Introduction to Networks, Reference Models

8/17/2019 Introduction to Networks, Reference Models

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Computer Networks Introduction, Reference Models

VR-10, CS6004 The Physical Layer

Mukesh Chinta

Asst Prof, CSE, VRSEC

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UNIT I

Introduction : Uses of Computer Networks, Network Hardware, LANs, MANs, WANs, Network

Software.

Reference Models: The OSI Reference Model, TCP/IP Reference Model, the comparison of OSI,

and TCP/IP reference models.

The Physical Layer : Guided transmission media: Magnetic Media, Twisted Pair, CoaxialCable, and Fiber Optics.

A computer network is an interconnected collection of autonomous computers able to

exchange information. A computer network usually require users to explicitly login onto one

machine, explicitly submit jobs remotely,explicitly move files/data around the network.

In a Distributed system, the existence of multiple autonomous computers in a computer

network is transparent to the user. The operating system automatically allocates jobs to

processors, moves files among various computers without explicit user intervention.

Def: “ A network is simply a collection of computers or other hardware devices that are

connected together, either physically or logically, using special hardware and software, to allow

them to exchange information and cooperate. Networking is the term that describes the

processes involved in designing, implementing, upgrading, managing and otherwise working

with networks and network technologies.”

Networking has become an indispensable part of modern society in every aspect of life.


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Some of the important properties of the networks are:

Scope: A network should provide services to several applications

Scalability: A network should operate efficiently when deployed on a small-scale as well

as on a large-scale

Robustness: A network should operate in spite of failures or lost data

Self-Stabilization: A network, after a failure or other problem, should return to normal

(or near normal) without human intervention

Autoconfigurability: A network should optimize its own parameters in order to achieve

better performance

Safety: A network should prevent failures as well as prevent failures from affecting

other areas of the network

Configurability: A network’s parameters should be configurable to improve

performance Determinism: Two networks with identical conditions should yield identical results

Migration: It should be possible to add new features to a network without disruption of

network service

Applications of Computer Networks

Business network applications

Resource sharing: Data, programs, equipment are available to users regardless of their

physical location.

High reliability: Files and databases could be duplicated on multiple machines. Multiple

CPUs prevent total system loss.

Economically sound: Networked micro computers using the client-server model offer better

price/performance ratio than mainframes.

Communication Medium: Networks provide powerful, effiencient and fast communication

among the employees via electronic mail (email), IP telephony or VOIP, Desktop sharing etc.

Home (personal) network applications

Access to remote information: Financial information, database access, the Web,newsgroups, Wikipedia etc.

Person to person communication: Email, voice, videoconferencing, instant messaging, social

networking etc.

Interactive entertainment: Video on demand, interactive TV (IPTV), networked games.


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Electronic commerce (e-commerce): Many forms such as home shopping, finance and bills

management are getting common. Some common forms of e-commerce are given below:

Mobile Users

Connectivity to the internet: Smart phones and other mobile devices are dependent on

internet for providing advacned services like GPS etc to the users. Wireless Hotspots are

found everywhere now-a-days which enable people to connect to the internet on their

devices

M-commerce: Short text messages from the mobile are used to authorize payments for

food in vending machines, movie tickets, and other small items instead of cash and credit

cards.

NFC (Near Field Communication): Mobile can act as an RFID smartcard and interact with a

nearby reader for payment.

Network Hardware

Computer Networks can be categorized based on many dimensions, but two dimensions

standout namely as transmission technology and scale.

Data communications networks can be generally categorized as either point-to-point or

multipoint. A point-to-point configuration involves only two locations or stations, whereas a

multipoint configuration involves three or more stations.


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A two-point circuit involves the transfer of digital information between a mainframe computer

and a personal computer, two mainframe computers or two data communications networks. A

multi-point network is generally used to interconnect a single mainframe computer (host) to

many personal computers or to interconnect many personal computers and capacity of the

channel is either Spatially shared : Devices can use the link simultaneously or Timeshared : Users

take turns

Transmission Modes

There are four modes of transmission for data communications circuits:

In simplex mode(SX), the communication is unidirectional, as on a one-way street. Only

one of the two devices on a link can transmit; the other can only receive. Commercial radio

broadcasting is an example. Simplex lines are also called receive-only, transmit-only or one-

way-only lines.

In half-duplex(HDX) mode, 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. The half-

duplex mode is used in cases where there is no need for communication in both directions at

the same time; the entire capacity of the channel can be utilized for each direction. Citizens

band (CB) radio is an example where push to talk (PTT) is to be pressed or depressed while

sending and transmitting.

In full-duplex mode(FDX) (called duplex), both stations can transmit and receive

simultaneously. One common example of full-duplex communication is the telephone network.

The full-duplex mode is used when communication in both directions is required all the time.

The capacity of the channel must be divided between the two directions.


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Based on the mode of transmission, computer networks are divided into point-to-point

networks or broadcast networks.

Broadcast networks use a single communication channel shared by all computers in the

network. Short messages (packets) are sent by any machine and received by all other

computers on the network. An address field within the packet specifies the intended recipient.

Others receiving this packet simply ignore it. Broadcast systems generally also allow the

possibility of addressing a packet to all destinations by using a special code in the address field.

