UNIT I-DATA COMMUNICATION

PART A1. What is meant by half duplex?

Each Station can both transmit and receive but not at the same time. When one device is sending the other can only receive and vice versa.

Direction of data at time 1

Direction of data at time 2

In a half duplex transmission, the entire capacity of a channel is taken over by whichever of the two devices is transmitting at the time.

2. List down the Network Criteria Parameters?PerformanceReliabilitySecurity

3. List the advantages of star topology?1. Less expensive than mesh topology2. Easy to install and reconfigure due to only one link and one I/O part to

connect it to any number of others.3. Robustness4. Easy fault identification and fault isolation.

4.which OSI layers are network supportive and which are user supportive layers?

Sol: The physical layer, data link and network layers network support layers and session, presentation, application layers are user support layer. Transport layer links network support and user support layers.

5. Write the advantages of Optical fiber over twisted pair and coaxial cable.

Sol. Noise resistance of optical fiber is very high.Without requiring regeneration optical signal can run for many miles-less signal attenuation.

Bandwidth is very high.

Station Station

6. what is peer to peer process?

Sol. Between machines layer-x on one machine can communicate with layer-x of another machine. The process on each machine that communicate at a given layer are called peer to peer process.

7.What is modem and give its various standards.

The word modem is a composite word that refers to the two functional entities that make up the device: A signal modulator and a signal demodulator.A modulator creates a bandpass analog signal from binary data and a demodulator recovers the binary data from the modulated signal.

Its standards are v.32,v.32bis,v.34,v.70,v.90

8.Discuss about the voltage levels of RS-232 standard.

The RS-232 standard defines the voltage levels that correspond to logical one and logical zero levels. Valid signals are plus or minus 3 to 15 volts. The range near zero volts is not a valid RS-232 level; logic one is defined as a negative voltage, the signal condition is called marking, and has the functional significance of OFF. Logic zero is positive, the signal condition is spacing, and has the function ON.

9.Give some serial interfaces similar to RS-232.

RS-422 (a high-speed system similar to RS-232 but with differential signaling) RS-423 (a high-speed system similar to RS-422 but with unbalanced

signaling) RS-449 (a functional and mechanical interface that used RS-422 and RS-423

signals - it never caught on like RS-232 and was withdrawn by the EIA) RS-485 (a descendant of RS-422 that can be used as a bus in multidrop

configurations) MIL-STD-188 (a system like RS-232 but with better impedance and rise time

control) EIA-530 (a high-speed system using RS-422 or RS-423 electrical properties in

an EIA-232 pinout configuration, thus combining the best of both; supersedes RS-449)

PART B-UNIT I

1. Explain the mesh and star topologies of the Network in detail with diagram.

Sol:Physical Topology: It refers to the way in which a network is laid out physically.

Two or more links form a topology. The topology of a network is the geometric representation of all the links and linking devices (nodes) to one another. There are four basic topologies possible:Mesh, Star, Bus and Ring.

Categories of Topology

Mesh:In this topology every device has a dedicated point-to-point link to every other

device. The term dedicated means that the link carries traffic only between the two devices it connects. To find the number of physical links in a fully connected mesh network with n nodes we need

i.e. each and every node must be connected to n-1 nodes if each physical link allows communication in both direction, we can divide the no of links by 2

Topology

Mesh Star Bus Ring

n (n-1)/2

Advantages:1. Dedicated links guarantees that each connection can carry its own load thus

eliminating the traffic problems that an occur when links must be shared by multiple devices.

2. It is robust.3. Privacy or Security4. Fault identification and fault isolation easy.

Disadvantages:1. The amount of cabling and number of I/O parts required.2. The hardware required to connect to each link can be expensive.

Star Topology:In this each device has a dedicated point-to-point link only to a central controller

usually called a hub. Star topology does not allow direct traffic between devices. The controller acts as an exchange. If one device wants t send data to another it sends data to the controller, which then relays the data to the other connected device.

