Sub Code:EC2352 Sub Name: Computer Networks Dept: ECE Sem ... 1.pdf · 8. What is the difference between a passive and an active hub? An active hub contains a repeater that regenerates

A. Joyce -AP/CSE-MAHALAKSHMI ENGINEERING COLLEGE Page 1

Sub Code:EC2352 Sub Name: Computer Networks

Dept: ECE Sem/Year: VI/III

UNIT-I

PART-A

1. Differentiate guided and unguided transmission medium.(AUC MAY’13)

Guided transmission medium Unguided transmission medium

Guided indicate, medium is contained have

any within the physical boundary

Unguided medium does not have

any physical boundary.

Transmission takes place through wire It’s a wireless transmission medium.

2. State the role of digital subscriber line.(AUC MAY’13)

Digital subscriber line (DSL) technology is one of the most promising for supporting high-

speed digital communication over the existing local loops.

However, like a cable modem, a DSL circuit is much faster than a regular phone

connection, even though the wires it uses are copper like a typical phone line.

in asymmetric DSL (ADSL) connection allows download speeds of up to about 1.5

megabits (not megabytes) per second, and upload speeds of 128 kilobits per second.

3. What is TCP/IP?(AUC NOV ’12)

TCP/IP is two separate protocols, TCP and IP, that are used together.

The Internet Protocol standard dictates how packets of information are sent out over

networks. IP has a packet-addressing method that lets any computer on the Internet

forward a packet to another computer that is a step (or more) closer to the packet's

recipient.

The Transmission Control Protocol ensures the reliability of data transmission across

Internet connected networks. TCP checks packets for errors and submits requests for

re-transmissions if errors are found; it also will return the multiple packets of a message

into a proper, original sequence when the message reaches its destination

4. Compare datagram networks with virtual circuit subnets. (AUC NOV ’12)

http://www.techterms.com/definition/adsl

http://www.computerhope.com/jargon/i/ip.htm

http://www.computerhope.com/jargon/p/packet.htm


5. What is meant by data communication?(AUC NOV’11)

Data communication is the exchange of data (in the form of 1s and 0s) between two devices via

some form of transmission medium (such as a wire cable).

6. For n devices in a network, what is the no of cable links required for mesh and ring

topology. (AUC NOV’11)

Mesh topology – n (n-1)/2

Ring topology – n

7. What are the three criteria necessary for an effective and efficient network?

The most important criteria are performance, reliability and security. Performance of the network

depends on number of users, type of transmission medium, the capabilities of the connected

h/w and the efficiency of the s/w.

Reliability is measured by frequency of failure, the time it takes a link to recover from

the failure and the network’s robustness in a catastrophe.

Security issues include protecting data from unauthorized access and viruses.

8. What is the difference between a passive and an active hub?

An active hub contains a repeater that regenerates the received bit patterns before sending

them out. A passive hub provides a simple physical connection between the attached devices.

9. Distinguish between peer-to-peer relationship and a primary-secondary relationship. Peer-to-peer relationship: All the devices share the link equally. Primary-secondary relationship: One device controls traffic and the others must transmit

through it.

10. Assume 6 devices are arranged in a mesh topology. How many cables are needed? How many ports are needed for each device? Number of cables=n (n-1)/2=6(6-1)/2=15 Number of ports per device=n-1=6-1=5

11. Explain cross talk and what is needed to reduce it?


Effect of one wire on another is called as cross talk. One wire will be the sending antenna and

the other wire will be the receiving antenna. We can use the shielded twisted pair cable or

coaxial cable for transmission, which contains metal foil to reduce cross talk.

11. Give the relationship between propagation speed and propagation time?

Propagation time = distance / propagation speed

The time required for a signal or a bit to travel from one point to another is called Propagation

time.

Propagation speed is the distance, a signal or a bit travel through a medium in one second.

12. What are the criteria used to evaluate transmission medium?

The criteria used to evaluate transmission medium are Throughput Propagation speed Propagation time Wavelength

13. What is refraction?

The phenomenon related to the bending of light when it passes from one medium to another.

14. Discuss the mode for propagating light along optical channels.

There are two modes for propagating light along optical channels, Multimode: Multiple beams from a light source move through the core in different paths.

Single mode: Fiber with extremely small diameter that limits beams to a few angles ,resulting in an almost horizontal beam.

15. Group the OSI layers by function.

The seven layers of the OSI model belonging to three subgroups. Physical, datalink and

network layers are the network support layers; they deal with the physical aspects of moving

data from one device to another. Session, presentation and application layers are the user

support layers; they allow interoperability among unrelated software systems. The transport

layer ensures end-to-end reliable data transmission.

16. What are the functions of a DTE? What are the functions of a DCE? Data terminal equipment is a device that is an information source or an information sink. It is

connected to a network through a DCE .Data circuit-terminating equipment is a device used as

an interface between a DTE and a network.

17. What does the electrical specification of EIA-232 describe? The electrical specification of EIA-232 defines that signals other than data must be sent using

OFF as less than -3 volts and ON as greater than +3 volts. The data must be transmitted using

NRZ-L encoding.