Most localized networks are broadcast networks. A variant of broadcasting called multicasting

in which transmission is done to a subset of machines. One possible scheme is to reserve one

bit to indicate multicasting. The remaining n - 1 address bits can hold a group number. Each

machine can ''subscribe'' to any or all of the groups. When a packet is sent to a certain group, it

is delivered to all machines subscribing to that group.

In contrast, point-to-point networks consist of many connections between individual

pairs of machines. To go from the source to the destination, a packet on this type of network

may have to first visit one or more intermediate machines. Point-to-point transmission with

one sender and one receiver is sometimes called unicasting. Generally, smaller, geographically

localized networks tend to use broadcasting, whereas larger networks usually are point-to-

point.

Networks are also classified according to their scale. Distance is an important metric for

classification because different techniques are used at different scales. Three main categories

of networks are LAN{Local Area Networks}, MAN{Metropolitan Area Networks} and

WAN{Wide Area Networks}.

Other categories of networks are SAN{storage area network or sever area network},

PAN{Personal Area Network}, CAN{Cluster Area Networks or Campus area networks} etc


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LAN {Local Area Networks}

A local area network (LAN) is a network that connects computers and devices in a limited

geographical area such as home, school, computer laboratory, office building, or closely

positioned group of buildings. LANs use a network operating system to provide two-way

communications at bit rates in the range of 10 Mbps to 100 Mbps. In addition to operating in a

limited space, LANs are also typically owned, controlled, and managed by a single person or

organization. They also tend to use certain connectivity technologies, primarily Ethernet and

Token Ring.

LANs are distinguished from other kinds of networks by three characteristics: (1) their size, (2)

their transmission technology, and (3) their topology.

LANs are restricted in size, which means that the worst-case transmission time is bounded and

known in advance, which simplifies network management. LAN’s often use a transmission

technology consisting of a single cable to which all the machines are attached. Traditional LAN’s

run at speeds of 10 to 100 Mbps, have very low delay and make few transmission errors. {A

megabit is 1,000,000 bits and Mbps means Megabits per second. Megabytes mean 1,048,076

bits}.

Network topology refers to the way a network is laid out either physically or logically. A

topology describes the configuration of a network and influences the networks cost and

performance. Various topologies are possible for broadcast LAN’s.

Five basic topologies are bus, ring, star, tree and mesh.


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Star topology: A star topology is designed with each node (file server, workstations, and

peripherals) connected directly to a central network hub, switch, or concentrator. Data on a

star network passes through the hub, switch, or concentrator before continuing to its

destination. The hub, switch, or concentrator manages and controls all functions of the

network. It also acts as a repeater for the data flow.

Bus topology Bus networks use a common backbone to connect all devices. A single cable,

(the backbone) functions as a shared communication medium that devices attach or tap into

with an interface connector. A device wanting to communicate with another device on the

network sends a broadcast message onto the wire that all other devices see, but only the

intended recipient actually accepts and processes the message. The bus topology is the

simplest and most common method of interconnecting computers. The two ends of the

transmission line never touch to form a complete loop. A bus topology is also known as

multidrop or linear bus or a horizontal bus.

Ring topology

In a ring network (sometimes called a loop), every device has exactly two

neighbours for communication purposes. All messages travel through a ring in the same

direction (either "clockwise" or "counter clockwise"). All the stations are interconnected in

tandem (series) to form a closed loop or circle. Transmissions are unidirectional and must

propagate through all the stations in the loop. Each computer acts like a repeater and the ring

topology is similar to bus or star topologies.


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Mesh topology The mesh topology incorporates a unique network design in which each

computer on the network connects to every other, creating a point-to-point connection

between every device on the network. Unlike each of the previous topologies, messages sent

on a mesh network can take any of several possible paths from source to destination. A mesh

network in which every device connects to every other is called a full mesh. A disadvantage is

that, a mesh network with n nodes must have n(n-1)/2 links and each node must have n-1 I/Oports (links).

Hybrid topology This topology (sometimes called mixed topology) is simply combining two or

more of the traditional topologies to form a larger, more complex topology. Main aim is being

able to share the advantages of different topologies.


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MAN {Metropolitan Area Network}

A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of

buildings to entire cities. Its geographic scope falls between a WAN and LAN. A MAN might be a

single network like the cable television network or it usually interconnects a number of local

area networks (LANs) using a high-capacity backbone technology, such as fiber-optical links,

and provides up-link services to wide area networks and the Internet.

A metropolitan area network based on cable TV

MANs typically operate at speeds of 1.5 Mbps to 10 Mbps and range from five miles to a few

hundred miles in length. A MAN (like a WAN) is not generally owned by a single organization.

The MAN, its communications links and equipment are generally owned by either a consortium

of users or by a single network provider who sells the service to the users. DQDB {DistributedQueue Dual Bus}, is the metropolitan area network standard for data communication. It is

specified in the IEEE 802.6 standard. Using DQDB, networks can be up to 20 miles (30 km) long

and operate at speeds of 34 to 155 Mbit/s.

It consits of two unidirectional buses (cables) to which all the computers are connected. Each

bus has a head-end, which initiates transmission activity. Traffic destined for a computer to the

right of the sender uses the upper bus and to the left uses the lower one.


