but related layer, each of which define a segment of the process of moving information across a network. Undertaking the fundamentals of the OSI model provides a solid basis for exploring data communication.

3. What are the function of network layer. Explain?

Ans:- The network layer is resposible for the source-to-destination delivery of a packet possibly across multiple network. Where data link layer over the delivery of the packet between two system on the same network, the network layer ensures that each packet gets from point of origin to its final destination. If the two systems are connected to the same link then there would be no requirement for the network layer. However if the two systems are attached to different network with connecting device between the network, then this layer plays a crucial role in getting the packet from source to destination.

The Network layer provided for the transfer of data in the form of packet across the communication networks. It establishes, maintains and terminates logical and physical connections across multiple interconnected networks. A key aspect of this transfer is the routing of packets from thee source to the destination machine typically traversing a number of transmission likes and network nodes where routing is carried out.

4. Explain the functionalities and the service offered by the transport layer of the OSI model.

Ans:- The Open Systems Interconnect (OSI) model has seven layers. This article describes and explains them, beginning with the ‘lowest’ in the hierarchy (the physical) and proceeding to the ‘highest’ (the application). The layers are stacked this way:
Application
Presentation
Session
Transport
Network
Data Link
Physical

PHYSICAL LAYER
The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It describes the electrical/optical, mechanical, and functional interfaces to the physical medium, and carries the signals for all of the higher layers. It provides:
Data encoding: modifies the simple digital signal pattern (1s and 0s) used by the PC to better accommodate the characteristics of the physical medium, and to aid in bit and frame synchronization. It determines:

What signal state represents a binary 1
How the receiving station knows when a “bit-time” starts
How the receiving station delimits a frame
Physical medium attachment, accommodating various possibilities in the medium:

Will an external transceiver (MAU) be used to connect to the medium?
How many pins do the connectors have and what is each pin used for?
Transmission technique: determines whether the encoded bits will be transmitted by baseband (digital) or broadband (analog) signaling.
Physical medium transmission: transmits bits as electrical or optical signals appropriate for the physical medium, and determines:

What physical medium options can be used
How many volts/db should be used to represent a given signal state, using a given physical medium
DATA LINK LAYER
The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link. To do this, the data link layer provides:

Link establishment and termination: establishes and terminates the logical link between two nodes.
Frame traffic control: tells the transmitting node to “back-off” when no frame buffers are available.
Frame sequencing: transmits/receives frames sequentially.
Frame acknowledgment: provides/expects frame acknowledgments. Detects and recovers from errors that occur in the physical layer by retransmitting non-acknowledged frames and handling duplicate frame receipt.
Frame delimiting: creates and recognizes frame boundaries.
Frame error checking: checks received frames for integrity.
Media access management: determines when the node “has the right” to use the physical medium.
NETWORK LAYER
The network layer controls the operation of the subnet, deciding which physical path the data should take based on network conditions, priority of service, and other factors. It provides:

Routing: routes frames among networks.
Subnet traffic control: routers (network layer intermediate systems) can instruct a sending station to “throttle back” its frame transmission when the router’s buffer fills up.
Frame fragmentation: if it determines that a downstream router’s maximum transmission unit (MTU) size is less than the frame size, a router can fragment a frame for transmission and re-assembly at the destination station.
Logical-physical address mapping: translates logical addresses, or names, into physical addresses.
Subnet usage accounting: has accounting functions to keep track of frames forwarded by subnet intermediate systems, to produce billing information.TRANSPORT LAYER
The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers.

The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery.

The transport layer provides:
Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
Message traffic control: tells the transmitting station to “back-off” when no message buffers are available.
Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).
Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame.

5.What are the functionality of session layer of OSI model?

Ans:– Functionalities of Session Layer:-

• The session layer organizes and synchronizes the exchange of data between application processes.
• It works with the application layer to provide simple data sets called synchronization points that let an application know how the transmission and reception of data are progressing. In simplified terms, the session layer can be thought of as a timing and flow control layer.
• The session layer is the network dialog controller. It establishes, maintains, and synchronizes the interaction between communicating systems. Specifically its responsibilities include:

Dialog control: This allows two systems to enter into a dialog. It allows the communication between two processes to take place either in half-duplex (one way at a time) or full duplex (both ways simultaneously).

Synchronization: The session layer allows a process to add checkpoints (synchronization points) into a stream of data. For example, if a system is sending a file of 2,000 pages, it is advisable to insert checkpoints after every 100 pages to ensure that each 100 page is received and acknowledged independently. The reception of acknowledgement for these checkpoints ensures to the sending system that data up to the corresponding checkpoint is received properly by the receiving end system.

An example where session layer plays a crucial role is the file transfer application when used to transfer huge files (downloading of huge files).  When huge files are transferred across the network, if the network connection speed is not high, it would take very long time for transfer. There is every chance that network connection would break during this transfer and the user is required to start a fresh to do the file transfer, and there is no guarantee that the same would not repeat, making it sometime practically impossible to download very huge files. If the checkpoints are used by the file transfer application then, this problem can be sorted out easily.  Suppose the connection fails during the 1034th page, then after reconnecting we can proceed from the last checkpoint, i.e. from page 1001. During retransmission time the only pages that are resent for the second time are 1001 to 1034 as pages till 1000 are received and acknowledged. If checkpoints were not there, we have to start from page 1 every time we try to reconnect.

6. What are the layer present in TCP/IP model? Explain the layer which provides the transport functionalities along with the protocols defined for that layer.

Ans:-

7. Make a brief comparison between OSI and TCP/IP models.

. Ans.OSI Model:-

• The OSI architecture is a de-jure (according to law) standard.
• The focus in the OSI world has always been more on the standard than the implementation of the standard.
•  The OSI model is used as a reference model to make comparisons.
• The OSI reference model was devised before the protocols were implemented.
• The ordering means that the model was not biased toward one particular set of protocols, which made it quite general

TCP/IP Model:-

• The TCP/IP architecture is a de-facto (in reality) standard.
• With the TCP/IP model, the protocols came first, and the model was just a description of the existing protocols.

The TCP/IP model is not used to describe other models.