PART-B

1. (i) Explain the TCP/IP reference model with a neat sketch(AUC MAY’13)

TCP/IP

Protocols

Protocols are the rules which govern accurate and dependable communication between two or

more parties. Network protocols ar; rules governing information exchange between two or more

computers on a Network. TCP/IP can handle communication on a very small scale to a very

large scale (Internet).

TCP/ICP

During 1960 the Advanced Research Projects Agency of Department of Defence (US)

sponsored the development of ARPAnet. During 1980 a new of protocols including TCP/IP was

developed. ARPA net was split into ARPA net for research and M1LNET for military

applications.

Types of services

Connection oriented services

Connectionless services

Connection oriented and connectionless services

In case of connection oriented service, there is no need to supply destination address. But in the

case of connectionless service, there is a need to supply the destination address.

Fig (a) shows a time line of the typical scenario that takes place for a connection oriented

transfer-first the server is started then sometimes later a client is started that connects to the

server.

Fig (b) above fig shows a time line of the typical scenario that takes place for a connectionless

protocol, the system calls are different. The client does not establish a connection with the

server Instead the client just sends a datagram to the server, using the send to system call

which requires the address of the destination as a parameter.

Similarly the server does not have to accept a connection from a client. Instead a server just

issues a recv from system call that waits until data arrive from some client. The recvfrom returns

the network address of client process.

TCP/IP suite


The official name of TCP/IP protocol is TCP/IP internet protocol suite. TCP/IP stands for

Transmission control protocol/internet protocol. The TCP/IP protocol suite is a set of protocols

that allows communication across multiple diverse networks.

The TCP/IP protocols are defined in documents called Request for comments (RFC). RFCs are

maintained by the Network Information Center (NIC), the organization that handles the Address

registration for the/Internet RFCs define a number of Applications, the most widely used being

TELNET, FTP and so on.

TCP/IP is normally considered to be a 4 layer system. The layers are application layer, transport

layer, internet layer, and host to network layer.

Host to Network layer is also called physical and data link layer of OSI Reference model. This

layer cannot define any protocol. It is responsible for accepting and transmitting IP datagrams.

In the Application layer in TCP/IP can be equated in the combination of session, presentation

and application layer of OSI reference model.

TCP/IP suite Relation of protocol in the TCP/IP suite

ICMP – Internet Control Message Protocol: The protocol that handle error and control

information between gateways and hosts. While ICMP messages are transmitted using IP

datagrams, these messages are generated by TCP/IP networking s/w itself not user processes.

Specially routers and hosts use ICMP to send reports of problem

IP-Internet Protocol: IP is a protocol that provides the packet delivery service for TCP, UDP and

ICMP. TCP/IP standard protocol defines IP datagram as the unit of information passed across

an internet.


ARP-Address Resolution Protocol: The protocol that maps an internet address into a hardware

address. It finds physical address using logical address. .

RARPReverse Address Resolution Protocol: The protocol that maps the hardware address into

an internet address.

TCP – Transmission Control Protocol: A connection oriented protocol that provides a reliable,

full duplex, byte stream for a user process. Most Internet application programs use TCP. Since

TCP uses IP, the entire protocol suite is called TCP/IP protocol “family.

UDP – User Datagram Protocol: A connectionless protocol for the user processes. Unlike TCP,

which is reliable protocol, there is no guaranteed that UDP datagrams ever reach their intended

destination.

FTP – File Transfer Protocol: TCP/IP standard, high level protocol for transferring files from one

machine to another. It uses two connections to transfer file. Control and data connection. SMTP

– Simple Mail Transfer Protocol: TCP/IP standard protocol for transferring electronic messages

from one machine to another.

SNMP – Simple Network Management Protocol: A protocol used to manage device such as

hosts, routers and printers.

TELNET- Remote Login: TCP/IP protocol for remote terminal services. TELNET allows an

interactive user on the client system to start a login session on a remote system.

TFTP – Trivial file Transfer Protocol

It is a simple file transfer protocol. It is not as complex as FTP. It does not have much code and

consumes less memory It’ can be used on small machines.

(ii) Compare the performance of TCP/IP and ISO/OSI reference model.

Transmission Control Protocol is used by Internet applications like email, world wide web, FTP,

etc. TCP/IP was developed by the Department of Defense (DOD) to connect various devices to

a common network (Internet).

The main purpose behind developing the protocol was to build a robust and automatically

recovering phone line failure while on the battlefield. On the other hand, Open Systems

Interconnection was developed by the International Organization for Standardization (ISO). This

model was made up of two components, namely, seven-layer model and the subset of

protocols.


TCP/IP is a four-layered structure, with each layer having their individual protocol. Let us have a

look at the four layers:

Link Layer

The layer includes the physical and logical connections from the host's link. It is also known as

Network Access layer and Network Interface layer. It explains how the data is transmitted from

the host, through the network. The physical connectors like the coaxial cables, twisted pair

wires, the optical fiber, interface cards, etc., are a part of this layer. This layer can be used to

connect different network types like ATM, Token ring, Ethernet, LAN, etc.