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WAN {Wide Area Networks}

Wide area networks are the oldest type of data communications network that provide

relatively slow-speed, long-distance transmission of data, voice and video information over

relatively large and widely dispersed geographical areas, such as country or entire continent.

WAN contains a collection of machines (hosts) intended for running user (i.e.,

application) programs. The hosts are connected by a communication subnet, or just subnet. The

hosts are owned by the customers, whereas the communication subnet is typically owned and

operated by a telephone company or Internet service provider. The job of the subnet is to carry

messages from host to host. In most wide area networks, the subnet consists of two distinct

components: transmission lines and switching elements. Transmission lines move bits between

machines. They can be made of copper wire, optical fiber, or even radio links. Switching

elements are specialized computers that connect three or more transmission lines. When data

arrive on an incoming line, the switching element must choose an outgoing line on which to

forward them. Router is the common name given for a switching element.

If two routers that do not share a transmission line wish to communicate, they must do this

indirectly, via other routers. When 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 entirety, stored

there until the required output line is free, and then forwarded. A subnet organized according

to this principle is called a store-and-forward or packet-switched subnet . When a process on

some host has a message to be sent to a process on some other host, the sending host first cutsthe message into packets, each one bearing its number in the sequence. These packets are then

injected into the network one at a time in quick succession. The packets are transported

individually over the network and deposited at the receiving host, where they are reassembled

into the original message and delivered to the receiving process. Routing decisions are made

locally according to a routing algorithm.


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Two types of WAN’s are VPN {Virtual Private Network} and ISP {Internet Service Provider}

network. Compared to the dedi-cated arrangement, a VPN has the usual advantage of

virtualization, which is that it provides flexible reuse of aresource (Internetconnectivity). Some

WAN’s use wireless technologies and examples are satellite systems and cellular telephone

network.

PAN {Personal Area Networks}

PANs(Personal Area Networks) let devices communicate over the range of a person. A common

example is awireless network thatconnects a computer with its peripherals. companies got

together to design a short-range wireless

network called Bluetooth to connect

these components without wires.

Bluetooth networks use the master-slave

paradigm. The system unit (the PC) is

normally the master, talking to the

mouse, keyboard, etc., as slaves.

Themaster tells the slaves what

addresses to use, when they can

broadcast, how long they can transmit,

what frequencies they can use, and so

on. Bluetooth can be used inother

settings like connecting a headset to a

mobile phone, linking up a digital music

player or mobile phone to a car stereo etc. PANs can also be built with othertechnologies that

communicate over short ranges, such as RFID on smartcards and library books.

Internetworks

People on one network want to communicate with other network’s necessiating the need for

connection of different and probably incompatible networks. A collection of inter connected

networks is called an internetwork or internet. The Internet uses ISP networksto connect

enterprise networks, home networks, and many other networks. A network is formed by the

combination of a subnet and its hosts. Gateway is the common name given to the machine that

makes a connection between two or more networks and provides the necessary translation,

both in terms of hardware and software. Gateways are distinguished by the layer at which they

operate in the protocol hierarchy


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Circuit Switched vs Packet switched networks

In ciruit switched netwroks, a dedicated circuit is established across a set of links. Example is

the telephone network. An end-to-end permanent connection is maintained. Once the

communication is complete, the connection is ended and the links are released.

Advantages: Guaranteed bandwidth, reliable communication, simple data routing, low per-

packet overhead.

Disadvantages: wasted bandwidth, blocked connections, connection set-up delay.


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In packet switched networks, data is split into blcoks called packets and each packet contains

indentification information. Packets traverrse the network individually. Destination address is

used to forward packets.

Advantages: Multiplexing, service, adaption to congestion and failures

Disadvantages: No guaranteed bandwidth, per packet overhead, complex end-to-end control,

delay and congestion.

An intranet is a private network that is contained within an enterprise. It may consist of

many interlinked local area networks and also use leased lines in the wide area network. An

intranet uses TCP/IP, HTTP, and other Internet protocols and in general looks like a private

version of the Internet. With tunneling, companies can send private messages through the

public network, using the public network with special encryption/decryption and other security

safeguards to connect.

Network Software

Protocol Hierarchies

To reduce their design complexity, most networks are organized as a stack of layers or levels,

each one built upon the one below it. The purpose of each layer is to offer certain services to

the higher layers, shielding those layers from the details of how the offered services are actually

implemented. Layer n on one machine carries on a conversation with layer n on another

machine. The rules and conventions used in this conversation are collectively known as the

layer n protocol. A protocol is an agreement between the communicating parties on how

communication is to proceed. A five layer network model is shown below.


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A communication protocol is a set of rules allowing end users (computers) and network devices

to send and receive data in an orderly and structured manner. A protocol consists of three key

rules: syntax (format of the data), semantics (interpretation of data), and timing (when to send

and when to respond, with what speed).

The entities comprising the corresponding layers on different machines are called peers.

The peers may be processes, hardware devices, or even human beings. Peers communicate

using the protocols. No data are directly transferred from layer n on one machine to layer n on

another machine. Instead, each layer passes data and control information to the layer

immediately below it, until the lowest layer is reached. Below layer 1 is the physical medium

through which actual communication occurs. Between each pair of adjacent layers is an

interface. The interface defines which primitive operations and services the lower layer makes

available to the upper one. A set of layers and protocols is called a network architecture. A list

of protocols used by a certain system, one protocol per layer, is called a protocol stack.

