BC0048- COMPUTER NETWORKS

BCA-SEMESTER-4

Question 1. Explain

(i) Merits and de-merits of Layered Architecture

(ii) Connection -oriented VS connectionless Services

Answer1:a) Merits and de-merits of layered Architecture

Advantage of layered Architecture .

(i) Any given layer can be modified or updating without affecting the other layers.

(ii) Modulazition by means of layering simplifying the overall design.

(iii) Different layers can be assigned to different standards , committees , and design teams.

(iv) Mechanisms like packet-switching, circuit-switching may be used without effecting more than one layer.

(v) Different mechanism may be plugged in all different layers.

(vi) The relation between control function can be better understood .

(vii) Common lower levels may be shared by different higher levels.

(viii) Function ( especially at lower levels) may be removed from software to hardware and micro codes.

(ix) Increase the compatibility of difference machines.

Disadvantages of Layer Architecture.

(i) Total overhead is higher.

(ii) Two communicating machines may have to use certain functions which they could do without layers.

(iii) As technology changes the function may not be in the most cost-effective layer.

b) connection- oriented VS connection service.

Layered can offer two type of services to the layer above them. They are connection oriented and connection less. Connection oriented service is modelled after telephone system . To use this service, the service user first establishes the connection and then release the connection. In most of the cases the order is

preserved so that bits arrived at receiver in the same order as they were send by the transmitter . In some cases when a connection is establishes the source , the subnet and the receiver conduct negotiation of certain parameters like the maximum size of the messages, quality of services(QOs) required and other issues .

We have another type of service called Connection less service. This is modelled after the postal system . Here each message carries the full destination address, and each one is routed through the system independent of each others . Here messages may not arrive at the receiver in the same order as they were sent , as it depends on the route each messages takes on the way to the destination. Six different types of services are summarized in table 2.1

Connection

oriented

Connection

less

Comparison of Different services

Question2. Define LAN, WAN, and Differentiate them.

Answer2. LAN: LAN stands for Local Area Networks. They are privately owned networks within a single building or campus of up to few kilometres in size . Most of LAN's use Bus or ring topology for connection and is illustrate as show in fig. 1.5 . They are used to connect personal computers and workstation in a company offices and factories to share resources and exchange information. Traditional LANs at speed of 10Mbps to 100 Mbps , have low delay (microseconds) and make very few errors. Newer LANs operate at 10Gbps. Various topologies are possible for broadcast LANs.

Services Example

Reliable messages stream Sequence of pages

Reliable byte stream Remote login

Unreliable Connection Digitized voice

Unreliable datagram Electronic junk mail

Acknowledge datagram Registered mail

Request- reply Database query

Computer Cable

(a)

(b)

Figure 1.5 LANs using (a) Bus topology (b) Ring topology

WAN: WAN stand for Wide Area Network .Wan spans a large geographical area often a continent or country . WAN contains a collection of machines, traditionally called as hosts. These hosts can be on LANs and are connected by a subnet or also called communication subnet. The hosts are owned by a customer or are personal computers . the communication subnets are owned by a telephone company or internet service provider . The subnet carries the messages from hosts to hosts ,each host is connected to a LAN on which a router is present. sometimes a host may be directly to a router. The collection of communication lines and router is called a communication subnet.

In most WANs the network contains many transmission lines each connecting a pair of routers. A packet is sent from one router to another via one or more intermediate routers. The packet is received at each intermediate router in its entirely . That is store the packet in full unit the required output lines is free, and then forwards or packet switched subnet. Not all WANs are packet switched . A second possibility for a WAN is a satellite system . Satellite networks are inherently broadcast network.

Subnet Router

LAN

Question3. Explain the TCP / IP Reference Model with diagram.

Answer3. The TCP/IP reference model is the network used in the current Internet architecture .It was created in the 1970s by DARPA for use in developing the internet's protocols, and structure of the internet is still closely reflected by the TCP/IP model . it has fever les rigidly defined layers than the commonly referenced OSI model , and thus provides an easier fit for real world protocols. It is considered as the grandfather of the internet the ARPANET . This was a research network sponsored by the Department of defence in the in the United State

A goal was of continuing the conversation between source and destination even if transmission went out of operation. The reference model was named after two of its main protocol , TCP (Transmission Control Protocol) and IP .Different names are given to the layer by different document and different numbers of layers are shown by different documents. There are versions of this model with four layers and with five layers. The original four-layer version of the model has layers as shown in figure 2.7 It consists of the following four layers

Layers 4 - Process Layer or Application Layer:

This is where the "higher level" protocols such as FTP,HTP, etc. operate. The original TCP/IP specification described a number of different application that fit in to the top layer of the protocol stack. these application include Telnet, FTP,SMTP and DNS.

Layers 3- Host- TO - Host (Transport ) Layer:

This is where flow - control and connection protocols exist, such as TCP. This layer deals with opening and maintaining connections, ensuring that packets are in fact

received. The transport layer is the interface between the application layer and complex hardware of the network. It is designed to allow peer entities on the source and destination hosts to carry on conservations . Data may be user data or control data. Two modes are available, full - duplex and half duplex .In full - duplex operation both sides can transmit and receive data simultaneously, whereas in half duplex, a side can only send or receive at one time.