Internet Layer


This layer is also known as the Network Layer. The main function of this layer is to route the

data to its destination. The data that is received by the link layer is made into data packets (IP

datagrams). The data packets contain the source and the destination IP address or logical

address. These packets are sent on any network and are delivered independently. This

indicates that the data is not received in the same order as it was sent. The protocols at this

layer are IP (Internet Protocol), ICMP (Internet Control Message Protocol), etc.

Transport Layer

This layer is responsible for providing datagram services to the Application layer. This layer

allows the host and the destination devices to communicate with each other for exchanging

messages, irrespective of the underlying network type. Error control, congestion control, flow

control, etc., are handled by the transport layer. The protocol that this layer uses is TCP

(Transmission Control Protocol) and UDP (User Datagram Protocol). TCP gives a reliable, end-

to-end, connection-oriented data transfer, while UDP provides unreliable, connectionless data

transfer between two computers.

Application Layer

It provides the user interface for communication. This is the layer where email, web browsers or

FTP run. The protocols in this layer are FTP, SMTP, HTTP, etc.

OSI model

The Open Systems Interconnected (OSI) model divides the network into seven layers and

explains the routing of the data from source to destination. It is a theoretical model which

explains the working of the networks. It was developed by the International Organization for

Standardization (ISO) for their own network suite. Here are the details of OSI's seven layers:

Physical Layer

As the name suggests, this is the layer where the physical connection between two computers

takes place. The data is transmitted via this physical medium to the destination's physical layer.

The popular protocols at this layer are Fast Ethernet, ATM, RS232, etc.

Data Link Layer

The main function of this layer is to convert the data packets received from the upper layer into

frames, and route the same to the physical layer. Error detection and correction is done at this

layer, thus making it a reliable layer in the model. It establishes a logical link between the nodes

and transmit frames sequentially.

Network Layer


The main function of this layer is to translate the network address into physical MAC address.

The data has to be routed to its intended destination on the network.

This layer is also responsible to determine the efficient route for transmitting the data to its

destination. While doing so, it has to manage problems like network congestion, switching

problems, etc. The protocols used here are IP, ICMP, IGMP, IPX, etc.

Transport Layer

This layer provides end-to-end delivery of data between two nodes. It divides data into different

packets before transmitting it.

On receipt of these packets, the data is reassembled and forwarded to the next layer. If the data

is lost in transmission or has errors, then this layer recovers the lost data and transmits the

same.

Session Layer

This layer is responsible to establish and terminate connections between two communicating

machines.

This connection is known as a session, hence the name. It establishes full-duplex, half-duplex

and simplex connection for communication. The sessions are also used to keep a track of the

connections to the web server.

Presentation Layer

The data conversion takes place at this layer. The data that it receives from the application layer

is converted into a suitable format that is recognized by the computer.

For example, the conversion of a file from .wav to .mp3 takes place at this layer.

Application Layer

This layer provides a user interface by interacting with the running application. E-mail, FTP, web

browsers, etc., are the network applications that run on this layer.

The entire communication industry stands on the backbone of TCP/IP and OSI reference model.

It is absolutely vital to learn the above differences, if anyone wants to be an expert in the field of

communication.


2. Explain in detail about network dependent and network independent layers of OSI

reference model. (AUC MAY’13,NOV’12,nov’11)

ISO / OSI MODEL:

ISO refers International Standards Organization was established in 1947, it is a multinational

body dedicated to worldwide agreement on international standards.

OSI refers to Open System Interconnection that covers all aspects of network communication. It

is a standard of ISO.

Here open system is a model that allows any two different systems to communicate regardless

of their underlying architecture. Mainly, it is not a protocol it is just a model.

OSI MODEL

The open system interconnection model is a layered framework. It has seven separate but

interrelated layers. Each layer having unique responsibilities.

ARCHITECTURE

The architecture of OSI model is a layered architecture. The seven layers are,

1. Physical layer

2. Datalink layer

ORGANIZATION OF LAYERS


The seven layers are arranged by three sub groups.

1. Network Support Layers

2. User Support Layers

3. Intermediate Layer

Network Support Layers:

Physical, Datalink and Network layers come under the group. They deal with

the physical aspects of the data such as electrical specifications, physical

connections, physical addressing, and transport timing and reliability.

User Support Layers:

Session, Presentation and Application layers comes under the

group. They deal with the interoperability between the software systems. Intermediate

Layer

The transport layer is the intermediate layer between the network support

and the user support layers.

FUNCTIONS OF THE LAYERS PHYSICAL LAYER

The physical layer coordinates the functions required to transmit a bit

stream over a physical medium. It deals with the mechanical and electrical

specifications of the interface and the transmission medium.

The functions are,


1. Physical Characteristics Of Interfaces and Media:

It defines the electrical and mechanical characteristics of the

interface and the media.

It defines the types of transmission medium

2. Representation of Bits

To transmit the stream of bits they must be encoded into signal.

It defines the type of encoding weather electrical or optical.

3. Data Rate

It defines the transmission rate i.e. the number of bits sent per

second.