A message, M, is produced by an application process running in layer 5 and given to

layer 4 for transmission. Layer 4 puts a header in front of the message to identify the message

and passes the result to layer 3.

Information flow in a five layer model

As layer 3 imposes a limit on the size of the message transmitted, it must break up incoming

messages into smaller units, packets, prepending a layer 3 header to each packet. In this

example, M is split into two parts, M1 and M2. Layer 3 decides which of the outgoing lines to


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use and passes the packets to layer 2. Layer 2 adds not only a header to each piece, but also a

trailer, and gives the resulting unit to layer 1 for physical transmission. At the receiving machine

the message moves upward, from layer to layer, with headers being stripped off as it

progresses. None of the headers for layers below n are passed up to layer n.

Advantages of Layering architecture

Interoperability - Layering promotes greater interoperability between devices from

different manufacturers and even between different generations of the same type of device

from the same manufacturer.

Reduction of the Domino Effect - Another very important advantage of a layered protocol

system is that it helps to prevent changes in one layer from affecting other layers. This helps

to expedite technology development.

Modularity –

Task Segmentation - Breaking a large complex system into smaller more manageable

subcomponents allows for easier development and implementation of new technologies; as

well as facilitating human comprehension of what may be very diverse and complex

systems.

Enhanced Troubleshooting and Fault Identification - Troubleshooting and fault

identification are made considerably easier thus resolution times are greatly reduced.

Layering allows for examination in isolation of subcomponents as well as the whole.

Rapid Application Development (RAD) - Work loads can be evenly distributed which means

that multiple activities can be conducted in parallel thereby reducing the time taken todevelop, debug, optimize and package new technologies ready for production

implementation.

Promotion of Multi-Vendor Development - Layering promotes multi-vendor development

through the standardization of networking components at both the hardware and software

levels because of the clear and precise delineation of responsibilities that layering brings to

the developers' table.

Standardization and Certification - The layered approach to networking protocol

specifications facilitates a more streamlined and simplified standardization and certification

process.

Portability - Layered networking protocols are much easier to port from one system or

architecture to another


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Design Issues for the Layers

1.

Addressing: Mechanism for identifying senders and receivers, on the network need

some form of addressing. There are multiple processes running on one machine. Some

means is needed for a process on one machine to specify with whom it wants to

communicate.2. Error Control: There may be erroneous transmission due to several problems during

communication. These are due to problem in communication circuits, physical medium,

due to thermal noise and interference. Many error detecting and error correcting codes

are known, but both ends of the connection must agree on which one being used. In

addition, the receiver must have some mechanism of telling the sender which messages

have been received correctly and which has not.

3.

Flow control: If there is a fast sender at one end sending data to a slow receiver, then

there must be flow control mechanism to control the loss of data by slow receivers.

There are several mechanisms used for flow control such as increasing buffer size atreceivers, slow down the fast sender, and so on. Some process will not be in position to

accept arbitrarily long messages. Then, there must be some mechanism

to disassembling, transmitting and then reassembling messages.

4.

Multiplexing / demultiplexing: If the data has to be transmitted on transmission media

separately, it is inconvenient or expensive to setup separate connection for each pair of

communicating processes. So, multiplexing is needed in the physical layer at sender end

and demultiplexing is need at the receiver end.

5.

Routing: When data has to be transmitted from source to destination, there may be

multiple paths between them. An optimized (shortest) route must be chosen. Thisdecision is made on the basis of several routing algorithms, which chooses optimized

route to the destination.

Layers can offer two types of services namely connection oriented service and connectionless

service.

Connection oriented service:

The service user first establishes a connection, uses the connection and then releases the

connection. Once the connection is established between source and destination, the path is

fixed. The data transmission takes place through this path established. The order of the

messages sent will be same at the receiver end. Services are reliable and there is no loss of

data. Most of the time, reliable service provides acknowledgement is an overhead and adds

delay.


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Connectionless Services:

In this type of services, no connection is established between source and destination. Here

there is no fixed path. Therefore, the messages must carry full destination address and each

one of these messages are sent independent of each other. Messages sent will not be delivered

at the destination in the same order. Thus, grouping and ordering is required at the receiver

end, and the services are not reliable.

There is no acknowledgement confirmation from the receiver. Unreliable connectionless

service is often called datagram service, which does not return an acknowledgement to the

sender. In some cases, establishing a connection to send one short messages is needed. But

reliability is required, and then acknowledgement datagram service can be used for these

applications.

Another service is the request-reply service. In this type of service, the sender transmits a

single datagram containing a request from the client side. Then at the other end, server reply

will contain the answer. Request-reply is commonly used to implement communication in the

client-server model.

Service Primitives

A service is formally specified by a set of primitives (operations) available to a user process to

access the service. These primitives tell the service to perform some action or report on an

action taken by a peer entity.


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The set of primitives available depends on the nature of the service being provided. The

primitives for connection-oriented service are different from those of connectionless service.