Advantages:1. Less expensive than mesh topology.2. Easy to install and reconfigure due to only one link and one I/O part to connect it

to any number of others.3. Robustness4. Easy fault identification and fault isolation.

Disadvantages:1. Dependency of the whole topology on one single point, the hub. If the hub goes

down, the whole system is dead.2. Cabling required more than bus and ring.

Station

Station Station

Station Station

Start Topology is used in LAN’s and High speed LAN’s.

2. Explain the bus,ring and hybrid topologies of the Network in detail with diagram.

Sol:

Bus Topology:It is a multipoint, one long cable acts as a backbone to link all the devices in a

network. Nodes are connected to the bus cable by drop links and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that link the splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core

Advantages:1. Ease of Installation2. It uses less cabling than mesh or star topologies.

Disadvantages:1. Difficult reconnection and fault isolation2. Fault or break in the bus cable stops all transmission.

TapBus Topology

Station Station Station Station

Cable End

Cable End

Station Station Station

Drop Line

Ring Topology:In a ring topology each device has a dedicated point-to-point link with

only the two devices on either side of it A Signal is passed along the ring in one direction from device to device until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along.

Advantages:1. Easy to install and reconfigure.2. Fault isolation is simplified.

Disadvantages:1. Unidirectional traffic.2. A break in the ring can disable the entire network.

StationStation

Repeater

Station Station

Station Station

Hybrid Topology: A network can be hybrid. We can have a main star topology with each

branch connecting several stations in a bus topology.

3. Explain the following:1.Protocols and standards2.Line configuration

Sol:

1.Protocols: The protocol is a set of rules that govern data communication;the key elements of a protocol or syntax ,semantics and timing.

Syntax: It refers to the structure or format of the data,meaning the order in which they are presented.

Semantics: It refers to the meaning of each section of bit

Station Station Station

Station Station Station

Station Station Station

Hub

Timing: It refers to two characteristics :when data should be sent and how fast they can be sent.

Standards: It provides a model for development that makes it possible for a product to work regardless of the individual manufacturers .They are essential in creating and maintaining an open and competitive market for equipment manufacturer.

2.Line configuration:It refers to the way two or more communication devices attached to a link. A link

is the physical communication pathway that transfer data from one device to another.

Types:

1.Point to pointIt provides a dedicared links between two devices.thentire capacity of he

channel is reserved for transmission between those two devices.

Eg: micro wave and satellite links

2.Multipoint:

it is one in which more than two specific devices share a single link. In this environment the capacity of the channel is shared either spatially or temporally.

4.Explain the Different categories of networks in details with diagram?There are three different categories 1.LAN 2. WAN 3.MAN

Local area network

It is usually privately owned and link the devices in a single office, building or campus. A LAN can be simple as two PC and a printer in someone’s office. Currently LAN size is limited to a few kilometers. In addition to size LAN’s are distinguished other types of networks by their transmission media and topology. In general, a given LAN will use only one type of transmission medium. The most common LAN topologies are Bus, star, and ring.

Entirely LAN’s had data rates in the 4 to 16 Mbps. Today speeds are normally 100 or 1000 Mbps. Wireless LAN are the newest evolution in LAN Technology.

LAN connecting 6 computers in a HUB

Wide area network(WAN):

It provides long distance transmission of data,image, audio and video information large geographic areas that may comprise a country,a continent or even the whole world.A WAN can be as complex as the backbones that connect the internetor as simple as a dial up line that connects a home computer to the internet.We normally refer to the first as a Switched WAN and to second as Point-to Point WAN.

The Switched WAN connects the end systems which usually comprise a router that connects to another LAN or WAN.

The Point –to-Point WAN is normally a line leashed from a telephone or a cable TV provider that connects a home computer or a small LAN to an Internet service provider.

End system

End system

End System

Switched WAN

A Router

Router Router

Router Router

RouterB

C

COMPUTERMODEM MODEM ISP

POINT-TO-POINT WAN

Metropolitan area networks (MAN):It is a Network with a size between a LAN and a WAN.