4. Synchronization of Bits

The sender and receiver must be synchronized at bit level.

5. Line Configuration

It defines the type of connection between the devices.

Two types of connection are,

1. point to point

2. multipoint

6. Physical Topology

It defines how devices are connected to make a network.

Five topologies are,

1. mesh

2. star

3. tree

4. bus


5. ring

7. Transmission Mode

It defines the direction of transmission between devices.

Three types of transmission are,

1. simplex

2. half duplex

3. full duplex

DATALINK LAYER

Datalink layer responsible for node-to-node delivery.

The responsibilities of Datalink layer are,

1. Framing

It divides the stream of bits received from network layer into manageable data

units called frames.

2. Physical Addressing

It adds a header that defines the physical address of the sender

and the receiver.


If the sender and the receiver are in different networks, then the

receiver address is the address of the device which connects the

two networks.

3. Flow Control

It imposes a flow control mechanism used to ensure the data

rate at the sender and the receiver should be same.

4. Error Control

To improve the reliability the Datalink layer adds a trailer

which contains the error control mechanism like CRC,

Checksum etc.

5. Access Control

When two or more devices connected at the same link, then the

Datalink layer used to determine which device has control

over the link at any given time.

NETWORK LAYER

When the sender is in one network and the receiver is in some

other network then the network layer has the responsibility for the source to

destination delivery.

The responsibilities are,

1. Logical Addressing


If a packet passes the network boundary that is when the sender and

receiver are places in different network then the network layer adds a

header that defines the logical address of the devices.

2. Routing

When more than one networks connected and to form an

internetwork, the connecting devices route the packet to its final

destination.

Network layer provides this mechanism.

TRANSPORT LAYER

The network layer is responsible for the end to end delivery of the

entire message. It ensures that the whole message arrives in order and intact. It

ensures the error control and flow control at source to destination level.


1. Service point Addressing

A single computer can often run several programs at the same time.

The transport layer gets the entire message to the correct process on


that computer.

It adds a header that defines the port address which used to identify the

exact process on the receiver.

2. Segmentation and Reassembly

A message is divided into manageable units called as segments.

Each segment is reassembled after received that information at the

receiver end.

To make this efficient each segment contains a sequence number.

3. Connection Control

The transport layer creates a connection between the two end ports.

It involves three steps. They are,

1. connection establishment

2. data transmission

3. connection discard

4. Flow Control

Flow control is performed at end to end level

5. Error Control

Error control is performed at end to end level.

SESSION LAYER

It acts as a dialog controller. It establishes, maintains and synchronizes the

interaction between the communication devices.



1. Dialog Control

The session layer allows two systems to enter into a dialog.

It allows the communication between the devices.

2. Synchronization

It adds a synchronization points into a stream of bits.

PRESENTATION LAYER

The presentation layer is responsible for the semantics and the syntax of the

information exchanged.


1. Translation


Different systems use different encoding systems.

The presentation layer is responsible for interoperability between

different systems.

The presentation layer t the sender side translates the information from the

sender dependent format to a common format. Likewise, at the receiver

side presentation layer translate the information from common format to

receiver dependent format.

2. Encryption

To ensure security encryption/decryption is used

Encryption means transforms the original information to another form

Decryption means retrieve the original information from the

encrypted data

3. Compression

It used to reduce the number of bits to be transmitted.

APPLICATION LAYER

The application layer enables the user to access the network. It provides

interfaces between the users to the network.


1. Network Virtual Terminal

It is a software version of a physical terminal and allows a user to log on to a

remote host.

2. File Transfer, Access, and Management


It allows a user to access files in a remote computer, retrieve files, and

manage or control files in a remote computer.

3. Mail Services

It provides the basis for e-mail forwarding and storage.

4. Directory Services

It provides distributed database sources and access for global information about

various objects and services

3. Explain the different types of switching networks and mention it’s advantages and

disadvantages. (AUC NOV ’12)

SWITCHING

Whenever we have multiple devices, we have the problem of how to connect them

to make one to one communication possible. Point to point connection between every pair

of devices are make the network very huge also cost inefficient.

A switched network consists of a series of interlinked nodes, called switches.

Switches are hardware and / or software devices capable of creating temporary

connections between two or more devices linked to the switch but not to each other.

There are three methods in switching are,

1. circuit switching

2. packet switching


3. message switching

CIRCUIT SWITCHING

Circuit switching creates a direct physical connection between two devices such as

phones or computers. A circuit switch is a device with n input and m output that creates a

temporary connection between an input link and an output link. The number of inputs does

not have to match the number of outputs. An n by n folded switch can connect n lines in

full duplex mode.

Circuit switching today can use either of two technologies:

1. space division switches

2. time division switches

SPACE DIVISION SWITCHES

In space division switches, the paths in the circuit are separated from each other

spatially. It is very useful in analog networks.

Crossbar switches:

A crossbar switch connects n inputs to m outputs in a grid, using electronic micro

switches at each cross point. The major limitation is number of cross points required. To

connect n inputs to m outputs it needs n * m cross points.