The Relationship of Services to Protocols

A service is a set of primitives (operations) that a layer provides to the layer above it. The

service defines what operations the layer is prepared to perform on behalf of its users, but it

says nothing at all about how these operations are implemented. A service relates to an

interface between two layers, with the

lower layer being the service provider

and the upper layer being the service

user.

A protocol , in contrast, is a set of rules

governing the format and meaning of the

packets, or messages that are exchanged by the peer entities within a layer. Entities use

protocols to implement their service definitions. They are free to change their protocols at will,

provided they do not change the service visible to their users. Services relate to the interfaces

between layers, whereas protocols relate to the packets sent between peer entities on

different machines.

The active elements in each layer are often called entities, which can either be a software entity

or hardware entity. Entities in the same layer on different machines are called peer entities.

Services are available at SAP’s which have an unique address for identification. Layer n SAP’s

are the places where layer N+1 can access the services offered.


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Reference Model

A reference model is a conceptual layout that describes how communication between devices

should occur. A reference model has many advantages such as it defines standards for buildingnetwork components thereby permitting multiple-vendor development and also defines which

functions should be performed at each layer of the model thereby promoting the

standardization of network.

The OSI Reference Model

International standard organization (ISO) established a committee in 1977 to develop

architecture for computer communication and the OSI model is the result of this effort. In 1983,

ISO published a document called ‘The Basic Reference Model for Open Systems

Interconnection’, which visualizes network protocols as a seven-layered model. In 1984, the

Open Systems Interconnection (OSI) reference model was approved as an international

standard for communications architecture.

OSI is a standard reference model for communication between end users in a network. The

term Open system means a set of protocols using which a system can communicate with any

other system irrespective of the differences in their underlying hardware and software. The OSI

reference model divides the problem of moving information between computers over a

network medium into SEVEN smaller and more manageable problemsIn 1983, Day and Zimmerman laid down certain principles that were applied to arrive at the

seven layers can be briefly summarized as follows:

1.

A layer should be created where a different abstraction is needed.

2. Each layer should perform a well-defined function.

3. The function of each layer should be chosen with an eye toward defining internationally

standardized protocols.

4. The layer boundaries should be chosen to minimize the information flow across the

interfaces.

5. The number of layers should be large enough that distinct functions need not be thrown

together in the same layer out of necessity and small enough that the architecture does not

become unwieldy


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. OSI model consists of a seven separate but related layers namely, physical, data link,

network, transport, session, presentation and application layers.

The lower 4 layers (transport, network, data link and physical —Layers 4, 3, 2, and 1) are

concerned with the flow of data from end to end through the network. The upper four layers of

the OSI model (application, presentation and session—Layers 7, 6 and 5) are orientated more

toward services to the applications. Data is encapsulated with the necessary protocol

information as it moves down the layers before network transit.

Physical Layer {the physical layer is responsible for transmitting individual bits from one node to the next}

The physical layer is the lowest layer of the OSI hierarchy and coordinates the functions

required to transmit a bit stream over a physical medium. It also defines the procedures and

functions that physical devices and interfaces have to perform for transmission occur. The

physical layer specifies the type of transmission medium and the transmission mode (simplex,

half duplex or full duplex) and the physical, electrical, functional and procedural standards for

accessing data communication networks.

Transmission media defined by the physical layer include metallic cable, optical fiber cable or

wireless radio-wave propagation. The physical layer also includes the carrier system used to

propagate the data signals between points in the network. The carrier systems are simply


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communication systems that carry data through a system using either metallic or optical fiber

cables or wireless arrangements such as microwave, satellites and cellular radio systems.

Data-link Layer {the data link layer is responsible for transmitting frames from one node to the next}

The data link layer transforms the physical layer, a raw transmission facility, to a reliable

link and is responsible for node-to-node delivery. It makes the physical layer appear error free

to the upper layer (network layer).

The data link layer packages data from the physical layer into groups called blocks, frames or

packets. If frames are to be distributed to different systems on the network, the data link layer

adds a header to the frame to define the physical address of the sender (source address) and/or

receiver (destination address) of the frame. The data-link layer provides flow-control, access-

control, and error-control.

Network Layer {is responsible for the delivery of individual packets from the source host to the destination host}

The network layer provides details that enable data to be routed between devices in an

environment using multiple networks, subnetworks or both. This is responsible for addressing

messages and data so they are sent to the correct destination, and for translating logical

addresses and names (like a machine name FLAME) into physical addresses. This layer is also

responsible for finding a path through the network to the destination computer.


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The network layer provides the upper layers of the hierarchy with independence from the data

transmission and switching technologies used to interconnect systems. Networking

components that operate at the network layer include routers and their software.

Transport Layer {is responsible for delivery of a message from one process to another}

The transport layer controls and ensures the end-to-end integrity of the data message

propagated through the network between two devices, providing the reliable, transparent

transfer of data between two endpoints.

Transport layer responsibilites includes message routing, segmenting, error recovery

and two types of basic services to an upper-layer protocol: connection oriented and

connectionless. The transport layer is the highest layer in the OSI hierarchy in terms of

communicatons and may provide data tracking, connection flow control, sequencing of data,

error checking, and application addressing and identification.

Session Layer {responsible for dialog control and synchronization}Session layer, some times called the dialog controller provides mechanism for

controlling the dialogue between the two end systems. It defines how to start, control and end

conversations (called sessions) between applications.