Layer2- Internet or Internetworking Layer :

This layer defines IP addresses, with many routing schemes for navigating packets from one IP address to another . The job of the network layer is to inject packets into any network and have them travel independently to the destination. The layer defines IP for official packets format and protocol . Packet routing is a major job of this protocol

layer - 1 Network Access Layer:

This layer describes the physical equipment necessary for communication such as twisted pair cables the signalling used on that equipment and the low level protocols using that signalling . The host-to-Network layer interface the TCP/IP protocol stack to the physical network . It varies from implemented to implementation, with vendors supplying their own version.

TCP/IP

Rep. Model

TCP/IP Network Protocol

Application

Transport

Internetwork

Host to Network

Question4. write a short not on

(i) Connection establishment

(ii) Connection Release

Answer4. Connection Establishment: Establishing a connection sounds easy but in practice it is not so. The problem occur when the network can lose, store and duplicate packets. The major problem is the existence of delayed duplicate . It can be solved in various ways but not of them work satisfactorily .

one way is to use throw away transport addresses. Here each time a transport addresses is needed , a new one is generated . When a connection is released , the address is discarded.

Another possibility is to give each connection a connection identifier that is a sequence number incremented for each connection established. This number is chosen by initiating party . After each connection is released, each transport entity could update a table listing absolute connections this scheme has basic flaws: it required each transport entity to maintain a certain amount of history information indefinitely . If a machine crashes and loses its memory it will no longer know which connection identifiers have already been used.

Another mechanism is to kill off the aged packets . Packet lifetime can be restricted to a know maximum using one of the following ways.

1) Restricted subnet design

2) Putting a hop counter in each packet

3)Time stamping each packet.

ii) Connection Release: Releasing a connection is easier than establishing the connection . There are two style of terminating / releasing a connection . They are:

Symmetric release Asymmetric release

Asymmetric release is the way the telephone system works. Here when one party hangs up , the connection is broken . Asymmetric release is abrupt and may result in data lose . Symmetric release treats the connection as two separate unidirectional connection and required each one to be released separately .

Releasing a connection means that the transport entity removes the information about the connection from its table of connection and signals the owner who had started that connection.

Question5. Write notes on (i) Non - persistent CSMA (ii) 1 - Persistent CSMA

Answer5. CSMA Protocol: Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control (MAC) protocol in which a node verifies the absence of other traffic before transmitting on a shared physical medium such as an electrical bus , or a band of electromagnetic spectrum .

"Carrier Sense" describes the fact that a transmitter listens for a carrier wave before trying to send . That is , it tries to defect the presence of an encoded signal from anther station before attempting to transmit. It a carrier is sensed, the node wait for the transmission in progress to finish before initiating its own transmission.

Multiple Access. Describe the fact that multiple nodes send and receive on the medium . Transmission by one node are generally received by all other nodes using the medium .

Collisions. Concurrent transmission by multiple node results in frame collisions . The multiple transmission interface with each other so that all area garbed and receives are unable to distinguish the overlapping received signals from each other . It is impossible to entirely prevent collision in CSMA networks, but there are three ways to address them.

In pure

CSMA, only the carrier sense is used to avoid collisions . If two nodes try to send a frame at nearly the same time , neither defects a carrier so both begins transmitting.

Non-Persistent CSMA

Non - Persistent CSMA is less greedy . the algorithm is given below:

1. Sense the channel.

2. IF the channel is idle , THEN transmit.

3. If the channel is busy , THEN wait a random amount of time and start over.

1-Persistent CSMA

1-Persistent CSMA is selfish . The algorithm is given below:

1. Sense the channel

2.IF the channel is idle, THEN transmit

3. IF the channel busy , THEN continue to listen until channel is idle.

Now transmit immediately.

Question 6. Describe Circuit switching and message switching.

Answer6. Circuit switching :

A circuit switching network is one that establishes s dedicated circuit (or channel) between nodes and terminals before the users may communicate. Each circuit that is dedicated cannot be used by other callers until the circuit is released and a new connection is set up . Even if no actual communication is taking place in dedicated circuit then , that channel still remains unavailable to the users. Channel that are available for a new calls at be set up are said to be idle. Circuit switching is used for ordinary telephone calls . It allows communication equipment and circuits, to be shared among users. Each user has sole access to a circuit (functionally equipment to a pair of copper wires) during network use.

Circuit switching can be relatively inefficient because capacity is wasted on connections when are set up but are not in continuous use ,on the other hand , the connection is immediately available and capacity is guaranteed until is disconnected

Communication using circuit switching involves three phases discussed below:

1. Connection establishment : Before any signal can be transmitted , an end to end circuit must be established .

2. Data transfer : Information can now be transmitted from source through the network to the destination using the dedicated path established.

3. Termination: After some period of data transfer , the connection is terminated.

Message switching : Message switching was the precursor of packet switching , where messages were routed in their entirely and hop at a time . It was first introduced by Leonard Kleinrock in 1961. Message switching are nowadays mostly implemented over packet -switched or circuit-switched data networks.

Hop-by-hop Telex forwarding are examples of message switching system. E-mail is another example of a message switching system .When this form of switching is used , no physical path is established in advanced in between sender or receiver . Instead , when the sender has a block of data to be sent, it is stored in the first switching office (i.e. router ) then forwarded later at one hop at a time.

Each block is received in its entity form , inspected for errors and then forwarded or - transmitted , it is a form of store and forward network . Data is transmitted into the network and stored in a switch. The network transfers the data from switch to switch when it is convenient to do so , as such the data is not transfer in real time . Blocking cannot occur , however long delay can happen . The source and destination terminal

need not be compatible, since conversions are done by the message switching networks.