Multistage switches:

It combines crossbar switches in several stages. The design of multistage switches

are depends on the number of stages and the number of switches required in each stage.

Normally the middle stage has fewer switches than the first and last stages. They

provide several options for connecting each pair of linked devices is known as

multipath.

TIME DIVISION SWITCHES

Time division switches uses time division multiplexing to achieve switching. There

are two methods are,

1. time slot interchange

2. TDM buds

Time slot interchange:

A time slot interchange is used to order the slots based on the desired connections. It

consists of random access memory with several memory locations. The RAM fills up the

incoming data from time slots in the order received. Slots are then sent out in an order based


on the decisions of a control unit.

TDM bus:

The input and output lines are connected to a high speed bus through the input

and output gates. Each input gate is closed during one of the four time slots. During the

same time slot, only one output gate is also closed. This pair of gates allows a burst of

data to be transmitted from one specific input line to one specific output line using the bus.

The control unit opens and closes the gates according to switching need.

PUBLIC SWITCHED TELEPHONE NETWORK:

An example of a circuit switched network is the public switched telephone

network. The switching centers are organized as five classes are,

Regional office

Sectional office

Primary office

Toll office

End office

DISADVANTAGES:

1. Circuit switching is less suited to data and other non voice communications.

2. less data rate

3. inflexible


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4. No priorities allowed.

PACKET SWITCHING

In packet switched network, data are transmitted in discrete units of potentially

variable length blocks called packets. The maximum length of the packet is established by

the network. Longer transmission is broken up in to multiple packets.

There are two popular methods are,

1. datagram approach

2. virtual circuit approach

DATAGRAM APPROACH:

In the datagram approach to packet switching, each packet is treated

independently from all there. Even when one packet represents just a piece of a

multipacket transmission, the networks treats it as though it existed alone. Packets in this

technology are referred to datagram.

The datagram approach can be used to deliver four packets from station A to

station X. In this example, all four packets belong to same message but may go by

different paths to reach their destination.

This approach can cause the datagram of a transmission to arrive at their

destination out of order .It is responsibility of transport layer in most protocols to reorder the

data grams before passing them on to the destination port.

The link joining each pair of nodes can contain multiple channels. Each of these

channels is capable, in turn, of carrying data grams either from several different sources or


from one source. Multiplexing can be done using TDM or FDM.

Devices A and B are sending data grams to devices X and Y. Some paths use one

channel while other uses more than one. the bottom link is carrying two packets from

different sources in the same direction. The link on the right, however, is carrying

data grams in two directions.

VIRTUAL CIRCIUT APPROACH:

In the virtual circuit approach to packet switching, the relationship

between all packets belonging to a message or session is preserved. A single route is

chosen between sender and receiver at the beginning of the session. When the data are

sent, all packets of the transmission travel one after another along that route.

Today, virtual circuit transmission is implemented in two formats: switched

virtual circuit (SVC) & permanent virtual circuit (PVC).

SVC:

The switched virtual circuit (SVC) format is comparable conceptually to dial-up lines

in circuit switching. In this method, a virtual circuit is created whenever it is needed and exists

only for the duration of the specific exchange.

PVC:

Permanent virtual circuits (PVC) are comparable to leased lines in circuit

switching. In this method, the same virtual circuit is provided between two users on a

continuous basis. The circuit is dedicated to the specific users. No one else can use it &,

because it is always in place, it can be used without connection establishment &

connection termination. Whereas two SVC users may get a different route every time they

request a connection, two PVC users always get the same route.

CIRCUIT SWITCHED CONNECTION VS VIRTUAL CIRCUIT CONNECTION:

Although it seems that a circuit-switched connection & a virtual-circuit

connection are the same, there are differences:

Paths versus route:

A circuit-switched connection creates a path between points. The physical

path is created by setting the switches for the duration of the dial(dial-up line) or the

duration of the lease(lease line).A virtual circuit connection creates a route between two

points. This means each switch creates an entry in its routing table for the duration of

thesession (SVC) or duration of the lease (PVC).

Whenever, the switch receives a packet belonging to a virtual connection, it checks the

table for the corresponding entry & routes the packet out of one of its interfaces.


Dedicated versus sharing:

In a circuit-switched connection, the links that make a path are dedicated;

they cannot be used by other connections. In a virtual circuit connection, the links that

make a route can be shared by other connections.

MESSAGE SWITCHING:

Message switching is best known by the descriptive term store and forward. In this

mechanism, a node receives a message, stores it until the appropriate route is free, then

sends it along.

Store & forward is considered a switching technique because there is no direct link

between the sender and receiver of a transmission. A message is delivered to the node

along one path then rerouted along another to its destination.

message switching, the messages are stored relayed from secondary storage

(disk), while in packet switching the packets are stored and forwarded from primary storage

(RAM).

Message switching was common in the 1960s & 1970s.The primary use has been to

provide high-level network services for unintelligent devices. Since such devices have been

replaced, this type of switch has virtually disappeared. Also, the delays inherent in the

process, as well as requirements for large-capacity storage media at each node, make it

popular for direct communication.