Session layer protocols provide the logical connection entities at the application layer. These

applications include file transfer protocols and sending email. Session responsibilities include


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network log-on and log-off procedures and user authentication. Session layer characteristics

include virtual connections between applications, entities, synchronization of data flow for

recovery purposes, creation of dialogue units and activity units, connection parameter

negotiation, and partitioning services into functional groups.

Presentation Layer {responsible for translation, compression, and encryption}

The presentation layer provides independence to the application processes by

addressing any code or syntax conversion necessary to present the data to the network in a

common communications format. It specifies how end-user applications should format the

data.

The presentation layer translated between different data formats and protocols. Presentation

functions include data file formatting, encoding, encryption and decryption of data messages,

dialogue procedures, data compression algorithms, synchronization, interruption, and

termination.

Application Layer {responsible for providing services to the user}

The application layer is the highest layer in the hierarchy and is analogous to the general

manager of the network by providing access to the OSI environment.

The applications layer provides distributed information services and controls the sequence of

activities within and application and also the sequence of events between the computer

application and the user of another application. The application layer communicates directly


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with the user’s application program. User application processes require application layer

service elements to access the networking environment. The service elements are of two types:

CASEs (common application service elements) satisfying particular needs of application

processes like association control, concurrence and recovery. The second type is SASE (specific

application service elements) which include TCP/IP stack, FTP, SNMP, Telnet and SMTP.

OSI Model Data F low

The sending process passes data to the application layer. The application layer attaches

an application header and then passes the frame to the presentation layer.


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TCP/IP model has four different layers.

1. Internet Layer

The internet layer is the linchpin that holds the whole architecture together. Its job is to permit

hosts to inject packets into any network and have them travel independently to the destination

(potentially on a different network). They may even arrive in a completely different order thanthey were sent, in which case it is the job of higher layers to rearrange them, if in-order delivery

is desired. Internet Protocol (IP) is the most important protocol in this layer. It is a

connectionless protocol and does not provide reliability, flow control, or error recovery. P

provides a routing function that attempts to deliver transmitted messages to their destination.

A message unit in an IP network is called an IP datagram. This is the basic unit of information

transmitted across TCP/IP networks. Other internetwork-layer protocols are IP, ICMP {Internet

Control Message Protocol}, IGMP {Internet Group Management Protocol}, ARP {Address

Resolution Protocol}, and RARP {Reverse ARP}.

2. Transport Layer

The layer above the internet layer in the TCP/IP model is the transport layer and its designed to

allow peer entities on the source and destination hosts to carry on a conversation, just as in the

OSI transport layer. Two end-to-end transport protocols namely TCP {Transmission Control

Protocol} and UDP {User Datagram Protocol} have been defined.

TCP is a reliable connection-oriented protocol that permits a byte stream originating on one

machine to be transported without error on any machine in the internet. It divides the

incoming byte stream into discrete message and passes each one onto the internet layer. At the

destination, the receiving TCP process collects the received message into the output stream.

TCP deals with flow control to make sure a fast sender cannot swamp a slow receiver with more

message than it can handle.

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3. Application Layer

In TCP/IP model, session or presentation layer are not present. Application layer is present on

the top of the Transport layer. It includes all the higher-level protocols which are virtual

terminal (TELNET), file transfer (FTP) and electronic mail (SMTP). The virtual terminal protocol

permits a user on one machine to log into a distant machine and work there. The file transfer

protocol offers a way to move data efficiently from one machine to another. Electronic mail

was used for file transfer purpose but later a specialized protocol SMTP, was developed for it.

FTP was designed to permit reliable transfer of files over different platforms and it uses TCP to

ensure reliability. HTTP permits applications such as browsers to upload and download web

pages. It makes use of TCP at the transport layer again to check reliability. HTTP (Hyper Text

Transfer Protocol) is a connectionless protocol that sends a request, receives a response and

then disconnects the connection. HTTP delivers HTML documents plus all the other

components supported within HTML such as JavaScript, Visual script and applets. By using TCP,

SMTP sends email to other computers that support the TCP/IP protocol suite. SMTP (Simple

Mail Transfer Protocol) provides an extension to the local mail services that existed in the early

years of LANs. It supervises the email sending from the local mail host to a remote mail host. It

is not reliable for accepting mail from local users or distributing received mail to recipients this

is the responsibility of the local mail system. For the transport of network management

information, SNMP (Simple Network Management Protocol) is used as standardized protocol.

Managed network devices can be cross-examined by a computer running to return details

about their status and level of activity. To reduce the overhead of network traffic, SNMP uses

UDP at the transport layer.

4.

Host to Network Layer

The network interface layer, also called the link layer or the data-link layer, is the interface to

the actual network hardware. This interface may or may not provide reliable delivery, and may

be packet or stream oriented. In fact, TCP/IP does not specify any protocol here, but can use

almost any network interface available, which illustrates the flexibility of the IP layer. Examples

are IEEE 802.2, X.25,ATM, FDDI, and even SNA.TCP/IP specifications do not describe or

standardize any network-layer protocols, they only standardize ways of accessing those

protocols from the internetwork layer.


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A Comparison of the OSI and TCP/IP Reference Models

OSI and TCP/IP models are based on the concept of a stack of independent protocols. Functions

of the layers are more or less similar.