It is normally covers the area inside a town or city. It is designed for customers who need high speed connectivity normally to the internet and have a end points spread over a city or part of city. A good example of a MAN is the part of the telephone company network that can provide a high speed DSL line to the customer.

5 .List the layers of OSI model and Explain.

Sol.:OSI model is nothing but the open systems interconnection(osi)model. Control is

passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy

1. Physical layer,2. Data link layer3. Network layer4. Transport layer,5. Session layer6. Presentation layer, 7. Application layer

Application(Layer 7):

This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer.

Presentation(Layer 6):

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Session(Layer 5):

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Transport(Layer 4):This layer provides transparent transfer of data between end systems, or hosts, and

is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Network(Layer 3):This layer provides switching and routing technologies, creating logical paths,

known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Data Link(Layer 2):At this layer, data packets are encoded and decoded into bits. It furnishes

transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

physical(Layer 1):This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components.

Application layerPresentation layerSession layertransport layernetwork layerdatalink layerphysical layer

6. (a) Write about Guided Transmission media. Sol: Computers and other telecommunication devices use signals to represent data. these signals are transmitted from one device to another in the form of electromagnetic energy. these electromagnetic signals can travel through vaccum,air or other transmission media.

Coaxial Cable

Pair of conductors separated by insulation

Offers longer distances and better speeds than twisted pair, due to better shielding. Used for cable TV and local-area networks. Had been widely used in telephone

systems, but optical fibre is now assuming this task.

Baseband Coaxial Cable 50-ohm cable, commonly used for digital transmission.

Broadband Coaxial Cable 75-ohm cable, commonly used for analog transmission.

Coaxial Cable consists of 2 conductors. The inner conductor is held inside an insulator with the other conductor woven around it providing a shield. An insulating protective coating called a jacket covers the outer conductor. The outer shield protects the inner conductor from outside electrical signals. The distance between the outer conductor (shield) and inner conductor plus the type of material used for insulating the inner conductor determine the cable properties or impedance. Typical impedances for coaxial cables are 75 ohms for Cable TV, 50 ohms for Ethernet Thinnet and Thicknet. The excellent control of the impedance characteristics of the cable allow higher data rates to be transferred than Twisted Pair cable.

OPTICAL FIBER:

Three components: light source, transmission system, and a detector The detector generates an electric pulse when hit by light 1-a pulse of light; 0-missing pulse of light. optical rays travel in glass or plastic core

When light move from one medium to another it bend at the boundary. The amount of bending depends on the properties of the media.

Light at shallow angles propagate along the fibre, and those that are less than critical angle are absorbed in the jacket

The cladding is a glass or plastic with properties that differ from those of the core Used in long distance communication, in locations having small amount of space,

and with reduction in price is starting to get also to LANs. Not affected by external electromagnetic fields, and do not radiate energy. Hence,

providing high degree of security from eavesdropping. Provide for multimode of propagation at different angles of reflections. Cause

signal elements to spread out in time, which limits the rate in which data can be accurately received.

Reduction of the radius of the core implies less reflected angles. Single mode is achieved with sufficient small radius.

A multimode graded index transmission is obtained by varying the index of reflection of the core to improve on the multi mode option without resolving to the cost of single mode. (index of reflection=speed in vacuum / speed in medium.) .

Coaxial cable 500 Mbps 350 MHz 1-10 km

optical fibre 2 Gbps 2 GHz 10-100 km

6.(b) .Explain different types line coding in data communication.

Sol:

Characteristics of coding:

Signal element; Data element Data rate; Signal rate Bandwidth Baseline Wandering DC component Self-synchronizing Built-in Error Detection Immunity to Noise and Interference Complexity

Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite.

In Manchester and differential Manchester encoding, the transitionat the middle of the bit is used for synchronization

polar-RZ coding:

Polar biphase: Manchester and differential Manchester schemes