4. Describe the circuit switched networks, data gram networks and cable networks with

suitable diagrams(AUC NOV’11)(REFER Q.No.3)

5.explain in detail about addressing?

ADDRESSING

Four levels of addresses are used in an internet employing the TCP/IP protocols:physical (link)

addresses, logical (IP) addresses, port addresses, and specific

addresses

Each address is related to a specific layer in the TCPIIP architecture, as shown in

Figure


Physical Addresses

The physical address, also known as the link address, is the address of a node as defined by its

LAN or WAN. It is included in the frame used by the data link layer. It is the lowest-level

address.

The physical addresses have authority over the network (LAN or WAN). The size and format of

these addresses vary depending on the network. For example, Ethernet uses a 6-byte (48-bit)

physical address that is imprinted on the network interface card(NIC)

Port Addresses


The IP address and the physical address are necessary for a quantity of data to travel from a

source to the destination host. However, arrival at the destination host is not thefinal objective of

data communications on the Internet. A system that sends nothing but data from one computer

to another is not complete. computers are devices that can run multiple processes at the same

time. The end objective of Internet communication is a process communicating with another

process

Logical Addresses

Logical addresses are necessary for universal communications that are independent of

underlying physical networks. Physical addresses are not adequate in an internetwork

environment where different networks can have different address formats. A universal

addressing system is needed in which each host can be identified uniquely, regardless

of the underlying physical network.The logical addresses are designed for this purpose. A

logical address in the Internetis currently a 32-bit address that can uniquely define a host

connected to the Internet.

6. . Explain guided media differ from unguided media? Explain the three types of guided

media and two types of unguided media

Twisted pair cable

A twisted pair consists of two conductors (normally copper), each with its own plastic insulation,

twisted together, One of the wires is used to carry signals to the receiver, and the other is used

only as a ground reference.

The receiver uses the difference between the two. In addition to the signal sent by the sender

on one of the wires, interference (noise) and crosstalk may affect both wires and create

unwanted signals. If the two wires are parallel, the effect of these unwanted signals is not the

same in both wires because they are at different locations relative to the noise or crosstalk

sources

Unshielded Versus Shielded Twisted-Pair Cable

The most common twisted-pair cable used in communications is referred to as unshielded twisted-

pair (UTP). IBM has also produced a version of twisted-pair cable for its use called shielded twisted-

pair (STP). STP cable has a metal foil or braided mesh covering that encases each pair of insulated


conductors. Although metal casing improves the quality of cable by preventing the penetration of

noise or crosstalk, it is bulkier and more expensive. Metal shield

Plastic cover

Coaxial cable

coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an

insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a

combination of the two. The outer metallic wrapping serves both as a shield against noise and

as the second conductor, which completes the circuit. This outer conductor is also enclosed in

an insulating sheath, and the whole cable is protected by a plastic cover

Fiber optic cable

A fiber-optic cable is made of glass or plastic and transmits signals in the form of light.

Light travels in a straight line as long as it is moving through a single uniform substance.

If a ray of light traveling through one substance suddenly enters another substance (of a d

different density), the ray changes direction

Optical fibers use reflection to guide light through a channel. A glass or plastic core is

surrounded by a cladding of less dense glass or plastic. The difference in density of the two

materials must be such that a beam of light moving through the core is reflected off the cladding

instead of being refracted into it.


The subscriber channel (SC) connector is used for cable TV. It uses a push/pull locking

system. The straight-tip (ST) connector is used for connecting cable to networking devices. It

uses a bayonet locking system and is more reliable than SC. MT-RJ is a connector that is the

same size as RJ45.

Performance

The plot of attenuation versus wavelength in phenomenon in fiber-optic cable. Attenuation is

flatter than in the case of twisted-pair cable and coaxial cable. The performance is such that we

need fewer (actually 10 times less) repeaters when we use fiber-optic cable.

Applications

Fiber-optic cable is often found in backbone networks because its wide bandwidth is cost-

effective.

Radio waves are used for multicast communications, such as radio and television, and paging

systems.

Microwaves are used for unicast communication such as cellular telephones, satellite networks,

and wireless LANs.

Infrared signals can be used for short-range communication in a closed area using line-of-sight

propagation.

7.Explain in detail about Dial –up modems.

Traditional telephone lines can carry frequencies between 300 and 3300 Hz, givingthem a

bandwidth of 3000 Hz. All this range is used for transmitting voice, where agreat deal of

interference and distortion can be accepted without loss of intelligibility. The effective bandwidth

of a telephone line being used for data transmissionis 2400 Hz, covering the range from 600 to

3000 Hz. Note that today some telephone lines are capable of handling greater bandwidth than

traditional lines. modem design is still based on traditional capability.

Telephone line bandwidth


The term 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. A demodulator recovers the binary data from the modulated signal

Modulation/demodulation

Modem Standards

V.32 and V.32bis

The V.32 modem uses a combined modulation and encoding technique called trellis coded

modulation. Trellis is essentially QAM plus a redundant bit. The data stream is divided into 4-bit

sections. Instead of a quadbit (4-bit pattern), however, a pentabit (5-bit pattern) is transmitted.