Three concepts are central to OSI Model: Services, Interfaces, and Protocols.

Services: This definition tells what the layer does. It defines the layers semantics.

Interface: It tells the process above it how to access it.

Peer Protocols: Protocols used in a layer are the layer’s own business.

TCP/IP model did not clearly distinguish between service, interface and protocol. The protocols

in the OSI model are better hidden than in the TCP/IP model and can be replaced relatively

easily as the technology changes. It is learnt that the OSI model was devised before the

protocols were invented. This ordering means that the model was not biased toward oneparticular set of protocols. Downside of this ordering is that the designers did not have much

experience with the subject and did not have good idea of which functionality to put in which

layer. The committee originally expected that each country would have one network ,run by the

government and using the OSI Protocols, so no thought was given to internetworking.

With TCP/IP, the protocols came first and the model was really just a description of existing

protocols. Protocols fit the model perfectly. The only trouble was that the model did not fit any

other protocols stacks. The difference between two models is the number of layers: the OSI

model has seven layers and the TCP/IP has four layers .Both have (inter)network, transport andApplication layers, but the other layers are different.

Another difference is in the area of connectionless and connection oriented communication.

OSI Model supports connection oriented communication in transport layer, whereas in network

layer it supports both connectionless and connection oriented. The TCP/IP model has only one

mode in the network layer but supports both modes in transport layer.

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Physical Layer

The physical layer is the first layer of the Open System Interconnection Model (OSI Model). The

physical layer deals with bit-level transmission between different devices and supportselectrical or mechanical interfaces connecting to the physical medium for synchronized

communication.

Analog data is defined as the data having continuous states and digital data is defined as the

data having discrete states. Analog signal is a signal that passes through and includes a wide

range of varying values of intensity over a period of time, whereas a signal that has only a finite

range of values is called as a digital signal.


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Periodic signal exhibits a specific signal pattern that repeats over time, whereas non-periodic

(aperiodic) does not repeat any specific signal pattern. A composite signal is a collection of one

or more signals having different frequencies, amplitude and phases. It can be periodic or

aperiodic signal.

The time required to transmit a character depends on both the encoding method and the

signaling speed (i.e., the modulation rate - the number of times/sec the signal changes its

voltage)

• Baud (D) - the number of changes per second

• Bandwidth (H) - the range of frequencies that is passed by a channel. The transmitted

signal is constrained by the transmitter and the nature of the transmission medium in

cycles/sec (hertz)

• Channel capacity (C) – the rate at which data can be transmitted over a given channel

under given conditions.{This is also referred to as data rate (R)}

Nyquist theorem: To transmit at a transmission rate of fb Hz requires a minimum bandwidth of

Hmin = fb/2 Hz → This specifies the maximum data rate for the noiseless case as:

Shannon’s theorem: If information rate does not exceed channel capacity, there exists a

coding technique such that information can be transmitted over a noisy channel, error free. The

channel capacity provides the maximum possible data rate for the general noisy case as:

Network data rate is limited by the choice of medium and communication technology

Some of the advantages of digital transmission are:

1.

Repeaters are used for long distance communication does not induce noise as with the

case of amplifiers in analog transmission.

2. Digital data & digitalized analog data can be easily encrypted and decrypted.

3.

With LSI & VLSI, size and cost of digital equipment has been significantly reduced.4. Multiplexing can be done easily in order to increase the channel capacity.


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Transmission Media

The transmission medium is the physical path between transmitter and receiver in a data

transmission system. It is included in the physical layer of the OSI protocol hierarchy. The

transmission medium is usually free space, metallic cable, or fiber-optic cable. The information

is usually a signal that is the result of a conversion of data from another form.

Transmission media can be generally categorized as either unguided or guided. Guided

Transmission Media uses a "cabling" system (or some sort of conductor) that guides the data

signals along a specific path. The data signals are bound by the "cabling" system. Guided Media

is also known as Bound Media. The conductor directs the signal propagating down it. Only

devices physically connected to the medium can receive signals propagating down a guided

transmission medium. Examples of guided transmission media are copper wire and optical

fiber.

Unguided Transmission Media consists of a means for the data signals to travel but

nothing to guide them along a specific path. The data signals are not bound to a cabling media

and as such are often called Unbound Media. Unguided transmission media are wireless

systems. Signals propagating down an unguided transmission medium are available to anyone

who has a device capable of receiving them.

A physical facility is one that occupies space and has weight as opposed to wireless

media such as earth’s atmosphere or a vacuum and includes metallic cables and optical cables.

Metallic transmission lines includes open-wire, twin-lead, and twisted-pair copper wire as wellas coaxial cable, and optical fibers include plastic- and glass-core fibers encapsulated in various

kinds of cladding materials.


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Guided Media

Various types of physical medium exist and each one has its own niche in terms of bandwidth,

delay, cost, ease of installation and maintenance. For guided transmission media, the

transmission capacity, in terms of either data rate or bandwidth, depends critically on the

distance and on whether the medium is point-to-point or multipoint.

Magnetic Media

One of the most common ways to transport data from one computer to another is to write

them onto magnetic tape or removable media (e.g., recordable DVDs), physically transport the

tape or disks to the destination machine, and read them back in again. Though this is more cost

effective, especially for applications in which high bandwidth or cost per bit transported is the

key factor, the delay characteristics are very poor.