The value of the extra bit is calculated from the values of the data bits. The extra bit is used for

error detection.

The Y.32 calls for 32-QAM with a baud rate of 2400. Because only 4 bits of eachpentabit

represent data, the resulting data rate is 4 x 2400 = 9600 bps. The constellation diagram and

bandwidth are shown in figure

The V.32bis modem was the first of the ITU-T standards to support 14,400-bpstransmission.

The Y.32bis uses 128-QAM transmission (7 bits/baud with I bit for errorcontrol) at a rate of 2400

baud (2400 x 6 = 14,400 bps).

The V.32 and V.32bis constellation and bandwidth


V:90

V.90 modems with a bit rate of 56,000 bpsare available; these are called 56K modems. These

modems may be used only if oneparty is using digital signaling (such as through an Internet

provider). They are asymmetricin that the downloading rate (flow of data from the Internet

service provider tothe PC) is a maximum of 56 kbps, while the uploading rate (flow of data from

the PC to the Internet provider) can be a maximum of 33.6 kbps.

V:92

The standard above V90 is called ~92. These modems can adjust their speed, and ifthe noise

allows, they can upload data at the rate of 48 kbps. The downloading rate is still 56 kbps. The

modem has additional features. For example, the modem can interruptthe Internet connection

when there is an incoming call if the line has call-waiting service.

8. Describe in detail about DSL.

DIGITAL SUBSCRUBER LINE

DSL, to provide higher-speed access to the Internet. Digital subscriber line (DSL) technology is

one of the most promising for supporting high-speed digital communication over the existing

local loops. DSL technology is a set of technologies, each differing in the first letter (ADSL,

VDSL, HDSL, and SDSL). The set is often referred to as xDSL, where x can be replaced by A,

V, H, or S.

ADSL


ADSL is an asymmetric communication technology designed for residential users;it is not

suitable for businesses. ADSL, like a 56K modem, provides higher speed (bit rate) in the

downstream direction (from the Internet to the resident) than in the upstream direction (from the

resident to the Internet). That is the reason it is called asymmetric.

Using Existing Local Loops: The existing local loops can handle bandwidths up to 1.1 MHz.

Adaptive Technology: ADSL is an adaptive technology. The system uses a data rate based on

the condition of the local loop line.

Discrete Multitone Technique

The modulation technique that has become standard for ADSL is called the discrete multitone

technique (DMT) which combines QAM and FDM. Typically, an available bandwidth of 1.104

MHz is divided into 256 channels. Each channel uses a bandwidth of 4.312 kHz, as shown in

Figure

Voice. Channel 0 is reserved for voice communication.

Idle. Channels 1 to 5 are not used and provide a gap between voice and data

communication.

Bandwidth division in ADSL

Upstream data and control. Channels 6 to 30 (25 channels) are used for upstream

data transfer and control. One channel is for control, and 24 channels are for data


transfer. If there are 24 channels, each using 4 kHz (out of 4.312 kHz available) with

QAM modulation

Downstream data and control. Channels 31 to 255 (225 channels) are used for

downstream data transfer and control. One channel is for control, and 224 channels are

for data.

ADSL Lite The installation of splitters at the border of the premises and the new wiring for the dataline can

be expensive and impractical enough to dissuade most subscribers. A new version of ADSL

technology called ADSL Lite (or Universal ADSL or splitter less ADSL) is available for these

subscribers. This technology allows an ASDL Lite modem to be plugged directly into a

telephone jack and connected to the computer.

HDSL

The high-bit-rate digital subscriber line (HDSL) was designed as an alternative tothe T-lline

(1.544 Mbps). The T-1line uses alternate mark inversion (AMI) encoding,which is very

susceptible to attenuation at high frequencies. This limits the length of a T-l line to 3200 ft (1

km).

SDSL The symmetric digital subscriber line (SDSL) is a one twisted-pair version of HDSL.It provides

full-duplex symmetric communication supporting up to 768 kbps in each direction. SDSL, which

provides symmetric communication.

VDSL

The very high-bit-rate digital subscriber line (VDSL), an alternative approach that is similar to

ADSL, uses coaxial, fiber-optic, or twisted-pair cable for short distances. The modulating

technique is DMT. It provides a range of bit rates (25 to 55 Mbps) for upstream communication

at distances of 3000 to 10,000 ft. The downstream rate is normally 3.2 Mbps.

9. write short notes on cable TV networks.

The cable TV network started as a video service provider, but it has moved to the business of

Internet access.

Traditional Cable Networks

Cable TV started to distribute broadcast video signals to locations with poor or no reception in

the late 1940s. It was called community antenna TV (CATV) because an antenna at the top of a


tall hill or building received the signals from the TV stations and distributed them, via coaxial

cables, to the community

The cable TV office, called the head end, receives video signals from broadcasting stations and

feeds the signals into coaxial cables. The signals became weaker and weaker with distance, so

amplifiers were installed through the network to renew the signals.