Twisted pair

Twisted pair is the simplest, oldest and low priced cable medium. It is made up of two insulated

copper wires about 1mm thick, twisted around each other in a continuous spiral. The purpose

of twisting the wires is to reduce electrical interference (or noise) from similar pairs close by.

The most common application of the twisted pair is the telephone system. Twisted pairs can

run several kilometers without amplification, but for longer distances, repeaters are needed.

Twisted pairs can be used for transmitting either analog or digital signals. The bandwidth

depends on the thickness of the wire and the distance traveled, but several megabits/sec can

be achieved for a few kilometers in many cases. Two basic types of twisted-pair cable exist:

Unshielded twisted pair (UTP) and shielded twisted pair (STP).

Unshielded Twisted Pair (UTP)

An UTP cable (category 5) is one of the most popular LAN cables. This cable consists of 4twisted pairs of metal wires (that means there are 8 wires in the cable). Each pair is twisted

with a different number of twists per inch to help eliminate interference from adjacent pairs

and other electrical devices. Each twisted pair consists of two metal conductors that are

insulated separately with their own coloured plastic insulation.


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UTP cable relies solely on the cancellation effect produced by twisted wire pairs to limit the

signal degradation caused by electromagnetic interference and radio frequency interference.

To further reduce crosstalk, the number of twists in the wire pairs varies. UTP cable is often

installed using a Registered Jack 45 (RJ-45) connector. The RJ-45 is an eight-wire connector used

commonly to connect computers onto a local area network (LAN), especially Ethernet. UTP

cables are suited for both data and voice transmissions; hence are commonly used in telephone

systems. They are also widely used in DSL lines, 10Base-T and 100Base-T local area networks.

UTP has several advantages as it is the cheapest media, easy to install and maintain. It also

occupies less space. It is the fastest copper-based medium today. Different categories of UTP

are:


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Shielded Twisted Pair (STP)

This cable has a metal foil or braided-mesh covering that covers each pair of insulated

conductors. The metal foil is used to prevent infiltration of electromagnetic noise. This shield

also helps to eliminate crosstalk.

STP reduces electrical noise both within the cable and from outside the cable. STP is suited for

environments with electrical interference and also provides better performance at higher data

rates. But the extra shielding makes the STP cables quite bulky and more expensive that UTPcables. Also, the metallic shielding must be ground at both ends. Or else it acts as an antenna

picking up unwanted signals.

Coaxial cable

It is also one of the common transmission medium (called as coax) in current day data

communications. They are also relatively inexpensive, but

most costly than UTP on a per-unit length.

A coaxial cable consists of a stiff central conductor (copper

wire) as the core, surrounded by an insulating material. The

insulator is encased by a cylindrical conductor, often as a

closely-woven braided mesh. The outer conductor is covered

in a protective plastic sheath known as jacket. Although

coaxial cabling is difficult to install, it is highly resistant to


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signal interference. It can support greater cable lengths between network devices and greater

bandwidth than twisted-pair cable. Coaxial cables are capable of transmitting data at a fast rate

of 10Mbps.

Thicknet and Thinnet are two varieties of coaxial cable, but rarely used. Ethernet can run

approx 100mts (328 feet) with UTP, while coaxial cable increases this distance to 500mts (1640

feet). The RG numbering system used with coaxial cables refers to cables approved by U.S.

Department of Defense (DoD).

Categories of coaxial cablesTo connect coaxial cable to devices, it is necessary to use coaxial connectors. The most

common type of connector is the Bayone-Neill-Concelman, or BNC, connectors. BNC connectors

are sometimes referred to as bayonet mount, as they can be easily twisted on or off.

There are three types: the BNC connector, the BNC T connector, the BNC terminator. Coaxial

cable applications include analog and digital telephone networks, cable TV networks, Ethernet

LANs and short range connections.

Fiber-optic Cable

Fiber-optic cable or optical fiber consists of thin glass fibers that can carry information in the

form of visible light. The typical optical fiber consists of a very narrow strand of glass or plasticcalled the core . Around the core is a concentric layer of less dense glass or plastic called the

cladding , whose refractive index is less than that of the core. The outer most layer of the cable

is known as the jacket, which shields the cladding and the core from moisture, crushing and

abrasion. Optical fibers transmit a beam of light by means of total internal reflection. When a

light beam from a source enters the core, the core refracts the light and guides the light along


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its path. The cladding reflects the light back into the core and prevents it from escaping through

the medium.

Fiber optic cables support two modes of propagating light, which are multimode and single

mode. In multimode, many beams from a light source traverse along multiple paths and at

multiple angles. In single mode, the beams propagate almost horizontally.

LED or LASER (Light Amplification by Stimulated Emission of Radiation) acts as the source

converting electric pulse to light pulses and photodiode acts as receiver doing viceversa. Fiber

optic cables uses 3 types of connectors, which are :

SC (Subscriber Connector)- used to connect cable TV

ST (Straight Tip)- to connect network devices

MT-RJ (Mechanical Transfer-Registered Jack)- for network applications.


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Documents

Introduction to Networks, Reference Models