Hybrid Fiber-Coaxial (HFC) Network

The second generation of cable networks is called a hybrid fiber-coaxial (HFC) network. The

network uses a combination of fiber-optic and coaxial cable. The transmission medium from the

cable TV office to a box, called the fiber node, is optical fiber; from the fiber node through the

neighborhood and into the house is still coaxial cable. Figure shows a schematic diagram of an

HFC network.

Modulation and distributionof signals are done here; the signals are then fed to the fiber nodes

through fiber-optic cables. The fiber node splits the analog signals so that the same signal is

sent to each coaxial cable. Each coaxial cable serves up to 1000 subscribers.

The use offiber-optic cable reduces the need for amplifiers down to eight or less. One reason for

moving from traditional to hybrid infrastructure is to make the cable network bidirectional (two-

way).

10.Explain in detail about cable TV for data transfer


Cable companies are now competing with telephone companies for the residential customer

who wants high-speed data transferDSL uses the existing unshielded twisted-pair cable, which

is very susceptible to interference. This imposes an upper limit on the data rate.

Bandwidth

This coaxial cable has a bandwidth that rangesfrom 5 to 750 MHz (approximately). To provide

Internet access, the cable company has divided this bandwidth into three bands: video,

downstream data, and upstream data,

Division of coaxial cable band by CATV

Downstream Video Band

The downstream video band occupies frequencies from 54 to 550 MHz. Since each TV channel

occupies 6 MHz, this can accommodate more than 80 channels.

Downstream Data Band

The downstream data (from the Internet to the subscriber premises) occupies the upper band,

from 550 to 750 MHz. This band is also divided into 6-MHz channels.

Modulation:Downstream data band uses the 64-QAM (or possibly 256-QAM)modulation

technique.

Data Rate :There is 6 bits/baud in 64-QAM. One bit is used for forward error correction; this

leaves 5 bits of data per baud.

Upstream Data Band

The upstream data (from the subscriber premises to the Internet) occupies the lower band, from

5 to 42 MHz. This band is also divided into 6-MHz channels.

Modulation The upstream data band uses lower frequencies that are more susceptible to noise

and interference.

Data Rate There are 2 bitslbaud in QPSK. The standard specifies 1 Hz for each baud; this

means that, theoretically, upstream data can be sent at 12 Mbps (2 bitslHz x 6 MHz).However,

the data rate is usually less than 12 Mbps.

Sharing

Both upstream and downstream bands are shared by the subscribers.

Upstream Sharing

The upstream data bandwidth is 37 MHz. This means that there are only six 6-MHzchannels

available in the upstream direction. The cable provider allocates one channel, statically or

dynamically, for a group of subscribers. If one subscriber wants to send data, contends for the

channel with others who want access; the subscriber must wait until the channel is available.

Downstream Sharing

The downstream band has 33 channels of 6 MHz. A cable provider probably has more than 33

subscribers; therefore,each channel must be shared between a group of subscribers. If there

are data for any of the subscribers in the group, the data are sent to that channel. Each


subscriber is sent the data. But since each subscriber also has an address registered with the

provider; the cable modem for the group matches the address carried with the data to the

address assigned by the provider. If the address matches, the data are kept; otherwise, they are

discarded.

CMandCMTS

To use a cable network for data transmission, we need two key devices: a cable modem(CM)

and a cable modem transmission system (CMTS).

CM

The cable modem (CM) is installed on the subscriber premises. It is similar to an ADSLModem

CMTS

The cable modem transmission system (CMTS) is installed inside the distribution

hub by the cable company. It receives data from the Internet and passes them to the

combiner, which sends them to the subscriber. The CMTS also receives data from the

subscriber and passes them to the Internet.

Cable modem (CM)

Cable modem transmission system (CMTS)

Data Transmission Schemes: DOeSIS

Multimedia

Cable Network Systems (MCNS), called Data Over Cable System Interface Specification

(DOCSIS). DOCSIS defines all the protocols necessary to transport data from a CMTS to aCM.

Upstream Communication

The following is a very simplified version of the protocol defined by DOCSIS for upstream

communication. It describes the steps that must be followed by a CM:


1. The CM checks the downstream channels for a specific packet periodically sent by the

CMTS. The packet asks any new CM to announce itself on a specific upstream channel.

2. The CMTS sends a packet to the CM, defining its allocated downstream and upstream

channels.

3. The CM then starts a process, called ranging, which determines the distance between the

CM and CMTS. This process is required for synchronization between all CMs and CMTSs for

the minislots used for timesharing of the upstream channels.

4. The CM sends a packet to the ISP, asking for the Internet address.

5. The CM and CMTS then exchange some packets to establish security parameters, which are

needed for a public network such as cable TV.

6. The CM sends its unique identifier to the CMTS.

7. Upstream communication can start in the allocated upstream channel; the CM can contend

for the minislots to send data.

Downstream Communication

In the downstream direction, the communication is much simpler. There is no contention

because there is only one sender. The CMTS sends the packet with the address of the

receiving eM, using the allocated downstream channel.

Documents

Sub Code:EC2352 Sub Name: Computer Networks Dept: ECE Sem ... 1.pdf · 8. What is the difference between a passive and an active hub? An active hub contains a repeater that regenerates